The Geology of the LLWR Site and

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LLWR ESC
The Geology of the LLWR Site and
Surrounding Region
Uisdean Michie
John Hunter
George Towler
QRS-1443Y-R1
Version 2.0 (Final)
October 2010
Document History
Title:
LLWR ESC
Subtitle:
The Geology of the LLWR Site and Surrounding Region
Client:
LLWR
Document Number:
QRS-1443Y-R1
Version Number:
Version 0.1
Notes:
Draft report submitted to peer review workshop involving
Date: March 2010
LLWR, NNL, Halcrow, BGS and Serco
Prepared by:
Uisdean Michie, John Hunter, George Towler
Reviewed by:
Uisdean Michie and John Hunter are the principal authors.
They cross-checked each others work, while George Towler
provided an overall review.
Version Number:
Version 1.0 (draft)
Notes:
Draft final report submitted to LLWR for review.
Prepared by:
Uisdean Michie and John Hunter
Reviewed by:
George Towler
Version Number:
Version 1.0 (Final)
Notes:
Final report following workshop and LLWR review
Prepared by:
Reviewed by:
George Towler
Approved by:
George Towler
Version Number:
Version 2.0 (DraftFinal)
Notes:
Incorporate updates to 3D Geological Model
Quintessa Limited
The Hub, 14 Station Road
Henley-on-Thames
Oxfordshire RG9 1AY
United Kingdom
Date: April 2010
Date: 23 October 2010
Tel: +44 (0) 1491 636246
Fax: +44 (0) 1491 636247
[email protected]
www.quintessa.org
www.quintessa-online.com
Prepared by:
Reviewed by:
George Towler
Version Number:
Version 2.0 (Final)
Notes:
Outstanding reference added. No other changes.
Prepared by:
George Towler
Reviewed by:
George Towler
Approved by:
George Towler
This report uses data extracted from British Geological Survey (BGS) maps and supporting
documents under the LLWR license IPR/117-4DR
QRS-1443Y-R1, Version 2.0 (Final)
Summary
This report is an updated and comprehensive review of the geology of the LLWR site
and its wider region. It is one of a number of reports produced to underpin the 2011
Environmental Safety Case (ESC) that describe the understanding of the LLWR site
and its wider environs. The geological understanding supports other aspects of the site
understanding, particularly the hydrogeological understanding, but also the
engineering design, design optimisation, and hence performance assessment.
The geology of the LLWR site is described in the context of the wider regional
Quaternary geology, both onshore and offshore under the East Irish Sea, and is related
to recent syntheses of the geological understanding by the British Geological Survey
(BGS) and others. The report also describes how understanding of the geology of the
LLWR site has changed with time, and why the current lithofacies approach is
considered to be the most appropriate approach to underpin the ESC.
The LLWR site is located on a generally thick sequence of unconsolidated Late
Quaternary sediments, which were deposited on an irregular surface of Triassic
Sherwood Sandstone bedrock. The Quaternary sediments consist of a wide range of
material types of significantly different properties, which have complex spatial
relationships. They are the focus of the geological interpretation because the LLWR is
developed within these sediments, and because the nature of the Quaternary
sediments is an important control on the long-term safety of the site: in particular with
regards to the site hydrogeology and aqueous contaminant transport; and the
vulnerability of the site to coastal erosion.
The complexity of the Quaternary sediments has been revealed as a consequence of the
extensive investigations that have been undertaken at the LLWR, including drilling a
very large number of site investigation boreholes. A much sparser dataset would hide
the complexity. As a consequence of this complexity, despite the very large amount of
data available for the LLWR site and wider region, uncertainties remain, particularly
with regards to the chronostratigraphic (time) correlation of individual deposits.
However, it is only uncertainty regarding the spatial distribution of materials that is
important to the ESC.
An important factor in the comprehensive geological understanding of the site is its
interpretation within the well-defined framework of the regional onshore and offshore
geology. There are still some spatial data limitations, for example only limited borehole
data are available at depth and between the site and the coast, relative to the very large
amount of shallow borehole data. However, it is proposed that collection of large
i
amounts of additional deep and offsite borehole data is unlikely to have significant
consequences for the ESC. This is because sufficient understanding is available
regarding the key parts of the system to identify any hydrogeologically significant
features that might be present (e.g. incised channels), and to define length scales of
heterogeneity and explore their impacts. More generally, it can be stated that due to the
very large data set, the natural variability and complexity can be understood and
bounded in safety assessment calculations. Therefore the geological interpretation is
considered to be appropriate to underpin the ESC.
A summary of the recent approach to the geological interpretation and developments
subsequent to the Schedule 9 Requirement 2 site understanding update, provided to
the Environment Agency in 2008, is given below.
Prior to 2007, the geological understanding of the Quaternary glacigenic sediments
underlying the LLWR site and its surrounding area had been described in a series of
widely-read reports which had been prepared by a succession of several different
authors. All of these reports utilised ‘layered sequence’ models to characterise the
nature of the glacial deposits, i.e., the preserved sediments were defined as a sequential
accumulation of layers of variable mixtures of clay, silt, sand and gravel material.
Individual layers were assigned to particular glaciation processes and events that
occurred during the Quaternary. These models made significant assumptions when
assigning such processes and events to the resulting individual layers of sediment and
also when identifying the boundaries between the layers and correlating these layers
between borehole logs.
Because of the limited number of surface outcrops of Quaternary deposits in the LLWR
area and the adjacent inland district, both of the principal layered sequence models
(i.e., the LLWR model for the site and the British Geological Survey (BGS) model for
the surrounding district) were constructed from a limited series of two-dimensional
cross-sections using a small number of selected boreholes – usually those boreholes
that had been drilled through the complete Quaternary succession to intersect the
underlying surface of the Triassic sandstone bedrock. The authors of these models
found difficulty in equating the layered sequences of sediment identified within the
LLWR (on-site) using one set of borehole logs, with those identified in the surrounding
inland area (off-site, or regional area) from other borehole logs, and also with the
preserved glacial sediments underlying the nearby seabed (offshore) identified from
geophysical surveying and offshore boreholes.
The correlation problems between the models were compounded by the spatial data
gap (i.e., the lack of boreholes) between the LLWR boreholes east (inland) of the site
and the area to the NE investigated in detail by the BGS as part of the Nirex
investigations. This lack of simple correlation between different sets of layered
ii
sequences resulted in the adoption of different stratigraphical names for the on-site,
off-site and offshore sequences. Both of the principal layered sequence models also had
difficulty in accommodating some of the variable features in the glacigenic deposits
that had been discovered by different phases of borehole drilling within the LLWR.
In 2007, the present authors prepared a report which reassessed the database of
historical borehole lithology logs and which, utilising most of the available data rather
than a limited selection of boreholes, proposed an alternative representation of the
LLWR Quaternary geology – termed a ‘lithofacies’ model. In this model, more
emphasis was placed upon descriptive analysis of the sedimentary materials and fewer
assumptions and inferences were made regarding the processes and events that may
have formed them. This approach was selected because the spatial distribution of
material types is the key input from the geological understanding to the ESC
(including the engineering design, hydrogeology, etc).
The 2007 model subdivided the sedimentary deposits into lithofacies units composed
of similar material types and the report outlined the procedure used to identify the
likely boundaries between the units. Some of the lithofacies units were shown to occur
as partial sequences of layers. However, this spatial arrangement is not a rigid
requirement and the model easily accommodates the spatial variability of sediment
types that is known from the borehole drilling records and also the apparent lateral
discontinuities that separate them.
In 2008 and 2009 two phases of additional borehole drilling were completed within the
LLWR and in the immediate off-site area on the coastal side. Geophysical surveying
and other scientific studies were also conducted. The lithology logs from the recent
boreholes provide new information that in the opinion of the authors, supports the
lithofacies model as a valid characterisation of the LLWR site. This report updates,
clarifies and refines the lithofacies model, but there are no major changes to the
framework established earlier.
The 2007 report described the spatial distribution of materials at the regional scale as a
number of lithofacies units (labelled A to D), while the spatial distribution of materials
at the site scale was described as a separate number of lithofacies packages (labelled
LP1 to LP7). Further work for this report has enabled the site and regional scale
interpretations to be integrated, and the current interpretation now uses a unified set of
lithofacies units, with the previous local LP nomenclature being considered
unnecessary and therefore replaced by the A-D units. These units can be mapped to the
BGS regional lithostratigraphy in a general sense. However, the BGS’ lithostratigraphy
has not been adopted because it is less focused on the spatial distribution of material
types, and because considerable uncertainties arise when trying to describe the
complex sequence of deposits at the LLWR using the BGS lithostratigraphy, e.g. in a
iii
given LLWR borehole there may be more or fewer till units than described by the BGS’
lithostratigraphy.
This report describes the spatial distribution of materials for the LLWR site and wider
region using a significant number of maps and cross-sections, which utilise almost all
the available data. These form the basis for a refined 3D model of the geology of the
LLWR site and surrounding area and are being incorporated into an updated 3D
computer model of the geology. The large amount of detailed geological data assessed
for this study is presented in this report using a comprehensive set of (2D) crosssections and also some boundary surfaces presented as rendered 3D images. Some of
the cross-sections for the regional area are diagrammatic, with a large vertical
exaggeration, to illustrate the details of the information from borehole logs. The
sections for the LLWR site provide a comprehensive network across the site.
Since Version 1 of this report was issued in June 2010, two further reports have been
prepared to accompany the updated 3D computer geological model. In the process of
digitising geological cross-sections for that work, some minor inconsistencies between
the exact positions of certain lithofacies boundaries on a few intersecting cross-sections
were revealed. Adjustments have been made to the relevant cross-sections to eliminate
these inconsistencies and to make the 2D cross-sections directly comparable to the 3D
model. The amended cross-sections, appropriately identified, have been substituted
into Version 2 of this report.
There is currently ongoing work to investigate the palaeo coastal evolution in the area
of the Drigg spit. The results from these studies will be available for inclusion into the
next update of the geological interpretation.
iv
Contents
Glossary
1
1
Introduction
4
2
Geology of the LLWR area of West Cumbria
7
2.1 Bedrock geology
9
2.2 Quaternary sediments
2.2.1 Application of a lithofacies approach
3
13
16
2.3 Sources of data
18
2.4 Lithological description of superficial deposits
21
Regional scale lithofacies units
23
3.1 Regional lithostratigraphical units
23
3.1.1 Age sequence and correlation of the Quaternary lithostratigraphical units
29
3.2 Offshore seismic stratigraphy
31
3.2.1 Correlation of the offshore seismic stratigraphy and the onshore
lithostratigraphy
4
36
3.3 Development of the regional lithofacies units
36
3.4 Lithofacies Units
43
Characterisation of the Quaternary geology at the LLWR site
51
4.1 The concepts of ‘Lithostratigraphical Formations’ and ‘Lithofacies’
57
4.2 Additional review of data for this report
59
4.3 Updated contour map of sandstone bedrock surface
61
4.4 Updated LLWR lithofacies model: the east-west cross-section and southwestnortheast section 8
64
4.5 Updated LLWR lithofacies model: northwest-southeast cross-sections 1 & 2 71
4.6 Updated LLWR lithofacies model: northwest-southeast cross-section 5
80
4.7 Discussion of uncertainties
83
5
Integration of the regional scale and site scale lithofacies packages
86
6
Application of the lithofacies units to hydrogeological modelling
89
6.1 Heterogeneity
92
7
Conclusions
94
8
References
95
Appendix A – Development of the understanding of the Quaternary sequence for
the area of the LLWR
99
Appendix B – LLWR site borehole cross-sections
117
v
Appendix C – Summary of lithology code consolidation
137
Appendix D - Lithofacies colour palette
143
Appendix E - Adaptation of 2D cross-sections into a 3D model
144
vi
Glossary
Term
Description
Deglaciation
Time period at the end of a glaciation when ice sheets melted
and retreated from their maximum extent. Climate reversals and
ice sheet dynamics during the deglacial period produce
readvances and oscillations in the position of the margin of the
ice sheet.
Diamict
A non genetic term that describes unsorted and predominantly
massive sediment consisting of variable admixtures of grain
sizes, which may range from clay to boulders. If the diamict
material can be shown to be of glacial origin, then the glacial
diamict is classified as till.
Glacial deposits
Sediments deposited directly by a glacier or ice sheet.
Glacigenic
A deposit that is glacially related, but not necessarily deposited
directly by ice.
Glacioeustatic
Global changes in sea level due to the transfer of water from the
sea to ice sheets on land.
Glaciomarine
Sediments deposited marginal to ice sheets in marine
environments.
Glaciotectonic
Deformation of previously deposited glacigenic sediments by
vertical compression or lateral movement of later ice cover.
Interfluve
An area of land separating two rivers. In the floodplain of
meandering and braided river systems, the interfluves often
consist of areas of finer-grained (silty, clayey) sediments that
have accumulated from repeated flooding events from the same
rivers and which separate active river channels and their
associated sand bars. However in the context of this report, the
term is applied to the non-fluvial glacigenic material (generally
clayey) that separates interpreted fluvial channel deposits (sands
and gravels) that may have formed from either surface or subglacial meltwater drainage.
Isostatic
The depression of the underlying bedrock and adjoining
1
depression
marginal area due to the weight of overlying ice. Bedrock at a
greater distance from the ice sheet can be raised due to deeper
sub-crustal movement of material from beneath the ice sheet.
Late Devensian
The British name for the sediments deposited on the present
land area of the British Isles during the time period from about
30,000 to 10,000 years ago when large ice sheets developed and
expanded over the northern parts of North America, Europe and
Asia. (The name Devensian is derived from the Latin name for
the River Dee in Cheshire, an area where there are thick deposits
from the glaciation. The equivalent name Weichselian is used in
Europe and for the deposits under the North Sea and other
offshore areas.}
Lithofacies
The physical (and partly chemical) characteristics of a body of
sediment that distinguish it from other sediments. It includes
the internal features of the sediment unit and the external
relationships with adjoining sediments. This approach allows
for the discontinuous nature of layers of a particular sediment
type, and lateral variation into different sediment types of the
same age.
Lithostratigraphy
The formal vertical subdivision and age correlation of sequences
of layers of sediments (beds) by means of sediment type. This
approach relies on bed-for-bed correlation and can lead to
assumptions of lateral continuity of units.
Outwash
Generally sandy and gravelly sediments deposited by meltwater
outside an ice sheet margin.
Palaeovalley
A valley infilled by younger sediments.
Pinning point
An upstanding bedrock feature that forms a stable position over
a period of time for the ice margin during ice advance or retreat.
Subaqueous
Sediments deposited into a body of water.
Subglacial
The complex range of environments beneath an ice sheet.
Towards the margin of an ice sheet, subglacial meltwater can be
very important in modifying the sediments deposited.
Till
Genetic term for the unsorted admixture of debris eroded and
deposited by an ice sheet and not significantly reworked by
2
meltwater. The thickest till deposits are formed at the margins of
ice sheets. Where the subglacial deposits are generally
structureless and contain boulder-sized clasts in a clay matrix,
they are often termed boulder clay. Tills deposited directly
beneath an ice sheet are termed lodgement till, which may be
deformed by the movement and weight of the ice (deformation
till). More clay-rich tills termed melt–out and flow tills are
formed by the deposition of material from within or above the
ice sheet during the later stages of the melting of stagnant ice.
Transgression
The rise in the sea over the land surface due to rise in the
relative sea level. Sediments deposited during the transgression
show lateral facies changes from coarse beach deposits adjacent
to the land to deeper water silts and muds offshore.
Tunnel valley
A deep channel cut by a ‘river’ of subglacial meltwater into
older sediments or bedrock. Because the water is confined by the
overlying ice sheet, incision can develop to well below
contemporaneous sea level. Infilling of the channel can be by a
range of sediments from coarse gravel to marine silts and muds.
Unconformity
A break in deposition of a sequence of sediments. It is
particularly noted where there was removal (erosion) and/or
tilting of the older sediments, and subsequent deposition of the
younger sediments across different layers of the older sediments
(angular unconformity).
3
1
Introduction
This report describes the geology of the LLWR site and the surrounding region. It
updates the lithofacies (i.e., sediment material type) model of the site’s Quaternary
geology, developed by Hunter et al. (2007a) in response to review of the 2002 Drigg
PCSC. The report does not present a complete 3-dimensional (3D) reconstruction of the
different lithofacies that comprise the Quaternary deposits beneath the LLWR site, and
adjacent areas of interest (e.g. potential groundwater transport paths). However, this
report approximates a 3D reconstruction by means of a considerable number of 2dimensional geological cross-sections orientated in different directions. A regional
scale 3D geological model, with some localised refinement of the LLWR site, has
previously been developed based on the first iteration of the lithofacies approach. The
3D model has been amended to incorporate the updated and detailed lithofacies
interpretation at both the regional and the LLWR site-scale that is described in this
report (Hunter 2010, and Smith 2010a).
The key purpose of the development of a reliable geological understanding of the
LLWR site is to provide an adequate representation of the site’s Quaternary geology
for use in hydrogeological modelling, in studies of contaminant transport, in civil
engineering (vault development) work, and in studies of coastal erosion. The report
provides a clear explanation of why the lithofacies model is the most appropriate to
meet this purpose. The report provides the information needed to support the
development of the 3D geological model (Smith, 2009a, 2010a), and other geological
information required for hydrogeological modelling, etc and production of an ESC.
The LLWR site and its surrounding area has been subjected to intensive surface and
sub-surface investigation for at least the previous three decades, yet despite this
scientific effort, the detailed character of the late Quaternary and Holocene sediments
underlying the LLWR site is still not completely understood, particularly with regard
to the sequence of depositional events and the chronostratigraphic (time) relationships
between different deposits. This is partly a consequence of the complexity of the local
geology resulting from the location of the site where the Irish Sea and inland ice
masses coalesced during the last glacial maximum, partly because most of the geology
is concealed and can only be interpreted from borehole records and partly because of
the high level of geological understanding expected by regulators for a licensed
repository site. The complexity has also been revealed as a consequence of the very
large number of boreholes that have been drilled at the LLWR site. A much sparser
dataset would hide the complexity.
The geology of the LLWR site is described in the context of the regional Quaternary
geology surrounding the site and also of the nearby offshore seabed sediments. The
4
lithofacies approach is applied at the site and regional scales. The report explains how
the lithofacies approach provides a regional framework for understanding the complex
nature of the thick deposits of Quaternary sediments preserved beneath the LLWR site
on a concealed bedrock surface that has significant topographical relief. The
advantages of this lithofacies approach in providing the basis for the production of 3D
models of the spatial distribution of the material properties of the late Quaternary
sediments in the region surrounding the LLWR site, and in detail for the LLWR site are
discussed.
Uncertainties and problems identified from incorporation of new data from the LLWR
site into the 3D geological model prior to this report (Smith, 2009a, 2009b) have been
addressed and new information resulting from ongoing investigations such as the
geophysical studies (Halcrow, 2010) have been included where relevant.
To achieve its objective, it is necessary for the report to include a summarised ‘audit
trail’ of previous interpretations of the LLWR site showing how the geological
understanding of the Quaternary deposits beneath the site has evolved and how the
lithofacies
model
compares
to
the
alternative
models
as
a
representative
characterisation of the site.
Section 2 describes the bedrock and Quaternary geology of the LLWR area of the West
Cumbria (the regional geology). Section 3 then describes the development of lithofacies
units at the regional scale, the associated material types and relationship with the BGS’
lithostratigraphy. Section 4 describes the Quaternary lithologies at the LLWR site and
Section 5 describes the relationships between the site and regional lithofacies. Section 6
describes the likely hydrogeological properties of the lithofacies units as an input to
hydrogeological modelling. Section 7 draws conclusions regarding the status of the
geological interpretation for the ESC.
Five Appendices are also presented. Appendix A provides further information
summarising the development of the classification of the sediment units in the
Quaternary sequence leading to the lithofacies framework.
The
successive
classifications of the sediment sequence in selected cross sections of the LLWR site at
key stages in the development of the geological understanding are presented and
reviewed, and their relationship the lithofacies framework identified. Appendix B
provides a complete suite of detailed cross-sections that illustrate the geology of the
LLWR site and adjacent areas of interest. Appendix C outlines how the extensive
database of lithological descriptions of borehole samples from the LLWR has been
rationalised and condensed to enable the lithofacies cross-sections of the site to be
produced. Appendix D gives details of the lithofacies colour palette so that users of this
report can maintain a consistent colour scheme in future studies. Finally, Appendix E
5
summarises the process used to adapt the geological information contained in the 2D
cross-sections for incorporation into the 3D computer model.
6
2
Geology of the LLWR area of West
Cumbria
This section describes the bedrock and Quaternary geology of the LLWR area of west
Cumbria, i.e. the regional geology. It includes the geological setting of the LLWR site,
and provides the background to development of the regional lithofacies units for the
Quaternary deposits described in Section 3. It also describes the sources and the
varying quality of the available data and defines the principal concepts that are used to
explain the site-scale lithofacies units that are described in more detail in Section 4.
Because the LLWR is a surface facility constructed within a generally thick sequence of
Quaternary sediments, the nature and spatial relationships of the Quaternary
sediments are the most important features of the geology for the ESC, engineering, etc.
Therefore the emphasis in this description of the current understanding of the geology
is on the Quaternary sediments. The nature of the underlying bedrock is only outlined
where relevant to the hydrogeological modelling of the area around the LLWR.
There is a broad division into the older rocks forming the higher area of the Lake
District in the east, and the younger sedimentary rocks forming the bedrock to the
coastal plain and the sediments under the East Irish Sea in the west. The older rocks are
metamorphic (deformed and recrystallised at great depth), or igneous (originally
molten), either formed at surface (volcanic and pyroclastic) or intruded into earlier
rocks at depth (granite). These older rocks also occur at depth below the younger
sedimentary rocks to the west and north.
Figure 2-1 is a compilation of information on the thickness of the Quaternary sediments
of the west Cumbria and offshore district from the Nirex studies of the Quaternary of
the area (Nirex, 1997a). These unconsolidated sediments were deposited on bedrock by
processes related to the advance of ice sheets over the area and subsequent post-glacial
deposition. All the deposits exposed at surface are considered to be the result of the
latest ice age and younger sedimentation. Onshore deposits more than 10 m thick are
confined to the valleys of the main rivers crossing the area from the uplands of the
Lake District to the Irish Sea. These valleys were deepened by river erosion at times of
lower sea levels and by valley glaciers and are now infilled with both glacial and postglacial deposits.
In the thickest onshore sequence, situated east and south of the LLWR, these deposits
overlie a concealed older sequence of lacustrine and marine to estuarine sediments,
situated beneath a level of 15 to 20 m below present sea level. (Note, the sea level
during their deposition is uncertain. The marine sediments were deposited on the
seabed below a rising/transgressing sea level that probably rose from below -40 m to
7
above -15 m. The marginal sandy facies adjacent to bedrock reaches to above -15 m and
probably consists of a beach and onshore sequence.)
The BGS Memoir on the geology of the west Cumbria district (Akhurst et al., 1997)
provides a full description of the mapped subdivision and genetic interpretation of the
Quaternary sediments both onshore and offshore. The interpretation of the Quaternary
sequence by the BGS is focussed on the age relationships of the sediments to the
advances and retreats of the eastern margin of the ice sheet that spread down the East
Irish Sea from an ice centre in Scotland, as shown on the inset to Figure 2-1.
Two of the advances of the margin of the ice sheet after a retreat into the Irish Sea
during deglaciation are shown on Figure 2-1 as the Gosforth Oscillation and the later
Scottish Readvance. A more detailed study of the age relationships and depositional
history of the deposits in the Lower Wasdale area, which comprises the lower part of
the valleys of the River Irt and River Mite and includes the area around the LLWR, is
provided by an article by Merritt and Auton (2000). These studies are summarised and
placed in their wider regional context in the BGS Regional Geology guide to Northern
England and the Isle of Man (Stone et al., 2010).
These geological studies are primarily concerned with understanding the origin of the
deposits and their stratigraphical (time) relationships. However, this study is primarily
concerned with understanding the nature of the deposits (i.e. material types) and their
spatial relationships. Therefore for this study and earlier related studies (Michie et al.,
2007; Hunter et al., 2007b), the nature of the deposits was used as the basis for their
investigation and classification of units. However, the BGS studies of the genesis of the
Quaternary sediments were used to support the interpretation of the spatial
relationships of the units.
8
Figure 2-1. Thickness of Quaternary sediments across the west Cumbria and
adjacent offshore area
2.1 Bedrock geology
Figure 2-2 shows the simplified bedrock geology of west Cumbria and the adjacent
offshore area. In the Lake District, the metamorphic and igneous bedrock consists of
the slates of the Skiddaw Group, dominantly in the north, the volcanic rocks of the
9
Borrowdale Volcanic Group, dominantly in the central area, and the granitic rocks of
the Ennerdale intrusion in the north-east and the Eskdale intrusion in the south. These
older rocks are all of Ordovician age, from about 500 to 460 million years old. They
extend at depth under the younger sedimentary rocks to the west and north, and are
collectively referred to as basement rocks.
In the north of the west Cumbria area, the basement rocks are overlain by a
sedimentary sequence of Carboniferous age (350 to 300 million years old) rocks,
including Limestone, Coal Measures and Sandstone. In turn, these are overlain by a
younger sedimentary sequence, the Cumbrian Coast Group of Permian age (290 to 260
million years old), which also oversteps across the Carboniferous rocks to rest directly
on the basement rocks. Above the Permian age strata, there is a thick succession of
sandstones of Triassic age, which forms the bedrock to the coastal zone and much of
the adjacent offshore area.
Figure 2-2. Simplified bedrock geology of West Cumbria and the adjacent offshore
area
Some of the younger sedimentary rocks forming the bedrock to the coastal area are
well exposed in cliffs up to 100 m high at St Bees Head, which is formed of the St Bees
Sandstone, the lowest unit of the Sherwood Sandstone Group of Triassic age, about 240
10
million years old. However, the Quaternary sediments cover the sedimentary rocks
over most of the coastal plain to the south, except for the valley of the River Calder,
which cuts into the sandstone bedrock. This mostly consists of the Calder Sandstone,
the middle unit of the Sherwood Sandstone Group. The transition to the Ormskirk
Sandstone, the uppermost unit of the Sherwood Sandstone Group, is present near the
coast, and this unit also forms low outcrops on the beach at Seascale.
Figure 2-3 shows the bedrock geology of the LLWR and surrounding area, onshore and
offshore. In the east of the area, the bedrock is formed of the igneous and metamorphic
rocks of the Lake District. The oldest rocks are a small area of Skiddaw Group on
Muncaster Fell east of Ravenglass. These have been brought up from depth by the
intrusion of the Eskdale granite into the Borrowdale Volcanic Group. About 4 km to
the east of the LLWR, a major fault (structural displacement) forms the boundary
between the basement (igneous and metamorphic rocks) of the Lake District uplands
and the younger sedimentary rocks of the coastal zone, which are downthrown to the
west by several hundred metres. Therefore the complete succession of the sedimentary
rocks above the older igneous and metamorphic rocks is not present at the bedrock
surface to the east of the LLWR area. However, to the north of the fault zone that runs
inland from the Seascale area, north of the LLWR, the sedimentary sequence above the
igneous and metamorphic rocks is present at the bedrock surface.
The basal unit is the Brockram, a unit formed of large clasts of the Borrowdale Volcanic
Group in a muddy matrix. Above this unit, there is an upward succession of St Bees
Sandstone, Calder Sandstone and then Ormskirk Sandstone , which forms the bedrock
under the LLWR. Faulting associated with the SW-NE trending Seascale Fault Zone
affects the Ormskirk Sandstone north of the LLWR site. Studies by Nirex of the likely
effect of faulting on the hydrogeological properties of the sandstone concluded that
there was unlikely to be any significant effect on the permeability of an already
permeable sandstone.
One of the Nirex deep boreholes (BH13) was drilled to the north of the LLWR site and
provided a complete succession of the sedimentary strata above the Borrowdale
Volcanic Group, which was reached at a depth of about 1636 m (1617 m below sea
level). From the surface, there was about 34 m of Quaternary sediment and
disaggregated sandstone (the informal ‘Snellings Sand’ unit) over the Ormskirk
Sandstone bedrock. The Ormskirk Sandstone occurred to about 176 m depth, with
surface-related weathering effects noted in the top 30 m. The sandstone is reddish
brown, fine to coarse, moderately to well sorted, generally poorly cemented and
friable. Two main facies were identified; a predominant aeolian (wind blown) dune
facies and a damp interdune facies.
11
Below the Ormskirk Sandstone, the Calder Sandstone occurs to 646 m depth and is a
generally coarser-grained sandstone. The Calder Sandstone contains both fluvial and
aeolian units, with the top of the uppermost fluvial unit defining the top of the
formation and the base of the lowest aeolian unit defining the base of the formation.
Figure 2-3. Simplified bedrock geology of the LLWR and surrounding area
Below this, the St Bees Sandstone occurs to 1270 m depth and is an entirely fluvial
formation of fine- to medium-grained sandstone. The base of the St Bees Sandstone is
transitional to the St Bees Shales, which extend to about 1442 m depth. The shales are
transitional to a 30 m thickness of the St Bees Evaporites, which contain layers of
anhydrite (calcium sulphate), a poorly-soluble evaporite mineral. These overlie and
12
interbed with the Brockram, a coarse conglomerate, which extends to 1590 m depth.
Between this and the Borrowdale Volcanic Group, a 46 m section of Carboniferous
Limestone is preserved.
The Ormskirk Sandstone forms the bedrock to the LLWR site and most of the coastal
area around the site. The strata are tilted (dip) to the west and so younger strata are
present to the west under the area offshore from the LLWR. About 2 km offshore, the
overlying Mercia Mudstone Group forms the bedrock under the sea. This is a thick
sequence of reddish brown mudstones, with layers of dolomitic mudstone and
anhydrite. Thick layers of halite, natural rock salt, are preserved at depth below the
zone of near-surface dissolution.
2.2 Quaternary sediments
The glacial deposits covering the bedrock in the west Cumbria area were deposited
from either the ice sheet that infilled the Irish Sea in the west, or the local Lake District
ice cap in the east, depending upon the position of their margins at specific times. At
the maximum of the Late Devensian glaciation from 27,000 to 22,000 years Before
Present (BP), the ice sheet and ice cap coalesced into the British-Irish ice sheet, which
also coalesced with the Scandinavian ice sheet. However, the Lake District ice cap
maintained an independent existence, probably because the high-level ice cap was
frozen to the underlying substrate. In contrast, the lower-level ice sheet flowing down
the floor of the Irish Sea was highly mobile.
The East Irish Sea between the coast of west Cumbria and the Isle of Man is shallow,
generally less than 50 m deep and the bed is fairly flat, but this conceals the infilling of
an incised bedrock surface with a variable thickness of Quaternary sediments. The
Coulderton Channel shown on Figure 2-1 is the most easterly of a set of such incised
and infilled channels running sub-parallel to the coast. These incisions may have
formed during earlier, more extensive, glacial episodes of the 2 million year duration of
the Quaternary, when there was deep erosion of the East Irish Sea basin. Within the
channels, the thickness of Quaternary sediments can exceed 100 m.
In addition, there are some transverse incisions extending out from the valleys of the
major rivers. Offshore seismic surveys and earlier boreholes drilled through the
Quaternary sediments by the BGS have revealed that there is a upward succession
consisting of basal sands and gravels with diamictons, overlain by lacustrine, marine
and glaciomarine sediments, a thin till layer, and then a cover of post-glacial marine
sediments on the sea floor. The nature of the sediments provides evidence for major
rises and falls in the relative sea level.
13
Between the Isle of Man and the Scottish coast, offshore boreholes penetrated at the
base of the Quaternary sequence a similar succession of laminated glaciolacustrine
clays, which pass upwards into marine silts and sands between 50 and 70 m below
current sea level. These are capped by a layer of till, which is covered by post-glacial
marine sediments. The age of the older lacustrine and marine sequences below the till
is uncertain.
In the north of the Isle of Man, boreholes have revealed a 250 m thickness of
Quaternary sediments, of which the lowest 140 m is concealed below current sea level
and includes marine deposits over 60 m below current sea level. The upper succession
is of interbedded tills and sands and gravels, deposited during the advances, retreats
and readvances of the late Devensian glaciation. In the south of the Isle of Man, a
radiocarbon date of about 36,000 years Before Present was determined for peat layers
buried beneath the Late Devensian glacigenic deposits. No glacial deposits underlie the
peat layer, which indicates that there was no significant Early or Mid Devensian
glaciation affecting the East Irish Sea basin.
The study by Heathcote and Michie (2004) showed that Relative Sea Level in the Irish
Sea area was more than below -50 m below present sea level from about 110,000 to
35,000 years Before Present, and during this time the East Irish Sea was not present.
Instead the area would have been a tundra environment with lakes, rivers and local
valley glaciers. However, after 35,000 years Before Present, the weight of the growing
British and Irish Ice Sheet caused depression of the crust around the ice sheet and
produced a rise in Relative Sea Level, sufficient to allow the sea to advance into the
area and deposit marine and glaciomarine sediments to infill the incisions in the
surface. The influx of a shallow sea in the area adjacent to the growing ice sheet in
Scotland, and the presence of an infilled bed of sub-horizontal clay-rich sediments
allowed rapid southward expansion of the Late Devensian ice sheet along the axis of
the East Irish Sea.
Such an interpretation of glaciomarine effects generating fast-flowing ice streams
during the initial advance of the ice sheet is similar to the interpretation presented for
the later stages of the ice sheet by Eyles and McCabe (1989, 1991). At its maximum
development, the ice sheet completely covered the Isle of Man, with indications of an
ice thickness of over 800 m. This thickness of ice would have remained grounded on
the floor of the East Irish Sea basin even when the inferred Relative Sea Level in the
area rose to over 25 m above current sea level at the maximum of the Late Devensian
glaciation (Heathcote and Michie, 2004).
However, the eastern margin of the ice sheet only gradually advanced over the coast of
west Cumbria. Before the arrival of the main Irish Sea ice sheet, local valley glaciers
from the Lake District ice cap flowed down the palaeovalley of the River Irt from the
14
Wastwater incised basin and deeply eroded the valley, particularly over the sandstone
bedrock in the lower part of Wasdale. The detailed studies by the BGS of the glacial
deposits identified that the maximum inland extent of the ice from the Irish Sea ice
sheet was probably only slightly inland of the limit reached by the ice sheet during
deglaciation, shown on Figure 2-1 as the “Limit of Gosforth Oscillation of the Irish Sea
ice sheet.” The inset to the figure also illustrates the general form of the ice sheet
during deglaciation.
A similar relationship applied during the early stage of arrival of the Irish Sea ice sheet,
when the valley glacier retreated up Wasdale, probably due to rising relative sea level
as the bedrock was depressed. This situation lead to the ponding of glacial meltwater
into large glacial lakes against the barrier of the Irish Sea ice sheet, which was pinned
to the bedrock high of Muncaster Fell, situated between the River Mite and the River
Esk, and other areas of bedrock. Meltwater channels across Muncaster Fell and lake
beaches on the hillsides indicate that the lakes reached elevations of over 100 m above
present sea level. Laminated silts and muds (varves) were deposited in the lakes, as
were outwash deposits of sand and gravel in deltas. The lakes also formed reservoirs of
large bodies of water that could be released subglacially to form tunnel valleys if the
Irish Sea ice sheet thinned and/or lifted off its base.
Figure 2-1 also shows the position that the BGS identified for the inland “Limit of the
Scottish Readvance of the Irish Sea ice sheet” at a later stage of the deglaciation,
probably before 16,000 yrs BP. This advance of the ice sheet inland produced folding of
the older Quaternary sediments along the coast south of St Bees Head and to the south
of Seascale, both environments where the sediments could be compressed against
bedrock ridges.
Inland, the glacitectonic effects were more limited and lead to the production of lowangle thrusting at the margins of till and sand units, and the development of small
normal faults due to flattening of the unconsolidated sediments. After the melting of
the Irish Sea ice sheet and its final retreat from the west Cumbria area, silts and sands
were deposited in local hollows and vegetation gradually colonised the tundra
environment. This lead to the formation of peat, the oldest layers of which in the
Hallsenna Moor area, NE of the LLWR site, were dated to 15,770 calendar years BP
(Nirex, 1997b).
During the post-glacial phase, relative sea level initially fell to tens of metres below the
present sea level as the bedrock rose following removal of the weight of overlying ice.
Then there was a rapid rise in sea level due to melting of the continental ice sheets
(Heathcote and Michie, 2004). Relative sea level reached a maximum of several metres
above present sea level in the mid-Holocene, around 7000 years before present. During
that time, raised beach deposits formed to 8 m above present sea level along the coast
15
and marine silts were deposited up to similar elevation adjacent to the estuary of the
rivers Irt and Mite, including the southern edge of the LLWR site. The post-glacial
development of the estuary of the rivers Irt, Mite and Esk was studied by Kelly and
Emptage (1992), based on interpretation of low-tide air photographs and field
mapping. Work is currently in progress to refine their interpretation in the area of the
LLWR using information from new geophysical surveys and shallow drilling.
The units defined by the BGS from their mapping and related studies of the
Quaternary sediments in the west Cumbria area are described in Section 3.1. However,
these units cannot readily be applied to hydrogeological modelling across the area,
because of the difficulty of identifying the properties of the units and the spatial extent
of the multiplicity of thin units with only limited development. In the BGS regional
guide for Northern England (Stone et al., 2010) it is noted that; “The sequence of events
that occurred during the MLD [Main Late Devensian] glaciation is not fully
understood since there is insufficient geochronological control, some phenomena result
from more than one phase of glaciation, and the stratigraphical record is beset with
difficulties of regional correlation.”
2.2.1 Application of a lithofacies approach
One of the key findings of the review by Bond (2006) of the geological interpretation
provided to support the 2002 Post-Closure Safety Case (PCSC) for the site (BNFL,
2002a) was that the geological understanding presented was not optimal to underpin
the update of the 2002 PCSC hydrogeological interpretation. Instead of presenting a
description of the spatial distribution of material properties for the Quaternary
sediments necessary for modelling groundwater flow and radionuclide transport, the
description focussed on describing the stratigraphy (time sequence) of geological
events that had affected the sediments.
In the report on the geological interpretation (BNFL, 2002a) that supported the 2002
PCSC, an event stratigraphy approach was adopted. Events are relatively short
occurrences, in a relative geological timescale, of phenomena that affect the deposition
or removal of sediments to produce features that can be correlated across an area. The
BGS lithostratigraphical framework for the onshore Quaternary deposits of the west
Cumbria area was also based on the application of an event stratigraphy, using
features such as the interpreted advances and retreats of the margins of the Irish Sea ice
sheet. For the BNFL (2002a) geological interpretation of the Drigg site, the BGS event
stratigraphy was adopted as the framework, but a site-specific set of units were
defined, which were different from the BGS lithostratigraphical units.
16
The summary of the 2002 geological interpretation states that, “This allowed the
development of an event stratigraphy for the geological sequence and provided the
framework to allow the prediction of the lithological and structural distribution across
the Drigg site.” and; “The event stratigraphy framework allowed a generalised
conceptual understanding of the regional depositional events to be developed. This
allowed the identification of glacial and post-glacial features that are likely to be
present at the Drigg site based on knowledge of the Quaternary events and the
resultant depositional and post-depositional geomorphological processes.” This event
stratigraphy approach for the LLWR site produced units that could not be confidently
correlated across the site, nor with the BGS lithostratigraphical units, and which did
not have well-defined material properties.
The requirement for an approach that identified and correlated common units with
well-defined material properties across the region and at the LLWR site was
considered in the 2007 review (Michie et al. 2007: Hunter et al. 2007a,b). That review
identified that a lithofacies approach would provide these necessary features. The
lithofacies approach was developed in Canada for the study of outcrop sections and
borehole cores from the complex suite of sediments deposited by braided river
deposits (Miall. 1978) and extended to the study of glacial diamict sequences by Eyles
et al. (1983). The BGS applied these lithofacies classifications to the interpretive logging
of the outcrops, pits and sediment cores from the boreholes drilled as part of the Nirex
Quaternary characterisation programme (Nirex,1997a).
The availability of regional lithofacies information from the Nirex/BGS investigations
and comparable lithological information from the LLWR site investigations, which
could be interpreted and assembled as lithofacies, allowed the application of the
lithofacies approach to be undertaken during the 2007 update of the geological
interpretation. Instead of using an event stratigraphy to provide a framework for
interpretation of the possible distribution of units, the lithofacies approach works
directly from the identification and correlation of well-defined units of similar material
properties. Lithofacies units are not constrained to have formed in the same time
interval, they may cut across other lithofacies units of different material properties, and
they may also be discontinuous.
Subsequent interpretation of the spatial distribution, nature and relationships of the
lithofacies units may allow the determination of their stratigraphical relationships and
refinement of the lithofacies units into individual units of stratigraphical significance.
The lithofacies approach avoids the problems related to a requirement to determine the
stratigraphy of the sediments as an initial and main purpose of the study. The
following Section 3 explains how the lithofacies approach was developed. The
lithofacies approach is further described in the context of the site geology in Section 4.
17
2.3 Sources of data
The principal sources of data that can be used for characterising the Quaternary
deposits beneath the LLWR site have been outlined by Hunter et al. (2007b), who also
summarised the data-processing procedures that have been used to codify and
condense the wide range of borehole lithology descriptions that comprise the bulk of
the recorded information from the LLWR site. The variable quality of these descriptive
lithology data and the limitations and caveats associated with their use are also
described by the same authors. Despite some limitations, it is considered that the
lithological data-set is a valuable source of information which can be interrogated to
reveal gross patterns of distribution of different sediment types sufficient for
characterising the Quaternary deposits at the LLWR.
Additional sources of data have become available since the 2007 report by Hunter et al.
most of which have been utilised in the preparation of this report. These additional
sources have been appended to the list originally shown in the 2007 report to produce
an updated, chronological list of principal data sources as follows:
‘Historical’: 79 boreholes drilled when the LLWR was a Royal Ordnance
Factory.
1962: the first three ‘old series’ boreholes were drilled.
1962 – 1975: a further four ‘old series’ boreholes were drilled.
1977: the Institute of Geological Sciences (IGS) (now the BGS) investigated the
site on behalf of BNFL. The study included examination of available geological
exposures in trenches, stream banks, and coastal sections on the Drigg beach. It
was followed by 28 DDS (Drigg Disposal Site) series boreholes drilled between
November 1977 and January 1979.
1981: a further 14 hydrogeological boreholes (DDS31 to DDS35 and DDS41 to
DDS49) were drilled. This additional information led to a revised
understanding of the geology and hydrogeology of the site, which is described
by Williams et al. (1985).
1981 – 1989: a phase of site investigations was instigated, which included more
DDS series boreholes drilled at various times.
1989 – 1995: several individual boreholes were drilled as part of a number of
separate site investigations.
18
1995 - 1996: as part of its Quaternary Characterisation Programme of the coastal
area of west Cumbria, Nirex drilled 15 boreholes in the area that includes the
LLWR site. The boreholes were sited following a programme of surface
mapping, excavation of pits, and shallow ground geophysics to identify
localities that would provide key information of the relationships of the
Quaternary sediments and their hydrogeological characteristics. Most of the
boreholes, drilled to a maximum depth of about 68 m, penetrated the
Quaternary sequence into bedrock. Summary logs of the boreholes are shown
as Figures 4 and 5 in the article by Merritt and Auton (2000), which also
provides references to the Nirex reports in which the detailed logs and their
interpretation are presented.
1995 – 2002: the Drigg Site Characterisation Programme (DSCP) involved
drilling a network of 41 boreholes on-site and 8 boreholes off-site in the first
phase of work, followed by clusters of between 2 and 4 closely-spaced
boreholes drilled to different depths at 12 locations on-site (the C1 to C12
cluster borehole sets). Various geophysical surveys (resisitivity tomography
and seismic reflection) were also undertaken.
2004 – 2005: the Drigg Vaults Phase II programme of site investigation involved
drilling 34 boreholes both on-site and off-site.
2006 - 2007: the Drigg Vaults Phase III programme (Stages 1 to 5) of site
investigation involved drilling 43 boreholes and excavating 17 trial pits (the
8400, 8500 and 8600 series boreholes). These boreholes were located both inside
the LLWR and also off-site, on the western (seaward) side. Note: the descriptive
lithology data generated by these intrusive investigations were not available in
time for incorporation into Hunter et al (2007).
2009: the Drigg Vaults Phase III, Stage 6 programme of site investigation
involved drilling 14 boreholes (8600 and 8700 series) to be used as groundwater
monitoring wells.
2009: geological logging of the Vault 9 trench excavation (Smith, 2009a)
2009: digitisation of historical lithology descriptions from Royal Ordnance
Factory (ROF) boreholes dating from the 1940s (Smith, 2009b).
2009: the offsite geophysical survey conducted by Halcrow-RSK (Halcrow,
2010).
2010: LLWR coastal investigation studies (not completed at the time this report
was prepared).
19
[Note: copies of all of the site investigation records listed above and used in creating
the 3D geological model described in this and in other reports are held by LLWR in
digital AGS format].
The depths of recently-drilled Drigg Vaults Phase III (Stages 1-6) boreholes vary from
10 m to more than 60 m. Of the total number of these boreholes, 12 were drilled to a
depth sufficient to penetrate through the entire thickness of Quaternary deposits and
intersect the sandstone bedrock beneath. A further 8 boreholes were drilled to a depth
sufficient to penetrate through almost the entire thickness of Quaternary deposits but
did not reach the sandstone bedrock. These 20 deep boreholes greatly assist the
interpretation of the Quaternary geology of the LLWR site at intermediate depths and
also improve the definition of the concealed surface of the sandstone bedrock.
The historical ROF (Royal Ordnance Factory) borehole data, comprising lithological
descriptions from a total of 79 boreholes drilled principally within the LLWR site, were
converted into a digital format suitable for appending to the main database of LLWR
borehole lithology descriptions by Smith (2009b). The final depths of the majority of
these boreholes are quite shallow – the average depth is 6.8 m and the deepest only
reaches 10.9 m. Therefore their value to site understanding is limited to improving the
definition of the near-surface deposits in areas of the site where no recent boreholes
have been drilled. The quality of these ROF data is also not as good as the data from
later borehole drilling programmes and in many cases the lithology descriptions
consist of only a few words. Also, narrow borehole intervals of slightly different
material, which by modern standards would be recorded separately on logs, have
probably been combined together into single composite units in many cases.
Another recent report prepared by Smith (2009a) describes in detail the exposure of
shallow glacigenic sediments revealed temporarily during the excavation of a 200 m
long trench for Vault 9 at the LLWR. The report provides a valuable record of the
sedimentary material types, depositional fabrics and gentle deformation structures
preserved just below the land surface (maximum depth of 6 m) in one area of the
LLWR.
The most recent study made available for the authors to consider during the
preparation of this report is the compilation of the 2009 off-site geophysical survey
conducted by Halcrow-RSK (2010). Because most of the borehole lithology data utilised
in the preparation of this report is located inside the LLWR site, only partial lengths of
some of the resistivity profiles from the geophysical survey which either coincide with,
or are sufficiently close to the borehole cross-sections presented herein, could be used
for comparative purposes. The new geophysical data have not been used to construct
any of these borehole cross-sections, which are based entirely upon lithology
descriptions.
20
It should be noted that the geophysical investigations were focused on describing the
near-surface Quaternary deposits in the context of their vulnerability to coastal erosion,
and the coastal sediment balance. They were not optimised to investigate the deep
geology, or undertaken to directly support the geological interpretation. Work to
analyse the results of the geophysical investigations is ongoing at this time.
The new borehole, outcrop and geophysical data acquired since the 2007 report can, in
addition to expanding the LLWR geology database to characterise the site, offer a
means of testing the robustness of the various geological models of the LLWR
Quaternary deposits and, in particular, the provisional map of ‘channel’ and ‘interfluve
clay’ units shown as Figure 21 in Hunter et al (2007b).
2.4 Lithological description of superficial deposits
The bulk of the sample descriptions in the LLWR borehole lithology database are
engineering-type descriptions of disturbed drilling samples recorded to the BS-5930
standard (British Standards Institution, 1999). As such, they often lack valuable
information on sedimentary and glacitectonic fabrics and other useful micro-level
observations that may assist with the identification of a material as being deposited by
a particular genetic process. With borehole samples from glacial deposits, for example,
this limitation may introduce uncertainty in the correct identification of different types
of tills. To avoid onerous borehole logging work and voluminous records, materials
with slightly varying sedimentological character but similar engineering characteristics
are also often grouped together as a single unit in descriptive logs, thus losing the high
resolution logging detail that is associated with scientific borehole logging and which
may be critical in recognising potential event boundaries.
The BNFL lithology coding system for Quaternary materials that has been applied to
lithology descriptions in the LLWR borehole database is described by Hunter et al
(2007a). It is comparable to and probably based upon the AGS coding scheme (AGS,
1999). More recently, Cooper at al. (2006) have proposed a revised coding scheme for
unlithified sediments that can be applied to glacial deposits. However, for consistency
with Hunter et al (2007), the additional lithology descriptions appended to the LLWR
database for the Phase 3 and Stage 6 borehole samples during the preparation of this
report have been coded in the same manner as the original BNFL scheme. That
geotechnical scheme does not include categories for till and related glacigenic deposits.
The document published by McMillan and Powell (1999) contains some useful
definitions, as used by the BGS, of the various types of unconsolidated superficial
sedimentary materials that comprise glacigenic deposits. For instance, paragraph 4.18
on page 15 of that document contains a definition of ‘till’, i.e., material “deposited
21
directly by and deformed underneath a glacier. It consists predominantly of diamicton,
a mixed, unsorted sediment of sand and coarser grains set in a clay/silt matrix”. Tills
can be subdivided and classified into a series of genetic categories depending upon the
particular glacial environment in which they were deposited. Whereas such genetic tilltypes may be potentially recognisable from high-resolution logging of scientific
boreholes and in outcrops, identification of such genetic units with any degree of
confidence in BS-5930 engineering-type logs is probably not feasible and has not been
attempted in this report.
Consolidation of a large range of lithology codes into a limited set representing the
dominant grain-size ranges of sediment, i.e., clays, silts, sands, mixed sands and
gravels etc., is a commonly-used method of simplifying an otherwise complex data-set
for presentation on report figures and accompanying geological interpretation. With
glacigenic deposits, where sediments of varying grain-size are often mixed together in
an unsorted mass (diamicton), reasonable representation of separate sedimentary units
can be difficult to achieve and different outcomes may result when groups of lithology
codes are consolidated in slightly different combinations.
For the purpose of continuity, the same lithology code combinations adopted by
Hunter et al. (2007b) have been used for the preparation of cross-sections in this report.
This coding scheme appears to be satisfactory for distinguishing the relatively simple
but distinctive lithofacies units of clay-dominant tills, laminated silts and channel-fill
sands and gravels. The scheme is incapable of differentiating between the more subtle
varieties of glacial till and it is likely that any attempt to achieve this will introduce too
much uncertainty into the model. A summary of the code combinations is presented in
Appendix C.
22
3
Regional scale lithofacies units
This section describes the development of the regional lithofacies framework, and the
relationships between the lithofacies units and the BGS lithostratigraphy. The
methodology for reviewing the interpretation of the Quaternary sediments on a
regional scale was straightforward. Information on the lithofacies and lithological
descriptions of the materials examined in the Nirex and earlier studies of the west
Cumbria area was assembled and examined to identify units of characteristic material
properties relevant to hydrogeological modelling, etc. This focussed attention on the
range of sediment types (lithologies) present in the sequence of sediments including
clays, silts, sands and gravels, and diamictons, and their degree of consolidation.
A limited range of dominant lithologies or combinations of lithologies were identified
that formed assemblages within a small number of discrete units. It was recognised
that the set of vertical cross-sections (transects) showing the regional Quaternary units
that had been constructed during the Nirex investigations could be re-interpreted
based on these lithofacies units. Some additional schematic sections were constructed
as part of the process of determining the spatial distribution of the lithofacies units
across the area and across the LLWR site. These transects formed the original basis for
the 3D geological model (Smith, 2009a,b).
Accordingly, it is useful to first describe the setting and development of the regional
Quaternary units identified by the BGS studies as part of the Nirex investigations, and
then show how these were re-interpreted.
3.1 Regional lithostratigraphical units
Figure 3-1 is a map of the bedrock surface beneath the Quaternary sediments that was
derived from information compiled from seismic surveys and regional borehole data
(Nirex 1997a), supplemented by later information from the LLWR. This map shows
that the LLWR site is located over the northern margin of an incised palaeovalley that
underlies much of Lower Wasdale. The extent of the palaeovalley is illustrated by the
sea level contour (0 m), which extends from below the LLWR site well inland towards
Gosforth and up Lower Wasdale. The maximum depth of the incision into bedrock is
over 60 m below present sea level. From the LLWR site north to Sellafield, the bedrock
surface is above present day sea level and is generally covered by less than a 10 m
thickness of Quaternary sediments. Sandstone bedrock is exposed on the beach at
Seascale, in stream sections inland towards Gosforth, and in the River Calder at
Sellafield. Under the Sellafield site, to the north of the present course of the River
Calder, the thickness of the Quaternary sediments increases significantly over another
23
incised palaeovalley, which forms part of the major palaeovalley under the present
River Ehen, which is incised to a maximum depth of about 50 m below present sea
level.
Figure 3-1. Contour map of the concealed bedrock surface beneath Quaternary
sediments of the LLWR and surrounding area (dashed red line shows the boundary
between igneous and sedimentary bedrock)
The most comprehensive study of the Quaternary sediments in this area was by the
British Geological Survey (BGS), who presented their results in a number of reports for
24
Nirex (Nirex, 1997a), in the new memoir for the “Geology of the west Cumbria district”
(Akhurst et al., 1997) and in a published article (Merritt and Auton, 2000). These
studies established a highly detailed lithostratigraphy with 20 units (separate
members, or formations where individual members were not identified) in the
southern onshore area around the LLWR, which extends from Ravenglass north to
Sellafield and inland to Gosforth and Lower Wasdale. However, it was noted in the
article by Merritt and Auton (2000) that; “… many units are recognized primarily by
their unconformable bounding surfaces, rather than by their lithology alone…”.
This emphasis on stratigraphical features meant that some units (members) had
variable lithologies (material properties). Individual members were treated as
continuous layers in specific areas, but the same lithological layer could have different
names in adjoining areas. Members were assembled into a complex set of overlapping
formations, which were in turn aggregated into Groups. The post-glacial units were
included in the Solway Drift Group of estuarine, alluvial (river channel or coastal plain
environment), organic (peat), aeolian (wind-blown), and mass movement (scree etc,)
sediments, previously classified as Flandrian or Holocene post-glacial deposits.
The glacial/glacigenic units were subdivided into two Groups on the basis of the
source of the deposits. In the BGS Geological Memoir (Akhurst et al., 1997) and in the
article by the BGS geologists Merritt and Auton (2000), these were classified as the
Central Cumbria Drift Group of units mainly derived from the Lake District; and the
West Cumbria Drift Group of units principally derived from the Irish Sea basin and
adjacent coastal areas. Younger members of the West Cumbria Drift Group overlie the
members of the Central Cumbria Drift Group in the area below the LLWR site.
Subsequently, the BGS (McMillan, Hamblin and Merritt, 2005) reclassified some of the
members included in formations within the Solway Drift Group as belonging to the
Cumbria-Lancashire Catchments Subgroup, part of the Britannia Catchments Group
(for fluvial (river), mass movement, organic and cover sand deposits), and other
members as belonging to the Great Britain Coastal Deposits Group (for marine,
estuarine and coastal deposits, including beach and dune sands). They reclassified the
Central Cumbria Drift Group as the Central Cumbria Glacigenic Subgroup, part of the
Caledonia Glacigenic Group, and reclassified the West Cumbria Drift Group as the
Irish Sea Coast Glacigenic Subgroup of the Caledonia Glacigenic Group. This
reclassification of the Quaternary sediments was useful for stratigraphical studies
related to surface geological mapping.
The Quaternary sediments of the region around the LLWR site are very varied and
were mostly deposited by processes related to the ice sheets that covered the area
within the last 25,000 years, during the late Devensian period. These sediments include
tills (glacial diamictons), coarse fluvioglacial sands and gravels, and silts of
25
glaciolacustrine (lake) origin. The late Devensian sediments are locally deformed by
glacitectonic processes, generally related to local re-advances of the ice sheet during the
later stages of the glacial period.
Some of the land-based (terrestrial) glacial and glacigenic deposits extend beyond the
present day shoreline due to deposition during periods of lower sea level before the
crustal depression and related rise in relative sea level at the maximum of the late
Devensian Glaciation (Heathcote and Michie, 2004). Additionally, some glacial
deposits formed at the base of the grounded ice sheet on the floor of the offshore basin
during the period of raised sea level at the maximum of the Late Devensian glaciation.
These are overlain offshore by younger glaciomarine and marine sediments associated
with marine transgression during the deglacial phase of the glaciation and also during
the younger post-glacial rise in sea level. Significant falls in relative sea level during the
deglacial and post-glacial phase produced breaks in sedimentation (unconformities)
within the glaciomarine and marine sequences.
The inter-relationships of the various members of the Quaternary deposits were
presented in a series of transects across the area in Nirex Report no. SA/97/045 (Nirex,
1997b). These were later supplemented by the production of additional transects
through the area around the LLWR (Hunter et al., 2007b; Smith, 2008) as part of the
update to the understanding of the Quaternary geology of the LLWR area. These
transects included some of the boreholes drilled as part of the investigations of the
LLWR and allowed a comparison of the regional lithostratigraphical units of the BGS
with some of the detailed lithology units identified in the LLWR boreholes.
Figure 3-2 is a highly vertically exaggerated W-E section across the LLWR site and
Lower Wasdale to the Lake District foothills. The schematic section was compiled to
illustrate the correlation of the BGS Lithostratigraphical units identified in boreholes in
the LLWR area in the west (Off-site borehole E, C8 and C11) with the BGS
lithostratigraphical units defined in Nirex boreholes to the east (QBH 2A, QBH 16 and
AF 2). There is a major facies change identified in the area of borehole QBH 2A. Thick
units of lacustrine clays, silts and fine sands (the Holmside Clay Member and the
Whinneyhill Coppice Clay Member) are present to the east in Lower Wasdale and are
interpreted as the deposits of a deep lake dammed by the Irish Sea Ice Sheet across the
mouth of Lower Wasdale against the basement ridge of Muncaster Fell.
To the west of borehole QBH 2A, there is an alternating sequence of tills (glacial
diamicton) and sandy units, which were linked to the sequence exposed on the low
cliffs at Drigg beach. Below the sequence of alternating tills and sands, a sandy unit,
the Barn Scar Sand and Silt Member, is interpreted to thicken towards the coast, partly
at the expense of the underlying unit of thick till, the Holmrook Till Member. This unit
was interpreted as a deposit from the local Lake District Ice Cap, probably from a
26
valley glacier that flowed into Lower Wasdale. Below this local till unit, the marine to
estuarine or lacustrine deposits of the Glannoventia Formation and the Carleton Silt
formation are preserved in lower parts of the palaeovalley. Below these formations, a
basal till, the Maudsyke Till contained clasts only from the Lake District and was
interpreted as a local valley glacier from Wasdale. At the base of the sequence in a few
of the boreholes, the BGS identified a ‘weathered’ till unit, the Drigg Till Formation,
which was interpreted to contain in addition to Lake District clasts, a suite of “Scottish”
granite clasts.
27
28
Figure 3-2. West-east regional cross-section through the LLWR from the coast to the Lake District showing the BGS lithostratigraphical
units
3.1.1 Age sequence and correlation of the Quaternary
lithostratigraphical units
The BGS interpreted the sequence of lithostratigraphical units identified within the
Quaternary sediments of the onshore area in an age (time) sequence of environmental
changes. Their interpretation, which is presented fully in Merritt and Auton (2000), is
summarised and reviewed here from the base of the sequence upward, from oldest to
youngest, based on the section shown in Figure 3-2.
The weathered appearance of the Drigg Till and its sporadic occurrence at the base of
the Quaternary sequence was taken to indicate an early to mid Quaternary age, older
than the Devensian glacial deposits of the late Quaternary. Weathered diamictons/tills
and palaeosols (fossil soils) of unknown, pre-Devensian age are preserved in valleys
within the Lake District (Boardman, 1985), but are much more weathered than the
Drigg Till. The separate identity, age and origin of the diamicton identified as the
Drigg Till is regarded in this review as uncertain. The spatial relationships of its
identified occurrence suggest that it may actually be a variant of the diamicton
classified as the Maudsyke Till in the boreholes near the LLWR site, and to be an
altered form of basal tills or other coarse basal deposits elsewhere.
Fossil material within the Carleton Silt Formation and overlying Glannoventia
Formation indicates the upward transformation of a lake or estuary (ria) into fully
marine to glaciomarine conditions as a result of rising sea level. Fragments of marine
shell from the uppermost (youngest) member of the Glannoventia Formation, the
Kokoarrah Shelly Sand Member, were analysed for the degree of degradation of
contained amino acids. The peak height ratio of (D)- alloisoleucine to (L)-isoleucine
was interpreted to indicate an age of c. 60, 000 years (discussed in Merritt and Auton,
2000). However, review of the original analysis shows that the inferred age was not
corrected for temperature or other environmental factors and the age is highly
uncertain, but is more likely to be in the range 35,000 to 30,000 years Before Present.
The Holmrook Till Member overlies the estuarine to marine sequence and is
interpreted as a valley glacier that flowed down Wasdale from the Lake District ice
cap, before the arrival of the Irish Sea lobe of the British- Irish ice sheet offshore. With
the arrival of the major ice barrier offshore, meltwaters were ponded in the area of
Lower Wasdale to form deep lakes, which lead to retreat of the valley glacier up into
Wasdale. Sands, gravels and minor silts were deposited from the advancing Irish Sea
ice sheet in the west to form the Barn Scar Sand and Silt Member. At the same time,
laminated clays and silts were deposited in the deep lake environment of Lower
Wasdale to form the Whinneyhill Coppice Clay Member.
29
At the maximum of the Late Devensian Glaciation, the Irish Sea ice sheet advanced
over the Barn Scar Sand and Silt Member to deposit the Ravenglass Till. It continued
inland over the lake sediments to become confluent with the Lake District ice cap and
deposit the Green Croft Till Member. There was then a retreat of the margin of the Irish
Sea ice sheet. The outwash deposits of the Kirkland Wood Sand and Gravel Member
were deposited adjacent to the retreating ice sheet in the west. At the same time, the
valley glacier of the Lake District ice cap had retreated up into Wasdale. Another suite
of laminated silts and clays, the Holmside Clay Member, was deposited in the icedammed lake between the two ice masses. A readvance of the edge of the Irish Sea ice
sheet, termed the Gosforth Oscillation, deposited the Drigg Beach Till, which only
advanced a limited distance into Lower Wasdale. There was no significant readvance
of the valley glacier down Wasdale and lake deposits continued to accumulate in
Lower Wasdale.
Another retreat of the margin of the Irish Sea ice sheet lead to the deposition of the
outwash sands of the Drigg Holme Sand Member in the west and continued deposition
of lake sediments in the east. This was succeeded by another readvance of the edge of
the Irish Sea ice sheet, the Scottish Readvance, which deposited the Fishgarth Wood
Till, but to a generally shorter distance inland into Lower Wasdale than the Drigg
Beach Till. During the recession following the Scottish Readvance, an interbedded
sequence of laminated silty clays and sands was deposited in the coastal area of the
LLWR in shallow lakes and braided rivers. Within this Drigg Moorside Silt Member, a
thin massive clay unit is interpreted as melt-out or flow till, formed by the decay of
stagnant ice. To the east, fluvial and deltaic deposits of sand and gravel formed the
Mainsgate Wood Sand and Gravel Member, which covered much of the area of lake
deposits. An equivalent unit, the Peel Place Sand and Gravel Member is present to the
north and north-east of the area of the LLWR. These sands and gravels were deposited
as ice contact deposits, outwash and deltas. A thin clay unit on top of the Peel Place
Sand and Gravel may be an equivalent flow till unit to the unit within the Drigg
Moorside Silt.
Post-glacial deposits partially overlie, and infill hollows and erosion channels in, the
glacial and glacigenic deposits. Some of these deposits infill hollows (kettle holes)
formed by the melting of large isolated masses of stagnant ice. As part of the Nirex
investigations, the peat infilling the hollow at Hallsenna Moor to the NE of the LLWR
site was dated by Accelerator Mass Spectrometry of carbon-14 (Nirex, 1997b). The basal
peat layer produced an age of 15,770 calendar years Before Present and lies on top of a
sequence of organic-poor sediments. This indicates an age for the last glacial advance
over the area of more than 16,000 years Before Present.
This outline of the revised sequence of development of the onshore lithostratigraphical
units agrees with studies of similar thick sequences of Quaternary sediments adjacent
30
to the Irish Sea basin in the Isle of Man, eastern Ireland, NW England and North Wales.
The revised time sequence indicates that the majority of the sediments were deposited
in a fairly continuous manner from the period shortly before the arrival of the Irish Sea
ice lobe of the British-Irish ice sheet. Changes in the marine to non-marine nature of the
sediments can be directly related to the timing of major changes in relative sea level
accepted for the northern part of the Irish Sea over the period.
3.2 Offshore seismic stratigraphy
A separate stratigraphy was developed for the offshore Quaternary deposits adjacent
to the west Cumbria area before the development of the lithostratigraphical framework
for the onshore Quaternary deposits of west Cumbria, (Nirex, 1993; Nirex, 1997c). This
was based on the interpretation of detailed seismic surveys, with some calibration from
earlier BGS offshore boreholes in the area. A layered, but overlapping and partly crosscutting succession of seven seismically distinctive sequences was identified, which
were separated in part by unconformities.
Figure 3-3 is a transect offshore from the LLWR to illustrate the relationship of the
offshore seismic sequences along the SW-NE line shown on Figure 3-1. The offshore
part of the transect is based on seismic line 89-211, which was interpreted by Eaton and
Williams in Nirex Report 519 (1993). This interpreted seismic section is reproduced in
colour in a recent report on re-examination of the offshore seismic interpretation
(Smith, 2010b). That report also reviewed the correlation of the offshore and onshore
sequences, but is superseded by this report. For this study, the vertical scale of the
seismic section was adjusted from two-way time to physical depth based on estimates
of the seismic velocities. Additionally, the horizontal scale was calibrated to distance
and the position of the line was adjusted to match the mapped location.
31
32
Figure 3-3. SW-NE cross-section from offshore to the LLWR showing offshore seismic stratigraphical units and onshore BGS
lithostratigraphical units
The offshore portion of Figure 3-3 shows a simplified cross section of the relationship
of the seismic sequences. Extension of the section to the SW to cross the incised and
infilled Coulderton Channel shows the overlapping nature of the sequences and their
thickening offshore, especially into the channels incised into bedrock. The nature of the
seismic sequences, calibrated by correlation with the sequences intersected in the BGS
offshore boreholes, is described and interpreted in the following summary. This
includes discussion of the general relationships in the other seismic sections, not just
the section illustrated in Figure 3-3.
Sequence 1: Diamict, sand and gravel
This consists of an irregular assemblage of diamictons, sands and gravels deposited
under terrestrial (land surface) conditions. It is generally less than 10 m thick over
bedrock highs, but thicker sequences are preserved in channels and other bedrock
incisions. The sequence is interpreted as the accumulated deposits of the tundra (cold)
environment during the long period of low sea level in the early and mid Devensian
before the late Devensian glaciation. The deposits may include remnant deposits from
the previous glacial and interglacial cycle.
Sequence 2: Lacustrine
This sequence is generally only 2 to 5 m thick. It forms a draped deposit over Sequence
1 and is sometimes absent over topographic highs, but thickens in the channels and
incisions. It is interpreted as passive, low-energy deposition in a lacustrine (lake)
environment and as rain–out from floating ice in a glacio-lacustrine environment.
These conditions are considered to result from increasing crustal downwarping
towards the growing Scottish ice sheet that created large lakes, which covered the
earlier land surface.
Sequence 3: Marine
This sequence is dominated by layered clays/muds and silts, which have kilometres of
lateral continuity on seismic sections. It cuts across and overlaps the lower sequences,
and infills the channel features, where it can be tens of metres thick, but thins over the
topographic highs to approximately 5-10 m. It is interpreted as a transgressing marine
sequence that replaced the lake environment when increased crustal depression
allowed entry of the sea. Sea level continued to rise, producing clay/mud-rich marine
sediments deposited in deep water well below wave base in the western part of the
area. Rising sea level allowed the sequence to advance and rise towards the onshore
area to the NE, where more nearshore sediments were deposited.
33
Sequence 3A: Till
This is a locally preserved sequence of clay-rich till, generally no more than 10 m thick.
It rests on a planar unconformity, called the “X” unconformity (Nirex, 1993, 1997c),
which cuts across the underlying Sequence 3. Sequence 3A and the underlying
sequences are over-consolidated, that is their degree of compaction is much greater
than that due to the present thickness of overlying sediment. The degree of overconsolidation indicates a thickness of several hundred metres of ice applied for
thousands of years. This sequence is interpreted as the product of a continuous cover
of the Late Devensian Ice Sheet until the deglaciation phase. During deglaciation, the
margin of the ice sheet generally retreated to the north, but there were oscillations in
the position of the margin, with readvances and retreats. In the axis of the East Irish
Sea, this process in the marine environment caused erosion, reworking and partial
removal of the till. At a higher level on land, the similar oscillations in the position of
the ice margin produced interbedding and channelling of till and sand and gravel
units.
Sequence 4: Glaciomarine
This sequence is lithologically similar to Sequence 3, but is not over-consolidated. It is
dominated by very fine-grained silts and clays with occasional pebbles and shells,
which indicate deposition in cold-water conditions. The sequence thickens offshore
from <5 m towards the present coast to over 20 m thick in the west. It is interpreted as
glacio-marine deposits in relatively deep water during the later phase of the
deglaciation of the Irish Sea Ice Sheet. Pebbles in the dominantly muddy sequence are
interpreted as dropstones from floating ice. Possible iceberg, or ice-flow, ploughmarks
were interpreted in the lower part of the sequence. The base of the sequence is an
unconformity that has involved removal of the underlying Sequence 3A till in places,
particularly towards the coast.
Sequence 5: Marine regression
This sequence is not present in the offshore part of the transect shown in Figure 3-6. It
is only present from more than 7 km off the coast of west Cumbria and forms a
lenticular wedge reaching about 6 m thick in the west. It dominantly consists of fine
silty sand, but there is also a limited development of medium to coarse sand towards
the coast. The lower boundary is an unconformity, the “Y” unconformity, which
truncates the stratification of the underlying Sequence 4. The shelly fauna indicates
deposition in brackish and shallow subtidal environments under temperate conditions.
This sequence is interpreted as the initial post-glacial deposits during a period of much
lower relative sea level. The coarser sands are interpreted as an offshore bar and
34
mollusc shells from these sediments were dated to 8310 (±160) radiocarbon years BP
(Nirex, 1997d).
Sequence 6: Marine transgression
This sequence thickens from an edge at about 12 m below present sea level close
inshore to about 15 m thick at about 20 km offshore. It is dominated by layered silts
with varying proportions of sand and clay and overlaps all lower sequences. In the
SW-NE transect (Figure 3-3), sequence 6 rests directly on Sequence 4 with
unconformity and oversteps across lower sequences to the NE to rest directly on
bedrock highs. The deposits are interpreted as the passive infill of a rising sea level, the
main Holocene transgression. The rise in relative sea level involved still stands at some
levels, which produced some accumulations of coastal deposits. The post-glacial
sediments are continuous upwards with the present sea-bed sediments.
Table 1. Proposed correlation of offshore-onshore stratigraphies
OFFSHORE SEQUENCE
STRATIGRAPHY
ONSHORE LITHOSTRATIGRAPHY
Drigg Point Sand Formation; Ehen
Sequence 6: marine transgression
Alluvial Formation; upper part of Hall
Carleton Formation
Sequence 5: marine regression
Sequence 4: glaciomarine
Blelham Peat Formation
Lower part of Hall Carleton Formation
(estuarine and lower members)
Holmrook Till and all equivalent and
Sequence 3A: till
higher till/glacigenic units and
interbedded sand and gravel units, and
lacustrine units.
Sequence 3: marine
Glannoventia Formation
Sequence 2: lacustrine
Carleton Silt Formation
Sequence 1: diamict, sand and gravel
Maudsyke Till, Drigg Till, and basal
deposits
35
3.2.1 Correlation of the offshore seismic stratigraphy and
the onshore lithostratigraphy
Although there is a data gap in the nearshore zone between the offshore and onshore
sequences, this review using the improved understanding of the nature of the
sediments, their facies relationships and the unconformities present has identified their
simple and direct stratigraphical (time) correlation. This is shown in Table 1.
3.3 Development of the regional lithofacies units
More detailed review of the information from the numerous boreholes in the LLWR
and Sellafield areas showed that many of the BGS lithostratigraphical units did not
have the continuity indicated from the various transects and sections (Michie et al.,
2007). In particular, the numerous till units in the upper part of the sequence could not
confidently be correlated. A major purpose of identifying units within the Quaternary
sequence was to allow classification into units with similar material properties, e.g.
hydrogeological properties for modelling groundwater flow. This lead to the adoption
of a lithofacies approach to classification of the Quaternary sediments to identify units
of similar properties. These were directly related to the lithological/lithofacies logging
of the sediments identified in the boreholes.
On a regional scale suitable for general hydrogeological modelling, regional lithofacies
units of average thickness about 5 m or more could be identified. On the local scale of
the immediate LLWR area, individual units down to 1 m thickness or less could be
considered. The development of site scale lithofacies units is considered in Section 4.
Appendix A provides more detail of the lithofacies approach and the relationship of
the lithofacies units to the earlier classifications of the Quaternary sediments in the area
of the LLWR.
Figure 3-4 shows the re-interpretation of the regional West-East section, (Figure 3-2),
through the LLWR area to identify the regional lithofacies units. An additional
borehole, C7 within the LLWR site is included to show the lack of continuity of the
BGS lithostratigraphical units and the difficulty of correlation of the numerous clay/till
units in the upper part of the sequence. In addition, a large part of the middle part of
the sequence was reclassified as dominantly sand and not diamicton/till.
36
Figure 3-4. West-east cross-section through the LLWR from the coast to the Lake District foothills showing the regional lithofacies units
37
The lithofacies units were assigned the letter codes A to D, in a general sequence from
youngest to oldest, with the additional sub-division of the B lithofacies into B1 to B4
from west to east, corresponding to the transition from offshore marine to onshore
inland sediments, and recognising lateral facies changes across the area associated with
the palaeogeography and position of the edge of the ice sheets. The details of the
nature and distribution of the regional lithofacies units are described in Section 3.4, but
their characteristics and possible origin are summarised here:
A. Onshore and coastal unconsolidated deposits; a heterogeneous collection of
generally thin post-glacial deposits near the present land surface.
B1. Silt and fine sand; glaciomarine and marine deposits from about 2 km
offshore; interbedded diamictons/tills, and coarser sands and gravels are
concealed at depth infilling incised depressions into bedrock.
B2. Interbedded clayey diamicton (till) and sandy to gravelly layers; formed by
deposition from the advances and retreats of the Irish Sea ice sheet.
B3. Sand and gravel; glaciofluvial deposits formed marginal to and also under
the Irish Sea ice sheet.
B4. Laminated silt and clay; glaciolacustrine deposits formed in ice-dammed
lakes in Lower Wasdale.
C. Diamicton; generally sandy diamicton where derived from the Lake District
but more clay-rich under the East Irish Sea. Usually thin units over the bedrock
of the Lake District, but thicker accumulations deposited by the valley glacier
within Lower Wasdale.
D1. Dominantly coarse sands and gravels, with diamictons at the base and
margins of the unit against the irregular bedrock surface. These coarse deposits
pass laterally into medium-grained to finer sands and silts. The D1 lithofacies is
similar to the B3 lithofacies, but is more heterogeneous and more consolidated
(overconsolidated)
D2. Laminated clays, silts and fine sands, often dark coloured. They indicate an
upwards transition from lacustrine to estuarine and fully marine deposits.
These sediments are similar to the sediments of the B4 lithofacies, but are more
consolidated (overconsolidated).
Figure 3-5 is another schematic transect to illustrate the complexity and apparent
continuity of the BGS lithostratigraphical units identified in the boreholes along the
North-South transect shown on Figure 3-1. Figure 3-6 shows the corresponding
38
lithofacies interpretation of the same section as Figure 3-5, where the lithofacies units
correspond directly to the dominant lithologies and lithofacies identified in the
boreholes.
The actual relationships of the lithofacies units are more complicated than indicated
schematically in Figure 3-4 and Figure 3-6. The distribution of regional lithofacies units
was initially determined by reassigning all the lithostratigraphical units in the BGS
transects in Nirex Report SA/97/045 (Nirex, 1997b) and the new BGS transect in
Michie et al. (2007) to the new lithofacies units. From these two-dimensional transects,
a map of the surface distribution of the lithofacies was produced and a simplified
version is shown as Figure 3-7.
39
40
Figure 3-5. North-south regional cross-section through the LLWR showing the BGS lithostratigraphical units
Figure 3-6. North-south regional cross-section through the LLWR showing the regional lithofacies units
41
Figure 3-7. Map of the LLWR and surrounding area showing the surface distribution
of the regional lithofacies units
42
This map also shows as a dashed line the concealed extent and limit of the C lithofacies
unit. In detail, the limit of the unit is more complex and this is discussed for the LLWR
site area in Section 4.
A simple three-dimensional digital model of the Quaternary sediments across the area
was created (Hunter et al., 2007a). This was subsequently updated in Smith (2008) and
in later studies of new information from the LLWR site (Smith, 2009a). In the following
description, the general nature of the lithofacies units and their relationships to the
BGS lithostratigraphical units is summarised. Details of the nature of the BGS
lithostratigraphical units and their occurrence are provided in the publication by
Merritt and Auton (2000).
3.4 Lithofacies Units
Lithofacies Unit A
This unit comprises all the post-glacial (latest Devensian and Holocene/Recent)
sediments across the area and their distribution is shown in detail on the published
BGS Solid and Drift map of the area (British Geological Survey, 1999). The sediments
include scree and head (rock debris and clayey hillwash) on hill slopes, peat, alluvium,
alluvial fan and river terrace deposits, lacustrine deposits, blown sand, estuarine and
beach deposits, and offshore sediments, dominantly marine sands and muds. BGS
lithostratigraphical units onshore included in this lithofacies unit are the Hall Carleton
Formation, the Ehen Alluvial Formation, the Blelham (Peat) Formation and the Drigg
Point Sand Formation. This unit comprises a wide range of lithostratigraphical units
with
likely
markedly
different
hydrogeological
properties.
However,
these
lithostratigraphical units are of limited spatial extent and / or thickness such that it is
not practicable to distinguish and characterise them all at the regional scale. However,
they can be subdivided into a limited number of lithofacies units in the local area
around the LLWR site.
Lithofacies Unit B1
This unit includes offshore glaciomarine and marine deposits, which are partly
underlain by glacial, lacustrine and terrestrial deposits. The glaciomarine and marine
deposits partially infill deeply-incised depressions (palaeovalleys) cut down into the
pre-Quaternary bedrock. The offshore deposits are predominantly bedded muds, silts,
sands and minor gravels. These formed in a dominantly marine environment, even
those laid down during the Late Devensian glaciation, and therefore can be treated as a
single lithofacies assemblage. For the 2007 study (Hunter et al., 2007b), the B1
lithofacies was also taken to include the underlying glacial, lacustrine and terrestrial
43
deposits. This review has identified the correlation of the older sequence with the
regional lithofacies units. This subdivision of the offshore sequence is discussed below.
Offshore Lithofacies
In the 2007 studies, the detail of the offshore sequences was not addressed and the
correlation with the onshore units was not determined. The B1 lithofacies unit was
applied to encompass all of the offshore Quaternary sediments until the recognised
deficiencies in their understanding, nature and correlation could be resolved. That was
subsequently achieved and was summarised in Section 3.2 of this review. Because the
upper part of the offshore sediments is dominantly formed of clays/muds, silts and
fine-sands, their overall categorisation as B1 is a reasonable approximation. However, a
more detailed correlation is now possible.
Figure 3-3 shows the separate offshore seismic sequences and the onshore
lithostratigraphy in the SW-NE, offshore to onshore transect. Figure 3-8 shows the
integrated lithofacies interpretation of the transect. This shows that Sequence 1 is
equivalent to lithofacies unit D1, that Sequences 2 and 3 are equivalent to lithofacies
unit D2, that Sequence 3A is equivalent to lithofacies unit C, and that Sequences 4, (5)
and 6 are equivalent to lithofacies unit B1. Lithofacies unit B1 forms the sea floor and
covers all the underlying units. In the nearshore environment, the B1 unit forms only a
thin layer above the coarse deposits of lithofacies unit D1, which are similar to the
coarse sands and gravels of the B3 lithofacies unit.
Lithofacies Unit B2
This unit includes a sequence of interbedded tills (glacial diamictons), and sands and
gravels. All these deposits were principally derived from the Irish Sea and formed
during the Main Devensian glaciation and subsequent readvance stages. From the BGS
lithostratigraphical scheme, the lowest part of the unit is formed by the Ravenglass Till
Member, and upwards there is an alternation consisting of the Kirkland Wood Sand
and Gravel Member, the Drigg Beach Till Member, the Drigg Holme Sand Member, the
Fishgarth Wood Till Member and the Drigg Moorside Silt (including an unnamed Till)
member. All the till (glacial diamicton) units are noticeably clay-rich and the upper till
units are almost entirely clay, formed as melt-out or flow tills. North of the LLWR site,
the lithofacies cuts across older sediments to rest directly on the bedrock. Because this
lithofacies is the host for the LLWR, a brief summary of its characteristics as observed
at local coastal exposures is provided here.
The type locality of the Ravenglass Till Member is in low sea cliffs south of Ravenglass,
where 4 m of the till is exposed. It is a very stiff, moderate brown, massive, matrix
supported, sandy, silty, clay diamicton, which contains moderately well dispersed,
44
subangular to subrounded clasts up to boulder size. These clasts are of both Lake
District and more northerly/ coastal provenance. This mixed origin may be due to
reworking of clasts into the till from the underlying older deposits derived from the
Lake District. The lower boundary of the till is not seen at the outcrop, but in boreholes
the till is interpreted to rest unconformably on older sands and gravels. The upper
boundary of the till is exposed at the coastal outcrops near Ravenglass and is generally
erosional, being overlain by sands and gravels, and also glacitectonites associated with
the emplacement of younger till. However, the equivalent of the younger Drigg Beach
Till was not identified and the till overlying the sands and gravels (the Drigg Holme
Sand Member) was interpreted as the Fishgarth Wood Till.
The upper part of the overlying sequence of interbedded tills, and sand and gravels is
well exposed on the coastal section near Drigg west of the LLWR. On the foreshore and
base of the sea cliffs, 3 m of the Drigg Beach Till Member is exposed. This comprises a
dusky red to weak red, stiff, plastic, clayey silt diamicton with well dispersed sand
grains and subangular to well rounded pebbles up to 3 cm in diameter, and also some
reworked marine shells. The till is interpreted to have formed from a readvance of the
ice sheet from the Irish Sea.
Overlying the Drigg Beach Till Member with an undulating erosion surface is the
Drigg Holme Sand Member, which is about 2.5 m thickness of fine to medium-grained
sand with stringers of granule gravel and channels containing clast-supported gravel.
The member becomes increasingly affected by planar, subhorizontal shearing towards
the base of the next overlying till. These features could indicate that the member was
laid down as glaciofluvial outwash between two glacial readvances, but could be
produced sub-glacially by lifting/floating of a thin ice sheet margin.
The overlying Fishgarth Wood Till Member is only 0.8 m thick and is a hard, moderate
reddish brown, calcareous, massive, matrix-supported, sandy silty clay diamicton with
well-dispersed (rare), subangular to rounded clasts up to 15 cm diameter. Coal, black
shale and shell fragments are prominent. Conformably overlying the thin till is a 2.1 m
thick sequence of pebbly gravels and sands. These may be a thin lateral equivalent of
the upper part of the thick, coarse sand and gravels that form the Peel Place Sand and
Gravel Member inland that is part of the B3 lithofacies described next. There is a
planar, tectonised layer developed below the uppermost glacigenic unit, the Drigg
Moorside Silt (and Till) Member, which is 1 to 1.9 m thick.
Later deformation features are seen to affect the interbedded till, and sand and gravel
units at the Ravenglass and Drigg beach outcrops and elsewhere. These include folding
of the Ravenglass and Drigg Beach Till Members and complex faulting of the higher
units. Faulting includes low-angle (near horizontal) thrust structures, which can
duplicate the strata, and high-angle (near vertical) fault structures, which can disrupt
45
the continuity of layers. These deformation features are related to readvances, and
other marginal processes, of the ice sheet affecting the underlying deposits.
Lithofacies Unit B3
This unit is dominantly composed of thick sand and gravel deposits and appears to
have two main developments. A lower development occurs below the B2 unit in the
area of the LLWR site and appears as a series of channel developments cutting into
older deposits. Some of these channels also cut into and remove the lower tills within
the B2 unit and indicate a series of phases of incision during the development of the B2
lithofacies. The incisions were probably related to periodic subglacial release of icedammed lake water from Lower Wasdale. This lithofacies unit includes the BGS
lithostratigraphical unit, the Barn Scar Sand and Silt Member, but also includes parts of
the sequence previously defined as the underlying Holmrook Till unit, and also parts
of the overlying sequence where the till units are not present. Another development
occurs inland marginal to the B2 lithofacies unit, and further inland similar sands and
gravels
overlie
the
following
B4
lithofacies
unit.
The
equivalent
BGS
lithostratigraphical units are the Peel Place Sand and Gravel Member and the
Mainsgate Wood Sand and Gravel Member respectively.
At the Peel Place Quarry, about 2 km NE of the LLWR, the workings are exploiting
about 11.5 m thickness of the Peel Place Sand and Gravel Member. Excavations have
revealed an underlying sequence of about 1 m of lacustrine finely laminated sands and
silts. There is a gradational upwards transition into thickly interbedded, cross-stratified
sand and upwards fining gravel at the base of the Peel Place Sand and Gravel Member.
This mainly comprises well-bedded, cross-stratified gravel and sand that typically
forms an upwards coarsening sequence, from thickly interbedded sand and cobble
gravel, into gravel with low-angle planar cross-stratification and trough cross bedding.
Interbeds of sandy silt with weakly developed lamination occur towards the top of the
member. Above this, forming the top of the quarry face and underlying the land
surface behind is about 1 m thickness of a contorted succession of interstratified hard,
sandy, clayey diamicton (flow till) and silty fine-grained sand. This unit may be the
lateral equivalent of the uppermost and youngest till unit, within the Drigg Moorside
Silt (and Till) Member in the Drigg beach succession. In places, the diamicton is
overlain by a thin (up to 70 cm) bed of medium to fine grained sand, possibly of
aeolian origin.
Lithofacies Unit B4
This unit, which occurs in Lower Wasdale, dominantly inland of the glacial readvance
limit during the Gosforth Oscillation, consists of glaciolacustrine deposits of
46
interlaminated silts and clays, but also includes in the middle a till unit that is equated
with the Ravenglass Till Member. The BGS lithostratigraphical units are the
Whinneyhill Coppice Clay Member, the Green Croft Till Member and the Holmeside
Clay Member, all of which relatively clay–rich and of fairly uniform properties.
Lithofacies Unit C
This unit consists of till as well as sands, gravels, silts and clays, deposited across the
area from ice moving westwards from the Lake District and extending beyond the area
of the LLWR site at depth. The till is generally a sandy stony diamicton, in contrast to
the clay-rich upper till units in unit B2, and contains clasts predominantly of the
Borrowdale Volcanic Group, the Eskdale Granite and the Ennerdale Granophyre, all
derived from the Lake District. The BGS suggest that the till was deposited before the
build-up of the main Late Devensian ice sheet that travelled southwards along the Irish
Sea and coastal plain (Merritt and Auton, 2000).
The lithofacies unit is partially equivalent to the BGS lithostratigraphical unit, the
Holmrook Till Member at depth beneath Lower Wasdale and the LLWR Site, and the
equivalent Scale Beck Till Member at outcrop inland in the Lake District. However, it
was identified that the lithofacies unit under the LLWR site area is generally thinner
than the thickness of the Holmrook Till unit interpreted by the BGS, and is removed by
channel incision of the overlying B3 unit in places. Over the bedrock high to the north
of the LLWR site, the lithofacies unit is absent, probably removed by the Irish Sea ice
sheet advancing from the north-north-west.
In the offshore area, where only one till unit (Sequence 3A) is present, this is of the
same lithofacies as the C lithofacies in the onshore area. As explained earlier in Section
3.2 and shown in Table 1, Sequence 3A is stratigraphically (time) equivalent to all the
till and interbedded sand and gravel units of the onshore area. The fluctuations in the
position of the eastern margin of the ice sheet over the onshore area did not extend into
the area of the East Irish Sea. In that area, a continuous cover of grounded ice was
maintained until the final deglaciation and therefore there was no development of
interbedded tills, and sand and gravel deposits. Sequence 3A is equivalent to
lithofacies C in the offshore area, and was partly removed by erosion during the
marine regression and transgression in the post-glacial phase.
Lithofacies Unit D
In the onshore area, the D lithofacies consists of all the older sediments present at
depth infilling the palaeovalley in Lower Wasdale below the C lithofacies (there partly
equivalent to the BGS lithostratigraphical units of the Holmrook Till Member and Scale
47
Beck Till Member to the east), which oversteps the D lithofacies to the east to rest
directly on the bedrock as shown on Figure 3-3.
The basal part of the lithofacies may be significantly older than the overlying
sediments and is weathered in places and of mixed character. It contains a wide range
of sedimentary types ranging from diamictons of mixed provenance (containing clasts
from the Lake District, and from the floor of the Irish Sea to Southern Scotland) to
sands and silts. The upper part of the lithofacies consists dominantly of lacustrine to
glaciomarine clayey siltstones and fine sandstones.
There are little data with which to characterise these deeper sediments across the
Lower Wasdale area, but they can be separated into different subfacies under the
LLWR site and adjacent area where the density of data is greater, although still
relatively limited compared to the borehole information on the upper lithofacies.
In the offshore area, the D lithofacies is interpreted to be equivalent to offshore
Sequences 1, 2 and 3. These were described in Section 3.2 and can be summarised as
follows: Sequence 1 at the base consists of an irregular assemblage of diamictons, sands
and gravels; the overlying Sequence 2 consists of a thin drape of lacustrine sediments;
and Sequence 3 consists of layered marine sediments that overlap the lower sequences.
These sequences lie beneath the single till of Sequence 3A and the late-glacial to postglacial marine deposits of Sequences 4 to 6. Sequences 1, 2 and 3 were identified as
similar to the succession forming the D lithofacies in Lower Wasdale.
The consolidation of the sediments forming Sequences 1 and 3 was measured by
oedometer tests on undisturbed clay samples from offshore borehole B3 and reported
in the BNFL/Nirex report on the interpretation of their offshore high-resolution
seismic survey (Nirex, 1997e). The results showed overconsolidation due to loads of
over 780 kPa, much greater than the present sediment overburden, and were
interpreted to indicate the load from a grounded ice thickness of several hundred
metres, the main ice sheet.
For the 2007 study (Hunter et al., 2007b), the D lithofacies was treated as a single unit
of generally silty character, but this study has confirmed that the lithofacies can be
subdivided regionally as follows:
The D1 lithofacies dominantly consists of coarse sands and gravels, with diamictons at
the base and margins of the unit against the irregular bedrock surface. Some of the
diamictons are probably not glacigenic, but may have formed as talus/scree
accumulations of coarse debris adjacent to cliffs (suggested by Eaton et al., 1996/97), or
as lacustrine to marine beach deposits, which may have reworked older sediments.
48
These coarse deposits pass laterally into medium-grained to finer sands and silts,
which may have formed in nearshore environments.
The D1 lithofacies is similar to the B3 lithofacies, but is more heterogeneous and more
consolidated (overconsolidated). In the onshore area, the D1 lithofacies is equivalent to
the sandier parts of the BGS lithostratigraphical units, the Maudsyke Till Member and
the Drigg Till Formation, but also to the sandier parts of the Carleton Silt Formation
and the Glannoventia Formation. In the offshore area, the D1 lithofacies is equivalent
to Sequence 1.
The D2 lithofacies consists of laminated clays, silts and fine sands, often dark coloured.
They indicate an upwards transition from lacustrine to estuarine and fully marine
deposits. These sediments are similar to the sediments of the B4 lithofacies, but are
more consolidated (overconsolidated). Equivalent BGS lithostratigraphical units in the
onshore area include the Glannoventia Formation comprising the Kokoarrah Shelly
Sand Member, the Stubble Green Silt Member and the Carleton Hall Clay Member, and
the Carleton Silt Formation. The D2 lithofacies was also taken to include clay-rich basal
sections classified as parts of the Maudsyke Till Member and the Drigg Till Formation.
Equivalent units in the offshore area are Sequences 2 and 3.
At the LLWR, the detailed cross-sections presented in Section 4 indicate that it is
possible to sub-divide the D2 lithofacies into the lower fine-grained lacustrine and
estuarine deposits (termed D2) and upper interbedded silt and sand marine units
(termed D3). This detailed subdivision is not possible at the regional scale due to the
limited data available with which to characterise the D lithofacies. Therefore, in this
regional Section and Appendix A, the upper interbedded (brown) silt and sand marine
units are included within the D1 lithofacies. Within the site and in the adjacent area,
thick sands, either near-shore facies variants of the silty units or possibly incised fluvial
channels, are identified within the D lithofacies. On the regional sections, these sands
are also included in the D1 lithofacies. However, in the detailed sections, these are
identified as separate sand units and they have not been assigned to one of the named
sub-units of the D lithofacies for the present. The spatial distribution of these units as
currently understood is shown in the cross-sections in Section 4, but there is
uncertainty about their spatial distribution.
This review has identified the correlation between the regional lithofacies discussed
above and the lithofacies packages previously defined at the LLWR site. Therefore, the
classification of the Quaternary sediments was unified and the regional lithofacies
classification applied at the LLWR site, but with more detailed classification of the
lithofacies and higher-resolution identification of their spatial distribution. This is
discussed more fully in the following section.
49
50
Figure 3-8. SW-NE cross-section from offshore to the LLWR showing the regional lithofacies units
4
Characterisation of the Quaternary
geology at the LLWR site
As discussed in the earlier sections of this report, the methods used by previous
researchers to characterise the Quaternary sediments at the LLWR site (and also in the
surrounding region of west Cumbria) can be summarised in terms of three separate,
alternative models. These are:
The ‘events-based stratigraphy’ developed by G. Eaton and subsequently
applied by BNFL to characterise the LLWR site for the 2002 PCSC (BNFL,
2002a).
The regional lithostratigraphical framework developed by the BGS during
and subsequent to the Nirex deep repository investigations (Nirex, 1997a).
The ‘lithofacies’ concept proposed by Hunter et al. (2007b) for the
Reinterpretation of the LLWR Site Quaternary Geology (LLWR Lifetime
Project) in 2007.
Each of these models is based upon interpretations of essentially the same source data,
namely, outcrops of Quaternary sediments, sample lithology descriptions from site
investigation boreholes, geological descriptions from scientific investigation boreholes
and pits (varying from low to high quality), and non-intrusive geophysical surveying.
However, each model assessed the relevant importance of these separate sources of
data for the purpose of characterising the Quaternary geology at the LLWR site in
different ways.
Although each of the three geological models is ultimately linked to current
understanding of the glaciation history of north-western England during the period
known as the Main Late Devensian (MLD) (described in Section 3), each model
incorporates different assumptions for defining the processes involved in the events,
the sedimentary material deposited (or eroded) by an event and also for recognising
such events in the available data.
Furthermore, the objective of each of the three models was also slightly different: the
events-based stratigraphy model was developed to describe a general understanding
of the geology beneath the LLWR site and to be able to predict lithological distribution
across the site (BNFL, 2002a); the lithostratigraphical model is part of the standard
approach to regional geological understanding of Quaternary deposits throughout
Great Britain adopted by the BGS (McMillan et al., 2005); while the lithofacies model
developed by Hunter et al. (2007) was devised in response to the need to provide a
51
materials focused description of the geology of the LLWR for hydrogeological
modelling, etc.
The first two of these models were derived principally from the construction of a few
schematic geological cross-sections using selected groups of simplified borehole
lithology logs - usually the limited set of deeper boreholes that penetrate through the
entire Quaternary sequence. However, the variability of the Quaternary sedimentary
deposits beneath the LLWR site is sufficiently complicated such that slight changes in
the orientation of the cross sections and the particular groups of borehole logs selected
(and omitted) for each cross-section can result in markedly different interpretations of
the Quaternary geology and stratigraphy.
The third (lithofacies) model was derived from a non-selective interrogation of all of
the available borehole data. Such an approach was deemed to be necessary to
investigate and characterise the rapid changes in lithology that can be observed
between certain groups of relatively closely-spaced boreholes. This alternative
approach was also deemed to be appropriate for a lithology dataset that exhibits no
obvious layered sequence that can consistently, or confidently, be matched (unit for
unit) to the published BGS regional Quaternary lithostratigraphy.
In a general sense, some or many of the unit boundaries that define the different
lithofacies of the third model are also event boundaries of the type proposed for the
first (i.e., events-based stratigraphy) model. Similarly, the lithofacies units can be
generally related to the BGS’ lithostratigraphy, as described in Section 3. However,
analysis of the LLWR borehole lithology database in the context of lithofacies revealed
evidence for the existence of channel-like bodies of coarse-grained sediments (sands,
gravels and cobbles etc) which are interpreted as being deeply incised into underlying
layers of finer-grained sediment. Extensive coalescing of these channels also seems to
have resulted in the erosion of most of a layer of thick, clayey till that was probably
deposited across the entire LLWR site, but now only exists as limited remnants.
Evidence for the existence of the channel-like bodies of coarse-grained sediments can
be found in the database of borehole lithological descriptions recorded from the LLWR
site, where comparison of particular pairs of boreholes located in relatively close
proximity to each other show significant contrasts in lithology at approximately
equivalent depths. Two examples of such contrasting lithology in borehole pairs are
shown in Table 2 and Table 3. The boreholes are also plotted on the cross-sections
shown as Figure 4-9 (and Figure C-2) and Figure 4-12 (and Figure C-3).
52
Table 2. Comparison of sample lithology for boreholes 8646 and 7525 below 0 m OD
Borehole 7525
Borehole 8646
Top
elev
-0.91
-2.91
-3.11
Bottom
elev
-2.91
-3.11
-9.81
-9.81
-14.61
-14.61
-16.11
-16.11
-16.61
-16.61
-17.11
Lithology description
Firm to stiff brown slightly
sandy slightly gravelly CLAY.
Gravel is subangular to rounded
fine to coarse of basalt and
granite.
Grey/brown fine to coarse
SAND and subangular to
subrounded fine to coarse
GRAVEL of mixed igneous and
metamorphic lithologies.
Stiff brown slightly gravelly
slightly sandy CLAY with
occasional cobbles. Gravel is
subangular to rounded fine to
coarse of basalt and granite.
Reddish brown very clayey fine
to coarse SAND and subangular
to subrounded fine to coarse
GRAVEL of mixed igneous and
metamorphic lithologies. With
some subangular to
Red slightly gravelly fine to
coarse
SAND.
Gravel
is
subangular fine to coarse of
sandstone. (Highly Weathered
Sandstone)
Moderately weak to moderately
strong red fine to medium
grained SANDSTONE. Highly
weathered. Recovered as sandy
subangular fine to coarse gravel
sized fragments.
Moderately strong red fine to
medium grained SANDSTONE.
Moderately
weathered.
Recovered
as
subangular
cobbles and gravel sized
fragments.
End of borehole
Top
elev
Bottom
elev
0.15
-3.15
-3.15
-8.35
Very
dense
brown
slightly
gravelly very clayey SAND with
some thin bands of brown clay.
Gravel
is
subangular
to
subrounded fine to medium of
mudstone and igneous lithologies.
Very dense multicoloured sandy
very clayey angular to subrounded
fine to coarse GRAVEL of
sandstone,
mudstone,
shale,
limestone and igneous lithologies
with frequent subangular to
subrounded
cobbles
an…
(description unfinished)
End of borehole
53
Table 3. Comparison of sample lithology for boreholes C8 and 8649 below -10 m OD
Borehole 8649
Borehole C8
Top
elev
Bottom
elev
-10.87
-11.69
-11.69
-15.08
-15.08
-17.02
-17.02
-18.63
-18.63
-18.71
-18.71
-19.57
-19.57
-26.23
-26.23
-26.49
-26.49
-27.32
-27.32
-33.88
-33.88
-34.14
-34.14
-36.71
moderate brown very sandy
CLAY with many fine sand
bands
fine to coarse GRAVEL with a
silty sand matrix
fine to coarse GRAVEL with a
silty sand matrix
(no recovery)
Top
elev
-10.1
Bottom
elev
-16.1
-16.1
-18.6
-18.6
-20.1
-20.1
-31.6
olive grey silty CLAY
dark grey very silty fine SAND
olive grey laminated sandy SILT
brownish grey silty CLAY with
gravel
gravel
gravel
light brown silty fine SAND
with gravel
SANDSTONE
End of borehole
-31.6
-33.6
Brown gravelly fine to
coarse SAND. Gravel is
subrounded fine to medium
of basalt, sandstone and
granite.
Dark brown fine to coarse
SAND and subangular to
subrounded fine to medium
GRAVEL
of
basalt,
sandstone and granite.
Brown fine to medium
SAND
with
frequent
subrounded
cobbles
of
granite and basalt.
Multicoloured slightly sandy
subrounded
coarse
GRAVELS and COBBLES
of granite and basalt.
Moderately weak red fine to
medium
grained
SANDSTONE. Moderately
weathered.
End of borehole
In these two examples, thick borehole intersections of mixed sand, gravel and cobbles
occur at similar elevations and are laterally equivalent to comparable thicknesses of
finer-grained lithologies that will have been deposited by different sedimentary
processes. In the first example, at an intermediate depth, the stiff, slightly gravelly,
slightly sandy clay intersected in borehole 8646 most probably represents a thick till
and in this study it has been assigned to the ‘C’ lithofacies. In the second example, at a
deeper level, several metres of olive-grey, laminated, sandy silt most probably
represent widespread marine sediments of the Glannoventia Formation (BGS) and this
lithology has been assigned to lithofacies ‘D2’ in this study. In both of these examples,
the intersections of comparable thicknesses of mixed sand, gravel and cobbles in
boreholes 7525 and 8649 – here both assigned to lithofacies B3 – indicate deposition by
an entirely different process.
While glacitectonic deformation is one possible mechanism to explain the juxtaposition
of these contrasting lithologies, fluvial incision by sub-glacial outburst drainage is a
simpler and more realistic option when the surrounding palaeogeography during the
54
late Quaternary Period is taken into consideration. Periodic outbursts of glacial
meltwater (and sediment) from one or more ice-dammed lakes in the Wasdale area are
likely to have occurred while the water level in these lakes was significantly higher
than the relative sea-level at the time (and both the relative sea level and the level of
the water in the ice-dammed lakes was higher during the glacial phase than the
relative sea level, ordnance datum, at present). The incised channel exposure at
Nethertown (north of LLWR) is an excellent example of such a deposit (Figure 4-10).
If the existence of incised, sand- and gravel-filled channels is accepted, then such
features represent distinct ‘events’ whose boundaries may be deeply incised into
underlying sediment packages. Thus the lithofacies model allows for the possibility
that components of younger sedimentary units (i.e., events) are likely to occur within,
or even below, older units. This differential vertical displacement of events is matched
in the model by similar lateral displacements because sedimentary material of local
(i.e., Lake District ice) provenance is likely to exist co-mingled with material of Irish
Sea ice provenance. In the lithostratigraphical framework model, glacial deposits
derived from these two, different source areas are classified as separate geological
‘formations’.
In a simplistic sense, the lithofacies model of the Quaternary deposits at the LLWR site
can almost be regarded as an inverse representation of the events-based stratigraphy
model. This is because the latter model (illustrated by Figure 4-14) depicts the
Quaternary deposits at LLWR as consisting of a thick ‘Main Diamict Formation’,
composed principally of various types of till deposited by a MLD glacier, which is
overlain by a sequence of mainly fluvial and lacustrine sediments, whereas in the
lithofacies model there is no equivalent of the Main Diamict till assemblage and most
of the preserved, recognisable tills exist at a shallower depth and are seen to overlie a
thicker deposit of sand and gravel which is more likely to be of fluvial outwash origin.
In many of the cross-sections of the LLWR Quaternary geology prepared for this
report, the lithofacies units can give the appearance of being simply another layered
sequence model. This is the consequence of each cross-section representing only a
small, 2-dimensional slice of one part of the LLWR site. When these lithofacies units are
considered in 3 dimensions on a wider, semi-regional scale, particularly the B3 facies,
their localised spatial distribution (controlled in part by the bedrock topography and
the Wasdale palaeovalley) and their cross-cutting and lateral-transitional relationships
to adjacent glacial deposits become more evident. These important features of the
lithofacies model distinguish it from the alternative, layered-sequence models at the
LLWR site and some of the essential differences between the three models are
summarised in Table 4.
55
Table 4. Summary of the features of the 3 basic geological models used to
characterise the LLWR and the immediately adjacent areas of interest
Geological
Methodology
Principal assumptions
model
The Gosforth Oscillation glaciation event
compacted the underlying MLD sediments, thus
creating a density-contrast surface between the two
Used selected site
events which can be detected by geophysical
investigation boreholes from
(seismic) surveying.
Events-based
within and in the immediate
stratigraphy
vicinity of the LLWR site;
model of the
‘event units’ are identified
from each event can be identified as a unit and the
LLWR site
partly on the basis of seismic
boundaries separating these units can be identified
reflectors and partly upon
from borehole lithology descriptions (in
lithology.
combination with seismic reflections).
Each glacial event deposited sedimentary material
in an accumulating sequence and the materials
Each event unit may contain a wide variety of
material types, depending upon particular glacial
environments (e.g., sub-glacial, marginal ice etc.).
Used a limited number of
Regional
lithostratigraph
-ical framework
surrounding
the LLWR site
Individual, discrete lithostratigraphical units,
scattered outcrops, pits and
including very thin units, can be correlated as a
scientific boreholes, mainly
uniform, systematic, layered sequence across large
outside of LLWR, logged in
areas on a regional scale between scattered,
great detail and correlated
isolated boreholes, outcrops and other data
over large distances. Units
sources.
identified using a combination
of lithology, sedimentary
Each lithostratigraphical unit may consist of a
gradation of different material types.
fabric and additional ancillary
information.
Lithofacies
LLWR site scale
model of both
LLWR site and
the
surrounding
region
Correlation of similar material types between
Used as many of the site
boreholes in close proximity to each other as
investigation boreholes from
simple lithology units, with no inferred genesis,
within and around the LLWR
offers the least interpretive and the most
site as possible, basing
reproducible characterisation of the LLWR
correlations entirely upon
Quaternary deposits for hydrogeological
simplified lithological
modelling even if the resulting spatial distribution
descriptions to identify gross
of units is not a uniform sequence of layers.
patterns of distribution of
different material types.
Nearby coastal exposures are useful analogues for
the materials being described in samples from the
LLWR site borehole database.
56
Regional scale
The complex spatial distribution of bulk lithology
types seen within the LLWR site is supported by a
large dataset and there is nothing to suggest that
the site is radically different from the immediate
surrounding area.
Used the same limited
Ice–dammed lakes in Upper Wasdale formed
regional borehole dataset as
reservoirs of water and outwash sand and gravel
the lithostratigraphical
that could be released subglacially to form tunnel
framework model, but re-
valleys if the Irish Sea ice sheet thinned and/or
interpreted in the context of
lifted off its base. These tunnel valleys probably
simplified, bulk lithology and
incised deeply into older glacial and marine
with regard to the regional
deposits, replacing tills and laminated silts with
palaeogeographical setting.
mixtures of sand and gravel.
Several other palaeographical features of the
Wasdale valley area (described in Sections 2 & 3)
probably restricted ice movement and influenced
local depositional environments and sediment
facies.
4.1 The concepts of ‘Lithostratigraphical
Formations’ and ‘Lithofacies’
One recurring question which is often associated with the events-based stratigraphical
model is that the exact basis for determining the bounding discontinuities between the
separate event formations in many of the borehole lithological logs, often within an
interval of seemingly uniform or similar lithology, is not recorded and, despite
significant searching, could not be found for incorporation into this report. Similarly,
the distinction between inference (of deposition and provenance) and description is not
always clear.
McMillan et al. (2006, page 6, para 1.2) state that: “By their nature, many Quaternary
units are strictly allostratigraphical, i.e., [they are] defined and identified on the basis
of their bounding discontinuities”. Also, “The lithology of many Quaternary deposits
is determined by the medium of transport, the medium of deposition and by
provenance. Inferred genesis and provenance may aid lithostratigraphical classification
of heterogeneous superficial deposits but always there should be a clear distinction
made between inference and description”.
The lithofacies model proposed by Hunter et al. (2007a) attempts to follow these
important principles by avoiding both the attempted identification of stratigraphical
bounding surfaces and also the direct association of particular stratigraphical units
57
with established glaciation events and their assumed depositional processes.
(Although, the lithofacies can be related to the lithostratigraphy and hence such events:
Section 3). However, the lithofacies model does ascribe a general genetic origin to the
clayey, stony diamictons, which are assumed to be sub-glacial tills, and the thick,
elongate zones of sand and gravel, which are assumed to be either sub-glacial or subaerial melt-water channels.
Both the events-based stratigraphical model for the LLWR site and the regional
lithostratigraphical model for surrounding region utilise the term ‘formation’ in
naming the sedimentary units. McMillan et al. (2006, page 21), quote the definition of a
formation as “the primary formal unit of lithostratigraphical classification used to map,
describe and interpret the geology of a region”. Also, the formation “is generally
defined as the smallest mappable unit and has lithological characteristics that
distinguish it from adjacent formations”. The significance of regional dimensions for
the definition of geological formations is emphasised by the statement that “In Britain,
formations should be mappable and readily represented on a 1:50,000 scale map”.
McMillan et al. (2006) discuss the issues associated with the application of these
definitions to glacigenic deposits and note that the key tests (of a formation) include
demonstrating type section(s) where the top and base of the unit can be observed
(recognising that these boundaries may vary laterally) … and tracing lateral continuity
(accepting that lateral and vertical variation is likely). A geological formation may or
may not be divided into ‘members’, which are “lithologically distinctive units
interbedded within more regionally significant glacigenic formations”. Even lower in
stratigraphical rank than the ‘member’ is the ‘bed’, “commonly applied to distinctive
units that may be thin and laterally inextensive”. This discussion of terminology is not
of mere academic interest; the use of the term ‘formation’ in a geological model of the
LLWR Quaternary deposits implies that the sedimentary units have recognisable
bounding surfaces and a degree of lateral continuity.
The use of the concept of lithofacies has been described by McMillan and Powell
(1999). In paragraph 4.5 on page 8 of that report, it is stated that “Bodies of
unconsolidated sediment with specified physical and sedimentary characteristics are
commonly referred to either (as) a single lithofacies or a lithofacies association. BGS
geologists have in recent years employed lithofacies codes to enable a first-stage, nongenetic description of field and borehole sequences.”
Such a ‘first stage’ approach is ideally suited to characterising the Quaternary deposits
beneath the LLWR site where the limited number of outcrops reveal only a small part
of the stratigraphical sequence and where the bulk of the information is in the form of
engineering-type lithology descriptions from borehole logs. A comparison of Figure 3-2
and Figure 3-4 shows how a sequence of complex glacigenic sediments can be re58
interpreted using the lithofacies concept as a more realistic alternative to a layered
sequence model.
4.2 Additional review of data for this report
The borehole cross-sections presented in Hunter et al (2007b) were preliminary in
nature and were intended to interrogate the entire borehole lithology database on a
non-selective basis and without any pre-conceptions of stratigraphical equivalence or
depositional direction being applied. The outcome of that process was the observation
that the spatial distribution of Quaternary lithologies beneath the LLWR exhibits
distinctive lateral discontinuities between separate zones that apparently consist of
different lithologies (i.e., lithofacies). These lithofacies zones (which in 2007 were
termed lithofacies ‘packages’), are probably composite bodies of sediment resulting
from more than one single geological ‘event’, may be potentially mappable and could
exert significant influence in a hydrogeological regime.
As noted by Jefferies (2009), one obvious disadvantage with the cross-sections
presented in the 2007 report was that it was not possible to make direct comparisons
between those sections and the existing, more widely-used cross-sections from older
reports. Therefore the additional cross-sections presented in this report are intended to
redress the omission of such comparative sections in the 2007 report. The new crosssections also afford the opportunity of assessing how closely the lithofacies zones
defined in the 2007 report predicted the lithologies that were recorded in the recent
Phase 3 and Stage 6 boreholes, particularly at intermediate depths.
The additional geological studies of the LLWR site undertaken for this report included
the following:
Updating of the contour map of the sandstone bedrock surface underlying the
Quaternary deposits at LLWR.
Preparation of a set of borehole cross-sections approximately equivalent to Sections
1, 2 and 3 of Eaton (1996), all of which are approximately northwest-southeast in
orientation and generally parallel to the long axis of the LLWR site.
Preparation of four additional borehole cross-sections also orientated parallel to the
long axis of the LLWR site to fill the gap between the replicated Eaton sections. All
of the longitudinal sections are orientated with a ‘sea to shore’ viewpoint to enable
easier comparisons to be made with previously published work.
Preparation of ten borehole cross-sections orientated generally perpendicular to the
long axis of the LLWR site. Some of these sections were located and orientated to
59
investigate specific lithology zones identified on Figure 21 of Hunter et al. (2007b) –
the ‘channels’ map, while others were located to allow comparison with regional
transects and one of the Halcrow (2010) geophysical profiles. All of these crosssections have been extended to reach the beach so that the relative position of the
Drigg Beach Till and other exposed formations can be compared to the borehole
lithofacies units.
The locations of these new borehole cross-sections are shown on an index map of the
LLWR site as Figure 4-1. The cross-sections have been prepared in a similar manner to
the systematic ‘thick sections’ (i.e., ‘wide’ sections) described in Hunter et al (2007b)
using the software Surfer9 (©Golden Software). However, unlike the 2007 crosssections, the positions and orientations of these new sections have been chosen to
investigate specific geological features at the LLWR.
Another difference between the 2007 cross-sections and those prepared for this report
is their reduced width, typically 100 m (i.e., boreholes from up to 50 m on either side of
the cross-section lines have been ‘pulled’ into the sections) rather than the 200 m width
used in 2007. A further difference is the abandonment of the site-scale ‘Lithofacies
Package’, or ‘LP’ nomenclature for identifying the separate lithofacies units at the
LLWR and the adoption of the ‘A – B – C – D’ nomenclature previously used only for
the regional lithofacies (Figure 5-1). This merging of the two parallel naming systems
into a single nomenclature is now possible because of the improved understanding, as
part of this study, of the relationships between the site- and the regional-scale
lithofacies units and also their combined relationship with the BGS regional
lithostratigraphical framework.
The same RGB colour palette has been used for lithofacies units shown on the regional
and the LLWR maps and on the cross-sections to enable direct comparisons to be
made. The colour palette for the lithofacies units is attached to this report as Appendix
D. Also, wherever possible, the same colour scheme has been used to depict the
principal lithology types (as intersected by individual boreholes) shown on the
borehole 'sticks' on the various cross-sections. However, it should be noted that, for
improved clarity and contrast with the background, the colours used to depict
lithologies on the thinner borehole 'sticks' used for the LLWR site-scale cross-sections
in this section are brighter, primary colours and therefore have different RGB values to
the corresponding lithology colours used for the regional cross-sections in Section 3
60
Figure 4-1. Index map of LLWR cross-sections prepared for this report
4.3 Updated contour map of sandstone bedrock
surface
The contour map of the bedrock (the Triassic sandstone surface) concealed beneath the
Quaternary deposits at the LLWR site shown as Figure 15 in Hunter et al. (2007b) has
been updated utilising the additional data points acquired from the Phase 3 and Stage
61
6 boreholes. Most of the new sandstone bedrock intersection data did not materially
change the previous depiction of the buried sandstone bedrock landform, however the
information from three particular boreholes (BH8772, BH8774 and BH8776) have
resulted in the discovery of a new topographical feature in the landform.
This takes the form of a narrow, incised valley, or channel, orientated in a northwestsoutheast direction, parallel to the prominent sandstone bedrock ridge that underlies
the northern half of the LLWR site (and perpendicular to the axis of the Wasdale
palaeovalley). This narrow channel-like feature suggests that the concealed bedrock
landform beneath the LLWR site may not be a simple, glaciated (i.e., ‘U’-shaped)
Wasdale palaeovalley feature, but is possibly a more complicated, dissected
topography which has implications for reconstructing the history of glaciation events
during the Main Late Devensian period. The revised model of the sandstone bedrock
surface is shown in the form of a flat contour map as Figure 4-2 and in the form of a
shaded 3D relief image as Figure 4-3. Slices taken through this interpolated surface
have been used to depict the position of the sandstone bedrock on the borehole crosssections shown in subsequent sections of this report.
62
Figure 4-2. Contour map of the concealed sandstone bedrock surface beneath LLWR
63
Figure 4-3. Shaded 3D relief image of the concealed sandstone bedrock surface
beneath LLWR
4.4 Updated LLWR lithofacies model: the east-west
cross-section and southwest-northeast section 8
This section and subsequent sections of the report describe the updated understanding
of the nature of the Late Quaternary deposits preserved beneath the LLWR site,
expressed in terms of lithofacies, that has been derived from a review of the additional
available data listed in Section 2.3 above. One means of explaining this updated
understanding is to begin with the lithology zones (i.e., the channels and interfluves)
identified on Figure 21 in Hunter et al (2007b), which is shown here as Figure 4-4. On
Figure 4-4 are also shown the locations of the East-West (‘thick’) Slice 3 - Figure 18 of
Hunter et al. (2007b) reproduced below as Figure 4-5, and two new cross-sections
which have been prepared for this report: Figure 4-6 and Figure 4-7. All of these
borehole cross-sections intersect the larger clayey interfluve zone identified as zone ‘F’
on Figure 4-4.
64
The East-West Slice 3 from 2007 (Figure 4-5) shows how the glacigenic sediments were
re-interpreted as a set of LLWR site-scale lithofacies packages (LP1-LP7), namely; the
post-glacial peat and dune sands (LP1), a zone of interbedded clays (tills), sands and
gravels (LP2), a sand- and gravel-filled channel (LP3) (shown schematically), which is
apparently incised into a thick clay unit (LP4) and an underlying unit of silty sands
(LP5). The lowest part of the sequence is completed by an apparently deeper-water
deposit, LP6 and then some older tills and gravels (LP7). The thick clay unit (LP4) is
not identified specifically in this section, although its existence is implied by ‘zone F’
on the channels map and it is likely to have been intersected by boreholes C8 and
DDS6.
Figure 4-4. Map of LLWR showing the locations of selected borehole cross-sections
superimposed on lithofacies zones proposed in 2007
65
66
Figure 4-5. The East-West borehole thick-section 3 from Hunter et al. (2007b)
The updated East-West cross-section presented here as Figure 4-6 can be compared
directly with East-West Slice 3 from the 2007 report. It is also coincident with the
regional-scale cross-section shown as Figure 3-4 in Section 3.2.
On this figure, the different lithofacies units (A, B2, B3 etc) are represented by different
colours and are separated by thin grey lines which represent their bounding surfaces.
These lithofacies boundary lines have been ‘correlated ‘ between boreholes to connect
intersections with apparently similar bulk lithological characteristics, as shown on the
figure key and defined by the coding system described in Appendix D. Of necessity,
because of the nature of that facies and paucity of data at intermediate depths, the
stacked, coalesced channel deposits of the B3 lithofacies are depicted as a single unit in
a semi-schematic manner and without any attempt to show internal detail.
Similar to the correlated stratigraphical boundaries used in the layered sequence
models, these lithofacies boundaries are also subject to some uncertainty. Therefore the
particular arrangement and relationship of the separate lithofacies units shown on this
cross-section is only one of a number of alternative variations that can potentially be
made. Despite this uncertainty, the lines of correlation shown on this and on other
figures are recommended as being appropriate to the generality of the lithofacies
concept and to the lack of precision in the much of the source data.
In addition, the subtle, variable discontinuities between individual layers of thin,
interbedded clayey till, sand and gravel of the type described in the Vault 9 excavation
by Smith (2009a) and in Trench 3, as shown on Figure 62 in the 2002 PCSC (BNFL,
2002a), cannot be identified with any confidence from the quality of borehole lithology
descriptions and the spatial distribution of the boreholes. The method by which these
correlations were derived and interpreted should be sufficiently clear to the reader
from the information provided in this report. A longer discussion of the issues of
confidence and uncertainty is presented in Section 4.7.
67
68
Figure 4-6. New East-West borehole cross-section
The new information available from several additional boreholes completed in the last
two years supports the interpretation shown on E-W Slice 3 from the 2007 report by
emphasising the significant discontinuities that exist between the large, thick, sandygravely channel facies on each side of the cross-section and a prominent zone of
interbedded clays, sands and gravels located in the centre. The new information from
the recent boreholes also provides further evidence for repeated channel-incision of
earlier sedimentary deposits, most noticeably at the base of borehole 8649 and in the
upper half of borehole 8772.
The improved lateral continuity of the grey silty layer, assumed to be marine in origin,
is evident, except where it has been eroded by later channel-incision. The new
boreholes 8772 and 8774, in combination with ROF borehole 19, suggest that a
relatively uniform till overlying the B3 lithofacies unit on the west side may correlate
with the Drigg Beach Till. This potential correlation may be improved if the elevation
of ROF borehole 18, as recorded in the database, can be confirmed to be erroneous and
it is actually 3-4 m lower than shown. Unfortunately at the present time the transcribed
elevation of ROF borehole 18 cannot be verified because the original borehole logs are
not available to be consulted.
The configuration of the highest clayey till(s) has the appearance of having been
deposited across a ground surface that suffered variable compaction during the final
ice advance. Thus the remnants of the slightly older tills which are preserved in the
centre of the cross-section (and in the centre of the LLWR site) may have been
compressed into a hollow ‘depression’ between two, more resistive, channel lithofacies
zones on either side. Similar preservation of a B2 lithofacies zone of interbedded clayey
tills, sands and gravels in this central ‘depression’ can be seen on several of the other
cross-sections.
Also identified in the new East-West borehole cross-section is a more detailed
subdivision of the deeper silty facies into three separate units (D1, D2 & D3). The most
distinctive of these units is the D2 layer, which is usually described as a laminated silt
in varying shades of grey and can be easily distinguished from the more variable
sands, gravels and clays beneath it (D1). Overlying D2 is a unit (possibly a
stratigraphical layer) of sands with multiple thin layers of brown silt, distinguishable
by the change in colour compared to the underlying grey silt. This can also be
recognised in several boreholes in a few of the other cross-sections. It is now identified
separately as Unit D3.
69
70
Figure 4-7. SW-NE borehole cross-section 8
The new borehole cross-section SW-NE 8, shown above as Figure 4-7, was constructed
specifically to display the best combination of borehole logs which intersect the ‘thick
clay’ lithofacies unit from Hunter et al. (2007b). Previously identified as ‘LP4’ and now
re-named as lithofacies unit ‘C’ for consistency with the regional units, this unit is
presumably a zone of preserved till which was not removed by subsequent incisedchannel erosion. It has been intersected by one of the new boreholes (8647) close to the
eastern LLWR site boundary, suggesting that it may extend beyond the site on the
inland side. However, it is absent from another of the new boreholes (8772) located offsite on the western, coastal side, presumably where it was eroded by an incised
channel and replaced by a thick accumulation of sands and gravel (B3). The greater
thickness of clayey material recorded in the log for borehole DR4114 suggests that
lithofacies unit C may be a compound till, comprised of deposits produced by more
than one glaciation ‘event’.
This cross-section also shows a similar pattern of interbedded brown silts and sands
comprising the proposed D3 lithofacies unit in both the older offsite borehole D and
the recent Stage 6 borehole 8772, although the distinctive grey, silty D2 unit is absent in
borehole D. Borehole 8772 also intersected a clayey till at a shallow depth, similar to
that intersected in offsite borehole D. As suggested above for the East-West crosssection, this till may possibly correlate with the Drigg Beach Till, or it may be a
younger (and higher) till, i.e., the Drigg Beach Till could have been removed by
glaciofluvial erosion and channel incision.
4.5 Updated LLWR lithofacies model: northwestsoutheast cross-sections 1 & 2
Figure 4-8 is a map of the LLWR showing the locations of three new borehole crosssections which have been constructed for the purpose of enabling comparisons to be
made with existing cross-sections of the LLWR site showing the ‘events-based
stratigraphy’ and the BGS regional lithostratigraphy interpretations of the glacigenic
deposits (i.e., from Eaton, 1996; Nirex, 1997b; and BNFL, 2002a). The new cross-sections
are not exactly comparable to the older sections because the latter are constructed from
selected boreholes which have been ‘pulled into’ the sections from a wider spatial
extent than the borehole sets used for the new cross-sections. For example, NW-SE
section 1 does not include offsite borehole G nor the C10 cluster boreholes, which are
beyond the approximate 50m-wide selection zone (on both sides of the section line)
used for the new cross-sections.
The inclusion of selected boreholes from a wider offset distance into the new crosssections is no longer necessary because of the greater choice of more recently-drilled
deep boreholes which were not available for use by the previous authors. The inclusion
71
of wider-offset boreholes could also introduce distortions and unnecessary complexity
into the geological interpretations.
Figure 4-8. Map of LLWR showing the locations of new borehole cross-sections NWSE 1, NW-SE 2 and Sections 1 and 2 from Eaton (1996)
The new NW-SE borehole cross-section 1, presented here as Figure 4-9, is equivalent to
Section 1 from Eaton (1996), shown as Figure 4-11. The horizontal : vertical scale ratio
of 20:1 used by Eaton has been reduced to 10:1 for the new cross-section to make the
72
details easier to see. The same scale ratio of 10:1 has been used for the majority of the
other new borehole cross-sections presented in this section and in Appendix B.
The preserved remnant of the thick-clay Unit C is evident in the centre of the section,
apparently truncated by thick accumulations (20-25 m) of incised, channel-fill sands
and gravels. There was no equivalent of this particular cross-section in the 2007 report
by Hunter et al., but if one had been included, the remnant of Unit C would have been
defined by only a single borehole (DDS41) and the Unit B3 on the right (south-east)
side would have been defined principally by three boreholes (C9, C11 and DDS134).
The new boreholes from the Phase 3 and Stage 6 drilling programmes provide support
for this lithofacies interpretation because two more boreholes (8646 and 8647) have also
intersected Unit C in the vicinity of DDS41, while two boreholes (8584 and 8664) have
been drilled into Unit B3.
The thick accumulation of clayey till located around borehole cluster C4, which was
shown on the channels-interfluve map in the 2007 report (Figure 21 therein) as being
probably part of the same thick clay lithofacies intersected by borehole DDS41, is now
considered to be too high in elevation to be directly correlated to Unit C and it is
therefore more likely to be a compound till deposit derived from younger glaciation
events. This re-assessment of the correlation of the Unit C lithofacies has allowed a
revised update of the 2007 channels-interfluve map to be prepared (Figure 4-16).
Lithofacies Unit B3 is depicted, schematically, on this and on other borehole
cross-sections as almost a single sedimentary unit. In reality, these thick intersections of
sand and gravel are very likely to be compound stratigraphical units formed as a result
of multiple, separate fluvial incision events that occurred at different times. Individual,
linear channels in-filled with sand and gravel that have been incised into older
glaciofluvial sediments, similar to the example exposed in the sea cliffs at Nethertown
(Figure 4-10), are unlikely to be distinguishable as separate stratigraphical units from
the borehole log lithology descriptions.
These multiple channel incision events have probably coalesced to create the large
masses of slightly silty, slightly clayey, sand and gravel which are preserved today
beneath the LLWR site. In the context of this lithofacies, the occasional, thin
intersections of finer sand and silt shown on some of the boreholes within Unit B3 are
interpreted as evidence of fining upwards cycles related to one or more of the these
fluvial depositional events. These thin intersections of finer sediment may or may not
exhibit lateral continuity and therefore no attempt has been made to correlate them at
this stage of the lithofacies conceptual model.
73
74
Figure 4-9. NW-SE borehole cross-section 1
Incised
channel
deposits
Figure 4-10. Photograph of a gravel- and boulder-filled channel incised into coaly,
braided, stacked fluvial sand deposits at Nethertown sea cliffs. Height of cliffs
approximately 5 m. (J. Hunter, January 2007).
Section 1 by Eaton (1996) (Figure 4-11) also shows the thick-clay lithofacies zone, which
is identified in this report as Unit C – it is correctly correlated between boreholes
DDS41 and cluster borehole C10. Its correlation with the thick clay in offsite borehole G
is probably also correct. In this report, borehole C10 is included in NW-SE borehole
cross-section 6, while offsite borehole G is included in SW-NE borehole cross-section 4,
both of which are presented in Appendix B. These intersections through probable
separate remnants of Unit C by boreholes offsite-G and C10 can be understood by
reference to the revised map of the lateral distribution of that unit shown on Figure
4-16.
However, the combination in Eaton’s Section 1 of this clayey lithofacies (Unit C)
together with the laterally adjacent sands and gravels of lithofacies Unit B3 (intersected
by other boreholes) into a single stratigraphical unit called the Main Diamict Formation
(MDF) can no longer be supported by the presently available data. One reason for this
is that no distinctive bounding surfaces (represented in cross-section by lines of
correlation) for individual geological formations are suggested by the borehole
lithology logs. Furthermore, the accumulation of sands and gravels that comprise Unit
75
B3 were probably deposited by separate, younger geological event(s) and by a different
genetic process to the clayey diamict of Unit C. Even if the bounding surface of the
MDF was initially defined from the results of a geophysical survey, it is a reasonable
expectation that the ‘geophysical surface’ should have a reliably recognisable
expression in the borehole lithology log data, particularly in boreholes located away
from the geophysical survey lines.
The same borehole cross-section shown as Section 1 in Eaton (1996) was adapted by
BGS authors as part of the development of the regional lithostratigraphical framework
and published originally in a report by Nirex (1997a). It is reproduced in this report as
Figure 4-11. The relationship between these regional lithostratigraphical formations
and the lithofacies units proposed in this report are discussed in Section 3.
The new NW-SE borehole cross-section 2, presented here as Figure 4-12, is equivalent
to Section 2 from Eaton (1996), shown as Figure 4-13. As before, the new cross-section 2
includes most, but not all of the boreholes that were used to construct Eaton’s Section 2
because they are located beyond the 50 m-wide selection zone along the section line.
The remnant Unit C is only distinguishable in this new cross-section in cluster borehole
C8 (see SW-NE cross-section 8 in Figure 4-7 for a view of Unit C perpendicular to this
section). It may also be present forming part of a thick compound clayey till unit that
has been intersected by boreholes 8426 and 7524, but such a possibility cannot be
verified and at this higher-elevation, this zone thick clayey-till is currently being
assigned to the Unit B2. A more likely candidate for an extension of Unit C at the
north-west end of the section and of the LLWR site is an intersection of stiff, laminated,
silty clay between the elevations of –6.25 m and – 8.46 m in offsite borehole F.
The laminated, grey-silt D2 Unit is clearly correlatable between the older cluster
boreholes C8, C7 and C12, however the removal of the grey silt layer by erosion from a
narrow, deeply-incised, gravel-filled channel intersected by the Stage 6 borehole 8649 is
quite evident. The remarks made previously with regard to the MDF and the Unit B3
sands and gravels in Eaton’s Section 1 also apply to Eaton’s Section 2.
76
Figure 4-11. Section 1 from Eaton (1996)
77
78
Figure 4-13. Section 2 from Eaton (1996)
79
4.6 Updated LLWR lithofacies model: northwestsoutheast cross-section 5
Figure 4-14 shows a schematic cross-section of the LLWR site that was included with
the 2002 PCSC (BNFL, 2002a). Similar to the Eaton cross-sections, this has been drawn
using a horizontal : vertical scale ratio of 20:1 and it was produced without any
indication of the boreholes that had been used for the interpretation. For comparative
purposes, Figure 4-15 shows the closest of the new borehole cross-sections to the PCSC
section, i.e., NW-SE section 5. This has been constructed using a H:V scale ratio of 10:1
and its location is shown on Figure 4-8.
In the 2002 PCSC section, most of the lower half of the glacigenic deposits has been
interpreted and described as an accumulation of tills assigned to a single geological
formation (the MDF), shown by the dark red-brown colour. The new borehole crosssection shows a more detailed re-interpretation of the borehole lithology data which
reveals a different configuration of sedimentary materials and suggests a different
glacigenic history for the LLWR site. The most notable feature of these two crosssections is the contrasting interpretation of the mixed sand-and-gravel material, which
constitutes the bulk of the glacigenic deposits beneath the LLWR site.
The possibility of partial erosion of older glacigenic deposits by later fluvial channel
incision events is suggested by the shapes of some of the differently-coloured units
shown on the PCSC section and also by the removal of part of the lower laminated silt
unit. The preservation of a series of discontinuous, clayey tills in the upper part of the
section, shown on the PCSC section as the Pebbly Clay Formation (PCF), is also
consistent with the Unit B2 lithofacies shown on Figure 4-15. However the depiction of
the MDF as a single stratigraphical formation of mixed types of till which is laterally
continuous across the entire site makes no attempt to differentiate between the
separate spatial zones of clayey and sandy diamicton that are apparent from the
borehole lithology logs.
The new borehole cross-sections shown as individual figures in this section of the
report are presented together with all of the other cross-sections as a compete set of
new borehole cross-sections in Appendix B. The general discussion of the lithofacies
units described in this section applies equally to the other sections, which have been
provided to help visualise the lateral distribution of lithofacies in a pseudo-3dimensional context across the LLWR rather than to illustrate any particular new
features within the glacigenic deposits. The preparation this new set of borehole crosssections has also enabled the map showing the distribution of remnants of the ‘thick
clay’, i.e., lithofacies Unit C, originally shown as Figure 21 in Hunter et al (2007b), to be
revised and updated. This is presented below as Figure 4-16. The reduced areal extent
80
of Unit C shown in this figure is a consequence partly of the changed correlations of
zones of clayey till at the northern and southern ends of the site and partly a
consequence of improved borehole control in the centre of the site.
Figure 4-14. NW-SE cross-section through the LLWR from the 2002 PCSC (BNFL,
2002a)
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82
Figure 4-16. Map of LLWR showing the distribution of lithofacies Unit C revised
using information from the Phase 3 and Stage 6 boreholes
4.7 Discussion of uncertainties
The large amount of data available for the LLWR site that is presented, in condensed
format, on the detailed cross-sections shown in this section and also in Appendix B of
this report serves to emphasise the natural variability and complexity of these
83
glacigenic sediments. Such complexity is inevitably associated with a degree of
uncertainty which is difficult to quantify.
Nevertheless, it is proposed that the lithofacies model offers increased confidence in
the understanding of the spatial distribution of sedimentary materials beneath and
adjacent to the LLWR compared to alternative approaches because it maps the bulk
distribution of the principal material types. The principal uncertainties associated with
the lithofacies model are the correct identification of the bounding surfaces and the
heterogeneity of the material within each lithofacies unit. However, even if
modifications are made to the borehole lithology coding system, or to the consolidation
of the codes into the reduced set of main lithology groups (clays, silts, sands, gravels
etc.), it is unlikely that the bulk spatial distribution of these lithology groups will
change significantly, even if the exact boundaries of some lithofacies are not always
well-defined.
There is confidence in the evidence for the existence of apparently abrupt, large-scale
lateral discontinuities in bulk lithology. More examples of these discontinuities were
revealed by the Phase 3 and Stage 6 boreholes. There is also confidence in the evidence
for deep, downward-narrowing, erosive channel incision into underlying glacigenic
deposits by younger glaciofluvial drainage, which is assumed to have been caused by
periodic sub-glacial outbursts from ice-dammed lakes. Once again, more examples of
these features were discovered by the recent borehole drilling, including the very deep,
gravel-filled incision through the unit D silt layers intersected by borehole 8649 (Figure
4-12). It is difficult to imagine a feasible alternative explanation for this particular
example. Finally, the initial lithofacies model developed in 2007 was able to 'predict'
important intersections of units C & B3 (i.e., the thick stony, clayey till beneath the
centre of the site and the thick sequences of sand and gravel) made by some of the
Phase 3 and Stage 6 boreholes.
Some other examples of uncertainties include: the detailed genetic processes that
formed the principal lithofacies (including the exact origin of the tills); the significance
of smaller-scale sedimentological changes that occur within bulk lithofacies units (e.g.,
do the thin silty layers in unit B3 represent the upper layers of fining-upward fluvial
sequences?); and the correct correlation of separate, detached clayey lithofacies to other
similar lithofacies units. Fortunately, such uncertainties are of only limited significance
in the context of describing the bulk spatial distribution of material types.
Furthermore, while there is significant scope for alternative arguments regarding the
late Quaternary glacigenic history of the site and its surrounding region, and also for
the chronostratigraphic correlation of the glacial deposits, it is only the uncertainty
relating to the spatial distribution of materials that is of direct relevance to the ESC. A
significant advantage, therefore, of the lithofacies model for the LLWR site compared
84
to alternative models is that it is essentially independent of any future revisions of the
regional lithostratigraphical framework and nomenclature.
In addition to the data quality limitations, there are also some spatial limitations
associated with the LLWR borehole lithology dataset. For example, compared to the
very large amount of shallow borehole data, only limited borehole data are available
from the deeper Quaternary deposits beneath the LLWR site and also from the area
between the site and the coast. There is also a large spatial data gap between the beach
and the nearest ends of the offshore seismic survey lines. As a result, the boundaries
between different lithofacies units cannot be defined with confidence in these areas
and they will also be more sensitive to changes in lithology coding of the borehole logs
(see Appendix C) than in other areas of the site. However the lack of additional deep
and offsite borehole data may not necessarily have significant consequences for the
ESC and for hydrogeological modelling.
85
5
Integration of the regional scale and site
scale lithofacies packages
The re-interpretation of the site-scale glacigenic deposits beneath the LLWR and the
regional Quaternary lithostratigraphy of the surrounding area in the context of a
lithofacies model was first proposed by Hunter et al. (2007b) and Michie et al. (2007).
Figure 6 in Hunter et al. (2007b) was an attempt to summarise in a single table the
proposed inter-relationships between the lithofacies units identified in the glacigenic
deposits beneath the LLWR site and the likely correlatives to these units in the
surrounding region. This table, which has been reproduced in several subsequent
reports, sometimes in a slightly modified form, is also included here for convenience as
Figure 5-1.
Figure 5-1. Summary of the possible relationship between site and regional
lithofacies units proposed by Hunter et al. (2007b)
In these two reports from 2007, a separate system of temporary names was deliberately
adopted to identify the different lithofacies units (or ‘packages’ of glacial sediment)
that had been interpreted within the LLWR site and to enable them to be distinguished
from the regional-scale lithofacies units interpreted in the surrounding area. The sitescale lithofacies units/packages were named ‘LP1’ to ‘LP7’, from the youngest to the
86
oldest unit, in contrast to the simple alphabetic system (‘A’, ‘B’, ‘C’ etc) applied to the
regional lithofacies units.
The purpose of this deliberate separation of nomenclature was to avoid any premature
assumptions being made about correlations between one or more site-scale lithofacies
unit with any particular regional lithofacies unit without additional consideration of all
of the available data. The use of two separate nomenclatures was also intended to
avoid, with the exception of the Holmrook Till Member, any premature correlations
between the site-scale lithofacies units and the published BGS lithostratigraphical
framework.
The additional studies completed during the preparation of this report have improved
the level of confidence in the interpretation of the Late Quaternary site- and regionalscale glacigenic deposits using the lithofacies concept to a sufficient extent to allow a
single, integrated lithofacies framework to be proposed. In this integrated correlation,
it is proposed to abandon the site-scale ‘LP’ nomenclature and adopt the regional
alphabetic nomenclature of lithofacies at all scales. An updated version of Figure 5-1
incorporating these changes is therefore presented as Figure 5-2.
For consistency, the single (alphabetic) lithofacies nomenclature of Unit A, Unit B, Unit
C etc., has been adopted for use throughout this report. It has been used to describe the
regional lithofacies concepts in Sections 2 and 3, as well as the discussion of the sitescale lithofacies analysis in Section 4. Sections 2 and 3 also provide information
showing how this unified nomenclature may be correlated to the individually-named
formations and members in the BGS lithostratigraphical framework. It should be noted
in this context that for some of the BGS regional lithostratigraphical formations, which
in the case of concealed formations may have only be defined by a small number of
borehole intersections, more information may be available for these formations from
the greater number borehole intercepts within the LLWR site than are available in the
surrounding area.
Adoption of the single lithofacies nomenclature means that the silt sands assigned to
LP5 in 2007 are now assigned to unit B3, while the silt and older tills and gravels
previously assigned to LP6 and LP7 respectively are now assigned to unit D.
87
Drigg Point Sand, Ehen Alluvium Fm,
Blelham Peat, Hall Carleton Fm
Peel Place Sand & Gravel Mem,
Mainsgate Wood Sand & Gravel Mem
LLWR
Lithofacies A
Lithofacies B3
Lithofacies B2
Drigg Moorside Silt
Fishgarth Wood Till
Drigg Holme Sand
Drigg Beach Till
Kirkland Wood S & G
Ravenglass Till
Lithofacies
B1
Offshore
sequences
4, 5 & 6
Lithofacies B3
Barn Scar S & S
C Offshore
sequence 3A
Offshore
sequences
2&3
Offshore
sequence 1
Lithofacies B4
Holmside Clay
Green Croft Till
Whinneyhill Coppice Clay
Lithofacies C
Holmrook Till
Scale Beck Till
Lithofacies D
(D1-D3)
Glannoventia Fm
Carleton Silt Fm
Drigg Till? Maudsyke Till?
Figure 5-2. Updated summary of proposed correlation between site and regional
lithofacies units (schematic and not to scale)
88
6
Application of the lithofacies units to
hydrogeological modelling
As highlighted throughout this report, the lithofacies approach was chosen because it
focuses on describing the spatial distribution of Quaternary geological materials. The
spatial distribution of geological media, and their associated properties, is the key
geological issue of interest to the ‘end users’ of the geological interpretation.
This report describes the spatial distribution of materials at two scales:
the detailed distribution of materials, principally from a very large number of
boreholes, at the LLWR site and in its immediately adjacent area; and
the more generalised distribution of materials across the wider region.
These two scales of information are consistent with the available data, and also with
the
anticipated
requirements
for
hydrogeological
modelling.
The
regional
interpretation provides sufficient detail to underpin a regional scale groundwater flow
model that can be used to provide boundary conditions for a more detailed model of
the site and the adjacent areas of interest (i.e. those areas through which contaminant
transport might occur). The detailed description of the spatial distribution of materials
at the site is also suitable to underpin assessment of the impacts of heterogeneity for
groundwater flow, and in particular contaminant transport.
In some respects the regional approach is comparable to the hydrogeologial domains
approach applied in Nirex’s studies (McMillan et al., 2000). However, Nirex were
predominantly interested in the impact of the Quaternary materials on deep
groundwater recharge and discharge from depth. Therefore Nirex only discretised the
Quaternary deposits in plan, as geographical domains, not in section, as particular
layers. However, the LLWR is a near-surface disposal system such that it is the shallow
groundwater flow system that is of primary interest. The regional lithofacies approach
is more qualitative than Nirex’s quantitative approach, which was limited by its
reliance on spatially limited regional hydro-test data for the purposes of upscaling (the
programme of work to address these limitations was not completed when the Nirex
programme ceased in 1997).
The 3D regional geological model, which includes some refinement in the area of the
LLWR, is being used as the basis for hydrogeological modelling at the regional and site
scales. Additional studies are examining the impacts of smaller scale heterogeneities at
the site scale.
89
The anticipated hydrogeological properties of each of the lithofacies units are described
below as an input to the hydrogeological models. Heterogeneity is further considered
in Section 6.1.
Lithofacies Unit A
Lithofacies unit A comprises a wide range of geological materials including scree and
head (rock debris and clayey hillwash) on hill slopes, peat, alluvium, alluvial fan and
river terrace deposits, lacustrine deposits, blown sand, estuarine and beach deposits,
and offshore sediments, dominantly marine sands and muds. It also includes made
ground.
The individual deposits that have been grouped within lithofacies unit A are of limited
spatial extent and / or thickness such that it is not practicable to distinguish and
characterise them all at the regional scale. However, their differing properties may be
highly significant for local groundwater flows, and these will have to be recognised
within the site scale groundwater flow model, and potentially even in the regional
groundwater flow model for key deposits such as river alluvium.
In addition, lithofacies unit A includes the soil zone, where the hydrogeological
properties are influenced by vegetation and rooting zones, biological activity,
weathering and anthropogenic activity such as ploughing and drainage. The National
Soil Resources Institute (NSRI) at Cranfield University describes the soils in the area of
the LLWR as generally loamy and free draining, although this is most relevant to the
agricultural soils, rather than surface peat and till outcrops.
Lithofacies Unit B1
This unit includes offshore glaciomarine and marine deposits, which are partly
underlain by glacial, lacustrine and terrestrial deposits. The glaciomarine and marine
deposits partially infill deeply-incised depressions (palaeovalleys) cut down into the
pre-Quaternary bedrock. The offshore deposits are predominantly bedded muds, silts,
sands and minor gravels, and therefore can be treated as a single lithofacies
assemblage of generally low permeability.
Lithofacies Unit B2
This unit includes a sequence of interbedded tills (glacial diamictons), and sands and
gravels. All the till (glacial diamicton) units are noticeably clay-rich and the upper till
units are almost entirely clay, formed as melt-out or flow tills. The clay-rich tills are
anticipated to be of low permeability, while the sands and gravels are likely to be
permeable. The impacts of this contrast can be observed in the Drigg beach cliffs,
90
where groundwater seeps are observed at the interface between tills and overlying
sands.
As demonstrated in Section 4, this unit is structurally complex, with clays sands and
gravels all having variable lateral continuity. Where the unit is dominated by clays it is
likely to be of low permeability, and where it is dominated by sands it is likely to be
permeable. In some areas of the site the till and sand / gravel layers are relatively
continuous forming a sequence of impermeable and permeable layers. Such areas
might exhibit low vertical permeability but relatively high horizontal permeability.
This complexity might be upscaled and represented in groundwater flow models by
specification of an appropriate horizontal to vertical anisotropy.
Lithofacies Unit B3
This unit is dominantly composed of permeable to very permeable thick sand and
gravel deposits. The permeability of this unit will vary depending upon the proportion
of silt and clay fines in the matrix component and also upon the presence of siltyclayey layers that may represent the upper layers of fining-upward fluvial sequences.
Lithofacies Unit B4
This unit consists of glaciolacustrine deposits of interlaminated silts and clays, but also
includes in the middle a till unit. It is dominated by impermeable materials.
Lithofacies Unit C
This unit consists of till as well as sands, gravels, silts and clays. Within the LLWR the
till is generally a stony clay diamicton, in contrast to the clay-rich upper till units in
unit B2. This unit is anticipated to have a similar permeability to the clay-rich tills in
unit B2, but it is laterally discontinuous. It is likely to be much less permeable than the
thick sands and gravels in unit B3.
Lithofacies Unit D
Unit D1 comprises coarse sands and gravels, and diamiction. It is anticipated that D1
will have moderate to high permeability. Unit D2 comprises laminated clays, silts and
fine sands. It is anticipated that D2 will have moderate to low permeability. Unit D3
(where differentiated) comprises interbedded silt and sand. It is anticipated that D3
may have similar properties to B2.
There are few data with which to characterise these deeper sediments with confidence.
However, it is noted that unit D is generally restricted to the very deep parts of the
Wasdale Palaeovalley, and it is only present in the most southerly parts of the LLWR
91
site. This unit not anticipated to be of significant interest to the ESC, such that the
limited data is not considered to be a significant issue.
Bedrock
The upper parts of the Sherwood sandstone are weathered and in some places they
have degraded to unconsolidated material termed “Snellings Sand”. These Snellings
sands and the weathered bedrock are both anticipated to be permeable.
The bedrock underlying the site is Ormskirk sandstone, which is a permeable
dominantly cross-bedded aeolian sandstone which is likely to exhibit a significant
fracture-porosity. The Ormskirk sandstone dips to the west, and approximately 2 km
offshore it is overlain by the Triassic Mercia Mudstone Group, which comprises
relatively impermeable siltstones, claystones and fine grained sandstones, although the
permeability may be significant where there is significant fracturing.
6.1 Heterogeneity
One of the limitations of the lithofacies approach is that the complex near-surface
geology has been bulked into a two lithofacies units, i.e. A and B2. It is anticipated that
the small scale near-surface heterogeneity may be important in the context of local
groundwater flow paths and hence contaminant transport.
This is because the magnitude and temporal variability of hydrological processes at the
site is greatest in the soil zone, e.g. Henderson (2008), Black et al. (2009). This
complexity is reflected in the complex spatial and temporal patterns of H-3
contamination in shallow groundwaters (Hartley et al. 2009) and the historical
responses of H-3 concentrations in shallow groundwater to engineering activities, e.g.
dewatering of the trenches (BNFL, 2002b).
In particular these heterogeneities have the potential to:
Affect the lateral extent of contaminant transport in shallow groundwaters, and
hence the potential for contaminant transport to surface waters.
Affect the locations where contaminants migrate from shallow local groundwaters
into deeper regional groundwaters, for example focussing contaminant transport
paths through spatial discontinuities in the clays.
Heterogeneity at depth may also be important, particularly in terms of the potential
impacts associated with a groundwater well: well location and sustainable abstraction
rate; plume capture and dilution. However, the deeper deposits are apparently more
homogeneous (although this may be an artefact of significantly less data) and the scale
92
of heterogeneity is apparently larger such that it can be reflected in the definitions of
the lithofacies units.
Despite this limitation of the lithofacies units, the geological interpretation still
provides the information necessary to assess the length scales and impacts of
heterogeneity. There are a number of lines of evidence:
Direct observations from the Drigg beach cliffs, and site excavations including the
trench walls, Vault 9 excavation and trial pits (both onsite and on the beach).
Evidence from analogue exposures along the coast, e.g. the large scale incised
channel at Nethertown (Figure 4-10).
Evidence for continuity of distinct material types from boreholes and sections such
as those presented in this report.
Evidence for the continuity of distinct material types from continuum geophysics
data (Halcrow, 2010).
Inferred length scales based on facies information, i.e. the types of depositional
environments, the depositional mechanisms, the energy state of the depositional
environment, the resultant geological features and their potential length scales.
93
7
Conclusions
The present study has updated the understanding of the geology of the LLWR. It
explains how a lithofacies approach to classification of the Quaternary sediments was
developed and first applied in 2007, after it was identified that previous classifications
of the sediments were not optimal to support the ESC, e.g. for hydrogeological
modelling. The lithofacies units are directly related to the material properties of the
sediments and their relationship to the earlier classifications is illustrated.
The lithofacies framework is able to ‘accommodate’ laterally discontinuous units that
reflect the laterally discontinuous deposits observed at the LLWR. However, the
depositional history of the sediments is not ignored, and where facies information is
available it is used to inform correlation of deposits of similar materials and inform the
potential length scales of heterogeneity.
As a result of the current update, a set of lithofacies units are identified that unify the
previous separate set of units for the wider region and the LLWR site area. In addition,
the update has extended the detailed correlation of these units to the offshore sequence
under the East Irish Sea. The comprehensive understanding of the nature of the
sediments across the area shows that they were deposited in a fairly continuous
manner during the later part of the Quaternary. The situation of the LLWR site at a
position marginal to the ice sheet that occupied the East Irish Sea basin produced
spatial relationships of the lithofacies units that were not a sequence of continuous subhorizontal layers. Instead the lithofacies units at the LLWR site are laterally
discontinuous, with some younger sandy units cutting down into older units to form
thick bodies of sand and gravel.
The resultant geological interpretation is considered appropriate to robustly underpin
the ESC. Although uncertainties remain, the geological interpretation provides
sufficient evidence to bound these uncertainties within the ESC with good confidence.
The knowledge gained from the geological interpretation is enhanced within the ESC
when it is combined with other sources of evidence regarding the features and length
scales of interest, e.g. hydrogeological and hydrogeochemical data.
94
8
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98
Appendix A – Development of the
understanding of the Quaternary sequence for
the area of the LLWR
The development of the understanding of the nature of the Quaternary sequence is
illustrated by presenting the successive interpretations of 3 sections across and adjacent
to the site. The locations of the sections are shown on Figure A-1. Section 1 along the
NE side of the site provides 4 different classifications of the nature of the Quaternary
sediments intersected in boreholes drilled through the sequence.
A.1 Williams et al. (1985)
The first systematic attempt to classify and correlate the Quaternary sediments of the
area of the LLWR, then the Drigg Disposal Site, was by Williams et al. (1985) of the
Environmental Protection Unit of the British Geological Survey. Their study of the
geology and hydrogeology was based on only 35 boreholes, 7 of which were Old Series
(OS) boreholes with only basic (and suspect) lithological logs. The boreholes logged by
the BGS were of the initial phase of the DDS (Drigg Disposal Site) series and the BGS
logs included the category ‘boulder clay’, a category that was not included in later
geotechnical logging of boreholes on the site.
In their interpretation of the glacial and glacigenic deposits intersected in the boreholes
at the site, the BGS noted that they were unable to correlate lithological units with the
two major phase of glaciation identified in the Geological Survey of the wider area
(Trotter et al., 1937). The survey had recognised a complex and partially repeated
sequence of boulder clays, and sands and gravels and suggested that after the deposits
of the ‘Main Glaciation’ there was withdrawal of the ice sheet followed by the ‘Scottish
Re-advance’, which contained periods of glacial oscillation. In the 1985 report on the
Drigg site it was noted that: “The complicated series of events which produced the
deposits at Drigg makes it extremely difficult to correlate lithological units to a
particular phase and this has not been attempted.” Instead, the BGS study at LLWR set
up an independent stratigraphy for the site based on a succession upwards from the
bedrock of G1 to G8 glacigenic units and some post-glacial units.
The odd-numbered G units were identified as clays and boulder clays, and the evennumbered G units were identified as sands and gravels. The post-glacial units included
peat, alluvium (in stream courses), blown sand and made ground. Although the
Quaternary sequence in the boreholes was interpreted in a series of cross-sections as a
set of near continuous layers across the site, the study noted that: “In most cases these
99
cross-sections assume continuity of a given clay stratum between boreholes. This may
not necessarily be the case especially where glacial wash-out channels cut through
older sediments.” and also that: “ …variations especially with respect to the number of
horizons and their thickness frequently occur.”
Their interpretation of the Section 1 cross-section is shown as Figure A-2, which is redrawn to a uniform scale for this report from Figure 7(a) of their report, and the units
identified. The identification of the bedrock as St Bees Sandstone is due to that name
being applied at that time for all of the Triassic sandstone bedrock. Only later was the
Triassic sandstone in the west Cumbria area identified as the Sherwood Sandstone
Group and the name St Bees Sandstone restricted to the lowest formation of the group.
Figure A-1. Map of LLWR showing locations of Sections 1, 2 and 3 (as discussed in
text).
100
Figure A-2. Williams et al. (1985) Section 1
101
A.2 Eaton et al. (1996/7)
Subsequent drilling of a much greater number of boreholes across the site, including a
large number into the deeper bedrock identified in the southern part of the site,
revealed that the simple “layer cake” interpretation was not supported, particularly for
the layers of clay/till, which were identified as discontinuous in places. In addition, an
older sequence of sediments was identified below the sequence intersected in the
shallower boreholes examined by the BGS. This lead to the adoption of a new
classification of the Quaternary sediments based on an Event Stratigraphy approach
(Eaton et al., 1996, 1997).
An Event Stratigraphy approach determines the stratigraphy (time sequence) of
sedimentary sequences based on the identification of events, which are broadly defined
as comparatively short occurrences, in a relative geological sense, during the
sedimentary deposition. Such an approach had been proposed for interpretation of the
Quaternary sediments of the wider Irish Sea basin by Eyles and McCabe (1989). In
addition, Eaton and Curtis (1995) had produced an interpretation of the Quaternary
sediments under the adjacent area of the Irish Sea from the results of the offshore
shallow seismic survey and had developed a seismic stratigraphy consisting of several
individual sequences.
They correlated the offshore sequences with the onshore stratigraphy being developed
at that time by the BGS as part of the Nirex investigations of the Quaternary of the
wider area. Figure A-3 is their interpretation of Section 1, which shows both their event
stratigraphy units as well as a correlation along the right hand side with an early
version of the BGS lithostratigraphy.
The lowest unit shown in the section is the Lower Diamict Formation, which they
interpreted to have been deposited from an initial mid to early Devensian glacial event.
This was succeeded by the Marine Silt Formation, which was interpreted to have
formed during a lacustrine to marine/intertidal event. Following this the thick Main
Diamicton Formation was interpreted to have formed during the main phase of
Devensian glaciation. After the diamicton emplacement, a period of erosion and
reworking of the diamicton was envisaged, with deposition in glacial outwash
channels and channelized fluvial deposits to form the Fluvial Outwash Formation. The
Pebbly Clay Formation was considered to lie unconformably above the outwash
deposits and represent late Devensian till deposition associated with the Gosforth
Oscillation event. Above this the Lacustrine/Fluvial Formation was linked to
deposition during a period of sea level rise, followed by glacitectonic deformation of
this formation and the Pebbly Clay Formation by the Scottish Readvance of the Irish
Sea ice sheet, which produced thin deformation tills. The Holocene/Recent Formation
102
consisted of peats developed in kettle holes formed by the melting of blocks of
stagnant ice, and the youngest deposits of wind-blown (aeolian) sand.
The section showed that units such as the Pebbly Clay formation could be
discontinuous and repeated, and in places only sandy units were present above
bedrock. However, the section also shows that the lithologies intersected in the
boreholes did not have a direct relationship to the interpreted formations. In particular,
the Main Diamict Formation was interpreted in some boreholes to consist almost
entirely of sand, or sand and gravel lithologies. A major conclusion of the Eaton et al.
(1996/7) report was that;“The Main Diamict Formation deposited during the late Devensian was previously
described as a boulder clay. However, recent investigations have shown this till to be
highly variable but generally sand rather than clay-bound and can therefore not be
classed as a boulder clay.”
There were also some problems with interpretations of the logs and elevations of the
older DDS series boreholes. For example, borehole DDS 33 is common to this section
and the previous Williams et al. (1985) section. However, in the Williams et al. section,
the top of the borehole is correctly shown at its measured elevation of 19.39 m, but in
the Eaton et al. section it is shown several metres higher.
In addition, the lower part of the borehole in the Eaton et al. section is shown without
any lithological symbol, but is interpreted as equating to Fluvial Outwash Formation
on top of Main Diamict Formation. The Williams et al. section correctly shows that the
lower part of the borehole is formed of a ridge of sandstone bedrock. (Conversely, the
Williams et al. section incorrectly interpreted that bedrock could be present in the base
of Old Series borehole OS 6 in the middle of the section at about 12.5 m below sea level.
The later drilling shown in the Eaton et al. section shows that bedrock in that area is at
about 24 m below sea level.)
The imposition of a genetic framework based on an interpreted event stratigraphy for
the Quaternary sequence produced a mismatch for some of the units, particularly the
Main Diamict, between their unit description and their actual sediment type. This
produced difficulties in trying to assign hydrogeological properties to the units for
modelling of groundwater flow. For the 2002 PCSC, Cooper (2002) re-interpreted this
and other of the Eaton et al. sections to remove some of these problems, but his
revision of this section was not able to be found for this review.
103
104
Figure A-3. Eaton et al. (1996/7) Section 1
A.3 BGS, 1997 and 2000
In their report to Nirex (Nirex, 1997) on the lithostratigraphy of the west Cumbria area,
the BGS included a re-interpretation of the previous section. They did not examine the
borehole material or review the detailed logs, but based their re-interpretation on the
description of the section and their correlation of the sequence with the sequences
observed at outcrop or in Nirex boreholes in the area. The lithostratigraphical units are
described in detail in the Nirex report and also in a published article (Merritt and
Auton, 2000). Figure A-4 shows their interpretation of the section onto which the
corresponding borehole logs have been added. Comparison with the interpretation by
Eaton et al. shows that some of the mismatch between units and borehole lithology has
been reduced by the identification of separate till and sand units within what had been
classified as the Main Diamict. However, a major feature of the BGS interpretation is an
attempt to produce continuous clay/till units across the section, but this does not agree
with the lithologies intersected in the numerous boreholes on the LLWR site.
105
106
Figure A-4. BGS Section 1
A.4 Drigg PCSC (2002)
Cooper (2002) developed a number of sections in support of the 2002 PCSC using the
Event stratigraphy approach of Eaton. These detailed sections were not presented in
the PCSC geological report (BNFL, 2002). Instead two schematic cross-sections were
presented, orientated parallel and perpendicular to the long-axis of the site. These
schematic sections were used to explain the geological conceptual model. One of the
schematic sections is shown as Figure 4-14 in the main text of this report.
107
A.5 Michie et al. (2007)
As part of the 2007 review of the geology of the Quaternary sediments of the LLWR
area, the sequence in BGS section 1 was re-interpreted using the Regional Lithofacies
framework. The resulting interpretation was not included in the report (Michie et al.,
2007) because it was regarded as an intermediate stage in developing a detailed
lithofacies interpretation of the LLWR site. However, it is useful to include the regional
interpretation in this review of the development of the understanding of the
Quaternary sequence in the LLWR area.
Figure A-5 shows the classification and correlation of the borehole sequences into the
regional lithofacies units. The A unit at the surface consists of sands and peats, all of
post-glacial age. Under this is the B2 unit of alternating clay/clay diamicton (till) layers
and sand/gravel layers. These are partly removed by the B3 unit material of thick
sands and gravels, which also cut down into lower units. The C unit is of stony
diamicton, which may thicken or form compound layers with the overlying tills
adjacent to the bedrock high. The underlying D unit dominantly consists of laminated
muds and silts of estuarine to marine aspect below about 20 m below present sea level,
but consists of sands and coarser material where preserved at higher level adjacent to
the bedrock high. The basal conglomeratic material may be a beach facies formed
during a marine transgression.
Because of its method of construction based on actual borehole lithologies, this
interpretation of the section matches the units to the dominant lithologies. This
framework was applied in this report to the development of the detailed lithofacies
interpretation of sections across the LLWR using all the available borehole information.
108
Figure A-5. Michie et al. (2007) Section 1
109
A.6 Additional section comparisons
In order to illustrate more fully how the regional lithofacies interpretation was applied
to the development of understanding of the Quaternary sequence, some additional
cross sections are provided.
Section 2 is approximately along the SW side of the LLWR site. Figure A-6 shows the
Event Stratigraphy interpretation of Eaton et al. (1996/7) and Figure A-7 shows the
corresponding regional lithofacies interpretation. There was no corresponding BGS
interpretation of the section. The major difference in the regional lithofacies
interpretation of identifying a thick B3 development of sandy material instead of a
thick Main Diamict of variable characteristics is apparent.
Section 3 is a long section through a set of off-site boreholes, west and SW of the site
and parallel to the coast. Figure A-8 shows the Event Stratigraphy interpretation by
Eaton et al. (1996/7) and Figure A-9 shows the BGS interpretation. The BGS
interpretation is similar to the Eaton et al. interpretation except that the BGS show
more continuous clay/till units in the upper part of the section. The regional lithofacies
interpretation of the section is shown on Figure A-10. There are more similarities of this
interpretation to the other interpretations in identifying the development of thick
sandy material, but only the regional lithofacies interpretation marks the major
development of sand and gravel filled incisions that remove most of the C unit (lower
diamicton) and part of the D unit (marine facies). This 2007 interpretation was
confirmed by subsequent deep drilling in the area and the results are included in the
detailed lithofacies interpretation.
110
111
112
Figure A-7. Lithofacies units Section 2
113
114
Figure A-9. BGS Section 3
Figure A-10. Lithofacies units Section 3
115
A.7 References
BNFL, 2002. Drigg Post-Closure Safety Case. Geological Interpretation.
Cooper, S. 2002. Drigg Technical Programme. Geological cross-section lines. DTP 02/03
TN142 (DTP/185 for issue to The Environment Agency).
Eaton, G. P., Cooper, S. and Speakman, P. 1996 (revised1997). Drigg Site
Characterisation : Geological Interpretation (Interim Report), 258 pages. Geological
Consultants Report No. GEO/96/25. Prepared for BNFL.
Eaton, G. P. and Curtis, N. J. 1995. 2D transects through the Quaternary of West
Cumbria. Geological consultants limited report No. GEO/95/10. Prepared for Nirex.
Eyles, N. and McCabe, A. M. 1989. The Late Devensian (<22,000 BP) Irish Sea Basin: the
sedimentary record of a collapsed ice sheet margin. Quaternary Science Reviews, vol.8,
p 307-351.
Merritt, J. W. and Auton, C. A. 2000. An outline of the lithostratigraphy and
depositional history of Quaternary deposits in the Sellafield district, west Cumbria.
Proceedings of the Yorkshire Geological Society, vol. 53, pp129-154.
Michie, U., Smith, N., Towler G. & Hunter, J., 2007. LLWR Lifetime Project:
Reinterpretation of the LLWR regional Quaternary geology. Nexia Solutions report
(07)8346 – Quintessa report QRS-1354F RegionalGeologyV1.0.
Nirex, 1997. The Quaternary lithostratigraphy of the Sellafield district. Nirex Report
no. SA/97/045.
Trotter, F. M., Hollingworth, S. E., Eastwood. T. and Rose, W. C. C., 1937. Gosforth
District. Memoir of the Geological Survey of Great Britain, Sheet 37 (England and
Wales), 136 pp.
Williams, G. M., Stuart, A. and Holmes, D. C., 1985. Investigation of the geology of the
low-level radioactive waste burial site at Drigg, Cumbria. British Geological Survey
Report, Vol. 17, No. 3, 24 pp.
116
Appendix B – LLWR site borehole crosssections
This Appendix contains a complete set of the new borehole cross-sections constructed
during the preparation of this report, together with an index map showing their
locations across the LLWR site.
For some of the borehole cross-sections, it has been possible to overlay partial sections
of some of the Halcrow (2010) geophysical survey (resistivity) profiles where they are
either coincident with or in close proximity to the borehole sections. For use in these
borehole cross-sections, the resistivity profiles have been re-scaled and, in some cases,
the images have been reversed.
It should be noted that the analysis and interpretation of the spatial patterns of
resistivity values has not been completed at the time of writing of this report and
therefore any apparent lack of immediate and obvious correlation between the
resistivity patterns and the distribution of lithofacies units shown on the borehole
cross-sections should not be construed as an indication than either one or the other of
these methods of investigation is invalid. The geophysical survey work was
undertaken in support of the coastal erosion modelling and was optimised to
investigate the post-glacial Holocene deposits along the spit, rather than the detail of
the deeper, Late Devensian glacigenic deposits.
117
Figure B-1. Index map showing locations of borehole cross-sections
118
Figure B-2. NW-SE borehole cross-section 1
119
120
Figure B-4a. NW-SE borehole cross-section 3
121
122
Figure B-4b. NW-SE borehole cross-section 3 with resistivity data sumperimposed
Figure B-5a. NW-SE borehole cross-section 4
123
124
Figure B-5b. NW-SE borehole cross-section 4 with resistivity data sumperimposed
125
126
Figure B-8a. SW-NE borehole cross-section 1
127
128
Figure B-8b. SW-NE borehole cross-section 1 with resistivity data superimposed
Figure B-9. SW-NE borehole cross-section 2
129
130
131
132
133
134
135
136
Figure B-16. E-W borehole cross-section
Appendix C – Summary of lithology code
consolidation
C.1 The Drigg Site Characterisation Database
Similar to the study produced by Hunter et al. in 2007, this updated interpretation of
the Late Quaternary glacigenic deposits preserved beneath the LLWR site is based
upon an interrogation of the accumulated database of lithological descriptions of
Quaternary material recovered from boreholes (i.e., arisings). These descriptions are
derived from numerous programmes of site investigation that have been conducted at
the LLWR at various times since planning consent for the facility was originally
granted in 1957. The history of these different phases of site investigation has been
summarised in Section 2.3 of this report.
These various programmes of historical site investigation at the LLWR were
undertaken by a number of different operators for a variety of engineering projects.
They will have used slightly different methodologies and different quality assurance
and reporting standards. As a result, although a large quantity of descriptive
geological data is available for the LLWR site, the quality of these data is variable.
Some lithology descriptions are lengthy and detailed, while others are very brief. Some
descriptive lithology records relate to very thin borehole intersections (<0.5 m), while
others may represent thicknesses of several metres.
The written lithological descriptions of borehole arisings from all of the pre-2007 site
investigation drilling work described in Section 2.3 have been transcribed by previous
investigators into a digital database (called the Drigg Site Characterisation Project
database) and quality control checked. The same investigators have also assigned a
one- or two-character alphabetic code to each lithology record to facilitate the
computer-plotting of appropriate standard rock patterns on borehole log crosssections. These lithology codes are the means by which the large database of
descriptive text can be condensed into manageable data suitable for plotting on crosssections and then interpreted into geological conceptual models.
However, the reliability of the lithology codes for this purpose is no greater than the
variable quality of the original lithological descriptions in the database records. The
reliability also depends upon the judgement used in assigning the most appropriate
codes to the descriptions. Furthermore, in common with most large databases of this
type, the DSCP database will inevitably contain an unknown, but small percentage of
transcription errors which can result in occasional incorrect lithology codes.
137
The DSCP database also contains all of the available coordinates and elevations of the
surface locations of the boreholes. The small number of borehole records that lack
spatial coordinates have not been used in this study, however, where possible, for the
few boreholes that lacked only ground elevation values, these missing data were
estimated from the digital ground surface elevation model.
As part of the preparation of this report, the digitised lithological descriptions of
borehole arisings derived from recent site investigation work – data that were not
available for use in the report by Hunter et al. (2007) – have been appended to a copy
of the DSCP database by one of the present authors. These additional records have
been assigned the same type of lithology codes so that they are consistent with the
existing data.
C.2 Interpretation of the lithology data
The sample lithology descriptions stored in the Drigg Site Characterisation Project
database constitute the basic ‘raw’ data that have been used (selectively, and
sometimes in combination with geophysical surveys) for all of the previous studies of
the Quaternary deposits beneath the LLWR site. Some of these data (i.e., selected
individual boreholes) have also been used by the BGS to assist with constructing the
regional lithostratigraphical framework, while the entire database has been utilised by
Hunter et al. (2007) and by the authors of this report.
However, despite making use of the same raw data, the various studies of the LLWR
Quaternary geology that now exist, including this report, differ in their interpretations
of the data and accordingly can be classified into three separate models, as described in
Section 4 of this report.
A basic requirement for constructing the borehole cross-sections presented by Hunter
et al. (2007) and also for the enhanced cross-sections included in this report, is a
simplification of the larger set of lithology codes into a condensed set of groups which
are most likely to represent the dominant sedimentary material types. The condensed
grouping is shown in the following table, each group being distinguished by a colour
which matches the lithofacies colours plotted on the borehole ‘sticks’ used in the crosssections. The principle groups are: clays, silts, sands, mixed sands and gravels, cobbles,
peat and sandstone.
138
Code
Lithology
A
SAND
AS
Silty SAND
AC
Clayey SAND
A1
Silty SAND with gravel
A2
Clayey SAND with gravel
AG
SAND with gravel
GA
Gravel with sand
Q1
Clayey sand and gravel
Q3
SAND and GRAVEL
G
GRAVEL
GC
Clayey GRAVEL
CO
COBBLES
BO
BOULDERS
C
Clay
C3
Sandy CLAY with gravel
CA
Sandy CLAY
CG
Gravelly CLAY
CS
Silty CLAY
P
PEAT
PA
Sandy PEAT
PT
Clayey PEAT
S
SILT
SA
Sandy SILT
SC
Clayey SILT
SS
SANDSTONE
Condensed lithology
Sands
Sand and gravel
Cobbles and boulders
Clays
Peat
Silts
Sandstone
An example of the interpretation of the lithology descriptions and codes by the three
types of interpretive geological models for the LLWR site and the surrounding region
is shown in the following table using data from the log for borehole DDS134. This
borehole log has been used in cross-sections for all three models and it appears in
several borehole cross-sections in this report, including Figures 4-10, A-3, A-4 and A-5.
The borehole was drilled in 1989 and the log was transcribed and coded in the DSCP
database prior to 2007.
139
Descriptive lithology log for borehole DDS134
Top
elev
(mOD)
11.4
Bottom
elev
(mOD)
8.5
8.5
7.2
7.2
5.5
5.5
4.7
4.7
-1.3
-1.3
-5.3
-5.3
-9.3
-9.3
-10.3
-10.3
-13.3
-13.3
-14.3
140
Lithological description
Red brown slightly silty fine to
medium SAND with a trace of
fine to medium subangular
assorted gravel and
occasional pockets of very
silty CLAY and occasional
cobbles.
Red brown fine to coarse
assorted subangular to
subrounded sandy GRAVEL
with some subrounded
cobbles and occasional
boulders.
Red brown slightly sandy silty
CLAY, firm, with some
subrounded fine to coarse
assorted gravel and
occasional cobbles.
Grey fine to coarse angular to
subrounded GRAVEL and
COBBLES with occasional
pockets of very silty clay and
occasional boulders.
Grey fine to coarse
subangular to subrounded
sandy GRAVEL and
COBBLES with occasional
boulders.
Predominantly grey fine to
coarse subangular to angular
COBBLES. Assorted clasts of
granite, granodioritic and
basaltic rocks.
Predominantly medium-dark
grey green medium-coarse
subangular to subround
subspherical sandy GRAVEL.
Clasts of granitic,
granodioritic and basaltic
rock.
Predominantly grey green
fine to occasionally medium
subangular to subrounded
silty sandy GRAVEL.
fine to occasionally medium
subangular to subelongate
slightly sandy GRAVEL.
medium to occasionally
coarse subangular to
occasionally subrounded
slightly sandy GRAVEL with
occasional COBBLES.
Lith
code
Events-based
stratigraphy
BGS Regional
Lithostratigraphy
Condensed Lithology
Litho-facies
A
Lacustrine
Fluvial
Formation
FWT, DHS, DBT &
KWS members,
(undifferentiated)
(Gosforth Glacigenic
Formation)
Sand &
gravel
GA
CS
Pebbly Clay
Formation
G
Fluvial
Outwash
Formation
Clay
Ravenglass Till
Member (Seascale
Glacigenic
Formation)
GA
CO
Main Diamict
Formation
Barn Scar Sand and
Silt Member
(Seascale Glacigenic
Formation)
GA
GA
G
GA
B2
(interbedded
clayey tills,
sands and
gravels)
Holmrook Till
Member (Blengdale
Glacigenic
Formation)
Sand and
gravel
(with a
layer of
cobbles)
B3 (undifferentiated,
coalesced,
multichannel
sand and
gravels)
-14.3
-17.3
-17.3
-19.3
-19.3
-23.05
-23.05
-25.8
-25.8
-33.3
-33.3
-36.8
-38.32
-40.3
-45.28
-45.9
-45.9
-49.05
Red brown medium to
occasionally fine SAND with
some dark grey green fine
angular to subangular
GRAVEL, with a trace of red
brown silty sandy CLAY at
the base.
Medium to dark red brown
medium to occasionally
coarse SAND, with
increasing red brown silty
sandy CLAY towards the
base, firm to stiff.
medium to coarse subangular
to subrounded GRAVEL with
boulder fragments.
Red brown silty sandy CLAY,
firm, with occasional gravel
lenses.
Medium red brown to
occasionally orange red fine
to medium SAND with
occasional gravel lenses.
Orange red fine to medium
SAND with increasing dark
green fine to medium
GRAVEL.
NO CORE RECOVERY.
Cuttings description: Orange
red fine to medium grained
completely disintegrated
SANDSTONE, with some
dark green fine grained
gravel.
Medium red brown medium
grained moderately to weakly
cemented SANDSTONE soft
to weak, locally crumbly and
friable, with closely spaced
cross bedding.
NO CORE RECOVERY.
Cuttings description: Medium
red brown fine to medium
grained completely
disintegrated slightly
micaceous SANDSTONE,
with some dark mineral
plates of Mn02.
Note: the log continues to a
greater depth within the
sandstone bedrock
AG
Glannoventia
Formation (includes
an unnamed upper
sequence of silts,
fine sands & gravels
which are possibly
glacitectonised
deposits of the
Glannoventia Fm.)
A
G
CA
Marine Silt
Formation
Clay
Lower Diamict
Formation
Sand
A
AG
Drigg Till Formation
D1 (interbedded,
brown,
laminated
silts)
D2 & D3
(sands with
some gravel
– laterally
equivalent
to grey
laminated
silts)
SS
SS
Ormskirk
Sandstone
Ormskirk Sandstone
Sandstone
Sandstone
SS
This tabulated borehole log conveniently illustrates the ‘averaging’ of lithology
information that occurs during the conversion of moderately detailed engineering
141
descriptions into the one- and two-character codes. It also illustrates the differences
between the subsequent interpretation of both the descriptions and the codes by each
of the three geological models that have been developed to characterise the LLWR
Quaternary geology.
The different stratigraphical interpretation of most of the borehole lithology intervals
by the events-based stratigraphy and the BGS regional lithostratigraphy models is
immediately evident. Also evident is that the identification of certain borehole intervals
as tills by both of these models involves assumptions and inferences about genetic
processes which are not unequivocal, based as they are upon the descriptions shown in
the third column from the left.
The last two columns on the right hand side of the table show the simplified bulk
lithologies produced by condensing the original lithology codes and the lithofacies
units that accommodate them. Although the condensation of detailed descriptions into
generalised, bulk lithologies and lithofacies may appear to be an oversimplification, the
derived lithofacies units form coherent spatial zones in three dimensions beneath the
LLWR. Also, because the majority of the borehole logs in the DSCP database have been
utilised in the mapping of the lithofacies, the derivation of the lithofacies units is
evident from the data shown on the site cross-sections, as is the associated uncertainty
when some lithologies in individual boreholes do not appear to ‘fit’.
C.3 References
Hunter, J., Smith N., Towler G. & Michie, U., 2007. LLWR Lifetime Project:
Reinterpretation of the LLWR Site Quaternary Geology. Nexia Solutions report
(07)8345 – Quintessa report QRS-1354F LLWRGeologyV1.0.
142
Appendix D - Lithofacies colour palette
143
Appendix E - Adaptation of 2D cross-sections
into a 3D model
This appendix contains a brief summary of the contents of the report prepared by
Hunter (2010), which describes the procedure adopted to convert the 2D lithofacies
cross-sections of the LLWR site presented in this report into a format suitable for direct
importation into the 3D computer model of the Quaternary geology of LLWR and its
wider surrounding region. The Hunter (2010) report was prepared after Version 1 of
this report was completed. Some of the figures from Hunter (2010) are included in this
appendix (Appendix E) in Version 2 of this report, because they provide an additional
means of visualising the Quaternary lithofacies and therefore they complement the
cross-sections shown in the Section 4 and Appendix B.
Hunter (2010) gives a detailed description of the process of digitising the lithofacies
boundaries shown on the 2D vertical cross-sections across the LLWR site and the recombination of these data into a set of x-y-z coordinates representing lithofacies
boundary surfaces in a more horizontal orientation. By this means the 3D spatial
configuration of the different Quaternary lithofacies present beneath the LLWR site is
represented as a series of irregularly-shaped surfaces which are stacked sequentially
above each other. In the 3D model these spatial coordinates are combined with
additional data derived from sources external to the LLWR site to produce a single,
comprehensive regional model.
Paradoxically, these multiple, spatially-continuous surfaces, which resemble a
lithostratigraphical succession, are being used to represent a set of lithofacies
boundaries that are often spatially-discontinuous and are not part of an established
lithostratigraphy. This procedure is necessary because the 3D computer model is a
layered model and it requires the data in each layer to be continuous across the full
extent of the model, i.e., it does not accommodate lateral discontinuities within
individual layers. Furthermore, in order for the spatial complexity and interrelationship of certain lithofacies units to be reproduced as accurately as possible in 3D,
it was necessary to subdivide these particular units into multiple sub-units to enable
abrupt lateral discontinuities to be characterised in a digital form. Internal surfaces that
subdivide individual lithofacies units are entirely artificial in concept and have no
geological significance. When the series of stacked surfaces is combined together in the
model, the internal subdivisions within a lithofacies unit are no longer apparent
because the properties of each of the subdivisions are the same.
144
An additional level of spatial control was imposed upon the lithofacies boundary
surfaces by digitising data from imaginary, or ‘virtual’, boreholes. These additional xy-z coordinates were included to try to ensure that the interpolation algorithm used by
the 3D model produced reasonable and realistic surfaces in areas of the model where
actual borehole control is lacking, i.e., data-gaps. In some parts of the model there are
lateral spatial data gaps, requiring complete virtual boreholes to be inserted through
the entire thickness of the Quaternary deposits. Elsewhere, there is adequate geological
control from existing boreholes at a shallow depth level, but some of these boreholes
were required to be projected to the sandstone bedrock by means of ‘virtual depth
extensions’.
A limited selection of the figures presented in Hunter (2010) is shown below. Each of
these figures illustrates a digitised lithofacies boundary surface projected as an
apparently-solid, colour-rendered 3D view for easier visualisation. These surfaces are
usually composite in nature, i.e., different parts of the surface may separate different
lithofacies in different areas of the model and therefore they are not necessarily of the
same age across their spatial extents. They generally do not represent geological events
marking particular time intervals in the established Quaternary stratigraphy. The effect
of combining all of the digitised lithofacies boundary surfaces into a single stacked
sequence to represent the Quaternary lithofacies beneath the LLWR site only becomes
apparent when they are imported into the 3D computer model itself (Smith, 2010).
145
Figure E-1. Contour map representing the base of lithofacies unit A
146
Figure E-2. 3D surface representing the base of lithofacies unit A
Figure E-1 is a 2D contour map and Figure E-2 is a 3D surface representing the
digitised base of lithofacies unit A, which is comprised of all post-glacial deposits
together with the made ground inside the LLWR site. Therefore these two figures
should depict the likely topography of the final glacial land surface if the post-glacial
sediments are removed.
Both of these figures reveal the shallow, stream valley feature that intersects the southeastern half of the LLWR site, known as the Drigg Stream. The green line added to
Figure E-1 represents the spatial limit of the post-glacial peat layer which is intersected
by many of the boreholes within the LLWR site. The peat layer post-dates and is
draped across this shallow valley landform, including the higher-elevation ground to
the north-west, but it is absent from the slightly elevated area in the south-east corner.
This surface can be considered, in part, as a geological event boundary.
147
Figure E-3. Contour map of base of lithofacies unit A beneath regional land surface
Figure E-3 shows the Figure E-1 contour map of the base of lithofacies unit A visible
through a ‘window’ in a contour map of ground-surface LIDAR data for the
surrounding area, the latter shown as a greyscale to enhance the contrast. On this
figure the apparent continuity of the Drigg Stream valley (shown as a dashed-blue line)
from outside to within the LLWR site is evident, with the 15m contour line appearing
148
to define the stream bank. Some of the ‘dimples’ and irregularities in this lithofacies
surface, particularly at the north-western end, are the result of various engineering
excavations that have been made within the LLWR site. These excavations are an
integral part of the LIDAR topographical surface, i.e., the upper surface of the model,
and where they are of sufficient depth, they extend through lithofacies unit A into unit
B2 beneath.
Figure E-4. 3D surface representing an intermediate surface within the B3
lithofacies.
Figure E-4 is a 3D image of the surface created to subdivide the B3 lithofacies unit
within the LLW site into upper and lower sub-units. This procedure was necessary to
enable the discontinuous remnants of the C lithofacies unit to be digitised in their
correct, interpreted spatial relationship to B3. Although the surface is described in
Hunter (2010) as an artificial concept, it nevertheless enables the remnants of the C
lithofacies to be visualised as a single entity in a manner which is not so easy to achieve
using a combination of cross-sections and plan-view figures. Also visible on this image
is the onlap of the B3 lithofacies unit onto the sandstone bedrock hill, which protrudes
through it.
149
Figure E-5. 3D surface representing the base of the B3 lithofacies.
Figure E-5 shows a surface representing the base of the B3 lithofacies unit, which is a
composite unit interpreted to be the consequence of multiple, coalesced glaciofluvial
events, some of which formed incised, infilled channels eroded into the D lithofacies
units beneath. The shapes of two of these incised channel features are revealed on this
image.
Only a small proportion of the ~650 boreholes drilled within and around the LLWR
site penetrate below the B3 lithofacies. These deep boreholes provide valuable clues
about the nature of the deepest glacial deposits in this area, however much of the
geological interpretation between the boreholes is speculative, including the shapes
and orientation of these incised channels.
A complete set of similar contoured maps and rendered 3D images for all of the
digitised surfaces are presented in Hunter (2010).
E.1 References
Hunter, J. 2010. The Geology of the LLWR Site – adaptation of 2D cross-sections into a
3D model. Quintessa report QRS-1443ZD-R1 Version 1.0.
Smith, N. 2010. 3D Geological Interpretation of Geophysical Profiles and Further 3D
Geological Modelling at LLWR site and surrounding area. NNL Report No. (10) 11217.
150

The Geology of the LLWR Site and

Transcription

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