The geochemistry of concretions from the Kimmeridge Clay

Transcription

Sedimentology (1991) 38,79-106
The geochemistry of concretions from the Kimmeridge Clay Formation of southern
and eastern England
I. C. SCOTCHMAN*
Department of Geology, University of Shefield, Beaumont Building, Brookhill, Shefield S3 7HF. UK
ABSTRACT
Concretions from the Kimmeridge Clay Formation are of three types : calcareous concretions, septarian
calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone
facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe-reduction,
sulphate-reduction and decarboxylation zones.
The septarian concretions show a long and complex history, with early initiation of growth and
development spanning several phases of burial, each often resulting in the formation of septaria. Growth
apparently ceased in the transitional zone between the sulphate-reduction and the methanogenesis zones.
Very early growth in the Fe-reduction zones is also seen in one sample. The non-septarian concretions
began growth later within the sulphate-reduction zone and have had a simpler burial history while the
pyrite/calcite concretions show carbonate cementation in the sulphate-reduction-methanogenesistransition
zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs.
Development of concretions appears to be indirectly controlled by the sedimentation rate and
depositional environment, the latter determining the organic matter input to the sediments. Calcareous
concretions predominate in swell areas and during periods of low sedimentation rate in the basins with
poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur
in organic-rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan
dolomite nodule grew under very high sedimentation rates.
INTRODUCTION
Concretions can provide a valuable insight into the
geochemical evolution of a mudstone sequence and its
pore fluids during burial and diagenesis (Curtis &
Coleman, 1986). Concretion growth can span a large
time period, from burial of the mudstone beneath the
sediment-water interface to depths of 1 km or more.
The successive stages of cementation, with their
associated trends in bicarbonate source and cation
supply, reflect the processes affecting the mudstone
sequence during burial diagenesis and can be preserved in concretions, both as concretion-body cements and as cements infilling septarian cavities
(Curtis, Petrowski & Oertel, 1972; Raiswell, 1976;
Irwin, Curtis & Coleman, 1977; Hudson, 1978;
*Present address: Amoco (UK) Exploration Co., Amoco
House, West Gate, London W5 lXL, UK.
Coleman & Raiswell, 1981; Boles, Landis & Dale,
1985; Dix & Mullins, 1987; Astin & Scotchman, 1988;
Thyne & Boles, 1989). Concretions have been documented as forming within the sulphate-reduction (SR)
zone (Raiswell, 1976; Irwin et al., 1977; Hudson,
1978; Coleman & Raiswell, 1981; Pisciotto & Mahoney, 1981 ; Dix & Mullins, 1987) with growth continuing into the methanogenesis (Me) zone .below.
Concretionary carbonates have also been assigned to
the Me and decarboxylation (D) zones (Irwin et al.,
1977; Friedman & Murata, 1979; Irwin, 1980; Kelts
& McKenzie, 1982; Garrison, Kastner & Zenger,
1984; Tasse & Hesse, 1984; Hennessy & Knauth,
1985; Baker & Burns, 1985; Curtis & Coleman, 1986;
Burns & Baker, 1987; Scotchman, 1988; Burns, Baker
&Showers, 1988).
Various controls on diagenetic carbonate precipi79
80
I . C. Scotchman
tation have been demonstrated in organic-rich mudstones, sedimentation rate being a major control
(Burns & Baker, 1987; Scotchman, 1989). Precipitation occurs in the SR zone under lower sedimentationrate conditions and in the Me and D zones with high
sedimentation rates. Due to the lack of sulphate in
these Me and D zone porewaters, carbonates are
predominantly dolomitic (Baker & Kastner, 1981 ;
Burns & Baker, 1987 and references therein). Nodules
and concretions may also be preferentially located in
layers that are slightly richer in organic carbon than
overlying and underlying rocks (Bums et al., 1988).
This is apparent in the basinal facies Kimmeridge
Clay Formation section at Kimmeridge Bay, Dorset,
where diagenetic dolomitic cementstones such as the
Washing Ledge occur within oil-shale units. Biogenic
carbonate content of the mudstones can also affect
concretion development (Burns & Baker, 1987), the
largest concretions growing in the calcareous mudstone facies (Scotchman, 1989).
Study of diagenetic carbonates in the basinal facies
mudstones at Kimmeridge Bay (Irwin et al., 1977;
Coleman, Curtis & Irwin, 1979; Irwin, 1979a, 1980)
has shown that the early organic-matter based diagenetic zones of Curtis (1977,1980) are applicable to the
Kimmeridge Clay Formation. The aims of this paper
are to document and compare the evolution of
concretions and their cements from different locations
within the Kimmeridge Clay Formation of southern
and eastern England, to elucidate sedimentological
and diagenetic controls on their authigenesis and to
demonstrate the extension of Curtis’ zonal diagenetic
model into different depositional settings.
The stratigraphic framework provided by the
chronostratigraphic units of the Kimmeridge Clay
Formation bed scheme, which is palaeontologically
and lithologically defined (Gallois & Cox, 1976;
Gallois, 1979; Cox & Gallois, 1981), allows the effects
of parameters such as sedimentation rate, lithology,
organic matter content and burial depth on the
sequence of diagenetic reaction zones to be studied
(Scotchman, 1989). Comparisons can therefore be
made between areas of low sedimentation rate (swells)
with low burial rates, calcareous mudstones and poor
organic matter preservation and areas of high sedimentation rate (the basins) with high burial rates,
more organic-rich mudstones and good organic matter
preservation.
GEOLOGICAL S E T T I N G
The Upper Jurassic Kimmeridge Clay Formation
comprises a transgressive, cyclic sequence of organic-
rich and calcareous mudstones deposited during a
period of maximum eustatic rise and tectonic subsidence. The 48 beds recognized within the formation
(Gallois & Cox, 1974, 1976; Gallois, 1979; Cox &
Gallois, 1981) are continuous along the whole 200-km
outcrop from the type section at Kimmeridge Bay,
Dorset to North Yorkshire and over the subcrop in
southern England and the East Midlands except where
removed by post-Jurassic erosion (Fig. 1). Although
deposition took place in a relatively uniform environment, mudstone facies changes along the outcrop from
dominantly organic-rich mudstones to calcareous
mudstones and thickness variations within the beds
are apparent. These reflect the influence of the basin
and swell tectonics, which characterized the Jurassic
(Hallam, 1958; Hallam & Sellwood, 1976), the
sedimentation rate, the rate of burial and the oxygenation state of the bottom waters which was highly
variable and ranged from oxic to anoxic (Gallois,
1976; Irwin, 1979a, b; Tyson, Wilson & Downie,
1979; Scotchman, 1984; Wignall & Myers, 1988;
Wignall, 1989). Both the sedimentation rate and the
oxygenation state of the depositional waters influenced
the preservation of organic matter within the sediments (Didyk er al., 1978)and therefore were acontrol
on the course and duration of the early diagenetic
reactions within the mudstones as well as any future
hydrocarbon potential.
Burial history
The burial history of the Kimmeridge Clay was highly
variable across the depositional basin. In the Central
Channel and Weald Basins of southern England
deposition was continuous into the early Tertiary
when inversion, uplift and erosion associated with the
Alpine Orogeny occurred (Lake & Karner, 1987).
However, the northern flanks of these basins were
subjected to uplift and erosion at the end of the
Jurassic which removed much of the Kimmeridge
Clay. To the north, in the South Midlands, Wash and
eastern England areas, uplift and erosion took place
at the end of the Jurassic with deposition of Volgian
to Early Cretaceous sands. Further burial occurred in
the Late Cretaceous (Whittaker, 1985) prior to final
uplift and erosion in the Tertiary. The Cleveland
Basin had a burial history similar to that of the
southern England basins with continuous, faultcontrolled deposition and burial through to the Late
Cretaceous when basin inversion and uplift associated
with the Alpine Orogeny occurred (Hemingway &
Riddler, 1982).
Geochemistry of Kimmeridge Clay concretions
AREA OF
NON-DEPOSITION
,
. ..
KIMMERIDGE CLAY Fm.
OUTCROP
0
. ..
+
SWELL AXIS
FAULT
0
CORED BORE HOLE
+
OTHER SAMPLING
LOCATION
81
KIMMERIDGE CLAY Fm.
SUBCROP
Fig. 1. Location map showing the distribution of the Kimmeridge Clay Formation in southern and eastern England and the
concretion sampling localities.
82
I. C . Scotchman
FIELD DESCRIPTION AND
MORPHOLOGY OF THE
CONCRETIONS
The concretions tend to be restricted to specific
horizons within the formation, particularly the carbonate-rich units such as Beds 4, 18 and 30 (Fig. 2).
They occur either as separate bodies or as semicontinuous bands known as cementstones and are
oftenassociated with thedevelopment of bored erosion
surfaces (Gallois & Cox, 1974, 1976) indicating
hiatuses in sedimentation. The dolomitic ‘ledges’ at
Kimmeridge Bay (Arkell, 1947; Bellamy, 1977; Cox
& Gallois, 1981 ; Leddra et al., 1987; Feistner, 1989)
are perhaps the best known examples of these
cementstones. They appear to be a feature unique to
the Kimmeridge Bay section and are not present in
other parts of the onshore section. Their geochemistry
has been studied by Irwin et al. (1977), Irwin (1980,
1981) and more recently by Feistner (1989) and, apart
from general considerations, will not be further
discussed here.
Concretion morphologies range from the small
irregular pyritic nodules in Bed 38 at Cuddle,
Kimmeridge (sample KC), 0.1 1 m long and 0.04 m in
diameter, to large septarian calcareous concretions
such as those from Bed 18 at Haddenham or Bed 30
at Roslyn Pit which are flattened spheroidal in shape,
1.5 m in diameter and 0.5 m thick. The concretions
comprise a body with a fine-grained matrix of clay
minerals and quartz tightly cemented by micritic or
micro-spar carbonate, often incorporating shell fragments. The pyrite content, generally occurring as
framboids, is variable, ranging from below levels of
X-ray diffraction detectability in sample D179.95 from
Bed 4 at Donington-on-Bain borehole to over 50% in
sample KC where it forms the bulk of the matrix
cement.
The septarian concretions have similar morphologies to those reported by Lindholm (1974) and Boles
eta[. (1985) and exhibit single or multiple generations
ofcracking andcementation, sample D179.95 showing
at least five generations of cement. Septarian crack
morphology ranges from simple fractures which do
not reach the concretion outer edge, containing a
single generation of cement, to complex patterns with
two or more generations of cement often with voids in
the concretion centre, forming a polygonal pattern in
horizontal section and aligned fractures in the vertical
section (Astin, 1986; Astin & Scotchman, 1988).
Septarian cement stratigraphy is complex even in
those concretions with just two apparent cement
generations (Astin, 1986; Astin & Scotchman, 1988).
The first generation is generally a brown coloured,
fine-grained, axially orientated calcite, the brown
colour being due to inclusions of organic matter (T.
R. Astin, pers. comm.; Lindholm, 1974). Later
cements comprise one or two zones of clear to white
subhedral to euhedral crystals up to 1 mm in size,
often with curved crystal faces. The cements usually
show a coarsening crystal size into the fracture,
indicative of growth into a void (Bathurst, 1975).
Precipitation of the later generations of white cement
generally followed a second phase of fracturing, which
cuts across the first-generation cements and penetrates
further into the concretion body.
The non-septarian carbonate concretions have a
body with a less compact, fibrous matrix, suggesting
later initiation of growth compared to the septarian
concretions.
SEQUENCE OF EVENTS IN
CONCRETION GROWTH
Staining and textural relationships of the concretion
body microspar and void cements allow construction
of the concretion growth and septarian fracturing
history (Fig. 3). Growth history began initially with a
compact central core of the concretion, the matrix
becoming fibrous with further growth (Astin &
Scotchman, 1988).Septarianfracturingof the compact
core then began, the fractures becoming dilated.
Brown and later white calcite was precipitated in the
voids, lining the cavities concurrent with further
outwards growth of the concretion body. In many of
the concretions, growth and septarian fracturing
appears to have ceased at this time.
A later second septarian fracturing event occurred
in some of the concretions, the septaria cross-cutting
the earlier cement-filled fractures from the central
cavities into the fibrous matrix of the outer concretion
body, followed by precipitation of the second generation of calcite cement. Concretion growth and fracturing then appear to have ceased although some calcite
probably continued to precipitate in the cavities.
Further development is exhibited by the complex
concretion D179.95. This has a single cement in the
first-generation septarian fractures with second-generation fractures and cement cross-cutting the concretion body and first-generation septarian cements.
Later cements infill the second-generation fracture
voids with multiple generations of white and cream’
cements.
Geochemistry of Kimmeridge CIay concretions
83
Sands
4-2
45
coccolith-rich band
1
1
Mudstone, undifferentiated
Pale grey, very calcareous mudstone
H
Silty mudstone
Very shelly mudstone
Oil shale
Cementstone, concretion
- ---
Pentacrinus
6 66
Rhynchonellids
ppp p
Phosphatic pebblelnodulebed
Fig. 2. Stratigraphy of the Kimmeridge Clay Formation of
eastern England. Section shown is 105 m thick.
3
3ei
34
rhynchonelids
~ Q , Q
-Saccocoma-rich
band
Nannocardioceras
plasters with coccolith
-rich bands
3
Crussoliceras Band
Nanogyra-rich limestone
Saccocoma-rich band
F
Saccocoma-rich band
Nanogyra virgularich bed
27 26
25
24
-23
silty bed with
_rhynchonellids
-
SupracorallinaBed
A. eulepidus plasters
22
21
20
1
Pentacrinus Band
silty bed with rhynchonellids
and Lopha
silty bed with
- -rhynchonellids
-
>
Xenostephanus-rich band
silty bed. locally cemented
phosphatic nodule bed
phosphatic nodule bed
The non-septarian calcite concretions and nodules
with their less compact fibrous matrix appear to have
begun growth later, concurrent with fibrous matrix
development in the septarian concretions.
Relationship to burial history
As demonstrated by Astin & Scotchman (1988), the
concretion development sequence can be related to
burial history. In general, the early initiation of
concretion growth followed by two separate septarian
fracturing and cementation events can be equated to
a two-stage burial history. Rapid burial in the Late
Jurassic was followed by end Jurassic uplift with
further rapid burial in the Late Cretaceous before
final uplift in the Tertiary (Fig. 4) (Astin & Scotchman,
1988). Astin (1986) demonstrated that septarian crack
formation is favoured by higher than normal porefluid pressures and reduced horizontal stresses which
can occur at shallow depths during the rapid burial of
mud rocks (Bryant et ai., 1985). The presence of nonseptarian concretions from adjacent locations with a
similar burial history can be explained by the textural
evidence for their relatively later initiation which, by
comparison with septarian examples, was synchronous with the first phase of septarian fracturing and
cementation. Presumably their less compact, fibrous
body matrix made them less susceptible to the second
septarian ‘event’ associated with the Late Cretaceous
burial phase than the more rigid, compact and
cemented septarian concretions.
The constraints on concretion growth imposed by
the two-stage burial history typical of the eastern
84
I . C. Scotchman
EARLY-FORMED
COMPACT CONCRETION
CENTRE
\
England shelf can be illustrated by comparison with
the development of the cemented 'ledges' of the
Kimmeridge Bay section. Here the section had an
overall continuous burial history into theearly Tertiary
with rapid burial during the Kimmeridgian resulting
in the strong development of the dolomitic cementstones. Ample evidence of fracturing due to overpressuring and dewatering is present (Irwin, 1980; Leddra
et al., 1987; Feistner, 1989). When subsidence and
burial rates were reduced at the end of the Kimmeridgian, the septarian calcareous Rotunda Nodule
concretions developed under conditions similar to
those prevailing in eastern England.
In summary, the general textural sequence exhibited
by the septarian concretions suggests development
over a considerable period of time with a sequence of
cements in the concretion body culminating in fibrous
cement synchronous with septarian fracture infills.
The later generations of septarian cement post-date
the main phase of body matrix cementation. From
such considerations it can be demonstrated that
geological history, particularly burial and uplift,
exerted a strong control on growth and development
of the concretions (Fig. 5). It is suggested that the
LATER DEVELOPMEN1
OF FIBROUS MARGiN
CENTRAL VOID
'
FRACTURES AND
CEMENTS ASSOCIATED
LATE SEPTARIAN FRACTURES
WITH FIRST PHASE
AND CEMENT INFILL ASSOCIATED
OF BURIAL
WITH SECOND BURIAL PHASE
KEY
COMPACT CONCRETION CENTRE
FIBROUS MARGiN
EARLY BROWN SEPTARIAN C E M E N l
0
LATER WHITE SEPTARIAN CEMENT
Fig. 3. Schematic diagram showing the development of a
typical septarian concretion.
AGE (MA)
150
50
100
20
50
Oa
v
a
\
KI
I EARLY CRETACEOUS I
5I-
...
-a
'.
LATE CRET
-J
1
PALAEOGENE
80
120
hEOGENl
Fig. 4. Burial history curves for the Kimmeridge Clay Formation. 1. South Midlands Shelf; 2. Eastern England Shelf; 3. Vale
of Pewsey Basin; 4. Cleveland Basin; 5. Mid-Dorset High (Wytch Farm); 6. Central Channel Basin; 7. Weald Basin.
85
AGE (MA)
150
100
50
0
LOW
\
kxx
/
FRACTURING&
FIBROUS MATRIX
CEMENTS
HISTORY-SEPTARIAN
& NON-SEPTARIAN
CALCITE CONCRETIONS
HIGH
\‘
I
I
TWO-STAGE BURIAL
I
I
f
I
I
!
1
RAPID. CONT‘INUOUS
BURIAL- Fe-RICH
CALCITE & DOLOMITE
NODULES, CONCRETIONS
& CEMENTSTONES
Fig. 5. Summary of ‘concretion development in relation to geological history.
I
dolomitic ‘ledges’ formed during rapid, continuous
burial while concretions reflect a less rapid, punctuated
burial history.
SAMPLING PROCEDURE
Concretions were analysed from a number of beds at
locations shown in Fig. 1 to give both wide geographical spread and vertical range within the Kimmeridge
Clay Formation. Both borehole core and outcrop
samples were studied. Subsamples of the concretion
body were taken using a masonry drill across sawn
surfaces cut along the axis of the concretion while
individual septarian vein cements were sampled using
a 2-mm diamond-burr drill.
ANALYTICAL TECHNIQUES
The carbonate mineralogy of the concretions was
determined using potassium ferricyanide and alizarin
red S staining techniques (Dickson, 1966) on both
thin sections and sawn surfaces. X-ray diffraction
(XRD) analysis, used to confirm the staining results,
allowed the degree of cation substitution to be
determined from the carbonate mineral d-spacings.
The isotopic composition of the samples was
determined at the NERC Stable Isotope Facility at
the British Geological Survey’s Geochemical Division
Laboratory at Gray’s Inn Road, London and full
details of the methodology appear in Scotchman
(1989).
All carbonate carbon and oxygen isotope data are
reported in standard &notation relative to the PDB
standard using the phosphoric acid fractionation of
Friedman & O’Neill (1977). Reproducibility was
determined to be 0.05%,. Porewater oxygen values are
reported relative to the SMOW standard.
Analysis of the carbonate cation chemistry was
made using the inductively coupled plasma (ICP)
spectrometer in the Geology Department of King’s
College, London (Scotchman, 1989). The efficiencyof
the carbonate dissolution and extraction of cations
from other sources, notably apatite and clay minerals,
are sources of error (Raiswell, 1973) and reproducibility was determined to be f 13%from multiple analyses
of the NBS-lb argillaceouslimestone standard. Microprobe analyses of transects across the septarian
cements in concretions D179.95 and P30B were also
performed.
RESULTS
Full tabulated data are available as a supplementary
publication No. SUP 15002 available (upon payment)
from British Library Document Supply Centre, Boston
Spa, West Yorkshire, LS23 7BQ, UK, or upon request
from the author.
86
I . C . Scotchman
Carbonate mineralogy
Carbonate chemistry
Staining and XRD show that the concretions are
dominantly calcitic with ferroan dolomite being
present only in sample C134.05 from Marton Borehole.
Fe content increases across the concretions, the body
matrix and early brown and white septarian cement
generations comprising non-ferroan calcite, while the
later white cements are ferroan calcite. The pyritic,
non-septarian concretions from Kimmeridge Bay,
samples CHB and KC, are cemented by ferroan
calcite while in sample D179.95 the concretion matrix
and calcite cements are entirely ferroan.
The results of the carbonate chemical analysis show
that there is significant substitution of Mg and Fe for
Ca in the calcite lattice. Mg is concentrated in the
body and the earliest septarian cements and Fe in the
later septarian cements with up to 5.8 mol% MgC0,
and 8.9 mol% FeCO, in calcite. The ferroan dolomites
of sample C134.05 have the highest MgC03 and
FeCO, contents at 28.5 and 14.1 mol% respectively.
Mn and Sr occur in trace amounts.
DISCUSSION
Stable isotopes
The concretions show a wide range of isotopic
composition, 6I3C ranging between -0.6 and
-23.3%, and 6l80between - 1.1 and - 14.5%,.6I3C
and 6l80 show a two-stage general relationship
(Fig. 6 ):
Stable isotopes
The antipathetic and the sympathetic trends of isotope
evolution are clearly shown by the septarian concretions (Fig. 6) and are similar to those reported from
concretions in the Oxford Clay (Hudson & Friedman,
1976; Hudson, 1978) and the Lias Jet Rock (Coleman
& Raiswell, 1981). The two-stage relationship forms
part of a single evolutionary trend interpreted from
(1) a first stage with an antipathetic relationship,
shown by the majority of the samples,
(2) and a second stage with a sympathetic trend.
+
STAGE2 EVOLUTlONARY TRFND
x
9;
:r1
r
-9
bI3C LIGHTER - 6"O LIGHTER
Fe CONTENT t
4
KEY
0
CONCRETION BWY
+ 111 SEPTARIAN CEMENT
x
2nd SEPTARIAN CEYENl
0 LATER
CEYENT8
SR TO Me ZONE
--I/
b"0
+ PRIMARY CARBONATE + D ZONE CARBONATE
L-15
Fig. 6. Isotopic composition of the septarian concretions (omitting D179.95)showing the two-stage isotopic evolutionary trend.
textural evidence as a time trend. The first-stage
antipathetic trend covers early concretion growth
from initiation of the body with light carbon (depleted
in 13C)and heavy oxygen (enriched in I8O) through
successive body cements and septarian vein cements
with heavier carbon and lighter oxygen to the
concretion edge. The reversed second-stage trend to
lighter carbon and oxygen appears to be a late-stage
event, following the antipathetic trend in the latest
generations of septarian cements and in the septarian
concretion, C134.05, from Marton Borehole. The
isotopic evolutionary sequence therefore appears to
follow closely the two-stage sequence of concretion
development derived from textural and geological
history considerations (Astin & Scotchman, 1988).
87
whole and not just to the type section at Kimmeridge
Bay (Irwin et al., 1977; Irwin, 1979a, b). The variable
burial depths constrain the diagenetic level attained,
as demonstrated by Scotchman (1989).
Isotopic evolution: (2) non-septarianconcretions
The non-septarian concretions (Fig. 7) show an antipathetic trend similar to the septarian concretions but
merge with the marine carbonate field at 613C and
6180=O+2%o.This suggests that the source of most
of the heavy carbon is marine carbonate and that Me
zone carbonate, which typically has a 613C range of
1 to 15%, (Irwin et al., 1977), only forms a minor
component.
+
+
Carbonate source
Isotopic evolution: (1) septarian concretions
The earliest calcite cements have a very light 6I3C
composition of - 10 to -23%,, a heavy 6 l 8 0 composition of - 1 to - 3%, and, generally, a non-ferroan
composition reflecting an SR zone source for the
carbonate. This is confirmed by the high pyrite content
of the concretion bodies. The trend to heavier carbon
and lighter oxygen suggests mixing of the SR zone
carbonate with a heavy carbon component, by the
progressive addition of carbonate from either marine
or Me zone sources, or a mixture of both. The addition
of post-SR zone cements is also indicated by the
change to ferroan calcite in the later septarian cements.
The latest cements in concretions such as those
from Blackhead, Dorset (samples TA and TB) and
Marton concretion C134.05 show a further trend in
the ferroan calcites or dolomites to light 613C of
- 13.4 to - 14.5%, and light 6 l 8 0 ranging from -7.9
to - 8.6%,,. The highly ferroan nature of the carbonates
and their light oxygen composition suggests precipitation at elevated temperatures during deep burial,
the lightening trend of 613C suggesting an increasing
input of carbon from the thermal breakdown of
organic matter by processes such as decarboxylation
(Irwin et al., 1977). Maximum burial depths are
estimated at 2 km for Kimmeridge Bay and between
1.4 and 3 km for Marton (Table 2 of Scotchman, 1989;
Green, 1989b),sufficient for decarboxylationreactions
to have occurred at these locations.
The evolution of the concretions therefore appears
similar to that described for concretions from the Lias
(Coleman & Raiswell, 1981) and suggests that the
depth-related diagenetic zones of Curtis (1977, 1980,
1987) based on organic matter degradation are
applicable to the Kimmeridge Clay Formation as a
Carbon isotopic ratios of the concretions reflect the
isotopic composition of the CO, of the source or
diagenetic zone in which they developed (Curtis,
1977; Irwin et al., 1977; Wigley, Plummer & Pearson,
1978). The data suggest that initiation of concretion
growth took place during early burial, generally in the
SR zone. Exceptions to this are: (i) Kimmeridge Bay
pyrite/Fe-calcite concretions CHB and KC (Fig. 8)
which, by comparison with the isotopic evolutionary
sequence seen in the septarian concretions, indicate
later initiation at greater burial depths; (ii) the Marton
concretion C134.05 (Fig. 9) where the isotopic data
suggest even later initiation of growth at a burial
depth of about 1 km as suggested by their position on
the Me-D zone evolutionary trend of Irwin et al.
(1977); and (iii) D179.95 whichappears to have begun
growth earlier than the other septarian concretions, in
the post-oxic Fe-reduction (FeR) zone (Fig. lo).
Although SR zone carbonates typically reflect the
organic matter of 613C values of - 25 to - 30%,, they
can exhibit a wide range of values due to the addition
of carbonate from other sources. Heavier values
appear common due to mixing with normal marine
carbonate. A lighter carbon signature than usual for
the SR zone may also occur, resulting from the upward
diffusion of methane from an underlying Me zone and
its oxidation in the SR zone (to give carbonate of 613C
- 60 to - 70%,) (Gautier & Claypool, 1984). Raiswell
(1987) considers that upward diffusion of dissolved
carbonate from the Me zone of 613C at 0 to lo%,
would normally accompany the methane, resulting in
a carbonate of 6I3C - 25 to - 30%,, a composition
similar to that of the SR zone. However, while
confirming anaerobic methane oxidation as an active
process in marine sediments, Whiticar & Faber (1986)
+
-34
-22
-50
-1p
6 I3C
-16
-14
I
l
l
-12
1
1
-10
1
1
-8
1
I
-6
I
l
-4
I
7
1
8
RANGE
SR ZONE
TPANSITION
KEY
0
--7
CONCRETION BOOK
-9
-41
-43
L-I5
-MARINE CARBONATE
RANGE
LATE STAGE INITIATION OF CONCRETION
GROWTH IN SR-Me ZONE TRANSITION
6 "0
KEY
0 CONCRETlON EOVY
Fig. 8. Isotopic composition of carbonates within the pyrite/ferroancalcite nodulesfrom the basinal facies of the Kimmeridge
Clay Formation.
6 I3C
4I
I
6 r
,
~
10,
Me ZONE CARBONATE
Me ZONE
+ PRIMARY + D ZONE CARBONATE
KEY
0 C134.05 CONCRETION BODY
0
C134.05 SEPTARIAN CEMENT
+ NOCS 2111-1 FERROAN DOLOMITE SILTSTONE
*
BANDS (Scotchman, 1988)
FERROAN DOLOMITE CEMENTSTONES,
KIMMERIDGE BAY (Irwin el al, 19TI)
EVOLUTIONARY TREND
-.?
1'kg
b l3 C LIGHTER - b'' 0 LIGHTER
1-15
Fig. 9. Isotopic composition of sample C134.05 plotted on the Me-D zone evolutionary trend of Irwin et 01. (1977). Data from
Scotchman (1988) are also presented.
--5
SR ZONE MISSING 7
--7
K N
0
D179.95 CONCRETION BODY
-b
D179.95 1ST SEPTARIANCEMENT
X
D179.95 2ND SEPTARIANCEMENT
0
D179.95 LATER CEMENTS
Fig. 10. Isotopic compositionof carbonates from complex septarian concretion D179.95.
t:
b'80
l
90
I . C.Scotchman
show that the C 0 2 produced by this process lies in the
range 6I3C - 15 to -25%,, rendering the somewhat
complex model of Raiswell(l987) unnecessary. Thus,
light SR zone carbonate can result both from the
direct reduction of organic matter and from anaerobic
methane oxidation, while heavier values reflect the
addition of C 0 2 from the Me zone and/or marine
carbonate. Claypool & Kvenvolden (1983) note that
the lightest 6I3C values (-15 to -25%,) occur in
present-day anoxic sediments at the base of the SR
zone which suggests a methane oxidation component.
Beneath there is a transition zone into the Me zone
where 613C becomes heavier with depth. Typical
values in the Me zone are in the range 0-15%, (Irwin
et al., 1977; Curtis, 1978), although recent work by
Whiticar et al. (1 986), who report an average value of
- 6%, for C 0 2derived from methanogenesis, suggests
that light values may be more typical.
The concretions show a strong SR zone carbon
isotope signature, the lowest value recorded being
-25%, but, more typically, values fall in the range
-12 to -16%,. This suggests concretion growth
towards the base of the SR zone, the light carbonate
from the SR zone being diluted with marine carbonate
from dissolved shell material with an average 6I3Cof
zero, the lack of positive 6I3C values arguing against
mixing with Me zone carbonate. The light SR zone
carbonate could equally be derived from the bacterial
oxidation of organic matter or from the oxidation of
methane at the base of the SR zone: no evidence is
apparent to suggest which of these processes was the
major source of the light C 0 2 , and a combination of
both appears likely.
Successive cements in the concretion matrix and
septaria show an increasing influence of the heavier
carbonate component but still retain their light 613C
values even though the associated 6' '0values indicate
precipitation at burial depths in excess of 100 m. This
suggests the occurrence of a deep transition zone
beneath the SR zone, perhaps extending to burial
depths of several hundred metres, before Me zone
processes became dominant. The change to ferroan
calcite suggests that sulphate reduction was not active
in the transition zone but that light carbon remained
the major component of the carbonate in the porewaters below the SR zone in the absence of any Me
zone carbonate with a positive 6I3C signature. The
Me zone was apparently not of sufficient intensity to
modify the carbonate inherited from the SR zone and
concretion development appears to have ceased within
the SR-Me transition zone before Me zone conditions
were attained (Astin &Scotchman, 1988).
The pyrite/Fe-calcite concretions CHB and KC
appear to have formed relatively later (Fig. 8), their
heavy 6I3Cand lighter 6I8O values indicating infilling
of the earlier SR zone pyritic matrix by Fe-calcite in
the SR-Me transition zone. No early SR zone
carbonate appears to be present.
The final stage in the evolution of the carbonate
source is shown by the late-stage sympathetic isotopic
evolutionary trend in the final generations of septarian
cement in samples TA and TB and in C134.05 (Fig. 9).
While light carbon/light oxygen cements (6I3C
- lo%,,
-7%,) can result from an influx of
meteoric water bearing soil-derived C 0 2 (Hudson,
1977), this trend is suggestive of an increasing input
of ferroan light carbon/light oxygen carbonate from
the D zone which typically has 613C values of - 20%,
(Irwin et al., 1977; Curtis & Coleman, 1986). Precipitation appears to have taken place in the Me-D
transition zone as no 'end-member' Me or D zone
carbon isotope signatures are seen.
Oxygen isotopes
The regular trend of lightening 6"O with time in the
concretion cements can be used to compare the
relative timing of the carbonate-producing processes
as well as for palaeotemperature and precipitation
depth estimations. A considerable amount of l6O
enrichment is seen with time within individual
concretions, the largest range being from -1.9 to
- 10.8%, 6l'O in D179.95. The oxygen composition
of a carbonate is dependent on the precipitation
temperature and on the water composition. Using
Craig's (1965) palaeotemperature equation and assuming a Jurassic seawater 6"O composition of - 1.2%,
(Shackleton & Kennett, 1975), the earliest cements
with a 6"O of - 1-9%,give a palaeotemperature of
20°C, which is in line with published data on Late
Jurassic sea temperatures (Tan & Hudson, 1971;
Marshall & Ashton, 1980). Using this porewater
composition the final cement gives a palaeotemperature of 70°C which, assuming a sediment-water
interface temperature of 15°C (Irwin et al., 1977) and
a palaeogeothermal gradient of 28°C km-' (Irwin et
al., 1977),indicates precipitation at a depth of 2.0 km.
This is excessive for the eastern England shelf
(Scotchman, 1987).
Clearly, the porewater composition was different
from Jurassic seawater during precipitation of the
later cements. Oxygen depletion during burial is
shown by the concretion cements, and the relatively
well-controlled seawater 6"O value can therefore only
be used for palaeotemperature analysis for SR zone
carbonates. Here the porewaters must be marine as
the SR process requires constant downward replenishment to enable the reaction to proceed. Below the SR
zone is a transition to the closed system conditions of
the Me zone with no effective downward diffusion of
seawater; otherwise, in the presence of excess organic
matter, sulphate reduction would continue. It seems
likely that oxygen depletion occurred in the porewaters
under these closed system conditions, ' 0 becoming
more negative with time probably due both to
temperature effects and to mixing with upward
migrating basinal porewaters and/or meteoric waters.
Irwin et al. (1977) estimated a -4.9%, 6"O composition for porewaters at the top of the Me zone at
Kimmeridge Bay although this appears an unusually
160-rich composition for the marine environment.
Hennessy & Knauth (1985) cite temperature as the
cause of similar oxygen isotope variations in early
diagenetic dolomite concretions from the Monterey
Formation in California, but it is clearly apparent that
temperature alone is insufficient to account for such
large oxygen depletions in the Kimmeridge Clay and
lightening of the porewaters during diagenesis is
indicated.
Porewater composition and evolution
Evidence from the matrix of the earliest-formed
concretions suggests a dominantly marine porewater
with a high sulphate content and a 6l'O of around
-l%, during the initial stages of burial. 8'0 was
heaviest in the earlier post-oxic FeR zone, as shown
by the heavy, very early matrix cements in D179.95.
A progressive lightening of the porewater 6 l 8 0 in the
Me and D zones beneath the SR zone is indicated by
the later cements and is supported by other studies on
mudstones (e.g. Irwin et al., 1977; Coleman &
Raiswell, 1981; Hennessy & Knauth, 1985; Bums &
Baker, 1987).
Interpretation of the origins of these later cements
is clearly problematic as light 6l'O values can indicate
precipitation from either light waters at low temperatures (e.g. Emery et al., 1988) or heavy waters at high
temperatures (e.g. Woronick & Land, 1985; Sellwood
et al., 1987).Evidence from organic maturation studies
(Scotchman, 1987 and references therein) indicates
maximum temperatures of about 50°C were attained
during burial except in the centres of the basins where
temperatures reached 80-105°C (Ebukanson &
Kinghorn, 1985, 1986; Penn et al., 1987; Sellwood et
91
al., 1989; McLimans & Videtich, 1989; Green, 1989a,
b).
Studies of sedimentary basins have shown porewaters becoming enriched in 6''O with depth (Clayton
et al., 1966), heavy formation waters being reported
from Tertiary sandstone/shale sequences of the Gulf
Coast of the USA by Land & Prezbindowski (1981)
and Woronick&Land(1985). Suchecki&Land(1983)
further noted that "0 enriched water is an important
product of smectite illitization in a closed, rockbuffered system and showed that sandstone porewater
buffered by this reaction has a 6l'O of about + 6%, at
temperatures of 100-140°C. Fisher & Land's (1986)
study of the Wilcox sandstones also suggests a heavy
"0 porewater composition with a possible range of
-3 to +8%,. Thus hot porewaters at perhaps 80105°C with a possible composition of +3%, 6 ' * 0
(median of - 3 to + 8%, from Fisher & Land, 1986)
could have migrated up-dip from the basin centres,
forming a major component of the porewater system,
particularly during the second phase of burial in the
Late Cretaceous.
Meteoric waters are also an abundant source of
depleted water. They appear to play a major role in
the cementation of limestones (Hudson, 1977) and
have been invoked by Hudson (1978) and Martill &
Hudson (1989) to explain oxygen depletion in Oxford
Clay concretions. Martill & Hudson (1989) demonstrate that septarian cements in these were precipitated from meteoric waters from an underlying
sandstone aquifer.
The amount of depletion shown by meteoric waters
is latitude-dependent (Dansgaard, 1964) and ranges
from - 3 to - 1I%, 6 " 0 , - 7%, being a likely value
for Late Jurassic rainwater (Marshall & Ashton,
1980). Invasion of the low-permeability mudstones of
the Kimmeridge Clay by meteoric water may have
occurred, particularly on the shelf areas, the mudstones
being underlain by a regional aquifer, the Corallian
Sandstone, in southern England (Alexander, Black &
Brightman, 1987) and overlain by Volgian-Early
Cretaceous sandstones in eastern England. However,
porewater studies in the Harwell boreholes .on the
South Midlands Swell indicate that meteoric invasion
of the mudstones was a late-stage event in the
Quaternary (Brightman et al., 1985).
Finally, depletion of porewaters during diagenesis
has also been mooted (Irwin et al., 1977; Coleman &
Raiswell, 1981). Mechanisms proposed include depletion due to formation of diagenetic minerals and
ultrafiltration by clay minerals. Depletion of up to
- 3%, 6"O over a burial depth of 600 m was found
92
I. C . Scotchman
during the Deep Sea Drilling Project (Lawrence,
Gieskes & Anderson, 1976). However, this was related
to the formation of smectite from volcanic materials
at mid-ocean ridges, an unlikely scenario for the
Kimmeridge Clay. Depletion due to ultrafiltration of
porewaters by clay minerals has only been demonstrated on a laboratory scale (Coplen & Hanshaw,
1973;Benzel& Graf, 1984)and can also be discounted.
Modelling of the burial history and temperatures
may be used to differentiate between the two possible
porewater sources (Emery et al., 1988). Taking the
late cements of concretion D179.95 (6l80=- l04%,)
as an example, a burial depth of 445 m is obtained
(200 m of Upper Cretaceous Chalk, 65 m of Lower
Cretaceous plus 180 m present-day burial depth). A
30°C km-' geothermal gradient and bottom (water)
temperature of 20°C gives a burial temperature of
33°C. However, Allsop & Kirby (1985) estimate uplift
of 600m occurred during the Tertiary using shale
sonic log velocities while Green (1989a), using apatite
fission track analysis on the nearby Biscathorpe-1
well, indicates uplift of 1.1 km giving maximum burial
depth estimates of 780 and 1280m and maximum
temperatures of 40 and 58°C respectively. A cement
of 6"O composition - 10.8%,would have precipitated
from porewater of 6180 composition of - S%, at 43°C
and of -2.4%, at 58°C suggesting a mixed basinal
brine/meteoric porewater.
However, the above discussion neglects the effects
of fluid flow and particularly those of hot basinal
brines and of sediment overpressuring. Green (1989a)
records maximum palaeotemperatures of 80-100°C
for Triassic sediments in Biscathorpe-1. These are
some 1 km below the Kimmeridge Clay, and so
maximum temperatures must be reduced by 30°C to
give an estimated maximum of 50-70"C for concretion
D179.95. The late cements (-10.8%, 6l8O) would
have precipitated from porewater of 6l80composition
of - 1 to - 3.8%,.
Sediment overpressuring, which is strongly linked
to the formation of septarian fractures and fibrous
cement textures in the concretions at shallow burial
depths (Astin & Scotchman, 1988), has been shown to
cause increases in geothermal gradient in mudstones
due to decreased thermal conductivity in underconsolidated sediments (Lewis & Rose, 1970). It is suggested
that porewater temperatures were raised in the
overpressured zones where the septarian cements were
precipitated, Schmidt (1973) recording a 62% increase
in geothermal gradient from normal to overpressured
zones in the Gulf Coast of the USA. The geothermal
gradient estimate for the Kimmeridgian overpressure
zones can, by analogy, be increased from 30 to 49°C
km-'. Similar values have been recorded from the
Jurassic in the North Sea Viking Graben (Carstens &
Finstad, 1981) and from the Kimmeridge Clay section
in the Winterborne Kingston Borehole in Dorset
(Bloomer et al., 1982). Therefore, at maximum burial
depth (780-1 280 m), temperatures could have ranged
from 58 to 80°C assuming a 20°C surface temperature.
Porewater compositions which could have precipitated the final cement of D179.95 range from + 0.5 to
- 23%, 6I8O.
Consideration of the discussion above suggests
porewater composition was in the range +0.5 to
- 50%, 6l80during the second phase of burial in the
Cretaceous when the later generations of septarian
cement were precipitated. The mixing of depleted
meteoric waters introduced during the uplift phase
which would have particularly affected the shallow
buried swell and shelf areas, as demonstrated for late
cements of similar composition in Oxford Clay
concretions (Hudson, 1978; Martill & Hudson, 1989),
and a basinal porewater component with the original
marine porewater is therefore suggested as the cause
of the oxygen depletion observed through successive
cement generations. Other workers (e.g. Sass &
Kolodny, 1972; Hudson & Friedman, 1976; Irwin et
al., 1977; Coleman & Raiswell, 1981 ; Marshall, 1982;
Moore et al., 1983; Hennessy & Knauth, 1985; Dix &
Mullins, 1987) report similar depletions from latestage carbonate cements, suggesting that such processes are commonplace during burial diagenesis of
organic-rich mudrocks.
Timing of concretion growth
The septarian concretions show the largest span of
isotopic evolution and both individually and collectively show similar trends (Scotchman, 1984), with
the lightest carbon cements occurring in the body
centre and the heaviest in the latest septarian cements.
This suggests that the septarian concretions were
initiated very early, soon after burial below the
sediment-water interface and that their growth
continued over a long period of time, to substantial
burial depths. The septarian nature of these concretions appears due to their very early, pre-compactional
development and reflects their long and often complex
burial histories (Astin, 1986; Astin & Scotchman,
1988).
The timing of septarian cracking, although generally early, appears also to be variable if the isotopic
composition of the first-generation septarian cement
is used as an indicator. Very early cracking and
cementation is indicated by D179.79 where the firstgeneration cements are of similar composition to the
concretion body. In contrast, later septarian cracking
after the cessation of concretion body growth is
indicated by TA and TB while precipitation of
septarian cements concurrent with the cementation of
the body outer regions is suggested in QlS and P30A.
The septarian cements in H22.00 have an isotopic
composition midway between early and late cements
in the concretion body. Therefore from the relative
'age' of the septarian cements, cracking appears to
have occurred over a wide period of time. The
formation of cracks was probably promoted by
overpressuring in the mudstones at shallow depths
during periods of rapid burial (Aston, 1986). Overpressuring can occur in argillaceous sediments due to
high sedimentation rates and low permeability causing
porewaters to be trapped and, consequently, pressure
to exceed hydrostatic pressure. Present-day examples
include the Mississippi delta where overpressuring
exists in argillaceous sediments at burial depths as
shallow as 25-50 m (Bryant et al., 1985). Sedimentation rates are high (5-1 1 m 1000 yr- ') and permeability is low, ranging from
Darcys near-surface to
Darcys at 550 m burial. Furthermore, underconsolidation of the sediments could also result in the
preservation of higher than normal porosities leaving
sufficient void space to be infilled by later marine and
Me zone carbonates.
Cation chemistry
The ICP and microprobe data on the cation composition of the carbonate cements show that the majority
of the concretions shared a common porewater
evolution. The early cements comprise non-ferroan
calcite and the later cements ferroan calcite, following
the general two-stage evolutionary scheme. This
appears typical of many septarian concretions (Lindholm, 1974). Exceptions are samples C134.05 and
D179.95. Compositional trends within typical concretions are illustrated in Fig. 11. The microprobe
analyses of the septarian cements from samples P30B
and D179.95 confirm the cation trends seen in the
relatively coarse sampling for the ICP and isotope
study but show that the cement stratigraphy is very
complex, particularly in the latter case.
In order to interpret changes in porewater chemistry
the relationships of 613C and the Mg/Fe and Mn/Fe
ratios are discussed. There is a complex partitioning
behaviour of the cations (Ca, Mg, Fe and Mn) between
93
the different carbonate mineral phases, particularly
for Ca, Mg and Fe. Fe and Mn have very similar
geochemistries, thus, as suggested by Curtis & Coleman (1986), 6I3C and Mn/Fe ratio are used as the
parameters for interpretation of porewater chemistry
variations.
Calcium
Ca shows no common overall trend, the content
generally being inverse to the amounts of Mg and Fe
substitution in calcite. Sample D179.95 shows a trend
of increasing Ca with heavier 613C (Fig. 12) and
lighter 6 l 8 0 , which, coupled with a relatively high Sr
content, suggests a Ca source from aragonite dissolution.
Magnesium
The majority of samples show an antipathetic relationship with Fe, the molar Mg/Fe ratio decreasing with
time and with 6I3C from the concretion body through
the late septarian cements (Fig. 13). With 6l8O0,the
molar Mg/Fe ratio decreases with heavier values.
The microprobe traverse of the septarian cements
of sample P30B shows that Mg decreases from the
concretion body to the edge of the first-generation
fibrous brown cement but then increases through the
first-generation white cement prior to the hiatus. A
peak is reached in the early part of the secondgeneration white cement before a final gradual
decrease, the final late spar cement having a low Mg
content.
In sample D179.95 Mg and Fe both decrease from
the concretion body to the late-stage septarian cements, the molar Mg/Fe ratio decreasing with heavier
613C (Fig. 14) and with lighter 6"O (Fig. 15). The
microprobe traverse of the septarian in-fill shows a
relatively constant Mg content in the earliest cements,
the second-generation cement being Mg-poor. The
third cement has a similar Mg content to the firstgeneration cement. The late-stage cements in the vein
centre have the lowest Mg content. A good correlation
with the ICP data is seen.
In C134.05 the Mg and Fe content decreases with
increasing calcite relative to ferroan dolomite in the
later cements, the molar Mg/Fe ratio decreasing with
lighter 6I3C (Fig. 16) and 6 l 8 0 .
In general Mg shows a decrease with time and with
heavier hi3C (Fig. 13), suggesting a predominant
seawater source which declines towards the base of
the SR zone, paralleling the reduction in SR zone
I. C. Scotchman
94
0
0
I
Ol
Ij
O-?t?
LO
0
I
0 ,
0 0 ,
sd
B
a-
m
0
95
I . C. Scotchman
96
-
100-
-
1st GENERATION
CONCRETION
SEPTARIAN
CEMENT
BODY
LATE
SEPTARIAN
CEMENTS
CEMENTS
0
0
-
0
m
ae
-
90-
2
GENERATION
SEPTARIAN
CEMENT
-
80
,
-25
I
-20
-15
carbonate. A similar trend is reported in septarian
concretions by Boles et al. (1985) and Thyne & Boles
(1989). In the septarian cements the calcite crystal
habit corresponds to the Mg content (Folk, 1974), the
early Mg-rich cements being fibrous and the later
stage fill being Mg-poor and sparry. The later cements,
particularly the ferroan dolomites of C134.05, have
an increased Mg content which again appears to
decrease with time, suggesting a deep burial source of
Mg such as from mixing with basinal porewaters
where, at temperatures greater than 90°C, the smectite
illitization reaction can supply Mg and Fe (Kahle,
1965; Boles & Franks, 1979; McHargue & Price,
1982).
Iron
The majority of concretions, as typified by P30A,
show an increase in Fe with heavier 613C and lighter
6I8O, reflecting the decreasing influence of sulphate
reduction and the pyrite Fe ‘sink’. The earliest calcites
are non-ferroan but change to ferroan calcite in the
outer margins of the concretion bodies and in the later
septarian cements, reflecting the decreasing importance of pyrite formation.
The increasing input of Fe to the porewater system
in later burial is indicated by the ferroan dolomite of
the body and cements of sample C134.05 from the D
zone. In the latest cements the Fe content decreases
as the calcite to ferroan dolomite ratio increases. The
increase in Fe content in the septarian cements
appears related to the second burial phase, the ferroan
cements of the Kimmeridge Bay concentrations and
-10
-5
3
the ferroan dolomite from Marton suggesting the
influence of more rapid, deeper burial.
Sample D179.95 is an exception as even the earliest
calcite cements are very ferroan with up to 8.9 mol%
Fe. The very early formation of these ferroan calcites
is suggested by the very light 613C and heavy 6”O
values (- 18.6 to -23.3%, and -2.2 to - 1.7X0
respectively), the high Mg content and the highly
septarian nature of the concretion. In particular, the
613C values are much lighter than seen in other
concretions suggesting a very minor marine carbonate
component, indicative of early precipitation. Negligible amounts of pyrite (below the XRD detection
level) are associated with the ferroan calcite, suggesting that sulphate reduction was an unlikely source of
light carbonate. Early formation in the post-oxic FeR
zone (Froelich et al., 1979; Berner, 1981; Coleman,
1985) appears probable. Carbonates of the FeR zone
are characterized by light 613C of -25%,, heavy 6 l 8 0
of about - 1%,and a high Fe content (Coleman, 1985)
and, due to the predominant seawater composition of
the porewaters, a high Mg content is also likely. The
later SR zone appears not to be represented as nonferroan carbonates associated with pyrite are not
evident, the later cements showing an increased input
of marine carbonate probably from the dissolution of
aragonite shell material as evidenced by the high Sr
content. The concretion is located in Bed 4 from the
base of the Kimmeridge Clay which is a shallowwater condensed horizon (Gallois & Cox, 1976), a
depositional environment which probably favoured
FeR zone conditions.
K
15-
14-
13-
MUDSTONE
12CONCRETION BODY
11-
I s t GENERATION SEPTARIAN CEMENT
2nd GENERATION SEPTARIAN CEMENT
L
10-
9-
,
F151.45
yd
0
67m
;
I
6-
5-
4 -
3-
2-
1 -
Fig. 13. Mg/Fe ratio versus 513C(%J
for the septarian concretions excluding D179.95.
97
98
I. C. Scotchman
0.07
0.7
0.06
0.6
%
\
x 0.05
0
+
\
1
3.5 +
\
0
t
U
9
U
2nd 13rd GENERATION
SEPTARIAN
CEMENTS
0.04
d
1s1 GENERATION
SEPTARIAN
CEMENTS
m
Yc 0.03
CONCRETICN
BODY
CEMENTS
0.4 2
AN
1
S
LL
0.3 Z
0.02
0.2
0.01
0.1
-20
-10
-15
-5
6'3c
Fig. 14. Mg/Fe and Mn/Fe molar ratios versus b13C
a0)
for concretion D179.95.
0.08
0.8
0.07
0.7
0.06
0.6
0.05
0.5
+
'x
d
U
4
0%
U
5
2 0.04
SEPTARIAN
m
2ndl3rd GENERATION
Y
s
1st GENERATION
SEPTARIAN
CEMENT
\
0.03
K
5
0.4
CONCRETION
BODY
CEMENTS
m
P
0.3
0.02
0.2
o'oll
I
0.1
,
-12
-11
-10
-9
-6
-7
-6
-5
6l80
Fig. 15. Mg/Fe and Mn/Fe molar ratios versus 6'*0
a,,)
for concretion D179.95.
I
1
-4
-3
-2
-1
2
U
99
0.14
1
Cc
”
F
c
3
2
1
Fig. 16. Molar Mg/Fe ratio versus 6I3C Go)for nodule C134.05.
Manganese
Mn and Fe tend to be closely related in the majority
of concretions, with some showing little variation in
molar Mn/Fe ratio with 613C(Fig. 17). Other samples
show a general decrease in Mn/Fe ratio from the early
concretion body cements to the later septarian and
body cements. Variations are shown by sample P30B
where the Mn content of the early second-generation
carbonate adjacent to the hiatus is high while the Fe
content is gradually increasing. In the later calcites
the Mn content rapidly reduces to a low level due to
depletion of the porewaters while Fe increases and
remains at a high plateau level. Thus the porewater
system in the calcites initially favoured partition of
Mn into the calcites rather than Fe, suggesting
precipitation just below the redoxcline (Curtis &
Coleman, 1986), with conditions then changing to
favour Fe precipitation.
Strontium
The Sr content of the calcites tends to increase through
the septarian cements, suggesting that the dissolution
of mdrine carbonates, particularly aragonite from
shell material, was a source of carbonate in the later
cements. In addition, coccoliths are a further possible
source of Sr as their content is 3-5 times that of the
inorganic calcite re-precipitated from solution (Baker,
Gieskes & Elderfield, 1982; Elderfield et al., 1982).
Controls on concretion formation
The Kimmeridge Clay concretions are confined to
specific horizons within sequences deposited under
low to moderate sedimentation rates. In particular,
the concretions occur commonly as distinct horizons
in the calcareous mudstones of the Wash area where
the bed thicknesses indicate a relatively low sedimentation rate. As noted previously, the bands of
concretions often appear to be related to the development of hiatuses in the sequence, the development of
these concretionary bands apparently requiring a lull
in sedimentation as suggested by Raiswell’(1987).
This hypothesis could explain the confinement of
calcareous concretions to specific parts of the sequence
at Kimmeridge Bay which were deposited under lower
sedimentation rates, notably the Rotunda Nodules at
the top of the sequence which are related to the
development of a depositional hiatus. Otherwise in
the sequence, where deposition was under high
sedimentation rates, concretions are relatively rare,
the dominant diagenetic carbonates being the Me-D
100
I. C.Scotchman
613C
Fig. 17. Variation of molar Mn/Fe ratio with 613CGo)for the septarian concretions (excluding D179.95).
zone dolomitic cementstone bands (Irwin et al., 1977).
Where concretions do occur in the sequence they
comprise pyrite/Fe-calcite nodules which did not form
in distinct horizons.
Sedimentation rate appears to have a strong control
on the growth of the concretions (Scotchman, 1989)
(Fig. 18). Bands of micritic calcite concretions were
formed in the SR zone in areas with moderate to low
sedimentation rates, the bands growing during lulls in
sedimentation along favourable horizons. In contrast,
under high sedimentation rates, Me-D zone carbonates predominate, such as at Kimmeridge Bay. Again
these formed in distinct horizons which were probably
related to periods with lower sedimentation rates. At
Kimmeridge Bay, concretions occur only with pyrite/
Fe-calcite bodies, their growth spanning the SR-Me
zone transition. Calcite concretions which can be
related to the development of hiatuses are not seen
until the Rotunda Nodules in the shallow-water, lowsedimentation-rate mudstones at the top of the
sequence.
The ferroan dolomite/calcite concretion from the
Cleveland Basin at Marton appears to have grown in
the upper part of the D zone in very organic-rich
mudstones deposited under high sedimentation rates,
probably in a fault-controlled basin. Septarian development again appears to be related to rapid burial.
CONCRETION AUTHIGENESIS:
A SYNTHESIS
In the swell areas, concretion growth predominantly
began in the SR zone and appears to have been
confined to specific horizons within the mudrock
sequence. This was perhaps due to sedimentological
variations such as higher porosity and permeability or
increased primary carbonate content within these
layers creating suitable sites for concretion nucleation.
Study of the mudstones above and below these
horizons indicates that they were the source of
carbonate with little diagenetic carbonate precipitation while the concretion-bearing horizons acted as
'sinks' (Scotchman, 1984, 1989), the concretions
growing during lulls in sedimentation as proposed by
Raiswell(l987).
101
b13C
-20
-10
0
OXlC
1 FeR
SR
SR-Me
RANSlTlOh
%
c
t
cU
z
I
u1
c
t?
Me
2
D
CATAGENIC
ZONE
I
BASIN
SUB-AASIN
SH~LF
SW'ELL
Fig. 18. Model for the formation of concretions in the Kimmeridge Clay Formation illustrating the influence of sedimentation
rate on the 6I3C
a,,)composition and the development of the diagenetic zones.
The concretions developed in the SR zone with the
formation of the body of micritic calcite, largely by
infilling of pore space in the host mudstone although
displacive growth is often apparent. Overpressuring
resulting from rapid burial in the late Kimmeridgian
allowed septarian fracturing of the concretion body to
occur, the fractures being infilled by calcite cements
of the same composition as was forming the outer part
of the concretion body. By this time, sulphate
reduction was waning as the main carbonate source,
but nevertheless dominated the carbonate cements
precipitated during burial through the transition into
the Me zone below. Dissolution of marine carbonate,
mainly from shell material, provided an increasingly
significant carbonate source during later burial,
In easternEngland, where the septarian concretions
are best developed, uplift during the end-Jurassic
Cimmerian tectonic phase appears to have caused a
hiatus in cementation. A second period of rapid burial
in the Cretaceous caused overpressuring of the
mudstones, resulting in further septarian fracturing of
the concretions. This was closely followed by a second
phase of carbonate cement with a ferroan character,
precipitated from porewaters of a mixed basinal,
marine and meteoric origin. Cementation ceased with
uplift in the Tertiary, often leaving uncemented voids
in the septaria (Astin & Scotchman, 1988).
Variations in concretion authigenesis are exhibited
by the non-septarian, pyrite/ferroan calcite and
ferroan dolomite concretions. Growth of the non-
102
'
I . C. Scotchman
septarian calcite concretions appears to have been
initiated later than the septarian concretions, based
on isotopic data and the lack of septaria. By
comparison with the septarian concretions, this
occurred deeper in the SR zone: subsequent development followed similar trends. Their later development
appears to have precluded the formation of septaria,
probably as the effects of rapid burial, such as a buildup of differential stress, had been dissipated in the
host mudrocks prior to concretion growth. Growth
continued through, and appears to have ended in, the
SR zone as indicated by the relatively heavier 613C
and lighter 6"O compositions and non-ferroan nature
of the latest concretion body cements. As with the
similar septarian concretions, the calcite non-septarian concretions developed under regimes of low
sedimentation rate in calcareous mudstones.
The pyrite/ferroan calcite nodules and concretions
grew in organic-rich mudstones (TOC 2 5-6 wt%)
under high sedimentation rates. Development began
as pyrite concretions in the SR zone where conditions
appear to have been too acidic, due to intense sulphate
reduction, to allow precipitation of carbonate (as
indicated by the absence of non-ferroan calcite).
Sufficient alkalinity appears to have developed in the
transition to the Me zone beneath the SR zone,
probably by Fe reduction, which allowed the precipitation of ferroan calcite with less negative 613C values
and light 6I8O values that cemented the pyritic
concretion body. Cements show little isotopic evolution, the concretion rim calcites being slightly lighter
in 6I3C, perhaps suggesting a minor methane oxidation zone component. Concretion growth appears to
have ended before Me zone conditions were attained.
The ferroan dolomite/calcite concretion from the
Cleveland Basin at Marton appears to be unique and
to have formed during deep burial. The concretion
occurs in a silty, very organic-rich mudstone
(TOC=9.4 wt%; Scotchman, 1987) and grew under
conditions of high sedimentation rate and rapid
burial, in a fault-controlled basin. The trend of
successive lightening carbon and oxygen seen through
the concretion body and septarian fracture cements
indicates growth in the Me-D zone transition. Here
the influence of light C 0 2 from decarboxylation
reactions became greater as the supply of heavy C 0 2
from the Me zone decreased (Irwin, 1980; Hennessy
& Knauth, 1985; Scotchman, 1988).
Very early initiation of concretion growth within
the post-oxic FeR zone just below the sediment-water
interface is indicated by the light carbon, heavy
oxygen ferroan calcites of the concretion body of
D179.95. The lack of pyrite and the ferroan composition of the calcite indicate that sulphate reduction was
not responsible for the light carbon isotopic composition. The complex stratigraphy of ferroan calcite
cement within two generations of septarian cracks
indicates a relatively complex burial history compared
to the other septarian concretions. The concretion,
from Bed 4 at the base of the Lower Kimmeridge
Clay, appears to have grown entirely in the FeR zone
and continued into the Me zone transition. There is
no pyrite or non-ferroan calcite present to indicate the
intermediate development of sulphate reduction zone
conditions, although the concretion shows an isotopic
evolution trend common to the other septarian
concretions. This is suggestive of a very low sedimentation rate which is further confirmed by the condensed
and winnowed nature of the basal Lower Kimmeridge
Clay beds, which contain numerous hiatuses with
well-developed phosphatic nodules and bored surfaces. However, this evidence for a very low sedimentation rate conflicts with the proposed model for the
formation of septaria which requires the development
of overpressuring due to rapid burial. At present this
problem remains unresolved ; one explanation may be
that the calculated sedimentation rates are averages
and the thin sediments resulted from a long period of
reworking and erosion which followed a short period
of rapid sedimentation during which the concretions
grew.
CONCLUSIONS
The following conclusions can be drawn from the
geochemical study of Kimmeridge Clay Formation
concretions.
Concretion development in the Kimmeridge
Clay Formation is strongly influenced by the
sedimentation rate, the burial rate and the degree
of organic matter preservation during burial.
Under a low sedimentation rate with poor organic
matter preservation, large calcite concretions
develop, whereas higher sedimentation rates
with higher levels of organic preservation are
typified by the pyrite/Fe-calcite concretions.
Dolomites and Fe-dolomites occur only in organic-rich mudstones deposited under high sedimentation rates.
The concretions follow a general two-stage
isotopic evolutionary trend which appears related
to burial history and to the development of
sediment overpressuring during periods of rapid
burial.
Late-stage porewaters precipitating the ferroan
light carbon/light oxygen cements appear to be
predominantly of marine or basinal origin with
a meteoric component, as demonstrated by
Hudson (1978) for similar composition cements
in Oxford Clay concretions.
Systematic compositional variations both within
and between the concretions indicate common
porewater evolutionary trends across the Kimmeridge Clay depositional basin, which are
similar to those reported for other organic-rich
mudrock sequences (Hudson, 1978; Coleman &
Raiswell, 1981; Curtis & Coleman, 1986).
The development of septaria only occurs in early
formed concretions and appears related to rapid
burial and overpressuring (Astin & Scotchman,
1988).
The majority of concretions were initiated and
grew in the SR zone with growth continuing into
the SR-Me transition zone beneath.
The relative timing of initiation of concretion
growth is variable within the SR zone, ranging
from very early in the upper part of the zone to
late at the base of the zone.
(8) The large micritic calcite concretions appear to
be restricted to calcareous mudstone lithologies,
whereas the pyrite/Fe-calcite concretions occur
in the organic-rich mudstones.
(9) Late-stage ferroan dolomites appear restricted to
the D zone concretion.
(10) Development of the complex septarian concretion D179.79 appears to have occurred solely in
the FeR zone with no later SR zone influence, as
evidenced by isotopic and carbonate chemistry.
Also, stratigraphic evidence suggests growth in
a condensed horizon with a minimal sedimentation rate. Thus this concretion, with its complex
history of septarian development and similar
isotopic evolution to the other septarian concretions, poses a problem as it does not readily
appear to support the hypothesis relating septarian crack formation to rapid burial and overpressuring.
ACKNOWLEDGMENTS
Profs C . D. Curtis and M. L. Coleman are thanked
for their support and guidance during this work,
which was carried out under the tenure of an NERC
103
studentship. Dr J. N. Walsh (now of Royal Holloway
and Bedford College) is thanked for his assistance
with the ICP analyses. Microprobe analyses were
provided by Prof. C. D. Curtis and Dr T. R. Astin.
Amoco is thanked for providing typing and draughting
services. Prof. M. L. Coleman and Dr T. R. Astin are
thanked for their comments on an earlier version of
this paper, and Prof. C. D. Curtis and Dr J . D.
Hudson provided thought-provoking reviews.
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(Manuscript received 16 December 1988; revision received 13 June 1990)

The geochemistry of concretions from the Kimmeridge Clay

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