The diagenetic history of some septarian concretions from the

Sedimentology (1988) 35,349-368
The diagenetic history of some septarian concretions from the Kimmeridge Clay,
England
T. R. ASTIN
Department of Geology, University of Reading, Whiteknights, PO Box 227, Reading RG6 2AB
I. C. SCOTCHMAN
Department of Geology, University of Shefield, Beaumont Building, Brookhill, Shefield S3 7HF*
ABSTRACT
Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising
anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite-rich
zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with
fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased.
Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite
microspar crystals were enlarged.
The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about
550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with
septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200
and 500 m during the second burial.
Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of
which is marked by the pyrite-enriched zone. Dissolving shells, and possibly minor methanogenic
carbonate, slowly diluted sulphate reduction-zone carbonate during deeper burial. During early concretion
growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and
Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg
decreased as calcite precipitated, implying relatively closed systems for these elements.
Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge
Clay became overpressured during the later stages of both burials.
INTRODUCTION
Carbonate concretions are often the most obvious
diagenetic feature of mudstone formations and contain
important information about the diagenetic evolution
of the surrounding mudstones. Textural information
has been used to constrain the timing of concretion
growth by analysis of the degree of compaction
preserved in the concretion, either by comparison
with the adjacent shale (Weeks, 1957; Raiswell, 1971)
or by estimating porosity at the time of formation
* Present Address: Amoco (UK) Exploration Co., Amoco
House, 1 Stephen Street, London W1P 2AU.
from the amount of cement present (Raiswell, 1971;
Oertel& Curtis, 1972; Gautier, 1982).
Geochemical information has been extensively used
to infer the nature of diagenetic processes, their timing
and the pore-water chemistry of host shales during
burial diagenesis. Stable isotope information (from
carbon, oxygen and sulphur) has been used to infer
cement sources, microbial processes and diagenetic
temperatures within sediments (e.g. Hudson & Friedman, 1976; Irwin, Curtis & Coleman, 1977; Hudson,
1978 ;Coleman & Raiswell, 1981 ;Gautier & Claypool,
1984).Abundance of phases such as pyrite can confirm
349
T. R.Astin and I. C . Scotchman
350
the diagenetic mechanism of concretion formation
(Raiswell, 1976),while variation in cation composition
of calcite cement (Boles, Landis & Dale, 1985) has
been used to constrain pore-water chemistries.
Stages of concretion growth and the depths of
diagenetic zones derived from these studies are
generally relative only. This paper integrates textural
and geochemical data from large, texturally complicated septarian concretions from one locality in the
Upper Jurassic Kimmeridge Clay of England which
have a well-constrained burial history. This combination of information gives more precise data on depth
and timing of diagenetic reactions in the host shales.
A number of areas of interpretation of concretion
texture and chemistry are re-examined in the study.
These include the extent to which concretion-matrix
cements preserve primary porosity or contain secondarily modified or displacive textures, the depth range
over which concretions form and particular diagenetic
zones, especially the sulphate reduction zone, are
important and the links between concretion textures
and fluid pressure history in the host shales.
ROSWELL PIT, ELY
z c.
31
8.
32
fl
u
CRETACEOUS
1
1-1
)
Fissille mudsrone
Mossive mudslone
a
Septorion concretions
Shells
3 -L
Siltstone
Sample location
The concretions studied come from Roswell Pit, Ely
(British National Grid Reference TL 551 806), where
about 16m of Lower Kimmeridge Clay is present
(Roberts, 1892) (Fig. 1). The lowest part of the pit lies
within the Aulacostephanoides mutabilis zone, while
most of the section lies within the Aulacostephanus
eudoxus zone (Arkell, 1933). Concretions up to 1.5 m
across and 0.7 m thick occur at two horizons about
4 m apart (Fig. 1). They are variably spaced along the
quarry face, from almost coalescing to 20m apart.
The exposure is not extensive enough to assess their
distribution on bedding planes. Using the bed scheme
of Gallois & Cox (1976) the section extends from Bed
18 to Bed 32, with concretions occurring in beds 29
and 30 (R. W. Gallois, private communication).
CONCRETION DESCRIPTION
Field relationships
The concretions in the two horizons are very similar
and at outcrop are fractured and break up into large
blocks allowing examination of the internal structure
(Fig. 2A, B). They are typically flattened discs about
1.5 m across and 0.5 m thick on average, comprising
pale grey, fine-grained carbonate. Their most obvious
Fig. 1. Location map and stratigraphy of the Roswell Pit, Ely
(TL 551 806). The stratigraphic column is simplified from
R. W. Gallois (private communication), with bed numbers
corresponding to the scheme of Gallois &Cox (1976).
features are large septarian cavities lined with brown,
followed by white, calcite spar. Completely filled
septarian fractures link the central cavities and extend
from them towards the margins of concretions.
Although complex in detail, the major fractures
have a simple overall geometry, with dominantly
vertical fractures -in vertical section (Fig. 2A) and a
polygonal pattern in horizontal section along the
middle of concretions (Fig. 2B). There are two
generations of cracks, with later cracks filled with
white calcite cross-cutting earlier cracks filled with
brown calcite.
Slabbed sections
Half of one large concretion from the upper layer was
broken up and taken for laboratory analysis. The
samples were slabbed vertically to give an approximate cross section through the middle of a concretion
Kimmeridge Clay concretions
351
extensively fractured, with the formation of open
cavities. Cavities lined with brown calcite and veins
forming the first set of fractures do not extend beyond
the compact centres. White calcite lines these cavities
and fills a second set of fractures, some of which reopened along the lines of the earlier cracks, and some
of which cross-cut them. These later veins extend into
the bioturbated outer parts of the concretions, and
some isolated second episode veins occur at the ends
of the concretion (Figs 3, 4A, B). The cracks have a
variety of features, including. cross-cutting relationships with displacements, tapering and forking terminations and en echelon patterns, showing that they
are tensile, extensional veins formed after the concretion matrix (Astin, 1987).
Thin sections
Seventeen thin sections were made, selected to include
a complete traverse from the centre to the outside of
the concretion matrix and critical features such as
fossils and different fracture sets identified after
slabbing. Thin sections were stained with alizarin red
and potassium ferricyanide solution for microscopic
examination, and subsequently polished for cathodoluminescence examination.
Fig. 2. (A) A large concretion at outcrop shows mainly
vertical septarian cracks in vertical section. These cracks
and cavities are lined with brown followed by white calcite.
Florin coin is 25 mm diameter. (B) Horizontal section
through middle of part of a concretion showing a polygonal
crack pattern. Two generations of fractures are visible (I, 2).
Large pieces of replaced ammonite shell (am) are present on
the surface. Florin coin for scale.
(Fig. 3). The inner part of this concretion, extending
to about half the diameter, has a matrix of compact,
uniform blue-grey carbonate. The outer part is softer,
porous, and mottled, revealing extensive bioturbation
through which larger shells are often broken and
disturbed. Large ammonites are preserved as lines of
separated shell fragments suggesting that they were
bored, or burrowed through, before preservation in
the concretions. The outer chambers of large ammonites are relatively uncompacted in the inner part of
the concretion, but are strongly compacted in the
outer part, showing the latter to have formed after
some burial (Figs 3,4A).
The compact centres of concretions have been
Concretion matrix
The central compact zone is mostly made up of
anhedral calcite crystals 15-25 pm across with no
obvious preferred orientation (Fig. 5A). Locally, near
the centre, there are clots of coarser spar up to 100 km
across which grade into the finer matrix. Clay flakes
are separated and mostly located along grain boundaries. Locally, crystal clusters radiate from pyrite
showing calcite precipitation occurred after pyrite
formation. Towards the edge of the compact part of
the concretion, crystal size increases up to about 3050ym. The transition to the outer part of the
concretion occurs over about 10 mm with a change to
orientated, elongate microspar and an increase in
crystal size to about 70-80pm long and 25-40pm
wide. Fibrous microspar continues to the concretion
margin, with the degree of preferred orientation
increasing outward. Here, original sediment is present
as discrete, bedding-parallel domains of brown organic matter and clays located between crystals, with
some isolated clay flakes incorporated into crystals
(Fig. 5B). The abundance and degree of compaction
of clay rich domains increases outwards. In the
outermost 30-40 mm, the matrix is porous, with the
352
T. R . Astin and I. C. Scotchman
COMPACT
LATE
AMMONITE
CENTRE
SHELL
10 em
SEPTARIAN
FRACTURES
-
PYRITE
AYHON I T €
RICH
I
FIBROUS
MA
Fig. 3. Internal structure of one of the concretions from bed 30. The concretion broke into large blocks in the field and the
internal structure was constructed from cut surfaces through these blocks.
largest pores about the same size as the spar crystals
(Fig. 5C). Next to pores the clay minerals are less
compacted, in contrast to domains between crystals
which are strongly compacted, suggesting that the
spar crystals forced the original clay sediment apart.
However, some of the compaction of the clay domains
could have been caused by cement crystal growth,
reducing the overall amount of expansion caused by
cement growth. The amount of cement present in the
outer parts is about 80% of the total. If the present
porosity of the clay domains were known, limits could
be put on the amount of expansion which occurred
during cementation. Assuming no porosity remains in
the clay domains, an initial clay sediment porosity of
75%, and that all compaction occurred by crystal
growth, gives a minimum expansion of about 20%
during fibrous microspar growth. Further evidence of
this displacive growth comes from included fossils.
Some microfossils, especially fish scales, are rotated
parallel to the microspar elongation direction and
perpendicular to the bedding, and locally, larger fossils
are broken and the pieces displaced by microspar
(Fig. 5A, D). Thus the bioturbation texture preserved
in this part of the concretion (Fig. 4A) was first
compacted before, then expanded during, fibrous
microspar growth.
Orientated microspar grew perpendicular to the
edge of the concretion rather than to the bedding, so
that at concretion ends the preferred orientation is
horizontal, though not as strongly developed as
elsewhere. Shells have been preferred sites for crystal
nucleation, leading to a greater concentration of
cement adjacent to them and local weakening of the
preferred orientation of the spar.
Staining shows that nearly all of the microspar is
non-ferroan calcite. In the outermost 5 mm, microspar
crystals also have thin, ferroan calcite overgrowths on
the ends of the elongate crystals. Elsewhere, small
patches of ferroan calcite have filled in occasional
voids between the non-ferroan calcite microspar.
Cathodoluminescence (CL) shows that virtually all
the microspar cement crystals are unzoned (Fig. SA),
with luminescence intensity being dull in concretion
centres, becoming increasingly bright outwards. The
ferroan calcite additions to the outermost crystals
show up as two thin zones of dull followed by bright
cement (Fig. 5N).
Pyrite distribution
Small patches of pyrite, including both obvious
framboids and more randomly shaped aggregates, are
present scattered throughout the matrix. Pyrite is
markedly more abundant in the outer part of the
compact inner matrix, and extending along the
bedding away from this early stage of concretion
growth, where it was later incorporated into the
fibrous part of the concretion (Fig. 3). In this outer
zone, most shells (mainly oysters and ammonite shells)
are extensively pyritized (Fig. 5C). Pyritization therefore occurred after the centre of the concretion formed
Kimmeridge Clay concretions
Fig. 4. (A) A cut block from the middle to the edge of the
concretion. The inner matrix is compact (cornp) and the
outer matrix preserves extensive bioturbation (biot). A large
ammonite (am) is present with uncrushed body chamber
which has been burrowed (bored?) through to break the shell
into pieces. Scale is 50 mm. (B) Detail of (A), showing the
two crack generations. The first generation (1) has numerous
microveins extending through the ammonite chamber. The
ammonite shell (am) has helped control the location of large
cracks of this generation. The second generation crack (2) is
simpler and cross-cuts first generation cracks (arrowed),
ammonite shell, and matrix. Scale is 10 mrn.
but before septarian cracks formed and the fibrous
outer microspar grew.
Shell preservation textures
Apart from local pyritization, shells are preserved as
calcite. Originally calcitic fossils, such as worm tubes,
oysters and bryozoans and calcitic layers of originally
bi-mineralic bivalves, are preserved unaltered, often
with their integral organic components preserved.
353
Originally aragonitic fossils, such as gastropods,
ammonites, some bivalves and bivalve shell layers,
have been preserved mainly as casts, having dissolved
after microspar formation to leave voids which were
filled by calcite spar. Some of the cavities derived
from large pieces of ammonite shell remain unfilled to
the present day. Dissolution cavities were often
incorporated into the septarian crack network. Some
ammonite and bivalve shell layers, near the concretion
margin only, appear to be neomorphosed, as suggested
by complicated, sutured boundaries between the
calcite crystals now forming them.
When ammoniteshellsdissolved, the organic matrix
(presumably proteinaceous tissue) often remained and
contracted, sometimes tearing and splitting in the
process (Fig. 5F). As voids were cemented by calcite,
pieces of organic matric locally retarded the growth of
earlier cement crystals by interference, before being
incorporated into later cement crystals, giving rise to
quite complex textures in replaced ammonite shells
(Fig. 5E).
Staining shows that ammonite dissolution voids in
the inner and middle parts of the concretion were first
partly infilled with non-ferroan calcite, the remainder
being filled with ferroan calcite. Near to the concretion
margins ammonites and aragonitic layers in bivalves
are replaced solely by ferroan calcite. CL shows
complicated zoning within ammonite void fills. The
earliest spar is usually very bright, with local very
bright additions, then moderate to dull cement
following (Fig. 5G).
Septarian cements
The septarian cavities in concretion centres have
calcite spar fills which show the fullest cement
stratigraphy present in the concretions. Hand specimens show brown calcite overlain by two distinct
zones of white calcite, the inner zone being a denser
white than the outer part. There is a hiatus within this
cement sequence marked by a well defined line of
crystal terminations (Fig. 5H),showing that there
were two episodes of cement growth separated by a
time gap. The hiatus comes within the white calcite,
the inner, denser white area ending the first cycle of
cement growth in the cavities.
Staining shows that brown calcite is non-ferroan,
becoming ferroan into the white calcite of the first
generation. The later white calcite is also ferroan,
initially rich in inclusions suggesting early rapid
growth (Fig. 5H).Near to crystal terminations there
Fig. 5. (A) Anhedral calcite forming the inner part of the consretion seen by cathodoluminescence, surrounding a worm tube.
The calcitic worm tube has been fractured by several stages of first generation veins. Mircoveins (mv) have also modified some
grain boundaries of the microspar. All scales in Fig. 5 are 200 pm. (B) Fibrous microspar from the outer part of the concretion.
Clay-rich domains are now separated between the variable to elongate microspar crystals. (C) Pyritized oyster shell from the
transition zone between inner, compact (on the right) and outer fibrous (on the left) microspar. Veins cross-cutting the shell
post-date pyritization, and have no clear continuation into the matrix. (D) CL view of same oyster shell as in (C), showing
veins to be displacive, with veins through the fossil becoming an anastomosing vein network in the adjacent matrix. (E)
Ammonite preserved by dissolution and infilling with calcite spar. The organic component (org) of the shell remained after
dissolution and was incorporated into the later cement. (F) Cement sequence from an ammonite mould seen in CL. First
generation non-ferroan cements vary from early dark (la), to moderate (Ib) to bright (2). Second generation ferroan cement
(3) is dark. The CL colour of the earliest cement is similar to the microspar showing dissolution occurred about the time of
concretion growth. ( G )The hiatus between first and second generation cements in a septarian cavity cement sequence. (H)
Shows the boundary between microspar and septarian fracture, and a first generation septarian vein synchronous with early
septarian cement, and a second generation vein cross-cutting earlier cements and microspar. (J) Anastomosing network of
first generation veins within the inner part of the concretion. (K) Part of (J) seen in CL showing many more microveins than
are visible in ordinary light. The brightness of the veins shows that they post-date the early microspar, forming when the outer
parts of the concretion were forming. (L) Vein formed within 50 mm of concretion margin. The vein is made up of fibrous
calcite similar to the microspar, but lacking sediment domains. (M) CL view of part of (L) showing that the vein is synchronous
with microspar growth, and was later cross-cut by second generation veins parallel to the concretion margin. (N) CL view of
microspar close to concretion margin. Overgrowths (arrowed) of ferroan calcite have grown preferentially on faces parallel to
the margin, enhancing the fibrous crystal shape. The larger black areas are non-luminescent shells with smaller black areas
being clay sediment domains and pores. (0)Part of an infilled ammonite mould showing dark and light first generation
cements (1 a, 1b), and dark second generation cement (2), all with associated microveins cutting through the matrix.
356
T. R . Astin and I . C. Scotchman
is sometimes a zone of less strongly ferroan calcite.
CL shows that the early cement is dull, becoming
brighter outwards and then becomes very dull in the
ferroan zone. There are no sharp zone-boundaries.
The initial increase in brightness correlates with that
seen in the microspar cements (Table 1). The second
cycle cements are initially moderate to dull with a
streaky pattern parallel to the growth direction
possibly related to the inclusions orientated this way.
Outwards, the CL response becomes very dull. Near
the edge, there is a thin brighter zone (corresponding
to the less ferroan zone) and finally a sull zone (Table
1).
Septarian cracks
Cement textures in first generation veins vary from
coarsening outward textures suggestive of void filling,
to fibrous textures suggesting cement growth keeping
pace with vein widening. Marginal crystals in veins
are typically enlargements of microspar crystals so
that new crystal nucleation did not take place.
The microspar matrix in the concretion centres is
often densely cross-cut by first-generation veins defined by clear spar crystals only a little bigger than the
microspar matrix cement, and therefore quite difficult
to see in hand specimen (Fig. 51). Such veins terminate
and interlink larger fractures. Their number decreases
outwards, extending little further than the major
septarian veins. Their presence means that sampling
of ‘pure’ microspar in concretion centres for isotope
determination is difficult (see below).
At concretion margins a few irregular veins of nonferroan calcite occur, made of a mosaic of elongate
microspar crystals similar in size and shape to adjacent
matrix cement, but lacking sediment inclusions (Fig.
SK), showing that they formed at the same time as the
adjacent matrix. These veins overcome a potential
space problem caused by enlargement of the concretion circumference during growth.
Second-generation veins mostly have fibrous fills,
showing that they did not become voids, the ratio of
crack dilation to cementation being lower than for the
first generation. Only the large central cavities left
from the first cracking episode remained open at this
time. This set of veins have much smaller dilations
and simpler terminations than the first set, with few
associated microveins. Some small veins formed close
to and parallel to the concretion margin (Fig. 5L) at
this time.
CL reveals more complicated patterns of veining
than stained thin sections. First generation veins vary
in luminescence from dull to bright showing that they
formed through a series of cracking and annealing
events extending over much of the time of concretion
growth and non-ferroan cement formation in central
cavities (Table 1).
Fine hairline cracks, seen only by CL, are common
in the microspar matrix, especially close to the
concretion centre (Fig. SA, M) zig-zagging around the
anhedral spar crystals, adding thin zones to particular
faces. Thus much of the anhedral mosaic fabric of the
concretion centre has been partly modified with many
grain-boundary shapes being controlled by microcracks.
Deformation of the inner matrix must have been
considerable to accommodate the large dilations of
the inner cavities which account for between 10 and
20% by volume of the non-fibrous part of the
concretion. This was achieved by formation of both
larger veins and hairline veining, and probably by
local dissolution of matrix spar as well, though
evidence for this is not present. Any local dissolution
along grain boundaries would not be apparent in CL,
but would have further modified microspar-crystal
shape.
Displaced shell fragments
Fossils are commonly broken and separated, the gaps
being filled in with clear calcite, similar to the
microspar cement. Shell separation is often by 1001000 pm across each break, without obvious deformation of the adjacent matrix (Fig. SD).
CL shows that these fabrics are not due to
replacement, but to displacement by veins. These are
mostly of the first generation, often with complex
zonation. Strain associated with single wide veins
across fossils is distributed by large numbers of
hairline veins in adjacent microspar without noticeably affecting crystal size, and giving a ‘bow-tie’ type
of pattern (Fig. 5E).
Sequence of events in concretion growth
Staining and CL response of the microspar and void
filling cements allow correlation of concretion growth,
timing of shell replacements, and septarian fracturing
(Table I). In summary, initially a compact concretion
grew, about half the diameter of the present one.
Towards the end of compact concretion growth, shells
and sediment at the concretion horizon were heavily
pyritized. Concretion growth continued, the matrix
Dull
Dull
Moderote
Dull
Very
(Local Bright
Zone)
Moderale
Bright
Dull
:ATHOD0 .UMINESCENCE
tESPONSE
0
Fe
2%
J
J
2%
-
Mn
CATION CONCENTRATIONS
-5
10
-5
I'
ISOTOPES
Central
Matrix
MTUS
TEXTURAL
EVOLUTION
Septarion Central
Ammonite
Fractures Cavity
Cavity
I
hergrowths
On Outer
Microspar
Fibrous
Microspar
Iln_
Fractures
Cavities
-M-i_crospa
_ _ _r_ _ _ _ _
'yrite Zone
- - - ------- Central
'I 1
-10
6'3C
STABLE
Table 1. Summary of concretion cement stratigraphy, textures and interpretation
c. I - 5 m
Reduction
HISTORY
Develops
Maximum Burial Under
The Chalk c . 5 0 0 m .
Overpressure
Early Cretaceous Uplift
c. 1 3 0 m
Kimmeridge Clay
Maximum Burial by
Burial
Overpressure Develops
Over
c. lOOm of
Zone
Sulphate
BURIAL
T. R . Astin and I. C. Scotchman
358
becoming fibrous, while aragonitic shells within the
concretiondissolved. At this stage septarian fracturing
of the inner, compact matrix started, fracturing
continuing throughout the rest of concretion growth.
The inner fractures dilated strongly becoming cavities
lined with brown calcite of the same age as the outer
matrix. After concretion growth and septarian fracturing ceased, cement continued to form in shell and
septarian cavities. Cementation then stopped for a
while.
A second set of septarian fractures then formed,
extending from central cavities into the fibrous outer
microspar, synchronously with the start of renewed
calcite cementation. Small additions were made to the
outermost microspar crystals at this time. Again, after
fracturing ceased, some calcite continued to precipitate in cavities. This sequence can be related to the
burial history of the concretions.
BURIAL HISTORY
The burial history of the Ely concretions is fairly well
constrained (Astin, 1986).Sands occur unconformably
on the Kimmeridge Clay at the top of the Roswell pit
(Fig. l), which are broadly of ‘Lower Greensand’ age
(Roberts, 1892), between the Late Jurassic (Volgian)
and the Early Cretaceous (Aptian). R. W. Gallois
(private communication) suggests that they are of
Volgian age. Higher Kimmeridge Clay zones than
occur at Ely are preserved to the north (Gallois &
Cox, 1976) recording the Late Jurassic burial. These
beds must have been eroded, bringing the concretions
close to the surface where they stayed until the MidCretaceous when burial took place in the Late
Cretaceous under the Gault and Chalk (Fig. 6A, B).
Astin (1986) has shown that septarian crack
formation is favoured by higher than normal porefluid pressure and lower than normal horizontal stress,
both of which are promoted during active sedimentation and burial loading. This implies that the first
generation of fractures formed and dilated during the
Late Jurassic burial and the second generation formed
during the Late Cretaceous burial (Table 1). The
smaller crack dilation associated with the second
burial suggests that overpressure development was
less pronounced than in the earlier burial because the
Kimmeridge Clay was already partly compacted.
During uplift, temperature decreases favouring
carbonate undersaturation although COz solubility
may also decrease, favouring precipitation. The likely
overall effect is a break in calcite growth, which can
be correlated with the hiatus seen between the first
and second generation cements in cavities. Therefore,
the first-generation ferroan cements formed in cavities
after the first generation of fractures were formed,
probably close to maximum Late Jurassic burial. The
ferroan cements precipitated after the second fracturing episode in the central cavities could have formed
any time between the latest Cretaceous and the present
day, but cement formation was probably slow or
absent during the Tertiary uplift.
The temperature at the concretion horizon over
time was modelled (by N. Wilson) for comparison
with oxygen isotopic data from the carbonates (see
below). The thermal gradient in a simple stretched
crust model of basin formation is related to the timing
and amount of stretching, and the thermal conductivity of the sediments formed (McKenzie, 1978). The
subsidence history for Ely (Fig. 6) was used to derive
a stretching factor, and when that stretching occurred.
Variation with time of the heat flux into the base of
the sediments can then be estimated from the presentday heat flux in the area and the stretching factor.
This heat flux and the assumed surface temperature
and the thermal conductivities of the lithologies
present are used to calculate the thermal gradient.
The shape of the temperature curve through time is
mainly controlled by the thicknesses and thermal
conductivities of the accumulated sediments. The
main control on the absolute temperature reached at
any time is the assumed temperature at the sediment
surface. These were assumed to be 18°C in the
Mesozoic and 12°C in the Tertiary (Fig. 6C) which
seem realistic and give a good fit to the isotope data
(see below). Slight variations in these assumptions do
not greatly alter the resulting temperature/time diagram for the concretion horizon (Fig. 6C). Only if
there was a ‘thermal event’ with a dramatic increase
in heat flow at some time during the Mesozoic could
the actual temperatures have been much different.
CONCRETION CHEMISTRY
Cation chemistry of calcite
Semi-quantitative analyses and trends in calcite
composition were obtained from the polished thin
sections using a LINK EDX on a JEOL 840 SEM.
Magnesium, iron and manganese profiles were measured across both a central cavity cement sequence and
the microspar from centre to margin (Fig. 7).
Kimmeridge Clay concretions
A
150
1
]
L'
BURIAL
100
0 Ma
50
\
HISTORY
359
B
El
0 - 0
500~11
MAAST.
SANTON.
TURON.
CENON.
GAULT
_ _
RED BEDS
I
,
l
150
100
'
'
.
.
l
.
'
.
'
50
I
OMa
Fig. 6. (A) Burial history for the Ely concretion horizon. (B) Stratigraphy used in reconstructing the burial history. The preKimmeridge Clay section is taken from the Soham borehole (TL 59 74), the Kimmeridge Clay thickness given is that preserved
in the northern Wash (Gallois & Cox, 1976), and the Cretaceous section is that preserved in Norfolk (Larwood, 1961; Peake
& Hancock, 1961). (C) This diagram compares the model temperature of the concretion horizon during its burial history and
model temperature for the calcite cements given by their oxygen isotopic composition .taken from Table 3. The surface
temperatures assumed when constructing the temperature during burial are also shown. Stages I to 111 in the cements
correspond to those shown in Fig. 9.
Magnesium, iron, manganese and strontium concentrations were also measured by ICP spectroscopy
(Table 2) in acid-extracted carbonate from microspar
and cavity cements drilled out along a traverse parallel
to the thin sections analysed. These results are more
accurate, but less localized than the EDX results. The
two sets of results are in good general agreement, the
EDX giving similar absolute values to the ICP for
iron and manganese, but slightly overestimating
magnesium.
The principal points to emerge for each element are
as follows.
(1) Irmz
The general trend from low to high iron contents
inferred from stained thin sections is confirmed. The
360
T. R . Astin and I . C. Scotchman
I
CONCRETION
MATRIX
Fe
X
I
I
I
2 Yo -
I
-
I
(CENTRE)
+PYRITE
RICH
*
-FIBROUS
MICROSPAR
-(OurSIDE)
ZONE
CENTRAL
CAVITY
CFMFNT
CEMENT
2%
I
2%-
Mn
I -
&ARL+-
BROWN
1st
(MATRIX
CALCITE
-
GENERATION
GROWTH)
_ _ -W H I T E
_-
1) -2nd
CALCITE
~
@AT9
GENERATION -
calcite cement from within a septarian cavity. Data points, error ranges and a three point moving average are shown.
36 1
Kimmeridge Clay concretions
Table 2. Trace element composition of concretion carbonates in p.p.m., and percentage of solid residue in the matrix
Sample
Kim5
Kim4
Kim6
Kim7
Kim3
Kim2
Kim 1
Comments
Cements
Brown (Early)
I
Brown (Late)
Mainly White, first generation (mixed with
adjacent late brown)
White, second generation (early)
I
White, late
Mn
Fe
Mg
Sr
633
1081
925
1405
172
4366
4403
7249
1741
1198
1141
1697
22 1
176
174
228
6026
660
48 1
7706
6930
6774
2705
1409
727
127
259
215
191.
240
322
644
61 1
945
937
89 1
1208
528
408
608
894
860
I169
1334
1005
391 1
3271
2864
1321
2309
2343
2072
2089
2217
2629
243
236
222
219
221
249
242
259
254
%Solid residue
Matrix
Kim8
Kim9
Kim10
Kim14
Kim1 1
Kim15
Kim13
Kim16
Kim12
Centre
Edge
~~
~~
11.0
8.9
11.8
13.3
12.8
17.2
15.6
16.7
23.0
~
Notes (I) Analyses were reproducible within 5% in all cases, and often within 1% (11) The percentage of acetic acid insoluble
residue in the matrix samples is only a qualitative guide to the sediment porosity when that matrix formed, because of
modification of, and displacive growth by, the matrix carbonate
earliest formed, innermost microspar is slightly more
ferroan than the rest, iron contents decreasing in the
pyrite zone, showing that pyrite precipitation removed
most available reduced iron in solution (Curtis &
Spears, 1968; Raiswell, 1976). Iron contents remained
low during concretion growth, increasing in cavity
cements formed during the later stages of Late Jurassic
burial after concretion growth ceased.
Continued low iron contents in cements formed
during concretion growth below the sulphate reduction
zone reflect a balance between the rate of reduction of
Fe3' compounds in the sediment which was probably
slow (Curtis, 1980)and removal of iron into concretion
microspar which was pronounced because of the large
amount of carbonate formed. Only when the cement
formation nearly ceased did reduced iron accumulate
in the pore-water and in these cements. The iron
content of cements formed during the second burial
and uplift remained high with supply to and removal
of iron from pore water remaining roughly in balance
for the relatively small amount of carbonate formed.
During concretion growth, manganese contents slowly
increased then rose rapidly in cements formed after
concretion growth ceased because of reduction of
manganese present in the sediment. The rapid rise in
manganese is later than for iron suggesting that
manganese was less easily reduced than some of the
iron in this Kimmeridge Clay during deeper burial.
Manganese contents remain high at the start of the
second generation of cementation, soon falling back
to very low levels showing that manganese was no
longer being supplied to the pore water by reduction,
so that manganese depletion in pore water could occur
during cement growth.
( 3 ) Magnesium
Magnesium content initially decreased during concretion growth before increasing in the outer microspar
and in septarian cements formed after growth ceased.
It shows a slow fall during precipitation of second
generation cements, intermediate in pattern between
iron and manganese.
(2) Manganese
Manganese contents are very low in the early
microspar. Early oxidizing conditions in the sediment
above the sulphate reduction zone would have caused
an initially low MnZ+concentration in the pore water.
(4) Strontium
Strontium contents are low throughout. Rises in
strontium in cavity cements both late in the first
362
T. R. Astin and I. C. Scotchman
generation and early in the second generation of cavity
cements indicate aragonite shell dissolution as a
source of carbonate at these times.
The probe data for Fe, Mn and Mg show similar
contents in calcites immediately either side of the
hiatus between first and second episode cements (Fig.
7). Thus the pore water chemistry was not dramatically
altered during uplift in the Mid-Cretaceous, with no
new water introduced into the shale at this time.
Stable isotopic composition of carbonate cements
Methodology
Drilled samples from pieces of two different concretions from Bed 30 were first treated to remove organic
matter by low-temperature oxygen plasma ashing
using a Nanotech 100 Plasma Chemistry unit (Scotchman, 1984). Carbon dioxide was extracted from the
calcites by reaction with anhydrous phosphoric acid
at 2518°C (McCrea, 1950). Isotopic analyses of the
COz were made with a Micromass MM903 triplecollection mass spectrometer at the British Geological
Survey's Geochemistry Division, London, using the
I
Fe
1
6000
PPlll
Mn
0 '
4000
PPl
0 .
0
calcite fractionation factor of Friedman & O'Niel
(1977).
Concretionmicrospar
The two concretions show a similar isotopic evolution
from relatively light carbon and heavy oxygen in the
early microspar to heavier carbon and slightly lighter
oxygen in the later microspar (Fig. 9). The textural
complexity of the microspar revealed by the thin
sections suggests that some microspar samples used
for isotope analysis are likely to be mixtures of calcite
from different stages of concretion formation.
Three types of contamination of primary microspar
could occur: (I) The earliest microspar is likely to be
mixed with first-generation microveins from later on
in the history. These veins would increase the 613C
value and lead to the lightest carbon values not being
those nearest to the concretion centre where the
microveining is densest (Fig. 9). This effect could hide
the lightest carbon values originally present in the
concretion centre; (2) Outermost microspar is mixed
with second generation overgrowths and veins leading
to scatter towards unusually light oxygen as well as
heavier carbon values; (3) Small shells could be
sampled with the microspar leading to slightly heavier
carbon and heavier oxygen in the analyses. All these
effects may be present in the microspar analyses and
helps explain the scatter in the microspar results.
The inner microspar has 6I3C about - 102, (Fig.
9), confirming that concretion growth started within
the sulphate reduction zone. The outer, fibrous
microspar averages about 613C of - 5%,, implying
dilution and replacement of dissolved carbonate from
the sulphate reduction zone during deeper burial. A
sediment surface temperature of about 19°C and a
Late Jurassic seawater with 6180pDB
of about - 1.2%,
(Shackleton & Kennett, 1975) are consistent with the
heaviest oxygen values (Table 3). The variation in
oxygen composition is about l.4xO61sOpDB,equivalent to a temperature change of about 7°C during
concretion growth, corresponding well with the temperature increase inferred during the Kimmeridge
Clay burial (Fig. 6C).
'
centre - MATRIX
-
edge
Central cavity cements
1st
-
CEMENTS-2nd
Fig. 8. Trace element compositions obtained by acid
dissolution and ICP spectrometry of calcite microspar and
septarian cavity cements from traverses parallel to those in
Fig. 7.
First generation cavity cements have similar isotopic
compositions to the fibrous microspar, confirming that
they formed during the later stages of the Late Jurassic
burial (Fig. 9). All second generation calcites have
very uniformcarbon values (about - 2.3%,) continuing
363
Kimmeridge Clay concretions
3
6I0O
8' 8 0
-10
-5
-5
I
I
Late Jurossic
.'Late
Cretaceous
MATRIX
0
W
CEMENT
\
Deeper
Burial
-5
'
'
\\
5
Concretion
Growth
BROWN
WHITE
-10
1
10
Early
6'"c
P C
- I5
15
Interpretation
Data
Fig. 9. (A) Stable isotopic data for the calcite microspar and cavity cements. The lines join traverses within either cavity
cements or matrix from single specimens. (B) Interpretation of the isotopic data in terms of the different stages of the burial
history shown in Fig. 6a. Model temperatures computed from each of the stages 1-111 are shown in Fig. 6C.
Table 3. Isotopic composition of concretion carbonates
Sample
Sub-sample
Comments
613C
6l80
T T
-6.10
- 4.40
-3.15
-3.16
-2.41
-2.21
-2.29
-2.12
-2.12
25.6
25.1
22.5
21.6
21.1
23.6
20.9
- 2.46
- 2.29
- 2.28
- 2.25
-2 4 1
-3.28
-4.59
-5.61
-5.55
-7.83
-8.23
26.3
32.8
38.5
31.9
51.0
53.4
- 11.19
-1.81
19.5
-6.61
- 1.46
- 2.39
-2.11
-3.10
-5.51
23.6
28.3
38.0
Matrix
P30A
A2
A1
B
C
D
E
F
G
H
I
J
K
L
Edge
- 5.85
Centre
Cement
Brown
White (early)
White (latest)
-9.14
- 9.81
- 11.16
- 9.66
- 5.14
Matrix
P30B
A
B
C
D
Centre
Cement
Brown
White (early)
White (late)
Notes: (i) Temperatures are calculated using the equation of Craig (1965)
assuming a pore water 6180PDBcomposition of - 1.2%,, being the assumed
Jurassic seawater composition (Shackleton & Kennet, 1915).
T. R . Astin and I. C. Scotchman
364
the trend toward heavier carbon with time. White
calcite from early in the second generation of cement
has 6 l S 0values of - 3.7%, and -4.7%, corresponding
to temperatures of 28-33°C assuming porewater
retaining a 6l8O of - 1.2%, (Table 3). This implies
that the second episode of septarian fracturing was
initiated after about 200 m of Late Cretaceous burial
(Fig. 6C). Later calcites formed after septarian
fracturing ceased have 6 l 8 0 values from -5.6z0 to
- 8.2z0, corresponding to temperatures of 38-53°C
(Table 3). These temperatures are higher than the
35°C for a maximum Cretaceous burial of about 550 m
given by the model (Fig. 6).
DISCUSSION
Links between overpressure and concretion textures
Timing of septarian fracture formation
Septarian fracture formation requires a combination
of low horizontal stress and raised pore-fluid pressure,
both of which are favoured during rapid burial (Astin,
1986). This implies that the two generations of
septarian fractures in the Ely concretions formed
during each of the two episodes of burial, during the
Late Jurassic and the Late Cretaceous. The match of
oxygen isotopic compositions of calcite formed at
times of septarian fracturing with the model temperature history confirms this timing and constrains the
first generation of fractures formed during most of the
Late Jurassic burial, probably within the depth range
30-130 m, and the second generation to have formed
rather deeper, probably in the range 200-500m.
Similarly, Boles et al. (1985) demonstrate septarian
fracture growth occurring during later stages of
concretion growth using geochemical correlation of
septarian cements and concretion microspar, though
without specific depth constraints.
Fibrous microspar growth and overpressure
Correlation of microspar and septarian fracture filling
cements by chemistry and cathodoluminescence response shows that the fibrous outer microspar (comprising more than 70% by volume of the whole
concretion) grew at the same time as septarian fracture
formation (Table 1). During the second burial, the
association is repeated with limited fibrous microspar
growth in the margins of the concretion by the
addition of oriented overgrowths occurring coevally
with septarian fracturing.
Fibrous microspar growth in the Ely concretions is
shown to be displacive by the texture of compacted
domains of clay matrix separated by fibrous crystals,
and by the porosity at the time of cementation implied
by the volume of carbonate present (80-85%, Table 3)
being higher than the likely original porosity after
several tens of metres burial (65-75%). Fibrous,
displacive growth probably takes place under high
pore-fluid pressures where the fluid pressure is close
to the confining pressure (Shearman et al., 1972;
Marshall, 1982; Stoneley, 1983).
Synchronous formation of fibrous margins and
septarian fractures are a common feature of concretions in shales (e.g. Marshall, 1982; Boles e f a / . , 1985).
This strengthens the interpretation of these textures
being dependent on overpressure development. Their
presence implies that the Kimmeridge Clay became
overpressured during both burial episodes, probably
more strongly during the first burial, with pressure
release during uplift. Overpressure developed at
relatively shallow depths (50-500 m) in each burial, a
feature found today in Mississippi submarine fan
sediment (Bryant et a / . , 1985) where it also results
from undercompaction of rapidly deposited sediments.
Origin of the carbonate for the concretion
The isotopic trends within the Ely concretions are
qualitatively similar to many concretions from marine
sediments (Hudson, 1978; Marshall, 1982; Gautier &
Claypool, 1984). The centres of the Ely concretions
formed at shallow depth towards the base of an active
sulphate-reduction zone. The depth of this zone is
limited by the depth of diffusion of sulphate from the
sediment surface and is likely to be about 10 m at
most, even in this well-bioturbated sediment (Curtis,
1983), and probably much less. A hiatus in deposition
is required for the concretion horizon to remain in the
base of the sulphate-reduction zone long enough to
form the resulting concretions (Raiswell, 1987). The
concretions continued to grow below the limit of
sulphate reduction over about 130 m of burial, with
almost no growth during the second burial.
The 613C values of the cements formed can be used
to try to assess the contributions of carbonate from
different sources over this burial interval. Light carbon
isotopic signatures have been explained by fractionation during sulphate reduction of organic matter
(giving carbonate with 613C of -20 to 25%,) (e.g.
Irwin e t a / . , 1977; Hudson, 1978). In addition, upward
diffusion of methane from an underlying methano-
Kimmeridge Clay concretions
genic zone and its oxidation in the sulphate-reduction
zone (giving carbonate with 613C - 60%, to - 7073
lead to an overall lighter carbon signature than usual
for the sulphate reduction zone (Gautier & Claypool,
1984). Raiswell(l987) considers that upward diffusion
of dissolved carbonate from the underlying methanogenic zone (613C 0 to +lo%, for methanogenic
carbonate) should usually accompany the methane,
thus leading to an overall heavier signature difficult to
distinguish from that of sulphate reduction of organic
matter directly. Therefore the amount of methane
oxidation can control how light the carbon signature
is in sulphate reduction-zone carbonates, with heavier
signatures implying less methanogenesis below.
The earliest carbonate in the Ely concretions has a
613C of - 11.8%, which is relatively heavy for the
sulphate reduction zone. Later carbonate cements do
not get heavier than 6I3C -4z0 implying the absence
of a strongly methanogenic zone over most of the first
130 m of burial. Textural evidence shows that aragonitic shells (613C about Og0) were dissolving during
early concretion growth, and the sulphate reduction
zone signature found is probably due to about equal
amounts of carbonate from organic matter (including
oxidation of organic matter and methane in the
sulphate reduction zone and upward diffusion of
carbonate from the methanogenic zone) and from
shelf dissolution.
Carbonate formed over the rest of the first burial
has a 613C isotopic signature of about - 5%,. Carbonate inherited from the sulphate reduction zone must
have been diluted, for example with up to about 25%
of methanogenic carbonate or up to about 50% of
carbonate from dissolving shells. There is an absence
of any carbonate with a positive 613C signature of
clear methanogenic origin showing that the methanogenic zone was not sufficiently intense to modify the
carbonate inherited from the sulphate reduction zone.
In addition there is textural evidence that aragonite
shells were dissolved during concretion growth.
Furthermore, the maintenance of high alkalinity porewater down to 130 m of burial is required for continued
carbonate precipitation. This was probably maintained by a combination of iron reduction in the
mudstone (Coleman, 1985) and aragonite dissolution.
The large size of the Ely concretions, with over 70%
oftheir carbonate formed below the sulphate reduction
zone, suggests the importance of continued aragonite
dissolution during the Kimmeridge Clay burial, both
as the source for much of the concretion carbonate
and as the way of maintaining sufficient alkalinity for
carbonate precipitation.
365
During the second burial phase, relatively small
amounts of calcite were added to the concretion over
a much larger depth range, showing the absence of
any major source of new carbonate ions during this
time. The carbon isotopes show that the pore water
had become dominated by dissolved shell carbonate
by this stage, with the main cause of calcite precipitation again being aragonite dissolution. The small
amount of carbonate formed shows the near exhaustion of available aragonite in the host sediment.
These points show that methanogenesis was not
quantitatively important as a source of carbonate for
concretions in the Kimmeridge Clay at Ely, compared
to carbonate generated within the sulphate reduction
zone and from shells. At least 50% of all dissolved
carbonate down to 130 m depth was inherited from
the sulphate reduction zone. Therefore, simple correlations of carbon isotopic signatures indicative of
sulphate reduction and depth should be treated with
caution.
A similar isotopic pattern is found in Middle
Jurassic Oxford Clay concretions from the English
Midlands (Hudson, 1978). This pattern from bioturbated mudstones contrasts with that found in more
organic rich, bituminous shales in the Kimmeridge
Clay in S. England (Irwin et al., 1977; Irwin, 1980)
and in the Lower Jurassic Shales-with-Beef of S.
England (Marshall, 1982) where methanogenesis was
more important, with isotopically heavy methanogenic dolomite cements formed immediately below
the sulphate reduction zone. An intermediate situation
occurs in the Upper Cretaceous Gammon Shale of the
Great Plains, USA (Gautier, 1982; Gautier & Claypool, 1984) which apparently lacks heavy methanogenic cements formed during deeper burial, but does
have some carbonates derived from the oxidation of
methane in the sulphate reduction zone. This suggests
a control on the intensity of methanogenesis developed
in a shale from its original organic content and type.
Oxygen isotopic composition of the carbonates
Temperatures of formation deduced from oxygen
isotopic composition of cements formed during the
first (Kimmeridge) burial correlate well with temperatures inferred from the subsidence history (Fig. 6).
This shows that the oxygen isotopic composition of
the pore water was little modified from Jurassic
seawater during the first burial. However, there is
poorer agreement between the isotopic temperatures
deduced from second generation cements and temperatures calculated from the subsidence history. The
T. R.Astin and I. C. Scotchman
366
isotopic temperatures are higher (up to 53°C) compared with the maximum subsidence model temperature (35°C). Either the composition of the pore water
changed by up to - 3x0by the end of the second burial
(of about 550 m), or there was a thermal ‘event’during
the Cretaceous, with a period of higher heat-flow than
at present. Changes of up to -3%, in pore-water
composition over 600 m of burial have been found
during the Deep Sea Drilling Project (Lawrence,
Gieskes & Anderson, 1976) caused by clay mineral
reactions, especially the alteration of volcanic ash to
smectite. It seems unlikely that such a pronounced
shift in oxygen isotopic composition of pore water
could have been achieved in the Kimmeridge Clay
which is not rich in smectite, making it more likely
that there was an increase in heat flow associated with
the Late Cretaceous subsidence.
Evolution of pore-water chemistry during burial
diagenesis
Trends in trace element abundances within thecalcites
are similar in pattern to Oxford Clay concretions
(Hudson, 1978) and Moeraki boulders (Boles et al.,
1985). The Ely concretions have a much lower
magnesium content, presumably because of a lack of
Mg-calcite in the original sediment.
The two burials show quite different changes in
pore-water composition. In the first burial, the pore
water tended to act as an open reservoir, with iron,
manganese, and to a lesser extent, strontium and
magnesium being released from the sediment. But
fractionation into calcite delayed their accumulation
in the pore water until concretion growth had almost
ceased. In the second burial the sediment was
relatively depleted in mobile trace elements so the
pore water tended to be a closed reservoir, with
fractionation into the relatively small amount of
calcite precipitated leading to decreases in trace
element concentrations.
CONCLUSIONS
Integration of textural evidence, burial history and
chemistry of Kimmeridge Clay concretions has provided evidence about a number of processes of
concretion growth, diagenetic reactions in shales,
evolution of the shale pore-water with time and the
development of overpressure during burial.
Concretion fabrics
Detailed textural study of the Ely concretion shows
that neither the compact inner microspar nor the
fibrous outer microspar is a passive pore-filling as
assumed for many other concretions (Oertel & Curtis,
1972; Curtis, Petrowski & Oertel, 1972; Curtis,
Pearson & Somogyi, 1975; Raiswell, 1976; Gautier,
1982). The inner microspar is modified by microveining and possibly dissolution, and both the inner
compact and outer fibrous microspar is proven to be
displacive. So, porosity estimates from septarian
concretions (non-septarian concretions may not be
modified) based on the amount of carbonate present
can be overestimates and need not give accurate
information about depth of concretion formation.
The concretions give support for linking both
septarian fracturing and fibrous cement textures to
overpressure development. The oxygen isotopic composition of the cements filling fractures show they
formed and overpressure developed from about 30 m
depth during Late Jurassic burial and from about
200 m depth during Late Cretaceous burial.
Pore-water chemistry during diagenesis
Integration of burial history with oxygen isotopic
information from cements and septarian fracture
formation history provides a more accurate framework
than previously achieved for showing the chemistry
of shale formation water at particular times and
depths in the sediment column. Pore water composition inferred from cements and fluid-pressure information derived from concretion textures potentially
constrain the amounts and composition of water
available by compaction of the Kimmeridge Clay for
release into adjacent permeable formations during
burial.
For example, the study shows that dissolved
carbonate originating in the sulphate reduction zone
can persist during burial as a major component of
total dissolved carbonate well below the zone of active
sulphate reduction where the deeper methanogenic
zone is not well developed. Organic matter oxidation
by methanogenesis is volumetrically insignificant
below the sulphate reduction zone down to about
500 m burial depth in the Kimmeridge Clay at Ely.
ACKNOWLEDGMENTS
Isotope geochemistry was studied at the University of
Sheffield and the geochemical division of the British
Kimmeridge Clay concretions
Geological Survey at Gray's Inn Road, London under
the direction of M . L. Coleman (I.C.S.), for which the
financial support of the Natural Environment Research Council is acknowledged. Textures and trace
element geochemistry were studied at Reading University (T.R.A.), where D. Whitehead determined
cation compositions using the ICP spectrometer. N.
Wilson is thanked for helping with the computation
of a model thermal history. A. Cross and J. Watkins
assisted with the production of figures and photographs.
Among many others, M . L. Coleman and C. D.
Curtis are thanked for their support, advice and
encouragement, and J . Hudson and R. Raiswell gave
thought-provoking and helpful reviews which were
much appreciated. R. Raiswell very kindly provided
a manuscript about the origin of concretions.
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