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Sedimentology - Elsevier Publishing Company, Amsterdam-Printed
in The Netherlands
STRUCTURAL AND TEXTURAL EVIDENCE OF EARLY LITHIFICATION IN
FINE-GRAINED CARBONATE ROCKS
H. ZANKL Instituie of Geology and Paleontology, Technical
University of Berlin, Berlin (Germany)
(Received February 7, 1969)
SUMMARY
Absence of compaction, intraformational breccias, resedimention,
internal sediments and synsedimentary hardgrounds indicate early
lithification of fine- grained carbonate rocks. One of the factors
controlling early lithification is the purity of lime mud. Less
than 2% of insoluble residue (especially clay minerals) favours
cementation and recrystallisation before further sediment
accumulation causes compaction. Thus, early lithification is
terminated in or near the environ- ment of sedimentation.
“Electrodiagenesis” is considered to be a possible mecha- nism for
cementation.
INTRODUCTION
Observations showing early lithification under submarine
conditions in modern oceans are more and more frequent. By “early
lithification” I understand the transformation of sediment into
solid rock by cementation, without major overburden by sediment
accumulation, at a time when the pore space is still in direct
connexion with the marine environment or for a short time exposed
to subaerial conditions. Numerous authors have observed a submarine
early lithifica- tion of fine-grained carbonate sediments in
relatively large water depths, whereas observations on submarine
early lithification in shallow water are scarce. Some- what
different is the well known early lithification of marine sediments
under subaerial conditions; it is difficult to distinguish
lithification under a shallow marine environment from subaerial
lithification in ancient rocks.
FRIEDMAN (1964, p.806) described a lithified fine-grained
sediment (micrite) rich in high-magnesium calcite from the Atlantis
Seamount (Mid-Atlantic Ridge) at a depth of 300 m. Likewise,
indurated carbonate sediment was observed by GEVIRTZ and FRIEDMAN
(1966) from the Red Sea in depths of 1300-1700 m. In this case, it
was a material especially rich in aragonite. From the Mediterranean
Sea, FISCHER and GARRISON (1967) described limestone crusts which
were sampled by the Austrian “Pola” expedition (1890-1894) from a
depth of 2000 m. The
Sedimentology, 12 (1969) 241-256
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242 H. ZANKL
cement is stated to be magnesium-rich calcite. The same authors
also describe a submarine lithified limestone from the area of
Barbados from a water depth of 280-440 m, which is rich in
high-magnesium calcite and occurs in association with manganese
nodules. Submarine lithification and recrystallization leading from
high-magnesium calcite to Iow-magnesium calcite were described by
MILLI- MAN (1966) from several locations of the Mid-Atlantic Ridge.
Here, a very low rate of sedimentation is supposed to be the main
condition for lithification.
As mentioned above, no report on submarine early lithification
in a shallow water environment existed until recently. W. H. Taft
(personal communication, July 1968), however, found a lithified
carbonate sediment in a depth of 5 m on Yeliow Bank, New Providence
Platform (Bahamas) ; in this case, subaerial lithification can be
excluded. The cement consists of aragonite.
In contrast to the examples cited, subaerial cementation is well
known; for examples, see FRIEDMAN (1964).
In the following, a catalogue of characteristics will be
established on the base of examples, allowing to recognize early
lithification in fine-grained carbonate rocks of the past.
EARLY LITHIFICATION IN SHALLOW-WATER ENVIRONMENTS
Lenses of fine-grained limestone are intercalated in bedded reef
detritus in the central area of the Hohe GO11 Reef (Upper Triassic
Dachstein Formation in the northern Calcareous Alps). These lenses
reach a length of 25 m and a thick- ness of 120 cm in the center.
Their upper surface is even, and they are convex- shaped towards
the underlying strata. The original lime mud was deposited in small
still-water basins situated below the zone of turbulence of the
reef environ- ment (ZANKL, 1969). Since there are no indications of
subaerial desiccation and since the overlying sediment penetrates
into the borings of organisms, it may be supposed that submarine
conditions prevailed until sedimentation continued (Fig.1). The
internal structures of the limestone beds show that organisms
re-
Fig.1. Vertical section through the upper part of a fine-grained
limestone bed in the central reef area of the Hohe GO11 reef
complex (Upper Triassic, Dachsteinformation, northern Calcareous
Alps).
A borehole of unknown origin penetrated into the fine-grained
limestone (sediment I ) . The boring was closed in the upper part
at first by coarse fragments and then by a calcarenite and mud
matrix (sediment 2). The lower part of the boring remained open and
was coated by a dark grey palisade calcite crust (see arrow) with
euhedral crystal spires (black).Some fractures cut the bottom of
the cavity and the calcite crust. Then sediment 3 was filled into
the open cavity with some fragments floating in it. Infiltration of
sediment 3 followed probably the irregular wide fissure on the left
side. The remaining space is occupied by sparry cement (black). The
upper surface of the limestone bed is cut by erosion (in the
picture this contact is somewhat blurred by stylolites). The
overlying calcarenite contains plenty of angular fragments of the
fine-grained limestone below. A vague inclined bedding is marked by
dashed lines. (Peel, negative print.)
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 243
Sedimentology, 12 (1969) 241-256
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244 H. ZANKL
worked the sediment several times until it was lithified. At
first, they were burrow- ing within the soft sediment; their traces
are bioturbated structures. Then re- working continued in
semi-solid sediment, and finally the stabilized sediment was
intensely pierced by boring. Skeletal remains of probably burrowing
and boring organisms as holothurians, echinoids, gastropods, and
pelecypods were found (ZANKL, 1965). Probably, crustaceans existed
as well, as may be concluded from the irregular shape of some
burrows, the cross-section of which is similar to “stroma- tactis”
(SHINN, 1968).
Sedimentation started with a fine-grained mud (Fig. 1, sediment
n0.Z) which after lithification was transformed into a microsparite
with grains with a long diameter of 7 p in average and a medium
diameter of 5 p. The grains are irregularly shaped and the contacts
are curved.
After stabilization of sediment no.], sediment no.2 was
deposited in borings and voids. Sediment 110.2 consists of
intraclasts from sediment no.Z and bioclasts in a matrix of
microsparite with grain sizes ranging from 10 to 60 p . It is
difficult to decide whether sediment no.1 was already cemented
before infilling of sediment no.2 or whether it had undergone only
a slight induration without cementation.
Remaining cavities are coated with a calcite crust of the
palisade type with euhedral crystal spires towards the cavity.
After encrusting and before infilling of an internal sediment no.3,
the pre-existing sediments were already lithified, as it may be
deduced from fragmentation of the cavity bottom and the calcite
crust in Fig.1. Finally a medium to coarse equant calcite spar
fills the remaining space. Lithification of the sediment and
crystal growth of the palisade crust may be caused during the same
process of carbonate precipitation.
The upper surface of the fine-grained limestone beds is slightly
eroded showing a relief up to 10 cm. The contact towards the
overlying reef debris is sharp ; very often, a secondary
styloiithization is blurring the original contact. Angular
fragments of different size derived from the underlying
fine-grained limestone and debris from other sources are
resedimented within this relief.
Thus lithification was terminated before the following reef
debris was deposited. Naturally, we do not know how long the time
interval was between lithification of the lime mud and the
sedimentation of the overlying reef debris, nor do we know whether
the mud beds were temporarily buried under a thin cover of loose
sedi.ment and lithification took place under this cover, which
after- wards was eroded again.
Moreover, the well preserved cavities and borings show that
there was no compaction during lithification. Especially the
undeformed shape of originally circular sections through tubes and
spheres normal to the bedding plane are indicative of lack in
compaction.
In the backreef area of the Upper Triassic Dachstein Formation,
a bank facies is developed with shallow water environments
represented mainly by subtidal sediments (“Megalodon” beds) and
intercalated intertidal to supratidal deposits
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 245
(“Loferite”) (FISCHER, 1964; ZANKL, 1967). Calcilutites within
the intertidal member show a complex story of sedimentation and
lithification (Fig.2). A fine- grained lime mud (sediment n0.Z)
with several small gastropods was deposited and mixed up by
burrowing organisms which were active also after the first con-
solidation of the sediment. The burrows remained open at this stage
of diagenesis. These cavities were further opened, especially at
the roof, by mechanical and/or chemical erosion due to flowing
interstitial water, while on the bottom sedimenta- tion started
with an internal sediment no.2: a carbonate mud with many
intraclasts of sediment n0.Z. Before sediment no.2 was deposited,
the aragonite shells of the gastropods were leached away, leaving
also cavities. Shells of calcite remained undissolved. By this
time, sediment no.1 had to be stable enough to ensure that the
moulds of the shells were not compressed; even partial casts of
gastropod tubes with sediment no.Z are winding as self-supporting
spirals from the surrounding sediment into the moulds. The lower
parts of the moulds were then filled with internal sediment no.2; a
sparry cement fills the upper remaining void.
Sediment no.] is a microsparite with a maximum grain-size of 10
p , while the microsparite in sediment 110.2 ranges from 10 to 60 p
. The contact between the two types of microsparite is sharp at the
wall of the cavities as well as against the intraclasts included in
sediment no.2. On the other hand, the foundation of the internal
sparry cement is sharp towards sediment no.Z, whereas towards the
internal sediment no.2 a gradational transition is observed. This
means that crystallization which causes lithification, and probably
recrystallization which causes a microsparite fabric of interlocked
grain boundaries (Fig.6) was terminated in sediment no.1, before
sediment 110.2 filled the cavities. Crystallization and
recrystallization in sediment no.2 may be connected with the
development of sparry cement in the cavities, which is demonstrated
by a gradational transition.
Also in this case, lithification took place without compaction.
This is well demonstrated by the undeformed circular sections of
mud-filled tubes (Fig.2). It is difficult to decide whether
lithification of this intertidal sediment took place under
subaerial or under submarine conditions. The complete leaching of
aragonite in the shells may be indicative of a temporary subaerial
exposure.
EARLY LITHIFICATION IN MODERATE DEEP ENVIRONMENTS
The basin environment near the Upper Triassic Dachstein reef
complexes 1s characterized by a thin-bedded, red or white,
fine-grained formation called Hallstatter Limestone. Its rate of
sedimentation is one fifth compared to that of the Dachstein
Formation. There is no doubt about the submarine conditions during
sedimentation and early diagenesis. The only differences in opinion
are related to water depths ranging from 100 to about 1,000 m (for
discussion, see ZANKL, 1967).
The Hallstatter Limestone of Norian age is well exposed at the
Kalber
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246 H. ZANKL
Fig.2. Vertical section through a limestone bed in the back reef
area of the Hohe Go11 reef complex (see Fig.)). Light grey =
sediment I ; dark grey = internal sediment 2; black = calcite spar.
Irregular burrows filled with sediment 2 penetrate sediment I in
different direc- tions; further opening of the burrows by overhead
erosion is well demonstrated on the upper left side. The mould of a
gastropod (white arrow) shows a crescentic tube filling with
sediment I (light grey) and bottom filling with sediment 2 (dark
grey) after leaching of the aragonite shell; the remaining void is
filled by sparry cement (black). Note undeformed circular sections
in the middle of the upper part of the picture. (Thin section,
negative print.)
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 247
quarry near Berchtesgadenl. This massive white limestone is
superposed by thin- bedded red “Flaserknollen” limestone. Lenses or
beds of an intraformational breccia are interbedded with the white
limestone (Fig.3). The components (sedi- ment no.1) are angular to
subrounded ranging in size from I to 50 mm. They are embedded in a
fine-grained sediment 110.2 containing a considerable amount of
coquina. Tiny shells are sometimes abundant, giving a
grain-supported fabric with carbonate mud no.2 in the interstices
(Fig.3). The remaining voids contain sparry cement. The grain size
of the carbonate matrix in brecciated sediment no.1 ranges from a
micrite to fine-grained microsparite (2-10 p), whereas sediment
no.2 is a coarse-grained microsparite (40-100 p ) .
The tiny shells preserved their whole original curvature and are
not com-
Fig.3. Coquina with intraformational breccia; Hallstatter
Limestone, Upper Triassic (Karn), Kalberstein quarry in the
Berchtesgaden Alps. Light grey = sediment I (brecciated); dark grey
= sediment 2 (partially filling the interstices of the shells);
black = sparry cement. (Thin section, negative print.)
Samples of “Hallstatter Kalk” and special informations were
kindly provided by Dipl.- Geol. J. Rieche, Berlin.
Scdimentology, 12 (1969) 241-256
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248 H. ZANKL
pressed; they must have been supported by the sparry cement
before the pressure of additional overburden occurred.
Early lithification thus started with carbonate precipitation
and probably was finished by recrystallization in sediment no.1
which afterwards was fractured, with some fragments remaining in
place and others being transported over a very short distance.
Resedimentation followed, together with deposition of shell coquina
and lime mud (sediment no.2). Lithification of this mud and ciystal
growth of sparry cement in the interstices may also be one act, for
there is an obvious gradational transition in the border zone of
the fine-grained sediment no.2, whereas the contact between sparry
cement and sediment no.1 is sharp.
The Liassic strata of the northern Calcareous Alps are partly
characterized by thin-bedded red limestone. A comprehensive
sedimentological study of t!iis limestone was published recently by
JURGAN (1969). One process described by Jurgan is submarine
carbonate dissolution (“subsolution”, HEIM, 1924) which indicates,
in the case of dissolving a hardground, early lithification.
Subsolution takes place in an oxidizing environment during a period
of interrupted carbonate sedimentation in moderate water depth.
This environment must be very similar to that described by MILLMAN
(1966) from the North Atlantic, except that dissolu- tion processes
are not yet observed there.
The sharp-edged and deep subsolution relief at the surface of
the sediment as well as detached components demonstrate that a
solid rock was affected (Fig.4).
Lithification may have continued during a period of very slow
carbonate sedimentation in the uppermost sediment layer. If the
rate of sedimentation is very low, lithification follows sediment
accumulation. Increasing rate of sedimentation may interrupt
lithification, which continues again in the period of reduced sedi-
mentation. In this way, the frequent internal cavities of the Lower
Jurassic red limestones (FABRICIUS, 1966, p.50; WENDT, 1969, p.226)
may be explained by erosion of soft sediment beneath lithified
crusts and intraformational breccias may originate by fragmentation
of these crusts.
Emersion and subaerial conditions can be excluded as the cause
for lithifica- tion; this applies to all “hardgrounds” (VOIGT, 1959
; HOLLMANN, 1964) connected with subsolution structures.
One of the most impressive examples for submarine lithification
under conditions of reduced sedimentation was given by LINDSTROM
(1963) for Early Ordovician sediments of Scandinavia. During a
period of very slow carbonate sedimentation, an interruption of
deposition occurred several times, followed by dissolution of an
already lithified surface. Intraformational fold structures and the
interaction of boring organisms with subsolution phenomena are good
indicators for submarine lithification.
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 249
Fig.4. Vertical section through a subsolution relief (Upper
Hettangian/Lias) in Adnet Limestone from Adnet near Salzburg,
Austria. A fossiliferous limestone irregularly stained by
iron/manganese oxide (black) is corroded by subsolution; the
subsolution surface (upper part of the picture) is connected with a
deep corrosion cavity (running down to the right hand side, it ends
about 5 mm outside of the picture). This cavity may originally have
been a boring further opened by subsolution. Subsolution fragments
(black-rimmed) are floating in the younger sediment (grey) in the
center of the cavity. (Thin section, positive print.)
CHARACTERISTICS FOR EARLY LITHIFICATION
The most important characteristic for early lithification in
fine-grained carbonate rocks is, according to these examples, the
absence of compaction. The connexion between early lithificatioii
and compaction was at first observed by PRAY (1960). The absence of
compaction can be recognized by the undeformed synsedimentary
cavities which are due to: ( I ) the activity of boring organisms;
(2) the mechanical opening and enlarging by shrinkage or internal
erosion; and (3) the chemical dissolution of unstable carbonate
modifications. The conclueion that the cavities are synsedimentary
openings may be drawn from the fact that at least one or more
further sediment generations are embedded into the cavities before
or during the overlying sediment cover was deposited.
Moreover, sediment-filled undeformed shells, and their internal
moulds indicate lack of compaction, whereas crushed shells and
deformed internal moulds are considered under special conditions as
a criterion for compaction (EINSELE
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250 H. ZANKL
and MOSEBACH, 1955, pp.382-383 ; WELLNHOFER, 1964, p. 14).
Resedimentation of angular intraclasts at the surface of a bed or
in internal cavities is evident for early lithification as well.
Also intraformational breccias may give the same indication.
Finally, for moderate and deeper-water environments subsolution
acting on hardgrounds demonstrates early lithification.
In order to investigate the dependance of compaction on the
contents of non-carbonate minerals (FABRICIUS, 1966, p. 14), the
residues insoluble in diluted hydrochloric acid were extracted and
examined by an X-ray diffractometer. Their mineralogical
composition was determined by a semiquantitative method. In all
examples where lack of compaction was proved by undeformed organic
and sedimentary structures, the residues were less than 2 weight %,
and in the case of the Dachstein Limestone even less than 0.1 %
(Fig.5). The mineralogical composi- tion consists dominantly of
illite with subordinate kaolinite and chlorite. Quartz and some
feIdspar are traceable in each sample.
In order to prove these results, the quantity and quality of the
insoluble residue was determined in samples with a well known
degree of compaction. In strata of Late Jurassic age in Frankonia
(Neuburger Bankkalke), the compac- tion of several beds was
measured by WELLNHOFER (1964, p.14) on the degree of deformation of
pelecypods Pinna and Rollierellul. The ratio of length to height of
internal moulds of isolated and horizontal embedded ?hells of
Rollierellu was measured in beds without compaction, and the degree
of deformation of moulds in compacted beds was compared and taken
as equivalent to the degree of compac- tion. In one bed, the
compaction was measured by the deformation of moulds of Pinnu in
growth position vertical to the bedding plane. The results (Fig.5)
show a critical content of 2% of insoluble residue, above which
compaction starts and is augmenting with increasing residue. Also
in the Upper Jurassic limestone the main mineral content of the
residue is illite, besides some quartz. Consequently, it is a
content of clay of more than 2%, which has a remarkable influence
on comp- action. Besides, there may exist some other factors, as
the iron content or content of organic material in the sediment;
these were, however, not investigated.
The grain-size distribution of different sediment generations in
early lithified fine-grained limestones ranges usually from a fine
microsparite (5-10 p) to coarse microsparite (40-80 p).
The grain shapes studied in electron micrographs (Fig.6, 7) show
irregular, embayed and deeply interlocked grain boundaries
(amoeboid mosaic by FISCHER et al., 1967) which are typical for
recrystallized grains, whereas crystals freely grown in voids are
euhedral to subhedral, rhomb shaped and showing straight grain
boundaries (Fig.6). Skeletal fragments are characterized by
specific grain orientation. The main grain shape in all of the
studied samples is of the irregular type. ~ -_.__
Samples of the “Neuburger Bankkalke” were kindly made available
by Dr. W. Barthel, Miinchen.
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 25 1
. 10
.9
G R A l N - COM PACT ION Fig.5. Unsoluble residue versus
compaction. The composition is indicated by quartiles
or “traces”. I = Dachstein Limestone (Megalodon beds), Pass
Lueg, Austria. Average of 10 samples. Minerals: 75 % quartz and
feldspar (albite), 25 % clay minerals (mainly illite, some
kaolinite). 2 = Hallstatter Limestone, white massive facies of the
Kalberstein Quarry, Berchtes- gaden. Average of 3 samples. Minerals
not determined. 3 = Dachstein Limestone, lenses of fine-grained
facies in the central reef area, Hohe Goll. Minerals: 50 % clay
minerals (mainly illite, some kaolinite), 50% quartz and some
feldspar. 4 = Neuburger Limestone, bed no.22 (WELLNHOFER, 1964,
p.14), Upper Malm, Neuburg, Frankonia. Minerals: 75 % quartz, 25 %
clay minerals (mainly illite). 5 = Adneter Limestone, Upper
Hettangien (below subsolution surface), Adnet, Austria. Minerals
not determined. 6 = Neuburger Limestone, bed no.42 (Berriasella
bed). Location, see 4. Minerals: 50% quartz, 50 % clay minerals
(mainly illite, some kaolinite). 7 = Dachslein Limestone, A-member
of Lofer Cyclothem (FISCHER, 1964), Pass Lueg, Austria. Minerals:
75 % illite, 25 % quartz and some feldspar. 8 = Neuburger
Limestone, bed nr.102. Location, see 4. Minerals: 50% quartz, 50%
clay minerals (illite, some kaolinite). 9 = Neuburger Limestone,
bed nr.116 (Pinnu bed), compaction 20%. Location, see 4. Minerals
not determined. I0 = Neuburger Limestone, bed nr.116 (Pinnu-bed),
compac- tion 35 %. Location, see 4. Minerals: 75 % clay minerals
(mainly illite), 25 % quartz.
It was possible to demonstrate that in the case of two or more
generations of sediments in an early lithified carbonate rock, on
one hand, sharp contacts among each other or towards sparites may
occur, and that, on the other hand, reaction rims indicate a
gradational transition. The sharp contacts are indicative of an
independent crystallization, whereas reaction rims suggest a common
phase of crystallization. Frequently, sediment no.Z had already
obtained a stable fabric which was not affected by later
recrystallization. Therefore, one finds the fine- grained
microsparites of sediment no.1 surrounded by coarse-grained micros-
parites or sparites without any transition. Pure micrites (mean
grain size < 4 p) were not traceable among the carbonates
lacking compaction. In contrast, micritic carbonate grains are the
essential component in the samples with a high clay
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2 52 H. ZANKL
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EARLY LTTHIFICATION IN FINE-GRAINLD CARBONATE ROCKS 253
Fig.7. Neuburger Limestone, bed 110.22 (see Fig.5, no.4), Upper
Malm, Neuburg, Franconia. No compaction. Totally recrystallized
fabric, grain size ranging from 10 to 40p. Relicts of original
micrite indistinctly preserved. (Electron micrograph.)
mineral content of the Upper Jurassic beds. This is well in
agreement with the investigations of BAUSCH (1968) who determined a
distinct limit of 2% content of insoluble residue below which the
recrystallization of microsparite starts suddenly. The limestones
with more than 2% of insoluble residue of mainly clay are
predominantly composed of micritic grains. The same trend was
observed by MARSCHNER (1968) in beds of Early Keuper age in
northwestern Germany.
Also an influence is considered of clay mineral content on
crystallization processes causing lithification. These processes
may be cementation by carbonate precipitation in the pore space -
especially in coarse-grained sediments - or
Fig.6. Neuburger Limestone, bed no.102 (see Fig.5, 110.8). Upper
Malm, Neuburg, Franconia. Compaction 10 %. Three types of calcite
grains are demonstrated: Irregular shaped grains, micrite ( < 4
,u) or microsparite in size, more or less densely stippled by
inclusions. Long extended blade-shaped (euhedral) microspar (upper
right hand side) well orientated. Rhomb-shaped (euhedral) microspar
(lower center) with regular grain boundaries, less in- clusions.
The blade-shaped grains represent the organic fabric of a shell.
The rhomb-shaped crystals were filling a void by free crystal
growth. The irregular grains show a recrystal- lization fabric,
partially derived by an aggrading porphyroid recrystallization
(FOLK, 1965, p.23) from micrite into microsparite, or partially by
monocrystalline overgrowth replacing micrite outward from a coarse
crystal, for instance, the irregular microspar rich in inclusions
growing outward from the skeletal grains or from the void sparite.
(Electron micrograph.)
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254 H. ZANKL
a syntaxial overgrowth on monocrystalline nucleus, probably the
main process of cementation in carbonate muds. It is unknown
whether inversion of instable carbonates - aragonite or
high-magnesium calcite - to stable calcite takes place before,
during, or after cementation.
Thus, a clay mineral content below of over 2% is one of the main
factors which, by their influence on cementation and
recrystallization hamper or accelerate early lithification.
Consequently, beside cementation recrystallization has to be
considered also as one of the main processes during early
lithification. Local recrystallization under special conditions in
sediments rich in clay minerals as stated by M I ~ K (1968) cannot
effect the main process of lithification and may occur before or
after lithification.
From a comparison of electron micrographs of samples with
different stages of recrystallization (Fig.6, 7), the first step to
be identified is a micritic fabric which may be the result of
cementation and inversion. The next step is a porphyroid aggrading
recrystallization (FOLK, 1965, p.23) finished at last at an equal
sized microsparite. The factors are unknown which usually stop
recrystalliza- tion in sediment no.1 at a grain size of 5-10 p and
on the other hand effect a further aggrading recrystallization in
internal sediments to microsparite or sparite. The clay mineral
content in both sediments is usually below 2% and thus without
influence. If recrystallization is stopped during early diagenesis,
it cannot restart unless stress or metamorphic conditions occur. A
modification of this type of recrystallization occurs a t the rim
of originally coarser-grained crystals (skeletal grains or sparite)
where in orientated syntaxia! overgrowth the micritic matrix is
replaced (Fig.6).
THE PROCESSES OF EARLY DIAGENESIS IN FINE-GRAINED CARBONATE
SEDIMENTS
?he story of early diagenesis - the period between sedimentation
and lithification - begins with a first compaction of the
mud-supported sediment, which may be termed void compaction. A
first strong loss of fluids takes place in the uppermost parts of
the sediment during this void compaction. In the Florida Bay lime
mud, GINSBURG (1957, p.91) found a decrease of moisture content
from 260% at the surface to 1 0 0 ~ o at a depth of 0.3 m. Kogler
(in SARNTHEIN, 1967, p.124) noted a water content of 85-104% at the
surface of a carbonate mud in the Persian Golf and 47-62% of water
in a depth of 4 m. The remaining sedi- ment after void compaction
reflects a grain-supported fabric with a porosity of about 40-50%.
This fabric is alieady stabilized well enough to keep cavities open
(SHINN, 1968). The remaining pore space decreases only very slowly
by mechanical compaction effected by an overburden of sediment
accumulation. This process may be termed grain compaction, which is
also accompanied by grain dissolution especially according to the
carbonate minerals. A further loss of pore fluids by migration acts
as a transportation medium for the dissolved carbonate ions.
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EARLY LITHIFICATION IN FINE-GRAINED CARBONATE ROCKS 255
A good model of grain compaction is given by EBHARDT (1968)
using experimental data. This way, early diagenesis is a very long
lasting process until lithification is terminated under the
overburden of sediment accumulation.
If the conditions for cementation and recrystallization are
favourable in a pure carbonate sediment with a clay mineral content
of less than 2%, no grain compaction follows, and lithification is
terminated by cementation and recry- stallization in or near the
environment of sedimentation without an overburden. In this case,
further compaction is only possible by dissolution of carbonate
along stylolites during late diagenesis.
One problem arises from the observation of early lithification
without compaction: a great amount of solution has to be flowing
through the sediment carrying the carbonate material to fill a pore
space of about 40-50%. This may be possible from the underlying
sediments where carbonate is dissolved by comp- action pressure,
which may be the case when sediments rich in clay and pure
carbonate sediments are interbedded (for example, carbonate series
of the Upper Jurassic in southern Germany). But there are also
series of pure limestone of a thickness of more than 1,000 m,
especially in the Alps. Structural evidence for early lithification
without compaction is obvious in this series. A mechanism forcing
diagenetical processes in fine-grained mud sediments was stated by
SERRUYA et al. (1967). They found that electrical currents flowing
through sediments stimu- late cementation (“electrodiagenesis”).
These currents may be produced by ionic exchange processes. The
formation of authigenic minerals such as calcite, gibbsite,
limonite, hydrohematite etc. was proved by experiment. Also the
cementation of unconsolidated sand by calcite was caused by
bicarbonate solutions which flow through the sediment as a iesult
of application of electrical current.
Further inve5tigations on electrical currents and potentials are
necessary in modern carbonate sediments under different natural
conditions as watei depth, daily or seasonal changes of the
solutions and the different influences of organisms and their
decomposition products.
ACKNOWLEDGEMENTS
Preparing and photographing the electron micrographs was
performed at the Institute of Micromorphology (Prof. Dr. J.-G.
Helmcke and Dr. H. Newesely) of the Max-Planck-Gesellschaft,
Berlin-Dahlem. A financial support was granted b y Deutsche
Forschungsgemeinschaft. It is a pleasure to acknowledge my debt to
all who gave help during this study.
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