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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 environment of sedimentation. “Electrodiagenesis” is considered to be a possible mechanism 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 lithification 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

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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 thickness 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 environment (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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worked the sediment several times until it was lithified. At first, they were burrowing 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 afterwards 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



(“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 sedimentation 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.)



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 (sediment 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.

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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 dissolution 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 sedimentation. 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 lithification; 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.



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


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 composition 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 compaction 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 compaction. 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 compaction. 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.


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), compaction 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 microsparites 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


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 inclusions. 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 recrystallization 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.)


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 recrystallization 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 sediment 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.



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 recrystallization 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 compaction 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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EBHARDT, G., 1968. Experimental compaction of carbonate sediments. In: G. MULLER and G. M. FRIEDMAN (Editors), Recent Developments in Carbonate Sedimentology in Central Ectrope. Springer, Berlin, pp.58-65.

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