ABSTRACT

Laterally persistent horizons of interstratified chert-limestone-manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the Proterozoic Penganga Group of the Pranhita-Godavari Valley, South India. The association of intraformational limeclast conglomerate, turbidites, and pelagic micritic limestone indicates deposition of the chert-bearing horizons at the toe of a distally steepened ramp slope environment.

The chert is characterized by a wide variety of fabrics, including cryptocrystalline and microcrystalline quartz, equant megaquartz, and chalcedony. Cryptocrystalline and microcrystalline quartz are most common; they occur mainly as lenses of mosaic quartz and define laminae. Megaquartz, in contrast, occurs as irregular patches, as fenestroids within cryptocrystalline or microcrystalline fabrics, or in complex aggregates and disrupted mosaics.

The origin of the cryptocrystalline and microcrystalline quartz by maturation parallels the morphological evolution of many deep-sea cherts that form by maturation of biogenic opal-A, to quartz chert through an opal-CT stage. Within the quartz stage, the microcrystalline fabric formed by pervasive space-centered grain growth of cryptocrystalline crystals, and porphyroid growth resulted in porphyrotopic fabric. The megaquartz formed directly from pore water in continuity with maturation of opal-CT. The disrupted and curdled mosaics attest to high fluid activity within semilithified sediments. The variation in fabric types at the microscale suggests fabric evolution at low temperature. The presence of chert clasts in debris-flow conglomerates and development of a plethora of water-escape structures suggest maturation of silica at shallow burial depth. Maturation at shallow burial was favored by very low detrital content of these cherts and a Mg-rich pelagic carbonate depositional geochemical milieu. Estimated sedimentation rates from 2 to 10 mm ky^-1 closely match the average sedimentation rate from modern pelagic siliceous deposits.

INTRODUCTION

The biogenic origin of most Phanerozoic siliceous deposits is almost universally accepted (Hein et al. 1978; Hein et al. 1981; Maliva et al. 1989). Maturation of biosiliceous oozes during burial to microcrystalline chert through an opal-CT intermediary is known to have resulted in extensive formation of porcellanite and chert during all Phanerozoic periods (Grunau 1965; Mizutani 1970; Calvert 1974, 1983; Garrison 1974; Kastner et al. 1977; Robertson 1977; Hein et al. 1978; Hein et al. 1981; Williams and Crerar 1985; Maliva et al. 1989). The bedded siliceous rocks in the geologic record were characteristically deposited in deep continental-margin basins under the influence of coastal upwelling (Jenkyns and Winterer 1982; Hein and Karl 1983; Hein and Parrish 1987; Maliva et al. 1989; and many others). The origin of Proterozoic chert, however, still remains unresolved (Siever 1992). The silica constituting some Precambrian cherts is suspected to be of organic derivation (La Berge 1983; Hein 1987). Late Proterozoic cherts, on the other hand, are considered to be of inorganic origin by Maliva et al. (1989), though their inference is limited by a paucity of Proterozoic pelagic successions.

Bedded chert that hosts manganese oxide ore occurs in the unmetamorphosed Proterozoic Penganga Group, South India (Chaudhuri et al. 1989; Roy et al. 1990). The chert horizons are interbedded with limestones deposited at the base of the slope of a distally steepened ramp (Mukhopadhyay et al. 1997). They offer a unique opportunity to study their fabrics and fabric associations. We describe here different fabric types, fabric associations, and different stages of fabric evolution in these cherts and attempt to explore their implications with regard to the origin of Proterozoic bedded chert in the pelagic carbonate-forming environment. This work is based on field and optical microscopic studies of bedded cherts. Manganese oxides intercalated with the bedded chert were identified by ore microscopy and XRD analyses (see Roy et al. 1990). Intercalated carbonates and iron oxides were identified by XRD analysis (N.J. Beukes, written communication 1996).

GEOLOGIC SETTING

The Proterozoic Penganga Group is part of the Godavari Supergroup (Chaudhuri and Chanda 1991), located in the Pranhita-Godavari Valley (Fig. 1). The Penganga Group is divided into three formations: the Pranhita Sandstone, the Chanda Limestone, and the Sat Nala Shale, in ascending order (Fig. 2A). The limestone-shale succession has been repeated by a number of thrusts that placed the distal deposits on the top of relatively proximal deposits (Fig. 2B). The Chanda Limestone is dominantly micritic and is characterized by laterally continuous rhythmites 5-30 cm thick. It hosts a number of autoclastic limeclast conglomerates and calcarenites of mass-flow origin in its lower parts. Shallow-water carbonate components such as ooids or intraclasts, and siliciclastics coarser than mud, as well as current- or wave-generated structures, are conspicuously absent. The attributes collectively indicate that the carbonates were formed in a distally steepened, deep-ramp setting below storm wave base (Mukhopadhyay et al. 1997). The distal part of the ramp and the slope deposits are dominated by a c. 80-m-thick interval of siliceous gray limestone (YY in Figure 2B) with approximately 20-30 wt % silica, which occurs as randomly dispersed cryptocrystalline quartz.

The siliceous limestone interval is characterized by a number of autoclastic debris-flow limeclast conglomerates and thin layers of calciturbidites (Chaudhuri et al. 1989; Bose and Sarkar 1991; Bandopadhyay 1996; Mukhopadhyay et al. 1997). The conglomerates occur either as laterally extensive tabular beds or as sheet-like, shallow-channel-fill deposits. These beds are 50 cm to 3 m thick, and range between 10 and 500 m in length. Both clasts and matrix of the conglomerates are made up of siliceous lime mudstone. Angular, blocky, and ductilely deformed clasts of bedded chert occur in a few channelized conglomerate beds. Siliceous lime mudstone with interbedded debris-flow conglomerates and thin beds of calciturbidites represent a base-of-slope carbonate apron deposit (sensu Mullins and Cook 1986; Mukhopadhyay et al. 1997). At least two horizons of bedded chert occur in the apron deposits (Mukhopadhyay et al. 1997).

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MODE OF OCCURRENCE OF CHERT

Chert in the Chanda Limestone is mainly in bedded form. Bedded chert is exclusive to the siliceous gray limestone, and occurs in two modes: submillimeter- to millimeter-thick laterally discontinuous planar laminae, and laterally persistent beds intercalated with manganese ores. Millimeter-thick impersistent chert laminae at certain levels within siliceous gray limestone are bluish gray and commonly extend laterally for a few centimeters to a few tens of centimeters. The two intervals of bedded chert are approximately one meter thick and can be traced for several kilometers until truncated by faults. In these horizons several intervals of centimeter-thick bedded chert and jasper with thickness on the order of a decimeter are interstratified (Figs. 2C, 3) with beds of manganese oxide ore and carbonates. Carbonates consist of rhodochrosite, ankerite, kutnahorite, calcite, and Mn-calcite (N.J. Beukes, written communication 1996). Coarser clastics and shaly interbeds are conspicuously absent from these chert horizons. Beds vary in thickness from a few centimeters to a decimeter and are internally laminated at a submillimeter scale. Locally, the chert-jasper interval alternates with micritic limestones, and at one manganese ore quarry the limestone interval grades laterally into a manganese ore.

BEDDED CHERT

The bedded chert in the ore-bearing intervals and in the millimeter-thick discontinuous laminae within siliceous gray limestone is made up of more than 95% quartz. This quartz, which has an interlocking texture, is tough,

FIG. 1. Geological map of the Pranhita-Godavari Valley.

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compact, and aphanitic, with a vitreous luster and conchoidal fracture. The chert also lacks intergranular porosity and is nearly free of clay impurities.

Fabric

The millimeter-thick chert laminae within siliceous limestone are nearly isotropic and cryptocrystalline under the microscope. Tiny patches of microcrystalline fabric are developed locally. The thicker chert beds in the ore-bearing horizons, in contrast, show a wide variety of quartz fabrics and fabric assemblages, and a tripartite hierarchical scheme is adopted here for their description. We propose the term fabric element to describe individual morphotypes of quartz (cf. Folk and Weaver 1952). A combination of fabric elements gives rise to fabric types, and association of fabric types results in fabric mosaics. At the thin-section scale, chert texture may be represented by either a single fabric type or by various fabric mosaics.

Fabric Elements.--The elements include different morphotypes of quartz with different grain size and morphology (Folk and Weaver 1952; Hesse 1989):

a) Cryptocrystalline quartz: quartz crystals beyond the resolution of ordinary light microscope (< 2 µm; Fig. 4A).

b) Microcrystalline quartz: the quartz crystals may be equant, with subcircular to amoeboid outlines, or slightly elongate (Fig. 4B). The equant grains are 5-10 µm in diameter, whereas the elongate grains are 7-20 µm in length. The elongate grains commonly show radial extinction.

FIG. 2. A) Geological map of the Penganga Group, around Adilabad, Andhra Pradesh. B) Lithologic log of the proximal (XX) and distal (YY) successions of the Chanda Limestone. Note different color- and composition-defined intervals of the formation. Siliceous gray limestone of the distal section is equivalent to the brown and pink limestones of the proximal section. C) Lithologic log of the interstratified bedded chert-limestone-Mn-ore horizons at Borjam (a) and Kanpa (b) manganese mines.

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c) Chalcedony: chalcedony sheaves (Folk and Weaver 1952) occur as spherulites or half spherulitic fans (Fig. 4C) with radii of 10-50 µm. The sheaves are length fast and locally include patches of cryptocrystalline and microcrystalline quartz, which may be the manifestation of optical orientation of the chalcedony fibers parallel to the microscope axis.

d) Megacrystalline quartz: crystals are equant, elongate, and wedge-shaped (Fig. 4D). These are hypidiotopic to xenotopic and are 20-200 µm in diameter. Elongate and wedge-shaped crystals display radial extinction (cf. Mukhopadhyay and Chanda 1971). Locally, the wedge-shaped crystals radiate from a common point, giving rise to a stellate spherulitic form.

Fabric Types.--Five fabric types have been identified on the basis of mutual interrelationships between different fabric elements.

a) Cryptocrystalline fabric: the fabric is a semi-opaque to sub-translucent (as seen between crossed polarizers) aggregate of cryptocrystalline quartz (Fig. 4A). The fabric is commonly host to red and black inclusions.

b) Microcrystalline fabric: the fabric is composed mainly of microcrystalline and cryptocrystalline quartz, and subordinate chalcedony (Figs. 4B, 5). The chalcedony spherules are commonly 10-20 µm in diameter. Two end members have been recognized in this fabric: (1) equigranular microcrystalline fabric, comprising equant microcrystalline quartz, and (2) inequigranular microcrystalline fabric (Fig. 5), characterized by cryptocrystalline, equant, and elongate microcrystalline quartz and chalcedony. The inequigranular fabric commonly shows continuous size variation from cryptocrystalline to elongate microquartz through equant microquartz. A porphyrotopic (microporphyrotopic) character with randomly dispersed chalcedony and elongate microquartz crystals, embedded in a cryptocrystalline-to-microcrystalline groundmass, also developed in places.

c) Porphyrotopic fabric: the fabric consists of a groundmass of cryptocrystalline to microcrystalline quartz with randomly dispersed crystals of megaquartz of 20-40 µm size range (Fig. 6), or spherulitic chalcedony (Fig. 7). Megaquartz crystals are hypidiotopic to xenotopic and polygonal, with predominantly curved intercrystalline boundaries.

d) Inequigranular megaquartz fabric: this fabric is characterized by interlocking crystals of equant to elongate megaquartz ranging in size from 20 to 90 µm as well as half spherules of chalcedony. Quartz crystals are hypidiotopic with curved and straight intercrystalline boundaries (Fig. 8).

FIG. 3. Interstratified bedded chert (white) -Mn-ore (dark) sequence.

FIG. 4. A) Laminoid mosaic of cryptocrystalline (darker) and microcrystalline (clearer) fabrics. Note sharp as well as gradational contacts between fabric laminae and stringers of cryptocrystalline fabric intercalated within microcrystalline fabric laminae. Scale bar, 225 µm. B) Microcrystalline fabric with equant to elongate microcrystalline quartz and cryptocrystalline quartz. Scale bar, 150 µm. C) Microcrystalline fabric with fenestroid of fibrous chalcedony. Scale bar, 150 µm. D) Equigranular megaquartz fabric. Note elongate to wedge-shaped megaquartz crystals with curved intercrystalline boundaries. Note gradational boundaries with stringers of cryptocrystalline to microcrystalline fabric. Scale bar, 150 µm.

FIG. 5. Inequigranular microcrystalline fabric. Note a few wedge-like chalcedony microporphyrotopes in the microcrystalline groundmass. Scale bar, 150 µm.

FIG. 6. Porphyrotopic fabric. Randomly dispersed megaquartz porphyrotopes in a cryptocrystalline to microcrystalline groundmass. Scale bar, 150 µm.

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e) Equigranular megaquartz fabric: megaquartz crystals varying in size from 20 to 200 µm, and commonly between 60 and 75 µm, constitute this fabric (Fig. 4D). Crystals may be equant to elongate, with curved as well as straight intercrystalline boundaries. Elongate crystals show radial extinction whereas equant crystals show mildly undulose extinction.

Fabric Mosaics.--Fabric types vary both along and across laminae at the thin-section scale. Thinner laminae may comprise only one fabric, whereas others may consist of more than one fabric type with gradational contacts. Three distinct patterns of fabric assemblage have been identified and each has been termed a fabric mosaic: (a) laminoid mosaic, (b) fenestroid mosaic, and (c) disrupted mosaic.

a) Laminoid mosaic: this mosaic is characterized by submillimeter-scale laminae composed of a single fabric type, most commonly cryptocrystalline or microcrystalline fabric types. The two fabric types occur in laminae that commonly alternate with each other. Microcrystalline laminae often include wispy inclusions of cryptocrystalline fabric (Fig. 4A). Porphyrotopic and megaquartz fabric laminae are uncommon. Contacts between fabric laminae may be sharp or gradational (Fig. 4A).

b) Fenestroid mosaic: this mosaic consists of discontinuous, ovoid to lensoid patches of coarser fabrics within fine-grained fabrics (Fig. 9). Lens-shaped patches mimic "fenestrae" of open space-filling (cf. Schubel and Simonson 1990) or the bird's eye structures in carbonates (Shinn 1968). In contrast to open-space filling, however, the fenestroids show gradational contacts with the enclosing fabric and commonly contain relict clots of finer fabric. Moreover, the fenestroids do not show any sequence of crystal size or shape variation from periphery to center.

c) Disrupted mosaic: this is a two-component system comprising (1) cryptocrystalline to microcrystalline quartz and (2) megaquartz. It is characterized by contorted and disrupted primary laminae that are defined by laminoid mosaics of cryptocrystalline and microcrystalline fabrics and opaque inclusion trails. Deformation and disruption of laminae have taken place in varying degrees and styles. Deformation of lamination is mainly expressed as gentle undulations, strong contortion accompanied by disruption of laminae, water-escape pillars, and curdling of laminae, all of which are soft-sediment deformation features (cf. Lowe 1975).

The laminae deformed by gentle undulations consist mainly of cryptocrystalline and microcrystalline quartz (Fig. 9), whereas the zones of acute contortions (Fig. 10) and disruptions are occupied by microquartz as well as megaquartz. The zones of disruptions are interpreted to represent water-escape channels (see Lowe 1975, fig. 6c). Much of the escaping water rose upwards, forming pillars, which locally show the effects of upward drag in subjacent laminae and some brittle deformation (Fig. 11). These pillars apparently result from remobilization of the host laminae at a very early stage of diagenesis, when unlithified siliceous sediments were forcibly injected upwards, breaking through lithified layers (Fig. 10) or permeable zones.

A related feature is semitranslucent lenses or smeared-out wisps of cryptocrystalline to microcrystalline quartz. The boundaries between these lenses and the surrounding fabric types of megaquartz and chalcedony are vague (Fig. 12). This extremely heterogeneous mosaic resembles "structure grumeleuse" in carbonate rocks (sensu Cayeux 1935, p. 271; Bathurst 1975), and the term grumeleuse or curdled mosaic may be appropriate for this heterogeneous mosaic.

FIG. 7. Porphyrotopic fabric with randomly dispersed spherulitic chalcedony porphyrotopes in a cryptocrystalline to microcrystalline groundmass. Scale bar, 150 µm.

FIG. 8. Inequigranular megaquartz fabric. Note straight to curved intercrystalline boundaries of megaquartz crystals. Scale bar, 150 µm.

FIG. 9. Fenestroid mosaic. Note patches of megaquartz fenestroids within cryptocrystalline fabric, and small-scale contortions of laminae. Scale bar, 225 µm.

FIG. 10. Acute contortions in the internal laminae of the bedded chert. Note water-escape path originating from the synformal flank and running through the antiformal core, and in situ brecciation of cryptocrystalline fabric laminae. Scale bar, 5 mm.

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DISCUSSION

Fabric Development

The presence of soft-sediment deformation features such as water-escape structures and contorted and disrupted laminae suggests that the Penganga chert underwent a diagenetic evolution from unlithified siliceous precursor to rigid quartz chert. Microscale interlayering of different rheological states (cf. Lowe 1975) in a clay-free siliceous sediment might be related to differential rates of silica phase changes, e.g., from opal-A to opal-CT, or from opal-CT to quartz (cf. Reich and von Rad 1979; Snyder et al. 1983) during early diagenesis. The dominance of cryptocrystalline and microcrystalline quartz and very finely crystalline chalcedony suggest that an opal-CT phase was a precursor of the Penganga chert (cf. Maliva and Siever 1988).

The various fabric elements and fabric types in this chert exhibit gradational interrelationships at varying scales. The laminoid mosaic occurs generally as cryptocrystalline and microcrystalline fabrics, whereas different kinds of megaquartz fabrics occur essentially as fenestroid mosaics. The disrupted mosaic is the manifestation of extreme heterogeneity where all the fabric types coexist as irregular patches or fenestrae with their interareas. The fabric elements and their interrelationships are almost identical with those in recrystallized micritic limestone where microscale variation in the size of the microspars or gradation from micrite to microspars has been established as the result of space- or point-controlled neomorphic grain growth (Folk 1965). The presence of cryptocrystalline fabric within microcrystalline fabrics in the laminoid mosaic (Fig. 4A), gradational contacts among cryptocrystalline fabrics, microcrystalline fabrics and porphyrotopic fabrics, as well as predominance of curved intercrystalline boundaries of fabric elements (Figs. 4B, 6), collectively suggest fabric evolution by neomorphic grain growth (cf. Folk 1965; Bathurst 1975). The cryptocrystalline to microcrystalline transition in the laminoid mosaic suggests space-controlled coalescent grain growth. The porphyrotopic fabrics, on the other hand, represent point-controlled porphyroid growth. The growth-controlling factors for the fenestroid mosaic operated on a much larger scale than in the case of porphyrotopes. A continuum of the processes controlling the layer-parallel and point-controlled growth, as well as between different scales, probably finds expression in the gradational contacts in the fenestroid or curdled mosaics and cryptocrystalline to microcrystalline laminoid mosaics.

Variations in the fabrics within a microdomain clearly reflect that the factors that controlled grain growth varied in microscale. Recrystallization in siliceous deposits is essentially a crystallochemical process, as it is in limestones. The process is controlled by factors such as grain size, pore-water activity, and impurities in the system, as well as temperature and time (Kastner et al. 1977; Hein et al. 1978; Williams et al. 1985; Hein and Parrish 1987; Hesse 1989). Equigranular megaquartz with triple-point fabric is encountered in bedded chert within the thermal metamorphic aureole (Keller et al. 1985) and in the vicinity of volcanic rocks or igneous activity within the marine sediments (Robertson 1977). However, microscale fabric variation and the development of porphyrotopic fabric in the Penganga chert preclude any role of high temperature in recrystallization. General absence of recrystallization beyond the micrite stage in the host siliceous limestone also testifies against any significant effect of high temperature. Space-centered grain growth and the presence of coarse-grained fabrics in the water-escape pathways suggest a dominant role of pore water in fabric development. Siliceous and calcareous oozes as well as clayey mud are known to have very high depositional porosities of about 70% or more. During progressive diagenesis accompanying lithification, pores of different sizes and shapes develop by phase transformation of silica (Tribble et al. 1995).

FIG. 11. Pillar flanked by upward-curving dishes. Note piercing of superjacent laminae, and upward drag on the subjacent laminae. Scale bar, 150 µm.

FIG. 12. Curdled or grumeleuse mosaic. Note wispy islets of cryptocrystalline fabric surrounded by anastomosing network of coarser fabric types. Note gradational contacts of cryptocrystalline fabric islets with surrounding megaquartz fabric. Scale bar, 150 µm.

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In the opal-CT stage, channel-like pores separate areas of dense aggregates that form a rigid framework, whereas uniformly distributed tiny pores develop in the submicron-scale quartz stage (Tribble et al. 1995). The pores of different sizes and shapes could easily serve as conduits for sustained water movement and thereby affect recrystallization.

Pervasive recrystallization from micrite to microspar is common in carbonates (Folk 1965; Bathurst 1975). The cryptocrystalline-to-microcrystalline transition by coalescent grain growth in the laminoid mosaic might have been facilitated by uniform porosity distribution, high water content, and high surface area in the cryptocrystalline range. Presence of impurities in some layers perhaps inhibited grain growth, such that small stringers of cryptocrystalline fabric survived within the microcrystalline laminae. Porphyrotopic fabric might have resulted from seepage dewatering (sensu Lowe 1975) through diagenetically enhanced pores during the early stage of reduction in bulk volume. Porphyrotopic, fenestroid, and grumeleuse mosaics are likely to have developed preferentially within these water-escape routes. Differential recrystallization or cementation in different laminae or in different parts of a lamina is likely to induce permeability differences (cf. Thruston 1972), which may result in a plethora of water-escape structures.

Direct nucleation of quartz is too slow and difficult a process at surface temperatures (cf. Siever 1962) to generate cryptocrystalline and microcrystalline quartz on a rock-forming scale. The dominance of cryptocrystalline and microcrystalline quartz rather strongly supports an opal-CT precursor (cf. Maliva and Siever 1988). Transformation from a cryptocrystalline to a microcrystalline phase takes place by Ostwald ripening (Williams et al. 1985). The process is believed to be controlled by excess Gibbs free energy related to grain size and solubility difference at the solid-liquid interface, which is likely to impose a maximum limit of around 10 µm on the upper range of quartz crystal size (Maliva and Siever 1988). Directly precipitated quartz, on the other hand, is not constrained by any upper size limit (cf. Kastner et al. 1977), and precipitation of quartz directly from pore waters in carbonate sediments or in the presence of Mg ions or alkalinity has been reported by Lancelot (1973), Kastner et al. (1977), Hein et al. (1978), Williams et al. (1985), and others. The crystal size of quartz can thus be an indicator of its origin. Megaquartz formed from pore water when silica concentration dropped below opal-CT saturation (cf. Maliva and Siever 1988). Development of megaquartz fenestroids within cryptocrystalline and microcrystalline fabrics and localization of the former in the water-escape paths lend support to this idea.

Silicification by pore- and fracture-filling cementation or carbonate replacement has been advocated as a process of silica diagenesis in the Miocene Monterey Formation (Behl and Garrison 1994). However, we were unable to identify any fabric element or fabric, such as discontinuous chert beds or laminae that thicken and thin across cherts, that suggests large-scale silica cementation of the cryptocrystalline and microcrystalline quartz or carbonate replacement in the Penganga chert. Chert formed by silicification of precursor limestone normally occurs as nodules or as thin discontinuous layers within the host carbonates. There are so far no known examples of extensive bedded chert of replacement origin in the geologic record, and the bedded cherts of the Chanda Limestone are continuous over several kilometers. The fabric elements, fabrics, and fabric relationships, on the other hand, strongly weigh in favor of recrystallization of cryptocrystalline to microcrystalline quartz in the Penganga chert, and circumstantial evidence suggests an opal-CT or opal-A precursor to the quartz. The maturation pathway of silica established for the Penganga chert, therefore, closely follows the trend of diagenetic transformation of unlithified biogenous siliceous ooze to microcrystalline through cryptocrystalline quartz stages as recorded in Phanerozoic biogenic cherts (e.g., Heath and Moberly 1971; Heath 1973; von Rad and Rosch 1974; Wise and Weaver 1974; Keene 1975; Hamilton 1978; Hein et al. 1978; Hein 1987; Kano 1979; Isaacs 1981; Iijima and Utada 1983; Isaacs and Peterson 1987; Maliva et al. 1989), and the fabric evolution is consistent with, but not indicative of, biogenic derivation.

Controls on Fabric Evolution

The presence of angular chert clasts in the associated debris-flow conglomerates strongly indicates that maturation of the Penganga chert reached at least the opal-CT stage at shallow burial depth. Maturation to opal-CT at or near the sediment-water interface is further supported by the water-escape structures with forceful expulsion of unlithified sediments and disruption of rigid layers. The water-escape structures as well as variation in grain size on a microscopic scale and the porphyrotopic fabric collectively suggest that pore-water movement rather than temperature and burial depth was the decisive factor in diagenesis of this chert.

The concept of maturation of siliceous oozes as a function of burial depth (temperature) was introduced by Bramlette (1946). In the Bering Sea deposits, opal-A is transformed to opal-CT at temperatures between 35+ and 50°C, at a burial depth between 500 and 600 m (Hein et al. 1978). This transformation took place at in situ temperatures of 41° to 56°C, corresponding to a burial depths of 700 m in the Monterey Formation (Murata et al. 1977). Weaver and Wise (1972) proposed that the maturation process can be faster at very shallow burial depth under conditions of high heat flow.

Thermodynamic considerations suggest that the diagenetic pathways of silica are mainly a function of crystal shape and particle size (Williams et al. 1985). Temperature increases rates of diagenetic reactions by increasing silica concentration (Ernst and Calvert 1969; Stein and Kirkpatrick 1976; Hein et al. 1978; Williams and Crerar 1985), and maturation can take place over a wide range of temperatures (Murata and Larson 1975; Murata et al. 1977; Mizutani 1977; Piscotto 1981). Hein et al. (1978) also suggested that given enough time (c. 30 My) the maturation of opal-A to opal-CT may take place at shallow burial (< 300 m) and low temperature (< 30°C).

Experiments by Kastner et al. (1977) show that the rates of silica diagenesis are strongly enhanced by the kinetic influence of Mg^2- activity and alkalinity in pore water, particularly within carbonate-hosted sediments (see also Greenwood 1973; Lancelot 1973; Calvert 1974; Land 1979). Furthermore, the rate of transformation is significantly impeded by even a small concentration of clay minerals. Bohrmann et al. (1990), Bohrmann et al. (1992), and Bohrmann et al. (1994), from their studies on cores from the Maud Rise and Southwest Indian Ridge in the Weddell Sea, reported two instances of very early transformation of opal-A to opal-CT by c. 0.5 and 4.2 My at burial depths of 6 m and 4.7 m, respectively, and at approximately 0°C. Precipitation of opal-CT at shallow burial depth and within 1 My has also been reported by Behl and Garrison (1994) in the Miocene Monterey Formation, California. Bohrmann et al. (1994) attributed the early formation of opal-CT porcellanite layers at low temperature to high purity of the precursor diatomaceous ooze. In the pelagic environment, with high Mg²⁺ concentration, complexing of Mg²⁺ and OH^- ions in carbonate sediments reduces alkalinity and promotes CaCO₃ dissolution, and thus creates a continuous source of alkalinity (Kastner et al. 1977). The formation of opal-CT at shallow burial in the Penganga chert also appears to have been prompted by the near absence of clay minerals and other detrital materials coupled with the Mg^2--rich geochemical milieu of the pelagic carbonate depositional environment.

Sedimentologic Implications

The time-temperature (depth of burial) curve constructed by Hein et al. (1978) shows a negative relationship between the two variables and suggests that for a temperature range of 15-18°C, the maturation of opal-A to opal-CT may take about 30-40 My. The transformation of opal-A to opal-CT in the clay-free, carbonate-hosted Penganga chert, however, might have been much faster (cf. Kastner et al. 1977). Field and petrographic evidence

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lends support to this notion. The chert beds, the source of the chert clasts in the limestone debris-flow conglomerates, must have been lithified at shallow burial depth. There are only a few records of early opal-CT formation in pelagic siliceous sediments (Bohrmann et al. 1992; Behl and Garrison 1994). Bohrmann et al. (1994) reported very early opal-CT formation in the pure siliceous beds at a burial depth of 4.7 m within 4.2 My in the Maud Rise, and at 6 m depth within 0.5 My in the Southwest Indian Ridge of the Weddell Sea. In view of the close similarity of the nearly clay-free Penganga chert with the pure siliceous deposits of the Weddell Sea, it can be suggested that the precursor siliceous sediments of the Penganga chert had also rigidified under similar time-depth conditions. Assuming that the transformation from opal-A to opal-CT occurred within 10 m of burial and within a 1 to 4 My time span, the rate of sediment accumulation of the siliceous sediments would have been approximately 2-10 mm ky^-1. Biogenic silica in the modern pelagic environments is deposited at rates varying from 4-400 mm ky^-1 (Scholle et al. 1983). The average sedimentation rate of pelagic red clay and siliceous ooze in present day oceans is approximately 1-10 mm ky^-1 (Tucker and Wright 1992, p. 232) whereas the carbonate sedimentation rates in modern pelagic regime are 10-300 mm ky^-1, with an average of approximately 30 mm ky^-1 (see fig. 17 of Scholle et al. 1983). The deposition rate in the Proterozoic carbonate regime is possibly comparable with that of the Phanerozoic, because evolution of Proterozoic carbonate platforms is strikingly similar to that of their Phanerozoic counterparts (Grotzinger 1989). Little is known, on the other hand, about the rate of silica sedimentation in the pre-Jurassic, particularly those of the Proterozoic deep-sea origin (Maliva and Siever 1989). Despite the many uncertainties, the proposed rate of accumulation of the Penganga siliceous sediments may be comparable to the silica sedimentation rate in the modern pelagic regime.

CONCLUSIONS

In the Penganga bedded chert, several types of fabric elements, such as cryptocrystalline and microcrystalline quartz (5-20 µm), chalcedony, and megaquartz (2-200 µm), are identified. The cryptocrystalline and microcrystalline fabrics occur mainly as laminoid mosaics and define laminae. Alternation of cryptocrystalline and microcrystalline fabric laminae, the gradational contacts between them, and the presence of cryptocrystalline fabric stringers within the microcrystalline fabric laminae indicate variation in degree of recrystallization on a microscale, and fabric evolution by grain growth. Porphyrotopic, equigranular, and inequigranular megacrystalline fabrics mainly occur as fenestroids within microcrystalline and cryptocrystalline fabric laminae and were probably formed along pathways of water seepage. Curdled and disrupted mosaics reflect soft-sediment deformation along water-escape pathways of larger dimensions.

The dominance of cryptocrystalline and microcrystalline fabrics and their similarities to those of deep-sea and shelf biogenic cherts of various ages support the idea that the chert formed mainly by maturation of opal-CT. Variations in grain size within microdomains and presence of coarse-grained fabrics along water-escape pathways suggest that pore-water activity rather than high temperature played the key role in the fabric evolution. The maturation advanced at least to the point of imparting some rigidity to the siliceous layers at shallow burial depths, and was presumably promoted by the near absence of clay minerals, and high Mg²⁺ activity and high alkalinity of pore waters in a pelagic carbonate ramp setting. Despite many uncertainties the estimated sedimentation rate is comparable with that of the modern pelagic siliceous deposits.