Facies architecture and sequence development in a Neoproterozoic carbonate ramp: Lakheri Limestone Member, Vindhyan Supergroup, Central India

Precambrian Research 132 (2004) 29–53

Facies architecture and sequence development in a Neoproterozoic carbonate ramp: Lakheri Limestone Member, Vindhyan Supergroup, Central India Partha Pratim Chakraborty∗ Department of Applied Geology, Indian School of Mines, Dhanbad 826 004, India Received 2 October 2002; accepted 9 February 2004

Abstract Outcrop-based study of high-resolution sequence stratigraphy (intra- and interfacies sedimentologic clues) in Neoproterozoic Lakheri Limestone, Central India, reveals definite signals for relative sea level variations and paleogeographic shifts between shallow beach-shoreface and distal-shelf. Depositional cycles in this carbonate formation are bound by unconformities or disconformities of ravinement origin. On outcrop scale the unconformities are sharp and irregular and marked by definite depletion in ␦13 C values of limestones. In the study area the entire Lakheri Limestone succession is divided into four depositional ‘sequences’ bound by type I unconformities. Different variants of system tracts (transgressive, highstand and lowstand), resulting from differences in paleogeographic locations, basin floor physiography and variable rates of relative sea level change, constitute the sequences developed on Lakheri carbonate ramp. A possible forcing of intrabasinal tectonics (both extension and compression) on meter to tens of meter thick Lakheri ‘sequences’ is inferred. Facies impersistence (both along and across depositional strike) of Lakheri Limestone is an additional complexity in depositional architecture of carbonate ramp sequence. This possibly resulted from formation of depositional slope breaks through tectonic or depositional factors. Three-dimensional reconstruction of lowstand wedge brings out the control of rugged physiography of the basin floor (imparted through patchy biohermal growth) on facies distribution in this Neoproterozoic ramp succession. © 2004 Elsevier B.V. All rights reserved. Keywords: Neoproterozoic; Lakheri; Vindhyan; Sequence; Systems tract; Bioherm

1. Introduction Correlation of chronostratigraphic surfaces, the fundamental requirement for sequence analysis (Mitchum and Van Wagoner, 1991), is severely restricted in Precambrian successions in the absence of fossil control and commonly poor dating. Chemostratigraphic correlation (based on C- and Sr-isotope profile) of Neoproterozoic successions also becomes uncertain because ∗

Fax: +91-326-202380. E-mail address: partha [email protected] (P.P. Chakraborty).

of incomplete geological record and lack of information regarding time gaps (where and how long they are? Brasier and Shields, 2000). Ambiguity prevails in explaining the ␦13 C signatures recorded from carbonate successions of this time. In Neoproterozoic Lakheri Limestone, the concern of present study, the excursion from enriched (∼+3‰) to depleted (∼−4‰) values in secular ␦13 C profile is interpreted differently by different workers; for ex. signature of Precambrian/Cambrian boundary (Friedman and Chakraborty, 1997), signal for subaerial emergence and unconformity development (Sarkar et al., 1998) or proxy for

0301-9268/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2004.02.004

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glaciation (Ray et al., 2003). None of these conclusions, however, is backed by any attempt of documentation and collation of internal architecture, facies distribution and depositional evolution of this carbonate succession in space–time frame so as to sequester inter- and intrabasinal controls on its sedimentology and stratigraphy. Limitations are inherent to the applicability of sequence stratigraphy in Neoproterozoic successions, which whenever attempted are of low resolution and heavily hinged on lithostratigraphy at different scales of observation (Christe-Blick et al., 1995; Catuneanu and Eriksson, 1999; Bose et al., 2001). Understandably, students of Precambrian sedimentology suffer from poor environmental resolution compared to their Phanerozoic counterparts. Lack of shelf-slope break (Einsele, 1985) and sedimentation either sensitive to small scale sea-level fluctuations or biased with storm (Hallam, 1997; Taylor et al., 2001) do not al-

low cause–effect modeling between accommodation change and depositional stratal patterns in Proterozoic epicontinental basin fillings, as extensively practiced in Phanerozoic shallow marine successions. Sea-level fluctuations inferred from sequence analysis in such epicontinental basin fills are of low resolution, principally of first or second order related with supercontinent amalgamation and breakup. Appreciation of small-scale sedimentary features and syndepositional stress regime in relation to observed depositional cyclicity attempted only rarely (Bose et al., 2001). These leave many of the intrabasinal forcings unattended. The present paper attempts at identification of high-resolution depositional cyclicity vis-à-vis documentation of primary depositional and synsedimentary deformational features in Lakheri Limestone, exposed in parts of Central India (Fig. 1), in order to understand the controls on a carbonate ramp evolution in a period of earth history (the Neoproterozoic

Fig. 1. Outcrop map of the Bhander Formation and the bounding formations in the Son Valley, Central India (A). Detailed location map of the studied sections of Lakheri Limestone Member is shown on the upper right (B). Broad stratigraphic subdivision of Vindhyan Supergroup, stratigraphic position of Lakheri Limestone Member and measured litholog depicting gradational transition between Ganurgarh Shale and Lakheri Limestone are presented in the lower half (C).

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era) that experienced dramatic changes in lithosphere, biosphere and atmosphere (Dalziel, 1997; Hoffman et al., 1998). Such field-based high-resolution documentation is prerequisite to understand controls on Neoproterozoic ramp evolution, which are otherwise cursorily mentioned as ‘Cap carbonate’ on the basis of stable isotope (C and O) signals. Detailed facies analysis by Sarkar et al. (1996) provided the basic framework for ‘Sequence’ analysis in Neoproterozoic Lakheri Limestone ramp. The aim of this paper is to present (i) analysis of carbonate facies types of Lakheri Limestone in terms of Neoproterozoic ramp evolution, (ii) inter- and intrafacies sedimentological evidences to comprehend relative rise or fall in water level on a Precambrian low-gradient carbonate ramp succession that lack independent paleontologic clue, and (iii) spatially variable accommodation generation and ‘system tract’ development motif on a carbonate ramp that is physiographically rugged.

2. Geological setting The Neoproterozoic marine Lakheri Limestone Member belongs to the Bhander Formation (Bhattacharya, 1996), and occupies a position towards the top of the Meso- to Neoproterozoic Vindhyan Supergroup, Central India (Venkatachala et al., 1996; Fig. 1). Patchy exposures of this limestone member cover about 200 km2 study area in Satna district, Madhya Pradesh, India (Fig. 1). This limestone is bound by fine-grained siliciclastics of the Lower Bhander sandstone above and the Ganurgarh Shale below (Fig. 1). Both these bounding members are marginal marine in origin. Lower Bhander Sandstone represents a muddy tidal flat that suffered recurrent emergence (Chanda and Bhattacharya, 1982). Facies pattern in Ganurgarh Shale is suggestive of a muddy chenier plain that experienced occasional insurgence of storm (Chakraborty et al., 1998). The Lakheri Limestone Member is virtually undeformed, unmetamorphosed and the beds are subhorizontal. Omnipresent wave features such as bimodal cross-stratification and hummocky cross-stratification, dominant southwestward slope and depth related facies organization suggest southwestward opening low-gradient ramp geometry for the Lakheri carbonate succession (Sarkar et al., 1996). Dominant micritic character of limestone,

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well preserved primary features (stromatolite laminae, pelloidal and intraclastic texture in lime wackestone), low Mn/Sr ratio (Ray et al., 2003) and poor correlation between ␦13 C and ␦18 O values obtained from mineral separates (Ray et al., 2003) and bulk carbonate samples (Kumar et al., 2002) of Lakheri Limestone Member support the view of Chakraborty et al. (2002) that discarded any large scale deep burial diagenesis suffered by this unit. In absence of a radiometric database, biostratigraphic and chemostratigraphic signals provided clues on age of Lakheri Limestone succession. Venkatachala et al. (1996) summarized the available age proposals from earlier works (Sarkar, 1974; Kumar, 1976; Rao and Ghosh, 1977; Maithy and Meena, 1989) and favored middle to upper Riphean age (1350 ± 50–600 Ma) for Lakheri Member on the basis of stromatolite biostratigraphy and microfossil evidence. Recently, Kumar et al. (2002) have put 700–570 Ma age bracket for the entire Bhander Group on the basis of carbon, oxygen and strontium isotope geochemistry. Comparing Sr isotope signatures of carbonates from the Upper Vindhyans with first-order secular variation in seawater 87 Sr/86 Sr signature through the entire Precambrian time, Ray et al. (2003) also suggested its Mid-Neoproterozoic age (750–650 Ma). Upward gradational transition from the Ganurgarh Shale to the Lakheri Limestone without any significant shift in paleogeography indicates transgression, albeit of very small scale (Chakraborty, 1996). Sections, which expose this transition, show sandstone-shale changing upward to carbonate-shale (Fig. 1), maintaining shallow-marine signature all through. A total change in depositional condition from siliciclastic to carbonate with minor change in bathymetry indicates that the siliciclastic Ganurgarh Shale gave rise to a very low gradient surface on which a small rise in relative sea level resulted large-scale inundation. This observation supports the long held view that the Lakheri Limestone developed on an epiric ramp with uniform, slow subsidence (Chanda and Bhattacharya, 1982; Chaudhuri and Chanda, 1991).

3. Methodology In Proterozoic sedimentary successions recognition of isochronous surfaces is heavily dependent on ap-

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preciation of stratal stacking pattern and their bounding discontinuities (Christe-Blick et al., 1990). In absence of chronologic control, sedimentological logging and facies mapping provided the necessary background for this exercise. Better exposures of Lakheri Limestone from a number of shallow quarry sections spread over the study area and in Thomas River cutting (Fig. 1) offered opportunity to pursue this work. A total of 154 m outcrop sections from 17 localities was examined and vertical variations in stratal stacking pattern, texture and physical structure were documented. Thin sections of selected samples were examined to determine subtle textural variations across the key surfaces. Depositional processes and environments were interpreted from primary sedimentological features and vertical shifts in paleogeography were inferred from variation in relative bathymetry. ␦13 Ccarb signals were used as confirmatory evidence for identification of some of the unconformities (identified independently through sedimentologic clue) and early diagenetic meteoric cement on them. Seven representative measured sections were constructed with the data compiled from nearby exposures. These sections offered scope for reconstruction of depositional dip-parallel profile in the Lakheri Limestone succession on northeast- southwest transect. Correlations between these composite sections were done with the help of marker horizons represented by three unconformities (UNC), maximum flooding surfaces (mfs) and one laterally extensive inferred paleohorizontal (PH) surface. This PH plane served as a datum plane of primary importance for relative placement of the measured sections (discussed below). 3.1. Lithofacies and depositional environment The Lakheri Limestone Member has been subdivided into six facies types on the basis of lithology, bed geometry and sedimentary structures (Table 1). Many facies appear throughout the succession (Fig. 2), and others appear only once. Therefore, facies are listed in decreasing order of abundance. All the limestone facies types are interspersed with thin massflow beds, majority of which are interpreted as products of synsedimentary slides (Sarkar et al., 1996). The massflows are lenticular in shallow water facies and parallel sided within the deep-water facies. These massflow

units are not considered in composite litholog measurements of the studied sections (Fig. 2) because of their minor volumetric proportion (<5%) and lateral wedge-out character. 3.2. Facies A: stromatolite facies Occurrence of this facies is recorded in all the measured sections and at different stratigraphic levels (Fig. 2). Stromatolites of this facies are present with two broad geometry viz. stratiform and biohermal (Fig. 3a and b; Table 1); the latter is, by far, the more common type. Occasional clusters of microscale columns within stratiform type resemble ‘tufa’ stromatolites described by Grotzinger (1989). Within biohermal type there are two broad subtypes. In both subtypes, the individual columns are club shaped and their internal laminations terminate against the column margin. However, in one subtype columns are frequently branching, preferably oriented and discrete, while in the other subtype columns are radially arranged within composite forms, enveloped by sets of wrinkled lamina. The height and head diameter of the columns of the former subtype vary from 0.7 to 1.2 cm and from 2.5 to 3.5 cm, respectively and the same for the other subtype are 12–16 and 8–10 cm, respectively. Beukes and Lowe (1989) ascribed biohermal forms of stromatolites to relatively deeper water with respect to stratiform stomatolites. However, branching with preferred orientation of columns in one of the biohermal subtypes presumably indicate strong current or wave influence (Hoffman, 1976; Glumac and Walker, 1997). Common lateral transitions of this facies into planar and cross-stratified calcarenite indicate shoreface paleogeographic setting for both. In rare instances, the stromatolite facies is observed to maintain gradational contact with the plane laminated lime mudstone (discussed later) indicating occasional extension of this facies on to the shelf. 3.3. Facies B: planar and cross-stratified calcarenite Although present in each of the sections studied, best exposure of this facies can be observed at Sajjanpur section where it attained the maximum thickness (4.35 m; Fig. 2). This facies unit is tabular in geometry and terminates laterally against the stromatolite

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Table 1 Lithofacies of Lakheri Limestone Member and their inferred depositional environment (modified after Sarkar et al., 1998) Facies type

Description

Facies association

Environment

A. Stromatolite (SM)

Digitate stromatolite with branching columns of low relief Columnar stromatolite with non-linked vertically stacked columns, occasionally inclined Domal composite stromatolite with micritic wavy laminated hemispheriod Microbial laminite with thin, flat, planar, crinkly laminae, occasional presence of quartz silt grains, desiccation cracks present Intraclastic, oolitic or peliodal light-coloured calcarenite with tabular bed geometry. Internally plane laminated and cross-stratified with chevron and herringbone geometry. Wave cum current ripples on bedding surface; polymodal current pattern Sheet-like bed geometry Calcarenites internally wave ripple laminated and hummocky cross-stratified; shale units massive or plane laminated. Gutter casts at the sole of calcarenite beds Plane laminated with thick–thin alternations between calcareous laminae which occasionally show internal grading. Rare graded or ungraded relatively thicker beds of possible storm origin Plane laminated or massive shale Thin sandstone/siltstone interbeds with wave ripple laminations Red colored; coarse grained, poorly sorted, massive, plane laminated or trough cross-stratified

Common association with facies A and C; rarely overlies facies D; contact with facies C and D gradational; common intrafacies transition between domal and columnar variety

Agitated intertidal to shallow subtidal

B. Planar and cross-stratified Calcarenite (PCS)

C. Heterolithic Calcarenite-shale (HL)

D. Plane laminated lime mudstone (PLM)

E. Shale (SH)

F. Sandstone (SST)

Shallow subtidal

Protected subtidal extending upto inner shelf Restricted upper intertidal to supratidal

Sharp lateral and vertical transition to facies B

Wave- and tide agitated shoreface

Lower contact erosional, upper contact non-erosional Stated above

Storm dominated inner shelf

Grades into facies C, E and rarely facies B; contact with facies A invariably sharp, erosional

Offshore below storm wave base

Invariable sharp contact with overlying and underlying facies Contact with underlying facies erosional

Distal offshore with occasional insurgence of storm Fluvial

facies. Alternations between cross-stratifications of two different scales, large and small (average set thickness 18 and 7 cm, respectively) constitute the basic motif for this calcarenite facies (Table 1). The

larger cross-sets are predominantly planar, their arrangement replicate chevron commonly and herringbone locally. The smaller sets are locally chevron-like and show lateral transitions into planar laminations

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Fig. 2. Composite lithologs showing facies types of Lakheri Limestone in studied sections. Two surfaces viz. an interpreted paleohorizontal surface (PH plane) and Ganurgarh Shale–Lakheri Limestone contact are used primarily for section correlation. Placement of western most section Emilia is done based on interpreted unconformity UNC 1. Note presence of all the unconformities only towards shoreward section Aber.

over a lateral extent not more than 2 m (Fig. 3c). Straight crested near-symmetric ripples (wavelength and amplitude 12 and 0.8 cm, respectively) with N–S crestline trend are locally present at bed tops of these chevron cross-stratified sets. The cross-strata orientation reveals a bimodal (NE-SSW) pattern for the larger and a broad unimodal (eastward) pattern for the smaller sets. At places, the facies is also characterized by broad wavy laminations that resemble hummocky and swaly cross-stratifications (average wavelength and amplitude 80 and 10 cm, respectively). Asymmetric ripples with sinuous and bifurcating crests (wavelength and amplitude 17 and 0.85 cm, respectively) mantle this facies unit only towards its top

part. Migration direction of these ripples is unimodal (northeastward). The cross-stratification patterns indicate dominance of oscillatory flows. Uncommon herringbone cross-stratifications in the larger sets indicate also the tidal influence. The N–S trend of the symmetric ripples provides a reliable indicator of paleoshoreline alignment. The large-scale cross-stratifications with well-dispersed paleocurrent pattern indicate shallow water origin for this facies. The smaller group of cross-stratifications showing common transitions to plane laminations record unstable character of the flow switching rapidly between lower and upper flow regime. This presumably resulted from

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Fig. 3. Photographs of lithofacies A–D. (a) Composite stromatolite (coin diameter 2.6 cm). (b) Stratiform stromatolite (pen length 13 cm). (c) Lateral transition of the cross-stratification to planar laminations in facies B. (d) Gutter cast at the sole of calcarenite bed in facies C and (e) thick–thin laminae alternations in facies D. Note presence of debrite unit within facies D.

flow expansion over the ripple crests under heavy cloud of suspension load. Between the two groups of cross-sets, both bearing unequivocal wave signature, the large-scale cross-stratifications are interpreted as of fair weather and the smaller sets of storm origin (Johnson, 1977). The storm origin for the smaller sets is invoked for unstable nature of the flow, steep energy gradient and unimodal but widely dispersed paleocurrent pattern (Brenchley, 1985). The imprint of storm wave is apparent in hummocks and swales (Harms et al., 1975; Walker and Plint, 1992). Although typical of shelf origin, these structures are not uncommon in the nearshore zone, particularly in the lower shoreface region (Mukherjee et al., 1987; Pedersen, 1985; Bose and Chaudhury, 1990; De Celles and Cavazza, 1992; Krassay, 1994). Towards top part of this facies unit the asymmetric ripples with migration at a high angle to inferred paleoshoreline are evidently wave-cum-current generated in the shallow water paleogeography. 3.4. Facies C: heterolithic calcarenite-shale facies Unlike facies A and B, occurrence of this facies is regionally restricted and can be observed towards the upper part of the studied stratigraphic interval be-

tween Girgita and Babupur section (Fig. 2). Calcarenite sheets interbedded with either plane-laminated lime mudstone or greenish grey shale constitute this facies (Table 1). Composed of ooids, intraclasts and peloids of fine sand size, the calcarenite sheets display abundant wave-ripple lamination (De Raaf et al., 1977), hummocky cross-stratification (wavelength 14–22 cm and amplitude 6–10 cm) and unidirectional small-scale cross-stratification. The bases of the calcarenites are invariably sharp while their tops less sharp or even gradational. Sole of the beds are replete with gutter casts (Fig. 3d), prod and bounce marks. The orientation of the gutter casts is NE-SW. Bed tops of certain calcarenite units are mantled with wave ripples with wavelength and amplitude of 12 and 0.7 cm, respectively. The ripples trend in the same direction as that of gutters i.e. NE-SW. Repeated alternations between cross-laminated calcarenite and mudstone indicate frequent fluctuations of energy. The omnipresent wave features, such as HCS, identify the calcarenites as storm-laid beds while the mudstone beds are indigenous. Sharp, erosional base and near gradational top of the calcarenite beds reflect episodic and waning nature of the storm generated flow. Apparently, lowered wave base during storm caused calcarenite deposition in otherwise mud

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depositing environment. Occurrence of wave ripples on top of these storm beds suggests reworking of the storm-laid carbonate sands by fair weather wave. This facies, though formed largely between fair weather and storm wave base, also extended somewhat above the fair weather wave level. The orientation of gutter casts (NE-SW) at high angle to the paleoshoreline (N-S) suggests that carbonate sand transportation took place offshoreward with little evidence of veering by coriolis force (Leckie and Kristinik, 1989). 3.5. Facies D: plane laminated lime mudstone facies Except for the Aber section, exposures of this facies are recorded in all other measured sections (Fig. 2). This facies, constituted of lime mudstone, is characterized by millimeter-thick plane laminations (Fig. 3e). Each of the carbonate laminae is draped by a very thin (commonly less than a mm) veneer of argillaceous material. The carbonate laminae, in places, show normal grading. The calcareous laminae (ignoring argillaceous veneers) characteristically show thick–thin alternations. These lamina sets are occasionally intercalated with coarser (commonly sand and granule grade) and thicker (average thickness 4 cm) massflow beds (Fig. 3e), which are either massive or normally graded, and are invariably parallel sided. Fine grain size, planar and laterally persistent character and rhythmic thick–thin characters are reminiscent of tidal deposition (De Boer et al., 1989). The argillaceous, laterally persistent interlaminae of uniform thickness appear to be products of tidal slackening. The grading within the carbonate laminae suggest carbonate deposition from plumes and is consistent with delivery of carbonate sediment below the wave base during spring tides (cf. Williams, 1991). The alternating argillaceous laminae are interpreted as the result of usual slow siliciclastic mud settlement in the offshore. Presence of clasts derived from stromatolite facies in the massflow debrites within this facies suggests downslope position for this facies with respect to stromatolite facies. 3.6. Facies E: shale facies Best developments (9 m thick) of this facies are recorded at Babupur and Sagmania section where this facies gradationally overlies plane laminated

mudstone facies and, in turn, with a sharp contact underlies the sandstones of Lower Bhander Sandstone Member (Fig. 2). Internally, this facies besides being shale-dominated also characterized by regular thin interbeds of silt and very fine sand (Table 1). The shales are plane laminated and greenish grey in color. The coarser clastic interbeds (silt and fine sand) are wave ripple laminated and their base invariably sharp than their tops. The plane laminated greenish grey shale is indicative of a calm and dysaerobic environment. The sharp base and gradational top of the sand/silt interbeds indicate episodic deposition of the coarser clastics by waning flows. The internal wave ripple laminations within these interbeds strongly favor deposition during unusually strong storm events. Gradational transition between this facies and Plane laminated limestone is indicative of distal shelf paleogeography for this facies. 3.7. Facies F: sandstone facies This loosely cemented pink to red colored poorly sorted arkosic (feldspar content > 32%) sandstone facies (Table 1) is encountered only towards the topmost stratigraphic level of the Lakheri Limestone at the easternmost section of the studied sector at Aber (Fig. 2). Wedge shaped geometry of this facies is apparent in thinning of this facies westward. Maximum thickness recorded for this facies in Aber section is 1.5 m. Almost the entire facies unit is massive except the topmost (0.4 m) part where coset (average thickness 16 cm) of trough cross-stratifications and poorly developed plane laminations are present locally. Foreset measurements of trough cross-stratifications (n = 8) revealed unimodal southwestward paleocurrent direction. The top surface of this facies is rippled. The ripples have average wavelength and amplitude 8 and 2 cm, respectively. For major part of this facies unit it is difficult to infer the sand depositing mechanism. The massiveness, in absence of any evidence for intense bioturbation in the entire Lakheri Limestone, may be due to either rapid deposition or postdepositional obliteration of internal structures. Red color, poorly sorted arkosic character and unimodal paleocurrent pattern, however, suggest a possible fluvial origin for this facies unit (cf. Eriksson et al., 1998).

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Fig. 4. Schematic illustration of the depositional environments identified in the Lakheri Limestone succession (not to scale). See Figs. 2, 6 and 10 for explanation of symbols.

3.8. Facies arrangement and stacking patterns The facies types identified in the Lakheri Limestone Member (Table 1) dominantly range in paleogeography from shallow beach-shoreface to distal shelf beyond storm wave base (Fig. 4); fluvial incursion took place only in a single instance towards the topmost part of the succession. Transgressive and regressive trends are recognised through intrafacies and interfacies variations. Except for the most shoreward planar and cross-stratified calcarenite facies (facies B), which records only the regressive trend, all other facies types viz. stromatolite (facies A), heterolithic calcarenite-shale (facies C) and shale (facies E) record signals for both relative rise and fall in water level. Intrafacies expression of water level change recognized either through bed stacking pattern or by sedimentologic response viz. changes in stromatolite morphology, relative dominance of current and wave, etc. Interfacies expression is, however, more sharp and decisive; manifested either through change in lithology or transition between facies of different paleogeography. Facies successions those correspond to depositional cycles are bound either by unconformities or by disconformities. The latter are interpreted as ravinement surfaces (cf. Nummedal and Swift, 1987; Shirai and Tada, 2000) as they occur at the base of ‘deepening-up’ parasequences and at times mantled by transgressive lag. 3.8.1. Shallowing-upward trends These successions (3–9.5 m in thickness; average thickness ∼ 4.5 m) of widely varying expression rest

either on a deepening-upward succession or on another shoaling-upward succession with or without intervening thin lags of grainstone/packstone. One such calcarenite succession exposed at Sajjanpur area reveals an upward structural transition from hummocky and swaly (Leckie and Walker, 1982) cross-stratification to large-scale chevron cross-stratification to asymmetric combined flow ripples within planar and cross-stratified facies (facies B) (Fig. 5). Unidirectional asymmetric ripples (wavelength:amplitude ratio 20:1) mantle the section. From interpreted facies paleogeography and up section dominance of current over oscillation a landward paleogeographic shift is interpreted from the shoreface to a shallower regime. Analogous prograding parasequence has also been documented from the easternmost extremity of the studied stretch at Aber, where towards top of the Lakheri Limestone section planar and cross-stratified calcarenite facies (facies B) alternates with reddish marl. Red coloration and upward transition from interbedded marl-limestone to red arkosic sandstone attest the shallowing-up trend there. The coarsening-upward heterolithic calcarenite-shale facies (facies C) section of Sagmania (Fig. 6a) presents another example for shallowing trend. Bed thickness variation; wavelength, amplitude of the wave generated structures and ratios thereof (Fig. 6b), similar with many regressive siliciclastic storm successions; indicate upward increase in depositional energy. While wavelength and amplitude of wave formed structures increase upward, their ratio decrease. Up the section change in pebble-filled massflow channel geometry (from nearly parallel-sided to broad lentic-

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Fig. 5. Common structural sequence in shoaling-upward succession assimilated from field observations (hummocky cross-stratification-swaly cross-stratification-asymmetric wave ripples—beach structures-bed top swash ripples).

ular) with steady decrease in width:depth ratio further corroborates the regressive trend. Another significant change this succession witnessed is the upward change in interbed lithology from shale to plane laminated lime mudstone. Analogous regressive trend has been interpreted through shelf to shoreface progradation in a storm dominated setting under stable to low relative water level rise (Walker and Bergman, 1993; Brenchley et al., 1993). The shallowing up ‘parasequence’ building motif is also evident in a 4.5 m thick stromatolite section (facies A) at Emilia, where club-shaped stromatolite columns are detached from each other and inter-columnar areas filled with variable sized mat fragments. The columns show preferred orientation (northeastward) all through. Steady upward decrease in degree of consistency of column orientation is suggestive of shallowing and progressive dominance of current or wave. Another evidence of shallowing in this section comes from steady decline in stromatolite column height up the section, eventually to digitate form. Similar reduction in stromatolite size has been attributed as indicator for decrease in water depth

(Beukes and Lowe, 1989; Glumac and Walker, 2000). However, the lack of mudcrack, subaerial exposure features or vadose features in this section indicates that the shoaling of the depositional substrate continued only up to shallow subtidal domain (cf. Glumac and Walker, 2000), not beyond that. 3.8.2. Deepening-upward trends In Lakheri Limestone, transgressive trend is registered in stromatolite (facies A), heterolithic calcarenite-shale (facies C) and shale facies (facies E). Two different transgressive modes are recorded in stromatolite facies; one at Sajjanpur and Aber section and the other at Ramnoi section. In the former, transgression or drowning is inferred using the same parameters those were used for recognizing regressive trend in Emilia section. The upward change in morphology from separated columns (club shaped with strong preferred orientation) to composite form with higher column diameter and weak preferred orientation (Fig. 7) in a 25 m thick stromatolite section at Sajjanpur signals rise in water level (cf. Beukes and Lowe, 1989). The other transgressive trend is iden-

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Fig. 6. (a) Photomosaic of heterolithic (HL) succession with an intervening maximum flooding surface (mfs) indicated by the man (within white box) (a) and detailed litholog of the section with stratigraphic variation in calcarenite:shale ratio (b). Note thinning- and fining-up character below the ‘mfs’ and thickening- and coarsening-up trend above. Variations of different parameter up the section are shown in c.

tified in a 6 m thick stromatolite section at Ramnoi, where biostromal stromatolite is found to alternate with nearshore plane and cross-stratified calcarenite facies and finally overlain by thick (4.7 m) plane laminated lime mudstone facies of offshore origin. Similar to Sajjanpur and Aber, morphological expressions for increase in water depth are also noted in stromatolites of Ramnoi section. Transgressive trend is also recognized in the lower part of heterolithic calcarenite-shale facies (facies C)

succession at Sagmania section from bed thickness variation (Fig. 6a and b). Unlike the regressive subsection, where lime mudstone similar to facies D constitutes the indigenous intervals, in transgressive part the indigenous sediments are constituted entirely of shale similar to facies E. This compositional difference could have been related to decline in autochthonous carbonate production under high-gradient rise in water depth (Sami and Desrochers, 1992; Schlager, 1992). Similar transgressive history is also recorded in fining-

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Fig. 7. Transition from inclined columnar to domal, composite stromatolite geometry in a deepening-upward succession. Rose diagrams on the right of the figure are showing variations in stromatolite orientation through the succession. Note up-section weakening in preferred accretion on stromatolite column growth.

and thinning-upward transition in the lower part of shale facies at Satna-Aber area. Both the deepening-up successions discussed here are terminated by condensed intervals, which are granular lag at Sajjanpur section and dark gray shale at Sagmania section, respectively. The massive, dark grey granular lag units are essentially consists of calcarenite and calcirudite clasts with either clast-supported or matrix-supported fabric. The marked difference in lithologic expression of condensed interval is explained through paleogeographic difference between the two studied sections.

4. Chronostratigraphic surfaces The mainstay of the present study is the correlation of seven region-wise representative composite sections. Three surfaces surved the purpose as entire facies motif could be constructed considering them as reference. Two of them are conspicuous lithological

contacts those coincide with the boundary of Lakheri Limestone viz. the contact with the Ganurgarh Shale and the same with the Lower Bhander Sandstone. Within the Lakheri Limestone, the third key surface is at the top of the progradational heterolithic calcarenite-shale facies, where it sharply gives way upward to the planar and cross-stratified calcarenite facies. Sarkar et al. (1996) interpreted this sharp transition between the shoreface (facies B) and inner shelf (facies C) facies assemblage as primarily of bathymetry controlled and paleohorizontal (PH plane). Using this contact as isochronous surface and measuring its vertical distance from the member-bounding surfaces the depositional facies motif was framed (Fig. 8). Lateral and vertical variation in the facies motif readily reflects paleogeographic shifts that finally led to identification of three surfaces of basin wide regression (UNC 1–3) with marked exposure and erosion with or without discernible facies omission. On the outcrop scale, these unconformities are knife sharp and irregular. Irregularity is principally

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Fig. 8. Facies motif and systems tracts development across the Lakheri Limestone ramp in its depositional-dip profile. Possible locations and geometry of lowstand wedges (overlying the unconformities) are also shown. Detailed dip-parallel profile of the lowstand unit that overlies UNC 3 is given in Fig. 11.

because of formation of solution breccia, network of irregular-shaped dissolution voids (Fig. 9) and spar-filled desiccation cracks. Dissolution voids, particularly on unconformity 1 (UNC 1) and unconformity 2 (UNC 2), are filled with blocky non-ferroan calcite spars and multi-generation cements including large fibrous gypsum (Fig. 9). Occurrence of these features in selective stratigraphic intervals those also depict depletion in ␦13 C value (in comparison to the background values) prompt interpretation of these features related to subaerial exposure and unconformity development. Significant depletion (␦13 C value –1.57 to −2.9‰ in host carbonate and –4.45 to –6.11‰ in crack-filling sparry calcite cement) recorded in sam-

ples collected from these unconformity planes at the most shoreward section Aber (Sarkar et al., 1998) in comparison with immediately underlying and overlying limestones (average +2.8‰) confirm emergence and precipitation from organically charged meteoric water along those intervals (cf. Allan and Mathews, 1982; Booler and Tucker, 2002). All the three unconformities are recorded in close spacing only towards the basin margin (northeastward) at Aber, where depositional sequences are highly condensed and entire Lakheri Limestone succession is compressed (ca. 48 m thick) between its enveloping members (Figs. 2 and 8). Only the unconformity 3 (UNC 3), formed on stromatolite facies,

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Fig. 9. Network of dissolution voids (a) and solution breccia (b) on the unconformity planes within Lakheri Limestone (Matchstick length 4.5 cm, bar = 0.2 mm). Large fibrous gypsum as one of the constituents of meteroric cement (c) on unconformity planes.

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could be traced westward over 7 km distance. Unconformity 1 and 2 though could not be traced with certainty over long distances; their sharp incidence above deepest carbonate facies (facies D) undoubtedly indicates sharp and major paleogeographic shifts associated with their development.

5. Sequence stratigraphic framework for Lakheri Limestone ramp Between Ganurgarh Shale and the topmost unconformity UNC 4 the Lakheri Limestone succession is divided into four depositional sequences (Fig. 8); their thickness varying from 3.5 to 35 m. The basal Lakheri Limestone sequence documents the terminal history of the transgression that resulted siliciclastic chenier development (Ganurgarh Shale Member) on prograding braid plain system of Upper Rewa Sandstone (Chakraborty et al., 1998). Though most of the composite sections constructed for the studied stretch of the Lakheri ramp are bottom truncated (see Fig. 2); the Ganurgarh Shale–Lakheri Limestone transition exposed at sections Ramnoi, Aber and Jura allowed correlation between them and visualize, within the exposure limitation, the ramp configuration at its initiation phase. On aggraded planar muddy chenier substrate the Lakheri Limestone ramp initiated gradationally with continuing transgression. Such within-trend flooding that initiated carbonate deposition in place of siliciclastic mudstone is never in a position to serve as a systems tract or sequence boundary, which is why it is not considered as a surface of sequence stratigraphy for the present purpose (Catuneanu, 2002). Surfaces of downlap or maximum flooding can only be recognized in such small-scale sequence through approximate levels of deepest facies (Strasser, 1994). For this lowermost Lakheri Limestone sequence maximum flooding is recorded towards its shoreward part (Ramnoi section) where stromatolite facies alternating with planar and cross-stratified calcarenite facies eventually gives way upward to distal plane laminated lime mudstone facies (Figs. 8 and 10a). Absence of plane laminated mudstone or any other evidence of condensation in the most shoreward section (Aber section) suggests termination of this distal facies against some physiographic high on the east of Ramnoi. Basin physiography must

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have played significant role in spatially differential change in accomodation and consequent variable rate of change in bathymetry during rise or fall in water level (cf. Liu et al., 1998). Considerable thickness of shallow water stromatolite–calcarenite (facies A and B) assemblage at Aber, towards top of this sequence, indicates creation of a physiographic high through higher rate of carbonate production and aggradation that kept pace with increasing water level (‘Catch-up’ phase of Kendall and Schlager, 1981). Development of unconformity 1 (UNC 1) on deepening-up succession over most parts of this sequence indicates sharp fall in relative sea level though magnitude of this fall may not be large. In low-gradient ramp settings, the effect of even small fluctuation in relative sea level can have regional effect through large-scale migration of shoreline (Sarkar et al., 2001). The calcarenite unit (facies B) that overlies unconformity 1 records the lowstand deposition associated with the unconformity development. That the entire Lakheri Limestone ramp turned shallow following unconformity 1 is evident from the presence of calcarenite unit in the two extremities of the studied stretch (viz. Emilia and Aber). At Ramnoi section this calcarenite unit is absent and unconformity 1 is overlain by a thin (ca. 12 cm thick) matrix-free grainstone layer, which signals transgressive erosion (Swift et al., 1991) and its possible lag origin. The stromatolite/microbial laminite facies (facies A) in Aber and top-truncated Emilia section that overlies the lowstand calcarenite unit registers this transgressive event. Slow rate of this transgression and resultant low rate of accommodation space generation is evident in (i) relatively smaller thickness (3.5 m) for this sequence and (ii) shallowing-upward character in the stromatolite section that indicates rate of accommodation generation never outpaced rate of carbonate production. Similar shallowing-upward facies evolution pattern has been visualized as common depositional motif for many ancient shallow marine carbonates deposited on platforms or ramps (James, 1984). Maximum flooding is recorded in the occurrence of distal plane laminated lime mudstone facies at Sajjanpur section at the top part of this sequence (Figs. 8 and 10b). Lack of signature for maximum flooding at Aber section may be either (i) due to failure of sea level rise to overcome the rate of carbonate production/deposition in this shallow part of the basin where carbonate production rate is conceiv-

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Fig. 10. Schematic representation of basin configuration and spatially differential ‘Sequence-specific’ evolution pattern inferred from Lakheri Limestone Member.

ably higher or, (ii) maximum flooding product, even if present, was thin and eroded out in subsequent period of emergence and unconformity development. Progradational planar and cross-stratified calcarenite facies occupies the basal part of Sequence 3 where it overlies plane laminated lime mudstone facies at Sajjanpur and stromatolite facies at Aber (Figs. 8 and 10c). Resting on unconformity 2 (UNC 2) this calcarenite unit of shoreface paleogeography represents product of possible lowstand. The flooding that terminates this lowstand deposition resulted in the establishment of transgressive system tract (TST), identified at Sajjanpur section through variation in stromatolite morphology. A grainstone layer (ca. 0.5 m thick) of transgressive erosion origin tops this TST. This cement filled grainstone layer is indicative of condensation and steepening in the gradient of water-level rise (Abbott, 1998). The immediately overlying parasequence reveals distal plane laminated limestone (PLM) facies at its base followed by stromatolite (SM) fa-

cies that indicates shallowing and progradation (HST; compare Sagmania section). The grainstone layer thus records maximum flooding and marks shift in stacking motif from retrogradational to aggradational and progradational. Absence of this late-stage progradational motif in landward section at Aber reflects higher rate of carbonate production in mid ramp area (cf. Burchette and Wright, 1992). The Lakheri Limestone section that overlies unconformity 3 (UNC 3; Figs. 8 and 10d) constitutes the youngest and best exposed depositional sequence and displays highest variability in facies motif. Coarse, poorly sorted trough cross-stratified (with unimodal paleocurrent pattern) arkosic sandstone that overlies this sequence in Satna–Aber area, marks the upper bounding unconformity for this sequence that turns gradational (conformable) basinward (Dolni section). At the base of this sequence, immediately overlying UNC 3, the wedge shaped body (maximum thickness at Sajjanpur area) constituted principally of planar and

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cross-stratified calcarenite facies is interpreted as lowstand wedge (discussed in the later section). Onlapping relation of overlying HL facies with the wedge marks the initiation of transgression, recorded in retrogradational stacking in the lower half of heterolithic calcarenite-shale (HL) facies. Separated by a 32 cm thick shale interval (mfs; cf. Van Wagoner et al., 1988) the progradational highstand part of HL facies overlies the TST (Fig. 6). Understandably, towards the upper half, sedimentation rate exceeded rate of sea level rise, which caused turnaround from transgression to regression (Catuneanu and Eriksson, 1999). With continued shallowing digitate stromatolite (tufa; Grotzinger, 1989) crown the section on the west and planar and cross-stratified calcarenite facies acquire beach structure on the east. Contrary to this overall trend, in the central part of the studied stretch between Sagmania and Babupur, regression is evidently halted halfway and a transgressive lag (packstone) caps the regressive section. Plane laminated lime mudstone and shale facies succeed. The regressive trend was reestablished only in the top part as evident in the coarsening-upward trend in shale facies. 5.1. Basin physiography and lowstand deposition Average paleogeographic position of any particular basin location relative to the lowstand and highstand shoreline (i.e. the average paleobathymetry) exerts major control over the facies development, stratal packaging pattern and the local expression of bounding surfaces. Unlike siliciclastic settings where local paleogeography is function of variations in physical processes and available grain size, ramp geometry (particularly the biohermal build-ups) and carbonate production rate exert major controls on system tract development in carbonate systems (Emery and Myers, 1996). Facies geometry reconstruction from exposures and closely spaced boreholes in the Lakheri Limestone succession suggests high degree of irregularity in the shape of stromatolite facies in contrast to dominant tabular or sheet geometry of all other facies types (Sarkar et al., 1996). Stromatolite facies with its biohermal growth though imparted rugged physiography with local depositional slopes on the Lakheri basin floor, never achieved wave-baffling character of a reef, which is apparent in omnipresent

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wave signatures in all other facies types (cf. Read, 1982; Burchette and Wright, 1992). The hummocky ramp floor, however, resulted frequent spatial variation in facies types, particularly in form of accommodating lowstand wedge at the shelter of seaward shelf break of stromatolite banks. One such lowstand wedge (Fig. 11) overlies partially eroded randomly oriented stromatolite facies of composite geometry, constituted of calcarenite (facies B) with underlying and overlying strongly imbricated stromatolite subunits and extends for 15 km across depositional strike between Ramban and Babupur. While sharp and erosional base of the wedge that rests on and terminates shoreward against composite (without any preferred inclination) stromatolites coincides with the sequence bounding unconformity UNC 3 (Figs. 8 and 11), the within-wedge facies contacts between calcarenite and inclined stromatolite facies are expression for within-trend normal regressive or flooding surface without any major sequence stratigraphic significance (Catuneanu, 2003). With onlapping and downlapping relation heterolithic calcarenite-shale (facies C) overlies the wedge and finally rests on stromatolite facies in both northeastern and southwestern extremities (Fig. 11). The planar erosional contact of heterolithic facies with its underlying stromatolite facies on northeast of the wedge (marked by 2/3 grain thick granule layer rich in quartz; transgressive lag, Walker and Plint, 1992) turns out to a correlative conformity (sensu Van Wagoner et al., 1988) towards southwest, where heterolithic facies tend to conform broad undulatory geometry of underlying stromatolite facies. In the proximal section, the heterolithic calcarenite-shale unit lacks any TST component and the transgression is recorded only within the thin lag. The calcarenitic wedge internally shows upward transition hummocky-plane lamination-straight crested ripples (wavelength = 12 cm, amplitude = 1.5 cm) and finally mantled by cuspate ripples; a gradual progradation of shoreline with increasing dominance of current over wave. The shoreface and beach features within the wedge bear telltale signature for seaward shoreline retreat. Little direct evidence for this lowstand deposition, however, exists in the landward (northeastern) sector; may be due to transgressive erosion that erased lowstand terrestrial record, if any. Contrary to highstand shedding from steep-sided carbonate ramps, basinal carbonate deposition is

46 P.P. Chakraborty / Precambrian Research 132 (2004) 29–53 Fig. 11. Dominantly calcarenitic lowstand wedge body exposed between Sajjanpur, Sagmania and Babupur (see Fig. 1 for section locations). Note onlapping character of HL facies on to the wedge.

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Fig. 12. Signatures of extension and compression in Lakheri Limestone succession. Slides of (a) stepped, (b) planar and (c) concave-up geometry demonstrate the extension. Compression is depicted in small-scale thrust (d) and mesoscopic fold (e).

believed to be significant in low angle ramps and rimmed shelves in periods of sea level lowstand (Hunt and Tucker, 1992). Lowstand wedge, however, is not expected on a low gradient ramp like Lakheri Limestone, which is dominantly confined within shelf and nearshore domain and lacks shelf-slope break. Differential subsidence or high shelf gradient encourage lowstand wedge accommodation during regression and ensures its preservation during subsequent transgression. A third alternative that can be invoked for accommodation and preservation of such wedge on

a carbonate ramp like Lakheri Limestone is the spatially differential biohermal growth and generation of hummocky basement configuration.

6. Signatures of intrabasinal tectonics Although devoid of any regional post-depositional deformation, the Lakheri Limestone Member reveals meso- and micro-scale features those bear telltale signatures for basin-scale extension and compression

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in course of its depositional history. Sarkar et al. (1996) interpreted the cm to decimeter thick carbonate mass flow products interspersed within the Lakheri Limestone succession as of synsedimentary slide origin. Further, detailed investigation revealed features indicative of both extension and compression, recurrently occurring at different stratigraphic levels within this member. Most prominent among extensional features are low-angle listric faults and slides of variable geometry (viz. stepped, planar, concave-up, sinuous and ramifying; Fig. 12a–c) with deformations both within hanging wall and footwall. While small-scale rollovers, convolutions and block rotations are among those that characterize the hanging walls, the footwall deformations are identified through shrading (chaotically deformed laminae of possible creep origin; Coniglio, 1986) and enechelon sets of sigmoidal fractures. Microscopic attestation in favor of tension comes from synchronous ‘S’ shaped mineral growth within the cracks (in form of lenses or dominos associated with base of the slides) with progressive coaxial opening in the middle and centripetal growth near the wall (Nicolas, 1987) and orthogonal relationship between stylolites and tension fractures. In contrast, the compactional features are larger (meter scale) in dimension and present either in form of low angle thrusts or outcrop-scale folds. Conjugate sets of thrusts with planes striking at high angle to each other are noticed (Fig. 12d) within grainstone/packstone, which at their intersection resemble sharp-crested ripple like forms (wavelength = 55 cm, amplitude = 6 cm) on bedding plane exposure. Folds (average amplitude 0.85 m; 12e) observed at these levels are with vertical axial plane and found truncated at their top against succeeding level of extension. Bose et al. (2001) identified similar cm to decimeter scale signatures of intrabasinal extension and compression from other Members of Bhander Formation (viz. Sirbu Shale, Lower Bhander sandstone and Upper Bhander sandstone) and attributed them with possible paleoseismic events related with minor tilting in an overall sag stage of basinal development.

7. Discussion Distinction between types 1 and 2 character of a sequence, as conventionally defined, is difficult in

epicontinental basins in absence of shelf-slope break or depositional shoreline break (Taylor et al., 2001). Lowstand products on such ramp are dominantly detached and characterized by basinally isolated shallow marine forced regressive wedges (Ainsworth and Pattison, 1994; Mc Murray and Gawthorpe, 2001) those can be traced 10–20 km basinward from depositional shorelines. Undoubted recognition of unconformities from isolated exposures in such settings is heavily dependent on identification of deeper water facies immediately below the suspected lowstand products. Clear evidences of emergence together with abrupt basinward shift of facies along three intraformational unconformities (UNC 1–3) in the Lakheri Limestone succession bear telltale signatures for forced lowstands (Plint and Nummedal, 2000) and type 1 character for these sequence boundaries (Van Wagoner et al., 1988). Such basinward migration of shoreline is expected to be associated with encroachment of continental agencies, particularly when the carbonate ramp is attached with continental margin (cf. Emery and Myers, 1996). Absence of significant siliciclastic input, except above the topmost member-bounding unconformity of Lakheri Limestone, is corroborative with dominant muddy chenier paleogeography of the underlying member (i.e. Ganurgarh Shale) where rivers already attained graded character at or near base level. Lakheri Limestone developed with aggradation/retrogradation on this muddy coastal plain in response to slow rise in sea level and curtailed supply of siliciclastics. Except for the development of terminal unconformity, the intermittent sharp water level falls experienced by the Lakheri ramp were never drastic enough to disturb the prevailing alluvial equilibrium. The only encroachment of continental agency in form of fluvial sandstones above the terminal unconformity and its dominant arkosic character is suggestive of hinterland tectonism associated with unconformity development and transition of Lakheri ramp from carbonate to siliciclastic one. Facies inconsistency both along and across depositional strike in the proximal part of the Lakheri Limestone depositional sequences is principally related with development of lowstand wedges associated with stromatolite banks. In addition to variability in accommodation space and sediment supply, physiography has also been recognized (Church and Gawthorpe, 1997; Mc Murray and Gawthorpe,

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2001) as an important control on the temporal and spatial evolution of depositional systems. Differential shoreline migration associated with relative sea level change can result from along-strike variation in physiography (Mc Murray and Gawthorpe, 2001). On an otherwise low-gradient carbonate ramp the steep slope gradient of biohermal build-up can render relatively small basin-ward shift in the position of the shoreline for a given sea level fall, making space available for accommodation of the wedge. Placement of the sequence boundary in relation to such sharp-based lowstand wedge (dominated by shoreface sediments) is controversial; Posamentier et al. (1992) placed such wedge above sequence boundary within the lowstand system tract, whereas Hunt and Tucker (1992) placed the same deposits just below the sequence boundary, at the top of the underlying highstand system tract. The introduction of ‘falling stage system tract’ (FSST; Hunt and Gawthorpe, 2000) was aimed to resolve this problem making distinction between sequence boundary, transgressive and regressive surfaces of marine erosion and ravinement surface. Proust et al. (2001) highlighted a more complex situation in Kimmeridgian–Tithonian section of Northwestern France, where within shoreface sediments late lowstand wedge product (cf. Posamentier et al., 1992) follows ‘forced regressive wedge’ component (cf. Hunt and Tucker, 1992) with a sequence boundary in between. Despite availability of accommodation, formation of wedges in falling stage or at lowest sea level stand is not favored for Lakheri Limestone lowstands in view of their dominant calcarenitic composition and the general consensus on reduced physical weathering and diminished clast generation potential of carbonate ramps at their lowstands (Hunt and Tucker, 1992). Profusion of shallow-water wave signatures and progradational character of the wedges are indicative of their generation above sequence boundary with slow rise in sea level at late lowstand. Thick (14 m) heterolithic calcarenite-shale facies on distal Lakheri Limestone ramp bears signature for frequent storm action, which might had provided the necessary high energy required for calcarenite generation on such carbonate ramp (Seguret et al., 2001). Dominance of current over wave up the wedge is indicative of gradual filling with sediment supply exceeding the available accommodation space generated through slowly rising water level.

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Similar progradational history is also recorded from other attached lowstand wedge sections represented by single shoreface deposit (Ainsworth and Pattison, 1994). The upper bounding surface of the wedge at Maihar–Babupur section (Fig. 11) represents the transgressive surface above which the stacking pattern in heterolithic facies becomes retrogradational defining the transgressive system tract (TST). Amalgamation between sequence boundary and the transgressive surface was noticed only towards landward section of the wedge where sequence boundary is overlain by patchy grainstone lag of transgressive erosion origin. Significantly, TST overlying the lowstand wedge is thicker compared to its counterpart in further seaward section. This probably indicates generation of higher accommodation space at a given rate of water level rise in the area where the wedge formed. In absence of paleontologic or geochronologic data on possible time gaps represented by the unconformities the present work is not in a position to assign order to the observed depositional sequences of Lakheri Limestone. Correlation of these sequences across the field area, however, confirms that the forcing behind these inferred oscillations is regional one. On a siliciclastic-poor carbonate ramp such regional forcing can be surmised through climatic forcing including glacioeustasy or episodic tectonics. Time gap between Neoproterozoic glacial episodes (Stuartian, Marinoan etc.; in intervals spanning over 100 s of Ma) does not allow glacioeustacy to account for meter to tens of meter thick small-scale Lakheri Limestone depositional sequences. Eriksson (1999) has also addressed the problem of extending glacioeustacy as forcing mechanism for stratigraphic cyclicity in Precambrian successions. The most plausible forcing conceived for observed sea level cyclicity in Lakheri Limestone Member is intrabasinal tectonics (both extensional and compressional) that might had forced coeval transgression and regression through small-scale variation in available accommodation space. Though evidences for basin scale extension and compression are recognized under the present study, further high-resolution documentation is warranted to establish one-to-one correlation between intrabasinal tectonics and inferred relative sea level variation. The variable facies architecture between different sequences is interpreted as high-resolution sequence development motif under the mutual interaction between low to moderate

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amplitude relative sea level fluctuation and physiographically rugged carbonate ramp that witnessed differential biohermal growth. In absence of subsurface database, the present study had limited scope for providing possible range of complexity in the facies architecture that may remain hidden within the Proterozoic ramp carbonate successions.

8. Conclusion Lakheri Limestone developed with transgression over muddy coastal plain of Ganurgarh Shale and witnessed repeated transitions between beach/shoreface and offshore maintaining uniform low-gradient ramp geometry. Temporal paleobathymetric variations were inferred through intra- and interfacies variations and abrupt basin-ward shift of shallow water facies types were interpreted as definite signals for sea level fall. Besides the topmost unconformity across which the ramp turned carbonate to siliciclastic, three other type I intraformational unconformities with karst features on their landward parts have been recognized within the Lakheri Limestone succession. Marked depletion in 13 C (both within sediment and cement) compared to their underlying and overlying sediment column characterizes these unconformity surfaces, as commonly observed on Phanerozoic emergent surfaces. Bounded by these basin-scale unconformities the Lakheri Limestone is divisible into four time slices with lowstand, transgressive and highstand sea level history represented through various constituent facies tracts. Though sharp, the water level falls associated with the development of these intraformational unconformities were never drastic enough to destabilize the prevailing alluvial equilibrium so as to result significant siliciclastic input. Facies inconsistency recorded in the proximal part of the depositional sequences primarily emerged from lowstand wedges developed at the depositional slope breaks of physiographic highs generated through spatially differential biohermal growth. Facies contacts between the facies types enclosing the wedge turn conformable to unconformable landward. Meter scale thickness and persistence of the observed sequences over wide lateral stretch are suggestive of regional scale forcing induced by small-scale intrabasinal tectonics that oscillated between extension and compression.

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