Siliciclastic-carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone(Ariyalur, India)

Mixed siliciclastic-carbonate rocks constitute the Upper Cretaceous Garuda- mangalam Sandstone Formation, Ariyalur （India）, and offer an opportunity to look into the broad spectrum of mixing of compositionally and genetically different components. The palaeogeographic reconstruction indicates that deposition in the nearshore zone differed strongly in energy and active processes operatives due to the presence of a shore-parallel river-mouth bar. The western wing of the Mississippi bird-foot delta is considered to be a present-day analogon. Facies analysis in combination with petrography clearly shows the variability in palaeoenvironmental characteristics, both biogenic and non-biogenic. It also indicates dia- genetic uptake of carbonate that filled empty spaces and actively replaced original compo- nents. Chemical staining followed by limited application of cathodoluminescence and energy dispersive X-ray analysis （EDAX） hint at intricacies in mixing arising from the compositional variations in the carbonate components. A model of siliciclastic-carbonate sediment mixing, including both the depositional and diagenetic developments, is presented; it is aimed at generating a better overview of, and a deeper insight into, the physical and chemical mechanisms involved.

Microbial mat-related structures shared by both siliciclastic and carbonate formations

Microbiota has always been the dominant life form, records of which are preserved in delicate forms within siticiclastic rocks. More pronounced record in the form of stromatotites possibly obscured the fact that many of the same delicate structures may be recognizable within carbonate rocks too. The Neo- proterozoic Bhander Limestone in central India bears many such structures that are quintessentially similar to microbial mat-related structures reported from the Paleoproterozoic Chorhat Sandstone preserved within the same, Vindhyan Basin. Extensive microscopic, ultramicroscopic, and geochemical studies address the appre- hension that such bedding plane structures in carbonate rocks could be merely weathering products. Trapping, binding and stabilitization of sediment by microbial mats are atl evident. Preferred pyritization along the inferred, predefined microbial mats confirmed on the basis of EPMA （Electron Probe Microanalysis） results, and the enhanced carbon content along these mats layers and within suspected mat chips associated with them, are revealing. Raman spectroscopy, indeed, evinces enhanced kerogen content within both mats and mat chips. Interestingly, these microbial mat layers are recognized selectively within the lower of the two tiers of the Bhander Limestone. The lagoonal carbonate of the lower tier of the Bhander Limestone is muddy and contains a substantial proportion of silt-sized quartz grains that possibly impeded stromatolite growth. Stromatolites abound in the wave agitated upper tier of the Bhander Limestone which is dominated by oosparite. This paper provides evidence that the delicate microbial mat-related structures reported so far only from siticictastic rocks can al.so be recognized within carbonate formations, and hopes to stimulate the search for additional such features, more preferably within carbonates originated in shallow and quiet water.

Ancient rip current records and their implications: an example from the Cretaceous Ukra Member, Kutch, India

Poorly-sorted conglomerate patches rich in granules or sturdy fossils or both, and reddish mud matrix within the interstices stand out amidst fine-grained siliciclastic shelf sediments of the trangressive systems tract(TST) of the Lower Cretaceous Ukra Member, Kutch Basin, India. The siliciclastic shelf sediments contrast the conglomerates with their remarkable lateral extension. The fossils belong to a low-diversity group of sedentary bivalves that can be traced into the shoreface facies assemblage. The shelf sandstones are almost always sculpted by wave structures,especially hummocky cross-stratification while textures in the conglomerates suggest that the sediment settling was generally from suspensions. Textural variations in conglomerates reflect an immediate variation in flow viscosity prior to the downloading. The current structures obtained from the conglomerates record offshoreward palaeocurrent, in contrast to the shore-parallel palaeocurrent in the TST. The hummocky cross-stratified(HCS) beds are interpreted as seasonal storm deposits, while the conglomerate patches are taken as rip current deposits induced by waves of much longer periods. The glauconite-rich shale that alternates with conglomerates is probable fair-weather products. The conglomerates could not be recognized either in the coarse-grained shoreface deposits occupying the lower part of the overall fining-upward TST or in the coarsening-upward and glauconite-depleted highstand systems tract(HST). In contrast to the TST, the HST is dominantly tide-imprinted, having shore-normal palaeocurrent direction. It appears that intensification of waves and weakening of tides during transgression favored strong rip currents generation, which had presumably caused severe damage to the sea coast and to the shell banks growing preferably at the necks of the rip current channels. Rapid lateral facies transitions in the shoreface deposits at the basal part of the TST suggest enhanced irregularity in the coastline, possibly because of the mega cusps indented upon it. Frequency and intensity of storms enhanced during periods of global warming caused the transgression of the Early Cretaceous Ukra Sea.

Syn-sedimentary tectonics and facies analysis in a rift setting: Cretaceous Dalmiapuram Formation, Cauvery Basin, SE India

The Cretaceous(Albian-Cenomanian) Dalmiapuram Formation is one of the economically significant constituents in the hydrocarbon-producing Cauvery rift basin, SE India that opened up during the Late Jurassic e Early Cretaceous Gondwanaland fragmentation. The fossil-rich Dalmiapuram Formation,exposed at Ariyalur within the Pondicherry sub-basin of Cauvery Basin, rests in most places directly on the Archean basement and locally on the Lower Cretaceous(Barremian-Aptian) Basal Siliciclastic Formation. In the Dalmiapuram Formation, a facies association of tectonically-disturbed phase is sandwiched between two drastically quieter phases. The early syn-rift facies association(FA 1), records the first carbonate marine transgression within the basin, comprising a bar-lagoon system with occasionally storms affecting along the shore and a sheet-like non-recurrent biomicritic limestone bed on the shallow shelf that laterally grades into pyrite e glauconite-bearing dark-colored shale in the deeper shelf. Spectacular breccias together with varied kinds of mass-flow products comprise the syn-rift facies association(FA 2). While the breccias occur at the basin margin area, the latter extend in the deeper inland sea. Clast composition of the coarse clastics includes large, even block-sized limestone fragments and small fragments of granite and sandstone from the basement.Marl beds of quieter intervals between tectonic pulses occur in alternation with them. Faulted basal contact of the formation, and small grabens filled by multiple mass-flow packages bear the clear signature of the syntectonic activity localized contortions, slump folds, and pillow beds associated with mega slump/slide planes and joints, which corroborates this contention further. This phase of tectonic intervention is followed by another relatively quieter phase and accommodates the late syn-rift facies association(FA 3). A tidal bar-interbar shelf depositional system allowed a transgressive systems tract motif to grow eventually passing upwards into the Karai Shale Formation, whose contact with the Dalmiapuram Formation is gradational.

Lakshanhatti stromatolite, India： Biogenic or abiogenic？

Abstract The extraordinary lateral continuity of isopachous stromatotite laminae in the -87 m-thick Mesoproterozoic Lakshanhatti Dolomite （India） evinces chemical precipitation. Fan-shaped crystals grown on tamina surfaces further corroborate this contention; growth of fan-shaped crystats under the overhanging stromatolite column-margin indicates direct carbonate precipitation from ambient waters. The fan-shaped crystats are stacked up, separated onty by thin dark micritic taminae. In a retativety upper stratigraphic in- terval of the formation, tighter laminae characterized by a dotted texture and traversed by numerous winding tubutar voids change graduatty upwards into dark micritic taminae. Some sporadicatty distributed lenticutar intraciastic beds also have the similarly dark micritic coatings. Ctear carbonate cement crusts atso occur between laminae and between successive dark micritic coats around intraciasts. DuU cathodotuminescence （CL） characterizes this cement as weU as the cement lining within earty dia^enetic voids, in contrast, the taminae with dotted textures show dirty orange luminescence, white the dark micritic taminae and the dark micritic grain-coats display dear bright orange luminescence. Pyrite and its pseudomorphs are preferabty concentrated along the dark micritic laminae. Carbon content in these dark micritic components, whether laminae or coats, is much higher than in the lighter components, exceeding what can be accounted for their CaMg（COs）2 composition. A large part of this carbon is kerogen, ptausibly biogenic. The dark components are, therefore, reasonably, though not unequivocally, assumed to be microbial mats. Degradation of the mats might have given rise to the tight laminae with dotted textures. The fuzzy-margin tubes within the tight laminae probabty manifest the escape of gases generated during organic matter decomposition. Si-At-rich terrigenous fines thinty draping the dark carbonaceous taminae was possibty the resutt of baftling and trapping of terrigenous fines by filamentous microbiota. Dark carbonaceous taminae encasing intraciasts was considered to be the resutt of binding and stabitization by microbiota. Spike-tike growth of discrete laminae strongty suggests an occasionat breakdown of colonial homeostasis of phototrophic microbiota. The microbiat community thus appears to have prayed an active rote in stromatolite-buitding in the Lakshanhatti Dolomite Member, even though the simultaneous existence of direct carbonate precipitates from sea water indicates a hybrid origin of these stromatotites. Resting on shelf sandstone and being capped by dark offshore shale, the Lakshanhatti Dotomite had been deposited in distal offshore, but not at the great depth, perhaps in an epeiric sea. Progressive deepening inhibited direct carbonate precipitation, δ13C and δ18O values suggest normal open marine salinity during deposition.

Overprint of neotectonism along the course of River Chel,North Bengal,India

This paper aims to unveit neotectonic imprints in topography, drainage and sediments in the 46.25 km tong course of the River Chet from its source down to its attuvial fan at the base of the Himatayan Mountain Front in the Darjeeting-Jatpaiguri districts of India. A semi-circutar ridge detimits its primary catchment. Within confinement of this watershed basin the drainage pattern is composite being convergent atong the periphery and divergent on a butte inside. AtE these geomorphic neotectonic imprints are accom- panied by ramp and fiat structures and spectacutar mytonitization of rocks. High hypsometric index and convex shape of the hypsometric curve derived from the centrat near-straight course of the river between the primary catchment and the Main Frontat Thrust （MFT） atso reftects tectonic youthfutness of the river course. It is wett manifested atso in widety variabte stream index and stream gradient index ratios （SL/K） often exceeding 2. In response to neotectonism, this river course as a whore shifted westward between 1962 and 2007. Maximum reduction of the stream gradient on top of the MFT is etoquent enough about recent uptift of the thrust ridge. The high average stope gradient of canyon watt about 45.68° is wett consistent with this uptift. Very tow channet-width/vaUey-height ratio along the river further corroborates the uptift. The attuviat fan system of the River Chet is comprised of five morphogenetic fans stacked one above another with a tendency to shrink and shift progressively upstope. They differ from each other in terms of titt, axiat orientation, primary depositionat surface gradient and convexity in transverse section and thus present a writ of ongoing tectonism. Progressive upward increase in the share of distat crystattine rocks in ctast composition within attuviat fan package is a dear proxy for southerty advancement of the MFT. Concomitant increase in maximum ctast size is in good agreement with sediment source uptift. Art the five fans are, however, dormant now. Present-day River Chet deeply incises through ate of them and suggests further basement uplift in the context of frequent evidences of neotectonism att around, atthough the rote of climate remains uncertain in absence of adequate data.

Palaeogeography, palaeohydraulics and palaeoclimate of the Mio-Pliocene Siwalik Group, eastern India

South-southwestward palaeocurrent swerved to east-southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio-Pliocene foreland system within which the Siwalik Group depositd in Darjeel- ing-Jalpaiguri Districts, eastern India. Palaeocurrent pattern is found to be multi-modal on the fans, virtually unimodal on the axial channel zone and again multi-modal, more profoundly, on the flood plain. Coarse siliciclastic mass-flows were progressively eliminated and gave way to predominant bed-load transport downfan and the axial river, and then to suspensionload dominance in fine siliciclastics on the flood plain. Distal flood plain lacustrine sediment included most of the coals and the entire bulk of the dolomitic limestone. Further resolution in palaeogeography within the frame of aforementioned foursome facies associations is elicited in twenty-six distinctive facies altogether. Critical evaluation of chemical indices （CIA, CIW, ICV, PIA, as well as Rb/Sr ratio） for weathering and depleted （3180 values indicate a high precipitation rate. The contention is further corroborated by the high discharge rate calculated from cross-set thicknesses within the main channel deposits. Reconciliation of various relevant data sets collected or calculated from all known worksites along the entire 2000 km-Iong exposure belt of the Siwaliks along the Himalayan foothills reveal confluence of two tributaries, one from the west and the other from the east, close to the present study area before escaping onto the Indian plains. Channel parameters, channel-belt width and discharge thus attained maxima in the present study area. The precipitation rate and temperature increased eastward overall as a prelude to the modern trend in this regard. 13C enrichment indicates that the transition from C3 to C4 vegetation had already set in.

Palaeoenvironmental and biostratigraphic implications of microbial mat-related structures:Examples from the modern Gulf of Cambay and the Precambrian Vindhyan Basin,India

A stretch of the modern hypersaline coastal plain of the Gulf of Cambay was chosen to examine the distribution of the microbial mat-related structures(MRS) on siliciclastic sediments in the intertidal and supratidal zones.The abundance of MRS increases from the lower intertidal zone to the upper supratidal zone while the type of MRS records a systematic change.While the lower intertidal zone exhibits wrinkle structures,sieve-like surfaces and patchy ripples in places,the upper intertidal zone exhibits diverse MRS related to reduced current activity on the mat layer and intermittent exposure.MRS in the upper intertidal zone include wrinkle structures,sieve-like surfaces,gas domes,reticulated surfaces,multi-directional ripples,patchy ripples,rolled-up mat fragments,setulfs and occasional petee ridges and cracked mat surfaces.The lower supratidal zone is characterized by increased occurrence of petee ridges,gas domes and cracked mat surfaces compared to the upper intertidal zone.The upper supratidal zone is distinguished by the presence of abundant cracked mat surfaces,petee ridges,gas domes and wrinkle structures.The presence of cm-scale,disc-shaped microbial colonies(DMC) with a variety of internal structures is a unique feature of the Gulf of Cambay study area.While wrinkle structures occur in all the coastal zones,setulfs occur close to the boundary between the upper intertidal and lower supratidal zones.An attempt has been made to compare the distribution of MRS in this modern environment with those in the ~1.6 Ga Chorhat Sandstone of the Vindhyan Supergroup for high-resolution palaeoenvironmental interpretation.The upper part of the intertidal segment of the Chorhat Sandstone is distinguished from its lower part by the presence of abundant cracked mat surfaces,petee ridges and gas domes in the former,while wrinkle structures,Kinneyia,rolled-up mat fragments,patchy ripples and multi-directional ripples are equally abundant in both parts.The lower part of the intertidal segment of the Chorhat Sandstone is thus analogous to the upper intertidal zone of the modern Gulf of Cambay environment,while the upper part of the Chorhat intertidal segment reflects prolonged exposure close to the high tide line.The bottom-most part of the intertidal segment of the Chorhat Sandstone with fewer MRS corresponds to the lower intertidal zone at Cambay.Inferred disc-shaped microbial fossils within Vindhyan sandstones are analogous to the DMC found in the modern environment and these features do not have any biostratigraphic implication.