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Lakshanhatti stromatolite, India： Biogenic or abiogenic？

Journal of Palaeogeography 2016年第3期5卷 292-310页

作者： Adrita Choudhuri Subir Sarkar Wladyslaw Altermann Soumik Mukhopadhyay pradip k.bose Department of Geological Sciences Jadavpur University Kolkata 700032 India Department of Geology University of Pretoria Pretoria 0002 South Africa

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 baft

关键词： StromatoHte Lamina continuity Mesoproterozoic Lakshanhatti Dolomite Member Inorganicprecipitation kerogen enrichment Filamentous microbiota

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

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Journal of Palaeogeography 2014年第3期3卷 270-296页

作者： Sunipa Mandal Subir Sarkar Nivedita Chakraborty pradip k.bose Department of Geology Asutosh College Department of Geological Sciences Jadavpur University

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.

关键词： Mio-Pliocene Siwalik Group Darjeeling Himalayas spatial variation axialriver reconstruction vegetation

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

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Journal of Palaeogeography 2014年第3期3卷 233-256页

作者： Subir Sarkar Nivedita Chakraborty Anudeb Mandal Santanu Banerjee pradip k.bose Department of Geological Sciences Jadavpur University Department of Earth Sciences Indian Institute of Technology Bombay Powai Mumbai-400076 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.

关键词： mixed siliciciastic-carbonate succession Garudamangalam Sandstone Formation Upper Cretaceous river-mouth bar India

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Evolution of Neoproterozoic siliciclastic kerur Formation in the light of sequence stratigraphic framework: Badami Basin, karnataka, India

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Journal of Palaeogeography 2024年第4期13卷 706-737页

作者： Soumik Mukhopadhyay Nipun Ghosh pradip Samanta Arunava Sen Soumyadeep bose Department of Geological Sciences Jadavpur UniversityKolkata 700032India Atomic Minerals Directorate for Exploration and Research Shillong 793019India Department of Geology University of North BengalDarjeelingSiliguri 734013India

Field-based sedimentology,state of the art facies analysis and sequence stratigraphic framework analysis have revealed the controls of local and global tectonics,basin-marginal slope,climate and changes in relative sea level (RSL) over the sedimentation pattern and evolution of a Neoproterozoic kerur Formation within the Badami Group of kaladgi Supergroup in *** entire succession shows three major cycles of *** study and fluvial architectural elemental analysis suggest considerable variations in depositional environments as well as palaeogeography.A transition from basin-margin alluvial cone deposits to braided system,initially with fluctuating ephemeral flows then to a steadier semi-perennial nature,is discernible within the 1^(st) cycle,in response to decreasing depositional slope with rising water *** initial alluvial cone and braided ephemeral streams of high slope areas is designated as a product of low accommodation systems tract (LAST),while the semi-perennial system with steadier flows,representing the axial river of the initial rift valley,appears to be a product of high-accommodation systems tract (HAST).The 2^(nd) cycle begins with a perennial and steady braided river system and grades upward to a shallow marine succession,comprising wave-dominated,well-sorted sandstone,with a granular transgressive lag at the ***,the bottommost fluvial interval of the 2^(nd) cycle constitutes the lowstand systems tract (LST).The marine succession represents deposits of outer shelf offshore to foreshore-beach settings and is composed of an initially deepening and fining upward transgressive systems tract (TST),followed by a coarsening and shallowing upward highstand systems tract (HST) with a maximum marine flooding surface (MFS) in between,demarcated by a shale-rich condensed *** 3^(rd) cycle,with its prograding alluvial fan and aggrading braided fluvial deposits and restricted occurrence,represents only the low accommodation systems

关键词： Neoproterozoic kerur Formation Facies and architectural elemental analysis Palaeogeography Sequence stratigraphy Basin evolution

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