Asru K. Chaudhuri

The Purana Basins of Southern Cratonic Province of India - A Case for Mesoproterozoic Fossil Rifts

Abstract Since its cratonization in the Palaeoproterozoic, southern peninsular India witnessed the development of a number of large intracratonic sedimentary basins, traditionally referred to as Purana basins, spanning in age from late Palaeoproterozoic through Neoproterozoic. The localization of these intracratonic basins, namely Pranhita-Godavari (PG), Chattisgarh and Cuddapah basins, is apparently controlled by pre-existing sutures and/or weak zones. The PG basin and the Cuddapah basin host unconformity-bound, thick, sediment-dominated successions attesting to several cycles of fluvial-shallow marine to shelf-slope-basin sedimentation. Deposition was punctuated by block uplifts resulting in local hiatuses and/or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites. Basin-margin deep faults apparently played a role in the facies distribution in these basins. Based on these features we propose that these basins initiated as continental rifts which, however, never opened up into a full-fledged ocean basin, but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the Chattisgarh succession. Spatial distribution of facies and sediment thickness in the Cuddapah and PG basins suggest that an open seaway existed to the east of the south Indian cratonic province during the Mesoproterozoic, while similar criteria point to the existence of an open seaway north of the Chattisgarh basin. Development history, including nature of inversion, suggest that the southern cratonic province of India existed as a single large continental mass since the Mesoproterozoic, in spite of episodes of supercontinent build-up and fragmentation involving India and East Gondwana during the Proterozoic.

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500‐Ma Adjustment in Indian Proterozoic Stratigraphy

The Chattisgarh Basin of east central India and many unmetamorphosed Proterozoic sedimentary basins of Peninsular India have been considered mostly Neoproterozoic (1000–545 Ma) in age. A newly recognized succession of rhyolitic ignimbrite, ash beds, and volcaniclastic sandstones near the top of the ∼2.2‐km‐thick sedimentary fill of the Chattisgarh Basin is a chronostratigraphic marker. Euhedral igneous zircons from these units give U‐Pb SHRIMP ages of 990–1020 Ma, indicating that the basin fill beneath this marker horizon is pre‐Neoproterozoic. On the basis of newly reported zircon ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A retreating fan-delta system in the Neoproterozoic Chattisgarh rift basin, central India: Major controls on its evolution

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Stratigraphic architecture of the Proterozoic succession in the eastern Chattisgarth Basin, India: tectonic implications

\end{document} Ma from the basal part of the Vindhyan Basin and accepting the consensus that all virtually undeformed and unmetamorphosed craton‐interior Proterozoic sedimentary basins in peninsular India are approximately coeval, we conclude that these basins are approximately Mesoproterozoic (1600–1000 Ma) in age. The reassigned age for these rocks (1650 to 900 or possibly ∼1000 Ma), up to 500 Ma in variance with the current notion (∼1100 to ∼518 Ma; Naqvi 2005), calls for a thorough rethinking of contemporary models concerning tectonics, sedimentation, and other geological activity that affected the Indian shield in the Proterozoic Era.

Proterozoic Rifting in the Pranhita-Godavari Valley: Implication on India-Antarctica Linkage

Abstract The Peninsular India hosts extensive record of Mesoproterozoic, and Neoproterozoic successions in several mobile belts, and cratonic basins. The successions provide excellent opportunities for chronostratigraphic classification, in tune with the chronometric classification adopted by IUGS for inter-regional correlation on a global scale. Major tectono-thermal events at 1000–950 Ma in the mobile belts, correlatable with the Grenville orogeny may be considered as the datum for Meso-Neoproterozoic classification in India. Principles of chronostratigraphic classification, however, can not be applied yet to the cratonic successions of India because of inadequate radiometric data, paucity of biostratigraphic studies, and lack of regionally correlatable stratigraphic or palaeoclimatic datum. The kimberlite magmatism which affected the Peninsular India on a continental scale at about 1100 Ma, holds the key to the identification of Neoproterozoic successions of the cratonic basins. Thus, the stratigraphically confined diamond-bearing conglomerates and/or the tuffs associated with kimberlites, may be considered as the datum to define the base of the Neoproterozoic, fixed at about 1000 Ma. Accordingly, the Rewa, and Bhander Groups in the Vindhyan basin, the Kurnool Group in the Cuddapah basin, the Jagdalpur Formation in the Indravati basin, and the Sullavai Group in the Pranhita-Godavari basin are taken to represent the Neoproterozoic successions in the Peninsular India. The Chattisgarh Group in the central India, the lower part of the Marwar Supergroup in western Rajasthan, the Badami Group in the Kaladgi basin, and the Bhima Group are the other “possible Neoproterozoics” in the Peninsula. The closing phase of the Mesoproterozoic in all these basins are characterised by stable shelf lithologic associations attesting to high crustal stability. The Neoproterozoic basins, by contrast, mark a new phase of rifting, and extension, and the basin fills exhibit signatures of initial instability which evolved with time into a more stable platformal condition. A major episode of sea level rise has been recorded in most of the basins. The riftogenic origin, and evolution of the basins are comparable with the history of Neoproterozoic basins of Australia though there is no unequivocal record of glaciation in the Indian formations.

Fluvial-aeolian interactions in a Proterozoic alluvial plain: example from the Mancheral Quartzite, Sullavai Group, Pranhita-Godavari Valley, India

A thick wedge of immature siliciclastics characterizes the basal part of the basin-fill succession in the eastern part of the Neoproterozoic Chattisgarh Basin. It comprises a mosaic of mass-flow–deposited conglomerate and pebbly sandstone, coarse-grained sandy braided stream deposits, wave-lag pebble sheets, high-energy, wave-dominated foreshore and upper shoreface facies, and wave- and tide-dominated lower shoreface facies. The succession is characterized by strong lateral and vertical facies variation. The facies associations and stacking pattern point to the development of an alluvial-fan to fan-delta complex, with outbuilding of delta lobes onto a high-energy shallow-marine shelf via a high-gradient, narrow delta plain. The delta-front deposits of coarse-grained sand interfinger and grade into prodelta mud in the seaward direction. The dominance of coarse-grained sandy detritus with a high content of fresh feldspar grains points to mechanical weathering of granites and gneisses of the cratonic basement and a semiarid climate. The sediment caliber and the climate exerted a major control on the facies and on processes of sediment transfer from the fan to the shelf. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime related to the extension of a cratonic rift basin. Repeated fault-controlled uplift of the source, followed by subsidence and transgression, generated multiple fining-upward cycles and a retreating fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced regressions, repeatedly disrupting the fining-upward pattern. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Retreating fan deltas are rarely reported and may be a stratigraphic response of marine-connected rift basins at the early stage of extension. The delta stratigraphy is analogous to the submarine fan stratigraphy. In the case where basin shales act as hydrocarbon sources, the fan deltas are potential hydrocarbon reservoirs. The coarse-grained lobes can be excellent reservoirs, with the enclosing prodelta muds acting as seals to vertical migration of hydrocarbon. The coarse-grained deposits of the delta plain and proximal delta front, occurring as laterally coalesced, large sand bodies blanketed by transgressive prodelta mud, might also form good reservoirs.

Trace fossils in middle to late triassic fluvial redbeds, pranhita‐godavari valley, south India

ABSTRACT Chemical characteristics of glauconite and ferric illite derived from alteration of K-feldspar in three Proterozoic formations of India are studied. The phases show widely varying octahedral and tetrahedral cation contents, but K-content is nearly constant. Co-existence of glauconite and ferric illite is interpreted as due to influx of fresh water in a marine-dominated sequence. A continuum in composition between illitic and glauconitic minerals at high alkali content is established. No correlation between composition of glauconite and environment of deposition or degree of diagenesis is deduced. Ferric illite in fluvial environment contains less Mg than that in a marine-dominated one. Direct pseudomorphism of K-feldspars by glauconite and ferric illite in their respective chemical en ironments controlled largely by the availability of Fe and Mg is indicated in this study. Chemical characteristics of glauconite and ferric illite in an environment with high aK+ induced by dissolution of K-feldspar are markedly different from those described earlier.

Stratigraphy and palaeogeography of the Godavari Supergroup in the south-central Pranhita-Godavari Valley, south India

Abstract The basin-fill succession of the Proterozoic eastern Chattisgarth Basin comprises a sandstone–shale–conglomerate-dominated shallow marine assemblage, and an assemblage of limestone–shale–pyroclastics deposited in off-shelf–slope–base of slope environments. The proximal and distal lithologic assemblages developed as lateral equivalents, and show signatures of sea level fluctuations amenable to chronostratigraphic correlation. The stratigraphic architecture of the proximal assemblage is characterized by cyclic deposition on different scales. The largest order cyclicity is represented by alternating sandstone sequences and mudstone-dominated sequences, of the order of several tens of metres. The smallest order cyclicity is represented by metre-scale units, punctuated aggradation cycles (PAC), bounded by isochronous surfaces. The lowermost large-scale sandstone sequence, the Lohardih sandstone, unconformably overlies the highly deformed rocks of the gneissic basement complex. The sequence was deposited as alluvial fan–fan-delta complexes, controlled by basin margin faulting. The upper two sandstones, the Daihan sandstone and the Kansapathar sandstone, represent two different phases of major sand accumulation in fan- or river-deltas. Their rapid emplacement over muddy shelf deposits along erosional surfaces signaled abrupt sea level fall, and basinward migration of facies belts. Generation and accumulation of very large volumes of sand and their rapid influx into the basin, accompanied by major changes in depositional motif, and in the circulation system in the basin, collectively indicate that larger order sand-depositing cycles were caused by tectonic uplift of the cratonic basement. The smallest order cycles, the PACs, on the other hand, may be attributed to glacioeustacy, though tectonic control cannot be ruled out. The sand-deficient, mud-dominated large-scale sequences are attributed to major transgression during periods of relative tectonic quiescence, and subsidence of the cratonic hinterland. Episodic tectonic uplift and derivation of sands from similar quartzo-feldspathic sources suggest repeated rifting of the cratonic basement and expansion of the rift into a major marine basin.