SEDIMENTOLOGY AND PALAEOGEOGRAPHY OF CENOMANIAN NIMAR SANDSTONE, CENTRAL INDIA

Abstract

Cretaceous global sea level rise affected almost all the continents including India, resulting in inundations of huge landmasses. Evidences of late Cretaceous marine transgression is chronicled in different basins (namely, Kutch, Bengal-Assam, Shillong, Cauvery, few parts of Himalaya, and Son-Narmada rift valley, etc.,) in the western, north-eastern, southern and central part of the Indian subcontinent. Sedimentological study of those sedimentary basins reveals important clues to understand the late Cretaceous palaeoclimatic and palaeotectonic changes in the Indian subcontinent. The Son-Narmada rift valley in central India is a repository of such deposit in the Bagh Group, which bears the evidences of late Cretaceous inundation of the Indian subcontinent. The present thesis is an attempt to assess the Nimar Sandstone, the lower most lithounit within the Bagh Group, which chronicles the evidences of first marine incursion in the area. The research work aims to delineate the palaeogeographic changes in Central India during late Cretaceous time based on detailed facies architecture, sediment-organism interaction pattern, geochemical analysis and sequence stratigraphic study of the Nimar Sandstone. Detailed sedimentological study of the Nimar Sandstone from excellently preserved and exposed sections reveals total seventeen facies types, which are clubbed under five distinct facies associations. Channel-fill facies association (FA-1) consists of clast supported conglomerate facies (1A), matrix supported conglomerate facies (1B), pebbly sandstone facies (1C) and trough cross stratified sandstone facies (1D). Overall lenticular shaped geometry of this facies association, presence of both matrix and clast supported conglomerates, dominance of immature sediments and alternate pebble-rich and pebble-poor sandstones indicate fluctuation in energy within fluvial channel filling system. Overbank facies association (FA-2) consisting of sandstone-mudstone interbedded facies (2A) and plane bedded sandstone facies (2B) with dominance of fine clastics in a fining-upward succession suggests deposition in overbank/ flood plains of the channels. The fluvial-dominated fluvio-tidal facies association (FA-3) consists of (i) large-scale trough cross stratified sandstone facies (3A) and (ii) planar cross-stratified sandstone facies (3B), both characterized by tidaly-reworked features like mud draped foresets (tidal bundles) with reactivation surfaces. Such sand-dominated facies with influence of tidal current indicates a fluvial-dominated tide–influenced channel bar depositional ii | P a g e setting. Tide-dominated fluvio-tidal facies association (FA-4) is more tide dominated within a fluvio-tidal interactive system, consists of sandstone mudstone heterolithic facies (4A), mud clast conglomerate facies (4B), plane laminated fine-grained sandstone-mudstone facies (4C), bioturbated sandstone facies (4D) and green sandstone facies (4E). This facies association is enriched with various tidal features, changing into wave reworked tidal features towards upsection. Dominance of lenticular bedding and wavy bedding with bi-directional cross-strata, sigmoidal strata bundles and laterally accreted tidal bundles confirm an inter-tidal to sub-tidal flat setting for deposition of FA-4. Predominance of tidal features along with very low wave reworking indicates a relatively sheltered situation, where waves were not strong enough for reworking the tidal sediments. Shore facies association (FA-5) consists of coarsening up to fining up sandstone sequence of fossil bearing sandstone facies (5A), Thalassinoides–Ophiomorpha bearing thinly laminated sandstone/mudstone facies (5B), wave ripples bearing sandstone facies (5C) and massive mudstone facies (5D). Presence of marine body fossils, round crested ripples, interference ripples and ladder back ripples indicates deposition in wave dominated beach condition. Gradual increase in mud content and decreasing bioturbation intensity towards the up section indicates increasing water depth resulting in more suspension deposition. Overall facies architectural pattern of the Nimar Sandstone and fining upward succession indicates a tide-dominated wave led sheltered estuarine setting. Based on the lateral and vertical distribution of the facies association, three major palaeodepositional conditions are envisaged - (i) a river dominated (FA-1) inner estuary with an fluvial channel and overbank setting (FA-2), changing into localized bay-head delta zones when influenced by tidal current (FA-3), (ii) tide dominated central estuary zone, forming dominantly inter-tidal to sub-tidal setting in the middle part of the succession, and (iii) an outer estuary zone where wave-reworked open shore condition prevailed with sub-tidal flat environment. Predominance of tidal features in FA-3 and FA-4, and wave reworked tidal features in upper part of FA-4 and FA-5 helps to understand precisely the palaeodepositional environment. The major tide-generated primary structures in the study area are laterally accreted strata bundle with reactivation surface, bi-directional cross strata set with development of herringbone cross strata, vertically accreted strata bundles, sigmoidal strata, and cyclical tidal rhythmites. From the eastern to western part and towards the up section of the studied area, increasing dominance iii | P a g e of wave generated structures and sequential gradation of tide-to wave-generated structures signify gradual shift in depositional condition from intertidal to subtidal to open marine setting within an estuarine condition. Appearances of SSDS are restricted in particular beds and they are delimited by undeformed bed. The major SSDS from Nimar Sandstone includes convolute laminae, load and flame structures, pseudonodules, slump structures, contorted beds, syn-sedimentary faults and sand dykes. SSDS are mainly associated with the sandstone and mudstone beds of FA-3 and FA-4. The SSDS-bearing beds are identified as seismites. Association of the seismites with fluvio-tidal facies association indicates a correlation between marine incursion and reactivation of faults, which triggered basinal subsidence and shift in depositional condition from fluvial to fluvio-tidal interactive. The studied estuarine sediments are characterized by abundant trace fossils. Altogether fourteen ichnoforms, grouped in four ichnofacies are identified from the study area namely, mixed Skolithos-Glossifungites ichnofacies, mixed Cruziana-Glossifungites ichnofacies, Glossifungites ichnofacies and mixed Skolithos-Glossifungites ichnofacies. Overall ichnodiversity is low and bioturbation intensity gradually increases towards western part of the study area with presence of both suspension- and deposit-feeding burrows. Distribution of trace fossils signifies pattern of sediment-organism interaction in response to changes in energy condition, fluctuating salinity and oxygen availability during sedimentation under fluvio-marine interactive system. Geochemically the rocks show wide range of major oxide concentrations. Major oxide ratios decipher a transition from non-marine to marine depositional environment, with prevalent humid climatic condition in a passive margin setting. Various discrimination diagrams of the major elements oxides decipher a quatzosedimentary to granodioritic provenance. The geochemical interpretations related with depositional setting validate the proposed estuarine depositional model for the Nimar Sandstone. Integration and interpretation of facies architecture, ichnofabric and geochemical character point to an west to eastward marine encroachment during the Nimar sedimentation. Overall fining up succession of the Nimar Sandstone constitutes part of a 2nd order transgressive system tract (TST) within the Bagh Group of rocks, delimited by unconformity at the base and a iv | P a g e flooding zone at the top (the overlying Nodular Limestone). This 2nd order TST is further subdivided into two stacked transgressive cycles - a lower transgressive cycle (TL), and upper transgressive cycle (TU), representing the 3rd order transgressive systems tracts (3rd order TSTs). TL incorporating fluvial sediments of FA-1 and FA-2 mostly, with some fluvio-tidal sediment of FA-3 whereas, TU includes fluvio-tidal and shore sediments of FA-3, FA-4 and FA-5. Each 3rd order TST is further divided into higher frequency (lower rank) regressive-transgressive cycles (T-R cycles), and are identified as the parasequences of 4th order. Overall stratal architecture and multiproxy evidences of marine transgression events in central India during late Cretaceous time suggest creation of accommodation space by global eustatic sea level rise in association with tectonic subsidence (reactivation of Son-Narmada South fault).