Physico-chemical tuning of palaeogeographic shifts: Bhuj formation, Kutch, India

Abstract

The long-standing dilemma between two extremes in palaeogeographic interpretation is resolved for the basal 115 m of the eastern exposures of the Lower Cretaceous Bhuj Formation, at the top of the hydrocarbon prospective Mesozoics of Kutch, India. An explicit record of a mega-scale event of relative sea level rise is found sandwiched between two fluvial stratigraphic intervals that amalgamate only at the eastern extremity of the exposure of the Formation. A localized estuary in the middle had its axis roughly parallel to the pre and post-inundation river channels, although a tributary made a significant detour. Variation in the fine sandstone-mudstone lithology documents decline in depositional energy in the mid-estuary amenable to coal formation. Sediment within the estuary had been routed through both the ends, palaeocurrent turning shore-parallel only at the estuary-mouth. Exception arises in case of some isolated lenses of very poorly sorted and coarse-grained deposits depicting flash floods in the tributary. Burrow characteristics change delicately in response to the inferred changes in the depositional scenario along the estuary.Resting on a coarsening upward ramp the initially fining and then coarsening upward estuarine succession is prograding, though not steadily. It was wave-dominated, but tide left imprint behind the estuary mouth bar gaining in intensity because of valley constriction. Macroscale changes in relative sea level, perhaps eliciting local tectonics, are recorded in the transient hysteresis in the stratigraphic trend with or without palaeocurrent diversions.The fluvial stratigraphic intervals encasing the marine interval have, in terms of architectural elements, many commonalities, but also some significant disparities. The latter indicates channel meandering in the pre-inundation period and braiding after inundation. Unlike the marine sandstones, bar-channel sandstones of the riverine deposits are distinctly less sorted, bimodal, even polymodal in grain-size distribution. Mineralogical composition identifies the sandstones as craton-derived, but their geochemical distinctions point to variation in parent rock composition under the maze of palaeogeographic overprints.