A Sequence Stratigraphic Interpretation of the Late Pleistocene Estuarine Incised Valley-Fill Sediments of Western Gujarat, India

Sourav Saha¹, Anupam Ghosh¹, Stuart Burley², Santanu Banerjee¹, and Pratul K. Saraswati^1
1Earth Sciences, Indian Institute of Technology, Bombay, Mumbai, India
²Basin Dynamics Research Group, School of Earth Sciences, University of Keele, Staffordshire, United Kingdom

This study provides a sequence stratigraphic interpretation of superbly exposed Pleistocene sections along the estuary of the Gulf of Khambhat based on an understanding of modern depositional processes, satellite imagery, micropalaeontology and detailed outcrop observations in a time-constrained framework.

The Pleistocene section begins with marine clay corresponding to the relative sea level (RSL) highstand ~120 Ka. This is overlain by an estuarine tidal bar complex, in turn capped by a ravinement surface represented by a granule bed. Sea level fall at ~100-90 Ka exposed this surface and resulted in pronounced soilification represented by a rooted horizon. The estuarine succession was locally incised creating a valley up to 5 m deep. Fluvial-originated pebbly sandstones occurring immediately above the sequence boundary represent a LST. The fall in relative sea level is represented by marine silt-shale alternations exhibiting a shallowing-upward trend towards the open sea.

During the succeeding RSL rise the incised Palaeo-valley was flooded and estuarine sediments were deposited (~70-60 Ka). A transgressive, chocolate brown and green clay (~60 Ka) containing abundant planktonic foraminifera deposited above the valley-fill represents the MFS extending up the palaeo-valleys for ~100 km. The estuarine succession bounded between the fluvial deposits and the maximum flooding surface represents a TST. A further fall in sea level resulted in the development of red-coloured terrestrial argillaceous sand with calcrete. Eolian sands capping the topmost part of the section are directly related to the Holocene lowstand (~15-18 Ka). A transgressive surface marks the base of the overlying TST and separates estuarine deposits from underlying fluvial channel deposits (LST). Pleistocene successions exposed along the western coastline of Gujarat thus represent the deposits of an estuarine incised-valley fill complex.

*Project sponsored at IIT Bombay by the Tapti Concession JV Partnership which includes ONGC, Reliance Industries Ltd and BG India.