Assessing the climatically triggered aggradation-incision processes in a dryland environment during the Late Quaternary period from Shetrunji River basin (Saurashtra), India

Abstract

The present study is an attempt to infer the response of the dryland fluvial sequences to paleoenvironmental change since the last glacial maximum (LGM) from the semi-arid Saurashtra region, a basaltic terrain of the Late Cretaceous age (Deccan Trap), of Western India. We employed a conventional sedimentological analysis supported by sediment geochemistry, which was constrained by optical dating. Based on detailed field investigation, we identified five terrace sequences from the valley fill deposit in the Shetrunji River basin since 18 ka (i.e. Post LGM). The sedimentary archives expose the deposition during three alternate phases of aggradation and incision, marked on the basis of morphostratigraphy and timing of landform evolution. The coastal region of western India experienced a widespread valley aggradation in response to the rising sea stand and under gradual strengthening of the monsoonal conditions around the 18–9 ka period. The Early Holocene climatic optima (9–7.5 ka) coupled with lower than present but rising sea stand, triggered the intense erosion in the valleys of Shetrunji River basin, leading to erosion/lack of sediment preservation. This was followed by the Middle Holocene period (7.5–5 ka), which experienced valley aggradation. This was ended by the incision post 5.5 ka in the lower reaches of the Shetrunji River. The sea stand was similar to present-day stand, as evidenced by the change from fluvial to the estuarine environment in the top sequence of Site-SL at 5.5 ka. The Late Holocene relative sea stand lowering ~3 ka, led to an incision in the estuarine terrace of about 2 m. The causal mechanism for triggering of this incision is most likely the relative fall in sea stand to present-day level from its Middle Holocene high stand. The Late Holocene period (3–1 ka) shows aggradation in upper reach in fluctuating hydrodynamic conditions with seasonality, followed by a relatively good monsoonal condition at < 1 ka, leading to the incision due to climate-tectonic interplay. These pulses of sedimentation could be recognized and explored as aided by basin-scale approach as in the present study. Site-specific studies do not enjoy such liberty.