Holocene evolution of the Banni Plain at the north-east margin of the Arabian Sea: Constraints from a ca 50 m long sediment core

Abstract

Holocene evolutionary history of the Banni Plain in the Great Rann of the Kachchh Basin is reconstructed from a subsurface sediment core of ca 50 m. Detailed data on textural and lithofacies variations, grain-size analysis, environmental magnetism and accelerator mass spectrometry ¹⁴C dates on seven samples were generated on the sediment core retrieved from the Banni Plain near Berada. A high-resolution record extending back to 10 ka has been reconstructed from the top ca 40 m of the core section comprising shallow marine sediments. The core is divisible into five depositional units. The basal part is a fluvial depositional unit followed upward by estuarine, sub-tidal, intertidal and supra-tidal environments. The sediment accumulation rate is highest in the sub-tidal to intertidal facies (1.9 cm year⁻¹) and decreases towards the supra-tidal facies to 0.09 cm year⁻¹. Environmental magnetic analysis, χlf coupled with the S-ratio, indicates high magnetic mineral concentrations during the Early Holocene, suggesting a wet period accompanied by high monsoon precipitation. This is followed by the onset of semi-arid conditions in the Great Rann of the Kachchh Basin as indicated by the low values of the χlf and S-ratios. A westward and northward shift in the shoreline towards the deeper part of the basin is suggested during the Late Holocene, which is coupled with aridity and reduced monsoonal conditions. The change in depositional pattern from the retrogradational deposit of fluvial (Unit 1) to estuarine sediment (Unit 2), progressing to sub-tidal (Unit 3), is attributed to sea-level transgression followed by regressive intertidal (Unit 4) to supra-tidal deposition (Unit 5), culminating in complete withdrawal of the sea, aided by tectonic uplift, during the Late Holocene. The results reveal that the sediment accumulation rates and depositional environments changed over time in response to changes in sea level from minima to maxima and then eventually to the present level.