Aggradation history and luminescence chronology of Late Quaternary semi-arid sequences of the Sabarmati basin, Gujarat, Western India

Abstract

The Gujarat alluvial plain in the semi-arid desert margin of the Thar desert contains a record of continental Quaternary sequences. These sequences were built by drainages originating in the Aravalli Hills and flowing westward into the Indian Ocean. Stratigraphic studies of a part of the Late Quaternary succession in the Sabarmati basin have resulted in a fourfold lithostratigraphic scheme-the Waghpur Formation (Conglomeratic Member; Sand Member); the Mahesana Formation (Lower Heterolithic Member; Upper Sand Member); the Akhaj Formation, and the Sabarmati Formation. Facies analysis, distribution of conglomerates and cross-bed azimuths of the clastic facies of the Waghpur Formation were used to infer NE-SW trending semi-arid alluvial fans. The deposition of the overlying Heterolithic Member took place in ephemeral streams influenced by episodic flows and aeolian reworking. The upper member of the Mahesana Formation represents aeolian and older levee deposits. Feasibility studies of Thermoluminescence (TL) and Infra-red Stimulated Luminescence (IRSL) dating on the Sabarmati succession yielded stratigraphically consistent ages ranging from ~5 ka to ≥300 ka. For two samples, TL ages on quartz and IRSL ages on feldspar separates, were consistent. These results, together with the stratigraphic and petrographic data reveal the following events in the accumulation history of the Sabarmati sequence: 1. (1) Significant aggradation at=300 ka (gravel-sand) and at 58 ka (sand), and 2. (2) Pedogenesis bracketed to 58-39 ka The Sabarmati river in its middle stretches follows a regional slope-deviatory course interpreted previously to be a neotectonic adjustment within the Cambay graben. Our chronological data suggest that this adjustment occurred, possibly, within the past 39 ka.