AGGRADATION HISTORY AND LUMINESCENCE CHRONOLOGY OF LATE QUATERNARY SEMIARID SEQUENCES OF THE SABARMATI BASIN, GUJARAT, WESTERN INDIA

The Gujarat alluvial plain in the semi-arid desert margin of the Thar desert contains a record of continental Quaternary sequences. These se quences were built by drainages originating in the Aravalli Hills and flowing westward into the Indian Ocean. Stratigraphic studies of a par t of the Late Quaternary succession in the Sabarmati basin have result ed in a fourfold lithostratigraphic scheme-the Waghpur Formation (Cong lomeratic Member; Sand Member); the Mahesana Formation (Lower Heteroli thic Member; Upper Sand Member); the Akhaj Formation, and the Sabarmat i Formation. Facies analysis, distribution of conglomerates and cross- bed azimuths of the elastic 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 influen ced by episodic flows and aeolian reworking. The upper member of the M ahesana Formation represents aeolian and older levee deposits. Feasibi lity studies of Thermoluminescence (TL) and Infra-red Stimulated Lumin escence (IRSL) dating on the Sabarmati succession yielded stratigraphi cally consistent ages ranging from similar to 5 ka to greater than or equal to 300 ka. For two samples, TL ages on quartz and IRSL ages on f eldspar separates, were consistent. These results, together with the s tratigraphic and petrographic data reveal the following events in the accumulation history of the Sabarmati sequence: (1) Significant aggrad ation at greater than or equal to 300 ka (gravel-sand) and at 58 ka (s and), and (2) Pedogenesis bracketed to 58-39 ka The Sabarmati river in its middle stretches follows a regional slope-deviatory course interp reted previously to be a neotectonic adjustment within the Cambay grab en. Our chronological data suggest that this adjustment occurred, poss ibly, within the past 39 ka.