Allogenic Control on Late Quaternary Continental Sedimentation in the Mahi River Basin, Western India

The late Quaternary continental succession exposed along the cliff-like banks (15-35 m) of the Mahi river basin comprises sediments of fluvial and aeolian origin and provides an opportunity to assess palaeoclimatic changes in western India. Seven different facies are recognized in the exposed sediment succession. The trough cross-stratified conglomerates (Gt) formed due to downstream migration of trains of sinuous-crested dunes in shallow channels, whereas the formation of planar cross-stratified gravels (Gp) is attributed to downstream accretion of the avalanching front of a mid-channel bar, The horizontally stratified sands (Sh₁ ) represent overbank deposits, while similar facies (Sh₂ ) owes its origin to infrequent sheet flooding events. The trough cross-stratified sands (St) appear to be a product of low-stage river processes. The 'loess-like' massive silts (Sim) are of aeolian origin. The calcic palaeosol facies (P) are represented by vertisols at the base and brown soils and calcic red soils towards the top indicating changing palaeoenvironments.

The overall sediment succession was built through three major climate-controlled aggradation phases. Aggradation phase 1, which is represented by gravel-bed rivers and coexistent vertic palaeosols, indicates a significantly wetter climate. Aggradation phase 2 points to a comparatively drier period during which suspended-load rivers existed. Within this phase, red soil formation suggests a climatic amelioration event around 40 ka B.P. to 23 ka B.P. Aggradation phase 3 reflects a period of extreme aridity and is manifested as the capping aeolian facies. This phase is thought to be in tandem with the Last Glacial Stage event. Stabilization of these dunes may have taken place around 6 ka B.P. as suggested by the Thar Desert palynological and lake records.

The modern Mahi channel flows due ssw. However, the palaeocurrent trends of the exposed sediment bodies reveal that the ancient drainage network in the northern sections (upper reaches) flowed due SW-SSW and in the southern sections (lower reaches) SSE, SSW and W. The present day course of Mahi is relatively younger and follows structural lineaments that developed during the Holocene (after 6 ka B.P.). The aIluvial sedimentation record is unrelated to the modern Mahi river, but is a product of ancient late Quaternary rivers which now stand either fully or partially destroyed.