DESERT QUATERNARY FORMATIONS AND THEIR MORPHOSTRATIGRAPHY - IMPLICATIONS FOR THE EVOLUTIONARY HISTORY OF THE THAR

Morphogenetic manifestations and stratigraphy of the Quaternary alluvi al and aeolian formations of the desertic tract of Rajasthan have been described. The stratigraphy is based on a study of deep sections, (ap proximately 15 m) from different locations. Additionally, well-logs ha ve been used to prepare the isopach maps of these formations. It is in ferred that during the major part of the Pleistocene, the region witne ssed widespread alluvial sedimentation. Though the region was well-wat ered then, the presence of large masses of lime coupled with negligibl e in- situ weathering suggests, that over-all the environment must hav e been predominantly semi-arid. Presence of nodular and lithic calcret e bands suggests an episodic character of this sedimentation. The form and distribution of surficial calcretes (including polycyclic lime no dules) suggest several cycles of calcrete formation. Thermoluminescenc e dating of aeolian sands sampled from 28 to 65 metre depths suggest t hat the antiquity of the aeolian dynamism extends to at least 100 ka B .P. The period from c.25 to 50 ka B.P. consistently shows the presence of wetter interludes with weaker aeolian dynamism, a fluvial redistri bution of sediments and formation of nodular calcretes and gypcretes. The last major aeolian activity peaked at LGM in the Sahara, the Austr alian and the Kalahari deserts. However, the evidence on Thar suggests a peak in aeolian activity post-dating the LGM.