Desert Quaternary Formations and their Morphostratigraphy: Implications for the Evolutionary History of the Thar

Morphogenetic manifestations and stratigraphy of the Quaternary alluvial and aeolian formations of the desertic tract of Rajasthan have been described. The stratigraphy is based on a study of deep sections (-15 m) from different locations. Additionally, well-logs have been used to prepare the isopach maps of these formations. It is inferred that during the major part of the Pleistocene, the region witnessed widespread alluvial sedimentation. Though the region was well-watered then, the presence of large masses of lime coupled with negligible in-situ weathering suggests, that over-all the environment must have been predominantly semi-arid. Presence of nodular and lithic calcrete bands suggests an episodic character of this sedimentation. The form and distribution of surficial calcretes (including polycyclic lime nodules) suggest several cycles of calcrete formation.

Thermoluminescence dating of aeolian sands sampled from 28 to 65 metre depths suggest that the antiquity of the aeolian dynamism extends to atleast 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 redistribution of sediments and formation of nodular calcretes and gypcretes. The last major aeolian activity peaked at LGM in the Sahara, the Australian and the Kalahari deserts. However, the evidence on Thar suggests a peak in aeolian activity post-dating the LGM.