Geochemistry of the loessic sediments on Delhi ridge, eastern Thar desert, Rajasthan: implications for exogenic processes

Quartzite ridges in the Delhi region include in their local depressions thick piles of unconsolidated, unstratified, yellowish brown, homogenous loamy sediments. These Delhi ridge sediments (DRS) have textural, mineralogical and geochemical characteristics similar to those of loess sediments. Geochemically, DRS, just as other loess sediments, are similar to average exposed upper continental crust of Taylor and McLennan [Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, London, 311 pp.]. The calculated chemical index of alteration [Nesbitt, H.W., Young, G.M., 1984. Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochim. Cosmochim. Acta 54, 1523-1534; Nesbitt, H.W., Young, G. M., 1989. Formation and diagenesis of weathering profiles. J. Geol. 97, 129-147.] and other chemical parameters suggest a provenance of chemically less weathered rocks. During the process of wind transport and deposition, not only the dilution effect of silica on the REE abundance was counterbalanced but also the REE pattern was kept similar to the UCC and PAAS by the appropriate concentration and mixing of the REE bearing phases. The immediate source to these sediments is likely to be mechanically ground sediments of glacial origin. The last phase of the upliftment of the Himalayas, accompanied by Pleistocene glaciation could have provided a vast glacio-fluvial outwash plain. The aeolian processes associated with prevailing W to SW winds in the hot arid condition seem to have been selectively transporting fertile silt materials leaving behind infertile desert sands. (C) 1999 Elsevier Science B.V. All rights reserved.