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 thi ck 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. Geoche mically, 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 ch emical 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 concentrati on and mixing of the REE bearing phases. The immediate source to these sedi ments is likely to be mechanically ground sediments of glacial origin. The last phase of the upliftment of the Himalayas, accompanied by Pleistocene g laciation could have provided a vast glacio-fluvial outwash plain. The aeol ian processes associated with prevailing W to SW winds in the hot arid cond ition seem to have been selectively transporting fertile silt materials lea ving behind infertile desert sands. (C) 1999 Elsevier Science B.V. All righ ts reserved.