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Authors
Affiliations
1 Department of Geology, University of Delhi, Delhi, IN
2 Max-Planck-Institut fuer Chemie, Air Chemistry Department, Palaeoatmos. Res. Group,
Postfach 3060, D-6500 Mainz, West Germany, DE
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 26, No 4 (1985), Pagination: 245-254
Abstract
Stromatolitic phosphorites and carbonates from the Proterozoic Aravalli sediments of Western India, have been subjected to organo-geochemical studies, involving a modified gc-ms technique. The stromatolitic phosphorites have an average value of 0.22% TOC which is significantly different from the average TOC value (0.64%) for all types of phosphorite from the Indian subcontinent. With only a few exceptions, the isoprenoids-pristane and phytane and the n-alkanes, account for about 30% of the total amount of volatiles (1-20 ppm). Normal alkanes occur in the range of n-c12 to n-c31 with maxima between n-c17 and n-c21 . No odd/even predominance has been observed. Besides pristane and phytane, other isoprenoids identified in these rocks are iSO·C16 and iSO-C18. The low pristane to phytane ratio of 1.14 suggests a reducing micro-environment within the laminae of shallow-water cyanobacterial/algal stromatolites. The phosphate is assumed to be precipitated within the pore-waters under anoxic conditions through the activities of biological agents, which is supported by geological and geochemical data. The overall basinal environment, however, remained oxidizing, as is evident from the associated sedimentary structures.
