Radioactive elements in the sediments of the upper proterozoic Chhattisgarh basin, India, as indicators of palaeoenvironments

The crescent-shaped Chhattisgarh basin occupies an area of ˜33 000 km2 in southcentral India (bounded by 20° 30° N-22° 20°N, 80° 46'E-83° 46'E). The Upper Proterozoic sediments in the basin, which may be of epicontinental or stable shelf sediment origin (Schnitzer, 1971), are included in the Chhattisgarh System. This System is divisible into the lower Chandarpur Group and the upper Raipur Group. Murti (1980) worked out the stratigraphic succession of the central part of the basin and summarized the present status of knowledge of the geology of the basin. Palaeocurrent and grain size data, major and trace elements, oxygen and carbon isotopes have been employed to delineate the environments of sedimentation of the Chhattisgarh basin (Murti and Aswathanarayana, 1982). In view of the usefulness of the contents of radioactive elements and Th/U ratios in the study of environments of sedimentation (Rogers and Adams, 1969), thorium, uranium and potassium contents of various sedimentary rocks (34 samples) from the central part of the basin have been determined by gamma-ray scintillation spectrometry. About 400 g of each sample (-120 mesh) was assayed with an integral assembly consisting of a 4 in (10.2 cm) × 2 in (5.1 cm) NaI (T1) crystal, coupled to 3 in (7.6 cm) RCA-8054 photomultiplier tube (M/S Nuclear Supplies, USA). The instrumental set-up and analytical procedure, have been described in detail by Rao (1974). The description and radioelement contents of the different stratigraphic units (arranged in stratigraphic sequence) are given in Table I