Terminal Proterozoic-Cambrian sequences in India: a review with special reference to Precambrian-Cambrian Boundary

Abstract

The Terminal Proterozoic marine sequences, grading into Cambrian, are present in the Krol Basin (Lesser Himalaya), the Kashmir, Spiti-Zanskar and Kumaun Basins (Higher Himalaya) and in Marwar Basin (western India). These sequences postdate a major tectono-thermal event associated with Cadomian Orogeny and rest on eroded older successions which include well-dated Malani Igneous Suite (Malani Rhyolite 745 ±10 Ma; Siwana Granite 731± 14 Ma) in Western India and over the Salkhala Group with granites (745 ± 50 Ma) or the Simla Group in the Himalaya. This cycle of sedimentation terminated with Pan-African Orogeny in Late Cambrian. These sequences are dominantly siliciclastic in basins in Higher Himalaya while those in Krol Basin and Marwar Basin show development of thick carbonate-evaporite facies with or without phosphorite. From the Upper Vindhyan and Bhima Groups in central and south India, respectively a Chuaria-Tawuia Assemblage along with sphaeromorphida Acritarch of Early Neoproterozoic age has been recorded. The δ13C values range from +1.3 to +4.0 ‰ PDB and that of δ 18O from -5 to -9‰ PDB. These are, thus, not considered part of this sequence. In absence of age-determinating biota and radiometric dates from the basal part of the succession, the lower boundary of the Terminal Proterozoic cannot be delineated and dated. However, a significant depletion in values may be taken to mark the lower boundary.
 The upper boundary of the Terminal Proterozoic (Precambrian-Cambrian Boundary) cannot be demarcated in terms of GSSP due to the absence of trace fossils of Zone-1 (Harlaneilla podolika Zone) and Zone-II (Phycodes pedum Zone) in carbonate facies in Krol Basin and poor documentation of siliciclastic facies in Kashmir and Spiti-Zanskar Basins. The trace fossils correlative with ichnozone-III occur in all sections in the Himalayas and are useful for regional and global correlations. However, a significant depletion in δ13C values has been recorded in the carbonate facies of the Krol Basin between the horizons yielding Ediacaran and small shelly fossils of Meishucunian Zone-I. This has also been recorded in the Marwar Basin below the Phosphorite Bed. This depletion may be correlated with that recorded from the Precambrian-Cambrian transition in Siberian Platform, Anti Atlas Mountains, Morocco, China, etc. It may be taken into consideration to define and mark the boundary in the absence of trace fossils.