Clay mineralogy of Visakhapatnam shelf sediments, east coast of India

Abstract

The shelf sediments off Visakhapatnam along the east coast of India were studied in detail for their clay mineralogy with the objectives of understanding the provenance, transport pathways and dispersal pattern of the sediments. In addition to the seabed samples, residual soils and fluvial sediments from the hinterland were also studied for comparative purposes. Illite, smectite and kaolinite, constitute, in that order of abundance, the chief clay mineral groups in the shelf sediments beyond the sand zone. In the sand zone in water depths of less than 30 m smectite is dominant. A high smectite zone and a high illite zone were identified on the inner shelf, the high smectite zone being confined to the sand zone and paralleling the coast. This zone is influenced mostly by the Godavari River smectite derived from the Deccan Traps and transported northward by longshore currents. The high illite zone covers the entire area beyond the sand zone where the influence of longshore currents is negligible. It is supplied by the Rivers Sarada and Gostani and by ephemeral streams draining the Eastern Ghat. The conspicuous absence of chlorite and paucity of kaolinite in the shelf sediments suggests that the influence of the river-borne sediments from the north is negligible. Comparison with onshore sediments shows that provenance influences clay mineral composition, whereas northeasterly littoral drift influences their distribution.