Significance of relic carbonate deposits along the central and Southwestern margin of India for late Quaternary environmental and sea level changes

Abstract

Environmental and sea level indicators were investigated using dredge samples from late Quaternary carbonate deposits along the shelf break between Goa and Cape Comorin, India. Geomorphic features in the area were identified from sonar profiles and included isolated patch reefs with a relief of up to 10 m, and linear reefs with reliefs between 2 and 15 m. The main clast types recovered from these features include fragmented corals and carbonate nodules dominated by either encrusted foraminifera or coralline algae. Some of these clast types are clearly of shallow-water origin. Fragments of reef-forming Poritid corals, for example, were collected off Mangalore at depths of 110-105 m and dated between 11,520 and 12,610 ¹⁴C years BP (13.42-14.77 ka). Nodules of similar age dominated by Lithothamnium and capped by foraminiferal veneers were also collected at -90 m off Cape Comorin. Their altered algal tissues are consistent with formation in shallow water, high-energy conditions. In contrast, nodules recovered off Kochi and Mangalore-Goa are of deeper water origin, younger in age (10,980-7350 ¹⁴C years BP), and are dominated by Gypsina encrustations with volumetrically less algal encrustation. They show cyclic succession of foraminiferal-algal, or foraminiferal-algal-coral laminations in which the algal species are typical of deeper waters. The age and elevation of corals and shallow-water nodules are both consistent with published glacio-eustatic sea-level curves. In addition, the alternate micro-encrustations of foraminifera, algae and encrusting corals could indicate changing conditions from nutrient-rich and turbid to nutrient-poor and clear water that may be attributable to seasonal variations in sediment flux caused by monsoons.