Occurrence of lime mud in the outer continental shelf off Visakhapatnam-Pentakota, east-coast of India

Abstract

Lime mud with a thickness of > 2.4 m occurs over an area of 1000 km(2) off Visakhapatnam-Pentakota, east coast of India. It occurs beyond 90 m carbonate ridge up to 220 m isobath in the outer continental shelf, covered by a veneer of clayey/calcareous sand. Lime mud is greyish white to creamy white and fine grained. It is made up of aragonite crystals and associated with skeletal fragments with negligible detritals. Chemical analysis showed CaO (46.36-50.9%), MgO (0.4-1.2%) and acid insolubles (2.82-4.38%). Based on the morphology of the outer shelf, nature of associated constituents and its proximity to the 90 m carbonate ridge, it is presumed that the 90 m ridge has acted as a source for the lime mud. Mechanical disintegration of the constituents of the 90 m ridge and precipitation from seawater are considered responsible for the formation of the lime mud. Sea bed morphology and sediment distribution suggest that localisation and thickness of the lime mud is controlled by palaeotopographic undulations in the outer continental shelf.