Algal microboring on ooids: An evidence of Holocene sea transgression in eastern India

Abstract

Ooid concentrated sediments occur at about 75 and 120 m depths on the outer continental shelf off Chennai. They are associated with sediments containing predominantly of terrigenous and authigenic green clay facies . Quartz, foraminifers and green grains are present in the nucleus of the ooids. SEM studies on the surfaces of ooids revealed pits formed mechanically in high energy conditions and pits and channels tailored by endolithic algae. Ooids associated with microborings probably indicate their formation in shallow marine environment and their concentration at two specific depths indicate sea level still stands close to ooid depths. Presence of terraces and reefal barriers at depths around 60 and 110 m on this margins also support palaeo-strand lines at these depths and ooid formation in two successive phases of Holocene transgression.