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

Ooid concentrated sediments occur at about 75 and 120 m depths on the outer continental shelf off Chennai. They are associated with sediments containi ng predominantly of terrigenous and authigenic green clay facies. Quartz, f oraminifers and green grains are present in the nucleus of the ooids. SEM s tudies on the surfaces of ooids revealed pits formed mechanically in high e nergy conditions and pits and channels tailored by endolithic algae. Ooids associated with microborings probably indicate their formation in shallow m arine environment and their concentration at two specific depths indicate s ea 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 o f Holocene transgression.