Diagenesis of Holocene reef and associated beachrock of certain coral islands, Gulf of Mannar, India: Implication on climate and sea level

The reef and associated beachrock from certain Gulf of Mannar islands (Rameswaram, Kurusadai, Shingle and Appa Island) were studied to assess the diagenetic evidences. Sixty samples were collected from marine terraces and reef platforms. The samples comprised of coral rubbles, shell fragments and lithic fractions. The presence of corals in the form of framework or isolated patches on the reef flat suggests the rapid increase of accommodation and probably absence of terrigenous and siliciclastic inputs. Moreover, the massive coral heads above the transgressive phase suggest the maximum flooding and relatively deepest facies. The freshwater dissolution, association of marine and meteoric cements suggest the semi-arid climatic condition with marine diagenesis during sea level lowstands and recharge of freshwater lenses during periodic rainfalls. In addition, the interaction of these mixed carbonate, siliciclastic sediments results in silicification of carbonate components. The reef associated beachrock were deposited in low energy environment with some amount of terrigenous matters derived from Precambrian basement rocks and transported into reef area by ephemeral streams and longshore sediment transport. The incorporation of coral fragments within the siliciclastic sediments are most probably due to the erosion and re-deposition of the sediments.