Facies analysis and paleoenvironmental interpretation of Piacenzian carbonate deposits from the Guitar Formation of Car Nicobar Island,India

摘要

Facies characterization of Piacenzian(late Pliocene) carbonate sediments of the Guitar Formation in Car Nicobar Island, India and the subsequent integration of paleoecological data have been applied to interpret the paleoenvironment of the coralline algal-reef deposits. Thin-section analysis reveals that Amphiroa, Corallina and Jania are the dominant geniculate corallines, while Lithothamnion, Mesophyllum, Phymatolithon, Lithophyllum, Spongites and Lithoporella are the major non-geniculate corallines contributing to the sedimentary facies. Numerous small and larger benthic foraminifera also dominate the biogenic assemblages. Corals, barnacle shells, echinoid spines, fragments of bryozoans,mollusks and ostracodes are the subordinate constituents. Grainstones dominate the studied facies while packstones and boundstones(with wackestone elements) are the sub-lithofacies showing a fair representation. Six carbonate facies presenting a complete reef complex have been distinguished that were deposited in shallow intertidal, back-reef shelf/lagoon, reef and deeper fore-reef shelf settings. Evidences of coralline algal and benthic foraminiferal assemblages, taphonomic signatures of abrasion and fragmentation, grain size, angularity and encrustation indicate a shallow to relatively deeper bathymetric horizon of approximately 10-60 m that corresponds to a regime of high to moderate hydrodynamic conditions.