Stratigraphy and Depositional Environments of Neogene Limestones of Andaman-Nicobar Islands, Northern Indian Ocean

Abstract

Carbonate rocks are widespread on the Andaman-Nicobar Islands, northern Indian ocean, ranging in age from Early Miocene to Pleistocene. Lithologically they range from deep water to shallow watcr calcarenites.

Thin section analysis of the limestones has revealed the following major lithofacies: lithic foraminiferal-algal grainstone-packstone; lithic foraminiferal-algal wackstone-mudstone; volcanoclastic packstone-wackstone/radiolarian cherts; foraminiferal packstone; coralgal boundstone and coarse lithic foraminiferal-algal-molluscan packstone grainstone. They exhibit sequential changes in their fabrics, biogenic and clastic constituents. Depths of deposition ranged from high energy, shallow neritic (lithic foraminiferal- algal grainstone-packstone facies) at the begnining of Early Miocene to progressively lower bathyal marked successively by lithic foraminiferal-algal wackstone-mudstone, volcanoclastic packstone-wackstone and radiolarian chert facies. The sudden influx of glass shards and tephras in thc later Early Miocene suggests that progressive deepening of the basin occurred contemporaneously with explosive volcanism in neighboring areas. During this interval of basinal subsidence, the limestones received abundant argillaceous material from subaerial erosion of nearby volcanic terrain. Gradual reduction in the frequency of glass shards and pumice from Middle Miocene (Ongeian Stage) onwards indicates culmination of volcanic eruptions and basinal stability. Gradual shallowing to subtidal and patch reef (Coralgal facies) environments began to form from the Middle Miocene onwards at Little Andaman, Havelock, Nicholson and other islands. However, deep basinal conditions prevailed during the Late Miocene to Early Pliocene at Neill, Sir Hugh Rose and Car Nicobar Islands. These different conditions may be attributed to tectonism in the Andaman sea region which occurred during the close of Middle Miocene. Thereafter, younger limestone facies of Plio-Pleistocene age, reflect a shoaling upwards to probable intertidal conditions due to rapid infilling and vertical uplift of the basin floor.

No hydrocarbons have been observed in the limestone sequences. However, the regional stratigraphic and petrographic studies reveal the presence of potential reservoir rocks in the Ongeian-Havelockian limestones of Middle Miocene age on the Andaman-Nicobar Islands.