Coralline algae and benthic foraminifera from the long formation (middle Miocene) of the Little Andaman Island, India: Biofacies analysis, systematics and palaeoenvironmental implications

摘要

Biofacies analysis is used to understand the palaeoenvironmental implications of late middle Miocene (Serravallian) algal-foraminiferal assemblages and carbonate deposition pertaining to the Hut Bay section of the Little Andaman Island, India. This study is based on the novel dataset of coralline algae and benthic foraminifera evaluated from the thin section analysis. Systematic descriptions of the abundant algal-foraminiferal taxa have been provided. The Long Formation in the studied succession contains rich assemblages of coralline algae and benthic foraminifera with moderate abundance of corals and sparse occurrences of echinoderms, gastropods, molluscs, bryozoans and barnacles. Based on the biogenic sedimentary components, Little Andaman Island was positioned in a tropical shelf environment at this time. The depth gradient of algal assemblages shows affinity to numerous other carbonate settings globally. The fossil benthic communities are indicative of thriving in a meso-oligotrophic regime. Dominance of coralline algae and larger benthic foraminifera indicates deposition in upper photic zone to the upper part of the lower photic zone. Frequent alternations of lower-energy, mud-supported wackestones with higher-energy, grain-supported grainstones and packstones suggest the likely incidence of cyclones/storms during the depositional period. The facies gradients and high incidence of taphonomic signatures such as abrasion and fragmentation support the likelihood of parautochthonous to allochthonous deposition for the bulk of the succession.