Rhodolith-forming coralline red algae in the CaCO₃ biofactory — A case study from the Serravallian of tropical northeastern Indian Ocean

Rhodolith-forming non-geniculate coralline red algae have been recorded from the Long Formation, exposed in four different outcrops at Little Andaman Island (Hut Bay) in the northeastern Indian Ocean. The non-geniculate corallines are represented by species of Sporolithon Heydrich, 1897, Mesophyllum Lemoine, 1928, Lithothamnion Heydrich, 1897, Phymatolithon Foslie, 1898, Lithoporella (Foslie) Foslie, 1909, Spongites Kützing, 1841, Neogoniolithon Setchell & Mason, 1943 and Lithophyllum Philippi, 1837. The algal assemblages also include geniculate corallines belonging to the genera Amphiroa Lamouroux, 1812 and Corallina Linnaeus, 1758. In addition, larger benthic foraminifers and few planktonic foraminifers also have been identified in thin section analysis. Based on the earlier study carried out on planktonic foraminifers, the Long Formation has been dated as Serravallian (late middle Miocene) and chronostratigraphically, the Long Formation has been included in the Ongeian Regional Stage. In the rhodolith-forming non-geniculate corallines, various growth forms and taphonomic features have been recognized. Diagenesis affected the studied material by micritization, cementation and compaction. The four outcrops are dominated by bioclastic wackestone and packstone composed of coralline red algae, benthic and planktonic foraminifers, echinoid spines and unidentified coral fragments. The rhodolith-forming coralline red algae, the growth forms and taphonomic features in non-geniculate corallines and the characteristic benthic foraminifers are indicative of a moderate energy depositional environment. This study indicates that the carbonate production was considerably high during the Serravallian of the tropical northeastern Indian Ocean.