Late Miocene radiolarian biostratigraphy and paleoceanography of Sawai Bay Formation, Neill Island, Andamans, India

Abstract

Stichocorys peregrina, Didymocyrtis penultima and Didymocyrtis antepenultima Late Miocene radiolarian zones are encountered from mudstone strata of Sawai Bay Formation, Neill Island, Andamans. Percentage data of forty-five coarser taxonomic groups of radiolarians were subjected to Q-mode cluster analysis. Cluster A comprises Stichocorys, Phormospyris, Eucyrtidium, Lamprocyclas, Acrosphaera and Cycladophora groups. Cluster B was divided into subclusters at 0.83 level of clustering. Subcluster B₁ comprised Didymocyrtis-Diartus, Pyloniids, Collosphaera, Phacodiscids, Pterocorythids, Pterocanium and Spongodiscids, whereas subcluster B₂ comprised Porodiscus, Euchitonids, Stylodictya-Stylochlamydium, Spongopyle, Phormostichoartus, Lophophaenids and Lithelius groups. Based on the ecology of the modern homeomorphs of the dominant radiolarian groups, it is suggested that cluster A and cluster B indicate colder and warmer periods, respectively. SubclusterB₁ indicates surface-water fauna and subcluster B₂ represents subsurface water fauna. Dominance of subcluster B₂ in Didymocyrtis antepenultima zone (8.5-7.2 Ma) suggests that subsurface fauna was thriving more probably due to the monsoonal upwelling during warmer periods. This finding is also substantiated with diatom/radiolaria ratio. Presence and absence of deep (1200-2000m) and intermediate (700-1200m) water dwelling radiolarians like the Plectopyramids, Botryostrobus and Sethoperinids groups indicate basinal shallowing during Late Miocene. It may be due to subduction of the Indian plate below the Asian plate, coupled with huge sediment discharged from the Irrawaddy River of Burma during monsoon dominated warmer periods (5.0-6.3 and 8.5-7.7 Ma) in Late Miocene.