Late Neogene Radiolarian Events in Andaman-Nicobar Islands, Northeast Indian Ocean

Abstract

Eleven radiolarian events are identified in the Late Miocene to Early Pliocene sequences of the Andaman-Nicobar Islands. Stratigraphic ranges of important taxa are discussed and events are compared with those in sediments from the Central Indian Basin and Site 214 in the Indian Ocean and Sites 289, 586, 573 and 503 in the Pacific Ocean. INTRODUCTION Deep-water Neogene sediments of the Andaman-Nicobar Islands in the Northeast Indian Ocean contain a rich and diverse radiolarian assemblage. However, radiolarian studies have been confined to a few publications (Jacob and Shrivastava 1952; Singh and Vimal 1973; Srinivasan et al. 1983; Sharma and Sharma 1988; Gupta and Srinivasan 1992; Sharma and Singh 1993; Sharma et al. 1993; and Mahapatra and Sharma 1994). The present study is the continuation of earlier analyses of the radiolarians in the sediments from Neill and Car Nicobar Islands by Sharma and Sharma (1988, 1989) and Sharma and Singh (1993). These earlier papers documented the radiolarian taxa and identified the radiolarian zones. The main purpose of the present paper is to identify radiolarian events and to examine these in relation to those recorded in the equatorial Indian and Pacific Oceans. MATERIALS AND METHODS Three sections located at Neill and Car Nicobar Islands have been examined in this study (text-figure 1). The sections contain rich and well preserved Radiolaria and provide an almost continuous record from Late Miocene to Early Pliocene. Thirty two samples from the East Coast Section, Neill Island, and sixty samples from Passa Bridge and Sawai Bay Sections, Car Nicobar Island, are examined. The stratigraphic disposition of the samples, lithologies and zonal scheme are illustrated in textfigure 2. Two to six slides for each sample were prepared from the washed fraction coarser than 63ptm. For Neill Island, slides additional to those studied by Sharma and Sharma (1989) were examined. This resulted in minor revision in the distribution of Stichocorys delmontensis and Stichocorys peregrina compared to that reported by Sharma and Sharma (op. cit.). BIOSTRATIGRAPHY AND EVENTS Based on the core samples from the Central Indian Basin, Johnson et al. (1989) using synchronous radiolarian events <0.4 Ma difference) in the tropical Indian and Pacific Oceans proposed eleven zones for the Plio-Pleistocene interval. Based on the scheme of Johnson et al., Sharma and Singh (1993) assigned the Car Nicobar sequence to Phormostichoartus doliolum, Phormostichoartus fistula and Stichocorys peregrina (non Stichocorys peregrina Zone of Riedel and Sanfilippo 1978) Zones in ascending order of age. Using the zonal scheme of Riedel and Sanfilippo (1978), the Car Nicobar sequence is assigned to Spongaster pentas Zone (Sharma and Singh, op. cit.). The events (morphotypic bottom, 'Bm' and morphotypic top, 'Tm') identified in the Car Nicobar sequence are Spongaster tetras Bm, Didymocyrtis tetrathalamus Bm, Amphirhopalum ypsilon Bm, in the Phomostichoartus doliolum Zone, and Anthocyrtidium pliocenica Tm in the Stichocorys peregrina Zone (non Stichocorys peregrina Zone of Riedel and Sanfilippo 1978). In addition, Phormostichoartus doliolum Tm and Phormostichoartus fistula Tm, are used to demarcate the zonal boundaries. Biostratigraphy of the sequence at Neill Island can be found in Sharma and Sharma (1989), according to which the sequence is divided into Didymocyrtis penultima and Stichocorys peregrina Zones of Riedel and Sanfilippo (1978) (text-figure 2). In terms of zonation by Johnson et al. (1989), the section from sample Mf 529 upwards can be assigned to the Anthocyrtidium prolatum Zone, as Anthocyrtidium prolatum first appears in sample Mf 529 and ranges through the younger samples of the examined section. The events recognised in the sequence at Neill Island, viz., Anthocyrtidium jenghisi Bm, Anthocyrtidium prolatum Bm and Spongaster pentas Bm, occur within the Stichocorys peregrina Zone. In the text we have used the zonal nomenclature of Johnson et al. (op. cit.) for the Car Nicobar sequence, for it provides a more detailed zonal scheme for the Plio-Pleistocene. However, for the Late Miocene-Early Pliocene Neill Island sequence, the zonation of Riedel and Sanfilippo (1978) has been applied. The identified events have been compared with those reported from the tropical Indian and Pacific Oceans (Table 1). Absolute ages are those of Johnson et al. (1989) and Caulet et al. (1993), except in three cases, where data from Johnson and Nigrini (1985b), Nigrini and Caulet (1988) and Theyer et al. (1978) have been used. SELECTION OF DSDP SITES FOR COMPARISON I an effort to find taxa comparable to those in the studied sections and in DSDP sites in the Indian and Pacific Oceans, we micropaleontology, vol. 43, no. 1, pp. 41-50, text-figures 1-2, plate 1, tables 1-2, appendix 1, 1997 41 This content downloaded from 207.46.13.193 on Thu, 08 Sep 2016 05:51:04 UTC All use subject to http://about.jstor.org/terms V. Sharma and Surender Singh: Late Neogene Radiolarian events in Andaman-Nicobar Islands, northeast Indian Ocean