Early Neogene multiple microfossil biostratigraphy, John Lawrence Island, Andaman Sea

Planktic foraminifera, calcareous nannofossils and radiolaria were examined from Early Neogene sequence of John Lawrence Island, Andaman Sea in order to compare and contrast the stratigraphic ranges of various microfossil gro ups to achieve a better biostratigraphic resolution. Based on stratigraphic ranges of planktic foraminifera, the examined samples from the South Point section of John Lawrence Island are referable to the upper part of the Glo bigerinatella insueta Zone and lower part of Praeorbulina glomerosa Zone of Srinivasan (1988). These zones also correspond to Globigerinoides bispheri cus (M4b) Zone and Praeorbulina sicana-Praeorbulina glomerosa (M5a) Zone of Berggren et al. (1995) delineating the Early / Middle Miocene boundary. Th e boundary between these two zones demarcates the Jarawaian / Inglisian Sta ge (Early / Middle Miocene) boundary. The calcareous nannofossil assemblage recovered from the examined samples is assigned to Helicosphaera ampliaper ta (NN4) Zone of Martini (1971), whereas radiolarian studies reveal the sam ples to be referable to Calocycletta costata Zone of Reidel and Sanfilippo (1970). Berggren et al. (1995b) estimated an age range of 16.7-16.1 Ma for planktic foraminiferal zones (M4b-M5a) and 18.3-15.6 Ma for the calcareous nannofos sil zone (NN4). According to Nigrini (1985) the radiolarian Calocycletta co stata Zone ranges in age from 17.3-15.7 Ma. A comparison of above estimated ages reveals the age data based on planktic foraminifera to be more precis e than the calcareous nannofossils and radiolaria. Thus, based on comparati ve age data the sequence examined at John Lawrence Island represents the up per part of calcareous nannofossil Helicosphaera ampliaperta (NN4) Zone and also the upper part of the radiolaria Calocycletta costata Zone. The inconsistency in the distribution and abundance of any single microfoss il group throughout the examined section shows the need for employing multi ple microfossil biostratigraphic approach. The study reveals that various m icrofossil groups serve as complimentary to each other for attaining enhanc ed biostratigraphic resolution.