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

摘要

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 groups 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 Globigerinatella insueta Zone and lower part of Praeorbulina glomerosa Zone of Srinivasan (1988). These zones also correspond to Globigerinoides bisphericus (M4b) Zone and Praeorbulina sicana-Praeorbulina glomerosa (M5a) Zone of Berggren et al. (1995) delineating the Early/Middle Miocene boundary. The boundary between these two zones demarcates the Jarawaian/Inglisian Stage (Early/Middle Miocene) boundary. The calcareous nannofossil assemblage recovered from the examined samples is assigned to Helicosphaera ampliaperta (NN4) Zone of Martini (1971), whereas radiolarian studies reveal the samples 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 nannofossil zone (NN4). According to Nigrini (1985) the radiolarian Calocycletta costata Zone ranges in age from 17.3-15.7 Ma. Acomparison of above estimated ages reveals the age data based on planktic foraminifera to be more precise than the calcareous nannofossils and radiolaria. Thus, based on comparative age data the sequence examined at John Lawrence Island represents the upper part of calcareous nannofossil Helicosphaera ampliaperta (NN4) Zone and also the upper part of the radiolaria Calocycletta costata lane. The inconsistency in the distribution and abundance of any single microfossil group throughout the examined section shows the need for employing multiple microfossil biostratigraphic approach. The study reveals that various microfossil groups serve as complimentary to each other for attaining enhanced biostratigraphic resolution.