Foraminiferal sequence biostratigraphy of the Olig0-Miocene Janjukian strata from Torquay, southeastern Australia

Abstract

The foraminiferal fauna from two holes near the coastal section of marls and limestones at Torquay is dominated by inner to mid-shelf benthic forms, especially the cibicidids, discorbids and milliolids. Planktonic species are rare and rarely age-diagnostic. A cluster analysis of the species occurrences and relative abundances identified four assemblages, A to D up-section. These assemblages also closely correspond to lithological changes characterising lithostratigraphic units: Angahook Formation (assemblage A), lower and upper Jan Juc Formation (B, C) and Puebla Clay (D). Biofacies trends based on the relative abundances of inner and outer neritic taxa led to the recognition of third-order sequences and boundaries equivalent to TB1.1 to TB1.4, confirming a previous identification on sedimentological grounds. They demonstrate that foraminiferal assemblages were directly influenced by third-order sea-level fluctuations and can be used to predict third-order sequences. The long-uncertain regional Oligocene-Miocene boundary is placed at the Jan Juc Formation - Puebla Clay contact, across which there was a major faunal change. Several benthic forms disappeared, at least temporarily: Cibicidoides perforatus, Amphistegina lessonii and Pararotalia mackayi. The typical Miocene planktonic taxa Globoquadrina dehiscens and Globoturborotalita brazieri made their first appearance. These events were associated with an increase in inner neritic benthos signalling a low sea-level, consistent with the contemporaneous global pattern.