Middle and Late Eocene Great Australian Bight lithobiostratigraphy and stepwise evolution of the southern Australian continental margin

Abstract

Middle to Upper Eocene carbonates recovered at ODP Leg 182 sites from the Great Australian Bight comprise three packages: (i) a 43–40 Ma quartzose limestone and packstone package; (ii) a 39–37 Ma wackestone package with ooze; and (iii) an Upper Eocene fine‐grained wackestone package mainly 36.5–35 Ma in age. Bounding these sediment packages are unconformities inferred to have occurred at ca 43, ca 39, ca 37 and ca 34 Ma, coinciding with four major third‐order sequence boundaries. Rapid changes in sea‐floor spreading probably caused these unconformities, while a stable high sea‐level between spreading pulses led to sediment packaging from offshore to coastal basins. The regional sea‐level changes in these Middle to Late Eocene times, when glacioeustatic influence was minimal, were mainly driven by plate tectonics. Biofacies indicate a rapid subsidence, faster offshore than nearshore, during the first phase of accelerated sea‐floor spreading between Australia and Antarctica around 43 Ma. Most significant environmental changes affecting sedimentation along the southern Australian margin at that time were the influx of warm waters from the Indian Ocean and the initial development of a stratified water mass in the deepening and widening Australo‐Antarctic Gulf. The warm (surface) water supported plankton from the subtropics and migratory and endemic larger benthos including foraminifers,while the fertile deeper waters were sites of chert accumulation. The bloom of carbonate‐generating bryozoans contributed to widespread carbonate sedimentation that progressed towards the southeastern margin ∼4 million years later. Subsequent carbonate packaging and unconformities, reflecting a stepwise accelerated sea‐floor spreading pattern, persisted into the terminal Eocene before the onset of global glaciation.