Foraminiferal biostratigraphy and depositional environments of the mid-Cenozoic Abrakurrie Limestone, Eucla Basin, southern Australia

The Abrakurrie Limestone from the central part of the southern Australian margin contains abundant bryozoan, molluscan and other calcareous skeletons including many poorly preserved foraminifers. Limited planktonic and benthic foraminiferal data suggest that the limestone was deposited during the Late Oligocene and Early Miocene. It is believed to have been deposited at about 28-21 Ma, as the local manifestation of eustatic second-order supercycle TB1. The limestone accumulated mainly in water depths around 100 m, for its benthic fauna was largely composed of mid to outer shelf taxa, particularly the cibicidids and eponidids. There was a major faunal change between the infauna-dominated assemblage in the middle, and probably also lower, Abrakurrie Limestone and the epifauna-dominated assemblage in the upper member. Amphistegina lessonii made its first but rare and episodic appearance in the lower member, and the species became abundant and widespread only in the upper member. Likewise, such species as Elphidium spp. and Pararotalia mackayi, which prefer a warmer water habitat, become more common in the upper member. Therefore, a well-lit, warmer, oligotrophic environment is inferred for the upper Abrakurrie Limestone, while the infauna-rich assemblages from the middle and lower members indicate relatively cooler, mesotrophic conditions similar to those which were prevailing in other parts of southern Australia. The scarcity of the plankton together with a low benthic diversity may indicate that the Eucla Basin was a swell-dominated, embayed shelf during the deposition of the Abrakurrie Limestone.