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 Austral ian margin contains abundant bryozoan, molluscan and other calcareous skeletons including many poorly preserved foraminifers. Limited plankt onic and benthic foraminiferal data suggest that the limestone was dep osited during the Late Oligocene and Early Miocene. It is believed to have been deposited at about 28-21 Ma, as the local manifestation of e ustatic second-order supercycle TB1. The limestone accumulated mainly in water depths around 100 m, for its benthic fauna was largely compos ed of mid to outer shelf taxa, particularly the cibicidids and eponidi ds. There was a major faunal change between the infauna-dominated asse mblage in the middle, and probably also lower, Abrakurrie Limestone an d 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 upp er member. Likewise, such species as Elphidium spp. and Pararotalia ma ckayi, 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 coo ler, mesotrophic conditions similar to those which were prevailing in other parts of southern Australia. The scarcity of the plankton togeth er with a low benthic diversity may indicate that the Eucla Basin was a swell-dominated, embayed shelf during the deposition of the Abrakurr ie Limestone.