PALEOECOLOGY OF COOL-WATER, SUBTIDAL CYCLES IN MID-CENOZOIC LIMESTONES, EUCLA PLATFORM, SOUTHERN AUSTRALIA

The open-shelf, subtidal, bryozoan-rich Abrakurrie Limestone beneath t he Nullarbor Plain (Eucla Platform) is cyclic at the meter-scale. Best developed cycles are asymmetric and comprise three distinct parts wit h a capping hardground. The basal part (A) is a thin, coarse, grainsto ne or rudstone that is rich in robust bryozoan and epifaunal echinoid fragments and pecten bivalves, reflecting growth and accumulation in g enerally high energy, hard bottom environments. The middle part (B), i nterpreted as a low-energy, sub-swellbase accumulation, is a burrowed to planar cross-laminated fine grainstone or packstone with a low dive rsity, delicate-branching bryozoan assemblage and little else except s cattered infaunal echinoids and pectens. The upper part (C) is a burro wed, abundantly fossiliferous (bryozoan, bivalve, echinoid) rudstone o r floatstone. Upward increases in the numbers and diversity of Mg-calc ite and aragonitic cheilostome bryozoans, (especially erect rigid, fla t robust branching and nodular/arborescent types), gastropods, infauna l bivalves, and infaunal echinoids points to a high-energy environment . Sediments at the top of C are variably cemented by inclusion-rich ma rine cement (now calcite) that formed a hardground which was subsequen tly physically and biologically eroded and stained by iron oxides duri ng a period of non-deposition. Sediments from the next overlying cycle succeed cements in uppermost intergranular pores and fill open crusta cean burrows. Variably developed cyclicity is interpreted, on the basi s of comparable Holocene cool-water shelf sediments, to reflect deposi tion in generally sub-photic environments that ranged from just below swell base (B) upwards towards the zone of wave abrasion (A and C). Ha rdgrounds (H) formed when the seafloor was within the zone of wave abr asion. Shallowing and deepening of these critical interfaces was contr olled by fluctuating sea level and/or climatic change. Cycles are not correlatable over distances of more than 10 hm, probably because their style and preservation is dependent upon capricious seafloor cementat ion The nature of sedimentation in similar high-energy environments wa s influenced by substrate. These cycles have attributes in common with shallowing-upward, high-energy successions that culminate in subaeria l exposure. In this instance, however, accumulation space was determin ed, not by sea level, but by the depth of wave abrasion.