PLIOCENE OYSTER BUILDUPS, NORWEST BEND FORMATION, SOUTH AUSTRALIA: BRACKISH WATER END-MEMBER OF THE REEF SPECTRUM

Monospecific oyster bioherms and biostromes created by the extant, soft-bottom oyster, Ostrea angasi, are prolific in Late Pliocene carbonates of the Murray Basin. These buildups are pioneer communities that formed through arrested ecological succession, each stage of which was linked to a separate phase of marine inundation.

The Stabilization Stage of reef development is restricted to the early transgressive systems tract when topographic highs were flooded and populated by dense aggregations of oyster clusters. The Colonization Stage is developed in transgressive and highstand systems tracts when increased accommodation led to the vertical and lateral accretion of oyster communities into large buildups. Bioherms formed in areas of low sedimentation and seafloor disturbance, both of which allowed rapid community growth. Biostromes in contrast, record buildup development on an unstable seafloor where episodic, catastrophic burial by storm sediment created conditions of repetitive seafloor stabilization and colonization. Subaqueous dunes formed as part of the falling stage systems tract when river currents reworked shell material from bioherms and biostromes into large-scale, subaqueous bedforms.

Brackish water conditions in the paleoestuary, a turbid water column, mesotrophic nutrient levels, and high spatial homogeneity of oyster buildups, together prevented recruitment of specialized, stenohaline organisms that typify the Diversification Stage of other metazoan reefs. Comparison with modern, low-diversity oyster communities indicate high fecundity, rapid growth rate, and exogenic nutrient cycling enabled oysters in the Norwest Bend Formation to thrive within this nutrient-rich setting.