MODERN HETEROZOAN CARBONATE SEDIMENTATION UNDER INVERSE ESTUARINE CIRCULATION; SPENCER GULF, SOUTH AUSTRALIA

Spencer Gulf is the largest marine embayment along the southern Australian coastline. It extends northwards ~ 400 km into the continental interior but water depths only reach 60 m. There are no permanent rivers bringing fresh water or terrigenous sediments to the Gulf. The warm-temperate semiarid climate results in seasonal heating and high evaporation. Salinities range from normal marine at the gulf mouth up to 48 ‰ at the inland head, leading to a pattern of inverse estuarine circulation. This situation is imposed on an overall internal clockwise gyre circulation that brings mostly normal oligotrophic marine waters into the Gulf from the west that are modified therein and exit the Gulf in the east as a mesotrophic saline outflow. The warm-temperate sediments are a spatial mixture of heterozoan carbonate gravels, sands, and muds. Bryozoans, rhodoliths, bivalves, and gastropods form gravels and some sands (via bioerosion). Small benthic foraminifera (SBF), delicate bryozoans, articulated corallines, and echinoids generate most sand grains. Mud is created by the comminution of fragile components such as thin crustose corallines and articulated bryozoans. These components form three megafacies: 1) Bryozoan-Bivalve-SBF Sands and Gravels, 2) Rhodolith Gravels, and 3) Bivalve Sands, Gravels and Muds. Bryozoan-Bivalve-SBF Sands and Gravels cover open seafloors with near-normal salinities in deep southern and central Gulf regions. Rhodolith Gravels are present in the southwestern corner of the gulf mouth and around bathymetric highs where currents of normal marine waters from the shelf sweep the seafloor. In the northern Spencer Gulf, tidal channels cut between seagrass banks to water depths of 25 m. These channels experience strong tidal currents and are also floored with coarse rhodolith gravels. Bivalve Sands, Gravels and Muds are associated with seagrass banks in the shallowest areas of the Gulf. Muddy bivalve deposits cover the tranquil seafloor in the northern Gulf, where high salinities restrict the growth of other organisms (e.g., corallines, bryozoans, and echinoids). This study of Spencer Gulf directly links modern sedimentation to oceanographic controls, providing a key to interpreting paleoceanography from the rock record and to predicting how future changes will affect marine environments.