CARBONATE MUD, GLORIOUS MUD FROM THE SHALLOW COOL-WATER MARINE REALM, SOUTH AUSTRALIA

Fine-grained carbonate muds from shallow temperate systems are rarely discussed and the origin of such particles (<63 µm) is poorly understood. Shallow, cool-water carbonate provinces are most often characterized by course-grained sediments along high-energy coasts, but many of these seafloors also have abundant carbonate muds. Spencer Gulf is a large, shallow embayment along the southern Australian margin. Benthic depositional environments within the gulf include seagrass meadows, sand barrens, and rhodolith pavements. Its sediments contain mixtures of carbonate gravels, sands, and muds; muds constitute up to 84% volumetrically. These muds are deposited in a variety of shallow environments (<50 meters water depth) due to the semi-protected nature of this gulf, and the presence of baffling organisms (seagrasses and rhodoliths). Fragments of bivalves, benthic foraminifera, coralline algae, and bryozoans dominate sands and gravels. Morphologies of mud grains (silts and clays) were investigated using scanning electron microscopy point-counting techniques. They are predominantly composed of skeletal fragments (bivalves, benthic foraminifera, ascidians, echinoderms, and coralline algae). Minor components include diatoms, sponge spicules, coccoliths, spirorbids, and planktonic foraminifera. Most muds are interpreted to have formed by breakdown of larger grains via maceration. The maceration process causes fragmentation along natural planes of weakness within carbonate shells and tests. X-ray diffraction analysis confirms that these muds contain combinations of intermediate-Mg calcite, low-Mg calcite and aragonite.

Marine carbonate mud has mostly been investigated in shallow, tropical settings and deep, offshore environments, and facies models for ancient systems reflect this. Spencer Gulf muds differ in mineralogy and composition from those of classic models of carbonate mud deposition. Unlike shallow-water, tropical carbonate muds, these temperate muds are not dominated by aragonite and there is no evidence of carbonate precipitation from seawater. Pelagic organisms only contribute trivial amounts to these deposits, in contrast to muds of deep slope deposits.