SYNSEDIMENTARY DETRITAL DOLOMITE IN QUATERNARY COOL-WATER CARBONATE SEDIMENTS, LACEPEDE SHELF, SOUTH-AUSTRALIA

Carbonate sediments on the Lacepede shelf contain up to 25% colorless to orange to dark red dolomite particles, as either single rhombs or rhomb clusters. This dolomite is present in variable amounts across the shelf in quartzose bivalve-bryozoan sediments, which are a mixture of modern and relict late Pleistocene particles. Rhombs range from sharply edged to abraded to completely rounded. Abraded aggregates are loosely cemented by high-Mg calcite and in places have pristine, unabraded rhombs attached. The dolomite is Ca rich (approximately 44 mol% MgCO3), Fe poor, and intricately zoned as seen under cathodoluminescence. Sr isotopes are compatible with precipitation from seawater of modem composition; crystals from one locality have a C-14 age of 28 ka. Stable isotopes of carbon and oxygen are similar to those of dolomite precipitated from seawater. The size, composition, and geochemistry of the crystals are incompatible with their originating in nearby Holocene Coorong and older Tertiary dolomites. The dolomite is interpreted as having formed initially during late Pleistocene time, a period of fluctuating sea level on the Lacepede shelf, in mildly reducing sediments just below the sediment-water interface. The crystals have since been reworked somewhat, and dolomite may be forming on these nuclei today. Such scattered dolomite could be interpreted as postdepositional in ancient limestones. When these sediments are buried, the rhombs might act as nuclei for further precipitation of dolomite from subsurface fluids.