Multicycle Holocene Dolomite in Cool-Water Carbonate Sediments, Lacepede Shelf, South Australia

BONE, YVONNE, University of Adelaide, Adelaide, South Australia, NOEL P. JAMES, Queen's University, Kingston, Ontario, Canada, and T. KURTIS KYSER, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Modern carbonate sediments on the Lacepede Shelf contain up to 10% dolomite particles, as either single rhombs or clusters. The rhombs vary from sharply edged crystals and nonabraded to slightly worn to completely rounded rhombs. These three groups probably represent different times of formation and transportation during Holocene glacio-eustatic sea-level changes. The abraded aggregates that are loosely cemented by calcite occasionally have pristine rhombs attached. Color varies from transparent and colorless or light orange to dark red, without apparent pattern. Cathodoluminescence shows distinctive zoning, analogous to nearby mid-Cenozoic dolomites. Similarly the dolomite is Ca-rich (43 mole % Mg). It occurs within bryozoan-bivalve sediments that are a mixture of relict and modern b oclastic

components, across the entire shelf, from siliciclastic sediments debouching from the Murray River to completely carbonate sediments at the shelf edge. Greatest concentrations are found adjacent to seafloor highs, sites of abundant bryozoan, sponge, and coralline algae growth.

Stable isotope values, however, are compatible with precipitation from seawater, similar to those of associated living brachiopods and Mg-rich bryozoans. Sr isotopes confirm the time of formation as "modern," unlike the mid-Cenozoic time of formation for similar Tertiary dolomites. These multicycle rhombs and rhomb clusters may therefore be the nucleii for epitaxial precipitation of dolomite either on the modern sea floor or later during burial.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)