Petrogenesis of Cenozoic Temperate Water Carbonates, South Australia: Model for Shallow-Burial or Meteoric Diagenesis of Phanerozoic Shallow Water Calcite Sediments

Noel P. James, Yvonne Bone

The Gambier Limestone is one of several extensive, shallow water, shelf carbonates of Eocene to Miocene age exposed along the southern margin of Australia. It is a muddy to grainy bryozoan calcarenite, with accessory benthonic foraminifers and echinoids. The sediments, originally composed almost entirely of calcite or Mg-calcite, have been in vadose and phreatic environments for over 10 m.y., yet are virtually unlithified. The only cements of any consequence are epitaxial on echinoids. Numerous karst features, dolines, caves, speleothems, and surface karren attest to prolonged residence in the meteoric zone. The Gambier is presently one of the best freshwater aquifers in Australia, with most flow intergranular and through sediments with over 30% porosity. Cementation is b minor intergranular pressure solution, which has developed under overburden of less than 100 m. The overlying Naracoorte Limestone (Miocene), a calcarenite of warmer water aspect, contains numerous aragonite molds and is cemented marble-hard.

We propose that such temperate-water calcite limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than aragonite-rich tropical sediments. It is probable that many similar early and middle Paleozoic calcite limestones may have been in the meteoric zone for prolonged periods, yet contain little or no petrographic or geochemical record of such exposure.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.