ABSTRACT

The Gambler 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 Ma, 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 Gambler Limestone is presently one of the best fresh water aquuifers in Australia. Most flow occurs through intergranular pores in sediments with over 30% porosity. Cementation is by 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 well-cemented.

We propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are 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.