Abstract

The Abrakurrie Limestone is a bryozoan-rich, cool-water, open shelf unit of Oligocene--middle Miocene age beneath the Nullarbor Plain. The sediments have been in the meteoric diagenetic environment, never buried more than a few hundred meters, since middle Miocene time. The unit is composed of meter-scale subtidal cycles, each of which is capped by an erosion surface. The upper several centimeters of most cycles are well lithified, forming distinctive hard layers in otherwise friable carbonates. Intergranular cements in these layers are now isopachous rinds of low-magnesium calcite. Crystals are inclusion-rich, generally fibrous, and have a speckled cathodoluminescence signature. Cements in intergranular pores at the tops of cycles are overlain by interstitial marine sediment. The cements are interpreted to have been marine precipitates in the form of magnesium calcite originally containing from 8-12 mole % MgCO 3 and having a delta 18 O value of -1.0 + or - 0.5 per thousand versus PDB. Stable carbon and oxygen isotope values in these interpreted marine hardgrounds are distinctly more positive than values from underlying soft sediments, reflecting alteration under lower water-rock ratio conditions due to reduced permeabilities. Covariance in stable isotope values from different localities suggests diagenesis in groundwaters of mixed composition. Fluid inclusions indicate neomorphism in fluids with salinities equal to or slightly greater than seawater. Alteration is envisaged to have taken place in a paleogroundwater system of variably mixed saline continental waters and seawater. Occurrence of similar to identical cemented layers and cements in coeval cool-water New Zealand shelf carbonates suggests that such cementation was a sporadic but widespread phenomenon on mid-Cenozoic platforms in this area. By inference, cool-water carbonates of all ages may contain units lithified on the seafloor by cements containing low to intermediate amounts of magnesium. Cool water is not an inhibitor to synsedimentary cementation.

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