Origin of dolomitization on the Barbwire Terrace, Canning Basin, Western Australia

Abstract

ABSTRACTA thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment.Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution.Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=−2 to +1‰ (PDB)) than the regional dolomite type (δ18O=−9 to −2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=−2‰ (PDB).The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near‐surface environment with a major recrystallization event superimposed during burial diagenesis.