Sedimentology, stable‐isotope geochemistry and palaeoenvironment of dolostones capping late Precambrian glacial sequences in Australia

Abstract

Thin (<10 m), extensive dolostones conformably capping late Precambrian glaciogenic rocks in the Kimberley region and the Adelaide Geosyncline commonly comprise micritic dolomite apparently of ‘primary’ or early diagenetic origin. Their aphanitic texture, the presence of algal and cryptalgal lamination, tepee structures and intraclastic rocks, and the lack of saline evaporites or their pseudomorphs, suggest deposition mainly in supratidal to lagoonal environments under a seasonally evaporitic/humid climate. Less common oolitic or pelletal rocks and a finely laminated shaly dolostone facies also suggest peritidal to marine influences.

The ‘cap’ dolostones (omitting possible large erratics) have a δ¹⁸O mean of —6.4 ± 1.9% vs. PDB and a δ¹³C mean of —1.9 ± 2.1% vs. PDB. The oxygen mean is comparable to that determined for other late Precambrian ‘primary’ or early diagenetic dolostones not associated with glaciogenic rocks and presumably deposited under warm conditions. The carbon mean, in contrast, is lighter than that determined for other late Precambrian dolostones.

The sedimentological and oxygen‐isotope data are consistent with relatively high formation‐temperatures for the cap dolostones. Abrupt climatic warming at the close of late Precambrian glacial epochs is implied.