ABSTRACT

The Second Red Bed Member, which is the lowest part of the Dawson Bay Formation, is composed mainly of red, brown and greenish grey mudstones, with a thickness ranging from 9.8 ft (3 m) to 39.4 ft (12 m). In the uppermost part of the member, a thin dolomitic unit is present, varying from several inches (centimeters) to 7.9 in (20 cm) thick. The unit consists of massive and laminated dolostones, and dolomitic intraclast breccia. The dolomitic sediments are characterized by tepee structures, stromatolites and mudcracks. Analysis by scanning electron microscopy reveals that the dolomitic rocks are composed mainly of microcrystalline dolomite, with significant amounts of clay minerals and minor evaporites. Most dolomite is present as well crystallized rhombs, from 1 to 4 microns in size. This dolomitic unit is widespread across the Canadian part of the Williston Basin. It does not grade laterally or vertically into intertidal or subtidal sediments. Additionally, no marine fossils were found in the dolostone. The depositional environment of the dolostone differs from that of a typical sabkha, and has many analogous features to the modern Coorong lakes of Australia.