Petrography of the Southesk-Cairn Carbonate Complex, Western Alberta, Canada: ABSTRACT

Abstract

Regional and petrographic studies suggest that a specialized reef association was responsible for the build-up of the carbonate mass. Although dolomitization has preferentially affected certain formational members, some original textures and rock constituents may be recognized. At the Mount Dalhousie type section, the complex has divided into lower Cairn and overlying Southesk formations. The coarsely crystalline, light gray dolomites of the Peechee member, Southesk formation, were originally medium and coarse calcarenites with some areas of algal-stabilized sediment. The whole is suggestive of a depositional environment with strong currents and local sheltered areas. Where dolomitization is incomplete, there is a preference for replacement of integranular sparry calcite. It is suggested that this is an early diagenetic event that took place prior to complete infilling of porosity by calcite precipitation. The Arcs member, in contrast, consists dominantly of microcrystalline calcite with eyes of clear sparry calcite (dismicrite and birdseye limestone), with sporadically distributed areas of calcarenite and layered algal limestone. Comminuted skeletal remains are abundant, although locally, delicate undamaged ostracod shells occur; the latter partly infilled with microcrystalline calcite constitute geopetal fabrics and indicate quiet micro-environments where the fine calcite or aragonite particles settled out. A relatively quiet environment is postulated during deposition of most of this member, with algal-stabilized areas building up locally to further restrict circulation. The algal material was in turn frequently torn up, presumably during storms, and incorporated as intercalated calc renites in the predominantly microcrystalline rock sequence of the Arcs member. The overlying Ronde member, composed of calcarenite, microcrystalline limestone, silty limestone, silty dolomite and siltstone, probably accumulated under unsettled conditions. Globular stromatoporoids appear to have been characteristic of turbulent reef-margin areas and colonial corals of rather deeper and turbid environments. The sedimentary record terminates at an unconformity beneath the overlying Sassenach formation. Viewed as a whole, the carbonate complex has been extensively dolomitized. Various types of dolostone have been mapped and studied with a view to equating them with limestone facies, establishing a reef model and interpreting the dolomitized sequence. End_of_Article - Last_Page 536------------