Depositional Environments of the Upper Devonian Birdbear Formation, Saskatchewan
Halabura, Stephen Philip
The Upper Devonian, Upper Frasnian Birdbear Formation of Saskatchewan is the uppermost carbonate-evaporite cycle of a thick carbonate sequence deposited in a shallow, epicratonic sea .. The formation is equivalent to the Birdbear (Nisku) Formation of North Dakota and Montana, and approximately equivalent to the Nisku sequences of Alberta. It is divided into an upper and lower member, with the dividing boundary at the base of the first anhydrite bed. In the lower member, six lithofacies are discriminated. They are the products of the last and most exclusive incursion of the Frasnian seas into Saskatchewan and beyond, and the ensuing period of standstill. This final flooding had started already in late Duperow and continued into earlymost Birdbear time. The rocks are primarily limestones, dolomitized limestones, dolomite, and some anhydrite which were laid down in environments ranging from moderate-high energy subtidal and intertidal, to low energy, backshoal-lagoonal. The upper member is characterized by seven lithofacies dominated by various types of micritic and pelletoidal limestones, dolomites and anhydrites, and some intraformational breccias. The sequences formed during the regression of the Frasnian sea and, as a result, are the record of ever-decreasing water depth and increasing salinity. The depositional environments range from restricted-lagoonal to supratidal. Mapping of the thicknesses of various units within the Birdbear, coupled with core examination from certain key wells, allow the reconstruction of the paleogeography of the Saskathcewan Shelf during Birdbear time. This shelf was broken into two distinct depocentres by two large platforms, one centered in west-central Saskatchewan, the other in eastern Saskatchewan. The rocks of the depositional center to the northwest of the platforms are very similar to those of the Nisku Formation of eastern Alberta and are correlated with them. To the south of the platforms, the rocks are similar or identical to those found in the Williston Basin area of the North American Plains. These are the "typical" Birdbear rocks. During the final phase of the Frasnian transgression in earlymost Birdbear time, an argillaceous, lowermost unit was deposited across Saskatchewan. After the transgression peaked, a period of sea level standstill occurred during which high-energy, open-marine carbonates were deposited, including mechanically-piled shoals and mounds in some locations. The platforms were the sites of tidal-flat to supratidal (sabkha) environments. Following this interim period, the regression of the sea began and evaporites, both supratidal and subaqueous, with algal and chemically precipitated carbonates, filled the basin. The only area to escape the effects of this regression was the northwest, where sedimentation continued to be primarily subtidal to intertidal. The Birdbear Formation has been post-depositionally altered by early and late dolomitization and by secondary anhydritization. The dolomitization is significant from an economic viewpoint, as selective dolomitization has led to the development of porosity and possibly to the accumulation of hydrocarbons in stratigraphic traps. To date, the only production, however, is from structural traps.