ABSTRACT

The Besa River Formation is a black shale of Devono-Mississippian age which varies in thickness from less than 1,000 ft. to more than 7,000 ft. Results of X-ray diffraction and X-ray fluorescence studies are utilized to provide mineralogical and geochemical criteria for correlation within the formation and into equivalent shelf deposits to the east.

The correlations presented indicate that the thin Besa River Shale sections of the Northern Rockies are fondothemic in character, and represent continuous deposition in deep water through a period extending from Givetian to Chesterian time. Thicker sections to the east and south represent the clinothem, and the undathem is represented by barrier reef complexes, blanket shelf carbonates and interbedded calcareous shales and limestones.

Petrographic studies and relationship of mineral facies to basin topography suggest that the quartz fraction of the Besa River is largely of organic or chemical origin. The evidence suggests a southeasterly provenance for the clay minerals, and the difficulty in visualizing transport of these clays across the evaporite basins of southern Alberta and Saskatchewan leads to the suggestion that the clays were derived either from volcanic ash and moved to the shelf margins, or that they were generated within the basin from dilute solutions of their constituents.

It is shown that depth of water is not a critical factor in the formation of the thin black radioactive shales of the Exshaw Formation and the Muskwa Member of the Horn River Formation. The position of these shales as the basal phase of a shale transgression across shelf carbonates is noted, and the hypothesis is advanced that resorption of shelf evaporites by the transgressing seas leads to density stratification which provides the physico-chemical environment required for the formation of this unique lithotype.