Abstract

Although the mudstone-dominated Loon River Formation of northwestern Alberta has clear value as a mappable lithostratigraphic unit, its limits have previously not been well defined. The formation definition is here brought into line with the North American Stratigraphic Code with the delineation of upper, lower and lateral boundaries, and the definition of a composite type section with its lower part in the cored well 00/14-18-123-19W5M/0, and an upper part consisting of outcrop sections along the Boyer and Ponton rivers.

Wireline-log, core and outcrop correlations indicate that regional transgressive/flooding surfaces TS1 and TS11, which respectively mark the base of the Spirit River Formation and top of the Peace River Formation in the Peace River area, mark the base and top of the Loon River Formation farther north. Transgressive/flooding surface TS9, at the top of the Spirit River Formation in the south, can be traced northward as an allostratigraphic surface to subdivide the Loon River Formation into lower and upper intervals.

The Spirit River and laterally equivalent lower Loon River interval form a northward prograding and thinning clastic wedge. New biostratigraphic data and correlation with the Spirit River Formation indicate that the lower Loon River Formation is of Early to Middle Albian age. In contrast, the Peace River–upper Loon River interval is thicker to the north. Initial mapping of discontinuities within the upper Loon River Formation indicates that it may consist of a series of northward offlapping sedimentary wedges. New U–Pb and ⁴⁰Ar/³⁹Ar ages for two bentonite beds, age-diagnostic dinocysts, and foraminifera of the Haplophragmoides multiplum Subzone indicate a Middle Albian age for the basal part of the upper Loon River Formation and confirm wireline-log correlation with the basal Harmon Member of the Peace River Formation to the south. The age of the uppermost part of the upper Loon River Formation is less certain. Foraminifera indicative of the Late Albian Miliammina manitobensis Zone and Late Albian dinocysts in samples from the overlying Shaftesbury Formation provide an upper age limit, and these strata may be as young as Upper Albian.