Jurassic strata of northwestern (and west-central) Alberta and northeastern British Columbia

The Jurassic succession in northwestern Alberta and northeastern British Columbia consists of two main sequences separated by a regional unconformity. The unconformity, at the base of the higher, foredeep sequence, reflects the initiation of the Columbian Orogeny in the early Late Jurassic. The lower sequence, comprising Hettangian to Pliensbachian Lower Fernie shelf limestone and shale and the Toarcian to Aalenian Poker Chip Shale member of the Fernie Formation, is a thin, stable platform succession dominated by shale and limestone. The upper package, comprising mainly Upper Jurassic Upper Fernie shale and sandstone, and the Nikanassin Formation and the Minnes Group, is an easterly tapering orogenic clastic wedge filling the foredeep to the west, with thin shelf shale and sandstone equivalents to the east. The absence or near-absence of Middle Jurassic strata indicates probable pre-Late Jurassic uplift and erosion. The informal Lower Fernie shelf limestone and clastic unit of northwestern Alberta and northeastern British Columbia is commonly but incorrectly called the Nordegg Member (Fernie Formation). The Nordegg Member proper is a banded black chert and limestone facies developed far to the south, in west-central Alberta. The northern Lower Fernie strata were deposited on the shelf north of a cherty carbonate platform that developed in west-central Alberta. The Nordegg is probably a western slope facies of that platform. New palynological data suggest that the Poker Chip Shale extends into the Aalenian in northwestern Alberta, the first documented occurrence of Aalenian strata in the Western Canada Basin. Glauconitic and sideritic sandstones originally assigned to the Rock Creek Member in the Peace River area of northwestern Alberta are probably basal strata of the Upper Jurassic Upper Fernie shale and sandstone orogenic wedge. This sequence grades upward into interbedded sandstone and shale of the Nikanassin Formation and lower part of the Minnes Group. The Nikanassin and lower Minnes are facies equivalent and contain the transition into the Lower Cretaceous. The lingering effects of differential compaction over different fault blocks along the locus of the ancient Peace River Arch are recorded in minor thickness variations in the Jurassic units.