ABSTRACT

The Jurassic of northeastern British Columbia and northwestern Alberta consists of two parts separated by a disconformity, at which the Middle Jurassic is absent or reduced north of about 54°N latitude. The magnitude of the hiatus (presumably indicating uplift) between the stable craton deposits below (Lower Jurassic) and the initial deposits of the Columbian foredeep above (Upper Jurassic), is thus much greater than it is farther south in the Western Canada Sedimentary Basin.

The lower package is a thin, starved shelf succession of cratonic platform-miogeocline origin, disconformably overlying a relatively smooth surface on top of Upper Triassic rocks in the west and Carboniferous rocks in the east. Over most of its subsurface distribution, the lower package is platy, organic-rich, dark, phosphatic siltstone, generally (but incorrectly in strict terms) referred to the "Nordegg Member" of the Fernie Formation. In the Rocky Mountain Foothills to the west, additional beds near the base of the Jurassic yield Lower Hettangian ammonites, while a platy, dark siltstone unit (perhaps equivalent to the northern subsurface "Nordegg" of common usage) contains Pliensbachian ammonites.

The upper part (Upper Jurassic) is a thin, widespread shale-sandstone package over most of its subsurface extent. It thickens dramatically westward within the Rocky Mountain Foothills, where both cherty, orogenically (westerly) derived foredeep-fill deposits and quartzose, cratonic (easterly) derived sandstones are present. In outcrops in northeastern British Columbia, a thin, dark, upper Fernie shale is overlain gradationally by the Monteith Formation (sandstone). The Jurassic-Cretaceous transition is low in the Monteith. In the adjacent subsurface, and southward within northeastern British Columbia and western Alberta, this interval is generally referred to as the Nikanassin Formation.

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