Exposure of the Colorado Group-equivalent strata in the Alberta Plains is largely limited to northeastern Alberta, where units above the Pelican Formation have been assigned to the Labiche Formation. In this report, we present new data from measured sections of outcrop of Colorado Group-equivalent strata on the Athabasca River. We determine whether terminology from southern Alberta (Westgate, Fish Scales, Belle Fourche and Second White Specks Formations) can be established in northeastern Alberta, partly replacing the use of the Labiche Formation.
Two outcrop sections, T79-R17W4-01 (Stony Rapids) and T81-R17W4-01 (Pelican Cliffs), were measured on the Athabasca River in northeastern Alberta and a natural gamma-ray log was recorded for the Stony Rapids section. Combined, the measured sections traverse the stratigraphy of the Pelican, Westgate, Fish Scales and Belle Fourche Formations. The lower part of the Pelican Formation consists of interbedded sandstone and mudstone. The upper part is dominated by a thick, cliff-forming, siderite-cemented sandstone, showing swaley and hummocky cross-stratification. The Pelican Formation is overlain by sulphur-stained, dark grey shale of the Westgate Formation. A prominent, indurated bioclastic bed with very high gamma-ray counts marks the base of the Fish Scales Formation. We observed two 1-2 cm thick bentonite units in the otherwise shale-dominated Fish Scales Formation. The contact between the Fish Scales and Belle Fourche Formations is probably not exposed and, based on comparisons with nearby downhole geophysical well logs, may lie within a covered interval above the bentonite units. The exposed lower part of the Belle Fourche Formation is shale dominated with silty lenses, coarsening to siltstone with sandy siltstone lenses towards the top.
Based on our observations presented in this report, we conclude that the lithostratigraphic divisions of the Colorado Group defined in the subsurface of southern Alberta can be applied to the Colorado Group in northeastern Alberta.