The Horseshoe Canyon Formation in southern Alberta: surface and subsurface stratigraphic architecture, sedimentology, and resource potential

Upper Campanian-Maastrichtian strata of the Horseshoe Canyon Formation have received little modern stratigraphic analysis on a regional scale, even though they are well exposed in the famous "Drumheller Badlands" along the Red Deer River, are present in thousands of drillholes in southern Alberta, and are endowed with significant coal and gas resources. Hence there is no regionally applicable subdivision, subsurface mapping, or appreciation of reservoir geometries and exploration strategies. Regional stratigraphic analysis of integrated surface and subsurface data from the Horseshoe Canyon Formation of south-central Alberta indicates the unit represents a third-order, southeastward-thinning, progradational system that advanced hundreds of kilometres into the Bearpaw marine basin over about 6 to 7 million years of Late Cretaceous time. The upper boundary of the Horseshoe Canyon Formation is herein formally revised to coincide with the regional unconformity present between strata of the Whitemud sandstone and the dark shale of the Battle Formation. The former "Whitemud Formation" is therefore downgraded to an informal pedogenic unit beneath this surface. In addition, the unit is clearly subdivisible into five previously unrecognized, regionally mappable informal lithostratigraphic tongues. In this study area, these comprise stacked composite, primarily regressive units that extend toward the southeast and are separated by tongues of marine shale extending toward the northwest from the main body of coeval Bearpaw Formation shale. In ascending order, these new informal tongues are here referred to as Strathmore, Hoodoo, Midland, Tolman, and Carbon, and are characterized by thinly interbedded sandstone, siltstone, coal, and various types of paleosol horizons. The tongues identified here may eventually warrant formal member status but at present, the definitions of their boundaries are inadequate for formalization. Paleoflow indicators suggest the direction of general sediment dispersal was toward the east-southeast, but that over time, dispersal tended more toward the east. Differences in paleosol type and abundance, and in coal thickness and abundance, suggest a climatic trend of most humid in the Strathmore to most arid in the Tolman, then back to humid again in the Carbon. The five Horseshoe Canyon nonmarine tongues, and the Bearpaw marine tongues that separate them, form a stacked set of asymmetrical, shallowing-upward, fourth-order regressive cycles with regional extents. In ascending order, these cycles are A) lower Bearpaw-Strathmore tongues, B) middle Bearpaw-Hoodoo tongues, C) upper Bearpaw-Midland tongues, D) Drumheller marine-Tolman tongues, and E) Carbon tongue. The locus of shoreline-related sandy deposition and the penetration of marine influence shifted southeastward (basinward) through time, denoting an overall regressive trend. The lowest Bearpaw marine tongue includes the maximum transgression phase immediately overlying the Belly River Group, whereas the subaerial unconformity at the top of the Carbon tongue represents the maximum regression surface. This unconformable top of the Horseshoe Canyon Formation is marked by the distinctive Whitemud sandstone, here interpreted as an intensely altered spodosol horizon, and is overlain by the black mudstone of the Battle Formation. Both the third-order transgressive-regressive sequence of the Dinosaur Park-Bearpaw-Horseshoe Canyon formations, and the contained fourth-order regressive cycles, are the result of fundamental regional tectonic controls on subsidence and sediment supply. In addition, nested within each fourth-order cycle is a series of thinner, asymmetrical, coarsening-upward, fifth-order, regressive subunits that mimic the thinning and fining trends of the larger cycles. These units include separate mappable reservoir or aquifer trends of channel, estuarine and shoreface character, and several of them have been studied in detail by other authors at the famous Hoodoos-Willow Creek location. The Horseshoe Canyon Formation contains potential for significant and under-exploited shallow gas, coalbed methane, and groundwater resources. More focussed exploration efforts resulting from the conclusions of this study may increase the economic benefits derived from these strata.

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