Abstract

Regional surface and subsurface correlation of the middle Belly River Group clastic wedge confirms that the Oldman Formation is present throughout southern and central Alberta. The Oldman generally thins to the north and northeast due to depositional thinning and to truncation by the overlying Dinosaur Park Formation. The Oldman Formation, primarily a transgressive fourth-order subunit, can be informally divided into two mappable units, referred to here as the lower Comrey Member sandstone and the "upper siltstone" member. The Comrey Member is dominated by light grey, fine to coarse sandstone with a sheet-like geometry over much of the map area. However, variations in thickness suggest that it is arranged in sharp-based linear WSW/ENE-oriented incised valley fill trends, up to 33 m thick, composed of stacked lenticular fining-upward channel units. Based on limited outcrop data from the southern part of the study area, paleoflow within these valleys was likely toward the eastern hemisphere. Corresponding clean sandstone trends (potential reservoir rock) are up to 25 m thick. The overlying "upper siltstone" member is dominated by pale-coloured, thinly interbedded noncalcareous mudstone and fine sandstone, with abundant rooting, convolute lamination and pedogenic horizons. The sediments of the Oldman Formation were derived from the west and southwest. The regionally consistent upward sequence of facies from erosional base to sandstone-dominated to mudstone-dominated can be interpreted to represent a fundamental reorientation of paleogeography related to an increasing rate of basin subsidence and consequent transgressive pressure in southern Alberta. A rapid and extensive base level fall, interpreted as the result of tectonic quiescence, basinwide rebound and erosion, was followed by deposition of an overall transgressive succession during increasing base level rise related to widespread subsidence during a phase of renewed tectonic thrust loading. The formation encloses known gas pools, but the presence of large areas of thick clean sandstone with no designated pools suggests further exploration possibilities. Included maps are intended as guides; for practical application, more detailed mapping of individual sandstone bodies is necessary.

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