Regional Geology of the Upper Blairmore Group and Bow Island Formation: a Subsurface Study in Southwestern Alberta

A comprehensive subsurface database was built from petrophysical logs of approximately 200 wells which penetrate the Lower Cretaceous successions from Townships 1 through 19, Ranges 19 west of the 4th meridian to the edge of the deformed belt (ranges 27 to 30 west of the 4th meridian). The subsurface database was used to construct a network of subsurface cross sections, including approximately 20 east-west and three north-south cross sections. Tops for the intertonguing marine and non-marine successions were identified from logs and outcrop work (in the deformed belt) for selected sites located proximal to areas with subsurface core-control. The outcrop and core sites were tied to subsurface logs by regional correlations. An intertonguing marine-nonmarine boundary was mapped throughout the study area to distinguish those hydrocarbon pools that would have a more fluvial setting (Upper Blairmore Group) from those with more marine environments of deposition (Upper Bow Island Formation). Log criteria for the distinction of marine and non-marine units included: the type of vertical changes in grain size (i.e. dominantly fining-upwards for non-marine; coarsening-upwards for marine) and the degree of lateral continuity of correlatable units (low for nonmarine; high for marine).

The progradational regressive successions reflect the input of dominantly fluvial sediments into the basin during times of tectonic quiescence. The two fluvial formations belonging to the upper Blairmore Group include the Beaver Mines and Mill Creek formations. The fluvial parts of the Beaver Mines and Mill Creek formations are separated by a marine incursion, associated with a transgressive phase, of the Lynx Creek Member. The fluvial upper part of the Mill Creek Formation is known as the Bruin Creek Member.

A change from dominantly more marine upper Bow Island sands and mudstones to continental or deltaic upper Blairmore (Mill Creek and Crowsnest formations) occurs across Ranges 26 and 27 W4 in the study area. This can be envisioned as an ancient shoreline with several embayments. Some wells drilled in this transition area will have marine Bow Island units, others will have continental successions.

In the region east of Range 25 W4, from Townships 1 through 19, Mannville-Bow Island terminology should be used. In areas west of Range 27 W4, near the limit of the deformed belt, Blairmore terminology should be used.

In the transition area from Range 26 to 27 W4, where both the Bow Island marine and the Mill Creek continental facies are found, both terminologies can be employed, depending upon which units are encountered. Care must be taken in this stratigraphic interpretation t be sure that the Lynx Creek marine units (part of the Blairmore Group) are not mistakenly identified as Bow Island marine successions.