Allostratigraphic Subdivision of the Upper Cretaceous Fish Scale Formation, Southwestern Alberta: Implication for the Forming Mechanism of the Barons Sandstone
Yongtai Yang1, Bogdan Varban2, and Andrew Miall1
1 University of Toronto, Toronto, ON
2 University of Western Ontario, London, ON
In industry, the sandstone units just above the base of the Fish Sacles zone are informally called the Barons Sandstone in southwestern Alberta. Although the Barons Sandstone is an oil producer, its relationship with its surrounding strata (Fish Scale Formation (FSF) and Belle Fourche Formation (BFF)) as well as its forming mechanism have not been clearly known. Based on examination of about 30 cores and 500 well logs, this study documents the detailed stratigraphic and sedimentological aspects of the FSF and the lower part of the BFF. The FSF is subdivided into two allomembers and major flooding surfaces are used to define their boundaries. These two allomembers are generally 5-15 m thick and each is characterized by progradation of sets of offlapping units. The offlapping units show coarsening-upward facies successions with sandstone at the top. The thickest sandbody of the Barons Sandstone is about 10 m in the lower allomember of the FSF. It coarsens upward from mudstone and rippled siltstone, through bioturbated and hummocky stratified fine sandstone, to trough crossbedded coarse sandstone. The sandbody is linear, oriented parallel to the northeast-southwest shoreline trends of the Mowry Sea. Downdip and updip, it rapidly thin out into mudstone. These characteristics suggest that the Barons Sandstone was mainly deposited in wave-dominated transgressive barrier islands. In addition, the FSF and the lowest allomember of the BFF are thought to be the transgressive systems tract of a third-order sequence composed of the FSF and BFF.