Secondary rock structures and the regional hydrogeology of claystone-rich cretaceous strata, Williston Basin, Saskatchewan, Canada

The geometry and spatial distribution of a polygonal fault system (PFS) and collapse features within Cretaceous strata (predominantly mudstones and claystones) were investigated using three high-resolution three-dimensional seismic datasets of the Williston Basin, Saskatchewan, Canada. Mapping of the planform geometry and fault throw distributions (throw–depth (T–z) profiling) shows that the PFS present in the Colorado Group and Pierre Fm has a vertical extent of 200–330 m. Variation in the lateral planform geometry is attributed to the relative rates of stress accumulation during early development of the planar faults and is constrained using sequence stratigraphic principles. The mean fault dip is 60° ± 12° (number of measurements, n = 225). The T–z profiles appear as partial C-type profiles, demonstrating that at least half of the total height of the PFS was removed during post-Cretaceous erosion. The presence of polygonal faults in equivalent strata of the Western Interior Sedimentary Basin (WISB) suggests the PFS described in the current study (1510 km2) may be present across the WISB. Collapse features, formed in response to dissolution cavities within underlying strata, crosscut the entire Cretaceous sequence and are subcircular in plan view with typical diameters of 350–450 m. These features are present in each of the datasets at a rate of 0.02–0.11 collapses/km2. The prevalence of collapses in areas where faults display modified throw distributions may suggest post-Cretaceous fault reactivation associated with Pleistocene glacial periods. Although these secondary rock structures likely affect groundwater and solute transport at the basin scale, the impact remains to be determined.

Evidence for Early Pleistocene glaciation from borecore stratigraphy in north-central Alberta, Canada

Pleistocene sediments collected in north-central Alberta, Canada, were subsampled and studied for paleomagnetic remanence characteristics. A magnetostratigraphy has been established for sediments previously assumed to represent multiple continental (Laurentide) glaciations but for which no geochronology was available. Based on the Quaternary record elsewhere in Alberta and Saskatchewan, it was thought that some of these sediments were deposited during pre-late Wisconsinan glaciations. The Quaternary sedimentary successions of north-central Alberta have a thickness up to 300 m within buried valleys and are composed of diamicts interbedded with glaciolacustrine and outwash sediments. Most of the sampled units are not accessible from outcrop, and their sedimentology and stratigraphy is derived from core data only. In 4 of 16 borecores sampled to date, diamict that correlates with the Bronson Lake Formation till is reversely magnetized, indicating an Early Pleistocene age. This formation is underlain by either Empress Formation sediments or Colorado Group shale, and is overlain by one or more normally magnetized glacigenic sedimentary units of the Bonnyville, Marie Creek, and Grand Centre formations, respectively. This new record of Early Pleistocene glaciation in north-central Alberta places the westernmost extent of earliest Laurentide ice at least 300 km farther west than its previously established limit in the Saskatoon and Regina regions of the Canadian Interior Plains, but still to the east of the maximum extent of the Late Wisconsinan (Late Pleistocene) Laurentide Ice Sheet, which extended into the foothills of the Alberta and Montana Rocky Mountains.

Correlation of physical surfaces, bentonites, and biozones in the Cretaceous Colorado Group from the Alberta Foothills to southwest Saskatchewan, and a revision of the Belle Fourche - Second White Specks formational boundary

New allostratigraphic correlations in west-central Alberta show that flooding surfaces in the Sunkay and basal Vimy members of the Blackstone Formation can be traced eastward from Burnt Timber Creek for >500 km to western Saskatchewan, and for 350 km northward into the Dunvegan and Kaskapau formations. At Burnt Timber Creek, a Miliammina manitobensis microfauna in the lowest 1 m of the Sunkay Member indicates equivalence to the Westgate Formation, and an overlying 70 cm conglomerate is correlative with the Fish Scales Formation. Overlying mudstones contain foraminifera of the Verneuilinoides perplexus Zone, indicative of the middle Cenomanian Belle Fourche Formation. The base of the Second White Specks Formation, as currently defined in core in eastern Alberta, equates in outcrop with the lower part of the Sunkay, rather than the base of the Vimy Member as previously thought. The base of the Second White Specks Formation should be placed about 35 m higher in the stratotype core. A bentonite located 9.3 m above the base of the Sunkay Member at Burnt Timber Creek lies near the highest occurrence of Evolutinella sp. aff. E. apricarius, indicating proximity to the "X" bentonite, recognized across the prairies and traceable north into allomember C of the Dunvegan alloformation. A thick bentonite 93.2 m above the base of the Burnt Timber Creek section lies 4 m above the base of the Vimy Member; this bentonite, traceable from northern British Columbia to New Mexico, lies a few metres below the Cenomanian–Turonian boundary and is here formally named the Bighorn River Bentonite.