ABSTRACT

The basal Cretaceous Iithosome of sandstone, siltstone and mudstone of the Western Interior is termed the Mannville Group and in Saskatchewan is distributed across most of the Province south of latitude 56 degrees. It is 100 to 800 feet thick and blankets an unconformity on southerly dipping Upper Jurassic to Cambrian strata. It is a somewhat rhythmic complex laid down under fluvial, deltaic and marine conditions governed:

(1) regionally, by episodic uplift of the Rockies and the Canadian Shield, and by penecontemporaneous subsidence in a reactivated Elk Point-Williston Basin and,

(2) locally, by the spread of salt sinks in the deeply underlying Middle Devonian Prairie Evaporite primed by moving Precambrian basement blocks.

Recognition of (1) and (2) leads to an apparent regional correlation of Mannville units based on concordance and non-cordance of marine datum units above, below and within the Mannville. Under concordance, deposition of the Mannville sediments are induced to have proceeded on low-gradient subsiding bottoms, and contacts of the rhythmic units to have maintained a pseudo-parallelism. Departures from concordance are revealed by:

(a) abnormal thickening of the Mannville section especially where such exceeds the stratigraphic relief of the truncated strata underlying the sub-Mannville unconformity.

(b) abnormal thinning of the Mannville, especially where accompanied by abnormal thickening of sub-Mannville strata.

By mapping the stratigraphy of the units (oldest to youngest) — Success (Lakota), Dina (McCloud), Cumming, Lloydminster, Rex, General Petroleum, Sparky, Waseca, McLaren-Pense —the following tectonic sequence is reconstructed.

(1) Post-Oxfordian uplift and the formation of a late Jurassic stripped plain across Jurassic and Paleozoic strata dipping southerly into the Williston basin;

(2) Mantling of this plain with deeply weathered terrestrial white kaolinitic (feldspathic), quartzose sandstones on the lower elevations and white tripolitic sandstones on the uplands underlain by the Mississippian Lodgepole limestones;

(3) Uplift of this plain in the region of the Williston Basin and generation of a deeply incised drainage system oriented northwesternly toward the McMurray tar sands basin of east-central Alberta from southeastern Saskatchewan;

(4) Reactivation and expansion of the Paleozoic Elk Point structural basin from central Alberta southeasterly into the Williston Basin region with progressive infilling by marine sediments coeval with heavy fluvial influxes from the west (biotite-chlorite green vacies), as well as the north and east (quartzkaolin white facies);

(5) Re-impositionof the dichotomy between the Williston and Elk Point basins by uplift of a broad arch across eastern Saskatchewan just south of latitude 52 degrees, and deep planation of the upwarped Mannville by a precursor of the Colorado sea (prepense).

These regional events were accompanied by general retreat of the northern and southwestern fronts of halite beds of the Prairie Evaporite to positions coincidental with the northern and southern flanks of the arch, and by the many local subsidences and structural reversals that displace the basal contact of the Mannville by as much as 600 feet.