Abstract

The Mannville Group of Saskatchewan is an unconformity-bounded clastic body comprising three main formations: Success, Cantuar and Pense.

The Success Formation, ranging in age from Kimmeridgian(?) to Neocomian, and distributed across the province as three large homologous bodies, is characterized by white, kaolinitic, sideritic and locally cherty, quartzose sandstones and mudstones up to 105 m thick. Its depositional slope was southward and southwestward from off the Precambrian Shield and its general depositional format is fluviatile-lacustrine, with some indications of early marine in the extreme south of the province. The Success is coeval with the Early Cretaceous Lakota Formation of the Williston basin, as well as the Jura-Cretaceous Morrison of the Fergus-Great Falls basin and, perhaps, the Swift sandstone of the Sweetgrass Arch.

In contrast, the Cantuar Formation is more varied in its lithological makeup and comprises three major facies:

a) Green biotite-chlorite rich tongues in the southwest (Atlas-Dimmock Creek Members) and in west-central Saskatchewan (Grand Rapids Formation). These yield eastward to the centrally distributed

b) Clearwater facies of marine, sequential or cyclic beds of black muds, bioturbated sands, mudstones and very argillaceous sandstones and well sorted sandstones; interrupted irregularly by fluviatile and deltaic marine channel sandstones, lagoonal mudstones and thin coals. The Clearwater facies grades eastward near Manitoba into

c) kaolinitic, quartzose sandstones feathering in from the Precambrian Shield. This facies also characterizes the underlying McMurray- Dina Member, which forms a regionally consistent unit of quartzose, locally kaolinitic sandstones, generally fluviatile to estuarine in depositional mode.

The Cantuar Formation overlies a high relief (100 m) unconformity and in general is a product of a marine invasion that extended from the Arctic to the Williston Basin. The seaway in Saskatchewan was relatively narrow and was thus more nearly estuarine. Provenances were the rising Cordillera (biotite-chlorite facies), older beds in the interfluves of the topography, as well as the Precambrian Shield.

The Pense Formation, wholly marine but retaining the cyclic depositional format of the Clearwater, overlies the Cantuar on an unconformity that locally cuts to the base of the latter on the Punnichy Arch. Four upward coarsening sequences of mudstone, bioturbated mudstone-sandstone and clean sandstones can be utilized as a yardstick for measuring the depth of each step in the subsidence of the seafloor.

Internal structural control of the Mannville basin was apparently exercised by basement blocks moving differentially along several major lineaments. These linears also controlled the paths of formation fluids, including meteoric influx. The hydrodynamic potentiometric field appears to have exercised some control on the location of the major oilfield for Mannville oil districts of the province. Source beds are deduced to have been Paleozoic strata to the south and southwest.