Abstract

The marine strata of the Colorado and Montana Groups (Middle Albian to Campanian) of Saskatchewan and Manitoba were laid down on two shelf areas, which were the sites of sand and silt sedimentation. They were separated by a basin proper, where mainly mud was deposited. A rapidly subsiding western shelf received abundant sand and silt from the Cordillera to the west, whereas a relatively quiescent eastern shelf from the adjacent Shield received comparatively minor amounts of sand. Ubiquitous sandstone-mudstone couplets, up to a few centimetres thick and characterized by current markings on the sandstone soles, and simple grading of different types of detritus (siliciclastic grains, fish-skeletal debris, Inoceramus prisms), are evidence for the widespread influence of storm-surge generated suspension currents as agents of sediment dispersal. Two main regressive-transgressive events are suggested by prominent northeastward-thinning sandstone wedges, formed on the western shelf and enveloped in basin mudstones: the Bow Island-Viking succession (Colorado Group) and the Belly River sequence and associated tongues (Montana Group). Nearshore/shoreline, proximal-shelf and distal-shelf environments are distinguished in the former on (he basis of lithology, sedimentary structures and trace fossils, whereas limited data permit only recognition of a generalized shelf facies in the latter. Clinobeds, delineated by bentonitic mudstones in the Viking Formation of western Saskatchewan, are considered to have originated as tidal sand ridges. The Ashville Sand of southern Manitoba was deposited on the eastern shelf as a widespread blanket facies and a localized “trough” facies. Trough initiation may have been a result of differential compaction of the underlying strata across a belt of erosional irregularities at the sub-Mesozoic unconformity. A similar origin is postulated for parts of the Viking-Newcastle succession, which is approximately equivalent and exhibits a pronounced variation in thickness and overall westward thinning across southeastern Saskatchewan. Sand dispersal across both shelf areas was influenced by basement structure, solution-generated collapse features associated with the Middle Devonian Prairie Evaporite, and compaction-formed surface irregularities.