Paleoenvironmental interpretations presented here for a portion of the Cretaceous Colorado Group marine shale succession in western Canada are based on the synthesis of biofacies, sedimentological and geochemical data. Vertical and lateral variations in foraminiferal, coccolith and dinoflagellate assemblages, in sediment fabric, structures and grain size, and in organic matter abundance and composition indicate shale deposition in a dynamic and variable basin setting. The upper Albian to middle Turonian Colorado Group shales were deposited during an overall eustatic sea-level rise punctuated by local, tectonically-induced, relative sea-level drops and variable circulation patterns. The upper Albian Westgate Formation was deposited during the initial stage of Mowry Sea transgression under a dominantly low-salinity, cool, Boreal watermass. Up to three coarsening-up cycles identified within this unit indicate local sea-level fluctuations or changes in sediment supply and/or distribution. The exclusively agglutinated foraminiferal assemblage is Boreal in affinity and reflects changes in substrate grain-size. Sedimentary structures and generally well-bioturbated sediment indicate deposition at or above storm-wave base beneath oxygenated bottom-waters. The basal lithology of the overlying lower Cenomanian Fish Scales Formation is a regionally extensive bioclastic conglomerate interpreted as either a wave-winnowed lag formed during a relative sea-level fall and subsequent rise, or a current-winnowed lag in deeper water. Deep-water bottom currents possibly were generated by mixing of the cool, low-salinity Boreal waters with warm, normal-salinity waters of Tethyan affinity as the Mowry Sea opened to the south forming the Western Interior Seaway (WIS). Organic matter is dominantly Type II, comprising a large component of marine algal material. The overlying barren, well-laminated sediments that comprise the bulk of the Fish Scales Formation were deposited under a stratified water column with anoxic bottom-waters and are characterized as a condensed section. The middle to upper Cenomanian Belle Fourche Formation conformably overlies the Fish Scales Formation. A regional sea-level drop occurred during Belle Fourche time as indicated by the progradation of Dunvegan deltaic sediments in northwestern Alberta. Widespread dysoxic conditions persisted throughout the middle to late Cenomanian in this region as shown by the limited agglutinated foraminiferal assemblage and sparse bioturbation. Increased detrital input is evident as an increase in silt content relative to the Fish Scales Formation and a re-introduction of significant amounts of Type III organic matter. The occurrence of numerous bioclastic conglomerates throughout the upper portion of the Belle Fourche Formation is possibly the result of relative sea-level drops affecting areas of different water depth with variable erosional intensity. Maximum transgression in latest Cenomanian to early Turonian time brought fully marine conditions and planktic Tethyan fauna into the Canadian portion of the WIS. This time period is represented by the Second White Specks Formation. Productivity in the upper water column was high and anoxic bottom waters preserved abundant Type II organic matter. Lateral facies variations and a diachronous introduction of Tethyan foraminifera and coccoliths to various parts of the basin indicate pathways of oceanic circulation. The influence of major Cordilleran detrital sources limited pelagic faunal development in the west. A significant unconformity in central Saskatchewan indicates local basin floor doming and subsequent erosion in late Turonian to Santonian time.

, , , , , , , , , ,
Cretaceous Research
Department of Earth Sciences

Schroder-Adams, C, Leckie, D.A. (D. A.), Bloch, J. (J.), Craig, J. (J.), McIntyre, D.J. (D. J.), & Adams, P.J. (P. J.). (1996). Paleoenvironmental changes in the Cretaceous (Albian to Turonian) Colorado group of western Canada: Microfossil, sedimentological and geochemical evidence. Cretaceous Research, 17(3), 311–365. doi:10.1006/cres.1996.0022