PALEOENVIRONMENTAL CHANGES IN THE CRETACEOUS (ALBIAN TO TURONIAN) COLORADO GROUP OF WESTERN CANADA - MICROFOSSIL, SEDIMENTOLOGICAL AND GEOCHEMICAL EVIDENCE

Paleoenvironmental interpretations presented here for a portion of the Cretaceous Colorado Group marine shale succession in western Canada a re based on the synthesis of biofacies, sedimentological and geochemic al data. Vertical and lateral variations in foraminiferal, coccolith a nd dinoflagellate assemblages, in sediment fabric, structures and grai n 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 over all 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 M owry Sea transgression under a dominantly low-salinity, cool, Boreal w atermass. Up to three coarsening-up cycles identified within this unit indicate local sea-level fluctuations or changes in sediment supply a nd/or distribution. The exclusively agglutinated foraminiferal assembl age is Boreal in affinity and reflects changes in substrate grain-size . Sedimentary structures and generally well-bioturbated sediment indic ate deposition at or above storm-wave base beneath oxygenated bottom-w aters. The basal lithology of the overlying lower Cenomanian Fish Scal es Formation is a regionally extensive bioclastic conglomerate interpr eted as either a wave-winnowed lag formed during a relative sea-level fall and subsequent rise, or a current-winnowed lag in deeper water. D eep-water bottom currents possibly were generated by mixing of the coo l, low-salinity Boreal waters with warm, normal-salinity waters of Tet hyan affinity as the Mowry Sea opened to the south forming the Western Interior Seaway (WIS). Organic matter is dominantly Type II, comprisi ng a large component of marine algal material. The overlying barren, w ell-laminated sediments that comprise the bulk of the Fish Scales Form ation were deposited under a stratified water column with anoxic botto m-waters and are characterized as a condensed section. The middle to u pper Cenomanian Belie Fourche Formation conformably overlies the Fish Scales Formation. A regional sea-level drop occurred during Belie Four che time as indicated by the progradation of Dunvegan deltaic sediment s in northwestern Alberta. Widespread dysoxic conditions persisted thr oughout the middle to late Cenomanian in this region as shown by the l imited agglutinated foraminiferal assemblage and sparse bioturbation. Increased detrital input is evident as an increase in silt content rel ative to the Fish Scales Formation and a re-introduction of significan t amounts of Type III organic matter. The occurrence of numerous biocl astic 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. Max imum 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 Whit e Specks Formation. Productivity in the upper water column was high an d anoxic bottom waters preserved abundant Type II organic matter. Late ral facies variations and a diachronous introduction of Tethyan forami nifera and coccoliths to various parts of the basin indicate pathways of oceanic circulation. The influence of major Cordilleran detrital so urces limited pelagic faunal development in the west. A significant un conformity in central Saskatchewan indicates local basin floor doming and subsequent erosion in late Turonian to Santonian time. (C) 1996 Ac ademic Press Limited.