The effect of paleotopography on the late Albian and Cenomanian sea-level record of the Canadian Cretaceous interior seaway

In western Canada, a major paleoenvironmental change at the Albian-Cenomani an boundary is related to a eustatic sea-level rise overprinted by a relati ve sea-level fall in conjunction with preexisting topography within the bas in. This paper shows the lateral variability of sedimentology, paleoecology , and biostratigraphy of the latest Albian to Cenomanian interval along the Canadian western margin of the Cretaceous interior seaway. This paper (1) provides an integrated depositional and paleoecological examination of the Albian and Cenomanian lithologic units in the northern Western Interior Sea way; (2) demonstrates the effect of antecedent paleotopography within the b asin at the time of the transgression; (3) documents the variability in sed imentary facies and paleoecology controlled by paleorelief; and (4) demonst rates the diachroneity, lateral extent, and extremity of the multiple uncon formities controlled by this paleorelief. An enigmatic aspect of the lowermost Cenomanian Fish Scales Formation has b een the regional occurrence of chert, quartz, and bioclastic pebbles associ ated with black anoxic shale. A regional north-south traverse across Albert a provides insight into this problem. In southwestern Alberta, chert- and q uartzite-pebble conglomerate and sandstone that are equivalent to the Fish Scales Formation-known as Barons Sandstone (subsurface) and Blairmore Grits (outcrop)-represent proximal shelf sedimentation. This coarse-grained sedi ment was flushed out during sea-level lowstand and then reworked by a subse quent transgression associated with the Belie Fourche Formation. A large pa leohigh existed in southwesternmost Alberta at this time. Northward, the Fi sh Scales Formation is bounded top and bottom by unconformities and conglom erate, indicating multiple sea-level fluctuations and deposition in a wave- influenced shelf environment. In west-central Alberta, preserved deposits o f the Fish Scales Formation indicate deposition in a nearshore setting. Far ther to the north and, most distally, in northwestern Alberta and else here to the east, the coarse component of the Fish Scales Formation is predomin antly gravel-sized fish and other vertebrate debris with significantly less siliciclastic detritus. Regionally, the unconformity at the base of Fish S cales-Barons becomes more pronounced southward and westward where the under lying shale of the Westgate Formation (or Westgate Member) has been eroded or was never deposited. The regional paleogeographic setting for the Barons Sandstone and Fish Scal es Formation indicates greater amounts of erosion and coarser-grained depos ition in the southwest associated with the paleohigh. Northward and eastwar d, there was greater accommodation space, less erosion associated with the unconformity, and finer-grained sediment.