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

Abstract

In western Canada, a major paleoenvironmental change at the Albian-Cenomanian boundary is related to a eustatic sea-level rise overprinted by a relative sea-level fall in conjunction with preexisting topography within the basin. 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 Seaway; (2) demonstrates the effect of antecedent paleotopography within the basin at the time of the transgression; (3) documents the variability in sedimentary facies and paleoecology controlled by paleorelief; and (4) demonstrates the diachroneity, lateral extent, and extremity of the multiple unconformities controlled by this paleorelief. An enigmatic aspect of the lowermost Cenomanian Fish Scales Formation has been the regional occurrence of chert, quartz, and bioclastic pebbles associated with black anoxic shale. A regional north-south traverse across Alberta provides insight into this problem. In southwestern Alberta, chert- and quartzite-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 sediment was flushed out during sea-level lowstand and then reworked by a subsequent transgression associated with the Belle Fourche Formation. A large paleohigh existed in southwesternmost Alberta at this time. Northward, the Fish Scales Formation is bounded top and bottom by unconformities and conglomerate, indicating multiple sea-level fluctuations and deposition in a wave-influenced shelf environment. In west-central Alberta, preserved deposits of the Fish Scales Formation indicate deposition in a nearshore setting. Farther to the north and, most distally, in northwestern Alberta and elsewhere to the east, the coarse component of the Fish Scales Formation is predominantly gravel-sized fish and other vertebrate debris with significantly less siliciclastic detritus. Regionally, the unconformity at the base of Fish Scales–Barons becomes more pronounced southward and westward where the underlying 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 Scales Formation indicates greater amounts of erosion and coarser-grained deposition in the southwest associated with the paleohigh. Northward and eastward, there was greater accommodation space, less erosion associated with the unconformity, and finer-grained sediment.