A Late Cretaceous (Cenomanian) marine vertebrate-rich bioclastic horizon from the northeastern margin of the Western Interior Seaway, Canada

Throughout the Cretaceous Western Canadian Sedimentary Basin, concentrations of remains of fossilized marine vertebrates & invertebrates (macrofossils) occur as dense bioclastic and bonebed horizons. In Manitoba and Saskatchewan, Canada, these horizons are particularly abundant within the Belle Fourche Member of the Ashville Formation, deposited along the eastern margin of the Cenomanian Western Interior Seaway (WIS). Concentrations of abundant bones, teeth, and scales of marine and, very rarely, terrestrial, vertebrates are found along with bivalve-rich horizons such as the regionally wide-spread Ostrea beloiti layer along the length of the Manitoba Escarpment.;A newly discovered locality near the southern end of the escarpment has yielded calcarenites, composed primarily of inoceramid-derived calcitic prisms and abundant, disarticulated marine vertebrate elements and microcoprolites, as well as oyster valves. Stratigraphically, these deposits are believed to be of early Middle Cenomanian age, originating lower in the Belle Fourche Member than previously described marine vertebrate bonebeds from the region.;The preserved vertebrate fossils reveal a diverse paleocommunity comprised of 12 chondrichthyan taxa, 13 osteichthyan taxa, and 2 reptilian taxa. Several of these, including Protosphyraena sp., Squalicorax falcatus, a salmoniform, Xiphactinus audax, Enchodus cf. E. gladiolus, Enchodus cf. E. petrosus, and Enchodus cf. E. shumardi represent earliest occurrences of these taxa in Canada and/or North America, as well as what may be the earliest evidence of eels (Order Anguilliformes) on this continent.;Taphonomic data indicates that the bioclasts were concentrated by physical processes such as winnowing, as well as by the siliciclastically-starved setting of the eastern margin of the WIS. Evidence of minor transportation, winnowing, and reworking are taken to indicate these bioclastic sediments were initially deposited relatively further offshore than other bonebeds known from the Manitoba Escarpment, at or just beneath storm wave base. Petrographic study of the bonebeds reveals that variability in the amount of inoceramid-derived prismatic calcite buried along with the concentrated vertebrate bioclasts may have affected the timing of cementation, and therefore the susceptibility of these deposits to further reworking.;Comparison with skeletal limestones known from the Greenhorn Limestone in the United States suggests that the calcarenites studied here were deposited in a similar manner: as the result of storms (i.e. as tempestites) impinging on the extremely shallowly sloping sea floor of the eastern margin of the Western Interior Seaway, during one of the frequent oscillations of sea-level during the Cenomanian. Cretaceous marine bioclastic and bonebed horizons are widespread in the WIS and may eventually serve as stratigraphic and sequence stratigraphic markers integrated into a basinwide framework.