The Odanah Member of the Pierre Shale in southwestern Manitoba is a porcelanite of Late Cretaceous (Campanian) age that was deposited on the eastern platform area of the Western Interior Seaway. Odanah sediments were derived from erosion of preexisting sediments in the Cordilleran areas to the west, from contemporaneous volcanism, and from accumulation of siliceous biogenic material. The mineral assemblage of the Odanah is dominated by cristobalite (opal-CT) with lesser amounts of quartz, mixed layer clays, feldspar, and illite. Cristobalite, derived from the diagenesis of diatoms, radiolarians, and siliceous sponge spicules, occurs as cryptocrystalline cement, bladed crystals, and lepispheres. Diatoms are considered to be the major source of biogenic silica in the Odanah. Chemical analyses of the Odanah give SiO 2 values which range from 70 to 85% (mean, 81%). Apart from silica, major element concentrations are consistently lower than those of the average shale. Major factors restricting Odanah deposition to a geographically limited part of the Interior Seaway may include: (a) reduced supply of terrigenous material from the west due to the presence of a paleotectonic barrier near the Manitoba-Saskatchewan border, and (b) increased diatom productivity in a zone of upwelling related to the southward flow of Arctic water within the Interior Seaway.