Viking Formation (Albian) at Eureka, Saskatchewan: A Transgressed and Degraded Shelf Sand Ridge

Abstract The Viking oil field at Eureka is part of the long (80 km), narrow (10 km) east-west–trending Dodsland-Hoosier offshore sand ridge in west-central Saskatchewan. The various facies of the ridge are encased in marine mudstones, and the ridge has formerly been interpreted as a tidal current deposit. However, core examination shows that the facies are burrowed and bioturbated and very few primary sedimentary structures are preserved. The sandstone facies are extensively bioturbated by Teichichnus, and contain no medium-scale cross-bedding, and no other tidal features such as reactivation surfaces, spring-neap cycles and tidal bundles. Cross sections constructed from well logs and cores demonstrate that the ridge overlies an erosion surface, and that at least three other erosion surfaces occur within the sand body. These surfaces strike eastwest, and dip very gently southward. Pebble veneers occur in patches on the erosion surfaces. We interpret the ridge as having formed from a plume of sand in a shallow sea, with its western end close to a deltaic distributary channel (similar to the plume east of the modern Nile delta). The regional Viking shoreline moved north or northeastward into the basin during a lowstand of relative sea level. During subsequent transgression, the delta was eroded and displaced an unknown distance southwestward. The ridge also shifted southward, and was finally detached from the distributary and abandoned on the shelf. Original slopes of the southern ridge margin were flattened to gradients of 0.0006–0.0009 by storm-generated currents. The internal erosion surfaces were probably formed during the overall transgression. Rates of sediment movement were relatively slow, resulting in almost complete bioturbation and, hence, destruction of primary sedimentary structures.