Deposition and Interpretation of Long, Narrow Sandbodies Underlain by a Basinwide Erosion Surface: Cardium Formation, Cretaceous Western Interior Seaway, Alberta, Canada

Abstract

ABSTRACT Sand bodies of the Cardium alloformation (Burnstick allomember) occur in three parallel linear trends in the Garrington, Caroline-Crossfield and Lochend oilfields. They are extremely long (up to 100 km), straight and narrow (up to 4 km) and are encased in marine mudstones, suggesting their former interpretation as offshore bars. Our study of 189 cores and over 800 resistivity well logs shows that, below the Burnstick, the Hornbeck allomember consists entirely of dark mudstones deposited in quiet water. The Hornbeck--Burnstick contact, where seen in cores, is extremely sharp. In cross sections, resistivity well log markers are truncated by the lower surface of the Burnstick, and the sand bodies rest in asymmetrical one-sided elongate scours, with erosional relief of up to 7 m. Above he scoured surface, there is a basal lag of mudstone intraclasts and chert pebbles, overlain by a sandier-upward succession of mudstones, sandstones and conglomerates. The sandstones are extensively burrowed but show some cross stratification. The Burnstick sandier-upward succession is overlain by black mudstones of the Raven River allomember. The morphology of the scoured surface, as defined in isopach maps, does not indicate its formation in an open marine setting. The long, straight, asymmetrical scours are parallel to regional strike and do not suggest subaerial erosion. Instead, we suggest that a major relative lowering of sea level first exposed the Hornbeck mudstones to subaerial erosion. Subsequently, marine transgression of this surface removed evidence of its subaerial exposure. Pauses in transgression (stillstands) resulted in incised shoreface profiles (the asymmetrical scours), and at this time rivers supplied coarse sediment to the shoreface. This sediment was reworked alongshore to form what are now the very long, straight narrow sand bodies. Continued transgression resulted in the burial of the shoreface sand bodies by black mudstone (basal part of the Raven River allomember). Incised shoreface environments provide an alternative to the offshore bar settings suggested for other long, linear sand bodies.