Samples of an Upper Cretaceous sandstone dredged from Pribilof Canyon, Bering Shelf margin, do not compare well with rocks of the Shumagin Formation of presumed Late Cretaceous age on Sanak Island. Contrary to repeated published inferences, the rocks from Pribilof Canyon do not appear to be strongly deformed. They show no evidence of slaty cleavage or penetrative deformation and were probably deposited in a shelf environment at neritic or possibly upper bathyal depth rather than in a trench or deep continental slope basin. The presence of shelf-facies rocks in Pribilof Canyon at the edge of the Bering Shelf leaves little room for a subduction-related accretionary terrane such as exists along the Shumagin shelf near Sanak Island.
The concept of strongly folded deep-water trench-facies rocks in Pribilof Canyon has led to the development of Late Cretaceous subduction-related scenarios that include magmatic arcs parallel to the hypothetical subduction zone. Alternative hypotheses include a strike-slip plate boundary along the Pribilof segment of the Bering margin, which is consistent with the petrographic character of the Pribilof Canyon rocks, or a transitional trench-transform boundary–for example, the modern central to western Aleutian Trench.