as an alternative to the two conventional theories described above, ketner proposed that "(1) left-lateral strike-slip faulting along the western margin of the north american continent, rather than plate convergence, was the engine of paleozoic tectonics in the region; (2) the roberts mountains allochthon, as such, does not exist, and the ordovician to devonian western facies assemblage was deposited essentially in situ; and (3) blocks of shelf carbonate rocks earlier thought to be exposures of the shelf in windows of the roberts mountains allochthon are slide blocks from the carbonate shelf. the slide blocks probably were dislodged by the alamo impact event of late devonian age." in this scheme, the deep-water aspects of the western facies assemblage are due to sea-level rise in the cambrian rather than displacement from an ocean basin.