Highlights—A Story of Incision

Incisions are stratigraphically important in that they record removal of previously deposited strata, and can represent sequence boundaries. Here, Ullah et al. describe scour surfaces at the top of the Ferron Sandstone Member (Cretaceous, Mancos Shale Formation, Henry Mountains, Utah) that include individual channel stories. This study shows that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multi-storey sand bodies. The results, coupled with modern analog studies, suggest that the maximum depth of confluence scours by autogenic process may reach up to four to five times the average depth of the incoming channels. Consequently, allogenic incised valleys (e.g., at sequence boundaries) can be defined in ancient systems only in situations wherein the erosional relief is more than five times average channel depth, markedly deeper than the thickest fully preserved storys, which in a braided stream are likely to represent confluence scour fills. Examples of autogenic modification of allogenically formed incised valleys suggest that both allogenic forcing and autogenic feedback can act simultaneously in fluvial systems.

Confluence scours versus incised valleys: examples from the Cretaceous Ferron Notom delta, southeastern Utah, U.S.A.by Mohammad S. Ullah, Janok P. Bhattacharya, and William R. Dupre