Abstract

Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges–Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth.

Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a single-story body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.