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Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, South East Utah

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 process may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours within 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 well-exposed confluence scours examples within a Cretaceous-age compound incised valley system in the Notom delta complex of the Ferron Sandstone Member, Utah this study shows that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multi-storey sand bodies. A confluence scour fill produces a single story body in which a 5th order scour is filled with unit bar foresets, which in turn are overlain by a 4th order surface capped by compound bar deposits. The storey thickness within the confluence scour does not represent the average channel depth since confluence scours allow preservation of the deepest parts of channels as well as fully preserved storeys. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundaries 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 storeys, which in a braided stream are likely to represent confluence scour fills.