Setulflike Scour-Remnant Features in a Neogene Volcaniclastic Flood Deposit, Central Oregon

Abstract

ABSTRACT Positive relief structures, resembling the inverse of flute marks, occur on a cohesive siltstone bed in nonmarine volcaniclastic sediments of the Deschutes Formation in central Oregon. Unlike setulfs and previously described scour remnant ridges, these structures increase in relief, rather than decrease, in the presumed downcurrent direction. The origin of these structures by sheet flow during floods is preferred over a wind origin, but the evidence is equivocal. The nature of flow necessary to produce these features is not known but may be related to unusual fluid boundary-layer conditions associated with high sediment-load transport.