Well-Preserved Cut-Bank Slumps from Paleocene Sediments, Powder River Basin, Wyoming: ABSTRACT

Abstract

Slump blocks are associated with bases of fluvial channels in the Paleocene Tongue River Member of the Fort Union Formation, southeastern Powder River basin, Wyoming. These blocks are composed of thin to medium beds of very fine-grained sandstone containing finer grained organic-rich partings. Slump blocks described from other areas are composed of very cohesive silt and clay; blocks composed predominantly of sand are rare or absent. Our blocks are significant in that the sands were cohesive enough to fail by slumping rather than flow. The sandstone beds within the blocks exhibit small-scale tensional and compressional deformation. Strikes of beds within the blocks are subparallel to paleocurrent measurements from overlying channels, and the average dip of 36/degrees/ is significantly greater than regional dip. The blocks, commonly wedge shaped and averaging 18 m long and 4 m thick, occur in horizontal zones that can be traced for as much as 400 m. The blocks are bounded by arcuate surfaces at the base and sides and commonly overlie mudstone or coaly carbonaceous shale. The tops are unconformably overlain by a chaotic layer averaging 0.8 m thick, composed of randomly oriented fragments of sandstone and abundant plant debris. Active channel fill erosionally overlies themore » chaotic layer.« less