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

Frances Wahl Pierce, Edward A. Johnson

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° 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 the chaotic layer.

These blocks represent rotational slumping of levee material due to oversteepening of cut banks by penecontemporaneous stream erosion. They failed by shear along arcuate fractures that penetrated below the thalweg to planes of slippage now represented by the mudstone and coaly carbonaceous shale below the blocks. The horizontal arrangement of the blocks records lateral migration of the channel. The chaotic layer represents the unconsolidated levee top that failed by liquefaction and flow. Preservation of these blocks is due to (1) subthalweg emplacement that removed them from active stream scour, (2) burial of the blocks by the part of the levee that fluidized during failure, and (3) subsequent rapid deposition of sand by an aggrading fluvial system.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.