The upper Muddy Sandstone in southeastern Colorado and the Terra Cotta member of the Dakota Sandstone in central Kansas both represent mid-Cretaceous (Albian/Cenomanian) fluvial deposition in the U.S. Western Interior between marine deposition of the Kiowa-Skull Creek and Greenhorn cycles.
These units also are characterized by intravalley cut-and-fill structures. These structures are composed of amalgamated channel-fill, lateral accretion, and overbank-fine elements; and are channel shaped with width/depth ratios near 14:1, and thicknesses near 7-12 m. At least in the case of the Muddy Sandstone examples, these structures are bound by larger-scale channel-shaped surfaces that represent cut-and-fill of large-scale valleys. These intravalley cut-and-fills thus exist on a scale larger than channel scours, and smaller than fluvial valley scours, and appear to result from primarily tectonic and climatic stimuli that result in temporary episodes of incision of channels into their own alluvium and subsequent filling.
Because these cut-and-fill structures may be filled with a
wide variety of lithofacies, the potential to generate permeability
contrasts across structure boundaries is high (as is exemplified
in the Terra Cotta member). Due to their linear nature, such structures
may serve as narrow conduits for petroleum migration as well.
As the processes that form these structures are common in most
all modern fluvial systems, these structures should be universal
to fluvial deposits. They are, however, very rarely reported from
ancient strata.