THE MISSING MUD BELTS IN THE ANCIENT RECORD

Mud belts are a common feature of modern inland seas but remain relatively poorly documented in ancient settings. Commonly associated with major river systems, mud belts are lensoid in cross-section, show internal sigmoidal clinoform geometries, and are generally positioned sub-parallel to shorelines. Such features can extend for hundreds of kilometers along strike and are commonly tens of kilometers in width. Diagnostic criteria for recognition of mud belts in fine-grained successions in sedimentary basins should be (1) lensoid geometries of strata positioned between marker beds (bentonite beds, minor flooding surfaces, etc.), and (2) a consistent component of unidirectional paleoshoreline-parallel coarse silt and very fine sand bedload transport, related to sediment remobilization by gyre currents. An ancient example of an interpreted mud belt of Cenomanian age from the Western Interior Seaway is presented. The feature, part of the Belle Fourche Member of the Frontier Formation, Powder River Basin, Wyoming, can be mapped for over 100km along strike and suggests a NW-SE oriented lowstand shoreline.