ABSTRACT

Marine fades occur at the base and top of the Campanian Judith River Formation in its type area in north-central Montana. The basal Parkman Sandstone Member, long recognized to be of marine origin, crops out throughout the type area. Sedimentary structures and facies assodations suggest a nearshore (shoreface/foreshore) origin. Marine rocks at the top of the Judith River Formation are restricted to the eastern half of the type area, where the upper 200 - 230 ft (60 - 70 m) of the formation consists almost exclusively of very fine- to fine-grained sandstone. This previously undescribed upper marine lithosome is characterized by tabular, trough, and swaley cross-stratification, shallow marine ichnofossils (Ophiomorpha, Skolithos, Teichichnus), and local concentrations of shark teeth, fish vertebrae, and marine reptile bones. Sedimentary structures and paleontological evidence suggest that this sandstone-dominated lithosome represents two stacked shoaling-upward shoreface successions. The upper succession shoals into foreshore and paralic deposits, and is overlain by dark grey shale of the Bearpaw Formation. Ravinement surfaces (D1, D2, D3) characterized by persistent lags of teeth, bones, and shell debris bound both shoaling-upward sequences. Stratigraphically equivalent facies in the western half of the type area are primarily non-marine, and include fine-grained sandstone bodies, extensive lignite/coal deposits, and carbonaceous mudstone beds with abundant plant debris and root traces. These strata are interpreted as deposits of low-sinuosity fluvial channels, swamps/marshes, and hydromorphic floodplains that bordered the western shoreline of the Cretaceous epeiric seaway. This genetically-linked package of lower coastal plain/near-shore marine strata accumulated during an overall rise in relative sea-level (initial stages of the Bearpaw transgression), and is part of a transgressive systems tract.