PALEOENVIRONMENTS OF VERTEBRATE-BEARING STRATA AT THE CRETACEOUS-PALEOGENE BOUNDARY IN NORTHEASTERN MONTANA AND SOUTHWESTERN NORTH DAKOTA (PALEOSOLS, SEDIMENTOLOGY, FACIES)

Exposures of the Cretaceous-Paleogene (K-P) Hell Creek and Tullock (=Ludlow) Formations in eastern Montana and western North Dakota demonstrate that biotic changes of the K-P transition in the Western Interior occurred concomitantly with paleoenvironmental changes. Paleoenvironments of the Hell Creek are reconstructed from four facies. Facies 1, interpreted as meander channel deposits, is a fine- to medium-grained volcanic lithic arenite with planar and trough southeast-oriented cross-stratification. Facies 2, interpreted as floodplain deposits, is siltstone with roots and paleosols. Facies 3, interpreted as point bar deposits, is a fine-grained arenite with lateral accretionary structures. Facies 4 is laterally discontinuous organic accumulations. Facies 5, diagnostic of the Tullock and Ludlow, is suspension-settled siltstone, claystone, and macerated plant laminae. Facies 4 and 5 reflect an increase in standing water characteristic of the K-P transition in this region.;The lenticular pattern left by fluvial deposition has stratigraphic implications. Individual coals and members established within the Hell Creek do not constitute predictive lithostratigraphic markers. Biostratigraphic indicators such as pollen, dinosaurs, and mammals, must be considered in the context of facies-dependent preservation and episodic deposition before originations and extinctions can be reliably reconstructed.;A preliminary stratigraphy based upon geochemical signatures of K-P volcanic ash deposits is established through neutron activation analysis.;Silty, sandy, organic, and volcanic soils are preserved. Pedogenic features include roots, microscopic segregations of amorphous material, birefringence fabric, and soil structures. O, A/E, Btg, Bhs, Bg, BC, Cg, and C horizons are recognized. During middle Hell Creek time, soils formed in a poorly drained, unstable environment. Gleying and segregation of metallic oxides suggest water table fluctuation. By latest Hell Creek time, a rise in the mean level of the water table is reflected by the occurrence of non-pedogenic inorganic and organic sediments indicative of ponding.