Fluvial architecture and late Cretaceous mammals of the Lance Formation, southwestern Bighorn Basin, Wyoming

This study comprises the first detailed study of fluvial architecture of the Lance Formation. Sandstone bodies of the basal Lance Formation exposed south of Meeteetse, Wyoming, here called channel belts, are composed of stacked channel-bar elements. Channel-bar elements are preserved deposits of low sinuosity (i.e., braided), sandy rivers. Avulsion-related mudstones separate channel belts. Regional variation in channel belt amalgamation and other evidence suggests presence of nested paleovalleys in the area of Gooseberry Creek. Paleovalleys imply bypass and time-equivalent deposition in the basinward direction; rapid progradation of deltas or shorefaces may have accompanied valley development in the Lance Formation. Paleovalleys can act as reservoirs and also hint at reservoirs further basinward in associated shoreline and marine facies.; The Hewett's Foresight Local Fauna is latest Cretaceous (Lancian) in age and comes mainly from University of Wyoming locality V-81013 northwest of Meeteetse. The fauna is dominated numerically by the multituberculate Mesodma and the marsupial Alphadon. Unlike all other well-sampled Lancian localities, the three common species of Mesodma from the Hewett's Foresight Local Fauna cannot be easily differentiated on the basis of their plotted tooth measurements. Alphadon marshi and A. jasoni are recognized, and this study is the first test of the revised diagnoses for these species that involves a statistically large sample of specimens from a single locality. The existence of Alphadon wilsoni is refuted. No new species are reported from the Hewett's Foresight Local Fauna, implying that the knowledge of Lancian mammals is reaching its apex. Previously unrecognized teeth of Pediomys krejcii and Turgidodon rhaister are figured, and a unique M3 may pertain to Batodon tenuis. The range of Cimolestes propalaeoryctes is extended south from the Flat Creek area of Montana.; Similar in faunal composition to that of the type Lance Formation, the Hewett's Foresight Local Fauna yields a minimum of 23 species of the 27 known from the type Lance fauna. Multituberculates and marsupials dominate both faunas, whereas eutherian mammals are relatively rare. A major point of difference between the two faunas, the Hewett's Foresight Local Fauna is dominated by didelphid marsupials while the localities of the type Lance Formation yield mostly pediomyid marsupials.