Paleocene Mammalian Biostratigraphy of the Carbon Basin, Southeastern Wyoming, and Age Constraints on Local Phases of Tectonism

Mammalian fossils from two principal collecting areas in the lower Hanna Formation of the Carbon Basin comprise three faunas, the Grayson Ridge, Halfway Hill, and Sand Creek faunas. The Grayson Ridge and Halfway Hill faunas are diverse, consisting cumulatively of 29 mammalian species, at least two of which are new. The faunas are approximately equivalent in age and are either latest lbrrejonian or earliest Tiffanian, or possibly sample both NALMAs. In any event, the faunas are very close in age to the lbrrejonian-Tiffanian boundary. Strata bearing the Grayson Ridge and Halfway Hill faunas were truncated by erosion, resulting in a previously unrecognized intraformational unconformity (IFU). The Sand Creek fauna consists of a meager sampling of mammalian fossils from directly above the unconformity and provides a Ti3-Ti5 age (middle or late Tiffanian) for strata overlying the unconformity. Fortuitous stratigraphic positioning of the faunas allowed age constraints to be placed upon two local phases of deformation. Simpson Ridge anticline, which separates the Hanna and Carbon basins, resulted from the first phase. Based upon lithologic correlation of the lower Ferris Formation at Simpson Ridge to its type section, development of Simpson Ridge began in the Lancian or Puercan time. Based upon fossil ages in the lower Hanna Formation, which onlaps the anticline, most, or all, of Simpson Ridge had formed by the early Tiffanian. A younger episode of deformation occurred no earlier than Ti4 (middle Tiffanian) and overprinted the folding of Simpson Ridge. Additionally, fossils from above and below the IFU suggest a hiatus of 1 to 3.5 m.y. in the lower Hanna Formation. Age constraints and orientations of Simpson Ridge anticline and an overprinting syncline suggest a local change in the direction of maximum shortening from early Paleocene to late Paleocene or Eocene.