ROLE OF LOWERED BASELEVELS IN THE GENERATION OF EARLY EOCENE BIOHORIZONS; WILLWOOD FORMATION, SOUTHERN BIGHORN BASIN, WYOMING

In the southern Bighorn Basin, the fossil vertebrate record of the ~770 meter-thick lower Eocene Willwood Formation is punctuated by three major intervals of faunal turnover that record the ”immigrations” (range expansions) and extinctions of several genera and species of early Eocene fossil mammals. These intervals have been designated Biohorizon A (within the ~195 to ~215 meter interval of the Willwood Formation), Biohorizon B (within the ~380 to ~430 meter interval), and the here-named Biohorizon D (tentatively placed within the ~585 to ~610 meter interval of the Willwood Formation). In lower Eocene rocks of the Wind River Basin, faunal change like that exemplified at the Bighorn Basin’s Biohorizon B occurs at the unconformable boundary between the Indian Meadows Formation and the overlying Lysite Member of the Wind River Formation, and faunal change like that of the Bighorn Basin’s Biohorizon D occurs at the boundary between the Lysite Member and overlying Lost Cabin Member of the Wind River Formation. Lithologically, the Lysite Member exhibits sediments derived largely from Paleozoic and Cretaceous sediments, whereas the Lost Cabin Member contains sediments largely derived from Paleozoic sediments and Precambrian granite; thus, the Lysite and Lost Cabin Members of the Wind River Formation document the progressive “unroofing” of the northerly adjacent Owl Creek and Bridger Mountains. The petrology of Bighorn Basin Willwood sediments across biohorizon boundaries is unknown; however, field studies indicate that the Willwood biohorizon intervals are marked by episodes of widespread erosion (massive cut-and-fill) in response to autocyclic baselevel lowering, probably in response to episodic uplift of one or more of the adjacent structural highs, including the Washakie Range, Owl Creek Mountains, Bridger Mountains, southern Bighorn Mountains and, possibly, movement along one or more of the several major basement-controlled lineaments associated with uplift of the Bighorn Mountains. In addition to downcutting, Willwood paleosol morphologies document that Biohorizons A and B are coincident with the advent of periods of less equable, more temporally spaced rainfall, probably related to the structural closure of the Bighorn Basin and elevation of the basin margins.