The mammalian paleontology and depositional environments of the Birch Creek local fauna (Pliocene: Blancan), Owyhee County, Idaho

The Birch Creek local fauna of Owyhee County, southwestern Idaho, contains a rich assemblage of late Blancan mammals including Stegomastodon, Gigantocamelus, Titanotylopus, Breztia, Equus stenonis anguinus, Procastoroides, Homotheylum, Canis, Trigonictis, Pliopotamys, Mictomys Ophiomys; and Sorex. The fauna contains a number of associated remains including nearly complete specimens of Equus stenonis anguinus and Homotherium crenandens. Biostratigraphic and lithostratigraphic correlation with fisson-track dated strata indicates the fauna is approximately 2.4 Ma and thus is slightly older than the Grandview local fauna.; Sediments exposed between the Birch Creek and Shoofly Creek drainages are interpreted as a lacustrine margin or paludal-fringe environment. The association of a diverse mammalian fauna with the paludal fringe of Glenns Ferry lake has not been documented before for a late Blancan fauna from Idaho. The vertebrate remains of the Birch Creek local fauna were entombed in sediments deposited in the paludal-fringe and shallow littoral zone of Glenns Ferry Lake, and thus are coeval with lacustrine deposits deposited closer to the basin center.; These sediments crop out in continuous exposures along the Birch Creek drainage for nearly 12 km. Sediments of the paludal fringe migrated basinward following the regressive shoreline of Glenns Ferry Lake. Beds yielding mammalian fossils overlie nearshore sands near the basin and grade into offshore muds toward the basin center. The fine-grained nature of the sediments, inferred depositional slope of the beds, lack of well-developed channel fills, and alteration of diatomite beds and paleosols support the interpretation that the sediments accumulated along the margin of the lake. Paludal-fringe deposits are overlain by distal, alluvial-fin sediments shed from the Owyhee Mountains to the west and southwest.; Paleoecological reconstruction suggests three biofacies exist within the fauna and these correspond to three biotopes. The first consists of Large, inshore fish associated with open Like sediments. The second biofacies consists of a mixed assemblage of small, marsh-dwelling fish and semiaquatic vertebrates or those preferring the mesic habitats of the paludal fringe. The third biofacies consists of large mammals that inhabited a steppe environment adjacent to the lake.