Geology, paleontology, and correlation of Eocene volcaniclastic rocks, southeast Absaroka Range, Hot Springs County, Wyoming

The report area includes about 430 km 2 in the southeast Absaroka Range and southwest Bighorn Basin, Hot Springs County, Wyo. It is bounded on the south by the Wind River Indian Reservation (along the South Fork of Owl Creek), and on the north by Grass Creek. The eastern part of the area is dominated by rugged badland topography; whereas, the western part is characterized by mountainous terrain; local relief in the area is about 1,200 m. Exposed sedimentary rocks are more than 3,500 m thick and include rocks of Devonian through Holocene age. Paleozoic rocks are restricted to the northern margin of the Owl Creek Mountains where they are overlapped by Tertiary volcaniclastics. Mesozoic rocks dip gently northward off the Owl Creek Mountains and are also exposed at the eastern margin of the Absaroka volcanic field, where they are unconformably overlain by essentially flat lying lower Eocene strata. Tertiary rocks are present throughout most of the report area. Four formations are recognized: the Will wood Formation of early Eocene age, and the Aycross, Tepee Trail, and Wiggins Formations of middle Eocene age. Each of these units is separated from older rocks by erosional or angular unconformities in the mapped area, and all, except the Willwood, are dominantly volcaniclastic. Quaternary deposits include alluvium, colluvium, landslide detritus, and large and small detached masses of Eocene rocks. Mapping indicates that the names Aycross Formation and Tepee Trail Formation are more appropriate for middle Eocene rocks in the southeast Absaroka Range than are the names Pitchfork Formation and "late basic breccia," respectively, used by earlier workers. Aycross and Tepee Trail rocks were recognized in the report area on the basis of equivalent lithologies, age, stratigraphic position, and sedimentary and tectonic framework with the type rocks at A-cross Ranch and Tepee Trail. These formations are mappable units in the report area. The Willwood Formation is characterized by brightly variegated mudstones, medium to fine sandstones, and pebble and roundstone conglomerates. Volcanic material is rare or absent in Willwood rocks, which are correlated with rocks of the upper part of the formation in the central Bighorn Basin; they are not considered basin margin equivalents of the Tatman Formation, as suggested by other workers. The Aycross Formation overlies the Willwood by erosional unconformity and is marked by the first appearance of abundant volcanic detritus in the sediments. Vo^anic roundstone conglomerates, andesitic tuff, and volcanic sandstone, mudstone, and shale are dominant. Kerogenic and ostracodal shales, carbonaceous mudstone, and green clay shales occur frequently near the base of the formation, where they generally contain little volcanic material. This lacustrine facies is correlated with similar rocks described in quadrangles to the north by other workers and with lacustrine rocks on Lysite Mountain; all are considered younger than Tatman rocks of the type locality. The Aycross Formation is middle Eocene in age; it is a correlative of the lower part of the Wapiti Formation on and north of Carter Mountain and is a correlative of the early basic breccia in the Yellowstone National Park region. The Tepee Trail Formation is typified by volcanic sandstone and conglomerates, lava flows, tuffs, and lahars that unconformably overlie the Aycross Formation. These rocks occur at least as far north as the south flank of Carter Mountain, where they were included in the Pitchfork Formation by earlier workers. Tepee Trail rocks in the report area are probably all middle Eocene in age and are in part equivalent to the upper part of the early basic breccia unit of the Yellowstonenorthern Absaroka region. Wiggins strata comprise castellate exposures of volcanic conglomerate, light-colored tuff, and volcanic sandstone and mudstone that lie both topographically and stratigraphic ally well above rocks referred to the Wiggins Formation north of the report area by earlier workers. The Wiggins is an equivalent of the late basic breccia unit in the northern Absaroka Range and Yellowstone National Park and is nearly all middle Eocene in age. The mapped area is at the boundary of three major late Laramide features: the western Owl Creek Mountains, the southwest Bighorn Basin, and the southeast Absaroka Range. Six principal episodes contributed to structural deformation during the Tertiary and Quaternary(? ): (1) post-Fort Union (Paleocene) and pre-Willwood (lower Eocene) folding and erosion; (2) post-Willwood and pre-Aycross (middle Eocene) uplift and erosion; (3) post-Aycross and pre-Tepee Trail (middle Eocene) folding and erosion; (4) Post-Tepee Trail uplift that caused disharmonic folding in the Aycross Formation and local unconformities between Tepee Trail and Wiggins strata; (5) later Tertiary (Miocene? ) regional uplift that initiated degradation in the BighornAbsaroka region and local intrusions that folded Wiggins rocks; and (6) late Tertiary or Quaternary volcanic activity in the Yellowstone area that triggered large-scale detachment faulting of Eocene rocks along the southeast front of the Absaroka Range. Detached rocks were largely derived from the Tepee Trail and Wiggins Formations; intense deformation associated with this faulting suggests that the masses must have been very thick to establish confining pressure. Fossil mammals recovered from the Aycross Formation are of early middle Eocene age and are correlatives of faunas from the Bridger Formation of southwest Wyoming and probably also of those from part of the Aycross Formation in its type area and the Wapiti Formation in the valley of the Shoshone River. Fossil turtles, invertebrates, and