Stratigraphy, vertebrate paleontology and depositional history of the Ogallala Group in Blanco and Yellowhouse canyons, northwestern Texas

The late Miocene Ogallala Group is up to 90 meters thick in exposures along the eastern escarpment of the High Plains in Crosby, Garza and Lubbock Counties, Texas. Two formations are mapped in this section. The lower formation can be divided into two members and is correlated with the early Clarendonian Land Mammal "Age". The upper formation contains early and late Hemphillian mammals. The geomorphic history of the area is reconstructed using depositional and biostratigraphic analysis of lithostratigraphic units. High-energy ephemeral streams first deposited sediment in valleys incised into Triassic and Cretaceous bedrock in early Clarendonian time. Later in Clarendonian time these fluvial sediments and areas of bedrock were covered by a blanket of eolian silty sand. In the late Clarendonian or early Hemphillian rivers began to downcut new valley systems. High-energy fluvial systems filled these incised valleys in the early Hemphillian. Ephemeral fluvial and eolian silty sand sheet deposits spread over most older bedrock highs on the High Plains in about the middle Hemphillian. Deposition ceased across the High Plains when fluvial input from the southern Rocky Mountains was cut off at the end of the Hemphillian. The caprock caliche formed at this time. Tectonic events in the Rocky Mountains appear to have controlled these geomorphic cycles in the study area and also to have influenced sedimentation in other areas. A significant break in sedimentation, seen in many areas of the central and western U.S., coincides with the Clarendonian/Hemphillian boundary. The faunal turnover at the Clarendonian/Hemphillian boundary in the Great Plains and the Rio Grande rift is probably accentuated by this hiatus. Vertebrate assemblages throughout the section are dominated by animals adapted to subhumid or semi-arid environments. Most of the ungulates are grazers. Ephemeral stream deposits, eolian silty sand sheets and caliche-rich paleosols also indicate subhumid or semi-arid conditions throughout the late Miocene. Faunal turnover and declines in diversity in the late Miocene are related to increasing aridity and possibly to a decrease in complexity of the savanna flora.