Two superposed faunas were discovered in southeastern Colorado at the Donnelly Ranch vertebrate site. Sediments containing a late Blancan assemblage are unconformably overlain by sediments containing a Rancho La Brean assemblage. The pre-Nebraskan Donnelly Ranch fauna is slightly older than the Sand Draw, Broadwater, and Blanco faunas. It is younger than the Curtis Ranch fauna. Faunal and paleomagnetic correlation indicate it is about 2.5 m.y. old. Tapirus, Geoche/one, and Sigmodon indicate the climate was mild and precipitation was much more effective than at present. Savanna, plains, and marsh communities are represented by the fauna.
The Sangamon Mesa De Maya fauna correlates best with the early Sangamon Cragin Quarry fauna. Mammals, molluscs, and pollen indicate that temperatures were more equable than present and precipitation may have been slightly more effective during cooler summers. The fauna consists primarily of grassland inhabitants but a marsh comÂ— munity is represented in artesian spring sediments.
The gross topographic and stratigraphic relations, sediment size analysis, fossil orientation, and sedimentary structures indicate stream, viscuous intermediate, and mudflow deposition on a low alluvial fan at the foot of Mesa De Maya. The sediment source was local consisting of basalt from Mesa De Maya, igneous, reworked metamorphic, and sedimentary rock from the Ogallala Formation, and sedimentary rock containing marine fossils from Cretaceous sediments. An artesian spring deposit consisting of predominantly silt size quartz particles was found overlying a gley soil. During the Sangamon, massive caliche formed in channel sands and nodular caliche formed in the mudflow deposits of the alluvial fan.
Potassium argon dates, paleomagnetism, and topographic position and correlation indicate that Mesa De Maya is older than Fowlis Mesa and the average age for both of these basalt flows is about 3.3 million years. The Mesa De Maya flows issued from local vents and flowed nearly to the present extent of the mesa. Scarp recession has been minor since the lavas erupted. Flow direction interpreted from air photographs indicate the flows were largely conÂ— trolled by topography and did not form an extensive sheet of lava from Mesa De Maya to Fowlis Mesa.
Erosion has carved deep canyons between the lobes of the flows and has caused a reversal of topography where hills of Ogallala once stood. More than 400 feet of Ogallala and Cretaceous sediments were removed prior to deposiÂ— tion of Pleistocene sediments. Erosion and deposition in the Donnelly Ranch area has been relatively minor since the Sangamon.