Over the past three field seasons at the Post research area, the survey team has recorded the skeletal remains of several extinct Ice Age animals within the Spring Creek beds. The Spring Creek beds are lake sediments left from an extinct lake that had formed during the Pleistocene. During this time, under more moist weather conditions, several paleolake basins formed along the eastern escarpment edge of the Southern High Plains. The exact age of the Spring Creek beds and their relation to other regional extinct paleolakes has puzzled researchers for several decades.
The Blanco basin, located in Crosbyton County, Texas is the largest known paleolake of the region. This basin has been the center of early Pleistocene animal research for over a century, and also is the only one dated. A volcanic ash layer is located within lake deposits of the Blanco basin, and another upper ash layer occurs above the lake sediments within the later wind-blown sediments of the Blackwater Draw Formation.
Throughout the Pleistocene (~2.6 mya-11,000 ka), sporadic eruptions from volcanoes in the Jemez Mountains region of north-central New Mexico and Yellowstone National Park in Wyoming blanketed the Great Plains. Where these ash lenses were preserved from erosion, they provided a reliable chronological marker for determining the age of sedimentation.
In the early 1970s, researchers dated two samples of the Blackwater Draw Formation ash layer, above the Blanco lake bed, and the estimated ages of these samples were 1.4 and 1.77 million years ago. This ash layer was identified as the Guaje ash that has its source in the Jemez Mountains in New Mexico. The lower ash layer within the Blanco paleo-lake sediments was dated to 2.8 million years ago. If these ages are correct, then the Blanco paleolake sediments date between ~2.8 – 1.4 million years ago.
At the Post research area, the Landmark team has discovered a thick deposit of volcanic ash located on an upland ridge. The research team currently is tracing the lateral extinct of the ash layer and mapping the Spring Creek lake basin using a drone.
During the 2019 field season, Landmark crew member collected samples from ash beds for ³⁹Ar/⁴⁰Ar dating at the Oregon State Geochronology lab (Figure 1). This Argon dating method now is regarded as the most reliable for determining the age of volcanic ash layers. Results of this work will help to narrow down the age of the Spring Creek beds and their relationship to other Southern High Plains paleolakes.
The Landmark team also is collecting ash samples from the Blanco basin to double check the ages of the 1970s results. Geologists at the time used fission-track dating, and some researchers have questioned these age determinations. Results of redating the Blanco ash layers and ascertaining the age of the ash found in the Spring Creek beds will help to refine the known ages of extinct Southern High Plains paleolakes and the extensive extinct animal remains within them.