ASSESSING POST-MIOCENE TILTING OF THE SOUTHERN HIGH PLAINS THROUGH PALEOSLOPE RECONSTRUCTIONS OF THE OGALLALA FORMATION

The Southern High Plains (SHP) is a broad plateau found in the panhandle of Texas and Eastern New Mexico that is capped by the erosionally resistant Caprock Caliche. Strata underlying the Caprock Caliche are interpreted to be fluvial and eolian deposits representing denudation of the Southern Rocky Mountains during the Miocene, forming the Ogallala Formation. The dimensionless slope of the modern surface of the SHP and the base of the Ogallala Formation are both 0.002. Slopes of this magnitude are common on depositional gradients within 35 km of the respective highlands. Given the concave-upward profile of fluvial gradients, slopes should progressively shallow in distances beyond 35 km. The Ogallala Formation that underlies the SHP is more than 200 km from its respective provenance region in the Southern Rocky Mountains. Considering this distance down fluvial profile, it seems unlikely that the modern slope represents the depositional slope of the Ogallala Formation. Paleoslope reconstructions based on the force balance between bed shear stress and the downslope component of gravity require estimates of median grain size and paleoflow depth. Data collected at Caprock Canyons State Park, along the eastern escarpment of the SHP, produced spatially averaged water depth (2.88 m) and median grain size (6.2 mm) which correspond to a dimensionless slope of 0.0002—an order of a magnitude less than the modern slope. Field data was collected between 350 and 400 km down dip of the modern slope from the Pecos River Drainage. Assuming a linear slope, this distance requires ~ 600-700 m of uplift in the Pecos River area relative to the eastern escarpment of the SHP to account for the tilt between calculated and observed slopes. Thickness of the retreating Cenozoic through Permian strata in the Pecos River drainage is ~ 1035 m. A simple backstripping model suggests that the crust would uplift ~ 525 m if a similar thickness and lithotype of strata was removed directly over the Pecos River near Santa Rosa, New Mexico. These results are smaller than the linear projections, but approach the amount required and are probably within the error of the paleoslope calculations. Thus, it appears that the SHP has been tilted since deposition of the Ogallala Formation, most likely as a result of isostatic adjustment as the Pecos River incised into eastern New Mexico.