Assessing post-Miocene tilting of the Southern High Plains through paleoslope reconstructions of the Ogallala Formation



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The Southern High Plains (SHP) is a physiographic region found in the panhandle of Texas and eastern New Mexico. The SHP is capped by an erosionally resistant caliche soil profile, the Caprock Caliche. Stratigraphy underlying the Caprock Caliche is largely fluvial and eolian deposition representing denudation of the Southern Rocky Mountains and Rio Grande Rift flank during the Miocene forming what is known as the Ogallala Formation. The dimensionless slope of the modern surface of the SHP and the base of the Ogallala Formation are both 0.0018. This slope is on the order of magnitude of proglacial alluvial fans, which do not hold a slope of this magnitude for more than 35 km from their source. Given the concave upward nature of fluvial downstream profiles, slopes should progressively shallow in distances past this 35 km. Deposits of the Ogallala Formation underlying the Caprock caliche are more than 300 km from their source land in the Southern Rocky Mountains. Considering this respective distance down fluvial profile, it seems unlikely that the modern slope is representative of the depositional slope of the Ogallala Formation. Paleoslope reconstructions were employed to estimate the slopes at which Ogallala Formation sediments were deposited at two sites using spatially averaged median grain size and paleoflow depth. Dimensionless depositional slopes were found to be 0.00018 and 0.00015—an order of magnitude less than the modern slopes of the surfaces bounding the Ogallala Formation. This implies the SHP has been tilted post-Miocene. Possible causes of tilting include: 1) recent tectonic activity, 2) isostatic uplift do to crustal heating, and 3) isostatic uplift due to erosion. No recent faulting has been reported in the study area capable of producing the amount of uplift necessary to reconcile the difference in slopes. A 2-D isostatic model was built to test effects of lithospheric heating, possibly related to the Rio Grande rift, and isostatic rebound due to incision of the Pecos and Canadian rivers. Lowering lithospheric mantle density produced uplift on the order of nearly one kilometer. Removal of crust by the Pecos River induced 75 meters of uplift in the model, and incision of the Canadian River to the north of the SHP produced 82 meters of uplift. These values are likely overestimates as they do not account for lithospheric rigidity. Nearly 600 meters of differential uplift must have occurred near the western escarpment of the SHP to reconcile the difference in depositional and modern slopes for the Ogallala Formation. If crustal rigidity and concurrent uplift along the eastern margin of the SHP are factored in, the uplift modeled along the western margin the SHP from mantle heating and river incision accounts for the expected differential uplift along the western margin.



Southern High Plains, Ogallala, Ogallala Formation, Miocene, Geology, Fluvial, Paleoslope, Slope