CHRONOSTRATIGRAPHIC ASSESSMENT OF ENIGMATIC REDBEDS AT POINT OF ROCKS, CIMARRON NATIONAL GRASSLAND, KANSAS: U-PB AGE CONSTRAINTS FROM DETRITAL ZIRCONS

A prominent bluff in extreme southwest Kansas, Point of Rocks, was one of the best-known landmarks for 19^th-century travelers on the Santa Fe Trail because it signaled proximity to natural springs in a long stretch with few sources of water. Point of Rocks is capped by distinct white beds of Neogene Ogallala Formation calcrete that overly beds of red, tan, and gray shale, siltstone, and sandstone. These unfossiliferous redbeds are currently assigned to the Jurassic System; however, their age has long been debated due to a lack of marker beds, index fossils, and nearby correlative outcrops. As a result, geologists over the years have assigned the rocks to systems ranging from the Permian to the Cretaceous. In this study, four stratigraphic sections were measured in the redbeds and three bulk samples were collected to determine the uranium-lead age distributions of detrital zircon (DZ) populations. The strata were interpreted as fluvio-estuarine deposits filling a paleovalley within older alluvial deposits. The youngest DZ age peaks were 274.75 Ma, 296.6 Ma, and 371.0 Ma, with a youngest single zircon age of 263.8 ± 12.1 Ma and a complete absence of Mesozoic age zircons. In addition, copper oxides along partings and fractures suggest that the redbeds once hosted Cu-sulfides, a common constituent of regional Permo-Triassic redbeds. The DZ data—in conjunction with the identification of the Permian Day Creek Dolomite marker bed in logs of nearby drilling tests—strongly suggests that the enigmatic redbeds cropping out at the base of Point of Rocks should be assigned to the Guadalupian Big Basin Formation, the uppermost Permian unit in Kansas.