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Older rocks underlying the Ogallala--red, tan, and gray beds of shale, siltstone, and sandstone--crop out for approximately 500 m along the base of the bluff (figs. These red beds are currently assigned to the Jurassic System (Zeller, 1968)1.
However, the rocks at this isolated exposure have always been problematic and, historically, have been classified as Triassic, Cretaceous, and Permian. J., 2013, Tracing clastic delivery to the Permian Delaware Basin, USA: Implications for paleogeography and circulation in westernmost equatorial Pangea: Journal of Sedimentary Research, v.
In a publication about the Permian red beds of Kansas, Norton (1939) suggested that "a small exposure in Morton County" (presumably referring to Point of Rocks) may be Big Basin Formation but offered no further evidence or explanation as to why he thought it might be Permian in age.
Point of Rocks, a high-relief bluff overlooking the Cimarron River valley in Morton County, Kansas, is capped by distinct white beds of Neogene Ogallala Formation calcrete that overlie red beds of shale, siltstone, and sandstone. R., 1967, Formation of red beds in modern and ancient deserts: Bulletin of the Geological Society of America, v. Figure 1--Map showing the historic Santa Fe Trail and the location of Point of Rocks in the Cimarron National Grassland, Morton County, southwestern Kansas. Figure 2--Aerial photo showing the location of the red beds at the base of the bluff, the overlying Ogallala Formation, and the parking area (road loop) atop the Point of Rocks landmark (photo by William C. Figure 3--Aerial view of the study area (modified from Google Earth, accessed March 3, 2015) showing location of measured sections and drillers' logs used to construct cross section A-A' (fig. Geologic contacts (yellow lines) between the Neogene Ogallala Formation (No), the Permian Big Basin Formation (Pbb), and Quaternary alluvium (Qal) are based on the results of this study and photo interpretation. Detrital zircon age peaks can be grouped into at least seven subpopulations with a youngest single zircon age of 263.8 ± 12.1 Ma, a more conservative age of 293.0 ± 6.95 Ma based on the youngest grouping of three grain ages overlapping at 2σ, and a complete absence of Mesozoic age zircons. Swineford, A., 1955, Petrography of Upper Permian rocks in south-central Kansas: Kansas Geological Survey, Bulletin 111, 179 p. In addition, copper oxides along partings and fractures suggest that the red beds once hosted copper sulfides, a common constituent of regional Permian-Triassic red beds. S., 2013, Permian dust in Oklahoma: Source and origin for Middle Permian (Flowerpot-Blaine) redbeds in western tropical Pangaea: Sedimentary Geology, v.