) but identified the overlying sandstone as the Lower Cretaceous Cheyenne Sandstone. ) Dockum group, an unnamed formation, and Triassic Dockum (?
Mc Laughlin (1942) classed the exposures at "Point Rock" (Point of Rocks) as Triassic (? ) Group, respectively, based on correlations with rocks of similar lithologies in Oklahoma, Texas, and New Mexico. Since Zeller was published in 1968, the age of the exposure has continued to be controversial, with various authors speculating about whether the rocks were Triassic (Kansas Geological Survey, 1991; Mc Ninch, 1996; Liggett and Zakrzewski, 1997; Liggett et al., 1998; Croxton et al., 1998) or Jurassic (Kume and Spinazola, 1985; Johnson et al., 2009). K., 1937, Geologic map of Kansas: Kansas Geological Survey, scale 0,000.
1), because it signaled proximity to natural springs in a long stretch with few sources of water.
Inset shows the locations of the measured sections (MS-1 to MS-4) and test hole 8 (Mc Laughlin, 1942; test hole 8, p. Uranium-lead age distributions of populations of detrital zircons (DZ) within a sedimentary unit yield trends that represent the relative contributions of weathered igneous material from particular events or provinces (e.g., Dickinson and Gehrels, 2009, 2010; Dickinson et al., 2010). Comparisons of DZ age distributions between strata can reveal changes in source areas and drainage networks over time. 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. Smith (1940) seemed to have deferred to Moore and Landes (1937) that the red beds are Triassic (? G., 1963, Changes in Kansas stratigraphic nomenclature: American Association of Petroleum Geologists, Bulletin, v. The youngest uranium-lead ages of grains in DZ populations are commonly used to constrain maximum depositional ages (Dickinson and Gehrels, 2010).
A growing database of DZ ages from various lithostratigraphic units offers opportunities to compare data from previously unknown samples when searching for similar provenance (Gehrels, 2012). P., 1995, Pangea and the paleogeography of the Permian; in, The Permian of Northern Pangea, vol. Lithologic units were characterized in the field by color, grain size, composition, texture, and primary and secondary sedimentary structures. Color was described from fresh hand samples using Munsell color charts (Munsell Color, 2000). G., 1951, The Kansas rock column: Kansas Geological Survey, Bulletin 89, 132 p. 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. This indirect approach has not provided a definitive solution.