Petrography, geochemistry, and clay mineralogy of a paleosol in the Dockum Group (Triassic), Texas Panhandle
A Triassic paleosol, informally known as the "Palo Duro geosol" in Texas and New Mexico, is the lowermost unit of the Triassic Dockum Group. This buried soil developed on sands of the Permian Quartermaster Formation in Texas. An excellent section of the paleosol is exposed at Caprock Canyons State Park, in Briscoe County, Texas. This paleosol marker bed is found around the Southern High Plains in Texas and New Mexico. A correlative unit, the "purple mottled" horizon, is also found in Triassic strata of Arizona and Colorado. The Palo Duro geosol is an Ultisol with two presumed major horizons identified here as A and B horizons. The A horizon consists of white sandstone with local accumulations of nodular and lamellar carbonate and chert layers at the top of the horizon. The B horizon consists of red, purple, and yellow mottled sandstone, siltstone, and mudstone. Petrographic analysis indicate that quartz, and to a lesser amount, sedimentary rock fragments are the major detrital grains in the entire profile. Iron oxide and chert cements predominate throughout the profile. Micromorphologic features observed in the profile include illuviation channels, voids, cutans, calcrete, and silcrete. Elemental enrichment and depletion trends in the Palo Duro geosol are correlated with those of modem soils with AI2O3 and Ti02 retained in the profile. Most of the elemental constituents are depleted from the profile relative to AI2O3 and Ti02. The clay minerals present in the profile are kaolinite, smectite, and illite. The Palo Duro geosol profile developed as a consequence of intense leaching under well drained environments in a climate with high precipitation rates. The occurrences of calcrete and silcrete samples suggests a climatic change from humid to dryer condition occurred later during soil formation. Isotopic data of several peodogenic calcrete and silcrete suggest that these micromorphologic features formed at low temperature («9 to 38°C) compatible with their origin as a product of soil formation.