CALCIC PEDOCOMPLEXES DELINEATE SEQUENCE BOUNDARIES IN TERTIARY STRATA OF THE GREAT PLAINS AND WESTERN USA

Calcic pedocomplexes are associated with regional unconformities that have approximate ages of 30 Ma, 20 Ma, and 4 Ma in the Great Plains and western USA. Where initially identified in southwestern Montana, the calcic pedocomplexes typically contain several partial soil profiles. In the most complete scenario, an individual profile may contain an argillic or argillic/calcareous (Bt or Btk) horizon, a K horizon, and a C horizon. The K horizon contains an upper laminated zone that is underlain by an indurated carbonate sheet. The calcic paleosols display carbonate morphology ranging from Stage IV to Stage VI.

The extensive occurrence of the calcic pedocomplexes/regional unconformity associations throughout this large area underscores their utility as a regional correlation tool in that the calcic pedocomplexes delineate sequence boundaries that occur between large-scale sedimentary packages. In addition to the pedocomplexes identified in southwestern Montana, pedocomplexes mark the 30 Ma regional unconformity in Badlands National Park, southwestern South Dakota, Banner County, western Nebraska, and Medicine Lodge Creek Valley, south-central Idaho; pedocomplexes identified at the 20 Ma regional unconformity occur in the South Killdeer Mountains, southwestern North Dakota, and Monroe Canyon, Nebraska; pedocomplexes recognized at the 4 Ma regional unconformity are found in Monroe Canyon, Nebraska, the Hagerman Fossil Beds National Monument, southwest Idaho, the Southern High Plains, Texas and New Mexico, Roswell-Carlsbad, southeastern New Mexico, Morman Mesa, southeastern Nevada, Vidal Junction, southern California, and the Pasco Basin in south-central Washington.