Calcic pedocomplexes—Regional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States

Abstract

Calcic pedocomplexes are associated with regional unconformities in the Great Plains and western United States that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. In southwestern Montana, the calcic pedocomplexes are readily identifiable on the surface, and a pedocomplex typically contains several partial soil profiles. In the most complete scenario, an individual profile may contain an argillic or argil - lic/calcareous (Bt or Btk) horizon, a K horizon, and a C horizon. Often, however, the Bt(k) horizon is truncated or can be entirely absent from an individual profile. The K horizon contains an upper laminated zone that is underlain by an indurated carbon- ate sheet. Carbonate nodules and chalky micritic matrix materials underlie the sheet carbonate. The calcic paleosols display carbonate morphology ranging from stage IV to stage VI. The calcic pedocomplexes also possess distinct physical properties that aid in subsurface identification. The combined density and velocity differences between paleosols and nonpedogenic strata result in bright reflections on seismic sections and distinct well-log signatures. Although the calcic pedocomplexes and regional unconformity associations were first described within Tertiary strata of southwestern Montana, the same associa - tions exist in numerous localities in the Great Plains and in other parts of the western United States. The extensive occurrence of the calcic paleosols and regional uncon- formity associations throughout this large area underscores their utility as a regional correlation tool. Moreover, the delineation of regional unconformities that are large- scale sequence boundaries by pedocomplexes has broad implications for continental sequence stratigraphy.