Sequence Stratigraphy and Paleosols in Continental Rocks - Examples from Cenozoic Deposits of the Great Plains and Western USA

Hanneman, Debra L. ¹, Charles J. Wideman² (1) Whitehall Geogroup, Inc, Whitehall, MT (2) Professor Emeritus, Montana Tech of the University of Montana, Butte, MT

Cenozoic continental deposits of the Great Plains and western USA can be separated into large-scale sequences that are variously marked by extensive paleosol formation, irregular topography developed on erosional surfaces, and/or angular discordance of strata. Abrupt changes in provenance or lithologies also may indicate a sequence boundary. Specifically, the interregional unconformities that bound these sequences within the Great Plains and western USA occur at about 55 Ma, 37 Ma, 30 Ma, 20 Ma, and 4 Ma. Hiatuses between sequences are variable in duration and can extend for several million years.

Paleosols are one of the more obvious sequence boundary markers. Because paleosols in many stages of formation may exist within a sequence, it is the advanced stages and pedocomplexes of paleosols that we use for identifying sequence-bounding surfaces. Calcic pedocomplexes are particularly useful in areas of arid to semiarid climatic regimes, and in fact delineate four of the five large-scale sequences identified throughout the Great Plains and much of the western USA. The calcic pedocomplexes are readily identifiable on the surface with a pedocomplex typically being several tens of feet thick and containing 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 calcic pedocomplexes also possess distinct physical properties that aid in subsurface identification. The combined density and velocity differences between paleosols and non-pedogenic strata result in bright reflections on seismic sections and distinct well log signatures.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana