Calcic Paleosols – Regional Sequence Boundary Indicators in Cenozoic Strata of Southwestern Montana

Debra L. Hanneman¹ and Charles J. Wideman^2
1 Whitehall Geogroup, Inc, Whitehall, MT
² Montana Tech of the University of Montana, Butte, MT

Calcic paleosols delineate regional sequence boundaries in Cenozoic strata of southwestern Montana. The bounding surfaces are recognized by the existence of stacks of calcic paleosols that exhibit advanced stages of carbonate morphology. Because of the nature of soil formation, calcic paleosol stacks may not exist as continuous features over all paleolandscapes. Consequently, laterally equivalent sequence boundaries may also be identified by erosional features, angular stratal relationships, and abrupt changes in provenance or lithologies.

The paleosol stacks appear within the subsurface as several relatively thin, high velocity/high density zones. Zone thickness ranges from 1 to 1.5 m; zones are stacked to thicknesses of up to 15 m. Density varies within the zones by as much as 0.6 g/cm3, and differs by as much as 0.9 g/cm3 from material immediately above these zones. Velocity differs by as much as 10 ft/ms from the overlying material. The combined density and velocity differences between paleosols and sediments result in bright reflections on seismic sections.

The use of a sequence stratigraphy with calcic paleosols as boundary indicators is a valuable explorationist tool for the deeper Cenozoic basins of southwestern Montana. Prior interpretations for these basins were based upon the assumption that only two units were present in the Cenozoic basin-fill, and that pre-Cenozoic rocks were the cause of reflections beneath these units. The application of a sequence stratigraphy for these basins reconciles basin-fill thicknesses interpreted from regional gravity surveys and seismic data collected from the deeper basins.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005