^PSCalcic Paleosols – Regional Sequence Boundary Indicators in Cenozoic Strata of Southwestern Montana*

By

Debra L. Hanneman¹ and Charles J. Wideman²

Search and Discovery Article #50019 (2005)

Posted September 18, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005)

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¹Whitehall GeoGroup, Inc., 107 Whitetail Road, Whitehall, MT 59759 ([email protected])

²Professor Emeritus, Montana Tech of the University of Montana, Butte, MT  59701

Abstract 

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. 

       Location maps. Southwestern Montana – the initial study area - with selected valleys identified on a digital relief image of southwestern Montana (Montana State Library, NRIS data bank, 2001). The valleys of southwestern Montana contain Cenozoic rocks and sediment.  The strata are largely of fluvial origin, although volcanic and eolian deposits also are a part of the Cenozoic rock record.  The valleys are typically segmented into individual basins, with some basin-fill attaining thicknesses of approximately 5000 meters.   

Summary

• Calcic paleosol stacks with a maximum carbonate morphology of Stage VI are associated with regional unconformities that have approximate ages of 30 Ma, 20 Ma, and 4 Ma.

• The calcic paleosols are easily identified in surface sections and have distinct physical properties that can be recognized in various types of geophysical data in the subsurface.

• The recognition of the calcic paleosol/unconformity association enables the separation of Cenozoic basin-fill into large-scale Unconformity Bounded Sequences, which can greatly enhance both surface and subsurface basin research.

• Although the 30 Ma, 20 Ma, and 4 Ma calcic paleosol stacks/regional unconformity associations were initially described in southwestern Montana, they have been traced throughout the Great Plains and western USA.

• The widespread extent of the calcic paleosols/regional unconformities associations enhances their utility as a regional correlation tool.

• Because the paleosol stacks delineate regional unconformities that are also large-scale sequence boundaries, the identification of the paleosol stack/unconformity association has broad implications for continental sequence stratigraphy.