STRATIGRAPHY

WHITEHALL GEOGROUP, INC.  specializes in stratigraphy of continental deposits.   We have primarily worked with the Cenozoic basin-fill of southwestern Montana in order to develop stratigraphic approaches to these types of deposits.   Sequence stratigraphy has enabled us to better understand the Cenozoic deposits of southwestern Montana and how they reflect the geologic history of this area. 

Key Publications:

• Sequence stratigraphy of Cenozoic continental rocks, southwestern Montana: Geol. Society of America Bulletin, v. 103, p. 1335-1345 (1991, Hanneman, D.L. and Wideman, C.J.).

    ABSTRACT

Because the presently used Bozeman Group lithostratigraphy is difficult to apply to Cenozoic basin-fill rocks in the Jefferson, Beaverhead, Melrose, and Divide (JBMD) valleys, southwestern Montana, we advocate the use of sequence stratigraphy. Surface sequence boundaries are often marked by calcic paleosol zones; these zones can be projected into the subsurface and tied to seismic reflection data. Density/velocity contrasts between calcic paleosol zones and overlying non-pedogenic rocks generate bright reflectors which define sequence boundaries. Sequence boundary delineation is further enhanced by reflection termination patterns, similar to those found in marine strata. Sequence age calibration is based upon vertebrate fossil and radiometric age data.

Five sequences, separated by basin-wide unconformities are recognized. Most sequence bounding unconformities are marked by paleosols, an occurrence not previously reported for Cenozoic strata of southwestern Montana. The sequences and their age ranges are (1) Bridgerian-Uintan (approximately 50 to 44 Ma) calc-alkaline volcanic flows and interstratified sedimentary rocks, (2) Duchesnian to Whitneyan (approximately 42 to 30 Ma) terrestrial sedimentary and volcanic rocks, (3) Arikareean (approximately 27 to 21 Ma) terrestrial sedimentary and volcanic rocks, (4) Barstovian to Blancan (approximately 16 to 4 Ma) terrestrial sedimentary and volcanic rocks, and (5) early Quaternary (approximately 1.8 Ma) to Holocene terrestrial sedimentary and volcanic rocks.

• Calcic paleosols: their use in subsurface stratigraphy: Amer. Association of Petroleum Geologists Bulletin, v. 78, p. 1360-1371 (Hanneman, D.L., Wideman, C.J., and Halvorson, J., 1994).

  ABSTRACT

  Analysis of well log data from Cenozoic basin-fill of the Deer Lodge Valley, southwestern Montana, provides evidence for identifying paleosols and paleosol stacks in the subsurface. The paleosol stacks are continental sequence boundary markers and appear as several relatively thin, high velocity/high density zones within basin-fill. Zone thickness ranges from 1 m to 1.5 m; zones are stacked up to as much as 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 and causes bright reflections on seismic sections. Synthetic seismograms are used to tie well log and seismic data.

  Based upon well log data and well cuttings analyses, the high velocity/high density zones are interpreted as limestone. The pedogenic origin of the limestone is shown by: (1) well cutting chips from the high velocity/high density zones that exhibit pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosol exposures.

• ABSTRACT -GSA Annual Meeting,Denver, Colorado, 1999.

  Age Reassessment of Some Units Within The Upper Cretaceous Beaverhead Group, Southwestern Montana. (HANNEMAN, Debra L., Whitehall Geogroup, Inc., 107 Whitetail Road, Whitehall, MT 59759, hanneman@ixi.net; and WIDEMAN, Charles J., Department of Geophysical Engineering, Montana Tech of the University of Montana, Butte, MT 59709).

  The Upper Cretaceous Beaverhead Group occurs in extreme southwestern Montana. Our detailed geologic mapping in the Melrose, Beaverhead, Sage Creek, and Red Rock valleys, southwestern Montana, indicates that in several areas of these valleys, what is currently shown on various public domain geologic maps as part of the Beaverhead Group can now be better age-constrained. Some units formerly mapped as part of the Beaverhead Group are not Upper Cretaceous, but instead range in age from Tertiary to Quaternary. In the Melrose Valley, on the northwest side of McCartney Mountain, previously mapped Beaverhead Group rocks are debris flows that are interbedded with Eocene volcanic flows. On the southeastern edge of the Beaverhead Valley, near Spring Canyon, Quaternary gravels derived from Tertiary Clarendonian-Hemphillian strata have been mapped as part of the Beaverhead Group. Debris flow deposits in the Sage Creek Valley, specifically located on the north side of lower Sage Creek, were mapped as Upper Cretaceous Beaverhead Group, but are interbedded with Tertiary Bridgerian and Uintan deposits. Formerly mapped Beaverhead strata in the Red Rock Valley, approximately 4 to 5 miles north of Dell, Montana, are Quaternary alluvial fan deposits. The recognition of various ages for deposits previously mapped as Upper Cretaceous Beaverhead Group should lead to reinterpretation of later Cretaceous to Cenozoic basin development in these areas of southwestern Montana.

   

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