Task 50 - deposition of lignites in the Fort Union Group and related strata of the northern Great Plains Page: 8 of 40
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the middle Clarkforkian. However, certain nonmarine molluscan taxa appear to be good indicators
of an early Wasatchian age, whether they occur in the San Juan Basin of New Mexico or on the
Wasatch Plateau of Utah. The present study shows that molluscan taxa appear capable of indicating
Clarkforkian biochrons in the absence of mammalian fossils in the Powder River Basin of
southeastern Montana and northeastern Wyoming. The approximate stratigraphic horizon of the
early and middle Clarkforkian boundary is near the level of the Anderson coal bed, while
Wasatchian-age strata occur just above the Arvada coal bed in the main body of the "Wasatch"
Formation. The boundary is documented by both the loss and introduction of different species of
mesogastropods and aquatic pulmonates, as well as a few species of unionid and pisidiid bivalves.
The Clarkforkian-Wasatchian boundary in the Willwood Formation of the Clarks Fork Basin can
be determined on the basis of these taxa and also by a well-differentiated and diverse terrestrial
ABBREVIATIONS AND NOTES
The following institutional abbreviations are used in this report: DOE (U.S. Department of
Energy), the EERC (Energy & Environmental Research Center), UND (University of North
Dakota, Department of Geology and Geological Engineering), and USGS (U.S. Geological
Survey). Miscellaneous abbreviations include the following: K/T (Cretaceous-Tertiary).
All observations were plotted on U.S. Geological Survey 7.5-minute series topographic
quadrangles. Geologists of the Montana Bureau of Mines and Geology are in the process of
producing a new 1:500,000-scale state geologic map. Preprint versions of maps at various scales
were graciously provided by Susan Vuke. The placement of some lithostratigraphic contacts and
member nomenclature were revised subsequent to field work undertaken for this project.
Fossil locality and geologic observation numbering systems are employed throughout this
report. All localities have an L-number for the purposes of computer-based data management. All
locality and geologic observation data are maintained in databases designed in Version 4 of Q&A*
by Symantec Corporation.
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Hartman, J.H.; Roth, B. & Kihm, A.J. Task 50 - deposition of lignites in the Fort Union Group and related strata of the northern Great Plains, report, August 11, 1997; Grand Forks, North Dakota. (https://digital.library.unt.edu/ark:/67531/metadc693470/m1/8/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.