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Scientific Investigations Report 2008-5033

The Yampa Bed—A Regionally Extensive Tonstein in the Williams Fork Formation, Northwestern Piceance Creek and Southern Sand Wash Basins, Colorado

By Michael E. Brownfield and Edward A. Johnson

Contributions to Stratigraphy

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Scientific Investigations Report
2008-5033 PDF (10.3 MB)

A regionally persistent and distinctive unit of Upper Cretaceous age is here formally named the Yampa Bed of the Williams Fork Formation for exposures in the Yampa, Danforth Hills, and Grand Hogback coal fields, Moffat and Routt Counties, northwest Colorado; the name is derived from the Yampa River valley. The type section was measured in the NE¼ SW¼ sec. 6, T. 5 N., R. 91 W., about 8 miles south of Craig, Colo., where the bed is 38 inches thick and lies within the C-D coal bed in the lower part of the Williams Fork Formation, about 165 feet above the Trout Creek Sandstone Member of the Iles Formation. The Yampa Bed is dated at 72.2 ± .1 mega-annum using the K-Ar method. Regionally, the Yampa Bed is a 0.5- to 5-ft-thick, regionally persistent tonstein that can be readily identified in several different lithofacies in the lower part of the Williams Fork Formation. The unit is useful as a regional datum in the correlation of facies within the Williams Fork, and it is easily recognized on geophysical logs by its low resistivity response. Evidence suggests that it is a diagenetically altered airfall ash.

Version 1.0

Posted June 2008

  • Plate 1 PDF (1.2 MB)
  • Subsurface Correlations of Coal and Related Rocks of the Upper Cretaceous Iles and Williams Fork Formations in the Yampa Coal Field, Northeastern Colorado
  • Plate 2 PDF (1.5 MB)
  • Subsurface Correlations of Coal and Related Rocks of the Iles and Williams Fork Formations in the Danforth Hills Coal Field, Northwestern Colorado

Suggested citation:

Brownfield, M.E., and Johnson, E.A., 2008, The Yampa Bed—A regionally extensive tonstein in the Williams Fork Formation, northwestern Piceance Creek and southern Sand Wash Basins: U.S. Geological Survey Scientific Investigations Report 2008–5033, 32 p.




Geologic Setting

Cretaceous Paleogeography

Stratigraphy of Upper Cretaceous Rocks in Northwestern Colorado

Mesaverde Group in the Yampa Coal Field

Detailed Stratigraphy of Coal-Bearing Upper Cretaceous Rocks

Iles Formation

Tow Creek Sandstone Member

Double-Ledge Sandstone

Trout Creek Sandstone Member

Williams Fork Formation

Sub-Twentymile Sandstone

Twentymile Sandstone Member

Big White Sandstone

Three White Sandstones

Depositional Setting of the Mesaverde Group

Origin, Description, and Distribution of the Yampa Bed

Yampa Coal Field

Eckman Park Measured Section

Fish Creek Canyon Measured Section

Ute Gulch Measured Section

Eagle Mine Measured Section

Lay, Colo., Measured Section

Danforth Hills Coal Field

Piceance and Sand Wash Basins

Geophysical Log Response

Yampa Bed as a Correlation Tool



References Cited


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