Scientific Investigations Report 2008-5033
Abstract
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. |
Posted June 2008
Subsurface Correlations of Coal and Related Rocks of the Upper Cretaceous Iles and Williams Fork Formations in the Yampa Coal Field, Northeastern Colorado
Subsurface Correlations of Coal and Related Rocks of the Iles and Williams Fork Formations in the Danforth Hills Coal Field, Northwestern Colorado
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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.
Abstract
Introduction
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
Summary
Acknowledgments
References Cited
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