Professional Paper 1777
The stratigraphy, correlation, mapping, and depositional history of coal-bearing strata in the Paleogene Fort Union and Wasatch Formations in the Powder River Basin were mainly based on measurement and description of outcrops during the early 20th century. Subsequently, the quality and quantity of data improved with (1) exploration and development of oil, gas, and coal during the middle 20th century and (2) the onset of coalbed methane (CBM) development during the late 20th and early 21st centuries that resulted in the drilling of more than 26,000 closely spaced wells with accompanying geophysical logs. The closeness of the data control points, which average 0.5 mi (805 m) apart, made for better accuracy in the subsurface delineation and correlation of coal beds that greatly facilitated the construction of regional stratigraphic cross sections and the assessment of resources.
The drillhole data show that coal beds previously mapped as merged coal zones, such as the Wyodak coal zone in the Wyoming part of the Powder River Basin, gradually thinned into several discontinuous beds and sequentially split into as many as 7 hierarchical orders westward and northward. The thinning and splitting of coal beds in these directions were accompanied by as much as a ten-fold increase in the thicknesses of sandstone-dominated intervals within the Wyodak coal zone. This probably resulted from thrust loading by the eastern front of the Bighorn uplift accompanied by vertical displacement along lineaments that caused subsidence of the western axial part of the Powder River Basin during Laramide deformation in Late Cretaceous and early Tertiary time. Accommodation space was thereby created for synsedimentary alluvial infilling that controlled thickening, thinning, splitting, pinching out, and areal distribution of coal beds. Equally important was differential subsidence between this main accommodation space and adjoining areas, which influenced the overlapping, for example, of the Dietz coal zone in Montana, over the Wyodak coal zone in Wyoming. Correlation in a circular track of the Wyodak coal zone in the southern part of the basin also demonstrates overlapping with lower coal zones. Recognition of this stratigraphic relationship has led to revision of the correlations and nomenclature of coal beds because of inconsistency within these zones as well as those below and above them, which have long been subjects of controversy. Also, it significantly changes the traditional coal bed-to-bed correlations, and estimates of coal and coalbed methane resources of these coal zones due to thinning and pinching out of beds. More notably, thickness isopach, orientation, and distribution of the merged Wyodak coal bodies in the south-southeast part of the basin suggest that differential movement of lineament zones active during the Cretaceous was not a major influence on coal accumulation during the Paleocene.
Improved knowledge of alluvial depositional environments as influenced by external and internal paleotectonic conditions within the Powder River Basin permits more accurate correlation, mapping, and resource estimation of the Fort Union and Wasatch coal beds. The result is a better understanding of the sedimentology of the basin infill deposits in relation to peat bog accumulation.
First posted September 29, 2010
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Flores, R.M., Spear, B.D., Kinney, S.A., Purchase, P.A., and Gallagher, C.M., 2010, After a century—Revised Paleogene coal stratigraphy, correlation, and deposition, Powder River Basin, Wyoming and Montana: U.S. Geological Survey Professional Paper 1777, 97 p., CD-ROM in pocket.
Objectives of Study
Historical Coal Geology and Stratigraphy
Early Mapping and Coal-Bed Nomenclature Problems
State-to-State Mapping Problems
Coal-Bed Splitting and Merging Problems
Availability of Data and Data Control-Point Spacing
Depositional Environments and Facies of Coal-Bearing Strata
New Sources of CBM Data
Revised Basinwide Coal Stratigraphy and Nomenclature
Depositional Concept of Coal Splitting in the Powder River Basin
Regional Cross-Section Lines
Wyodak Coal of Mapel (1973)
Cross Section H–H’
Cross Section N–N’
Basinwide Lateral Variation of the Wyodak Coal
Basin South of Gillette, Wyoming
Revised Nomenclature of Wyodak Coal South of Gillette, Wyoming
Basin North of Gillette, Wyoming
Revised Nomenclature of Wyodak Coal North of Gillette, Wyoming
North-South Cross Sections
Effects of Stratigraphic Overlap of Coal Zones
Paleodeposition of the Wyodak Coal Zone and Related Rocks
Paleotectonically Controlled Alluvial Drainage
Interplay of Alluvial Accommodation Space, Tectonics, and Coal Zone Overlap
Regional Paleogeographic and Paleotectonic Patterns of Merged Coal Bodies