UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY SUMMARY OF THE GEOLOGY, MINERAL RESOURCES, LANDSCAPE GEOCHEMISTRY, AND ENGINEERING GEOLOGIC CHARACTERISTICS OF THE NORTHERN POWDER RIVER COAL REGION, MONTANA PART A. Geology and mineral resources by W. J. Mapel and V. E. Swanson PART B.--Lands cape geochemistry by J. 0. Connor PART C.--Engineering geologic characteristics by F. W. Osterwald, compiler \ A. F. Chleborad, C. R. Dunrud, R. A. Farrow, T. C. Nichols, Jr., E. E. McGregor, C. H. Miller, J. Odum, H. W. Olsen, F. W. Osterwald, W. Z. Savage, and W. K. Smith OPEN-FILE REPORT 77-292 1977 This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards and nomenclature. CONTENTS Page Part A. Geology and mineral resources- . ._-- __... 1 Introduction- ----- - -- -_-__-.__. _- 1 Geographic features---- -- ------- . 1 Sources of information--- - - -.-- ..-._-.___ .... 4 Conversion of metric units- - ------------ ___.-___. -._ 4 Status of coal studies-- -- -. -- _- --. 4 Geologic setting--- ------------- ---- - -------- ---- 5 Rock units- __._--_ .. -_..__ _ 5 Hell Creek Formation--------- --- -- - ---- - -- 9 Fort Union Formation--- -------------- - --- ... 9 Wasatch Formation---- ---- -- -._ _. __-_.__.._.. 13 Terrace gravels -------- ---- -- - -- _._ 14 Valley-fill deposits 14 Structural features------ - -_.._ - ._._ 15 Folds ---- - _--._ __-_. _- -__ . .-_. 15 r au I L.S- _._._ .__-_. -__---..__.__.----._-__._... ID Coal deposits _- _ _ -- ___ - _. .__ 17 Coal in the lower part of the Fort Union Formation------------- 17 Coal in the Tongue River Member of the Fort Union Formation---- 21 Rosebud and associated coal beds- -- - -- -- 21 Knobloch coal bed- - -- _-__. .__...._ 22 Sawyer coal bed------ --- ------ -- 22 Brewster-Arnold and Cache coal beds -.--__ 22 Wall coal bed 23 Canyon coal bed------ ---- - -- 23 Dietz 2 and 1 coal beds------- __-_ - _._- 23 Anderson coal bed - 24 Roland coal bed 24 Coal resources--- -- --- ._---. _-_- _-_-. '24 Coal resources in thick, accessible deposits -- -- -- 24 Coal production-- - _.._.. __ .. .... 26 Coal quality and composition-- --. ------- ------ --- 26 Col strip area- -- --- -- - -- ---- - 30 Ashland area-- -- -- ---- - -- 35 Decker area---- _ . ___ 38 CONTENTS--continued Page Part A. Geology and mineral resources continued Composition of overburden- - -- 42 Oil and gas 46 Natural gas in coal 50 Bentonite - -- - -- _____________ 50 Construction materials-- -- - - ----- - 51 Clinker-- 51 Sand and gravel------ ---- --------- --- ___._-__- 53 Clay and shale--- ~ -------- -- _____________ 54 References cited, Part A------------ ---------------- _______ 54 Part B. Landscape geochemistry------------ - - - 1 Synopsis ------ -- -- -- - - _____ _ } Discussion- ------ _- -- __________ _ i References cited, Part B- -- -- - -- ---- 13 Part C. Engineering geologic characteristics--- ------ -___. l Seismic risks - _____ _ ______________ ._ .. 1 Geotechnical properties of the Fort Union Formation---------- - 2 A preliminary basis for prediction of natural slope stability----- 4 Foundation conditions-- -- -- 5 Open-pit coal mine slope stability----- ------ 6 ' Introduction- ------ - - __________ 6 General observations--- -- -- -- -- 6 Physical properties of Fort Union rocks --- ------ 7 Slope stability calculations-- -- . 14 Conclusions- -- --- ------ ____ - 17 Stability of underground coal mine workings 1.7 Surface subsidence effects from underground coal mining 18 Coal mine fires-- - - .----- - -- - 21 Seismic activity - 22 In-pi ace gasification - - _ - 22 Summary and conclusions------ - 23 Geotechnical studies-- ----- __ -- ___________ 24 References cited, Part C ---- -- - -- 26 CONTENTS--continued Illustrations Figures PART A Page Figure 1. Location of northern Powder River coal region -- 2 2. Area described - 3 3. Earlier coal reports, 1909-1969 6 4. Later coal reports, 1970-1977 7 5. Powder River Basin in relation to some nearby major structural features-- - .________-__--_- 8 6. Geologic map, northern Powder River coal region--- ------Folded 7. Structure contour map, Upper Cretaceous rocks---------- -16 8. Relations of coal beds from south to north- -------------18 9. Relations of coal beds from west to east- ----------------19 10. Locations of stratigraphic sections, figures 8 and 9- ---20 11. Map showing strippable coal deposits --- - -----------Folded 12. Locations of producing coal mines---- -- 27 13. Location of clay samples from the Tongue River Member of the Fort Union Formation------ ----- - ---------------------45 14. Oil and gas fields in and near the northern Powder River coal region-- ---- ---- -----47 15. Density of drilling for oil and gas------ -----------49 PART B Figure 1. Regression trends in Pannelid chlorochroa for concen­ trations of fluorine, selenium, strontium, and ash progressing east from the Dave Johnson power plant 5 2. Metal trends in sagebrush away from power plant 6 PART C Figure 1. Locations of drill holes from which cores were tested-- 3 2. Idealized cross section through an open pit in the Fort Union Formation ------- _ __ _ ________ -\ 3. Potential surfaces of failure for which safety factors were calculated---- - -- _________ 15 CONTENTS continued Page Tables PART A Table 1. Rock units at the surface and in the subsurface-------- 10 2. Estimated original coal resources- --___-_-___---__- 35 3. Reserves of coal in strippage deposits--- ------------ 28 4. Coal production from operating strip mines in 1975-------- 29 5. Average ash, sulfur, and heat value in coal deposits -- 31 6. Arithmetic mean, observed range, geometric mean, and geometric deviation of 36 elements in 490 Northern Great Plains province coal samples---- _ --- 32 7. Arithmetic mean, observed range, geometric mean, and geometric deviation of 15 major and minor oxides and trace elements in the ash of 490 Northern Great Plains province coal samples-------------------- ------------ 33 8. Composition of coal in the Rosebud and McKay beds, Colstrip area----- - - ---- - --------- __ 34 9. Average composition of ash and contents of seven additional trace elements in coal of the Rosebud and McKay beds, Colstrip area------------------------------------- 36 10. Composition of coal in the Knobloch and Flowers-Goodale beds, Ashland area- ---- --------- _-___ 37 11. Average composition of ash and contents of seven additional trace elements in coal of the Knobloch and Flowers-Goodale beds, Ashland area-------- - --- ---------- 39 12. Composition of coal in the Anderson, Dietz 1, and Dietz 2 beds, Decker area--- ------------ _ --- 49 13. Average composition of ash and contents of seven additional trace elements in coal of the Anderson, Dietz 1, and Dietz 2 beds, Decker area- --- ---- ______ --4] 14. Average chemical composition of sandstone, siltstone, and shale that comprise the overburden rock above the Knobloch coal bed near Ashland-- --- - 43 15. Chemical analyses of clay samples from the Tongue River Member of the Fort Union Formation - 44 CONTENTS continued Page Table 16, Oil and gas fields in and near the northern Powder River coal region--- ---- ------ 48 17. Test data on bentonite samples-- ____ _ _______ 52 PART B Table 1, Approximate maximum limits of chemical concentrations and derivative geochemical properties in some ordinary landscape materials in the Powder River Basin --- -- 7 Copper and molybdenum in sweetclover and pH in spoil materials from eight coal mines------------------------ 10 Some reported trace-element toxicities in plants and animals---------- --------------- __ ____________ 12 PART C Table 1, Results of triaxial tests- ----- --------- . ---_ 8 2. Results of Poisson's Ratio tests--------------- ------- 12 3. Results of Brazilian Cylinder tests--------- --------- 13 4. Safety factors calculated by the method of slices for the five potential failure surfaces shown in figure 3-- 16 Point load strengths of bedrock materials---------------- 25 SUMMARY OF THE GEOLOGY, MINERAL RESOURCES, AND ENGINEERING GEOLOGIC CHARACTERISTICS OF THE NORTHERN POWDER RIVER COAL REGION, MONTANA PART A. GEOLOGY AND MINERAL RESOURCES by W. J. Mapel and V. E. Swanson INTRODUCTION The Powder River coal region is a large area of coal-bearing rocks which together with the laterally continuous Fort Union coal region to the northeast occupies substantial parts of Wyoming, Montana, and North and South Dakota (Trumbull, 1960) (fig. 1). This report describes a part of this region, called here the northern Powder River coal region. As defined for the purposes of this report the northern Powder River coal region includes about 20,000 square km in southeastern Montana, extending roughly from the Montana-Wyoming boundary northward to the Yellowstone River near Forsyth and Miles City, and from an irregular north-trending line 3-15 km east of the Little Bighorn and Bighorn Rivers eastward to about the Little Powder and Powder Rivers (fig. 2). The area includes most of Powder River County, and parts of Custer, Big Horn, Rosebud, and Treasure Counties, Montana. The Northern Cheyenne and eastern part of the Crow Indian Reservations occupy about 4,600 square km in the western and central parts of the region. This report summarizes the geology and mineral resources in the northern Powder River coal region, with special emphasis on the coal resources. The information is intended for use in assessing environmental impacts of coal mining in the area. GEOGRAPHIC FEATURES Rosebud Creek, the Tongue River, and the Powder River, all northward or northeastward flowing