Geology of the Fort Peck Area, Garfield, McCone and Valley Counties Montana GEOLOGICAL SURVEY PROFESSIONAL PAPER 414-F Prepared as a part of a program of the Department of the Interior for development of the Missouri River basin Geology of the Fort Peck Area, Garfield, McCone and Valley Counties Montana By FRED S. JENSEN and HELEN D. VARNES SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 414-F Prepared as a part of a program of the Department of the Interior for development of the Missouri River basin UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Page Abstract_ _________-__-_.--___________--. Fl Post-Cretaceous stratigraphy Continued Introduction ______________________________ 1 Glacial deposits_________________________________ F31 General location and purpose of the work. 1 General discussion. _________________________ 31 Geography and elimate_________________ 2 Ground moraine. __ _________________________ 31 Fieldwork and acknowledgments_______ 2 Kintyre formation __________________________ 33 Outwash channel deposits____________________ 34 Cretaceous stratigraphy ____________________ 3 36 Montana group______________________ 3 Outwash terrace deposits -____________._-__._ 37 Post-Montana group formations_________ 4 Postglacial Quaternary and Recent deposits________ Alluvium __________________________________ 37 Subsurface bedrock formations __________ 5 Intermittent pond deposits_____ ______________ 39 Exposed bedrock formations____________ 5 Alluvial-colluvial deposits____ ________________ 40 Judith River formation _____________ 5 41 Bearpaw shale.________-__--_.-_-_- 5 Geologic history of the Tertiary and Quaternary deposits. Preglacial history. ______________________________ 41 Fox Hills sandstone.-___-_-_____-__ 11 Glacial history_________________________________ 41 Strata overlying the Montana group.____ 16 Regional background outlined by earlier studies. 42 Hell Creek formation_____________ 17 Glacial history of the Fort Peck area__________ 42 Structure _________________________________ 23 Drainage history________________________________ 43 Regional structural setting._____________ 23 Late-glacial and postglacial history of the Fort Peck Structural features in the Fort Peck area_ 23 44 Deformation at Tiger Butte_________ 24 Summary of natural resources. _________________ 46 46 Post-Cretaceous stratigraphy._______________ 26 Highway subgrade materials _______________ Sand and gravel __________________________ 46 General statement _____________________ 26 Riprap __________________________________ 46 Formations antedating glaciation________ 26 Ceramic clay_____________________________ 46 Tertiary deposits______________________ 27 Material for lining water-retaining structures_ 46 Flaxville formation_______________ 27 Lightweight aggregate_____________________ 46 Quaternary deposits______-__-_-_-____ 28 Selected references. ___________________________ 46 Wiota gravels. 28 Index _______________________________________ 49 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic map and sections of the Fort Peck area. 2. Bedrock geologic map and columnar section. 3. Geologic map of Tiger Butte. Page FIGURE 1. Index map of Fort Peck area, Montana.______________________________-_-_____-_-_-_-----_----------- Fl 2. Stratigraphic diagram showing formations of the Montana group. ____________________----.----.-----------. 4 3. Badland topography in the Bearpaw shale on the lower slopes of Milk River Hill_______________--------_-_ 5 4. Slacking and disintegrating of the Bearpaw shale exposed along the shoreline of the Fort Peck Reservoir. 6 5. Mechanical analyses of samples of weathered Bearpaw shale, Fox Hills sandstone, and the ground moraine. _ _ 8 6. Atterberg limits and soils classification of test samples_______________________-_-_-_-_-_-_-_----------- 12 7. Mass wasting of the Fox Hills sandstone causing mantling of the Bearpaw shale___________--_----___------ 13 8. Rimrock of the upper part of the Fox Hills sandstone_______________________-__--__--_----__----------- 13 9. Map showing thickness of the Fox Hills sandstone, including the Colgate member where present________-__-_ 14 10. Contact of the Colgate member with the overlying Hell Creek formation________________-______--___----- 17 11. Resistant lenses of sandstone in the Hell Creek formation. ______________________-_--_-_-_-_--_---------- 18 12. The Hell Creek formation capped by sandstone ledges.__________________________-___-_-______-_---_----. 19 13. Crossbedding and ovoid sandstone concretions in the Hell Creek formation__________________-___---------_. 20 14. Conglomeratic lenses of sandstone in the Hell Creek formation___________________-_-_-_---_-.-----------. 21 15. Soil profile of ground moraine._-__-______-_________-__________________-______-----------__----------. 33 16. Bedding of the Kintyre formation. ___________________________________________________________________ 35 17. Sketch of the Kintyre formation___________________________________________________---_-__----------. 35 18. Mechanical analyses of alluvium and outwash of terrace gravel__________________--_-_-_-_-____----------. 36 19. Sketch map showing drainage history of the vicinity of Fort Peck area__________________________---------. 43 ill SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGY OF THE FORT PECK AREA, GARFIELD, McCONE, AND VALLEY COUNTIES, MONTANA By FRED S. JENS EN and HELEN D. VAENES ABSTRACT Rivers and along their major tributaries, where alluviation The Fort Peck area, in the central part of northeast Mon­ has been dominant. tana, includes a little more than three 15-minute quadrangles and covers an area of about 600 square miles. The area is in INTRODUCTION the northern part of the Great Plains physiographic province GENERAL LOCATION AND PURPOSE OF THE WORK and is predominantly an undulating grassy treeless prairie upland which rises gradually to the north. The Milk and The Fort Peck area (fig. 1) includes about 600 square Missouri Rivers have carved their channels into this prairie miles in the central part of northeastern Montana in upland, so that steep scarps as much as 200 feet high border the wide bottom lands. South and southwest of the rivers, the surface rises abruptly and irregularly through a belt of "breaks" to a rolling dissected prairie with badland areas along many of the drainage courses. The exposed bedrock is Late Cretaceous in age; the oldest formation is the Judith River formation, which is exposed only in the complexly faulted Tiger Butte area. The overlying Bearpaw shale is the surface bedrock except for about 12 square miles in the southern part and in the extreme north­ east corner where the Bearpaw shale is overlain by the Fox Hills sandstone. The Hell Creek formation, generally con­ sidered to be latest Cretaceous in age, is the youngest bedrock in the Fort Peck area; it disconformably overlies the Fox Hills in an area of about 5 square miles. 1. uiasgow quadrangle The unconsolidated and semiconsolidated materials over­ 2. Nashua quadrangle lying the Cretaceous bedrock are Miocene or Pliocene or 3. Frazer quadrangle younger and represent later chapters in the long history of FIGURE 1. Index map of the Fort Peck area, Montana. intermittent regional uplift that was characteristic of the Great Plains province since early in the Tertiary. The oldest are preglacial sediments deposited on stream-cut plains and the northern part, of the Great Plains physiographic terraces. Of these, the Flaxville formation (Miocene or Plio­ province. It consists of the Glasgow, Nashua, and cene) is limited to a few small plateau remnants in the ex­ Frazer 15-minute quadrangles and a small additional treme northeast corner of the area. The younger Wiota gravels area of about 26 square miles southwest and southeast (Pleistocene) are extensively deposited, along the rivers and their major tributaries. of the Fort Peck Dam. The surflcial material over more than half the Fort Peck The development of the natural resources of the Mis­ area is ground moraine of Wisconsin (Pleistocene) age. At souri River basin has been aided since the middle one time the ground moraine probably covered the entire area, lOSO's by several government agencies, which are en­ but postglacial erosion has stripped it from major valleys and gaged in a long-term multiple-phase program. This from the hilly belt of shale and sandstone in the southern and southwestern parts of the area. program relates to mineral resources, efficient land use, The Kintyre formation, composed of fluviolacustrine silt and and the construction of dams and canals for flood con­ fine sand and clay, overlies the ground moraine in some parts trol, power, irrigation, and navigation. of the Milk and Missouri River valleys. The Kintyre forma­ Begun in 1933 and officially completed in 1941, the tion was deposited on stagnant ice of the wasting Wisconsin Fort Peck Dam, in the south-central part of the report glacier. Glacial outwash is limited to widely separated de­ area, stands 250 feet above the Missouri River. The posits in the rolling prairie upland and to terrace deposits. Since the disappearance of the glacier ice, erosion has been reservoir extends nearly 180 miles upstream and has. a dominant except in the trenches of the Milk and Missouri capacity of 20 million