Geologic Map of the Canyon Ferry Dam 30´× 60´ Quadrangle, West-Central Montana By Mitchell W. Reynolds and Theodore R. Brandt Adel Mountains Willow Mountain Sacajawea Mountain Mm Mann Mm Ml Gulch Mb Gates of the Coulter Canyon Rocky Mountains Ml Mm Mm *Mab Qgl Qa Missouri River Mm Scientific Investigations Map 2860 Canyon of the Missouri River above the Gates of the Rocky Mountains is incised through beds of the Upper and Lower Mississippian Mission Canyon Formation (Mm). On July 19, 1805, the Lewis and Clark expedition named the Gates of the 2005 Rocky Mountains, as the cliffs first appeared as a closed gate, then appeared to open as expedition boats changed course approaching the canyon mouth. Mann Gulch is the site of the infamous forest fire that took the lives of 13 U.S. Forest Service fire fighters in August 1949. View is north-northeast toward the Adel Mountains U.S. Department of the Interior from Upper Holter Lake in the northwest corner of the Canyon Ferry Dam quadrangle. Unit labels as on map explanation. U.S. Geological Survey Geologic Map of the Canyon Ferry Dam 30’ x 60’ Quadrangle, West-Central Montana By Mitchell W. Reynolds and Theodore R. Brandt Pamphlet to accompany Scientific Investigations Map 2860 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ This publication is available online at: http://pubs.usgs.gov/sim/2005/2860 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Reynolds, M.W., and Brandt, T.R., 2005, Geologic map of the Canyon Ferry Dam 30’ x 60’ quadrangle, west-central Montana: U.S. Geological Survey Scientific Investigations Map 2860, 32-p. pamphlet, 3 plates, scale 1:100,000. ISBN 0-607-95575-9 iii Contents Discussion ..................................................................................................................................................... 1 Introduction ......................................................................................................................................... 1 Geologic Units ..................................................................................................................................... 1 Geologic Structure ............................................................................................................................. 7 Acknowledgments ..................................................................................................................................... 12 Description of Map Units .......................................................................................................................... 13 References Cited ....................................................................................................................................... 30 iv Conversion Factors ________________________________________________________________ To convert Multiply by To obtain ________________________________________________________________ meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) 2 2 ________________________________________________________________square kilometer (km ) 0.3861 square mile (mi ) Discussion unpaved) and unpaved county roads serve the eastern part of the quadrangle. The geologic map was made as part of the Montana Introduction Investigations Project to provide new information on the stratigraphy, structure, and geologic history of the geologically The Canyon Ferry Dam quadrangle, scale 1:100,000, in complex area. Because of the geologic complexity, number west-central Montana, lies at the junction of four major tec­ of geologic units mapped, and detail mainly of new informa­ tonic elements: the Lewis and Clark tectonic zone, the Mon­ tion gathered, the geology is presented in three formats. Sheet tana disturbed belt, the Boulder batholith and related intrusive 1 contains the geologic map, structure sections, correlation bodies, and the continental autochthon (figs. 1 and 2, sheet 1). and list of map units, and index to major sources of geologic Basin and range structures overprint those tectonic elements. information incorporated on the map. Sheet 2 displays the The quadrangle encompasses about 4,200 km2 in the north­ geology on a base of shaded topographic relief, supplemented eastern part of the northern Rocky Mountains. Four mountain with selected color photographs, in order to aid interpretation ranges dominate the map area: the Big Belt Mountains, which of relations between geology and physiography and to enhance form the northwest-trending high backbone across the west- the understanding of the geology by visitors to an area known central part of the map area; the Elkhorn Mountains in the for its scenic beauty (map jacket; figs. 3, 4, 7, and 8), for southwest corner; the west end of the Little Belt Mountains in its historic significance (pamphlet cover), and for its well- the northeast corner; and the Adel Mountains in the northwest exposed spectacular geologic structure (figs. 3–8). The tec­ corner (fig. 1, sheet 1). Boulder Baldy, the highest peak in tonic map (fig. 2) on sheets 1–3 presents an interpretation of the Big Belt Mountains is 2,726 m high, and Woods Mountain relations among tectonic elements in the area. Sheet 3 empha­ in the western Little Belt Mountains is 2,294 m high. Local sizes the principal geologic structures and provides selected relief is as much as 1,065 m in the Big Belt Mountains (fig. 3, structural point data, including dips and strikes of stratified sheet 2, and Boulder Baldy area), and 800 m in the Tenderfoot rocks, layering in igneous rocks, and foliation or layering in Creek area (T. 14 N., R. 5 E.) of the Little Belt Mountains. metamorphic rocks, in context of the regional geology. Helena Valley and the north end of Townsend Valley lie west of the Big Belt Mountains, and the north end of the Smith River Valley lies east of those mountains (fig. 1, sheet 1). Geologic Units Drainage west from the crest of the Big Belt Mountains, from the Elkhorn Mountains, and from the western part of the Rocks and sediments in the Canyon Ferry Dam quadran­ Adel Mountains is to the Missouri River, which flows north- gle are assigned to 129 map units, described in the “Descrip­ northwest across the western part of the quadrangle. Drainage tion of Map Units” section, on the basis of rock or sediment east from the crest and from the Little Belt Mountains is to type and age. The discussion here only highlights strati­ the Smith River, which flows north across the eastern part of graphic relations among some strata and clarifies age designa­ the quadrangle. The Missouri River is dammed from south to tions or nomenclature for other units. north to form successively Canyon Ferry Lake, Hauser Lake, Paleoproterozoic units are metamorphic crystalline and Upper Holter and Holter Lakes. Northwest of Canyon rocks, including mainly granite gneiss and three other locally Ferry, meanders of the Missouri River are deeply incised restricted units of metasedimentary, metavolcanic, or sodic through the northwest end of the Big Belt Mountains (see map metaintrusive origin. These rocks are part of the continental jacket). Similarly, about 48 km directly east of the meander ­ autochthon, exposed in the northeast quarter of the quad­ ing reach of the Missouri River, meanders of the north-flowing rangle, which plunges west under the southeast terminus of the Smith River are incised through the west end of the Little Belt Montana disturbed belt (fig. 2, sheets 1–3). None of the units Mountains and bedrock ridges south of those mountains (fig. within the map area has been dated radiometrically, but in the 7, sheet 2). The lowest elevation in the quadrangle is 1,090 m Neihart and Sheep Creek areas of the White Sulphur Springs where the Missouri River leaves the quadrangle (sec. 9, T. 14 1:100,000-scale quadrangle that adjoins the Canyon Ferry N., R. 3 W.); by comparison, the Smith River flows from the Dam quadrangle on the east, Vogl and others (2004, p. 20, 30) quadrangle at an elevation of 1,208 m (sec. 6, T. 14 N., R. 4 E.). reported a maximum age of about 1,810 Ma for rocks equiva­ Helena, the State capital city, lies adjacent to the west- lent to the granite gneiss and ages ranging from about 1,867 to southwest edge of the quadrangle; the communities of East 1,842 Ma for older metamorphic rocks of the area. Helena, Canyon Ferry, and York are the main settlements A thick succession of strata of Mesoproterozoic age in within the quadrangle, all in the southwestern part. Urban six formations in the Belt Supergroup includes the lowest growth is expanding rapidly across the south flank of Hel­ strata of the supergroup known in the eastern part of the Belt ena Valley, north, east, and southeast of East Helena. Simi­ depositional basin. Within the Canyon Ferry Dam quadrangle, lar growth is occurring across the north flank of the valley, the Belt Supergroup is everywhere faulted against the Paleo­ west-northwest of Lake Helena. Principal transportation