University of North Dakota UND Scholarly Commons Theses and Dissertations Theses, Dissertations, and Senior Projects 1967 The geology of the Elliston area, western Montana Thomas R. Walker University of North Dakota Follow this and additional works at: https://commons.und.edu/theses Part of the Geology Commons Recommended Citation Walker, Thomas R., "The eg ology of the Elliston area, western Montana" (1967). Theses and Dissertations. 310. https://commons.und.edu/theses/310 This Thesis is brought to you for free and open access by the Theses, Dissertations, and Senior Projects at UND Scholarly Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UND Scholarly Commons. For more information, please contact [email protected]. THE GEOLOGY OF THE ELLISTON AREA WESTERN MONTANA by Thomas F. Walker.,,.... B,A. in Geology, University of Montana, 1962 A Thesis Submitted to the Faculty of the University of North Dakota In partial fulfillment of the requirements for the degree of Master of Arts Grand Forks, North Dakota January 1967 I I/ i :·, i ., .• l .. ,... This thesis submitted by Thomas F. Walker in partial fulfillment of the requirements for the Degree of Master of Arts in the University of North Dakota is hereby approved by the Committee under whom the work has been done. ~LL~:<Chairman ~~-cf-z ii .JI'-~-----·-. .......... #$-·"-"' -,- .. ·-·~-· 3_o_l_5~_,-·-· ·----- TABLE OF c·oNTENTS Page LIST OF ILLUSTRATIONS V ABSTRACT ...• vi INTRODUCTION . 1 Location l Topography and Drainage l Climate and Vegetation. 4 Previous Work 5 Present Work •..••• 6 Acknowledgements • 7 PRE-CENOZOIC STRATIGRAPHY 10 Precambrian ..... ll Cambrian System 13 Flathead Quartzite 14 Wolsey Shale 18 Meagher Limestone •• 20 Park Shale . , . 22 Hasmark Dolomite 24 Red Lion Formation 26 Devonian System •..... 29 M:i.ywood Fonn.ation •• 29 Jefferson Fonnation 31 Three Forks .Formation 34 Mississippian System .•••••• 38 :Madison Group ...••. 39 Pennsylvanian System •.••• .Amsden(?) Formation. Quadrant Quartzite .• 44 Permian System •...•• 46 Phosphoria Formation 46 Jurassic System •..•. 48 Sawtooth Formation ••••. 49 Swift Formation .• 50 Morrison Formation ••. 51 Cretaceous System ..•.. 52 Kootenai Formation 53 Colorado Group ••. 55 IGNEX:>US ROCKS .•.•.. 60 Elk.horn Mountains(?) Volcanics 60 Undifferentiated Andesitic Intrusives 61 l I iii ~-----------i r Page Boulder Batholith and Related Intrusives. 63 Tertiary Volcanics . 66 CENOZOIC STRATIGRAPHY. 72 Tertiary System 72 "Lake" beds • . ~(2 High level gravels ..••• 73 Tertiary and Quaternary Systems •••••••• 77 Gravels of the Avon Valley 77 Quaternary(?) System ...••• 81 Pre-Wisconsinan dri:f't(?) 81 Quaternary System •.••••.. 84 Wisconsinan glacial deposits .••• 84 Alluvium.. ·• . • . • • • • • • 85 Landslide deposits •• 86 Colluvium ••••• 86 ME':CAM.)RPHIC ROCKS. 89 STRUCTURE. 90 Pre-Laramide Structure. 91 Laramide Structures •••. 91 Post-Laramide Struc+,ures 96 GEOLOGIC HISTORY, 99 Pre-Tertiary Stratigraphic and Tectonic History. 99 Cenozoic depositional and tectonic history and evolution of landforms ••••.••••••• 101 APPENDIX I• GRAIN SIZE AND BEDDING THICKNESS GERMINOLOGY, ••.....•• 112 APPENDIX II, STRUCTURE SECTIONS 113 APPENDIX III. STRATIGRAPHIC SECTIONS. 117 BIBLIOGRAPHY. 132 iv - LIST OF JLLUSTRATIONS Pla:te Page I. Geologic Map of the Elliston Area, Western Montana ... In Pocket Figure l. Index Map. • • • • • • • • • • • • • • • • • • • • • • • 2 2. View East from Meyers Hill •••• 8 3. View East up Ophir Creek to Avon Valley Scarp 8 4. View East up Snowshoe Creek to Avon Valley Scarp •. 9 5. Blackfoot City Stock. 9 6. Correlation Chart . .. 12 7. Cross-Bedding in Flathead Quartzite 17 8. Cross-Bedding in Flathead Quartzite 17 9. Contact of Blackfoot City Stock with Wolsey Sha.le 67 10. Contact of Blackfoot City Stock with Meagher Limestone • 67 11. Hoodoos in Rbyolite. .. 71 12. High-Level Gravels on Meyers Hill 71 13. Panora.ma across Avon Ba.sin from Southwest 78 14; Panoramic. View North along Avon Valley Scarp • 80 15. View from Meyers Hill toward Greenhorn Mountain 82 16. Elliston Moraine 17. View down Little Blackfoot River Valley 18. Floodplain of the.Little Blackfoot River. 110 V l I ABSTRACT The Elliston area includes four townships located along the continental divide west of Helena, Montana on the northwest margin of the Boulder Batholith and within the Laramide disturbed belt. Strata ranging in age from Precambrian through recent are present, with all but thQ Ordovician, Silurian, and Triassic System& repr~s sented. The sedimentary rocks are mainly marine and continental carbonates, shales, and sandstones typical of a relatively stable shelf. The Precambrian Belt "Series" comprise the oldest rocks. The lower Cretaceous Blackleaf Fonnation is the youngest pre-Laramide unit. Post-Laramide sediments include fine-grained mid-Tertiary basin fills, terrace gravels, a moraine, and a mantle of coarse gravel in the north which may be the remnant of an early Pleistocene drift sheet. The oldest of four generat.ions of igneous rocks is a sequence of pre-Laramide latitic and andesitic volcanics and related sills, probably correlative with the Lake Cretaceous Elkhorn Mountains f Volcanics. A number of small post-Laramide granodioritic plutons, t-·. } satellitic to the Boulder Batl1olith, intrude the volcanics and older rocks, and are responsible for extensive thermal metamorphism. In the Avon basin there are extensive later post-Laramide Tertiary rhyolite flows, which pre-date the basin-fill sediments. In the Dog Creek basin area similar rhyolites occur which appear to overlie the dirft (?) gravels and may be of Quaternary age. The major structural feature is a large southeasterly plunging syncline, which is overthrust from the west by an adjacent snticline. These structures were modified during batholitb.ic intrusion by some secondary folding and much faulting. The Avon and Dog Creek basins are products of post-Laramide block-faulting. vi INTRODUCTION Location The Elliston area is located in centraJ.-western Montana about 10 miles west of Helena, in Lewis and Clark, and Powell counties. The area mapped consists of four townships mainly in the northern two-thirds of the Elliston Quadrangle but overlapping into the Avon Quadrangle to the west. On the south the boundary extends just out­ side of the township line, following the Little Blackfoot River and then U.S. Highway 12 up to McDonald Pass. Most of the area is easily accessible from U.S. Highway 12, which follows the southern boundary of the area. Good graded and drained roads extend northward to Mullan Pass on the east side and up Snowshoe Creek on the west side of the area. In the north a good unimproved road connects Marysville and Blackfoot City, giving good access to this part. An interconnecting network of poorer unim­ proved roads and jeep trails passable to most cars, except during and shortly after heavy rains, provides a means of traversing most of the country between the better roads mentioned above. Topography and Drainage Three of the townships mapped are mountainous, but about 29 square miJ.es of the southwestern township are in the Avon Valley, a smaJ.l intermontane basin. The mountains of the Dog Creek drainage in the ea,stern and northern parts of the map area are low and rounded (see Fig. 2); the mountains drained by the streams of the Avon Valley l 2 I ·-·-. ...., \' ·~ . : . '.·.· ~ r.._ <;., /. ~:tci~::J;~."· )'· ';~ I • . ,,.,",' : ... .".,.. ~ . '.~# -:.• ' ,........ , ·. .. STREAM S '"TERMONT/\NE CON Tt"" E"NT AL OI V/0( 3 have somewhat narrower) steeper-sided valleys) but are still low and smooth (Figs. 3,4). The mountainous topography is in an early mature stage of devel9pment; ridgecrests are mostly narrow) as are the valleys) with the exception of Dog Creek and its larger tributaries, which have relatively broad, alluvium-filled valleys. The Avon Valley is characterized by flat, gently sloping sUl"faces, with an area of higher, more rugged hills along the western border of the map area. Relief in this basin locally ranges between 100 and 300 feet; total relief is 843 feet. In the mountains local relief ranges in general between 500 and 1500 feet; total relief is 3170 feet. Most of the higher ridge crests, that is those forming the skyline when.seen from a dis­ tance, are in the 6800 to 7000 feet elevation range. Two rather mas­ sive mountains rise noticeably above this level - Black Mountain (elev. 8338) in the extreme northwest corner and Greenhorn Mountain (elev. 7505) in the northeastern part of the area (Fig.2). The Continental Divide extends through the area near to and paral­ leling the north and east boundaries and thus most of the area is west of the divide and is drained by the southflowing tributaries of the Little Blackfoot River, itself a tributary of the Clark Fork of the Columbia. The parts of the area east and north of the Continental Divide are drained by the headwaters of smaD. tributaries of the Mis­ souri River. Most of the larger streams in the area are perennial, but many of their tributaries are ephemeral, drying up by August. There is a noticeable adjustment of drainage to fractures in the bedrock. Bedrock in the area is not well exposed. ..,_-----------~....----~~------------""""·lr" _if}". 9 3.;p\!t 41. 5.4 P&.,~ A!Rh < J@; 4 Climate and Vegetation The climate is characterized by short warm summers and long cold winters. Average temperature for July, the warmest month is aoout 0 1 64°F; that for January, the coldest month is 21 F. The prevailing westerly winds provide a moderating influence for winter temperatures. Mean annual precipitation at Elliston is 21 in. (Konizeski, 1957). Snowfall is heavy and with the possible exception of July, may fall at any time of the year. On some of the higher forested northern and eastern slopes drifts may persist through most of the summer. Most of the precipitation falls as rain in May and June. The mountains are generally forested, although large grassy patches often occur on south facing slopes. Areas underlain by shale and hornfels, or gravel, also may be unf~rested,'being characterized 'by gentle grassy slopes.