Montana Bureau of Mines and Geology Montana Ground-Water Assessment Atlas 4, Part B, Map 4 A Department of Montana Tech of The University of Montana Open-File Version October 2006

R18W N R17W R16W R15W Explanation:

Well that penetrates bedrock 0 0 0

T21N 5 -

3

0 R31W MB-2 0 R30W Deep well from Norbeck (1980) 0 R29W Scale 1:150,000

T20N R32W Contour of bedrock, altitude above mean sea level (ft); irregular 500 4 051015 miles -2500 T21N 5 interval, dependent on data quality 0 0 3 1 1 5 2 0 - 2 5 2 2 0 0 0 3 0 0 5 0 5 0 0 0 Montana NAD 1983 0 0 0 0 0 Swan Range 0 0 0 3 0 -1 4 State Plane Coordinate System Approximate contour 5 0 500 2 3 0 0 0 0 0 4 0 - 0 0 0 0 10 0 0 0 00 R28W Inferred contour (from gravity geophysical data) 0 -500 4500 0 Basin fill: Quaternary sediments and Tertiary 47° 500 sedimentary rocks 1000 00 T19N T20N 15 47°

Bitterroot Range Tm Tertiary metamorphic rocks on Bitterroot Range front 2000 0 2500 R27W 0 3000 0 0 28 TKi Tertiary and Cretaceous intrusive rocks 5 3500 0 0 Lolo-Bitterroot Area 4000

0 0 T20N

0 9

3 2

4

115° 5

Pzb Paleozoic sedimentary rocks Swan 0 0

Clark Fork River valley Valley 5000 0 Saint Regis 10 Yb Proterozoic Belt Supergroup rocks 0 3 0 4 Ninemile Valley 2 0 0 HOLLAND LAKE 6 15 0 00 2600 Study area boundary Missoula Co. 2000 00 Mineral Co. Seeley 2500 3000 0 3 Missoula Valley Lake 3 Township boundary 000 35 00 2900 00 2600 0 0 28 T19N 4 Section boundary 6 0 0 0 3 0 0 2500 0 0 5 2500 0 42 0 2800 0 0 St. Regis Major road E 0 K East Missoula A 45 T19N 2700 Potomac L 00 H

G 00 Principal river or stream Missoula R 6 E 2600 2700 0 2 5 B 0

0 D

9 0 2 2 N

2900 0 I 8

2800 R26W Lake or reservoir L 0 6 0 St. Regis This map 0

0 0

0 0 6 Clark Fork River valley 2 River floodplain area 4

0 4

0 4 4

3000 0 6 0 0 0 alley City or town 0 0 VValley 00 2800 6 6 00 2 2600 7

0

s e s

2800 0 T18N

g

n n

i 2 R25W R24W n LAKE ALVA 7 Bitterroot 0 a a 0 Hamilton t R

n 2 AL t 80 u

2700 V 0 o

o T18N

A o

2700 o

r M r r Ravalli

e e

2 e

t

6 t r

t

2600 00 t i i i Co. LAKE INEZ

h

B B Darby p 420 p 0 R23W a S T17N 2700 4 00 2600 Yb 0 2800 2 3 7 6 00 3 0 2800 8 0 00 2800 4 2 Superior 0 0 R22W 2 2 Superior 60 9 0 0 2600 0 2600 2700 2700 T17N 2800 2900 T17N

3 2 8 8 3 0 6 0 0 0 0 0 Figure 2. Index map showing the principal cities, drainages, and SEELEY LAKE3 T16N 42 8 4 4 0 6 0 00 0 Pzb 0 3 0 0 2900 0 physiographic locations. 8 4 4 0 0 4 2 4 0 0 4 0 0 4 0 2 0 0 0 0 4 0 0 3 0 0 4 0 0 0 0 R14W 2600 2800 Trail Creek R20W Seeley Lake 4400 2700 R21W 4200

4200 4000 2800 3 2 Yb 0 0 4200 T16N 4 3800 33 000 00 3 Pzb Yb 800 2900 3 2600 3 60 4000 500 0 3800 PLACID LAKE 0 T16N 31 340 0 Altitude of the Bedrock Surface, Mineral and Missoula Counties, Montana 0 0 R18W 2 0 3300 3 4200 4 2600 600 2800 3000 MISSOULA CO 3 4000 200 0 2900 00 4200 3000 3 MINERAL CO SA by 2800 LM 320 00 O 0 1 N 0 2900 3 400 2700 3 0 L 3000 0 R19W A 2 30 4200 0 K 0 1 E 2700 0 MB-2 Larry N. Smith 00 T15N 5 3500 20 00

3000 0 2900 0 1 50 2 Pzb 0 5 114° 2900 0 3 47° 20 0 40 3100 00 0 Author's Note: This map is part of the Montana Bureau of Mines and Geology DISCUSSION 2700 150 3000 Pzb 2 0 0 700 2800 0 (MBMG) Ground-Water Assessment Atlas for the Lolo-Bitterroot Area ground- 9 Yb 2800 2 water characterization. It is intended to stand alone and describe a single Regional Configuration of the Bedrock Surface 250 3100 2 0 4 Yb hydrogeologic aspect of the study area, although many of the area's 90 Alberton 2 3100 0 0 0 T15N hydrogeologic features are interrelated. For an integrated view of the The altitude of the bedrock surface beneath basin-fill deposits was 4 3000 TKi 0 2900 00 hydrogeology of the Lolo-Bitterroot Area the reader is referred to Part A controlled by structural deformation, including faults that define some of the 3 36 8 (descriptive overview) and Part B (maps) of the Montana Ground-Water valley boundaries, and by erosion by streams subsequent to faulting. In most 0 0 T14N 3400 0 0

Assessment Atlas 4. areas, available data were not sufficient to fully distinguish each process. 2600 115° 3200 The contours show the position of the bedrock surface, which is reasonably 34 INTRODUCTION well constrained by drill-hole data along many of the basin margins; in these TKi 3000 00 2700 Pzb 25 3 areas the position is shown with solid contours. The map is poorly constrained 00 000 0 The distribution of bedrock at the land surface and its position in the in most of the basin centers where few or no wells penetrate bedrock; in 50 1 20 2600 0 subsurface are important for understanding ground-water resources. Bedrock, these areas dashed contours were used. 3 0 47° 0 0 1 1000 as the term is used here, is well-cemented or indurated rock which, in Mineral The bedrock surface is an unconformity between the bedrock and the 0 0 0 0 5 500 and Missoula counties, consists mostly of Proterozoic metasedimentary rocks overlying basin-fill units. In basin-margin areas, where Tertiary sedimentary 0 0 MB-4 of the Belt Supergroup, sparsely distributed Paleozoic sedimentary rocks, rocks are less than a few hundred feet thick, the bedrock is an important 1500 3500 and Cretaceous and Tertiary plutonic and metamorphic rocks. In the Lolo- aquifer. Bedrock aquifers are extensively used in areas along the southern 32 Bitterroot Area, ground water is produced from the bedrock only where it and northern portions of the Missoula Valley, in the Ninemile Valley, and in 00 3000 3000 3000 3100 is fractured; typically, unfractured bedrock is not sufficiently permeable to the Potomac area. 250 3200 3 0 T14N 0 200 be considered an aquifer. In some areas, fractured bedrock is the only available 0 0 3200 0 aquifer either because it is the only geologic unit present or because overlying Missoula Valley 50 basin-fill deposits have low permeability, are unsaturated, or are too thin. T13N 0 Because the bedrock is usually less permeable than the overlying basin fill, The bedrock surface beneath the Missoula Valley occurs at about sea Yb 1000 2 1 5 9 3 it forms the base of the productive basin-fill aquifer. level west and northwest of the city of Missoula, and rises irregularly to the 00 0 0 4 0

2

2 0 This map shows the altitude above sea level of the bedrock top beneath land surface at the valley margins. The irregular and complex nature of the 0 340 3500 5 0 3200 0 0 3 0 unconsolidated surficial sediments and partially consolidated basin-fill bedrock surface is shown by well-constrained gravity surveys in the center 0 0 0

0 0 3200 deposits. The altitude of the contact between bedrock and basin-fill deposits of the Missoula Valley and in the East Missoula area (Evans, 1998; Nyquest, 3000 0 6 0 3400 Pzb 3 80 was estimated from drill holes and geophysical surveys. In the Clark Fork, 2001). Movements along bounding faults have caused the basin to drop, 3 3100 4 Ninemile, Missoula, and Seeley-Swan valleys, bedrock units beneath the especially along the northeast margin, and have deformed the bedrock surface. 00 3 0 basin fill are mostly the Belt Supergroup. Paleozoic sedimentary rocks crop Evans (1998) interpreted the variations in the bedrock surface to be due to 3100 20 Missoula 0 3200 3400 1000500 Potomac out between the Missoula and Ninemile valleys and near Potomac. Tertiary transpressional and transtensional vertical movements as a result of strike- 3200 2800 2900 38 and Cretaceous granitic intrusive rocks occur locally, especially in southern slip movements along faults. The complexity shown in the southeastern 2900 3600 00 3100 Missoula County (fig. 1). Tertiary sedimentary rock is not considered bedrock portion of the Missoula Valley may be characteristic of the other, more poorly 2900 2900 0 for this map because it has sufficient intergranular permeability to be an mapped portions of the basin. Because of this structural complexity, depth 3000 3300 38 0 2900 3000 3300 TKi T13N 3 aquifer and is distinctly less lithified than the bedrock (Smith, 2006). to bedrock is difficult to estimate in the undrilled portions of the basin. 2 0 Development of ground-water resources in fractured bedrock aquifers 0 2800 2900 30 increased during the 1990s as land near the perimeters of valleys was Ninemile Valley 2700 Alberton 00 0 0 0 3 4000 5 0 8 2 0 3200 00 4000 subdivided. This map is intended to help estimate the altitude of, or depth 3 4 2800 3 0 to, bedrock in areas where wells have not yet been drilled. Depths to bedrock The geology and structure of the Ninemile Valley are poorly understood. 800 0 340000 36 0 2900 3000 36 00 can be calculated by subtracting a bedrock altitude derived from the map Because of dense vegetation cover, surficial mapping of the geologic units T12N 42 000 00 4 from a land-surface altitude derived from a topographic map. This map is is generalized (see McMurtrey and others, 1965; Mudge and Earhart, 1983). 0 Pzb 2900 440 3000 460 useful for estimating drilling depths and completion levels for water wells, Available mapping indicates at least two structural basins, separated by an 3800 0 as an aid in planning new subdivisions or other developments relying on area where Belt Supergroup bedrock is at the land surface. Sparse subsurface 2900 3 3 2800 3 4 0 0 R14W ground water. The map also shows areas where bedrock is near the land data in the southeastern and northwestern portions of the valley show that 32 0 0 surface, and areas where basin-fill aquifers may be absent. Wells completed part of the area is underlain by southwest-dipping bedrock surfaces. The 3400 00 in fractured bedrock commonly produce enough water for domestic purposes, structure of the northwestern portion of the valley is especially poorly known. TKi but rarely produce more than 30 gallons per minute (gpm); reported yields Pzb have a median value of 10 gpm. Clark Fork River Valley R15W

DATA SOURCES AND MAP CONSTRUCTION The altitude of bedrock shows as much as 500 ft of variation in narrow 3400 REFERENCES 0 portions of the Clark Fork River valley upstream and downstream of the 320 Yb Data used for contouring the altitude of the bedrock surface come from Missoula Valley. Even though the present valleys are narrow, an even narrower T12N descriptive lithologic logs of water wells and gravity geophysical surveys. channel cut in bedrock can typically be mapped in subsurface within the Crosby, G.W., 1984, Structural-geophysical interpretation of Swan Valley, 3 Montana, in McBane, J.D., and Garrison, P.B., eds., Northwest Montana 4 Of the nearly 12,500 water wells with lithologic logs in the northern Lolo- valleys. Erosion by the river was likely responsible for much of the relief 0 Bitterroot Area, a subset of 2,800 well logs was chosen to interpret the on the bedrock surface. Upstream of Missoula, the map shows a 100- to 300- and Adjacent Canada: Montana Geological Society 1984 Field Conference 0 positions of geologic units. Well logs were selected based on areal distribution, ft-deep incised channel that broadens and deepens towards East Missoula. and Symposium, p. 245–251. T11N depth, lithologic detail, and locational accuracy. About 35 percent (980 of Downstream of the Missoula Valley, between Alberton and Superior, a Evans, C.A., 1998, A constrained gravity model of the central Missoula Valley and the shape of the Ninemile fault: [unpubl. M.S. thesis] University R25W 2,800) of the well logs are interpreted to have been drilled into bedrock; channel as much as 500 ft deep was cut into bedrock. In the lower part of 3200 0 3500 0 3600 these locations are shown on the map. Logs from some wells that did not the downstream reach near St. Regis, faulted and folded Tertiary sedimentary of Montana, Missoula, 45 p. 5 penetrate bedrock also provided maximum possible bedrock altitudes. The rocks are exposed along the valley margins and present in the subsurface. Lewis, R.S., compiler, 1998a, Geologic map of the Butte 1° x 2° quadrangle: 3 Montana Bureau of Mines and Geology Open-File Report 363, 16 p.,

locations of about 100 of the non-visited wells were refined by use of cadastral Understanding of the structural configuration of this part of the Clark Fork 0

0 0

0

0 0

scale 1:250,000. 6 4

0

data; some well locations were confirmed by talking to the well owners. River valley is incomplete because of the sparse data. 0 3 3

2 0 R16W Logs of drill cuttings from two uranium exploration wells, drilled to Lewis, R.S., 1998b, Geologic map of the Montana part of the Missoula West TKi 3 3 30' x 60' quadrangle: Montana Bureau of Mines and Geology Open-File 2,515 and 2,907 ft deep, helped constrain the depth to bedrock in two areas Potomac area 3000

3200 Report 373, 20 p., scale 1:100,000. 3400 TKi where the depth to bedrock is greater than 2,000 ft (Norbeck, 1980). Data Tm 2800 from these wells were used to estimate parameters necessary to model gravity The Potomac area is in a north-dipping structural valley formed by Lonn, J.D., and McFaddan, M.D., 1999, Geologic map of the Wallace 30' T11N Sources of geologic mapping surveys of the contact between weakly consolidated Tertiary strata and the bedrock faulting on its northern margin. Lithologic logs from a few deep x 60' quadrangle: Montana Bureau of Mines and Geology Open-File R22W and geophysical data highly consolidated bedrock of the Missoula Valley (Evans, 1998). Other wells show that the Tertiary sedimentary rocks and Quaternary sediments Report 388, 15 p., scale 1:100,000. gravity surveys include two cross sections by McMurtrey and others (1965), in the basin are underlain by bedrock at depths of a few hundred to possibly Lonn, J.D., and Sears, J.W., 2001, Geology of the Bitterroot Valley, shaded a regional study of the Swan Valley by Crosby (1984), and a detailed study as much as a thousand feet. relief, not topography: Montana Bureau of Mines and Geology Open- R24W 114° 12. of the East Missoula area by Nyquest (2001) (location names are in fig. 2 File Report 441C, scale 1:48,000. 1. and studies are in fig. 3). Seeley-Swan Valley McMurtrey, R.G., Konizeski, R.L., and Brietkrietz, A., 1965, Geology and R23W Figure 1. Altitude of bedrock in Mineral and Missoula counties. R17W Depth-to-bedrock estimates derived from well-log and geophysical data ground-water resources of the Missoula basin, Montana: Montana Bureau were converted to altitudes above sea level by subtracting the depth values In the Seeley-Swan Valley, well drilling and geophysical data define the of Mines and Geology Bulletin 47, 36 p., 3 pls. from land-surface altitudes. Land-surface altitudes at well locations were bedrock surface. The bedrock surface is determined by well-log data near Mudge, M.R., and Earhart, R.L., 1983, Geologic and structure maps of the obtained from the 1:24,000-scale U.S. Geological Survey Digital Elevation the town of Seeley Lake. East of the town, the southwest-directed valley Choteau 1° x 2° quadrangle, western Montana: United States Geological Models (DEMs) using ArcInfo™ computer software. A comparison of well- along Trail Creek has a shape that suggests an erosional origin. The valley Survey Miscellaneous Geologic Investigation 1300, scale 1:250,000. 1. Lonn and McFaddan (1999) location altitudes determined from topographic maps for field-confirmed trending northwest of Seeley Lake is a half graben with the greatest movement Norbeck, P.M., 1980, Preliminary evaluation of deep aquifers in the Bitterroot 10. well locations with those derived from the DEMs showed that most DEM- along the Swan Range (Mudge and Earhart, 1983). Near and north of and Missoula valleys in western Montana: Montana Bureau of Mines 2. L.N. Smith (unpubl. mapping) 2. based values were within 20 ft of the field-determined values. Because of Lindbergh and Holland Lakes, geophysical data and a few deep wells show and Geology Open-File Report 46, 160 p. 3. Lewis (1998b) Nyquest, D.L., 2001, A depth to bedrock model of the Hellgate Canyon and imprecision in the well-log data, contours were drawn only where multiple structural complexity, with the altitude of the bedrock surface dropping to 4. Evans (1998) 11. data points supported the contour location. Contours were drawn by hand 1,500 ft above sea level. Few wells have been drilled along the valley margins Bandmann Flats area, western Montana using constrained inversion of 9. and then digitized. north of Lindbergh Lake. The deepest part of the Swan Valley is in northern gravity data: [unpubl. M.S. thesis] University of Montana, Missoula, 186 5. McMurtrey and others (1965) p. In areas of incomplete geologic mapping of Tertiary rocks and where Missoula County, where bedrock is greater than 3,000 ft below sea level 6. Lonn and Sears (2001) 4. there were no well penetrations to bedrock, the top of Belt Supergroup (Crosby, 1984). Consolidated Tertiary siltstone and carbonaceous shale have Smith, L.N., 2006, Hydrogeologic framework of the Lolo-Bitterroot Area, 7. Nyquest (2001) 77. bedrock was mapped with long-dashed (approximate) contours. Contours been sampled from a few of the deep wells in the area. Most of the basin fill Mineral, Missoula, and Ravalli counties, Montana (open-file version): 5. based entirely on geophysical data are shown as short-dashed lines. Because in the Seeley-Swan Valley is thought to be Tertiary sedimentary rocks (Crosby, Montana Bureau of Mines and Geology Ground-Water Assessment Atlas 8. Lewis (1998a) 3. data are unevenly distributed across the map and because the widths and 1984). 4B, Map 2, 1 sheet(s), scale 1:250,000. 8. depths of alluvial basins vary widely, contour intervals are not consistent. Van der Poel, W.I., 1979, A reconnaissance of the late Tertiary and Quaternary 9. Mudge and Earhart (1983) The contour interval is generally 200 ft in the main map; it is 100 ft in the ACKNOWLEDGMENTS geology, geomorphology, and contemporary surface hydrology of the 10.Witkind and Weber (1982) Rattlesnake Creek watershed, Missoula County, Montana: [unpubl. M.S. two enlarged inset maps. The contour interval in the main parts of the 11.Van der Poel (1979) Missoula and Swan valleys is 500 ft where only geophysical data were This work was supported by the Ground-Water Characterization Program thesis] University of Montana, Missoula, 85 p. 6. available. at the Montana Bureau of Mines and Geology. The map and text were Witkind, I.J., and Weber, W.M., 1982, Reconnaissance geologic map of the 12.Crosby (1984) Water-well driller logs and well locations are stored in the Ground-Water improved due to reviews by John LaFave, Tom Patton, Gary Icopini, John Big Fork-Avon environmental study area, Flathead, Lake, Lewis and Information Center database at the Montana Bureau of Mines and Geology. Metesh, Ed Deal, and Susan Barth. Clark, Missoula and Powell counties, Montana: United States Geological The outcrop areas of the bedrock were modified from digital versions of Survey Miscellaneous Geologic Investigation 1380, scale 1:125,000. Figure 3. Index to previous maps from which contacts and geologic maps as shown in figure 3. subsurface data were compiled.