Montana Bureau of Mines and Geology Montana Ground-Water Assessment Atlas 4, Part B, Map 5 A Department of Montana Tech of The University of Montana Open-File Version August 2006

R21W R20W 114° R19W Altitude of the Bedrock Surface in the R18W Explanation for map T12N Bitterroot Valley: Missoula and Ravalli Well that penetrates bedrock Lolo MB-12 Counties, Montana 3400 Deep well from Norbeck (1980) 3200 3200 T12N Yb by 500 Contour of bedrock, altitude above mean sea level (ft) Larry N. Smith Approximate contour Author’s Note: This map is part of the Montana Bureau of Mines and Geology

3 Inferred contour (from gravity geophysical data)

0 (MBMG) Ground-Water Assessment Atlas for the Lolo-Bitterroot Area ground-

0 0

3 water characterization. It is intended to stand alone and describe a single

T11N 2 0 Basin fill: Quaternary sediments and Tertiary hydrogeologic aspect of the study area, although many of the area’s 0

0 hydrogeologic features are interrelated. For an integrated view of the 0 0 sedimentary rocks

0 6

2 3

3400 hydrogeology of the Lolo-Bitterroot Area the reader is referred to Part A 3 (descriptive overview) and Part B (maps) of the Montana Ground-Water 0 Tgws 0 T11N Tertiary granodiorite of the Willow Creek and Skalkaho Stocks 0 Assessment Atlas 4.

3 Tm Tertiary metamorphic rocks on Bitterroot Range front INTRODUCTION

MISSOULA CO TKi Tertiary and Cretaceous intrusive rocks The distribution of bedrock at the land surface and its position in the subsurface are important for understanding ground-water resources. Bedrock, 4 0 TKi 0 42 as the term is used here, is well-cemented or indurated rock, which in southern RAVALLI CO 0 0 Yb 0 0 Proterozoic Belt Supergroup rocks 0 0 3 Missoula and Ravalli counties consists of Proterozoic metasedimentary rocks 8 0 0 Tm 6 0 3 4 3 0 3 3 8 0 0 0 3 0 0 2 0 of the Belt Supergroup and Cretaceous and Tertiary plutonic and metamorphic 4 0 3 0 0 6 3 0 8 0 00 2 0 3 Study area boundary rocks. In the Lolo-Bitterroot Area, ground water is produced from the bedrock TKi 3 26 0 2 0 0 0 0 only where it is fractured; typically, unfractured bedrock is not sufficiently

24 Township boundary permeable to be considered an aquifer. In some areas, fractured bedrock is 0 4 0 2 0 the only available aquifer either because it is the only geologic unit present 2 0

0 2 or because overlying basin-fill deposits have low permeability, are unsaturated,

T10N 0 0 0 4 Section boundary 8 0 or are thin. Because the bedrock is usually less permeable than the overlying

2

0 0 0 0 6 2000 basin fill, it forms the base of the productive basin-fill aquifer. 2 Major road 0 This map shows the altitude above sea level of bedrock beneath 0 1800 4 unconsolidated surficial sediments and partially consolidated basin-fill 2 1600 T10N Principal river or stream deposits (fig. 1). The altitude of the contact between bedrock and basin-fill 1400 MB-9 N deposits was estimated from drill holes and geophysical surveys. In the 400 3 3200 Bitterroot Range and Sapphire Mountains (fig. 2) and beneath the basin fill A 3000 Lake or reservoir 42 of the Bitterroot Valley, bedrock units are Belt Supergroup, Tertiary and 3 0 8 0 0 Cretaceous granitic intrusive rocks, and, especially in southern part of this MB-6 0 Active river floodplain area map, Tertiary volcanic rocks. Tertiary sedimentary rock is not considered 0

1600 0 4 bedrock here because it has sufficient intergranular permeability to be an Yb MB-8 4 A' 0 0 0 0 City or town aquifer and is distinctly less lithified than the bedrock (Smith, 2006). 1800 2 2 2 2 Development of ground-water resources in fractured bedrock aquifers 2000 has increased during the 1990s as land near the perimeter of the Bitterroot 2200 TKi Scale 1:125,000 Valley has been subdivided. This map is intended to help estimate the altitude 2 2400 20 2600 of, or depth to, bedrock in areas where wells have not yet been drilled. Depths 0 2800 1200 3000 051015 miles to bedrock can be calculated by subtracting the bedrock altitude at a location 0 2 14000 3200 4 6 0 from the land-surface altitude for the same location derived from a topographic T9N 2600 0 1 0 0 8 0 3400 1 0 0 0 map. This map is useful for estimating drilling depths and at what levels to 2 0 Montana NAD 1983 2 0 3600 2 0 State Plane Coordinate System complete water wells as an aid in planning new subdivisions or other 4 0 2 0 3800 developments relying on ground water. The map also shows areas where 6 0 2 0 2800 8 0 0 42 bedrock is near the land surface, and areas where basin-fill aquifers may be 2 0 00 00 0 4 T9N A A' 3 thin or absent. Wells completed in fractured bedrock commonly produce 46°30’ enough water for domestic purposes, but rarely produce more than 20 gallons Stevensville 5000 per minute (gpm); reported yields have a median value of 8 gpm. Tm MB-9 2200 MB-6 (136105) MB-8 DATA SOURCES AND MAP CONSTRUCTION (136053) Creek 2400

(59510) Threemile 0 0 0

0 0 0 46°30’ 4000 Data used for contouring the altitude of the bedrock surface come from 2 6 4 4400 3 3 3 Qsc Qsc 2600 descriptive lithologic logs of water wells and gravity geophysical surveys. 0 0 River Bitterroot 4 4200 Aquifers were interpreted for 4,200 of the more than 15,000 water wells 1 with lithologic logs in the southern Lolo-Bitterroot Area; the positions of 00 3000 16 geologic units were interpreted from 1,100 well logs. Well logs were selected 3000 3200 Tsc based on areal distribution, depth, lithologic detail, and locational accuracy. 1800 3400 Tsf 600 About 40 percent of the wells are interpreted to have been drilled into 3 0 3800 0 Yb 2000 0 0 TKi bedrock; the locations of these are shown on the map. Logs from some wells 0 0 4 4 T8N 2200 4 0 0 4 2 6 0 0 2000 that did not penetrate bedrock also provided maximum possible bedrock 2 8 0 0 2 0 0 0 altitudes. Fifty-six of the wells drilled to bedrock (or 13 percent) were located 3 0

2 Altitude (ft) above mean sea level 3 in the field. The locations of about 100 of the non-visited wells were refined 0 0 0 2200 0 8 0 0 0 by use of cadastral data; some well locations were confirmed by talking to 0 0 4 0 3 0 42 4 0 0 3 4 T8N 6 the well owners. 2400 3600 3 3 B B' 0 0 Logs from 5 research wells, 3 in cross section A–A' north of Stevensville 2 2600 VVictorictor 3 and 2 in cross section C–C' north of Hamilton, helped to constrain depths 3000 R18W Explanation for cross sections: to bedrock in the deep portions of the basin (Norbeck, 1980). Data from TKi 4800 Qsf 2800 these wells were used to estimate parameters necessary to model gravity Qsc Quaternary sediments–coarse-grained 1800 surveys of the contact between weakly consolidated Tertiary strata and the 16001400 1600 4000 Qsc 1200 2 River highly consolidated bedrock (Wells, 1989).

0 Poverty Flat 0 Qsf Quaternary sediments–fine-grained 800 0 Bitterroot Depth-to-bedrock estimates derived from well log and geophysical data 30002000 600 1 3 were converted to altitudes above sea level by subtracting the depth values 4000 0 400 0 6 Ts 0 Tertiary sedimentary rocks 3800 200 0 Yb 3200 from land-surface altitudes. Land-surface altitudes at well locations were 0 3000 3 8 obtained from the 1:24,000-scale U.S. Geological Survey Digital Elevation 4 0 Tm 0 0 Tsc Tertiary sedimentary rocks–coarse-grained 0 3600 0 Models (DEMs) using ArcInfo™ computer software. Comparison of well B 0 3400 location altitudes determined from topographic maps for field-confirmed Ts Tsf Tertiary sedimentary rocks–fine-grained 2000 well locations with those derived from the DEMs showed that most DEM- T7N based values were within 40 ft of the field-determined values. Because of Tertiary granodiorite of the Willow Creek Yb Tgws imprecision in the well log data, contours were drawn only where multiple 4000 and Skalkaho Stocks B' TKi data points supported the contour location. Contours were drawn by hand 1000 and then digitized. Tm Tertiary metamorphic rocks on Bitterroot In areas of incomplete geologic mapping of Tertiary rocks and where

800

Altitude (ft) above mean sea level Range front 2000 Yb there were no well penetrations to bedrock, the top of Belt Supergroup -200 4000 0 T7N 600 bedrock was mapped with long-dashed (approximate) contours. Contours 0 0 2200 TKi 0 0 0 Tertiary and Cretaceous igneous rocks 0 4 0 0 0 based entirely on geophysical data were shown as short-dashed lines. Water 3 2400 6 3

Tm 400 2 0 0 200 0 well driller logs and well locations are stored in the Ground-Water Information 0 6 8 3 3800 Yb Proterozoic Belt Supergroup rocks 3200 2 Center database at the Montana Bureau of Mines and Geology. The outcrop 200 400 CC' areas of the bedrock were modified from digital versions of geologic maps as shown in figure 3. 3600 600

1000 Inferred fault showing relative motion 3800 0 0 3000 1600 4 800 3 5000 DISCUSSION

2800 2000 Tgws MB-12 MB-11 Position and depth of well log near section 3200 (220916) The contours show the position of the bedrock surface, which is reasonably

(5418) bench 4000 Qsf well constrained by drill-hole data along many of the basin margins. Where MB-12 MB-1MB-111 4000 Qsc Tertiary sedimentary rocks are less than a few hundred feet thick along the Hamilton Heights C 0 2 mi T6N 0 basin perimeter, the bedrock is an important aquifer. Bedrock aquifers are 6 0 River Bitterroot 3 0 Tm extensively used near the western edge of the Bitterroot Valley within a mile 38 Vertical Exaggeration = 9 or two of the Bitterroot Range and along the eastern edge of the valley near 0 4200 00 3000 Yb Yb bedrock outcrops. The map is poorly constrained in the central part of the 4 C' valley where few or no wells penetrate bedrock. 200 The Bitterroot Valley was formed mostly by downdropping along a Hamilton Ts major fault system at the Bitterroot Range front, delineated by the mylonite 400 TKi T6N Tgws 2000 rock “'Tm” (Hyndman, 1980; Lonn and Berg, 1999). The structure is a 600 complex graben where slippage occurred mostly on the west side, the bedrock surface beneath the valley basically dips to the west, and the deepest parts 1200 800 of the basin tend to be on the west side (figs. 1, 4B). This structure is 1000 1000 1600 complicated by additional faults beneath the valley that show movement in Altitude (ft) above mean sea level 1400 2000 Yb both the same and the opposite directions as the west side valley-bounding 2200 TKi 2400 1800 fault (figs. 4A, 4C). The data suggest that there are two large sub-basins in 2600 the Bitterroot Valley, one near Hamilton and one north of Stevensville. The 2800 0 3000 northern basin has multiple structurally low areas. The development of sub- 3200 basins suggests that different amounts of extension may be accommodated 3400 Figure 4. Cross sections showing the depth of fill in the Bitterroot 4000 3800 3600 by cross-valley faults. However, such faulting has not been recognized in 4200 Valley. T5N TKi surficial geologic mapping or with subsurface data. 0 0 The northern sub-basin is illustrated along cross section A–A', where 60 380 Tgws 3 deep drilling encountered bedrock near the basin center and gravity measurements defined the shape of the basin (fig. 4A; Norbeck, 1980; Wells, 1989). North of Stevensville, the bedrock surface is as low as about 1,000 ft above sea level. The basin-fill sediment is relatively thick on the west side. T5N Lolo-Bitterroot Area The basin fill thins eastward from the valley center toward the Sapphire Mountains. The gravity data in the center of the valley suggest that the Swan River bedrock surface is disrupted by a series of north–south-trending faults (fig. Valley 4A). Saint Regis The southern sub-basin is suggested by gravity profiling along cross

4800 0 Ninemile Valley 0 sections near Hamilton (figs. 1, 4B, 4C). A deep research well (MB-11) near 0 6 0 4 4 cross section C was drilled to a bottom altitude of about 1,100 ft, but was 4 4200 Missoula Co. still in Tertiary sedimentary rocks at total depth. Gravity data suggest that Mineral Co. Seeley Missoula Valley Lake the bedrock surface extends below sea level along cross section B (fig. 4B). Each of the two sub-basins mapped in figure 1 have poorly constrained 38 00 dimensions, but the gravity data suggest that they are separate at depth. Because of this structural complexity, depth to bedrock is difficult to estimate 40 00 in the undrilled portions of the basin. The buried bedrock surface shows about 100 ft of relief in the narrow T4N East Missoula portions of the Bitterroot Valley in the reaches north of Lolo and south of Potomac Darby. Narrow channels cut in bedrock are typical in narrow river valleys Missoula bounded by bedrock. Erosion along the river in this region, possibly during glacial times, was likely responsible for much of the relief on the bedrock surface. T4N Lolo ACKNOWLEDGMENTS O M 3800 This work was supported by the Ground-Water Characterization Program CO 4 E 0 alley K 0 at the Montana Bureau of Mines and Geology. The map and text were LA 4 0 VValley 2 5 0 0 4800 0 This map improved due to reviews by John LaFave, Gary Icopini, Tom Patton, John 0 0 Metesh, Ed Deal, and Susan Barth.

R17W e s

4600 4 s

4 g n 0 n i 0 n REFERENCES Bitterroot

a a Hamilton t R Darby n

t

u Berg, R.B., and Lonn, J.D., 1999, The preliminary geologic map of the Nez

o

o Perce Pass 30' x 60' quadrangle, Montana, revised 1999: Montana Bureau o T3N o r M r r Ravalli of Mines and Geology Open-File Report 339, 14 p., scale 1:100,000.

e e e t

t r Hyndman, D.W., 1980, The Bitterroot dome-Sapphire tectonic block, an t t i

i

46° i Co.

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B B Darby p example of a plutonic-core gneiss-dome complex with its detached p a suprastructure, in Crittenden, M.D., Coney, P.J., and Davis, G. H., S Metamorphic Core Complexes, Geological Society of America Memoir 153, p. 427–443. Lewis, R.S., compiler, 1998a, Geologic map of the Butte 1° x 2° quadrangle: 4800 4 4 6 0 46° 0 0 Montana Bureau of Mines and Geology Open-File Report 363, 16 p., 0 0 4 4 4 2 scale 1:250,000. 0 0 0 0 T3N Lewis, R.S., 1998b, Geologic map of the Montana part of the Missoula West 30' x 60' quadrangle: Montana Bureau of Mines and Geology Open-File Report 373, 20 p., scale 1:100,000. 1. 2. 0 0 2 3. Lonn, J.D., and Berg, R.B., 1999, The preliminary geologic map of the 4 7. Hamilton 30' x 60' quadrangle, Montana, revised 1999, Montana Bureau of Mines and Geology Open-File Report 340, 6 p., scale 1:100,000. Lonn, J.D., and Sears, J.W., 2001, Geology of the Bitterroot Valley, shaded relief, not topography: Montana Bureau of Mines and Geology Open-

4 Figure 2. Index map showing the cities, drainages, and File Report 441C, scale 1:48,000. 0 0 0 physiographic locations. Norbeck, P.M., 1980, Preliminary evaluation of deep aquifers in the Bitterroot 1. Lewis (1998b) 4. and Missoula Valleys in western Montana: Montana Bureau of Mines T2N 2. Lonn and Sears (2001) and Geology Open-File Report 46, 160 p. 3. Lewis (1998a) Ruppel, E.T., O'Neill, J.M., and Lopez, D.A., 1993, Geologic map of the 4. Lonn and Berg (1999) Dillon 1° x 2° quadrangle, Montana and Idaho, United States Geological 5. Berg and Lonn (1999) Survey Miscellaneous Geologic Investigation 1803-H, scale 1:250,000. T2N 6. Ruppel and others (1993)93) Smith, L.N., 2006, Hydrogeologic framework of the Lolo-Bitterroot area, 7. Wells (1989) Mineral, Missoula, and Ravalli counties, Montana (open file version): 6. Montana Bureau of Mines and Geology Ground-Water Assessment Atlas 5. 4B, Map 2, scale 1:250,000. Wells, K.J., 1989, Digital filtering and modeling of the gravity field of the Bitterroot Valley, western Montana: [unpubl. M.S. thesis] University of Montana, Missoula, 86 p. R22W R21W R20W R19W R18W R17W 114° Figure 3. Index to previous maps from which contacts were Figure 1. Altitude of bedrock in the southern part of the Lolo- compiled. Bitterroot Area.