Montana Bureau of Mines and Geology Montana Ground-Water Assessment Atlas No. 2, Part B, Map 7 A Department of Montana Tech of The University of Montana December 2004 Altitude of and Depth to the Bedrock Surface: 24 W 23 W 22 W 21 W 20 W 19 W 114o o Flathead Lake Area, Flathead and Lake Counties, 3 48 30 20 2700 3100 0 3 0 3300 0 0 8 3 2 0 2 00 0 29 3 0 32 N 0 0 0 2900 0 350 3600 0 Montana 2 7 ? 0 0 0 0 3 3500 3200 3 0 3 7 0 0 3 2 0 3 2 5 80 0 0 0 by 2 0 8 00 3 0 3 4 3 0 0 3 0 6 1 3000 3 0 0 5 0 0 0 0 3 3 6 3 2900 0 Larry N. Smith 0 0 0 3000 2 3400 7 0 0 3200 31 N ? 1 Authors Note: This map is part of the Montana Bureau of Mines commonly greater than the nearby topographic relief (Fields and 3000 0 0 400 2800 ? 0 0 2 3100 3 90 3 200 and Geology (MBMG) Ground-Water Assessment Atlas for the others, 1985). 0 Flathead Lake Area ground-water characterization. It is intended to Fault boundaries between basins and uplifted mountain ranges ? 3200 300 0 2 2900 400 0 50 stand alone and describe a single hydrogeologic aspect of the study have not been mapped in detail on published surficial geologic maps, 9 00 0 2 5 2 2 00 2800 1000 0 2700 Whitefish 15 3000 area, although many of the areas hydrogeologic features are except for part of the Mission Fault on the southeastern side of the 00 3000 ? 3 1500 interrelated. For an integrated view of the hydrogeology of the Mission valley (Ostenaa and others, 1990, 1995). Inferred locations 1 0 0 200 3100 Flathead Lake Area the reader is referred to Part A (descriptive of basin-bounding faults in the area (fig. 6) were drawn based on 3100 2 32 2 2 9 overview) and Part B (maps) of the Montana Ground-Water Assessment trends in the structure contours on the bedrock surface and, in part, 2 1 0 3 0 7 8 0 0 8 0 5 0 0 0 3100 2 0 0 0 0 90 0 0 0 0 1 3200 Atlas No. 2. on published maps of Mudge and others (1982) and Harrison and 3 2800 0 1 0 0 0 3 3300 1 0 Columbia Falls 0 others (1986, 1992). The bedrock surface shows N-S- and NW-SE- 3000 3 0 00 0 0 0 4 3 INTRODUCTION trending, structurally controlled basins in the Swan, Flathead, Mission, 0 4 2000 0 3 3 0 1 Hungry Horse 0 0 0 3 2 2 2 0 0 7 2 Jocko, and Little Bitterroot valleys and the Irving Flats and Camas 6 0 0 0 0 0 0 0 2 0 0 9 0 5 2 The main map (fig. 1) shows the altitude above sea level of Prairie Basin areas (fig. 4). 0 0 0 0 0 0 3 the Belt Supergroup bedrock surface beneath unconsolidated surficial 3 1000 1 3100 0 0 sediments (sand, gravel, silt, and clay) and partly consolidated Tertiary 0 0 2600 1 30 N 3 sedimentary rocks. The contours on the bedrock surface show the 3 2 2 8 position of the unconformity on the Belt rocks (the geologic structure) 0 3 0 0 1 0 0 2 0 7 2 2 8 0 and, in places, the erosional shape of the valleys at depth (the 3 0 0 0 0 0 0 2900 0 0 paleogeography). These features offer clues to help unravel the geologic history. The smaller maps of the Kalispell and Mission 2 7 0 3 valley areas (figs. 2 and 3) show the estimated depth below the ground 0 2 0 1 0 5 0 0 surface one would have to drill to encounter the same bedrock surface 3 0 0 2 2 5 3 5 0 0 6 00 2 5 0 37 0 00 0 2 beneath these valleys. The depth to the bedrock surface outside of 0 3 9 1 0 0 0 0 0 0 0 1 0 0 3 0 0 areas in figures 2 and 3 can be calculated by subtracting a value Explanation 3 9 2600 2 3 2600 1 2 0 3 0 0 derived from the map from the land-surface altitude. 0 0 0 0 Fault, dashed where 3 2 2 1500 Development of ground-water resources in fractured bedrock 0 0 6 0 0 inferred; ball and bar on 00 0 30 aquifers has increased during the 1990's as land around the perimeters downthrown side 200 30 2 0 7 0 8 0 of valleys has been subdivided. These maps are intended to help 0 0 2 200 0 0 7 3000 0 estimate the altitude of, or depth to, bedrock in areas where wells Troughs in bedrock; 0 3300 3 400 dashed where 5 00 3 1500 have not yet been drilled and aid in planning new subdivisions or 3 1 discontinuous; arrow 0 0 other developments relying on ground water. 32 29 N 0 0 indicates inferred flow 0 900 90 2 These maps are useful for estimating drilling depths and at what direction 0 3200 0 5 1 levels to complete water wells. In areas where bedrock is present at 2 5 0 0 0 0 0 shallow depths there likely will be a lack of sand and gravel aquifer Outcrop of pre-Tertiary 3 2 7 0 2 rocks, predominantly 0 9 materials and wells must be completed in fractured bedrock. Because 3100 0 0 Proterozoic Belt 2 3600 3300 8 wells typically produce less water from fractured bedrock, than from 700 0 3 3500 0 8 0 Supergroup units 3600 0 alluvium, development of large-capacity wells is typically precluded 3 3 2 7 3 0 8 0 0 9 4 0 0 0 0 1 0 0 0 where bedrock is at shallow depths. The depth of the bedrock surface 0 3 3700 5 beneath confining units, such as till or silt and clay, is important to Mission Fault 0 3400 0 260 know in areas where new wells are being installed because in some 3600 0 2 3 5 6 3 places bedrock is the only available aquifer. 0 0 0 3 0 0 00 0 3 1 39 4 Bedrock units exposed around and presumably present beneath 2300 1 0 0 5 5 1000 0 2400 0 0 370 0 0 0 0 2500 Kalispell0 the valleys belong to the Belt Supergroup (Johns, 1970; Harrison and 3900 1500 2700 others, 1986, 1992). Ground water in Belt Supergroup rocks is 800 3 2 3 3 3 9 3 3600 10 0 0 generally produced from fractures; unfractured rock typically is not 0 0 7 0 0 0 3000 0 2 400 0 34 3 0 0 2 2 very permeable to water. Tertiary sedimentary rocks are not represented 6 0 0 0 0 0 0 0 28 N 0 3 1 0 1 2 3 1500 on these maps, but overlie Belt Supergroup bedrock and are apparently 0 3 0 0 2900 3 3000 8 3 0 3 0 0 7 8 50 0 0 3 0 3 locally extensive in the subsurface. They are exposed at the surface 0 6 8 3100 0 0 2 0 0 1500 32 0 0 0 0 0 5 3 8 0 0 2500 1000 1 north of Hungry Horse; west-northwest of Whitefish, in the upper 3 3 3300 5 1 3 0 3900 0 2 0 1000 3300 2 5 3 0 0 3 3500 9 0 3 50 32 5 0 ends of valleys within the Salish Mountains, south of Big Draw; and 7 3 0 0 0 0 3 4 0 0 0 0 2000 0 0 6 0 0 0 900 3 0 2000 0 3 3 3600 700 in the Jocko Valley (fig. 4; Smith, 2002; Smith and others, 2000). 8 0 0 9 0 0 0 0 0 0 4 9 200 3 3 2 27 Marginal quantities of ground water are produced from local, thin, 9 2 0 0 0 3 3 00 9 0 800 0 8 0 00 7 2 3500 2 0 600 0 3 4 2 0 0 60 sandy and gravelly beds in the Tertiary rocks. 0 0 0 0 3 400 3 6 6 3 7 00 0 50 0 30 2300 2 2 Figure 6. Location of known (Ostenaa and others, 1990, 1995) and 0 0 0 500 0 6 0 0 3 0 2 0 8 2200 2500 1 inferred basin-marginal faults (black), and approximate traces of 00 5 3200 2 0 300 Whitefish Range 0 3200 70 0 3 0 Whitefish Lake Teakettle 0 0 0 trough-shaped scours in bedrock (blue). 7 0 2 0 3 9 0 5 2 3 0 Mountain 0 0 7 2 2 3900 5 2100 0 0 2 3 3 0 8 3900 3600 0 3 0 3 0 8 00 2 0 0 0 0 3 0 34 3100 2 2 Badrock Canyon 0 0 0 3800 3400 0 2900 6 5 3300 0 3 0 2 0 0 9 0 Whitefish Columbia 0 0 Stillwater River valley 0 0 3900 3 Extent of Tertiary Rocks 34 2400 7 Stillwater River valley 3 00 6 2 Falls 3900 2 Hungry Horse 6 1500 00 0 0 0 3 0 0 3000 5 0 2700 0 0 3200 3400 3 0 2300 0 3 00 80 2600 0 35 0 0 Salish Mountains Swan 3 32 0 15 0 00 3 00 3 7 0 1 8 0 3 0 3 3 0 60 Basin-filling sedimentary rocks above Belt Supergroup rocks 8 0 2 2 0 0 2 0 0 8 3 0 0 7 3100 7 0 0 0 27 N 3900 3 2 include a section of clay-rich lower-to-middle Tertiary rocks and 0 3800 7 Kalispell valley 3 3100 0 300 0 3500 0 0 (upper Flathead 350 3 3 more poorly understood middle-to-upper Tertiary sedimentary rocks 0 3 0 0 3 330 2000 8 3400 0 3 0 0 River valley) 0 0 4 3400 0 0 0 0 3 that accumulated in various tectonic basins.