Montana Bureau of Mines and Geology Montana Ground-Water Assessment Atlas 7, Part B Map 3 A Department of Montana Tech of The University of Montana Open-File Version September 2008 111° R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E. R. 6 E. R. 7 E. 2592 Altitude of the top of the Madison Group in part of 2600

2579 2621 2658 Cascade County, Montana

by 2528 2668 2535 2649 2600 2633 240 Larry N. Smith 00 2649 0 26 2642 Morony Dam Belt Creek 2 Author's Note: This map is part of the Montana Bureau of Mines and Geology (MBMG) to the confluence of the Missouri River with Sand Coulee Creek. At a few locations where T. 21 N. 500 2738 2736 2292 Ground-Water Assessment Atlas for the Cascade-Teton Area ground-water characterization. data are sufficiently closely spaced, tributary canyons can be recognized. The most prominent 2657 It is intended to stand alone and describe a single hydrogeologic aspect of the study area, tributary canyon is that of the paleo-Sun River where it entered the Missouri from the northwest 2700 2700 although many of the area's hydrogeologic features are interrelated. For an integrated view in T. 20 N., R. 3 E., sections 25 and 36 (figs. 1, 5). of the hydrogeology of the Cascade-Teton Area the reader is referred to Part A (descriptive Outside of the Missouri River paleochannel, the top of the Madison beneath Jurassic and 2515 T. 21 N. overview) and Part B (maps) of the Montana Ground-Water Assessment Atlas 7. Cretaceous rocks is very irregular between the Great Falls International Airport and the Big 2732 2675 Bend, southwest of Great Falls (fig. 1). Local relief of 50–75 feet over distances of one-quarter 2800 2675 INTRODUCTION to one-half a mile is apparent in this area of dense well control. Data on the structure of the Missouri River 2468 Morrison Formation dark shale interval in the Great Falls area are insufficient to show detail The Mississippian Madison Group is an important and productive aquifer in Cascade in the structure above the Madison (fig. 6). The local relief most likely reflects paleotopography 2760 County and the Great Falls area. This map shows the altitude above sea level of the top of the on the erosional unconformity that defines the top of the Madison Group. 2720 Madison beneath younger sedimentary rocks and unconsolidated surficial sediments (fig. 1). Folding, faulting, and fracturing of the Madison Group and overlying rocks influence Giant Springs 2427 2865 2818 The altitude of the top of the Madison was estimated from well logs and outcrops. The Madison ground-water flow and development. A steepened northward dip of the Madison in the 2599 is the oldest rock unit and deepest aquifer used in the Cascade-Teton Area, except in the small northeastern Great Falls area, shown by the contour spacing near an altitude of 2,950 feet on 2857 areas where older rocks crop out in the Little Belt Mountains (figs. 1, 2). the map (fig. 1) may be an east-west trending normal fault. These fractures are transverse to 2900 Development of ground-water resources in the Madison aquifer began in the early 1900s, secondary folds mapped in the area, but are roughly parallel to N. 70 ° W. fractures at Giant 290 2918 0 but increased significantly during the 1990s as land near Great Falls was subdivided and Springs. Water is interpreted to travel along these fractures from the Madison through the 2900 2901 2965 developed for housing reliant on domestic wells. Feltis (1980) mapped the regional structure Kootenai Formation to Giant Springs (Lemke and Maughan, 1977).

2909 10 0 29 0

2816 G 0 9 2935 K 2910 290 of the Madison previously. Wilke (1983) mapped the subsurface altitude of the top of the 2900 2920 2 2916 2923 2950 2920 Madison in the 15 townships centered on Great Falls (fig. 4) using an unknown number of Sand Coulee and north flank of the Little Belt Mountain areas 2940 2960 2849 2693 2880 2958 water-well logs available at that time (the points were not shown on her map). The Madison 0 2980 3000 290 water wells drilled since the 1980s provided data necessary for refining these older maps. The area south of Great Falls and the Missouri River paleochannel is drained by north- 2991 00 3000 29 2 2 8 7 2775 00 GrGreateat Falls 3013 0 0 flowing streams from the Little Belt Mountains. well-log data from near the town of Sand 29 3114 0 0 2950 Geology Coulee show that Sand Coulee Creek and possibly Spring Coulee and other tributaries, were 2964 2960 incised to the Madison Group; Glacial Lake Great Falls deposits eventually buried the canyons. 2964 2967 0 3066 In Cascade County, the Madison Group is composed of two formations. The upper Mission The contours on figure 1 between the Sand Coulee Creek and Spring Coulee drainages show 47° 30' 0 2965 2971 3050 3050 8 2946 2 2977 3045 Canyon is mostly massive to thick-bedded, fossiliferous limestone with thin chert interbeds, a northeast-plunging anticline-syncline pair in T. 19 N., R. 4 E. This structure is also apparent 2695 and the underlying Lodgepole Limestone is a thin-bedded fossiliferous limestone and subordinate in the outcrop patterns of the overlying Jurassic and Cretaceous units shown on Vuke (2000), 3050 0 2907 2953 5 mudstone (fig. 3). Within the study area, an erosional unconformity affected total Madison and is therefore related to structural folding and not erosion. Additionally, the overlying coal 2770 2923 9 31 2620 2 0 3010 0 0 2583 300 Group thickness; thicknesses range from about 1,200 to 1,700 feet. Solution breccias of beds in the Morrison Formation are folded along the syncline near Sand Coulee Creek (fig. 00 47° 30' 2775 3 9 0 2 limestone and some sandstone clasts in a matrix of reddish shale and residual soils are common 6) and the syncline is not coincident with the creek valley (fig. 1). Therefore, this irregularity 0 3079 3108 0 0 3028 0 near the top of the formation. In central Montana, solution breccias, soils, and caverns developed in the Madison surface, shown by the 3,200-foot contour, is probably not the result of incision. 0 3021 0 2 5 0 30 7 2 5 3095 3087 0 T. 20 N. 3055 0 3100 2820 0 during erosion of the Madison rocks as early as the Mississippian (Petty, 2006), and periodically However, erosional removal of Jurassic rocks above the Madison in the stream channel near 3055 3035 3038 3 3160 3 3045 3054 0 0 2825 3042 15 3180 0 into the Jurassic (Mudge, 1972; Geraghty and Sears, 2004) when the region was subaerially Centerville, as shown by wells and the 3,300- and 3,400-foot contours, shows that some of 3005 3048 3100 3 0 2755 T. 20 N. 3013 3055 3064 3079 3104 3131 3136 exposed. Most caverns at the top of the Madison were filled with sediment when deposition the irregularities in the Madison surface were due to paleochannel incision by the Missouri 3038 3084 3171 3120 28 3 3030 3060 3014 3080 0 3038 3069 3160 0 0 3087 resumed. River and its tributaries before deposition of the glacial sediments. 0 3067 3073 3061 3068 3084 3065 3170 3148 3140 29 0 3066 3094 2938 00 3100 3140 00 The Sweetgrass Arch is a broad northwest-plunging anticline that extends from the Little On the north flank of the Little Belt Mountains, between the Smith River and Box Elder 3045 3040 3063 3090 3125 3104 31 3010 3050 3151 3105 3080 3090 3150 Belt Mountains, through Great Falls, and north into Canada (Perry, 1928; Lorenz, 1982). The Creek (T. 18–19 N., R. 2–5 E.) there are some irregular northeast and east-northeast structural 3076 3102 3099 3151 2916 2984 2990 3073 3094 3097 3115 3152 3152 2613 2975 2984 3077 3 3135 arch controls the general subsurface position of the Madison Group and other strata. The trends in the top of the Madison (fig. 1). These structures are coincident with some faults and 3025 3047 3100 3102 3119 3164 15 3120 3155 3014 3102 3077 0 3 3042 3068 0 3060 2883 3008 3015 3010 3117 2950 previous structure maps show the Sweetgrass Arch with nearly evenly spaced, smooth structure folds in the overlying Kootenai Formation (fig. 1; Vuke, 2000) and the Morrison coal interval 3015 3 3105 0 3200 2998 3016 05 0 2790 0 3085 3145 contours on the top of the Madison (Feltis, 1980; Wilke, 1983). (fig. 6). The sparse well-log data suggest locally faulted anticlines and synclines are continuous 2884 3045 3078 31 2770 2992 3030 3176 5 3175 2841 3030 3010 3047 3050 3082 0 310 Rocks of the Big Snowy, Amsden, and Ellis Groups overlie the Madison. These rocks along a trend of about N. 75º E. from about the Smith River to a few miles east of Belt. This 3068 3064 3 3100 3266 0 2891 2972 3020 3072 3100 0 3125 3000 3101 3102 3069 0 0 thicken southwestward towards the Little Belt Mountains, and easterly and westerly away tectonism must be Cretaceous or younger in age. 2980 3070 3080 0 00 3010 3070 3074 3000 3 3 3268 Belt Creek 3015 3127 10 3025 30903030 00 3110 0 from the Great Falls area. Jurassic erosion caused thickness changes across the arch. On the 3095 3042 3089 1 3122 3100 2900 3197 3210 3256 3276 3081 3135 3 3058 3135 3165 Box Elder Creek 3005 3090 3056 3199 axis of the arch, between Great Falls and Belt, all but the Swift Formation of the Ellis Group Belt area 2990 3062 3165 3132 2970 3175 3020 3084 2986 3122 2979 32 2991 3035 3135 3086 3252 0 3240 2810 3027 3012 3060 2997 3148 3100 0 3270 3206 is absent and the Swift is generally less than 20 feet thick. The Morrison Formation overlies 3124 3250 3202 3200 3189 0 3055 3050 3080 3115 3081 3081 2985 2997 0 2964 3000 00 3093 31433127,3115 300 3195 3272 the Swift, or the Madison where the Swift is absent (fig. 3). Cretaceous rocks of the Kootenai The top of the Madison Group near the city of Belt and along Belt Creek is at a lower 3 3020 3035 3070 3121 3116 3157 29653214 3212 3090 3082 Sand Coulee Creek 3195 3193 3214 3005 3185 3091 3070 3168 3103 3153 3147 3140 3000 3177 3140 3062 3084 3120 3146 3121 3007 and Blackleaf Formations also are gently folded by the Sweetgrass Arch, showing that the altitude than most adjacent areas (fig. 1). A number of wells near Belt indicate a northeast 2765 3119 3156 3137 31 3200

3045 3139 3187 2982 3152 3066 00 0 3172

3 3119 3141 3212 3204 3110 structure had a long history of activity that continued through the Cretaceous (Payenberg, trending depression in the bedrock that is not coincident with the trend of Belt Creek. The 3110 0 3094 0 3102 3152 3165 2983 3105 0 3228 5 3050 3128 0 2974 3157 31653154 3124 3192 3174 0 3260

0 3110 3142 3 3020 31613162 3180 1 3230 3015 3116 3100 3144 3088 3130 3160 3 2003; Payenberg and others, 2003). southern and southwestern part of the depression is poorly defined by well data. The northeastern 2605 3012 3121 3150 3160 3077 3070 3052 3063 3124 31653069 3167 3200 3040 3143 3124 3201 3171 Quaternary stream and glacial deposits locally overlie the Madison on the crest of the part of the depression, near Belt Butte, is near a small structural depression in T. 19 N., R. 7 3040 3077 3117 3128 3115 3140 3166 3170 330 3053 3130 3140 3160 3042 3239 3282 0 0 3039 3121 3148 3 3205 3226 0 3019 3035 3095 3140 3150 3250 3225 3224 33 Sweetgrass Arch. Prior to Pleistocene glaciation; the Missouri River flowed through an E., section 19 (fig. 1). Vuke, Berg, and others (2002) interpreted this depression as a collapse 3124 3225 0 3175 2900 3143 3115 0 3345 32 2890 3016 3045 3153 3113 3240 0 3171 3223 3185 0 3156 3025 3186 0 entrenched canyon, which is now lower Sand Coulee Creek valley. The trace of the pre-glacial structure where Cretaceous rocks collapsed into a void formed by dissolution of the Madison. 2913 3050 3202 3217 3231 3200 3024 3150 3132 3222 3050 channel extends through the Fields area to near the location of Morony Dam (figs. 2, 5; Fisher, Kravits (1981) recognized thickening and thinning of coal intervals in the upper Morrison 0 3138 3150 3138 3155 3237 3215 3205 3208 K 0 3125 3190 3228 3301 G 9 3104 3137 3155 3320 2 2908 2960 3138 3176 3190 3222 3244 3222 3282 1909; Alden, 1932). The advance of the Laurentide ice sheet during the most recent Pleistocene Formation between the town of Belt and the Smith River. He attributed localized subsidence 3006 3207 3206 3210 3233 3191 2951 3213 3229 3228 3230 3280 3292 3213 3035 3049 3105 3174 3208 3244 glaciation dammed the Missouri River forming Glacial Lake Great Falls. Lake sediment back- of the Madison Group in collapse structures due to karstic erosion as a factor in controlling 200 3215 3243 2937 2930 3000 3042 3110 3 3162 3212 3265 3205 3214 3208 0 filled the Missouri River valley and its tributaries, including the pre-glacial canyons cut into the distribution of coal depositional environments. Based on this previous work and the 3039 0 0 0 33 3296 G 3308 50 3027 3266 3240 2 3200 00 30 5 3288 0 G 3 1 3103 3026 1 3185 3218 3225 3282 Spring Coulee 3324 the Madison. Deglaciation and lake drainage resulted in rearrangement of the stream courses alignment of the east-northeast trending depression near Belt with the numerous structural 3 3229 3150 3030 3 3242 3303 3225 3312 K 3210 3322 to their present locations. As part of their report on the engineering and subsurface geology features between Belt Creek and the Smith River, the Belt-area depression may be related to 3031 3200 3220 3406 G 3326 3300 3290 3090 3140 3179 3217 31703195 3255 3253 3335 3318 3393 3 3188 of the Great Falls area, Lemke and Maughan (1977) mapped the thickness of Quaternary both structural deformation and collapse of karst features. The structure of the upper Morrison 3094 3105 3093 3190 200 3300 4 3350 3359 3 3264 3408 0 3293

3200 3 3093 3344 0 3263 3230 3325 0 deposits from boreholes drilled for that project and examined water-well logs. Their map Formation in this area shows the regional form of the Sweetgrass Arch with secondary structural 3124 3010 3289 3298 3238 3311 0 2 310

0 0 3047 3257 3298 3362 3237 3213 3 3220 3264 3258 3297 3 0 shows the meandering paleochannel of the Missouri River south and east of Great Falls irregularities (fig. 6). For instance, southwest of Belt, there is a structural high in the Morrison, 3245 3300 3304 0 3138 3167 3332 3360 3288 3350 340 3260 3270 3409 K 3290 3300 G 3234 3285 3369 represented on figures 1 and 5. defined by the 3,700-foot contour (fig. 6), that is coincident with structure in the Madison (fig. K 3 3246 3267 3310 00 3210 3305 3 31 0 3311 3286 0 The locations of coal exploration boreholes and coal outcrops in the upper Morrison 1). However, the basin in the Madison east and southeast of Belt (fig. 1) is not evident at the Smith River 3230 0 400 Sand Coulee 3295 0 33 3 3296 3312 3410 3130 3336 3269 T. 19 N. Formation in the central part of the map area (Kravits, 1981; Daniel and others, 1992) are Morrison coal interval (fig. 6). These geologic relationships suggest that some downwarping T. 19 N. 3125 3389 3418 3285 3161 Big Bend 0 3017 3412 3337 3419 shown in figure 6. The structure of the coal and laterally equivalent dark shale interval was in Jurassic rocks were a result solution collapse in the underlying Madison Group, and that 0 3152 0 3218 3255 1 3302 0 3420 3405 0 0 3255 3343 34 0 200 3 1 3329 0 3422 3355 3 used to understand stratigraphic relationships between the Madison and overlying formations. sedimentation at and below the coal interval filled in a basin. Basin features recognized at land 3 3216 3 3263 3273 3 3399 3405 3410 3187 3430 3403 3400 3176 surface in Cretaceous rocks east of Belt (fig. 1), suggest that local areas of karst subsidence 0 3002 3238 3246 3320 3435 0 0 3315 3348 3392 0 8 3200 3375 3435 3197 Ground water in the Madison Group continued into, or beyond, the Cretaceous (Vuke, Berg, and others, 7 2 3376 3332 2 3330 3372 3445 3402 3290 3222 3370 2002). 3050 3372 3325 0 3380 3320 0 3337 0 3314 Belt 32 b Water from the Madison aquifer is used for self-supplied and community domestic, stock, 0 0 3425 9 0 3322 3350 3105 2 0 3403 0 3430 3277 3 40 00 industrial, and minor irrigation purposes in the Great Falls area and south to the Little Belt SUMMARY 3397 3 Sand Coulee Creek 34 3462 3361 3340 3355 3301 Mountains. Ground water is produced from the Madison only where it is fractured or where 3550 3300 0 G solution cavities have developed; typically, the unfractured limestone is not sufficiently The altitude of the Madison Group top in Cascade County is highly variable, making 340 3338 3350 3284 00 3 3409 G 3 permeable to be considered an aquifer. The altitude of the subsurface and exposed Madison prediction of drilling depths more problematic in some areas than would be inferred from 3 500 3275 3 0 3497 3 3070 0 3430 3360 3354 Group is important to water users in Cascade County because that determines drilling depths earlier maps. The improved detail on this map should help people contemplating development 3270 3400 b 3585 K 3435 for wells. Most wells provide adequate amounts of water by penetrating a few tens of feet into of ground water from the Madison make more informed decisions. The variability is a result 3430 33033325 3560 the Madison; however, some wells on the flanks of the Little Belt Mountains must penetrate of structural deformation (folding and faulting) and a long history of erosion of the Madison. 3293 3558 K 3360 many hundreds of feet into the unit where the upper part of the Madison is undersaturated. The main structural feature is the Sweetgrass Arch—a broad anticline that extends north- Missouri River 3310 3388 3320 3299 3363 3350 Ground water in the Madison flows generally north and discharges to shallower formations northwest from the northern flank of the Little Belt Mountains, through the city of Great Falls, 0 3295 K Mm 60 3365 3450 0 3540 3 3394 3310 G 3359 or surface water, such as at Giant Springs where it flows in fractures through the overlying and into southern Canada. East-northeast/west-southwest oriented folds and faults disrupt the 0 3365 6 3420 3490 3 3385 3317 Kootenai Formation. Madison Group between Great Falls and the northern flank of the Little Belt Mountains. 3700 This map may be useful to individuals who must estimate drilling depths and completion Solution collapse from karstic erosion in the upper Madison is evident near the town of Belt. levels for water wells and as an aid in planning new subdivisions or other developments reliant The timing of the erosion and collapse began prior to and during Jurassic erosion and deposition G 35 0 2 0 6 3675 3421 on Madison Group ground water. Depths to the top of the Madison can be calculated by in the area, and possibly may have continued during and after the Cretaceous. 0 3308 3540 0 K K G subtracting the contoured altitude portrayed on the map from the land-surface altitude from The greatest local relief developed on the Madison Group was from stream erosion during 2 3405 5 a topographic map. The map also shows areas where the unit is exposed. Wells completed in pre-glacial, likely Pleistocene, incision by the Missouri River along a now buried paleochannel. 0 0 the Madison reportedly yield an average of 30 gallons per minute (gpm), but maximum reported The Missouri River eroded through the overlying Cretaceous and Jurassic strata, forming a 3660 00 3673 38 3660 3 yields exceed 500 gpm. narrow canyon in the Madison Group along what is now the lower reach of Sand Coulee 3720 4 360 3700 0 0 0 3693 Creek, Gibson Flats, Gerber, and Johnson Flats (Fisher, 1909; Alden, 1932; Lemke and 3764 DATA SOURCES AND MAP CONSTRUCTION Maughan, 1977). As the Pleistocene continental glacier advanced into the area, the Missouri 3

7 3

0 3712 3 0 was dammed forming Glacial Lake Great Falls. Lake sediments backfilled the Missouri River 0 3750 G G 0 Data used for contouring the altitude of the Madison Group bedrock surface came from valley and its tributaries. Deglaciation and lake drainage caused rearrangement of the lower 0 0 3666 water-well logs, wireline and lithologic logs of hydrocarbon exploration boreholes, and surface stream courses to their present locations. 5 K 3 G G 3275 geologic maps. Of the nearly 5,100 water wells with lithologic logs in Cascade County, about 3698 K 750 were interpreted to intersect the top of the Madison Group. Five logs from water wells ACKNOWLEDGMENTS 3602 K K 0 3595 T. 18 N. 360 K in Chouteau and Judith Basin Counties added control points north and east of Cascade County; K G G T. 18 N. there are no reported water wells in Teton County that encounter the Madison Group. All of This work was supported by the Ground-Water Characterization Program at the Montana K K the known penetrations of the Madison Group with confirmable locations were used in Bureau of Mines and Geology. The map and text were improved due to reviews by John 3629 0 G 40 contouring. Well locations were verified by field visits (139 wells) or by comparison of well- LaFave, Susan Vuke, John Metesh, Phyllis Hargrave, Tom Patton, Ed Deal, Susan Barth, and 3 3494 b and land-ownership records; some well locations were confirmed by talking to the well owners. Luke Buckley. G K 3500 Additionally, logs of wells that were drilled to aquifers above the Madison were used to 3653 constrain the maximum altitude of the Madison. Wireline or sample logs of oil and gas

0 exploration wells were used to map the regional structure of the Madison Group in the map REFERENCES 3500 0 K K 7 3600 3 area (30 logs) and in the region (340 logs). All locations, reported altitudes, and tops of 3657 formations were checked for consistency. Alden, W.C., 1932, Physiography and glacial geology of eastern Montana and adjacent areas: A structure map of the Jurassic coal or dark shale interval near the top of the Morrison U.S. Geological Survey Professional Paper 174, 133 p. 3 7 0 0 Formation (fig. 3) helps explain the timing of deformation (folding and faulting) for units Berg, R.B., 2002, Geologic map of the Valier 30' x 60' quadrangle: Montana Bureau of Mines 3970 below this horizon (fig. 6). Comparison of the structure portrayed on figure 1 with that shown and Geology Open-File Report 453, 10 p., scale 1:100,000. 3743 on figure 6 indicates whether folding or faulting occurred before or after deposition of the Berg, R.B., 2008, Geologic map of the Choteau 30' x 60' quadrangle, north central Montana: 3600 2 Jurassic Morrison coals in any given area. Figure 6 was constructed by contouring data from Montana Bureau of Mines and Geology Open-File Report 571, 14 p., scale 1:100,000. 5

0 0 3864 120 water-well logs, 17 other borehole logs, 44 coal exploration boreholes, and 177 surface Berg, R.B., and Vuke, S.M., 2002, Geologic map of the Fort Benton 30' x 60' quadrangle: locations of coal outcrops taken from the National Coal Resources Data System (NCRDS) Montana Bureau of Mines and Geology Open-File Report 460, 7 p., scale 1:100,000.

2 coal database (Kravits, 1981, and Daniel and others, 1992). Daniel, J.A., Bartholomew, M.J., and Murray, R.C., 1992, Geological characteristics of the 4 3500 3 0 4000 3290 8 0 0 0 0 All depth-to-formation values derived from well-log data were converted to altitudes above Stockett bed coal in the central Great Falls coal field, Montana: Montana Bureau of Mines 6 0 3 0 sea level by subtracting the depth values from land-surface altitudes. Land-surface altitudes and Geology Special Publication 102, p. 145–157. 70 K 3700 3 G at oil- and gas-well locations were taken from the logs themselves. For visited water-well logs, Feltis, R.D., 1980, Map showing configuration of the top of the Madison Group, Great Falls 0 0 0 4 80 land surface altitudes were obtained from the 1:24,000-scale U.S. Geological Survey topographic 1-degree by 2-degree quadrangle, north-central Montana: Montana Bureau of Mines and 3282 3 3 0 maps. Altitudes for the remaining unvisited wells were obtained from 1:24,000 topographic Geology Geologic Map 10, scale 1:250,000. 90 K 3281 3 0 Belt Creek maps for their inferred locations. The altitudes of the upper contact of Madison Group at Fisher, C.A., 1909, Geology and water resources of the Great Falls region, Montana: U.S. 0 1 3 3900 2736 0 00 outcrops mapped on 1:100,000-scale geologic maps were picked from topographic maps to Geological Survey Water-Supply Paper 221, 89 p. 0 0 3 4 K 3 aid contouring of the Madison. Faults and folds expressed in the surface formations in the area Geraghty, E.M., and Sears, J.W., 2004, Middle Jurassic forebulge migration, northern Montana 00 G K 38 south of Great Falls (fig. 1) were used to constrain contours in small areas. Because of Rockies: evidence from karstified and sand-filled extensional fracture sets: Geological 2 T imprecision in the well-log data, contours were typically drawn only where multiple data points Society of America Abstracts with Programs, v. 36, p. 208. 3 2660 K 0 0 K supported the contour placement. Contours were drawn by hand and then digitized. Kravits, C.M., 1981, West Belt prospect, Cascade County, Montana, geology and coal reserves: 2660 T 4000 The contour interval of figure 1 is 100 feet. However, because data are unevenly distributed unpublished preliminary report to Western Energy Company, Butte, 132 p. 0 390 across the map, a few supplementary contours could be drawn in areas with exceptional well Lemke, R.W., and Maughan, E.K., 1977, Engineering geology of the city of Great Falls and 0 3160 0 control, and a 500-foot contour interval was used south of Great Falls. Where well data poorly vicinity, Montana: U.S. Geological Survey Miscellaneous Investigations Map I-1025, scale 2 3 T. 17 N. 000 constrain the position of contours, dashed (approximate) contours are used. 1:24,000. 4 T. 17 N. Water-well driller logs and well locations are stored in the Ground-Water Information Lopez, D.A., 2001, Geologic map of the Conrad 30' x 60' quadrangle, north-central Montana: 00 45 Center database at the MBMG. Coal-, oil-, and gas-well log data are available through the Montana Bureau of Mines and Geology Open-File Report 444, 4 p., scale 1:100,000. MBMG. The outcrop areas of the bedrock were simplified from digital versions of published Lorenz, J.C., 1982, Lithospheric flexure and the history of the Sweetgrass Arch northwestern 3865 K 4 3867 geologic maps (fig. 4). Montana, in R.B. Powers, ed., Geologic Studies of the Cordilleran Thrust Belt: Rocky 5 K 00 3940 Mountain Association of Geologists, Denver, Colorado, p. 77–89. DISCUSSION Mudge, M.R., 1972, Pre-Quaternary rocks in the Sun River Canyon area, northwestern Montana: 2 2 0 U.S. Geological Survey Professional Paper 663-A, 142 p. 0 Mm G Regional Configuration of the Bedrock Surface Mudge, M.R., and Earhart, R.L., 1983, Geologic and structure maps of the Choteau 1º x 2º G quadrangle, western Montana: U.S. Geological Survey Miscellaneous Geologic Investigation 5 000 4318 The general configuration of the Madison top is a broad, north-northwest plunging anticline 1300, scale 1:250,000. Mm 4312 0 that begins with a broadly arching outcrop expression along the northern flank of the Little Payenberg, T.H.D., 2003, Evidence for the influence of the Sweetgrass Arch on sedimentation G K 450 Belt Mountains. South of Great Falls, the western flank of the anticline trends north-south and of the Late Cretaceous Eagle Formation, northern Montana, based on a high-resolution 0 K 500 Mm dips west. East and north of Belt, the Madison dips to the northeast. Near Great Falls, the arch allostratigraphic framework: Annual Meeting Expanded Abstracts - American Association K is slightly asymmetric with limbs dipping about 2 degrees southwest and about 1 degree of Petroleum Geologists, v. 12, p. 134. R. 1 E. R. 2 E. R. 3 E. R. 4 E. R. 5 E. R. 6 E. R. 7 E. northeast. Between these flanks, the crest of the broad anticline is irregular because of the Payenberg, T.H.D., Braman, D.R., and Miall, A.D., 2003, Depositional environments and 111° Base information is from scanned versions of USGS 7½- Figure 1. Altitude of the top of the Madison Group in parts of Cascade, Mississippian and Jurassic erosional unconformity and the erosion during the Pleistocene (fig. stratigraphic architecture of the late Cretaceous Milk River and Eagle Formations, southern minute quadrangles provided by the Montana Natural Chouteau, and Judith Basin Counties; map location and extent is shown on 1). In the northern part of the map, the more regular, northwest-plunging anticlinal form of Alberta and north-central Montana; relationships to shallow biogenic gas: Bulletin of Resource Information System http://nris.mt.gov/ Era Period Epoch Lithology Figure 4. Geologic mapping in the Cascade- figures 2 and 6. the Sweetgrass Arch is evident. Canadian Petroleum Geology, v. 51, p. 155–176. 111° Perry, E.S., 1928, The Kevin-Sunburst and other oil and gas fields of the Sweetgrass Arch: 111° 30' Teton study area recently completed at a scale Outline of map in figure 1 Berg (2002) Lopez (2001) Great Falls and Missouri River areas Montana Bureau of Mines and Geology Memoir 1, 41 p. 110 of 1:100,000 is shown by the colored tiles. The 48° Petty, D.M., 2006, Stratigraphic development of the Madison paleokarst in the southern 48° extent of figure 1 is shown by the uncolored 111° 30' The altitude of the Madison top in areas near and south of Great Falls is broadly anticlinal, Williston Basin area: The Mountain Geologist, v. 43, p. 263–281. Kootenai Fm central area. Wilke's (1983) subsurface map with minor secondary folding, and is highly irregular in detail. Abundant well data between Reynolds, M.W., and Brandt, T.R., 2005, Geologic map of the Canyon Ferry Dam 30' x 60' exent lies within bounds of figure 1. Great Falls and the Big Bend of the Missouri River clearly show the complex bedrock surface quadrangle, west-central Montana: U.S. Geological Survey Scientific Investigations Map 48° Muddy Creek 120 48° Sunburst Sandstone Mbr (figs. 1, 2). Secondary folding is evident in northeastern Great Falls area where a north-to- 2860, scale 1:100,000. Choteau Vuke, Colton, and others Berg and Vuke FLATHEAD CO. FLATHEAD Early Berg (2008) northeast plunging syncline-anticline pair is mapped near Giant Springs (fig. 1). Reynolds, M.W., and Brandt, T.R., 2007, Preliminary geologic map of the White Sulphur Million years ago (Ma) (2002) (2002) Box Elder Creek Teton River Cutbank Sandstone Mbr 111° Missouri River The most irregular area on the top of the Madison Group is the paleochannel of the Missouri Springs 30' x 60' quadrangle, Montana: U.S. Geological Survey Open-File Report 2006- 130 coal and black shale Sun River River incised into the top of the Madison Group beginning 4–5 miles southwest of Great Falls, 1329, scale 1:100,000. Morrison Fm upstream of the Big Bend, extending up Sand Coulee Creek through Gibson and Johnson Flats Vuke, S.M., 2000, Geologic map of the Great Falls South 30' x 60' quadrangle, central Montana:

Teton River CHOUTEAU CO. Cretaceous Wilke (1983) Figure 1 and then trending northeast toward Morony Dam (figs. 2, 5). The channel is best defined in Montana Bureau of Mines and Geology Open-File Report 407, scale 1:100,000. 15 ? about 80 well logs that show Quaternary deposits directly overlying Madison Group limestone Vuke, S.M., Berg, R.B., Colton, R.B., and O'Brien, H.E., 2002, Geologic map of the Belt 30' , 140 47° 30' 47° 30' 47° 30' Great Falls unconformity Great Falls (fig. 1, red dots). At these locations, the overlying Jurassic and Cretaceous sedimentary rocks x 60' quadrangle, central Montana: Montana Bureau of Mines and Geology Open-File 111° ? 47° 30' were eroded along the pre-glacial Missouri River and its tributaries, exposing the Madison Report 450, 16 p., 2 sheets, scale 1:100,000. SweetgrassTETTETONON Arch CO. Characterization Morrison Fm study area boundary 320 Group. Quaternary alluvial and lake sediments were later deposited in the canyon covering Vuke, S.M., Colton, R.B., and Fullerton, D.S., 2002, Geologic map of the Great Falls North CASCADE CO. 0 287 150 the Madison and extending onto adjacent eroded Swift, Morrison, and Kootenai Formations. 30' x 60' quadrangle: Montana Bureau of Mines and Geology Open-File Report 459, 10 / /87 Late /89 Figure 1 The well-log data show that the base of the Missouri River paleochannel decreases in p., scale 1:100,000. 33 Morony Dam Belt Creek Mudge and Earhart (1983) 00 altitude downstream along the subaerial channel towards the northeast. The sediment-filled Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., 2007, Geologic map of Montana: Swift Fm Belt Creek er Sun Riv canyon is generally about one-quarter to one-half mile wide, and has a buried local relief of Montana Bureau of Mines and Geology Geologic Map 62 , 73 p., 2 sheets, scale 1:500,000. Great Falls 160 3400 0 0 about 300 feet; at its narrowest, it is only about one-eighth mile wide with a buried local relief Wilke, K.R., 1983, Appraisal of water in bedrock aquifers, northern Cascade County, Montana: 47° 30' Fields Rierdon Fm 111° 30' Vuke (2000) Vuke, Berg, and 0 0 21 200 47° 30' Middle 3 34 35 Sand Coulee Ellis Gp others 3 00 of 200–250 feet. The maximum thickness of fill recognized in well logs was 495 feet near the Montana Bureau of Mines and Geology Memoir 54, 22 p., 2 sheets. Centerville Jurassic Piper Fm Sawtooth Fm (2002) downstream end. Quaternary sediments are commonly 300–350 feet thick from the Big Bend

S 47° a Belt 47° n G 3 LEWIS AND CLARK CO. d break and change in scale Sand Coulee Creek 60 C Belt Butte 111° 0 Big Bend Spring Coulee K Explanation o u 300 l unconformity Reynolds and Brandt Missouri River e Creek 87 N Spring Coulee e Box Elder / Reynolds and Brandt (2005) (2007) C Missouri River 0 Contour—shows altitude (ft) of the top of the Madison Group, dashed

r 0 3700

15 e 3300

e 5 , k Belt Creek 0 3 where approximate; contour interval 100 ft (50 ft and 500 ft in some Smith River 0 3700 4 /89 3 areas) Pennsylvanian Alaska Morony Dam Coal or dark shale 320 Tyler Fm Bench Fm Gibson Flats Quaternary alluvial, glacial-lake, and landslide deposits Group 20 miles Heath Fm Amsden Belt Creek altitude data 111° 30' Big 3 4312 Altitude (ft) from hydrocarbon-well log Otter Fm Snowy Sun River 70 Kibbey Fm Group Sun River Coal borehole 900 0 Carboniferous Great Falls 3

JUDITH BASIN CO. 0 ppaleochannel 80 3 3421 Altitude (ft) from water-well log aleochannel Water well 3 90 Mission Canyon Fm Johnson Flats 0 0 47° 340 0 380 Quaternary glacial till Mississippian Gp 7 4 0 47° Lodgepole Fm (A) Missouri River Coal outcrop 3 0 Little Belt Mountains Madison 0 Madison Group top eroded during the Quaternary and subsequently paleochannel 0 3421 111° 0 buried by Glacial Lake Great Falls deposits iver 0 PALEOZOIC MESOZOIC R Altitude of coal interval 6 N ri 3 MEAGHER CO. u 3700 Tertiary, Cretaceous, and Jurassic rocks and Mississippian Nonmarine Marine o (feet asl) G Anticline, large arrow shows plunge s Smith River 42 Big Snowy Group Conglomerate Sandstone, conglomerate is 00 4100 K Syncline, large arrow shows plunge Sandstone Siltstone, mudstone, claystone, M Big Bend Belt Township line 4300 Figure 2. Index map of the Cascade-Teton shale Siltstone, mudstone, claystone, shale Main stream Ground-Water Characterization Area showing Carbonate 00 T Monocline, arrows show dip 0 (B) Area of structural 0 0 42 Mm Madison Group rocks cited principal cities, drainages, and Study area boundary 0 0 00 41 00 Carbonate disruption 80 90 4 3 N physiographic locations. 3 3 4 Structural depression Figure 5. Simplified map showing irregularities b Figure 3. Stratigraphic section from the Madison Group to Belt Creek the Kootenai Formation. Big Snowy, Amsden, and Ellis Group on the subsurface Madison Group top. (A) (C) Area of Scale 1:75,000 Fault, bar and ball on downthrown side rocks are discontinuous across the Sweetgrass Arch. Modified location of the buried pre-glacial course of Sweetgrass Arch solution & collapse 0 5 10 miles from Vuke and others (2007). the Missouri River; (B) location of areas of 20 miles structural disruption; and (C) possible solution- collapse feature near Belt. Locations of creeks Figure 6. Altitude on top of coal or equivalent dark shale interval in the Projection: Montana State Plane and populated places referenced in the text Little Belt Mountains 111° are shown. upper Morrison Formation. Datum NAD83