Montana Bureau of Mines and Geology Montana Ground-Water Assessment Atlas 7, Map 3 A Department of Montana Tech of The University of Montana 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 5075 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 Great 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 3042 3045 3054 0 0 2825 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 3080 3060 3038 3069 3014 00 0 3087 3160 resumed. River and its tributaries before deposition of the glacial sediments. 0 3068 3067 3073 3061 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. 1819 N., R. 25 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.