The Montana Investigations Project produces modern detailed intermediate- and large-scale geologic maps for an area of about 12,600 km2 in west-central Montana, extending from the capital city of Helena east to near Harlowton, and from Neihart and Utica on the north to Martinsdale, Wilsall, and Clarkston on the south (fig. 1). The area encompasses three 1:100,000 quadrangles, including the Canyon Ferry Dam, White Sulphur Springs, and Townsend quadrangles, as well as parts of adjacent quadrangles on the south and west. In addition, the project is producing 1:24,000-scale maps for areas of complex geologic structure in the Big Belt (fig. 2) and Little Belt Mountains and areas at the apparent surface terminus of the Lewis and Clark tectonic zone.
Figure 1. Index map showing location of Montana Geologic Investigations Project and selected place names referred to in text. Project area shown in green; principal valleys shaded orange.
Figure 2. Geologic map of the northwest corner of the Canyon Ferry Dam 30' x 60' quadrangle, Montana (Reynolds and Brandt, 2005) showing the complex geology of the Montana disturbed belt terminating on the east against weakly deformed Paleozoic rocks of the continental crust and on the south along the trace of the Lewis and Clark tectonic zone. See the original published map for an explanation of the units. (Click for larger image of map detail)
The project provides new information on the stratigraphy, structure, and geologic history of this geologically complex area. Data are interpreted for exploration of natural resources, for interpretation of geologic factors related to groundwater and surface water availability, for land use and land management purposes, and for scenic recreational purposes. Geologic maps resulting from project work have been incorporated in the new Montana State geologic map, published by the Montana Bureau of Mines and Geology (2007).
A wide variety of geologic units and geologic structures are present across the extended area of the project and are represented on the geologic maps (fig. 3). Rocks range in age from Neoarchean [about 2,600-2,800 Ma (million years)] and Paleoproterozoic (about 1,870 to 1,790 Ma) to Pliocene (about 2 Ma). (Rocks of only the Ordovician, Silurian, and Triassic systems are absent.) Sedimentary deposits of Quaternary and Pleistocene age (younger than about 1.8 Ma), including sand, gravel, talus, glacial and landslide debris, overlie the bedrock. Metamorphic rocks of Neoarchean and Paleoproterozoic age are exposed in the northwestern part of the Little Belt Mountains, where they are overlain locally by a truncated sequence of Mesoproterozoic sedimentary rocks (about 1,600 to 1,250 Ma). These Mesoproterozoic rocks, as thick as about 7,500 m, belong to the Belt Supergroup and are widely exposed in the Big Belt Mountains and on the south flank and locally in the core of the Little Belt Mountains. Paleozoic and Mesozoic sedimentary rocks rest unconformably across the folded and faulted older rocks. Basal and uppermost Paleozoic rocks are mainly mudstone, siltstone and sandstone, whereas a thick sequence of limestone and dolostone forms the middle Paleozoic succession (figs. 3, 4, and 5). Stratified Mesozoic rocks, present on the west and north flanks of the Big Belt and Little Belt Mountains are sandstone, mudstone, volcaniclastic strata, and some extrusive igneous flows. Along the west edge of the project area, the sedimentary rocks are intruded by mainly Cretaceous granitic rocks of the Boulder batholith and related plutons (about 76 to 66 Ma); the northwest part is intruded by igneous rocks of the Upper Cretaceous Adel Mountains volcanic field (76-73 Ma), and the northern and central parts are intruded by Eocene-age (about 44-47 Ma) igneous rocks in small plutons, laccoliths, and sills. Younger Tertiary sedimentary rocks accumulated in fault-controlled valleys across the area.
| Figure 3. Generalized sequences of rocks exposed across the Montana Investigations Project area. Vertical ruling in columns denotes intervals in which the rock record is absent. Granitic rocks of the Boulder batholith and related plutons (Late Cretaceous) and Eocene igneous rocks of the Little Belt and Castle Mountains intrude rocks that range in age from Late Cretaceous to Mesoproterozoic and locally Paleoproterozoic. Thicknesses are diagrammatic. Unit colors on this figure approximate map colors on figure 2. | |||
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Figure 4. Disharmonic, stacked recumbent and asymmetric folds in the Avalanche Butte thrust plate, exposed on north side of Beaver Creek, secs. 28-31, T. 13 N., R. 1 W. Cliff of the Upper Devonian Jefferson Formation (Dj) (near canyon bottom, left) and repeated units of the overlying Lower Mississippian Lodgepole Limestone (MI) are overlain by recumbently folded rocks of the Lower Mississippian and Upper Devonian Three Forks Formation (MDt) and the Lodgepole Limestone. Rocks of the recumbently folded Hogback Mountain thrust plate are at center, right edge; bare patches in upper right edge are eroded across siltite and argillite beds of the Mesoproterozoic Greyson Formation (Yg) on upper plate of the Moors Mountain thrust fault, which overrides and truncates both the Hogback Mountain and Avalanche Butte thrust plates below. View is north-northwest from Hogback Mountain toward Willow Mountain (upper left) and Candle Mountain (upper center). Unit labels as on map explanation. In this view, unit 
consists of recumbently folded Upper and Middle Cambrian rocks, undivided.
Figure 5. Mississippian carbonate rocks form the carapice of the Little Belt Mountains. View is west up the Middle Fork of the Judith River.
Geologic structures contrast markedly across the project area. In the northwest corner (fig. 2) the Montana disturbed belt, a zone of folding and thrust faulting that extends north into Canada, terminates by detachment along an interval of evaporate deposits above gently deformed rocks of the continental crust. At the junction of the west-northwest-trending Lewis and Clark tectonic zone and the north-northwest-trending disturbed belt, strata are strongly folded and thrust in complications not otherwise present in the thrust belt to the northwest (fig. 4). The southwest part of the area is characterized by broad folds adjacent to the Boulder batholith and related plutons, and by thrust faults of northward-decreasing structural offset along the southwest flank of the Big Belt Mountains. Exposed across the Little Belt Mountains along the north and northeast part of the project area (fig. 5), is a complex anticlinorium cored by Eocene plutons and sills. The anticlinorium was active recurrently from Neoproterozoic into Tertiary time with the result that rock units of different ages are differentially truncated against either and/or both the north and south flanks of the major structure. Superposed across all the older structures are basin and range normal faults that downdrop valleys between the mountain blocks (fig. 1). These normal faults splay south from the Lewis and Clark tectonic zone to accommodate the differential movement between the extending earth's crust south of the zone and the relatively coherent crust north of the zone. Helena, Townsend, and Boulder River valleys in the western and southwestern part of the area, and the Smith River Valley in the center of the project area result from the crustal extension. Earthquake activity in parts of all these valleys results from continuing tectonic extension of the region.
[Note: 1:100,000-scale geologic maps by Reynolds and Brandt are available in paper and digital formats]
Harlan, S.S., Snee, L.W., Reynolds, M.W., Mehnert, H.H., Schmidt, R.G., Sheriff, S.D., and Irving, A.J., 2005, 40Ar/39Ar and K/Ar geochronology and tectonic significance of the Upper Cretaceous Adel Mountain Volcanics and spatially associated Tertiary igneous rocks, northwestern Montana: U.S. Geological Survey Professional Paper 1696, 29 p.
Reynolds, M.W., 2003, Geologic map of the Hogback Mountain quadrangle, Lewis and Clark Counties, Montana: U.S. Geological Survey Geologic Investigations Map I-2773, Scale 1:24,000.
Reynolds, M.W., compiler, Generalized bedrock geologic map of the Helena area, west-central Montana, scale 1:100,000, in Thamke, J. N., 2000, Hydrology of the Helena area bedrock, west-central Montana, 1993-1998, with a section on Geologic setting and a generalized bedrock geologic map by Mitchell W. Reynolds: U.S. Geological Survey Water-Resources Investigations Report, 00-4212, 119 p.
Reynolds, M.W., and Brandt, T.R., 2005, Geologic map of the Canyon Ferry Dam 30′ x 60′ quadrangle, west-central Montana: U.S. Geological Survey Scientific Investigations Map SIM-2860, scale 1:100,000.
Reynolds, M.W., and Brandt, T.R., 2006, Preliminary geologic map of the Townsend 30′ x 60′ quadrangle, Montana: U.S. Geological Survey Open-file Report 2006-1138, scale 1:100,000.
Reynolds, M.W., and Brandt, T.R., 2006, Preliminary geologic map of the White Sulphur Springs 30′ x 60′ quadrangle, Montana: U.S. Geological Survey Open-File Report 2006-1329, version 1.1 (2007), scale 1:100,000.
Reynolds, M.W., and Hays, W.H., 2003, Geologic map of the Nelson quadrangle, Lewis and Clark County, Montana: U.S. Geological Survey Geologic Investigations Map I-2774, Scale 1:24,000.
Reynolds, M.W., Miggins, D.P., and Snee, L.W., 2002, Age and tectonics of middle Tertiary basaltic volcanism and effects on the landscape of west-central Montana: Geological Society of America Abstracts with Programs, v. 34, no. 6, p. 409.
Snee, L.W., Reynolds, M.W., and Miggins, D.P., 2002, Late Cretaceous and Tertiary plutonism in west-central Montana—Pins in the end of compression and the beginning of extension: Geological Society of America Abstracts with Programs, v.34, no. 6, p. 43.
Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., compilers, 2007, Geologic Map of Montana: Montana Bureau of Mines and Geology, Geologic Map 62, scale 1:500,000.
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