(By U. M. Sahinen, Montana Bureau of Mines and Geology, and C. E. Erdmann, A. E. Weissenborn, and P. L. Weis, U.S. Geological Survey)
Montana is separable physiographically into three distinct, roughly parallel, northwestward-trending regions. Each comprises about one-third of the State, and the distinctive character of each is strongly influenced by the stratigraphy and structure of the underlying rocks The distribution of these rocks is shown in figure 3 (Perry, 1962). This has been generalized from a map that has been published by the U.S. Geological Survey in cooperation with the Montana Bureau of Mines and Geology (1955). (A newer, colored version is shown below as figure 3.)
The western third of Montana forms the eastern part of the Northern Rocky Mountains (Fenneman, 1931, pl. I, province l9; ch. V, pp. 183-224). This province is characterized by deeply dissected mountain uplands, separated by intermontane basins. These mountains have been carved by erosion from rocks that have been uplifted and in many places faulted and folded. Alden (1953) has recently described their surface features and glacial sculpturing. The southern half of this western mountain region has been invaded extensively by granitoid igneous rocks, the two principal bodies being the Idaho batholith south of Missoula and the Boulder batholith between Helena and Butte (Billingsley, 1916) (fig. 4). NOTE: Figure 4 is an oversized image which Internet Explorer may display in a low-resolution mode. To view the full image, move the cursor over the image and click the enlarger button which appears in the lower right corner. Volcanic activity in this region has resulted in lava flows, and beds of agglomerate and tuff, as well as small intrusive bodies.
The eastern border of the Northern Rocky Mountains is marked by a 10- to 30-mile-wide strip, known as the Disturbed Belt (fig. 5), that is distinguished by severe deformation in consequence of overthrust faulting. The topographic transition from the bold Rocky Mountain Front to the Great Plains Province, or Missouri Plateau (Fenneman, 1931, pl. I, provinces 13a, 13b; pp. 61-66), takes place over a short distance across the Disturbed Belt.
Most of the processes that contributed to the formation of the majestic western ranges also have been active in the Great Plains province of Montana, the principal differences being that the major components of diastrophism have been mostly vertical rather than tangential and were somewhat later in time. The result is a series of isolated mountain ranges that break the monotony of the seemingly endless plains. Some are formed as a result of block faulting by the intrusion of igneous stocks or laccolithic masses; still others are the remnants of volcanic piles. Some are simply broad welts without much topographic relief, but all are separated by gently inclined beds.
The eastern third of the State is devoid of mountains and, except along the west flank of the Cedar Creek anticline south of Glendive, the strata are nearly flat-lying (fig. 5). The surface features of the middle and eastern parts of Montana have been described in detail by Alden (1932).
The Precambrian rocks of Montana may be divided into two units, an older pre-Belt unit of metamorphic and intrusive rocks and a thick overlying unit of sedimentary rocks known as the Belt series.
PRE-BELT METAMORPHIC AND INTRUSIVE ROCKS
The rocks of this unit are exceedingly old and constitute the so-called basement complex ("Archeozoic" on Figure 3). Their principal areas of exposure in Montana are in the southern part of the State between Dillon and Livingston and between Livingston and Red Lodge. Other exposures of smaller extent occur in the Little Belt Mountains around Neihart and in the core of the Little Rocky Mountains south of Harlem. As shown by scattered drill holes, this ancient terrane also underlies much of the Plains area at depth.
Several groups of these highly metamorphosed rocks are known, the best exposed being the Pony and Cherry Creek groups and the Stillwater complex. The first two consist of marine sediments which were complexly folded, metamorphosed, and intruded by diabasic, gabbroic, and granitic igneous rocks before the deposition of the sedimentary rocks of the overlying Belt series (Tansley, et al., 1933; Heinrich and Rabbitt, 1960; Reid, 1957). The Stillwater complex of ultrabasic igneous rocks is exposed over a wide area in Park, Stillwater, and Sweet Grass Counties. Radioactive age measurements on three pre-Beltian rocks indicated ages of 1,690 million and 2,540 million years for these rocks (Hayden and Wehrenberg, 1959, pp. 1778-79).
Much of northwest Montana is occupied by rocks of the Belt series (Ross, 1959; Ross, in press; "Proterozoic" on Figure 3; these rocks are now referred to as the "Belt supergroup"), a sedimentary sequence of shallow-water marine origin 35,000 to 50,000 feet thick, that rests with great unconformity on the crystalline rocks below, and is in turn overlain by beds of Middle Cambrian age.
in Glacier National Park are made up of Belt rocks.
Regional low-grade metamorphism has altered the original sedimentary rocks--sandstone, silty shale, and carbonates--to quartzite, argillite, and impure dolomite. They are divided stratigraphically into four units which are, from oldest to youngest: the pre-Ravalli rocks, the Ravalli, the Piegan, and the Missoula groups (Ross, 1959, p. 17). Sills, dikes, and lava flows of Precambrian Age are present locally. On the basis of radioactivity measurements of two specimens of pitchblende from the Sunshine Mine, Coeur d'Alene district, Idaho, which is in strata correlative with the Ravalli group of Montana, Eckelmann and Kulp (1957, pp. 1129, 1130) concluded that the uranium mineralization occurred about 1,190 million years ago. The rocks that enclose the uranium minerals must therefore be still older (Wallace and others, 1960, p. 25).
Strata of Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian age comprise the Paleozoic era, which in Montana had an original thickness of approximately 10,000 feet. Rocks of this era, which is estimated to have had a duration of about 335 million years, have been deposited extensively over Montana (Sloss, 1950, pp. 423-451). Today, except for small isolated remnants, Paleozoic rocks have been completely removed by erosion west of longitude 113°40', or the meridian through a point about 16 miles east of Missoula. East of this line Paleozoic rocks are exposed in upturned belts along the Rocky Mountain front, surround the cores of the larger mountain masses of southwest Montana as well as some of those out on the Plains, and generally are in the subsurface wherever younger rocks are present.
This distribution has been outlined on a series of preliminary paleogeographic maps by Perry (1962, pp. 23-27); and the stratigraphic nomenclature of the various series, groups, and formations is given in further detail for nine separate areas in a chart which is included here through the courtesy of the Montana Oil and Gas Commission (fig. 6). NOTE: All references to Figure 6 will take you to an index to specific stratigraphic charts, taken from the U.S. Geological Survey's 1995 National Assessment of U.S. Oil & Gas Resources. The stratigraphic columns of figure 6 may display in Internet Explorer in small, low-resolution mode. To see the full image, move the cursor over the image and click the expander button that appears in the lower right. The stratigraphic names of the various formations in this chart may not in every case follow the customary usage of the U.S. Geological Survey. Practically all Paleozoic units shown are of marine origin. Dolomite and limestone are the preponderant rock types, although shale, siltstone, sandstone, and evaporites (gypsum, anhydrite, and salt) also are present. The vertical shading in the chart represents strata that are absent either because of nondeposition or because of subsequent uplift and erosion. Such breaks represent unconformities of various kinds, some of which are regional in extent. These unconformities are in places of economic significance because of the influence they may have had on the migration and accumulation of petroleum and natural gas.
Sedimentary formations of Mesozoic age with an aggregate thickness of about 5,00O feet crop out over about 55 percent of the area of Montana, chiefly in the central and eastern part of the State, where they have been brought to the surface on the northern end of the Black Hills uplift and the elongate Cedar Creek anticline. In western Montana limited exposures occur in belts adjacent to Paleozoic rocks in scattered mountainous areas. Deposition was continuous during most of the Jurassic and Cretaceous Periods, but in some areas much or even all of the Triassic strata is missing, the absence of these beds marking a great unconformity that separates the Mesozoic from the Paleozoic system over much of Montana (fig. 6).
The Mesozoic era, which had a duration of about 155 million years, is characterized by the deposition of both continental (terrestrial) and marine rocks. The Upper Cretaceous rocks in particular consist of a sequence of thick alternating wedges of marine and continental strata. Shale is the most abundant Mesozoic rock of marine origin, with sandstone next. Marine limestone is conspicuous in the Jurassic system. The terrestrial strata are chiefly mudstone, siltstone, and sandstone, with minor fresh-water limestone.
Rounded, brown-weathering boulders of granite
give the Boulder Batholith its name.
Tertiary system.--Throughout the Cenozoic, which is estimated to have had a duration of 60 or 65 millions, streams and rivers were reinvigorated by the Late Cretaceous-early Tertiary orogeny of the Northern Rocky Mountains. Vertical uplifts or tilting that may still be in progress swept great floods of rock waste over the Eastern Plains region of Montana and into deep structural basins and valleys within the mountains in the western part of the State. The original aggregate thickness of this continental detritus is difficult to estimate because of more or less continuous erosion and redeposition. Along the mountain front, however, and in some intermontane valleys, the order of thickness may have been as much as 4,000 to 6,000 feet, thinning to 3,500 to 4,000 feet in eastern Montana. Now more or less consolidated, these sediments comprise the Tertiary system which is subdivided into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. At no one place in Montana, however, was deposition continuous. Except for a few formational units of early Paleocene age which have been inserted to mark the top of the Mesozoic, the Tertiary system has been omitted from the correlation chart (fig. 6) for thus far no indigenous oil or gas has been found in rocks of that age in Montana.
Rocks of Paleocene age are distributed widely over the eastern Montana Plains (fig. 3) where they comprise the Fort Union formation which, in ascending order, consists of the Tullock, Lebo, and Tongue River members. The Wasatch formation of Eocene age consists of soft, variegated mudstone with minor beds of stream conglomerate, but has been almost completely stripped off by erosion. Its former extent, however, is indicated by remnants preserved in small down-faulted blocks in the vicinity of the Bearpaw Mountains. These remnants have been of immense assistance in the geologic dating of the volcanic rocks of that detached uplift.
Rocks of Oligocene and Miocene age are rare on the Eastern Plains, occurring only as small, thin outliers on the highest major stream divides, or as caps on isolated buttes. On the other hand, in western Montana thick sections of Oligocene and Miocene rocks, commonly referred to as "lakebeds," are well preserved in the fill of the intermontane valleys. Each is more or less a distinct unit, depending upon local environment. The valley sediments of western Montana, therefore, are highly variable sequences that involve mudstone, siltstone, and claystone, much of which may be tuffaceous, yet also contain thin interbeds of bentonite or crystal tuff, and more rarely, layers of diatomaceous earth. Other rock types prevalent are units of fissile bituminous shale, thin seams of impure coal, thick accumulations of stream conglomerate, and fine- to coarse-textured fanglomerate. Rocks of Paleocene age seem to be absent, and beds of proved Eocene age are uncommon. The youngest Tertiary formation on the Eastern Plains is the Flaxville formation, which probably is of Miocene or Pliocene age, but which may range from late Miocene into earliest Pleistocene. Most of the deposits are remnants of thin sheets of quartzite stream gravel derived from the Belt series that rest on high-level erosion surfaces or pediments. In the vicinity of the mountain ranges on the Plains, however, the Flaxville formation consists largely of local material from the uplift.
Pleistocene series.--At least twice during the Pleistocene epoch, which is considered to have had a duration of about 1 million years, the northern part of the Missouri Plateau (Fenneman, 1931, pl. 1, province 13a) has been the terminal area of continental ice sheets. Earlier ice sheets may also have extended into extreme northeastern Montana. Western Montana likewise has undergone extensive glaciation both from continental and from Alpine glaciers. Glacial deposits abound where the country formerly was covered by ice, and they vary in origin from simple boulder-clay or till in the ground moraine, which is most prevalent, through terminal moraines, kames, and eskers formed by melt water at ice contacts, to fluvioglacial outwash aprons and glacial lakebed clay and silt. The spectacular alpine scenery of western Montana is the result of glaciation, and the stream pattern, waterpower potential, and suitability of the land for agriculture--all have been profoundly affected by glacial activity.
Alluvium.--Alluvium consists of mud, sand, silt, gravel, reworked soil, or other detrital deposition by running water during Recent geologic time. Extensive accumulations make the flood plains along streams, where further concentration and sorting have formed low-level deposits of gravel and sand.
Geologic structures in Montana can be considered in the same threefold areal subdivisions as were topography and stratigraphy.
In eastern Montana structures, like topography, are subdued. Layered rocks in the plains area are flat-lying or dip gently as a result of broad, gentle flexures that form simple structures of large size. Major features, such as the Williston Basin and the Cedar Creek anticline ((fig. 5)) have a total structural relief of only a few hundred feet, yet extend for many tens of miles across the countryside. Many such structures are so broad and gentle that they can only be recognized by careful geologic mapping over large areas. Faults are few and most are of relatively minor displacement. For hundreds of millions of years, no violent structural deformations occurred in the Montana Plains.
Central Montana has structures that are closely correlative with its topography. The area is predominantly one of gently dipping rocks, which underlie the plains. In a number of places, however, the rocks are domed steeply over isolated mountain ranges of considerable size. Topographic and structural relief in these ranges are both measured in thousands of feet. Some of the ranges, as the Big Snowy Range, formed by vertical uplift alone, and are surrounded by steeply tilted, but otherwise comparatively undeformed rocks. Elsewhere, as in the Bearpaw and Pryor Mountains, faulting has complicated the structural picture. Some of the ranges were domed by igneous activity: the Little Rocky and Crazy Mountains are examples of mountains with laccolithic cores. In those, a variety of complex structures exist, in response to both overall folding and local disruption of the preexisting rocks by the intrusives. Faults are more numerous and of greater displacement in the plains of central Montana than they are farther east; they, like the folds, represent a transition zone between the east and west.
The Disturbed Belt marks the eastern edge of the complex and intensely deformed western third of the State. The belt itself is a complicated system of thrust faults, in places with compound thrust plates piled on top of one another. Elsewhere, as on the Lewis overthrust at the east edge of Glacier National Park, it is a single thrust sheet with at least 40 miles of horizontal displacement (Ross, 1959, p. 102). West of the Disturbed Belt are a system of linear mountain ranges and large intervening structural basins that show a great variety of structural features. Folds range from broad and open to steep, overturned, and to complex multiple systems formed through successive periods of movement in different directions. They range in size from great arches many miles across to intricate contortions on a microscopic scale. Faults are equally abundant and varied. Thrusts, normal faults, reverse faults, and strike-slip faults are known, and like the folds, they show a wide range in size and displacement. Some of the range front faults in southwestern Montana show evidence of comparatively recent movement, and the 1959 earthquake in Madison Valley indicates continued structural activity in at least part of the area through the present.
Unlike individual structures in central and eastern Montana, some of the structures in western Montana are segments of features that are continental or subcontinental in scope. The Disturbed Belt is a part of a structural system that marks the front of the Rocky Mountains for hundreds of miles north into Canada. The Lewis and Clark line cuts across parts of three States, and has been traced west, into eastern Washington. One segment of the line is represented by the Osburn Fault, which in western Montana, has a horizontal displacement of at least 12 miles (Wallace and others, 1960).
Igneous activity has had a marked effect on the structures of the area. Intrusives range in size from the Idaho batholith, one of the largest in North America, to small dikes, sills, plugs, and related intrusive bodies, some only a few inches in length. Associated structures are equally varied. They include profound deformation of surrounding rocks in places, and in some instances large masses of igneous rocks appear to have acted as buttresses that resisted deformation caused by later rock movements.
The nature of the structures of Montana, their variety and distribution, have an important bearing on the fuel and mineral resources of the State. Complex structures in areas that also contain igneous rocks are a favorable environment for mineralization, particularly for metallic minerals. Areas containing thick deposits of Paleozoic and Mesozoic sedimentary rocks that have been gently folded and domed are favorable for the discovery of oil and gas. Thus structure, like stratigraphy, must be understood in order to properly understand the distribution of natural resources in Montana.
The three parts into which Montana can be divided differ markedly in the mineral resources which are found therein. These differences are directly related to differences in the geology and structure of the underlying rocks which have been discussed in the preceding pages.
The metalliferous ore deposits of the State--and especially those of gold, silver, copper, lead, zinc, and tungsten--in most cases are closely associated with igneous intrusive rocks of intermediate to acidic composition, particularly those that were intruded in late Mesozoic or early Tertiary time. The ore deposits are found both in the intrusive and the invaded rocks, and most are related to fractures or other types of deformation. The host rocks may be of any age from Precambrian to Tertiary and may be of either sedimentary, igneous, or metamorphic origin. The sedimentary or volcanic rocks that were formed after the intrusive rocks were emplaced (and that in places cover them) contain few workable deposits.
Intrusive granitic bodies of late Cretaceous-Early Tertiary age such as the Idaho batholith, the Boulder batholith, and the Tobacco Root batholith are prevalent in the mountainous western third of the State (fig. 4). The concentration of metalliferous ore deposits in western Montana in and around these bodies and smaller satellite stocks is very striking (see maps in chapters on "Gold"; "Silver, Zinc, and Lead"; and "Tungsten") and accounts for the great mineral productivity of this part of the State.
In southwestern Montana deposits of talc, corundum, iron ore, graphite, sillimanite, and kyanite are found in Precambrian rocks of the Cherry Creek Group, and large deposits of chromite are found in the Precambrian Stillwater Complex in Stillwater and Sweet Grass Counties. In the western third of Montana sedimentary rocks of Paleozoic and Mesozoic age are actual or potential sources of phosphate rock, limestone, silica, crushed and dimension stone, clays, and other industrial minerals. Some bentonite has been mined from highly altered beds of volcanic ash in Tertiary sediments in intermontane basins. Oil and gas have not been found to date in this part of the State.
In central Montana metalliferous ore deposits have been mined in the Little Rocky Mountains, the North Moccasin Mountains, the Judith Mountains, the Little Belt Mountains, and other isolated ranges. As in western Montana, the deposits almost invariably are closely associated with granitic intrusive rocks of late Cretaceous or early Tertiary age.
Central Montana, however, is better known for the production of petroleum and natural gas. Such structures as the Cat Creek anticline, the Kevin-Sunburst dome, the Sweet Grass arch, and many others have been highly productive. In central Montana, Mesozoic sandstones are important reservoir rocks for the accumulation of petroleum and natural gas. Some Paleozoic formations have also been productive. Some Cretaceous sandstone, for example the third Cat Creek sand at the base of the Kootenai formation, the Virgelle sandstone member of the Eagle sandstone, and the Fox Hill sandstone, are valuable ground water aquifers--an important asset in a semiarid region. The Kootenai formation also in places provides clay suitable for brick and tile. Coal deposits underlie much of central Montana, most of the coal in the area being either in beds of Jurassic or late Cretaceous age.
No metalliferous ore deposits are present in the eastern third of the State, but this area contains over 90 percent of Montana's extensive coal reserves. The coal is in the Fort Union formation of Paleocene age and is especially widespread in the uppermost or Tongue River member. The eastern region also produces petroleum, principally from the west end of the highly productive Williston Basin and from the Cedar Creek anticline. In this area most of the petroleum production is from Paleozoic rocks, although some gas is derived from Cretaceous formations. Bentonite is mined from beds of Cretaceous age in Carter County.
In summary, in western Montana igneous, sedimentary, and metamorphic rocks are present, and geologic structures are complex. The region is dominantly a metalliferous province but has large resources of many nonmetallic minerals. In central Montana, igneous anti metamorphic rocks are much less abundant. Geologic structures are simple to complex. The resources of the area are chiefly petroleum, natural gas, coal, and some metals. The variety of nonmetallic resources is less than in western Montana. Eastern Montana is a sedimentary terrane. Geologic structures are simple and subdued. Mineral resources are chiefly petroleum, coal, and natural gas. Nonmetallic resources are chiefly clay, bentonite, and sand and gravel.
The above is a highly generalized and very incomplete description of Montana's resources of minerals and fuels, but it shows how different these resources are from one part of the State to another and how closely they are related to the geology and structure of the underlying rocks. A more detailed summary of the resources on a commodity-by-commodity basis is presented in the ensuing chapters.
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----- 1953, Physiography and glacial geology of western Montana and adjacent areas: U.S. Geol. Survey Prof. Paper 231, 200 p.
Billingsley, Paul, 1916, The Boulder batholith of Montana: Am. Inst. Mining Engineers Trans., v. 51, pp. 31-56.
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Ross, C. P., The classification and character of the Belt Series in northwest Montana: U.S. Geol. Survey Prof. Paper 346 (in press).
----- 1959, Geology of Glacier National Park and the Flathead region, northwestern Montana: U.S. Geol. Survey Prof. Paper 296, 125 p.
Ross, C. P., Andrews, D. A., and Witkind, I. J., 1955, Geologic map of Montana, scale 1:500,000, 2 sheets: U.S. Geol. Survey.
Sloss, L. L., 1950, Paleozoic sedimentation in Montana area: Am. Inst. Petroleum Geologists Bull., v. 34, no. 3, March 1950. pp. 423-451.
Reid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root Mountains, Madison County, Montana: Montana Bur. Mines and Geology Mem. 36, 25 p.
Tansley, Wilfred, Schafer, P. A., and Hart, L. H., 1933, A geological reconnaissance of the Tobacco Root Mountains, Madison County, Mont.: Montana Bur. Mines and Geology Mem. 9, 57 p.
Wallace, R. E., Griggs, A. B., Campbell, A B., and Hobbs, S. W., 1900, Tectonic setting of the Coeur d'Alene district, Idaho: U.S. Geol. Survey Prof. Paper 400-B, pp. 25-27.