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Middle Proterozoic Belt Supergroup, Western Montana part of MT Field Guides
From Great Falls, to Butte, Helena Glacier National Park and Spokane, this field trip crosses onto the leading edge of the Rocky Mountain thrust belt and proceeds to the stratiform copper-silver deposits in the Revett Formation of the Troy Mine in the main part of the Belt basin. The road log includes sections on the paleontology of the Middle Proterozoic Belt Supergroup, stromatolites of the Belt Supergroup (specific to Glacier National Park), Middle Proterozoic Tectonics of the Belt basin, and a sedimentologic and tectonic interpretation of the Belt Supergroup.

Topics: Surficial geology, Fossils, Metamorphic rocks, Structures, Sedimentary rocks, Igneous rocks
Geographic Location: Northwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Guide to the Chrome Mountain Area part of MT Field Guides
The Chrome Mountain area lies at nearly 10,000-feet elevation in the west-central part of the Stillwater Complex between the main part of the Boulder River and the headwaters of the East Boulder River. At Chrome Mountain, the olivine +/- bronzite +/- chromite cumulates of the Ultramafic Series are relatively well exposed and stratigraphically broadly resemble those at Mountain View. In addition, a distinctive rock type occurs: a fine-grained dunite which appears to have formed as a replacement of the ultramafic cumulates.

Topics: Igneous rocks, Resources, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Gish Mine Area part of MT Field Guides
The Gish mine is located in the Boulder River valley near the west end of the Stillwater Complex. At Gish, the Peridotite zone of the Ultramafic series is exposed, and features of particular interest include an excellent exposure of discordant dunite and a major chromite seam which has been correlated with the G chromitite elsewhere in the complex. Other notable features include: (1) cyclic units which exhibit some lateral variation, and (2) a possible unconformity that separates the series of cyclic units (olivine-bronzite cumulates and bronzite cumulates), which the discordant dunite intrudes, from the major series of olivine cumulates which enclose the main chromite seam.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Stillwater Complex Exposed in the West Fork Area part of MT Field Guides
The parts of the Stillwater Complex exposed along the West Fork of the Stillwater River were the site of early chromite exploration, followed by Cu-Ni-sulfide exploration and, finally, platinum-group-element (PGE) exploration. The best exposures of Troctolite-Anorthosite zone I (TAZ I) anywhere in the complex and the discovery outcrops of the J-M Reef occur in cliffs on the west side of the river and are readily accessible. Parking for these outcrops is immediately beyond the bridge across the West Fork of the Stillwater River at the Absaroka-Beartooth Wilderness boundary. The lower chromite seams in the Peridotite-zone cumulates are well-exposed in the West Fork area but require nearly a full day to visit on foot. These exposures provide the opportunity to trace chromite seams and to study the changes in their internal stratigraphy and chemistry. The West Fork area illustrates quite well that not all members in Peridotite-zone cyclic units are continuous.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Road Log to the Picket Pin Mountain, Chrome Mountain and Contact Mountain Areas part of MT Field Guides
This road log describes the geologic features along the access roads for three additional traverses: Guide to the Picket Pin Mountain Area, Guide to the Chrome Mountain Area, and A Traverse Through the Banded Series in the Contact Mountain Area. "This trip requires nearly 2 hours without stops. Beyond 12.5 miles, the road is rough and steep in places; a four-wheel-drive vehicle is recommended.

Topics: Igneous rocks, Sedimentary rocks, Structures
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

A Traverse Through the Banded Series in the Contact Mountain Area part of MT Field Guides
This traverse through the Banded Series of the Stillwater Complex follows the section described in detail by McCallum and others (1980)....Unusual or particularly interesting features are highlighted by the 22 localities along the traverse. In view of the nearly continuous outcrop, however, there is much of interest to see between localities, and full appreciation of the entire section would require several days. Nonetheless, most of the important rock types can be seen on a 1-day trip.

Topics: Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Geology of the Stillwater Complex Exposed in the Mountain View Area and on the West Side of Stillwater Canyon part of MT Field Guides
The part of the Stillwater Complex exposed on the west side of the Stillwater canyon has been the site of concentrated prospecting and geology activity since Jack Nye and Jimmy and Jonas Hedges found sulfide-rich rocks there in 1883. Readily seen in this area are well-studied and excellent exposures of: (1) most rock types, (2) ore deposits and mineralized occurrences, (3) structural features and (4) stratigraphic and age relations between the metamorphosed sedimentary rocks and the layered ultramafic-mafic stratiform intrusion. In addition, there are good exposures of a quartz monzonitic suite, which intrudes the complex and the metamorphosed sedimentary rocks, and of the Beartooth gneisses. Because the rocks in this area have been deformed by several episodes of faulting, a synopsis of the structural relations is given to clarify the geologic setting. Detailed overviews of the geology specific to the area are presented, and the geology of the more interesting exposures in the area is described in a road log that begins in the Ultramafic series, proceeds through the Basal series, the Banded series, and the quartz monzonite that cuts the complex, and ends in the Beartooth gneisses.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

A Traverse Across the Central Belt Basin From Bowmans Corner, Montana to East Hope, Idaho part of MT Field Guides
This road log highlights a variety of sedimentary rock types and structures along a transect of the Rocky Mountain Fold and Thrust Belt. The log follows Montana Highway 200, which crosses the central part of the Middle Proterozoic Belt basin from the Montana disturbed belt to Missoula, where it turns northwesterly into northern Idaho, traversing rocks of the western and northwestern Belt basin. The road log begins east of the Rocky Mountain front in the disturbed belt underlain by soft, Cretaceous shale and somewhat more resistant sandstone units. Next, the road log passes into the eastern thrust belt where thrust faults bring Belt rocks first over Cretaceous, then over Paleozoic rocks, and finally over Proterozoic rocks farther to the west. At Rogers Pass, the route crosses the Continental Divide and into the Ovando block where Cenozoic listric normal faults form the major structures. The leading edge of the western thrust belt is encountered at Bonner, MT. From Missoula, Highway 200 trends northwestward to northern Idaho and diagonally crosses the western part of the Belt basin. Changes in grain-size and sediment type observable from outcrops along this road log illustrate the evolution of sedimentary transport and facies tracts within the central part of the Belt basin.

Topics: Fossils, Sedimentary rocks, Igneous rocks, Metamorphic rocks, Structures, Surficial geology
Geographic Location: Northwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Field Guide, Little Belt Mountains part of MT Field Guides
This field guide extends from Monarch to Utica across the northeastern portion of the Little Belt Mountains. The trip, via Hughesville, Yogo Peak and the Yogo sapphire mines, provides an overview of Laramide igneous activity in the Little Belt Mountains of central Montana. The mountains were formed as a large anticline in the Late Cretaceous to late Paleocene or earliest Eocene. The forceful intrusions in the Little Belt Mountains by felsic, hence viscous magmas, contrast sharply with the low-viscosity, basic, alkaline extrusives in the Highwood Mountains to the north.

Topics: Fossils, Resources, Surficial geology, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Central Rocky Mountains Foreland Province

A Traverse Across the Northern Belt Basin From East Glacier Park, Montana to Bonners Ferry, Idaho part of MT Field Guides
This field guide examines differences between three segments of the Belt Basin along an east-west transect from East Glacier Park, Montana to Bonners Ferry, Idaho. Each segment is characterized not only by its structural style, but also by the suite of Belt rocks that comprise it. The easternmost segment consists of the Lewis thrust plate and associated thrusts as far west as Columbia Falls, Montana where it is terminated by the Rocky Mountain trench. This segment contains eastern Belt facies including: Altyn, Appekunny, Grinnell and Helena (Siyeh), Snowslip, Shepard, Mount Shields and McNamara formations. Fine sediments in these rocks, probably derived from a western continental source terrane, were deposited adjacent to the stable North American craton, where they were mixed with smaller amounts of coarse sand derived from an inferred coarse sand sheet that mantled the crystalline craton. The central segment extends from the Rocky Mountain trench to the Libby thrust system. It is characterized by broad, open folds that expose Belt rocks of the central part of the basin, including a thick section of Prichard, Burke, Revett, St. Regis, Empire, Wallace and Helena formations. These rocks are composed mostly of argillite, carbonate and fine-grained quartzite. The western segment includes the Libby thrust and Leonia fault system. It extends as far west as the Purcell trench. Exposed in this area are rocks from the Prichard through the Missoula Group. Rocks of the Ravalli Group and the Wallace Formation contain more fine-to medium-grained quartzite than those of the central segment, reflecting a western source. However, the Missoula Group rocks are finer grained and more calcareous than those of the type area, indicating that late in the Middle Proterozoic the western source terrane probably subsided and that the basin center shifted westward.

Topics: Fossils, Igneous rocks, Sedimentary rocks, Metamorphic rocks, Structures, Surficial geology
Geographic Location: Northwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

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