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Volcanism and Plutonism at Shallow Crustal Levels: The Elkhorn Mountains Volcanics and the Boulder Batholith, Southwestern Montana part of MT Field Guides
The Upper Cretaceous Elkhorn Mountains Volcanics (EMV) and Boulder batholith of southwestern Montana provide an example of a large-volume, epizonal, volcanic-plutonic complex whose deep level of erosion has exposed the cogenetic intrusive rocks while preserving sizeable portions of the volcanic field. Such a volcanic-plutonic association provides a unique opportunity for evaluation of many aspects of the evolution of a shallow-crustal magmatic system, such as geochemical relations of both the volcanic and plutonic rocks and the nature of intrusive-extrusive relationships at the present level of exposure.

Topics: Structures, Igneous rocks, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Early Proterozoic Geology of the Highland Mountains, Southwestern Montana, and Field Guide to the Basement Rocks that Compose the Highland Mountain Gneiss Dome part of MT Field Guides
The Highland Mountains are underlain by the largest of the northwesternmost exposures of basement crystalline rocks in southwestern Montana....Metasedimentary rocks in the Highland Mountains are in part lithologically similar to the Late Archean multilithologic sequence (in the Tobacco Root, Ruby, and the northern Madison and Gravelly Ranges), but are considerably thinner. In the Highland Mountains the individual beds of aluminous schist, marble, quartzite, and iron-formation extend for only short distances, but the assemblage as a whole is mappable. It nowhere exceeds 300 ft (100 m) in thickness and appears to pinch out to the north....these rocks in the Highlands, unlike those to the southeast, are overlain by more than 10,000 ft (3000 m) of aluminous biotite gneiss that may have been deposited as muds basinward from the Late Archean shelf edge.

Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Road Log from Bozeman to Specimen Creek via Gallatin Canyon and U.S. 191 part of MT Field Guides
This trip leads southward through the Gallatin Range to Specimen Creek in Yellowstone National Park. It affords an excellent worm's-eye view of the structure and stratigraphy of this range as revealed both laterally and vertically through the quietly beautiful Gallatin Canyon. The route also borders the east margin of the Madison Range and its spectacular Spanish Peaks uplift. The Gallatin and Madison Ranges are geologically and topographically similar, an essential difference being the thick cap of andesitic lava and breccia that covers the high parts of the Gallatin Range.

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

Field Guide to an Archean Transect, Eastern Beartooth Mountains, Montana-Wyoming part of MT Field Guides
The eastern Beartooth Mountains of Montana and Wyoming contain a record of crustal evolution that spans almost 1000 Ma and culminates in a major episode of crustal growth 2700-2800 Ma. The earlier record is sparse and complex as a result of extensive magmatism and intense metamorphism associated with Late Archean activity. In general, however, it appears that continental material was present in this area by roughly 3600 Ma, and that a stable continental shelf accumulated quartzites, iron-formation, and lesser amounts of pelitic to psammitic units interspersed with small volumes of mafic to silicic volcanic rocks. This cycle of accumulation was apparently terminated by an episode of granulite facies metamorphism 3300-3400 Ma, perhaps as a result of continent-continent collision....About 2800-2900 Ma, a second major cycle of crustal growth began that bears some resemblance to those associated with modern continent-ocean subduction zones....The first igneous rocks produced during this cycle were andesitic or dioritic rocks, both coarse and fine grained, that were subsequently metamorphosed to amphibolite facies....This interval is restricted by the presence of a granodioritic series (Long Lake granodiorite) that was intruded late in the kinematic cycle... (2779 Ma)...and marks the lower limit for the last major episode of regional metamorphism. The last and most volumetrically important rock produced during the cycle was the Long Lake granite. This unit composes approximately 80-90% of the eastern portion of the range and engulfs all older rock types. It appears to have been intruded about 2740 Ma....This excursion will attempt to view the evidence of these two major cycles in four main stops: (1) Hellroaring Plateau, (2) Lower Quad Creek, (3) Upper Quad Creek, and (4) Long Lake. Late Archean mafic dikes (at Beartooth Lake) will also be examined.

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

Field Guide; Belt Butte and Tiger Butte part of MT Field Guides
This field trip extends east from Great Falls across rolling glacial plains to the summit of Belt Butte, where its formation and the collapse structure adjacent to it will be discussed. In addition, the Tiger Butte laccolithic intrusion, contact metamorphic effects, associated dikes, and structure resulting from the intrusion will be examined.

Topics: Surficial geology, Hazards, Resources, Fossils, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Field Guide; Little Rocky Mountains part of MT Field Guides
This fieldtrip examines deformed Paleozoic sedimentary rocks on the margins of the Little Rocky Mountains, Tertiary intrusive rocks (porphyrys, magmatic-hydrothermal breccias, and dikes), and associated mineralization.

Topics: Structures, Resources, Fossils, Metamorphic rocks, Igneous rocks, Sedimentary rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

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

A Traverse Across the Eastern Belt Basin From Neihart to Townsend, Montana part of MT Field Guides
This road log focuses on Precambrian (Proterozoic) sedimentary rock deposited in the Helena embayment of the eastern Belt basin. It also includes a general description of the geology between exposures of Proterozoic strata, briefly describing occurrences of Precambrian igneous and metamorphic rocks, Paleozoic sedimentary rocks, and Tertiary igneous rocks. A stratigraphic column is provided for refeference.

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

The Yellowstone Plateau-Island Park Region part of MT Field Guides
The Yellowstone Plateau, at the center of one of the Earth's largest volcanic fields, spans the continental divide between the Northern and Middle Rocky Mountains at an average elevation of about 2.400 m. The eruptions of the Yellowstone Plateau volcanic field, entirely postdating 2.5 Ma, were exceedingly voluminous but are only the surficial expression of the emplacement of a batholithic volume of rhyolitic magma to high crustal levels. Although the latest eruptions were about 70,000 years ago, an immense hydrothermal system and a variety of geophysical characteristics indicate the continued presence of an active shallow magma chamber.

Topics: Structures, Surficial geology, Igneous rocks, Sedimentary rocks
Geographic Location: Yellowstone National Park
Geologic Province: Yellowstone Plateau

Heart Mountain Detachment Fault and Clastic Dikes of Fault Breccia, and Heart Mountain Break-Away Fault, Wyoming and Montana part of MT Field Guides
This field guide describes the geology of two sites near Silver Gate, MT (near Cooke City) where different features related to the Heart Mountain detachment are displayed. At the first site, several features pertaining to the origin of the Heart Mountain detachment fault can be examined, including: (1) severely deformed upper plate rocks in contact with undeformed lower plate rocks; (2) the character and composition of the fault breccia; (3) contacts of volcanic rocks with upper plate blocks and the Heart Mountain fault; and (4) dikes of carbonate fault-breccia injected into both upper plate blocks and overlying volcanic rocks. The second site is an exposure of the Heart Mountain break-away fault.

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

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