Integrating Research and Education > Montana-Yellowstone Geologic Field Guide Database > Search the Database

Search the Database

Use this page to search our collection of educationally-useful geologic field guides and road logs in Montana and Yellowstone. You may search the database by entering a keyword to search or choosing one of the listed terms for geologic topic, geographic location, or geologic province.


Help

Results 31 - 40 of 54 matches

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

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

Crazy Mountains, Montana part of MT Field Guides
This trip examines a variety of mid-Eocene alkalic (feldspathoidal) stocks, laccoliths, sills, and dikes emplaced into Cretaceous and Paleozoic sedimentary strata. These rocks include mafic and felsic varieties, and are both texturally and compositionally variable. Most of the mafic alkalic rocks are unique in the Montana Alkalic Province in having Na2O > K2O. They are unusual even among feldspathoidal rocks because they are stongly enriched in incompatible elements, and have Sr, Nd, and Pb isotopic compositions which reflect an ancient source having low Rb/Sr, Sm/Nd, and U/Pb.

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

Plutonism at Deep Crustal Levels: The Idaho Batholith, Montana and Idaho part of MT Field Guides
The Idaho batholith field trip traverses a well exposed cross-section of the northern Idaho batholith, briefly examines the broad aspects of this deep-seated granitoid batholith and its regionally metamorphosed country rocks, and considers the role of the synplutonic mafic magmas from the mantle in providing heat for melting of continental crustal rocks to form the more felsic main-phase units of the batholith.

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

Geology of the Highwood Mountains, Montana: A Survey of Magma Types and Sources part of MT Field Guides
The Highwood Mountains represent a dissected Eocene volcanic and intrusive complex composed of latites and highly potassic, mafic to felsic igneous rocks. Along with the Bearpaw Mountains and Eagle Buttes, they form a northeast-trending belt within the Montana Alkalic Province, and were all emplaced between 54 and 50 Ma. In the Highwoods an older suite of quartz-normative latites is overlain and intruded by a younger suite composed mostly of mafic phonolite flows and dikes, minette and trachyte dikes, and shonkinite-mafic syenite stocks. This trip examines a variety of dikes, sills, and stocks composed of these unusual rock types.

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

Shonkin Sag and Square Butte Laccoliths, Montana part of MT Field Guides
Shonkin Sag and Square Butte laccoliths are at the eastern edge of the Highwood Mountains as part of the Central Montana Alkalic Province. Rocks in the Highwood Mountains date to 50-53 Ma. Both laccoliths are internally differentiated and contain a variety of rock types, including shonkinite, syenite, and pegmatite.

Topics: Sedimentary rocks, Igneous rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Bearpaw Mountains, Montana part of MT Field Guides
This trip examines volcanic and shallow intrusive rocks in the Bearpaw Mountains which were emplaced during the period from about 54 to 50 Ma, in the late early Eocene and early middle Eocene. Within this area, wwo volcanic fields lie north and south of a central anticlinal arch in which uppermost Paleozoic and Upper Cretaceous sedimentary rocks and many intrusions are now exposed. The trip follows a route across the northern volcanic field into the core of the central arch to examine the variety of rock types in the Rocky Boy stock and the youngest volcanic rocks on Centennial Mountain (Chippewa Cree Tribe). Land traversed is on Hill County park land and private land; permission is required for access and collection of samples.

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

Missouri Breaks Diatremes, Montana part of MT Field Guides
The Missouri Breaks diatremes are some of the best exposed diatremes in the United States, and display remarkable subsidence features. The diatremes and associated intrusions are important in showing (1) genetic connections of alnoitic, kimberlitic, and carbonatitic magmas, (2) xenolith suites representing the upper mantle and lower, middle, and upper crust of the Eocene lithosphere, and (3) mechanisms of eruption and emplacement of volatile-rich magmas. Some of the diatremes are fairly accessible in good weather. However, the Missouri Breaks is a remote area in which most of the roads are unimproved tracks across clay-rich sedimentary rocks, and even a small amount of rain may immobile four-wheel drive vehicles and make steep grades terminally hazardous.

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

Smoky Butte Lamproite, Montana part of MT Field Guides
Smoky Butte is a composite pluglike mass, one of several in a 3-km-long zone of dikes and plugs of lamproite. K-Ar age is 27 +/- 3 Ma on phlogopite in a sample from the quarry on Smoky Butte and is the youngest igneous age in eastern Montana. The wall rocks are the Tullock Member of the Paleocene Fort Union Formation, immediately above the Tertiary-Cretaceous boundary. Dikes are discontinuous and show en echelon offsets probably related to intrusion mechanics rather than to faulting. Pockets of lamproite breccia and heterolithic breccia, containing vesicular lamproite fragments (locally pumaceous) and slightly metamorphosed fragments of Fort Union rocks, occur along the dikes and make up part or all of some of the irregular igneous bodies along the zone. Some dike margins show ropy pahoehoelike texture. Welded agglutinates are locally interlayered with bedded tuffs and occur as blocks wtihin some of the heterolithic breccia bodies.

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

Quaternary Geology and Faulting of the Helena Valley, Montana part of MT Field Guides
This field trip will give the participants an overview of the Quaternary deposits and late Cenozoic faulting which have shaped the Helena valley. The Helena valley is a NW-trending graben surrounded by bedrock highlands. Most of the western half of the valley is a young alluvial plain formed of coalescing alluvial outwash fans issuing from drainages flowing towards Lake Helena. The western valley is ringed by older pediment surfaces sloping gently down from the valley margins. The eastern half of the valley is underlain primarily by later Tertiary silts, sands and gravels uplifted and segmented by normal faulting. Atop these Tertiary deposits along the southern valley margin, early Quaternary alluvial deposits are preserved as hilltop remnants of a formerly much larger deposit. Late Cenozoic faulting shaped the Helena valley, creating the modern topography and influencing sedimentation patterns; however, only a few faults show evidence of late Quaternary offsets.

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