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Research about Montana

Source-linked reports with geographic coverage including Montana.

At least 901 records · Page 50Linked to original sources
USGS1974

Earthquake history of Montana

Montana is one of the most seismically active States in the Union. Since 1925, the State has experienced five shocks that reached intensity VIII or greater (Modified Mercalli Scale). During the same interval hundreds of less severe tremors were felt within the State. Montana's earthquake activity is concentrated mostly in the mountainous western third of the State which lies within a seismic zone that also includes southeastern Idaho, western Wyoming, and central Utah.

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USGS1973

Radiometric ages of intrusive rocks in the Little Belt Mountains, Montana

Radiometric ages indicate that most, if not all, of the major intrusions in the Little Belt Mountains, central Montana, were emplaced during the Eocene epoch, between 48 and 54 m.y. ago. In the Hughesville area, igneous activity continued, or was episodic until 42 m.y. ago. As a result of the continued igneous activity, radiometric ages in the Hughesville area can be interpreted either as primary ages or as reset ages.

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USGS1973

Boulder Batholith, Montana: A product of two contemporaneous but chemically distinct magma series

Rocks of the Late Cretaceous composite Boulder batholith, though successively emplaced in a relatively small segment of the Earth's crust within a very brief time span (78 to 68 m.y.), can be grouped chemically into two magma series: (1) the main series , defined principally by plutons in the central and northern parts of the batholith; and (2) the sodic series , defined mostly by plutons in the southern part. For any given SiO 2 content, the rocks of the main series tend to be higher in K 2 O and lower in Na 2 O than rocks of the sodic series. The chemical distinction between the two series proposed is also expressed by variation patterns for U, Th, Rb, and Sr abundances, by lead isotope compositions, but not by strontium isotope compositions. The prebatholith Elkhorn Mountains Volcanics (Late Cretaceous), especially the mafic members, are chemically and isotopically similar to the rocks of the main series, confirming geologic evidence of the genetic association between them. The postbatholith Lowland Creek Volcanics (early Eocene), though chemically more closely related to the sodic series, isotopically are more akin to, but slightly more radiogenic than, the main series. Post–Lowland Creek volcanic rocks (Miocene or Pliocene) are compositionally similar to the sodic series rocks. Spatial distribution of the batholith and the volcanic rocks exhibits a very crude chemical zonation of the region: for a given silica content, relatively more potassic rocks (main series and prebatholith volcanic rocks) tend to occur mainly in the north and east, whereas relatively more sodic rocks (sodic series and postbatholith volcanic rocks) predominate in the south and west. Available field, chemical, and isotopic evidence collectively suggests that the observed compositional variations for the Boulder batholith are most reasonably interpreted in terms of a model involving two magma series derived from two or more magma sources within the lower crust or upper mantle. These source regions are interred to vary chemically and isotopically, either laterally or vertically; in view of the rather small areal extent of the Boulder batholith, however, a vertically zoned source region is more probable.

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USGS1973

Evaluation of the ground-water supply at eight sites in Glacier National Park, northwestern Montana

Seven of eight test holes drilled in Glacier National Park derive water from the Quaternary alluvial or glacial deposits. The eighth test hole was dry. Aquifer tests indicated that production wells at the seven sites will yield enough water for domestic or campground uses. Estimated maximum pumping rates range from 4 to 50 gallons per minute (0.2 or 3.2 liters per second). The water is of excellent chemical quality and generally the major dissolved constituents are calcium and bicarbonate.

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USGS1973

Water resources data for Montana, water year 1972; part 2, water quality records

Water resources data for the 1972 water year for Montana include records of data for the chemical and physical characteristics of surface water. Data on the quality of surface water (chemical, temperature, and sediment) were collected from designated sampling sites at predetermined intervals such as once daily, weekly, monthly or less frequently. Records for a few pertinent water-quality stations in bordering States are also included. The records were collected by the Water Resources Division of the U.S. Geological Survey under the direction of George M. Pike, district chief. These data represent that portion of the National Water Data System collected by the U.S. Geological Survey and cooperating State and Federal agencies in Montana. The Geological Survey has published records of chemical quality, suspended sediment, and water temperatures since 1941 in an annual series of water-supply papers entitled, "Quality of Surface Waters of the United States." Beginning with the 1964 water year, water-quality records also have been re-leased by the Geological Survey in annual reports on a State-boundary basis. Distribution of these reports is limited; they are designed primarily for rapid release of data shortly after the end of the water year to meet local needs. These records will be published later in Geological Survey water-supply papers.

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USGS1973

Magnetizations of some Late Cretaceous glassy tuffs, volcanic breccias, and altered basalts of the Elkhorn Mountains volcanic field, Western Montana

New magnetization data for Late Cretaceous glassy welded tuffs, volcanic breccias, and altered basalts from the Elkhorn Mountains volcanic field, together with geologic, mineralogic, and K-Ar data, indicate that (1) the glassy tuffs have unusually strong, uniform remanent magnetizations which are reversely polarized, much of the remanence perhaps residing in submicroscopic single-domain iron oxide particles within the glass itself, (2) breccias emplaced at minimum temperatures of about 400°C have moderately dispersed remanent magnetization directions which are normally polarized, and (3) basalts that occur within a broad area of hydrothermal alteration have remanent magnetization directions which are normally, horizontally, and reversely polarized. Present and previous studies indicate the occurrence of from two to eight complete field reversal cycles during emplacement of the volcanic field about 78 m.y. ago. Among the rocks studied, only basalt from the Zosell mining district is important as a major aeromagnetic anomaly source.

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USGS1973

A Precambrian diamictite below the base of the Stillwater Complex, southwestern Montana

The metasedimentary rocks of Precambrian age below the base of the Stillwater Complex, southwestern Montana, contain a metamorphosed diamictite similar to diamictite in strata of late Precambrian and Early Cambrian age in Canada and the Western United States. The diamictite consists of a quartz-cordierite matrix containing an average 10-15 percent of rock fragments diverse in shape, size, angularity, lithologic types, and textures. The diamictite unit is poorly sorted, crops out discontinuously for 14 miles, and locally has a crude layering containing dropstones. Its origin is debatable, but the rock is suggestive of glacial marine deposition. The minimum age of the diamictite of the complex is 2,750 m.y. from U-Pb determination on zircon of intrusive quartz monzonite, the maximum 3,140 m.y. as determined on zircon from metasedimentary rocks interbedded with the diamictite.

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USGS1972

Principal facts for gravity stations in the Bearpaw Mountains and vicinity, Montana

Observed gravity values, station locations, terrain corrections, and Bouguer gravity data are provided in tabular form for approximately 500 gravity observations in north-central Montana. These data were used in preparation of -- Peterson, Donald L. and Rambo, William L., 1967, Bouguer gravity anomaly map of the Bearpaw Mountains and vicinity, Montana: U.S. Geol. Survey open-file report.

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