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Geology topics

Bruce L. Reed

Publications and source records attributed to Bruce L. Reed.

At least 19 recordsLinked to original sources
USGS2013

Geologic map of the east half of the Lime Hills 1:250,000-scale quadrangle, Alaska

This map is compiled from geologic mapping conducted between 1985 and 1992 by the U.S. Geological Survey as part of the Alaska Mineral Resource Assessment Program. That mapping built upon previous USGS work (1963–1988) unraveling the magmatic history of the Alaska–Aleutian Range batholith. Quaternary unit contacts depicted on this map are derived largely from aerial-photograph interpretation. K-Ar ages made prior to this study have been recalculated using 1977 decay constants. The east half of the Lime Hills 1:250,000-scale quadrangle includes part of the Alaska–Aleutian Range batholith and several sequences of sedimentary rocks or mixed sedimentary and volcanic rocks. The Alaska–Aleutian Range batholith contains rocks that represent three major igneous episodes, (1) Early and Middle Jurassic, (2) Late Cretaceous and early Tertiary, and (3) middle Tertiary; only rocks from the latter two episodes are found in this map area. The map area is one of very steep and rugged terrain; elevations range from a little under 1,000 ft (305 m) to 9,828 ft (2,996 m). Foot traverses are generally restricted to lowermost elevations. Areas suitable for helicopter landings can be scarce at higher elevations. Most of the area was mapped from the air, supplemented by direct examination of rocks where possible. This restricted access greatly complicates understanding some of the more complex geologic units. For example, we know there are plutons whose compositions vary from gabbro to granodiorite, but we have little insight as to how these phases are distributed and what their relations might be to each other. It is also possible that some of what we have described as compositionally complex plutons might actually be several distinct intrusions.

Alaska↗
USGS1979

The Alaskan Mineral Resource Assessment Program: Background information to accompany folio of geologic and mineral resource maps of the Talkeetna quadrangle, Alaska

The Talkeetna 1° by 3° quadrangle, which consists of about 17,155 km 2 in south-central Alaska, was investigated by integrated field and laboratory studies in the disciplines of geology, geochemistry, geophysics, and Landsat data interpretation for the purpose of assessing its mineral resource potential. Past mineral production has been limited to gold from the Yentna district, but the quadrangle contains potentially significant resources of tin and silver and possibly a few other commodities including chromite and copper. The results of the mineral resource assessment are given in a folio of maps which are accompanied by descriptive texts, diagrams, tables, and pertinent references. This Circular provides background information on these investigations and integrates the component maps. A bibliography cites both specific and general references to the geology and mineral deposits of the quadrangle.

Alaska↗
USGS1974

Offset plutons and history of movement along the McKinley segment of the Denali Fault system, Alaska

The Foraker and McGonagall plutons, bodies of granodiorite with nearly identical mineralogy and chemistry, are considered to be parts of a single igneous mass that has undergone right-lateral displacement of about 38 km along the McKinley segment of the Denali fault system since the igneous mass crystallized about 38 m.y. ago. These offset plutons place severe constraints on the amount and rate of movement along the McKinley segment since the beginning of the Oligocene Epoch. The 38-km displacement indicates an average rate of 0.1 cm/yr if movement began immediately after crystallization in early Oligocene time or an average rate of 0.4 cm/yr if movement began 10 m.y. ago in late Miocene time. These rates, however, are considerably less than Holocene movement rates measured along the fault further to the east, which suggests that the Holocene displacement rate is greater than the pre-Holocene rate or that right-lateral movement along the fault may diminish to the west.

GSA Bulletin↗
USGS1974

Chemical variations across the Alaska-Aleutian Range batholith

A study of 79 chemical analyses of plutonic rocks in the northern part of the Alaska-Aleutian Range batholith shows that K 2 O and SiO 2 in Late Cretaceous and Tertiary rocks decrease toward the Pacific margin and that Al 2 O 3 and CaO increase. Plots for Fe 2 O 3 , FeO, MgO, and TiO 2 suggest a possible increase toward the Pacific margin; Na 2 O, H 2 O+, and MnO show no significant trends across the batholith. Oxide trends for groups of plutons in the western and eastern parts of the batholith in general are opposite that of the groups combined. For the Jurassic plutonic rocks, K 2 O may decrease toward the Pacific margin; other oxides show considerable scatter, and meaningful trends are not readily apparent. The oxide trends across the batholith are similar to trends across the central Sierra Nevada batholith of California, with the exception of SiO 2 and A1 2 O 3 , which show no significant changes. In the Coast Range batholith of British Columbia, potassium also increases away from the Pacific margin. The increase in K 2 O toward the continent in the Alaska-Aleutian Range batholith is similar to that found across Quaternary volcanic island arcs, where K 2 O content increases directly with increasing depth to inclined seismic zones. This increase suggests that generation of magma may have taken place along, or above, a paleoseismic zone. Geologic evidence supports such a model for the Jurassic plutonic rocks, which represent the roots of an early Mesozoic magmatic arc that probably formed above a descending oceanic plate. However, the hypothesis that magma for Late Cretaceous and Tertiary plutonic rocks was generated along seismic zones does not fit available geologic evidence, and an anatectic model cannot be ruled out.

Alaska↗
USGS1973

Alaska-Aleutian range batholith: Geochronology, chemistry, and relation to circum-Pacific plutonism

Potassium-argon mineral ages and reconnaissance mapping of approximately 65,000 sq km in south-central Alaska indicate that the Mesozoic and Cenozoic plutonic rocks in the region were emplaced during three discrete intrusive epochs. Most of the plutonic rocks are part of the Alaska-Aleutian Range batholith; the remainder appear as outcrops in isolated plutons northeast of the main batholith. Maximum and minimum concordant mineral ages on coexisting biotite and hornblende are used to mark the beginning and ending, respectively, of each intrusive epoch. The oldest intrusive epoch, Early and Middle Jurassic, contains plutonic rocks emplaced between about 176 and 154 m.y. ago. Jurassic plutonism occurred along a magmatic arc at least 1,300 km long, extending from about 480 km southwest of Becharof Lake northeast to the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which represents the roots of the arc portion of an early Mesozoic arc-trench system, also continues southwest into the Bering Shelf. The associated trench is thought to be represented by an imbricated mélange of ophiolite and submarine lava with associated chert and argillite that occupies a belt 140 km southeast of the magmatic arc. Clastic sediments more than 4.5 km thick occupy the 140-km-wide arc-trench gap. Late Cretaceous and early Tertiary plutonic rocks, emplaced between about 83 and 58 m.y. ago, are found mainly in the northern part of the batholith and in isolated plutons to the northeast toward Mount McKinley. Middle Tertiary plutonic rocks ranging in age from 38 to 26 m.y. occur in two areas within the batholith and also in the Mount McKinley area. Although some of the Late Cretaceous and Tertiary plutons are associated with extrusive rocks, these plutons are largely post-tectonic, and magma was generated beneath both stable platform areas as well as beneath former eugeosynclinal regions in which deformation had essentially ceased. Magma for the Jurassic plutonic rocks appears to have been generated along or above an early Mesozoic subduction zone. The region southeast of the zone is considered to represent a classic example of continental accretion of eugeosynclinal sediments caused by under-thrusting of oceanic crust beneath an island arc. Magma generation for the Cretaceous and Tertiary plutons in the northern part of the batholith does not appear related to the early Mesozoic subduction zone, for it would require that the zone shift inland, or toward the continent, from its position during the Jurassic and then shift away from the continent once again to its present position.

GSA Bulletin↗
USGS1973

Timing of mesozoic and cenozoic plutonic events in circum-Pacific North America

Evaluation of isotopic ages of granitic intrusive rocks of large batholiths in circum-Pacific North America indicates that Mesozoic and Cenozoic plutonism was episodic but not periodic. Three intrusive epochs have been defined in the Alaska-Aleutian Range batholith of Alaska on the basis of concordant potassium-argon ages of coexisting biotite and hornblende or muscovite. Only two intrusive epochs based on concordant ages of mineral pairs and an older plutonic episode can be recognized in the Sierra Nevada and southern California batholiths. Detailed intrusive histories for other parts of Alaska, British Columbia, and Yukon Territory cannot be inferred from available data. The intrusive epochs in the western United States do not correlate with the epochs in the Alaska-Aleutian Range batholith. The spacing, duration, and synchroneity of intrusive epochs in the circum-Pacific batholiths of North America are not yet well established.

GSA Bulletin↗