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Geology topics

R. M. Tosdal

Publications and source records attributed to R. M. Tosdal.

At least 37 records · Page 2Linked to original sources
USGS1994

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

Geological Society of America Bulletin↗
USGS1992

The Conterminous United States Mineral Assessment Program: Background information to accompany folio of geologic, geochemical, geophysical, and mineral resource maps of the Ajo and Lukeville 1° by 2° quadrangles, Arizona

Encompassing about 21,000 km 2 in southwestern Arizona, the Ajo and Lukeville 1° by 2° quadrangles have been the subject of mineral resource investigations utilizing field and laboratory studies in the disciplines of geology, geochemistry, geophysics, and Landsat imagery. The results of these studies are published as a folio of maps, figures, and tables, with accompanying discussions. Past mineral production has been limited to copper from the Ajo Mining District. In addition to copper, the quadrangles contain potentially significant resources of gold and silver; a few other commodities, including molybdenum and evaporites, may also exist in the area as appreciable resources. This circular provides background information on the mineral deposits and on the investigations and integrates the information presented in the folio. The bibliography cites references to the geology, geochemistry, geophysics, and mineral deposits of the two quadrangles.

Arizona↗
USGS1987

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km 2 , and Picacho Peak (CDCA-355A), 23 km 2 , Wilderness Study Areas (WSA's) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSA's are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800's, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSA's, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSA's - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed. Four areas of possible exploration interest were identified within the WSA's. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist.

Journal of Geochemical Exploration↗
USGS1982

Geologic map of the Domeland Wilderness and contiguous roadless area, Kern and Tulare counties, California

The Wilderness Act (Public Law 88-577, September 3, 1964) and related acts require the U.S. Geological Survey and the U.S. Bureau of Mines to survey certain areas on Federal lands to determine their mineral-resource potential. Results must be made available to the public and be submitted to the President and the Congress. This report presents the results of a geologic survey of the Domeland Wilderness and contiguous roadless areas in the Sequoia National Forest, Kern and Tulare Counties, California. The Domeland Wilderness was established by Public Law 88-577 (1964). The Woodpecker Roadless Area (05206) and the Domeland Addition Roadless Areas (05207) were classified as further planning areas during the Second Roadless Area Review and Evaluation (RARE II) by the U.S. Forest Service, January 1979.

California↗