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At least 595 records · Page 33Linked to original sources
USGS1980

Petroleum potential, environmental geology, and the technology for exploration and development of the Kodiak lease sale area #61

The Kodiak lease area is along a convergent ocean margin where active subduction is probably the greatest single influence on the geology. This influence is indicated by the Aleutian Trench, the Aleutian chain of volcanoes, and a well developed Benioff zone of earthquakes. Crustal structure under the Kodiak Shelf is intermediate between continental and oceanic. The thickness of sedimentary rock is 8 ± 3 km, which is greater than beneath the island. The proposed lease—sale area is on a submerged shelf extending 100 km or more seaward from the Kodiak group of islands, and it is more than 400 km long. The Kodiak Shelf still retains a glacial topography which has been modified by tectonically uplifted banks along the shelf edge and across the shelf. These banks are readily detectable signs of recent tectonism. Not so easily detectable are three deep offshore Neogene basins formed by depression of an unsampled presumed Paleogene sedimentary section. The basin floors have subsided 5 to 7 km since middle (?) Miocene time; the basins are filled with late Miocene and younger sediment that is only gently deformed. A sudden increase in seismic velocity occurs across the contact between the basin fill and the presumed Paleogene rocks that underlie it. This discontinuity in seismic velocity, the smooth character of the basin surface, and the truncation of dipping beds beneath it, are the basis for inferring subaerial erosion of the Paleogene section. If this inference is correct, the structure in some places requires at least 3000 m of subsidence followed by an uplift of even greater magnitude in Neogene time. The vertical tectonism offshore might produce reservoir rock and different source rock than encountered onshore.

Alaska↗
USGS1980

Field studies of borehole gamma-ray spectrometer methods for mineral exploration; a selected bibliography

A finite-difference digital model was developed to simulate the effects of a proposed well field on the water levels in existing wells in the Salt River Indian Reservation, which is in the southeastern part of Paradise Valley in central Arizona. The model area includes about 600 square miles in Paradise Valley and the adjoining Salt River Valley. In 1975 about 37,500 acre-feet of ground water was withdrawn for irrigation in the Salt River Indian Reservation. The proposed well field would withdraw as much as 45,000 acre-feet per year of additional irrigation water north of the Arizona Canal. The model was calibrated by using measured water-level declines for 1923-76 and simulated declines for 1946-75. The calibrated model was then used to predict water-level declines, based on projected amounts of pumpage, after 20 years of pumping. The rate of water-level decline would be an additional 2 to 6 feet per year in existing wells after 20 years of pumping in the proposed well field. The model was more sensitive to changes in pumpage distribution and in pumpage and recharge amounts than to changes in transmissivity and specific yield. (USGS)

Open-File Report↗
USGS1983

Slides showing aeromagnetic and gravity data for regional mineral exploration in Colorado, New Mexico, and Arizona

Examples of aeromagnetic and gravity data over 1? x 2? areas are presented for regions near the Cripple Creek mining area, Colorado, and the Lordsburg-Tyrone-Silver City mining areas, southern New Mexico and Arizona. These data indicate broad crustal structures and compositional variations that are marked by magnetization and density contrasts. The focus is on anomalies that may signal large-dimension controlling structures for the emplacement of economic mineral deposits. An example is a continuous, quasi-linear, north-trending gradient in both gravity and magnetic data located west of Cripple Creek area along long. 105? 30? W. This trend correlates with two mineral deposits of the Southern Rocky Mountains Front Range. It also correlates in part with an area of volcanic rock and with a mapped fault complex (Elkhorn-Currant Creek-Else-Westcliffe). The trend is interpreted to indicate a continuous crustal fault system, although exposures of this system are discontinuous between areas of alluvium and volcanic-rock cover. Similar geophysical trends exist in the Silver City to Tyrone area, where northeast-and northwest-trending anomalies appear to be marked by intrusion and mineralization. In this area, northwest-trending alluvial basins favor the use of geophysics to infer economically accessible but hidden bedrock whose association with exposed mineralization seems possible. An example of an inferred broad and relatively shallow, but hidden bedrock complex in association with more areally-limited mineralization is the Victorio Mountains area about 34 mi (55 km) south-southeast of Tyrone, New Mexico. The mineralization is within faulted sediments whose outcrop covers a small portion of the geophysical anomaly-complex.

Open-File Report↗