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7 recordsLinked to original sources
USGS2014

Geophysical interpretation of ground‐water levels

The theory of rock‐pressure as a cause of artesian‐head dates back at least to early Grecian times. Thus the philosopher Thales, about 600 B.C., taught that the springs derive their water from the ocean through subterranean channels and that the water is lifted to the springs by rook‐pressure. The theory of rock‐pressure has had a number of recent advocates, chief among whom was the British geologist, J. W. Gregory, but apparently no effective attempt was made by any of them to apply critical data to the problem. In general, geologists and hydrologists have rejected rock‐pressure as a vague heresy and have assumed that the artesian formations function as perfectly rigid and inert containers, not recognizing the fact that the data in regard to the performance of wells appear to conflict with such an interpretation .

Eos, Transactions, American Geophysical Union↗
USGS1980

Geoscientists for international development

Professional societies are usually concerned with the advancement of scientific knowledge, but a relative newcomer to the international scene has a different focus - geoscience development in the Third World. David Hastings, a member of AGID, explains.

The British Geologist↗
USGS2011

Porphyry copper assessment of British Columbia and Yukon Territory, Canada: Chapter C in Global mineral resource assessment

The U.S. Geological Survey does regional, national, and global assessments of resources (mineral, energy, water, biologic) to provide science in support of land management and decision making. Mineral resource assessments provide a synthesis of available information about where mineral deposits are known and suspected to be in the Earth’s crust, which commodities may be present, and estimates of amounts of resources that may be present in undiscovered deposits. Canada is an important source of copper, consistently ranking as one of the top 10 world producers during the past decade (2000–2010). The preponderance of this production has been from porphyry-copper-type deposits in the western Canadian Cordillera. A probabilistic mineral resource assessment of undiscovered resources associated with porphyry copper deposits in western Canada was completed as part of a global mineral resource assessment. The purpose of the assessment was to (1) compile a database of known deposits and significant prospects, (2) delineate permissive areas (tracts) for undiscovered porphyry copper deposits that may be present in the upper kilometer (minimally) of the Earth’s crust, and (3) provide probabilistic estimates of amounts of copper (Cu), molybdenum (Mo), gold (Au), and silver (Ag) that could be contained in undiscovered porphyry copper deposits in the tracts. The study was done by the U.S. Geological Survey (USGS) in collaboration with geologists from the British Columbia Geological Survey, Yukon Geological Survey, and industry consultants. The database of known deposits and significant prospects includes an inventory of mineral resources in 89 known porphyry copper (and 2 related copper-bearing polymetallic vein) ore zones, representing 50 porphyry copper deposits, and lists key characteristics of 280 additional porphyry copper and related copper-bearing prospects, as indicated by currently available exploration results, which also are summarized. Resource and exploration and development activity are updated with information current through April 2010. The delineation of permissive tracts and probabilistic estimation of resources in undiscovered deposits were done using the USGS three-part mineral resource assessment approach. Permissive tracts are defined in accordance with descriptive models for porphyry copper deposits to include igneous rocks and known deposits and prospects within magmatic arcs related to convergent plate-margin boundary zones. Frequency distributions of total tonnages and average grades of thoroughly explored deposits were used as models for undiscovered deposits and include a new grade and tonnage model for calc-alkaline porphyry Cu±Mo±Au deposits in western Canada. Five permissive tracts for the occurrence of porphyry copper deposits were delineated: 2 island-arc tracts, 1 tract of transitional, mixed island-arc and continental arc affinities, and 2 continental arc tracts. In permissive tract 003pCu2001, calc-alkaline igneous rocks of Middle Triassic to Late Jurassic age in accreted island-arc terranes of the Intermontane belt are assessed for calc-alkaline porphyry Cu±Mo±Au deposits. The area of this tract is 175,250 km 2 . In 12 known deposits, the total reported tonnage of ore is 8,100 million metric tons (Mt) containing 24.6 Mt copper. An estimated 6.9 undiscovered deposits contain a calculated mean of 8.9 Mt copper and a median of 6.9 Mt copper. The spatial density for the 18.9 known plus estimated undiscovered deposits in this tract is approximately 11 deposits per 100,000 km 2 . In permissive tract 003pCu2002, alkaline igneous rocks of Middle Triassic to Late Jurassic age within the Intermontane accreted island-arc terranes are assessed for alkaline porphyry Cu-Au deposits. The area of this tract is 109,290 km 2 . In 12 known deposits the total reported tonnage of ore is 6,440 Mt, containing 20.9 Mt copper. An estimated 7 undiscovered deposits contain a calculated mean of 22 Mt copper and a median of 13 Mt copper. The spatial density for the 19 known plus estimated undiscovered deposits in this tract is approximately 17 deposits per 100,000 km 2 . In permissive tract 003pCu2003, calc-alkaline igneous rocks of Late Triassic to Early Cretaceous age within the accreted Insular terranes of mixed island-arc and continental arc affinities are assessed for calc-alkaline porphyry Cu±Mo±Au deposits. The area of this tract is 58,360 km 2 . The total tonnage of ore reported in the 2 known deposits is 1,160 Mt containing 3.17 Mt copper. An estimated 2.3 undiscovered deposits contain a calculated mean of 3 Mt copper and a median of 1.9 Mt copper. The spatial density for the 4.3 known plus estimated undiscovered deposits in this tract is approximately 7 deposits per 100,000 km 2 . In permissive tract 003pCu2004, calc-alkaline igneous rocks in continental magmatic arcs of Jurassic to Eocene age are assessed for porphyry Cu±Mo±Au deposits. The area of this tract is 639,500 km 2 . The total tonnage of ore reported for the 23 known deposits is 6,520 Mt containing 17.9 Mt copper. An estimated 9.6 undiscovered deposits contain a calculated mean of 13 Mt copper and a median of 11 Mt copper. The spatial density for the 32.6 known deposits plus the estimated undiscovered deposits in this tract is approximately 5 deposits per 100,000 km 2 . In permissive tract 003pCu2005, calc-alkaline igneous rocks in continental magmatic arcs of Oligocene to Pliocene age are assessed for porphyry Cu±Mo±Au deposits. The area of this tract is 32,840 km 2 . The total tonnage of ore reported for the 1 known deposit is 44.8 Mt containing 0.224 Mt copper. An estimated 1.4 undiscovered deposits contain a calculated mean of 1.8 Mt copper and a median of 0.72 Mt copper. The spatial density for the 2.4 known plus estimated undiscovered deposits in this permissive tract is approximately 7 deposits per 100,000 km 2 . Western Canada has been thoroughly explored for porphyry copper deposits. The total estimated copper contained in known deposits is about 66.8 Mt (based on 2010 data), as compared to a 49 Mt mean of estimated copper in undiscovered deposits and a 34 Mt median of estimated copper in undiscovered deposits. The copper contained in known porphyry copper deposits represents about 58 percent of the total of known and undiscovered porphyry copper deposits (based on mean values). About 86 percent of the increase in estimated copper resources between 1993 and 2009 resulted from the discovery of extensions to known deposits. Nevertheless, exploration for undiscovered deposits continues, especially in and around significant prospects and in parts of permissive tracts that are mostly hidden beneath younger volcanic, sedimentary, or vegetated surficial cover.

British Columbia, Yukon Territory↗
USGS2018

James Dwight Dana and John Strong Newberry in the US Pacific Northwest: The roots of American fluvialism

Recognition of the power of rivers to carve landscapes transformed geology and geomorphology in the late nineteenth century. Wide acceptance of this concept—then known as “fluvialism”—owes to many factors and people, several associated with exploration of western North America. Especially famous are the federal geographic and geologic surveys of the US Southwest with John Wesley Powell and Grove Karl Gilbert, which produced key insights regarding river processes. Yet earlier and less-known surveys also engaged young geologists embarking on tremendously influential careers, particularly the 1838–1842 US Exploring Expedition with James Dwight Dana and the 1853–1855 railroad surveys including John Strong Newberry. Informed but little constrained by European and British perspectives on landscape formation, Dana and Newberry built compelling cases for the erosive power of rivers, largely from observations in the US Pacific Northwest. They seeded the insights of the later southwestern surveys, Dana by his writings and station at Yale and his hugely influential Manual of Geology , published in 1863, and Newberry by becoming the first geologist to explore the dramatic river-carved canyons of the Southwest and then a forceful proponent of the federal surveys spotlighting the erosional landscapes. Newberry also gave Gilbert his start as a geologist. Although Dana and Newberry are renowned early American geologists, their geomorphic contributions were overshadowed by the works of Powell, Gilbert, and William Morris Davis. Yet Dana and Newberry were the first ardent American proponents of fluvialism, providing strong roots that in just a few decades transformed western geology, roots nourished in large measure by the geologically fertile landscapes of the US Pacific Northwest.

Journal of Geology↗
USGS2003

Nature, origin, and production characteristics of the Lower Silurian regional oil and gas accumulation, central Appalachian basin, United States

Low-permeability sandstones of the Lower Silurian regional oil and gas accumulation cover about 45,000 mi2 (117,000 km 2 ) of the Appalachian basin and may contain as much as 30 tcf of recoverable gas resources. Major reservoirs consist of the "Clinton" sandstone and Medina Group sandstones. The stratigraphically equivalent Tuscarora Sandstone increases the area of the Lower Silurian regional accumulation (LSRA) by another 30,000 mi2 (78,000 km 2 ). Approximately 8.7 tcf of gas and 400 million bbl of oil have been produced from the Clinton/Medina reservoirs since 1880. The eastern predominantly gas-bearing part of the LSRA is a basin-center gas accumulation, whereas the western part is a conventional oil and gas accumulation with hybrid features of a basin-center accumulation. The basin-center accumulations have pervasive gas saturation, water near irreducible saturation, and generally low fluid pressures. In contrast, the hybrid-conventional accumulations have less-pervasive oil and gas saturation, higher mobile-water saturation, and both normal and abnormally low fluid pressures. High mobile-water saturation in the hybrid-conventional reservoirs form the updip trap for the basin-center gas creating a broad transition zone, tens of miles wide, that has characteristics of both end-member accumulation types. Although the Tuscarora Sandstone part of the basin-center gas accumulation is pervasively saturated with gas, most of its constituent sandstone beds have low porosity and permeability. Commercial gas fields in the Tuscarora Sandstone are trapped in naturally fractured, faulted anticlines. The origin of the LSRA includes (1) generation of oil and gas from Ordovician black shales, (2) vertical migration through an overlying 1000-ft (305-m)-thick Ordovician shale; (3) abnormally high fluid pressure created by oil-to-gas transformation; (4) updip displacement of mobile pore water by overpressured gas; (5) entrapment of pervasive gas in the basin center; (6) postorogenic uplift and erosion, causing gas leakage and a marked reduction in fluid pressure. Most future natural-gas production in the Clinton/Medina sandstones is anticipated to come from the basin-center accumulation. The Tuscarora Sandstone has additional gas resources but typically low reservoir porosity and permeability, and the likelihood of low-energy (in British thermal units) gas reduce the incentive to explore for it.

Kentucky, Maryland, New York, Ohio, Pennsylvania, ↗
USGS1964

Subdivision of the San Lorenzo Formation (Eocene and Oligocene) west-central California

The San Lorenzo Formation was for many years considered synonymous with Oligocene Series. The formation name was extended, incorrectly in most cases, to rocks as far north as British Columbia and as far south as southern California. The formation in its type area was never adequately studied, resulting in fallacious concepts of its faunas and erroneous correlations. The San Lorenzo Formation has been subdivided into the Twobar Shale Member of Narizian (late Eocene) age and Rices Mudstone Member of Rufugian and Zemorrian (Eocene and Oligocene) age. Faunas and lithology suggest that the older member was deposited slowly in a bathyal, open-sea environment, whereas the Rices Mudstone Member seems to have been deposited rapidly in a restricted basin. Glauconite at the contact of the members suggests a stratigraphic break in the depositional sequence. Most, perhaps all, of the so-called characteristic San Lorenzo mollusks are from the upper (Zemorrian) part of the Rices Mudstone Member, or from the overlying Vaqueros Sandstone. The "transitional sandstone" of Arnold is Vaqueros, not San Lorenzo. Subdivision of the San Lorenzo Formation and the discovery of glauconitic marker beds provide additional stratigraphic control in an oil province characterized by poor exposures.

California↗
USGS1968

Lead and strontium isotope studies of the Boulder Batholith, Southwestern Montana

The isotopic composition of lead in feldspar varies widely from pluton to pluton of the Late Cretaceous Boulder batholith, encompassing the following ranges in isotopic values: 16.9-18.1 for Pb 206 /Pb 204 ; 15.4-15.7 for Pb 207 /Pb 204 ; and 37.7-38.5 for Pb 205 /Pb 204 . Although each pluton has a characteristic isotopic composition, the fact that Pb 206 /Pb 204 for the Butte Quartz Monzonite varies by 1.3 percent, compared to a range of about 8.6 percent for the batholith as a whole, suggests that perfect isotopic mixing of the magma was not always attained. Whole rock initial Sr 87 /Sr 88 for the entire batholith ranges from 0.705 to 0.710, comparable to ranges for the Sierra Nevada and British Columbia batholiths. Ore leads of the Butte district are isotopically very similar to feldspar leads of the host Butte Quartz Monzonite. This isotopic similarity was interpreted by Murthy and Patterson (1961a) to indicate complete mixing of independently derived feldspar lead and ore lead rather than close genetic relationship. However, detailed study of the only sample in the present investigation, from the Donald pluton, that clearly has mixed leads shows significant differences in lead isotopic composition between megacryst K-feldspar and both the groundmass K-feldspar and plagioclase, indicating that isotopic mixing of leads is not complete, even within a single hand specimen. Spatial relations of isotopic differences among the various plutons strongly suggest that complete mixing of leads of different isotopic composition from the magma did not occur in the Butte Quartz Monzonite nor in any other pluton of the batholith, and that the isotopic similarity between Butte ore and feldspar lead of the host rock indeed stems from a genetic association. Lead and strontium isotope ratios generally behave in a similar way; that is, the more radiogenic the lead in a rock, the more radiogenic the strontium. The source of the lead which could produce the isotope variations observed for the batholith is calculated to have a mean age of about 2,200 m.y., which is compatible with ages of Precambrian crystalline rocks in the region, and an isotopic makeup with U 238 /pb 204 <9, Th/U > 4, and Rb/Sr of 0.03-0.09. However, no large volumes of prebatholith rocks exposed in the Boulder batholith region are of the required isotopic composition, which is typical of basaltic-gabbroic or quartz dioritic chemical composition. Models involving complete melting of possible source materials to account for the isotope variations are considered and rejected. Mechanisms involving par.tial melting of lower crustal or upper mantle source material appear to be the most viable in explaining the observed isotopic compositions. Assimilation of upper crustal material (i.e., the Precambrian Belt and pre-Belt rocks, in which the lead and strontium are more radiogenic than in the batholith rocks) may have accompanied partial melting of the lower crust or upper mantle and indeed must have played a major role if the source material had a composition isotopically comparable to that observed for oceanic tholelites (i.e., low Th/U, U 238 /Pb 204 and Rb/Sr), but would be relatively unimportant if the source material were sufficiently radiogenic to begin with as might be expected from basalt, gabbro, or quartz diorite compositions 2,200 m.y. old. © 1968 Society of Economic Geologists, Inc.

Montana↗