SEARCH · Geology Reports

Results for “Modern Geology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 847 records · Page 47Linked to original sources

USGS1985

Carboniferous U-Pb age of the Sebago batholith, southwestern Maine: Metamorphic and tectonic implications

Two phases (pink and white granite) of the Sebago batholith of southwestern Maine have been dated by the U-Pb zircon method. Identical upper concordia intercepts of both rocks indicate an intrusive age of 325 ± 3 m.y. for the batholith. The lower intercept of the pink-phase sample, 114 ± 13 m.y., is inferred to represent episodic lead loss due to the intrusion of the nearby Cretaceous Pleasant Mountain stock. The lower intercept of the white-phase sample, 18 ± 21 m.y., suggests only modern dilatancy lead loss. Monazites have ages of 272 m.y. (pink) and 282 m.y. (white) which are thought to be cooling ages. Rb-Sr whole-rock data have low initial 87 Sr/ 86 Sr ratios of 0.7031 (pink) and 0.7053 (white). These data, in conjunction with published 40 Ar/ 39 Ar, Rb-Sr, K-Ar, and fission-track ages, suggest that little or no uplift occurred in this part of New England until the Permian and that the uplift rate from 275 m.y. to 225 m.y. was ∼3 times as rapid as was the rate for 225 m.y. to the present. The Carboniferous age of the Sebago batholith suggests that currently accepted metamorphic and tectonic interpretations for southwestern Maine and for east-central New Hampshire require revision.

Maine↗

USGS2021

Groundwater flow conceptualization of the Pahute Mesa–Oasis Valley Groundwater Basin, Nevada—A synthesis of geologic, hydrologic, hydraulic-property, and tritium data

This report provides a groundwater-flow conceptualization that integrates geologic, hydrologic, hydraulic-property, and radionuclide data in the Pahute Mesa–Oasis Valley (PMOV) groundwater basin, southern Nevada. Groundwater flow in the PMOV basin is of interest because 82 underground nuclear tests were detonated, most near or below the water table. A potentiometric map and nine sets of hydrostratigraphic and hydrologic cross sections supplement the conceptualization. Potentiometric contours indicate that groundwater in the PMOV basin generally flows south-southwest and discharges at Oasis Valley. Groundwater encounters an alternating sequence of low- and high-transmissivity rocks, referred to as dams and pools, respectively, as it moves from east to west across eastern Pahute Mesa. Flow from all Pahute Mesa nuclear tests is to Oasis Valley and is well-constrained by water-level data. Flow converges along a corridor of high transmissivity between Pahute Mesa and Oasis Valley. The location of the lateral PMOV basin boundary is well defined, and this boundary, with a few minor exceptions, represents a no-flow boundary. Some boundary uncertainty exists in the northeastern part of the basin, but potential flow-rate estimates across the northeastern boundary resulting from this uncertainty are small relative to the basin groundwater budget. Recharge in the PMOV basin is derived from episodic pulses of modern water and the diffuse percolation of old water (greater than 1,000 years). Episodic recharge is a minor recharge component observed as a rise in groundwater levels that occurs 3 months to 1 year following a wet winter. Minor amounts of episodic recharge through an unsaturated zone in excess of 1,000 feet (ft) requires preferential flow through faults and fractures. The dominant recharge component is slow, steady, diffuse percolation of old water through the unsaturated zone. A large component of old water recharging the groundwater system is consistent with observations of isotopically light deuterium and oxygen 18 compositions in water from wells on Pahute Mesa and central Oasis Valley. About half the recharge in the PMOV basin is derived from the eastern Pahute Mesa area. The remaining recharge is derived primarily from other highland areas including Timber Mountain, Belted and Kawich Ranges, and Black Mountain. The PMOV groundwater system is nearly steady state, where recharge is balanced by the 5,900 acre-feet per year of natural discharge at Oasis Valley. This assumption is reasonable because the basin is dominated by steady-state conditions, where long-term changes in groundwater storage are minimal. Total groundwater withdrawals from 1963 to 2018 have amounted to less than 10 percent of annual groundwater discharge and less than 0.2 percent of the basin’s groundwater storage. Therefore, present-day (2020) conditions are considered representative of predevelopment (pre-1950) conditions in nearly all areas of the basin. The lower PMOV basin boundary is defined at 4,000 ft below the water table to encompass all underground nuclear tests and tritium plumes. This boundary defines the lower boundary of radionuclide migration. However, nearly all flow and tritium transport occur in the upper 1,600 ft of the saturated zone because a transmissivity-with-depth relation indicates that greater than 90 percent of the transmissivity contributing to groundwater flow occurs within 1,600 ft of the water table. Rocks at deeper depths have low transmissivity because argillic and mineralized alterations plug the fractures. Volcanic rocks form the primary aquifers and confining units in the PMOV basin. Volcanic hydrogeologic units (HGUs) and hydrostratigraphic units (HSUs) have transmissivity distributions that span up to eight orders of magnitude with considerable overlap between distributions. Despite the large overlap between units, mean transmissivities of aquifers are one-to-two orders of magnitude greater than the confining units. However, all volcanic-rock HGUs and HSUs are composite units, meaning that they can function spatially as either an aquifer or confining unit .

Nevada↗

USGS1967

Chemical quality of surface water in the Allegheny River basin, Pennsylvania and New York

The Allegheny River is the principal source of water to many industries and to communities in the upper Ohio River Valley. The river and its many tributaries pass through 19 counties in northwestern and western Pennsylvania. The population in these counties exceeds 3 million. A major user of the Allegheny River is the city of Pittsburgh, which has a population greater than The Allegheny River is as basic to the economy of the upper Ohio River Valley in western Pennsylvania as are the rich deposits of bituminous coal, gas, and oil that underlie the drainage basin. During the past 5 years many streams that flow into the Allegheny have been low flowing because of droughts affecting much of the eastern United States. Consequently, the concentration of solutes in some streams has been unusually high because of wastes from coal mines and oil wells. These and other water-quality problems in the Allegheny River drainage basin are affecting the economic future of some areas in western Pennsylvania. Because of environmental factors such as climate, geology, and land and water uses, surface-water quality varies considerably throughout the river basin. The natural quality of headwater streams, for example, is affected by saltwater wastes from petroleum production. One of the streams most affected is Kinzua Creek, which had 2,900 parts per million chloride in a sample taken at Westline on September 2, 1959. However, after such streams as the Conewango, Brokenstraw, Tionesta, Oil, and French Creeks merge with the Allegheny River, the dissolved-solids and chloride concentrations are reduced by dilution. Central segments of the main river receive water from the Clarion River, Redbank, Mahoning, and Crooked Creeks after they have crossed the coal fields of west-central Pennsylvania. At times, therefore, these streams carry coal-mine wastes that are acidic. The Kiskiminetas River, which crosses these coal fields, discharged sulfuric acid into the Allegheny at a rate of 299 tons a day during the 1962 water year (October 1, 1961, to September 30, 1962). Mine water affects the quality of the Allegheny River most noticeably in its lower part where large withdrawals are made by the Pittsburgh Water Company at Aspinwall and the Wilkinsburg-Penn Joint Water Authority at Nadine. At these places raw river water is chemically .treated in modern treatment plants to control such objectionable characteristics as acidity and excessive concentrations of iron and manganese. Dissolved-solids content in the river varies along its entire length. In its upper reaches the water of the Allegheny River is a sodium chloride type, and at low flow, the sodium chloride is more than half the dissolved solids. In its lower reaches the water is a calcium sulfate .type, and at low flow the calcium sulfate is more than half the dissolved solids. In middle segments of the river from Franklin to Kittanning, water is more dilute and of a mixed type. Many small and several larger streams in the upper basin--such as the Conewango, Brokenstraw, Kinzua, Tionesta, and French Creeks--support large populations of game-fish. Even in segments of the Clarion River, Mahoning, and Redbank Creeks, which are at times affected by coal-mine wastes, fish are present. Although different species withstand varying amounts of contaminants in water, the continued presence of the fish indicates that the water is relatively pure and suitable for recreation and many other uses.

Water Supply Paper↗

USGS1970

Distribution of silver and copper in placer gold derived from the northeastern part of the Colorado Mineral Belt

Placer gold grains from the modern streams originating in the Colorado Mineral Belt were examined for silver and copper content on a quantitative basis utilizing the electron microprobe. The variation among grains from a particular locality is large, but the mean silver content of the interior of the placer gold grains from each locality and (or) the variation in copper content may be of value in distinguishing lode sources and gold mining districts. Microprobe analysis of the interior of gold grains is independent of chemical actions that affect the border of placer gold grains during their transport history, and it is shown that distinct compositional groups of different lode sources may be identified even in a single sample--information that may aid in recognizing the existence of concealed lodes that once contributed to a placer environment.

Colorado↗

USGS1990

Middle Jurassic syntectonic conglomerate in the Mt. Tallac roof pendant, northern Sierra Nevada, California

Middle Jurassic marine conglomeratic and debris-flow deposits in the Mt. Tallac roof pendant are interpreted to be syntectonic fault trough deposits. Similar deposits in similar stratigraphic successions throughout the northern Sierra Nevada demonstrate that the Middle Jurassic continental-margin arc north of latitude 39° was essentially marine, and possibly extensional or transtensional in nature. Recognition of Middle Jurassic syntectonic deposits in the northern Sierra Nevada establishes continuity of structural style between the arc in the northern Sierra Nevada and a portion of the arc south of latitude 39°, which has been interpreted as an ancient analog of the modern extensional or transtensional Central American arc. The Middle Jurassic marine conglomeratic deposits in the northern Sierra Nevada form one of three post–Late Triassic stratigraphic elements shared by the northern Sierra terrane and rocks of probable cratonal affinity in west-central Nevada. Upper Triassic limestone in the northern Sierra Nevada is linked faunally to correlative limestone of west-central Nevada. While correlation is speculative, Lower to Middle(?) Jurassic intervals of quartzose sandstone are common to parts of the Sailor Canyon Formation in the northern Sierra terrane and to rocks of the Auld Lang Syne Group, Boyer Ranch Formation, and Dunlap Formation of west-central Nevada. Middle Jurassic syntectonic conglomeratic deposits, such as in the Dunlap Formation of west-central Nevada, are now recognized in the northern Sierra terrane as well.

California↗

USGS1999

Tritium and 14C concentrations in unsaturated-zone gases at test hole UZB-2, Amargosa Desert Research Site, 1994-98: A section in U.S. Geological Survey Toxic Substances Hydrology Program: Proceedings of the technical meeting, Charleston, South Carolina, March 8-12, 1999: Volume 3 (Part C) (WRI 99-4018C)>

Tritium concentrations have been determined yearly since April 1994 from water-vapor samples collected at test hole UZB-2. The hole was drilled about 100 m (meters) south of the southwest corner of a commercial burial site for low-level radioactive wastes in September 1993. UZB-2 is equipped with ten 2.5-cm (centimeters) diameter air ports permanently installed in the unsaturated zone between the depths of 5.5 and 108.8 m below land surface. Depth to ground water is about 110 m. Additional sampling ports were driven by hand to depths of 0.5, 1.0 and 1.5 m in May 1997. Initial samples of water vapor collected in April 1994 showed elevated tritium concentrations of more than 100 TU (tritium units) from all 10 air ports, with a maximum concentration of 762±10 TU from an air port at a depth of 24.1 m. Subsequent tritium concentrations increased in all air ports, although tritium concentrations at depths of less than 34.1 m have remained relatively constant since July 1995. The largest observed increase in tritium has been at a depth of 47.9 m. There, tritium concentration has increased from 198±5 TU in April 1994 to 2,570±30 TU in June 1998. Large increases also have been measured in samples collected from air ports at depths of 106.4 and 108.8 m, just above the water table. During September and October 1998, carbon dioxide samples were collected from all ten air ports in UZB-2 and at a depth of 1.5 m, and analyzed for radioactive carbon-14 ( 14 C). 14 C concentrations are highest in air ports at depths less than 6 m where they exceed 2,000 pmc (percent modern carbon). Concentrations decrease rapidly in air ports at depth and are about 20 pmc below 94.2 m. However, at 47.9 meters, the 14 C concentration is 205±1 pmc, which is 2 to 4 times higher than concentrations in air ports immediately above and below. This depth corresponds to the largest tritium increase in UZB-2. Concentrations of both tritium and 14 C are greater than what could be expected from atmospheric fallout. The distribution of tritium and 14 C likely represent a complex pattern of lateral and vertical transport through the unsaturated zone from buried wastes to UZB-2.

Nevada↗

USGS1994

Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks

Many of the minor elements in seawater today have a concentration-depth profile similar to that of the biologically essential nutrients, NO-3 and PO3-4. They show a relative depletion in the photic zone and enrichment in the deep ocean. The difference between their surface- and deep-ocean values, normalized to the change in PO3-4, approaches the average of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plankton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies, particularly of sediments which accumulate in shelf-slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated. The interelement relations observed for some of these sediments approach the average values of plankton. These same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites, rocks which have a high content of marine fractions (e.g., organic matter, apatite, biogenic silica and carbonates). Many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity, and/or seawater with an elemental content different from that of the modern ocean, was required to account for their minor-element contents. However, the similarity in several of the minor-element ratios in many of these formations to minor-element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal seas, of the ocean today. The accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater. The suite of elements in this fraction reflects redox conditions in the bottom water, as determined by bacterial respiration. For example, high Mn, high Cr+V and high Mo concentrations, above those which can be attributed to the accumulation of planktonic matter, characterize accumulation under bottom-water oxidizing, denitrifying and sulfate-reducing conditions, respectively. ?? 1994.

Chemical Geology↗

USGS2023

Rapid active thrust faulting at the northern Alaska Range front

Plate convergence rates strongly influence seismicity and mountain building inboard of convergent margins, but the distribution and kinematics of structures accommodating farfield convergence can be elusive. In interior Alaska, Yakutat microplate convergence drives late Pleistocene–recent right slip on the Denali fault, but westward-decreasing slip rates leave substantial residual Yakutat motion unaccounted for. Here, we show that Northern Foothills thrust slip beneath the northern Alaska Range absorbs a modern 4.4 mm/yr geodetic velocity gradient equivalent to ~78% of the 5.6 mm/yr residual Yakutat convergence along the central Denali fault. Infrared-stimulated luminescence ages of strath terrace deposits (67–4 ka; six sites) quantify Totatlanika River bedrock incision across the 1947 M w 7.1 thrust earthquake epicentral region. Incision rates increase abruptly from <1 mm/yr to 4.8–5.6 mm/yr above the blind thrust tip near the range front. Rapid slip at 6.7 mm/yr on a steep thrust ramp beneath the northern Alaska Range front accommodates the geodetic gradient, drives rock uplift at rates matching measured incision rates, and implies that large earthquakes like the 1947 event may recur with 500–1400 yr frequency. Results illuminate focused seismogenic strain inboard of a complex convergent margin and prompt reevaluation of Alaska’s neotectonic framework.

Alaska↗

USGS2011

Stratigraphy and depositional environments of the upper Pleistocene Chemehuevi Formation along the lower Colorado River

The Chemehuevi Formation forms a conspicuous, widespread, and correlative set of nonmarine sediments lining the valleys of the Colorado River and several of its larger tributaries in the Basin and Range geologic province. These sediments have been examined by geologists since J. S. Newberry visited the region in 1857 and are widely cited in the geologic literature; however their origin remains unresolved and their stratigraphic context has been confused by inconsistent nomenclature and by conflicting interpretations of their origin. This is one of the most prominent stratigraphic units along the river below the Grand Canyon, and the formation records an important event or set of events in the history of the Colorado River. Here we summarize what is known about these deposits throughout their range, present new stratigraphic, sedimentologic, topographic, and tephrochronologic data, and formally define them as a lithostratigraphic unit. The Chemehuevi Formation consists primarily of a bluff-forming mud facies, consisting of gypsum-bearing, horizontally bedded sand, silt, and clay, and a slope-forming sand facies containing poorly bedded, well sorted, quartz rich sand and scattered gravel. The sedimentary characteristics and fossil assemblages of the two facies types suggest that they were deposited in flood plain and channel environments, respectively. In addition to these two primary facies, we identify three other mappable facies in the formation: a thick-bedded rhythmite facies, now drowned by Lake Mead; a valley-margin facies containing abundant locally derived sediment; and several tributary facies consisting of mixed fluvial and lacustrine deposits in the lower parts of major tributary valleys. Observations from the subsurface and at outcrops near the elevation of the modern flood plain suggest that the formation also contains a regional basal gravel member. Surveys of numerous outcrops using high-precision GPS demonstrate that although the sand facies commonly overlies the mud facies where the two are found together, contacts between the two occur over a range in elevation, and as a consequence, the sand and mud facies are similarly distributed both horizontally and vertically throughout the valley. Collectively, the outcrops of the formation lie below a smooth elevation envelope that slopes 50 percent more steeply than the historic (pre-Hoover Dam) valley, from nearly 150 m above the historic flood plain near the mouth of the Grand Canyon to less than 30 m above the flood plain at the head of the flood plain near Yuma, Arizona. The steepness of the valley at the peak of aggradation probably represents a depositional slope. Layers of fine grained volcanic tephra have been found below and within the Chemehuevi Formation at five widely separated sites, one of which is now submerged beneath Lake Mead. Major element geochemistry of glass shards from the four accessible tephra sites were analyzed. Three of the sampled tephra layers are interbedded within the Chemehuevi Formation, and a fourth tephra conformably underlies the formation. The three interbedded tephra layers are similar enough to one another that they are probably from the same eruptive unit, hereafter referred to as the Monkey Rock tephra bed. The other sample, which locally underlies the formation, is similar enough to the Monkey Rock tephra bed to suggest it is from the same volcanic source area; however, it may not be from the same eruption, and thus may not be the same age. On the basis of the stratigraphic contexts of chemically similar tephra layers found elsewhere in the Basin and Range, we suspect that the source area is the Mammoth Mountain dome complex in Long Valley, east-central California. Two samples of proximal Mammoth Mountain pumice were analyzed and produced geochemical signatures similar to all four of the Chemehuevi Formation tephra, supporting Mammoth Mountain as a possible source area. The Mammoth Mountain volcanic center produced eruptions between about 111±2 and 57±2 ka and was most active in the later part of this time interval, during Marine Oxygen Isotope (MOI) stage 4 (between 74 and 59 ka ago). Chemically similar tephra in cores from Owens Lake and Walker Lake are approximately 70 and 74 ky old, based on age models of those cores. Other lines of stratigraphic evidence from nine tephra-containing sections in the Basin and Range are also consistent with an age assignment for the Monkey Rock tephra of ~72 ky, near the beginning of MOI stage 4. We propose to designate the Chemehuevi Formation as a formal lithostratigraphic unit, and propose as the type section a well exposed outcrop near the ranger station at Katherine Landing, Arizona, in the Lake Mead National Recreation Area. This exposure shows the two dominant facies, an example of one of the four known tephra layers, and interbedded lenses of locally derived gravel. In the type section, as in many of the other examples of the formation, the sand facies overlies the mud facies on a conspicuous, abrupt erosional surface; however, nearby is a contiguous section demonstrating that the mud and sand facies interfinger. In addition to the type section, measured reference sections compiled here illustrate other important lithologic and stratigraphic features of the formation. Our preferred interpretation of the Chemehuevi Formation is that it contains the remnants of deposits formed during a single major episode of fluvial aggradation, during which the Colorado River filled its valley with a great volume of dominantly sand-size sediment. This would reflect an increase in the supply of sand-size sediment, and(or) a reduction in transport capacity below the mouth of Grand Canyon. The most likely cause for the aggradation is an extraordinary increase in sand supply, likely due to widespread climatic change. However, other explanations have not been ruled out. Other aggradation events predated the Chemehuevi Formation, and some smaller events may have postdated the formation. However, the Chemehuevi Formation contains the remnants of the most recent large magnitude (>100 m) aggradation of the Colorado River.

California;Nevada;Arizona↗

USGS2011

Site-specific seismic-hazard maps and deaggregation in the western United States using the NGA models for ground-motion prediction

The 2008 National Seismic Hazard Mapping Project (NSHMP) update for the conterminous United States employs several new ground-motion prediction equations which include modern empirical models of linear and nonlinear site response to local and regional earthquakes. The recent availability of attenuation functions incorporating site conditions via Vs30 values permits the calculation of site-specific hazard maps for a wide range of spectral accelerations. I compare alternative site specific hazard maps using Vs30 values estimated according to the methods of Wills and Clahan (2006), Wald and Allen (2007), and Yong and others (in press). These maps are presented for 5-hertz (Hz) and 3-second spectral accelerations having 2 percent probability of exceedance in 50 years for central California and the western part of southern California. Because these attenuations incorporate nonlinear site response for the larger ground motions, the site-specific probabilistic ground motions for the western United States can show either increases or decreases with respect to the firm-rock site condition. Furthermore, the ground motions on soil can be different from those that are predicted by applying National Earthquake Hazard Reduction Program recommendations for adjusting rock values to account for the soil column. One finding of this investigation is that at high spectral frequencies, strong differences in the site's Vs30 estimates often result in relatively small differences in probabilistic ground motion in western California or other tectonically active regions. In addition, this report shows how incorporating geologic site condition information alters the values of the dominating magnitudes and distances in deaggregation-5-Hz values for a site near San Quentin, Calif., and 5-Hz and 1-Hz values for Harbor Island near Seattle, Wash. These deaggregations show that the modal event can shift from a larger closer source to a more distant, perhaps smaller source when nonlinear soil behavior is explicitly included in the hazard integral. The potential shift in the mode when considering the soil column's effect ought to be carefully considered by engineers who select scenario events based in part on the distribution in magnitude, distance, and epsilon space.

Open-File Report↗

USGS2012

El Niño-Southern oscillation variability from the late cretaceous marca shale of California

Changes in the possible behavior of El Niño–Southern Oscillation (ENSO) with global warming have provoked interest in records of ENSO from past “greenhouse” climate states. The latest Cretaceous laminated Marca Shale of California permits a seasonal-scale reconstruction of water column flux events and hence interannual paleoclimate variability. The annual flux cycle resembles that of the modern Gulf of California with diatoms characteristic of spring upwelling blooms followed by silt and clay, and is consistent with the existence of a paleo–North American Monsoon that brought input of terrigenous sediment during summer storms and precipitation runoff. Variation is also indicated in the extent of water column oxygenation by differences in lamina preservation. Time series analysis of interannual variability in terrigenous sediment and diatom flux and in the degree of bioturbation indicates strong periodicities in the quasi-biennial (2.1–2.8 yr) and low-frequency (4.1–6.3 yr) bands both characteristic of ENSO forcing, as well as decadal frequencies. This evidence for robust Late Cretaceous ENSO variability does not support the theory of a “permanent El Niño,” in the sense of a continual El Niño–like state, in periods of warmer climate.

California↗

USGS2024

On algorithmically determined versus traditional macroseismic intensity assignments

The utility of macroseismic data, defined as the effects of earthquakes on humans and the built environment, has been increasingly recognized following the advent of online systems that now produce unprecedented volumes of macroseismic intensity information. Contributed reports from the U.S. Geological Survey “Did You Feel It?” (DYFI) system ( Wald et al ., 1999 ) are used to generate intensity values with an algorithm based on seminal work by Dengler and Dewey (1998) . The algorithm was developed initially to reproduce intensity values assigned by expert opinion using questionnaire results collected by telephone survey. In this article, I discuss reasons why intensity values from (self‐selected) DYFI responses can differ from values that would be assigned by expert opinion given more complete data from randomly selected participants. For example, with the data used by Dengler and Dewey (1998) , intensities near 4 could be determined from the percentage of people who felt shaking in each town. With less spatially rich data from self‐selected participants, this percentage often cannot be determined reliably. Audible noises are key additional diagnostic criteria for modified Mercalli intensity (MMI) 4, but, although the DYFI system includes a question about noise, following Dengler and Dewey (1998) , the DYFI algorithm does not include a noise indicator. At the upper end of the scale, as defined the DYFI algorithm yields a maximum intensity value of 9.05, nominally corresponding to peak ground acceleration of 75% g . These and other factors can result in DYFI values that are low compared to traditional MMI values assigned using expert opinion, even absent factors that can bias traditional MMI assignments. Modern ground‐motion intensity conversion equations determined using DYFI intensities are expected to be appropriate for DYFI intensities, but the results of this study suggest that biases may be introduced if DYFI and traditional intensities are assumed to be interchangeable.

Seismological Research Letters↗

USGS1996

Gold placers of the historical Fortymile River region, Alaska

The Fortymile River region in east-central Alaska has a long and colorful history as the site of the first major gold discovery in interior Alaska. Placer gold has been mined in the region nearly every year since its original discovery in 1886. Total gold production is approximately 500,000 troy ounces. Although many of the rich deposits have been mined, there still exist areas that contain gold. Areas of mined and unmined gold-bearing creek and terrace gravels are outlined on the accompanying geologic map. The early history of the Fortymile area centered on the small frontier settlement of Fortymile City located at the junction of the Fortymile and Yukon Rivers in Canadian territory. This was the supply and jumping-off point for prospectors who worked their way into Alaska up the Fortymile River and found gold on many of its tributaries. Hand mining, both underground and surface, using sluice boxes and (or) rockers were the earliest methods; later, hydraulicking, dredging, and draglining methods were used. More recently, bulldozers and elevated trammels have been used, as well as very portable floating suction dredges. The rich mining lore of the area is closely associated with events of the nearby world-famous Klondike District. Bedrock and placer geology and mining history of individual gold-rich creeks are herein updated. The Fortymile area, which is part of the Yukon-Tanana Upland, contains quartzite, schist, gneiss, amphibolite, marble, serpentinite, and granite overlain by basalt, sandstone, conglomerate, shale, tuff, and coal; overlying these rocks are several deposits of varying ages consisting of gold-bearing gravel and colluvium. The close spatial association of creeks containing placer gold and the gneiss, schist, amphibolite, and marble unit strongly suggests this metamorphic unit is the gold source. High terrace gravels record a time from the late Tertiary to early Pleistocene when the ancestral Fortymile River and its major tributaries, the North and South Forks, had floodplains roughly 1 to 2 miles (2-3 kilometers) wide and gradients of about 4 feet per mile (0.75 meters per kilometer). Base-level lowering during the post-early Pleistocene caused the rivers to cut into their floodplains and to develop the youthful characteristics they have today such as V-shaped canyons, narrow floodplains, and gradients of at least twice those of the old river. Colluvium marginal to creek deposits in steep-sided valleys is often gold bearing. Much of the unconsolidated gravel within the major drainages of the Fortymile River, South Fork, North Fork, and Mosquito Fork is colluvium. Heavy-mineral-concentrate samples from the gold-producing creeks and high terrace gravels contain varying amounts of magnetite (20 to 80 percent) and ilmenite (10 to 30 percent), and samples from creeks draining areas principally composed of metamorphic rocks contain abundant garnet (10 to 30 percent). Gold fineness ranges from 620 to 927, but it is difficult to attach any geologic significance to the fineness data. Most placer gold in the Fortymile River area has been recovered at, or near, the gravelbedrock contact. The lowermost 3.3 feet (1 meter) of gravel and the uppermost 1.6 feet (0.5 meter) of bedrock may contain as much as 80 to 90 percent of the gold that is ultimately recovered. Gold nuggets are rare and most of the gold recovered is in the form of flattened fragments less than .2 inches (5 millimeters) in greatest dimension. However, large gold nuggets have been found on Wade Creek; examples are ones of 25,33,56, and 70 ounces. Occasionally, large nuggets may still be found in the tailing piles along the creek. The Fortymile River and its tributaries the South Fork, Walker Fork, and Mosquito Fork, all of which at one time were the sites of bucket-line dredge operations, now are almost exclusively mined using floating suction dredges. Unmined gold-bearing gravel is present in the floodplain of the Walker Fork valley below Cherry Creek and in low (about 100 to 130 feet or 30 to 40 meters) terraces along the north side of Walker Fork and east side of Cherry Creek. Considering the locations of where most gold has been found in the South Fork valley both by the older bucket dredges and the modern suction dredges, it seems likely that the tributary drainages of Lost Chicken, Napoleon, Franklin, and Buckskin Creeks have supplied the bulk of the gold to the South Fork valley. A quarter acre (0.10 hectare), 130-foot-thick ( 40 meters) section of the high terrace gravels on the north side of Napoleon Creek was mined for placer gold and yielded values estimated to be $8.50 per cubic yard (or $6.50 per cubic meter) at $350 per troy ounce. The unmined high terrace gravels on the south side of Buckskin Creek contain gold; however, this gravel is only 3 to 6.5 feet (1 to 2 meters) thick. The search for a lode gold source in the Fortymile River region may be in vain, because substantially more gold than has been recovered from the placers can be derived by the gradual erosion of large volumes of source rocks that contain background mean gold amounts. Using Leon's mass balance equation, 5,167 metric tons of gold may exist in the placers of the Fortymile River region, less than 1 percent of the recovered amount of 15.6 tons. The largest gold resource remaining in the Fortymile River region is probably in the high terrace gravels exposed along many of the creeks and rivers. Until there is exploratory drilling or a comprehensive sampling program, the amount of gold in these gravels will remain unknown. Environmental constraints imposed by Federal and State agencies have slowed, but not stopped, placer mining in the Fortymile River area, and a significant gold price rise would result in more mining.

Alaska↗

USGS1979

Palladium, platinum, and rhodium concentrations in mafic and ultramafic rocks from the Kizildag and Guleman areas, Turkey, and the Faryab and Esfandagheh-Abdasht areas, Iran

The Kizildag and Guleman areas, Turkey, and the Faryab and Esfandagheh-Abdasht areas, Iran, have produced chromite from ophiolite complexes consisting of harzburgite tectonite, dunite tectonite containing chromitite, pyroxenite, wehrlite, and gabbro. Forty-six samples from these complexes were analyzed in order to investigate the possibility of platinum-group metals being present that could be produced as byproducts. The results, however, indicate concentrations of palladium, platinum, and rhodium ranging up to 46 ppb (parts per billion), 55 ppb, and 24 ppb, respectively. The concentration levels and ratios of these metals are similar to other alpine ultramafic bodies that have been analyzed by modern analytical techniques. Ten samples from massive sulfide deposits in the Gunes and Ergani-Maden areas, Turkey, and the Sheikh Ali mine, Iran, were analyzed also. The results of the analysis suggests a low potential for byproduct palladium, platinum, and rhodium production in these ophiolite-associated massive sulfide deposits. The four ultramafic and mafic areas in Turkey and Iran under consideration have, and are, producing in 1978 chromite from podiform chromitites in alpine-type complexes. Because of the known association of chromitites and platinum-group metals, a collection of chromitites was made to check their palladium, platinum, and rhodium content to determine if economic concentrations might exist. Also, inasmuch as such rocks are thought to represent parts of the oceanic crust and upper mantle, data on platinum-group elements from these rocks can provide information to design geochemical models for the distribution of these elements. During the field excursions of the CENTO Working Group on Volcanic and Intrusive Rocks and Their Associated Ore Deposits, the Kizildag and Gunes areas, Turkey, were sampled in 1974 and the Guleman area, Turkey; Faryab and Esfandagheh-Abdasht areas, Iran, were sampled in 1975. In addition, samples of massive sulfide ores from the Ergani-Maden area, Turkey, were also collected for analysis, and the results included here. In this report the geologic framework of these areas is briefly examined as a background for interpreting the concentrations of palladium, platinum, and rhodium in the chromitites and ultramafic and mafic rocks. Comparisons of these analyses with analyses from other areas containing similar rocks show few differences in concentration or proportions of these three platinum-group metals.

Open-File Report↗

USGS2014

Geologic and hydrogeologic characteristics of the Ogallala Formation and White River Group, Belvoir Ranch near Cheyenne, Laramie County, Wyoming

The geologic and hydrogeologic characteristics of Tertiary lithostratigraphic units (Ogallala Formation and White River Group) that typically compose or underlie the High Plains aquifer system in southeastern Wyoming were described physically and chemically, and evaluated at a location on the Belvoir Ranch in Laramie County, Wyoming. On the basis of this characterization and evaluation, three Tertiary lithostratigraphic units were identified using physical and chemical characteristics determined during this study and previous studies, and these three units were determined to be correlative with three identified hydrogeologic units composing the groundwater system at the study site—a high-yielding aquifer composed of the entire saturated thickness of the heterogeneous and coarse-grained fluvial sediments assigned to the Ogallala Formation (Ogallala aquifer); an underlying confining unit composed primarily of very fine-grained volcaniclastic sediments and mudrocks assigned to the Brule Formation of the White River Group and some additional underlying sediments that belong to either the Brule or Chadron Formation, or both (Brule confining unit); and an underlying low-yielding aquifer composed primarily of poorly sorted fluvial sediments assigned to the Chadron Formation of the White River Group (Chadron aquifer). Despite widely varying sediment heterogeneity and consolidation, some limited hydraulic connection throughout the full vertical extent of the Ogallala aquifer was indicated but not conclusively proven by interpretation of similar chemical and isotopic characteristics, modern apparent groundwater ages, and similar hydraulic-head responses measured continuously in two Ogallala aquifer monitoring wells installed for this study at two different widely separated (83 feet) depth intervals. Additional work beyond the scope of this study, such as aquifer tests, would be required to conclusively determine hydraulic connection within the Ogallala aquifer. Groundwater levels (hydraulic heads) measured continuously using water-level recorders in both monitoring wells completed in the Ogallala aquifer showed a consistent strong upward vertical gradient in the Ogallala aquifer, indicating the potential for water to move from deeper to shallower parts of the aquifer, regardless of the time of year and the presumed effects of pumping of public-supply and industrial wells in the area. Continuous measurement of groundwater levels in the shallowest monitoring well, installed near the water table, and examination of subsequently constructed water-level hydrographs indicated substantial groundwater recharge is likely during the spring of 2009 and 2010 from the ephemeral stream (Lone Tree Creek) located adjacent to the study site that flows primarily in response to spring snowmelt from the adjacent Laramie Mountains and surface runoff from precipitation events. Using the water-table fluctuation method, groundwater recharge was estimated to be about 13 inches for the period beginning in early October 2009 and ending in late June 2010, and about 4 inches for the period beginning in March 2011 and ending in early July 2011. Comparison of previously measured groundwater levels (hydraulic heads) and groundwater-quality characteristics in nearby monitoring wells completed in the Chadron aquifer with those measured in the two monitoring wells installed for this study in the Ogallala aquifer, combined with detailed lithologic characterization, strongly indicated the Brule confining unit hydraulically confines and isolates the Chadron aquifer from the overlying Ogallala aquifer, thus likely limiting hydraulic connection between the two units. Consequently, because of the impermeable nature of the Brule confining unit and resulting hydraulic separation of the Ogallala and Chadron aquifers, and compared with local and regional hydrostratigraphic definitions of the High Plains aquifer system, the groundwater system in Tertiary lithostratigraphic units overlying the Upper Cretaceous Lance Formation at the location studied on the Belvoir Ranch was defined as being composed of, from shallowest to deepest, the High Plains aquifer system (high-yielding Ogallala aquifer only, composed of the saturated Ogallala Formation); the Brule confining unit composed of the Brule Formation of the White River Group and an underlying fine-grained depth interval with sediments that belong to either the Brule or Chadron Formation, or both; and the low-yielding Chadron aquifer (composed of poorly sorted coarse-grained sediments with substantial fine-grained matrix material assigned to the Chadron Formation of the White River Group).

Wyoming↗

USGS2001

The interplay of species concepts, taxonomy, and conservation: lessons from the Hawaiian avifauna

The Hawaiian Islands, with their unique geological history and geographic position, provide an excellent natural laboratory in which to evaluate currently competing biological (BSC) and phylogenetic (PSC) concepts of the species. Although the BSC as historically applied in archipelagic situations is shown to be flawed in producing overlumped polytypic species, it nevertheless remains the preferable concept for most practical purposes. A review of the taxonomic history and species limits in Hawaiian birds under both concepts reveals that, when properly applied, the BSC yields a species total remarkably close to that produced under the PSC, contrary to what many proponents of the latter have supposed. We propose that the widespread adoption of the PSC for conservation purposes is potentially harmful. The PSC trivializes the species taxon and introduces new problems of deciding when a population becomes diagnosable, the possibility that species could appear and disappear in a reticulate fashion, and the likelihood that genetically diagnosable but phenotypically identical, and therefore not field identifiable, populations could be ranked as species. All of these problems negatively impact such things as constructing credible and politically defensible lists of endangered species, the prioritization of limited conservation resources, and the gathering of field data. We contend the BSC is arguably a more rational concept that better supports the activities of both scientific and nonprofessional observers. Biological species limits in oceanic archipelagoes worldwide need to be reevaluated using modern concepts and technologies before rational conservation decisions can be made.

Hawaii↗

USGS2025

An early Holocene wet period in the southwestern United States

Multiple generations of spring-fed streams traversed ∼800 km 2 of the Las Vegas Valley in southern Nevada between ca. 10.9 ka and 8.5 ka, depositing an extensive tufa network. The scale of this network and diversity of tufa morphologies is novel in North America and offers an opportunity to obtain quantitative paleoclimate data for the region during the early Holocene. We determined isotopic compositions and estimated past temperatures using clumped isotope data from early Holocene tufa on the valley floor (698 m) as well as tufa forming today at higher elevation in the nearby Spring Mountains at Cold Creek Spring (1856 m). Modern and fossil tufa yielded comparably low δ 18 O values, implying that source waters for both were derived from high-elevation winter precipitation. Clumped isotope temperatures of modern tufa average 15.8 ± 2.5 °C, aligning with mean summer temperatures of the emergent spring water, and indicate equilibrium conditions of tufa formation. The early Holocene tufa yielded similar clumped isotope temperatures, averaging 15.2 ± 3.9 °C, meaning it precipitated at temperatures that occur at much higher elevations today. The Las Vegas tufa record, combined with nearby and temporally correlative paleospring and lacustrine records, suggest that cool/wet conditions prevailed throughout the Mojave Desert during the early Holocene. These records also demonstrate that spring ecosystems responded to millennial-scale hydroclimate variations that supersede climate change driven solely by insolation. The previously unrecognized pattern of ecosystem response to hydroclimate documented here may assist in understanding climate drivers for the early Holocene and provide critical information for the fate of groundwater-dependent ecosystems in the southwestern United States.

Nevada↗

USGS2005

Evaluation of geohydrologic framework, recharge estimates and ground-water flow of the Joshua Tree area, San Bernardino County, California

Ground water historically has been the sole source of water supply for the community of Joshua Tree in the Joshua Tree ground-water subbasin of the Morongo ground-water basin in the southern Mojave Desert. The Joshua Basin Water District (JBWD) supplies water to the community from the underlying Joshua Tree ground-water subbasin. The JBWD is concerned with the long-term sustainability of the underlying aquifer. To help meet future demands, the JBWD plans to construct production wells in the adjacent Copper Mountain ground-water subbasin. As growth continues in the desert, there may be a need to import water to supplement the available ground-water resources. In order to manage the ground-water resources and to identify future mitigating measures, a thorough understanding of the ground-water system is needed. The purpose of this study was threefold: (1) improve the understanding of the geohydrologic framework of the Joshua Tree and Copper Mountain ground-water subbasins, (2) determine the distribution and quantity of recharge using field and numerical techniques, and (3) develop a ground-water flow model that can be used to help manage the water resources of the region. The geohydrologic framework was refined by collecting and interpreting water-level and water-quality data, geologic and electric logs, and gravity data. The water-bearing deposits in the Joshua Tree and Copper Mountain ground-water subbasins are Quarternary alluvial deposits and Tertiary sedimentary and volcanic deposits. The Quarternary alluvial deposits were divided into two aquifers (referred to as the 'upper' and the 'middle' alluvial aquifers), which are about 600 feet (ft) thick, and the Tertiary sedimentary and volcanic deposits were assigned to a single aquifer (referred to as the 'lower' aquifer), which is as thick as 1,500 ft. The ground-water quality of the Joshua Tree and Copper Mountain ground-water subbasins was defined by collecting 53 ground-water samples from 15 wells (10 in the Joshua Tree ground-water subbasin and 5 in the Copper Mountain ground-water subbasin) between 1980 and 2002 and analyzing the samples for major ions, nutrients, and selected trace elements. Selected samples also were analyzed for oxygen-18, deuterium, tritium, and carbon-14. The water-quality data indicated that dissolved solids and nitrate concentrations were below regulatory limits for potable water; however, fluoride concentrations in the lower aquifer exceeded regulatory limits. Arsenic concentrations and chromium concentrations were generally below regulatory limits; however, arsenic concentrations measured in water from wells perforated in the lower aquifer exceeded regulatory limits. The carbon-14 activities ranged from 2 to 72 percent modern carbon and are consistent with uncorrected ground-water ages (time since recharge) of about 32,300 to 2,700 years before present. The oxygen-18 and deuterium composition of water sampled from the upper aquifer is similar to the volume-weighted composition of present-day winter precipitation indicating that winter precipitation was the predominant source of ground-water recharge. Field studies, conducted during water years 2001 through 2003 to determine the distribution and quantity of recharge, included installation of instrumented boreholes in selected washes and at a nearby control site. Core material and cuttings from the boreholes were analyzed for physical, chemical, and hydraulic properties. Instruments installed in the boreholes were monitored to measure changes in matric potential and temperature. Borehole data were supplemented with temperature data collected from access tubes installed at additional sites along study washes. Streambed hydraulic properties and the response of instruments to infiltration were measured using infiltrometers. Physical and geochemical data collected away from the stream channels show that direct infiltration of precipitation to depths below the root zone and subsequent gro

Scientific Investigations Report↗