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At least 433 records · Page 24Linked to original sources
USGS2010

Nitrogen compounds in natural water—A review

Nitrogen compounds in natural water are significant in public health, agriculture, industry, and geochemistry. The many sources of nitrogen compounds and the deep involvement of nitrogen in the life processes of organisms makes the study of such compounds difficult. The sources include natural aerosols, precipitation, fixation by micro‐organisms in soil and water, decaying organic matter, and animal and industrial wastes, as well as probably undiscovered sources in consolidated and unconsolidated rocks. Nitrogen compounds are both oxidized and reduced by organisms. Some nitrogen compounds are adsorbed on clay. The theoretical end product in water and the compound probably most often determined is NO 3 −1 . The concentration of nitregen compounds ranges from 0.0 to >100 ppm (parts per million) in surface water and from 0.0 to >1000 ppm in groundwater. Seasonal fluctuations occur. Much further research is needed, including improvements in methods of analysis, further investigation of sources, and detailed study of the nitrogen cycle in small drainage basins. (Key words: Geochemistry; quality of water.) This paper is not subject to U.S. copyright. Published in 1966 by the American Geophysical Union.

Water Resources Research↗
USGS1970

Nuclear methods applied to uranium geochemistry

Stable and radioactive daughter products produced from nuclear disintegrations of uranium have proved useful in fundamental studies of the geochemistry of uranium in igneous rock and sedimentary environments and in ore deposits. Information gained from geochemical studies of uranium migration has been used to develop models for attempts to date archeological, geological, and oceanographic environments represented by samples of bone, wood, charcoal, continental and marine carbonates, marine sediments, and glacially derived soils. Recent improvements of nuclear instrumentation and techniques allowed accurate measurements of natural radioactive isotopes, and it is now believed that radioactive equilibrium between the long-lived isotopes of the two uranium decay series is more the exception than the rule in nature. It was assumed that the 234U and 238U isotopes were in equilibrium until Thurber confirmed that considerable separation between 234U daughter and 238U parent exists in nature. It now has been documented that the 234U content may range from 60% deficient to 500% in excess relative to 238U. An excess of 15% of 234U isotope in sea water is well documented. A summary of previous work has shown that geochemical fractionation of the radioactive nuclides in 238U and 235U decay series takes place in the hydrologic environment, resulting in depletion of 230Th and 231Pa with respect to their parents, 238U, 234U, and 235U, in water and a complementary enrichment of these daughter nuclides in some sediments. Subsequent assimilation of Uranium, essentially free of radioactive daughters, occurs in some specific types of deposits such as carbonates and phosphates.

IEEE Transactions on Nuclear Science↗
USGS2018

Geochemical investigation of weathering processes in a forested headwater catchment: Mass-balance weathering fluxes

Geochemical research on natural weathering has often been directed towards explanations of the chemical composition of surface water and ground water resulting from subsurface water-rock interactions. These interactions are often defined as the incongruent dissolution of primary silicates, such as feldspar, producing secondary weathering products, such as clay minerals and oxyhydroxides, and solute fluxes (Meunier and Velde, 1979). The chemical composition of the clay-mineral product is often ignored. However, in earlier investigations, the saprolitic weathering profile at the South Fork Brokenback Run (SFBR) watershed, Shenandoah National Park, Virginia, was characterized extensively in terms of its mineralogical and chemical composition (Piccoli, 1987; Pochatila et al., 2006; Jones et al., 2007) and its basic hydrology. O'Brien et al. (1997) attempted to determine the contribution of primary mineral weathering to observed stream chemistry at SFBR. Mass-balance model results, however, could provide only a rough estimate of the weathering reactions because idealized mineral compositions were utilized in the calculations. Making use of detailed information on the mineral occurrence in the regolith, the objective of the present study was to evaluate the effects of compositional variation on mineral-solute mass-balance modelling and to generate plausible quantitative weathering reactions that support both the chemical evolution of the surface water and ground water in the catchment, as well as the mineralogical evolution of the weathering profile. ?? 2008 The Mineralogical Society.

Conference Paper↗
USGS1978

Distribution of organic carbon and petroleum source rock potential of Cretaceous and lower Tertiary carbonates, south Florida basin: Preliminary results

Analyses of 134 core samples from the South Florida Basin show that the carbonates of Comanchean age are relatively richer in average organic carbon (0.41 percent) than those of Coahuilan age (0.28 percent), Gulfian age (0.18 percent) and Paleocene age (0.20 percent). They are also nearly twice as rich as the average world, wide carbonate (average 0.24 percent). The majority of carbonates have organic carbons less than 0.30 percent but the presence of many relatively organic rich beds composed of highly bituminous, argillaceous, highly stylolitic, and algal-bearing limestones and dolomites accounts for the higher percentage of organic carbon in some of the stratigraphic units. Carbonate rocks that contain greater than 0.4 percent organic carbon and that might be considered as possible petroleum sources were noted in almost each subdivision of the Coahuilan and Comanchean Series but particularly the units of Fredericksburg 'B', Trinity 'A', Trinity 'F', and Upper Sunniland. Possible source rocks have been ascribed by others to the Lower Sunniland, but lack of sufficient samples precluded any firm assessment in this initial report. In the shallower section of the basin, organic-rich carbonates containing as much as 3.2 percent organic carbon were observed in the lowermost part of the Gulfian Series and carbonate rocks with oil staining or 'dead' and 'live oil' were noted by others in the uppermost Gulfian and upper Cedar Keys Formation. It is questionable whether these shallower rocks are of sufficient thermal maturity to have generated commercial oil. The South Florida basin is still sparsely drilled and produces only from the Sunniland Limestone at an average depth of 11,500 feet (3500 m). Because the Sunniland contains good reservoir rocks and apparently adequate source rocks, and because the success rate of new oil field discoveries has increased in recent years, the chances of finding additional oil reserves in the Sunniland are promising. Furthermore, the presence of possible source rocks in many of the other stratigraphic units, in particular, the Fredericksburg, should give further impetus to exploring for other productive horizons.

Florida↗
USGS2007

Quaternarymagmatism in the Cascades - Geologic perspectives

Foreward The Cascade magmatic arc is a belt of Quaternary volcanoes that extends 1,250 km from Lassen Peak in northern California to Meager Mountain in Canada, above the subduction zone where the Juan de Fuca Plate plunges beneath the North American Plate. This Professional Paper presents a synthesis of the entire volcanic arc, addressing all 2,300 known Quaternary volcanoes, not just the 30 or so visually prominent peaks that comprise the volcanic skyline. Study of Cascade volcanoes goes back to the geological explorers of the late 19th century and the seminal investigations of Howel Williams in the 1920s and 1930s. However, major progress and application of modern scientific methods and instrumentation began only in the 1970s with the advent of systematic geological, geophysical, and geochemical studies of the entire arc. Initial stimulus from the USGS Geothermal Research Program was enhanced by the USGS Volcano Hazards Program following the 1980 eruption of Mount St. Helens. Together, these two USGS Programs have provided more than three decades of stable funding, staffing, and analytical support. This Professional Paper summarizes the resultant USGS data sets and integrates them with the parallel contributions of other investigators. The product is based upon an all-encompassing and definitive geological database, including chemical and isotopic analyses to characterize the rocks and geochronology to provide the critical time constraints. Until now, this massive amount of data has not been summarized, and a systematic and uniform interpretation firmly grounded in geological fact has been lacking. Herein lies the primary utility of this Cascade volume. It not only will be the mandatory starting point for new workers, but also will provide essential geological context to broaden the perspectives of current investigators of specific Cascade volcanoes. Wes Hildreth's insightful understanding of volcanic processes and his uncompromising scientific integrity make him uniquely qualified to present this synthesis. During more than three decades of volcanological studies, he has carried out comprehensive investigations of Mount Adams, Mount Baker, the Three Sisters, and the Simcoe Mountains Volcanic Field. He also brings a broad experience in other volcanic arcs, having conducted integrated field and laboratory investigations at several major volcanic centers in the Andes and the Aleutian arcs. His expertise and perspective have been further enhanced by in-depth petrologic studies of caldera environments, primarily in Long Valley, California, and Yellowstone. On the basis of all these field and laboratory investigations and exhaustive literature searches, he has published three definitive petrologic syntheses addressing the passage and transformation of basaltic magmas from their mantle sources through the crust to form the many types of volcanic manifestations at the Earth's surface. A major strength of this Professional Paper is that it adheres to data first and foremost, and only then correlates these data with relevant theories. Petrological and geophysical interpretation is left to the later sections of the volume, and even there is never allowed to stray from the pertinent databases. Hildreth's interpretations are not just idle speculations, but are carefully reasoned inferences firmly based on his thorough evaluation of the observational geological data. Professional Paper 1744 should not be skimmed lightly, in the hope that the salient points will quickly rub off. Instead, every section, indeed every paragraph, presents scholarly observations and insightful interpretations that demand careful and thoughtful study. This volume will influence and guide the course of Cascade investigations for decades to come.

British Columbia, California, Oregon, Washington↗
USGS1956

Geology of the Ralston Buttes district, Jefferson County, Colorado: a preliminary report

The Ralston Buttes district in Jefferson County is one of the most significant new uranium districts located east of the Continental Divide in Colorado. The district is east of the Colorado Front Range mineral belt, along the east front of the range. From November 1953 through October 1956, about 10,000 tons of uranium ore, much of which was high-grade pitchblende-bearing vein material, was shipped from the district. The ore occurs in deposits that range in size from bodies containing less than 50 tons to ore shoots containing over 1,000 tons. The only other mining activity in the area has been a sporadic production of beryl, feldspar, and scrap mica from Precambrian pegmatites, and quarrying of dimension stone, limestone, and clay from sedimentary rocks. Most of the Ralston Buttes district consists of complexly folded Precambrian metamorphic and igneous rocks - gneiss, schist, quartzite, amphibolite, and granodiorite. Paleozoic and Mesozoic sedimentary rocks crop out in the northeastern part of the district. These rocks are cut by northwesterly-trending fault systems of Laramide age and by small bodies of intrusive rocks that are Tertiary in age. The typical uranium deposits in the district are hydrothermal veins occupying openings in Laramide fault breccias or related fractures that cut the Precambrian rocks. Pitchblende and lesser amounts of secondary uranium minerals are associated with sparse base-mental sulfides in a gangue of carbonate minerals, potash feldspar, and, more rarely, quartz. Less common types of deposits consist of pitchblende and secondary uranium minerals that occupy fractures cutting pegmatites and quartz veins. The uranium deposits are concentrated in two areas, the Ralston Creek area and the Golden Gate Canyon area. The deposits in the Ralston Creek area are located along the Rogers fault system, and the deposits in the Golden Gate Canyon area are along the Hurricane Hill fault system. Two geologic factors were important to the localization of the uranium deposits: (1) favorable structural environment and (2) favorable host rocks. The deposits in each of the two major areas are located where a northwesterly-trending Laramide fault system splits into a complex network of faults. Also, most of the deposits appear to be localized where the faults cut Precambrian rocks rich in hornblende, biotite, or garnet and biotite. The ore controls recognized in this relatively new uranium district may have wider application in areas of similar geology elsewhere in the Front Range.

Colorado↗
USGS1978

Geohydrology of the northern Louisiana salt-dome basin pertinent to the storage of radioactive wastes; a progress report

Salt domes in northern Louisiana are being considered as possible storage sites for nuclear wastes. The domes are in an area that received regional sedimentation through early Tertiary (Eocene) time with lesser amounts of Quaternary deposits. The Cretaceous-Tertiary accumulation is a few thousand feet thick; the major sands are regional aquifers that extend far beyond the boundaries of the salt-dome basin. Because of multiple aquifers, structural deformation, and variations in the hydraulic characteristics of cap rock, the ground-water hydrology around a salt dome may be highly complex. The Sparta Sand is the most productive and heavily used regional aquifer. It is either penetrated by or overlies most of the domes. A fluid entering the Sparta flow system would move toward one of the pumping centers, all at or near municipalities that pump from the Sparta. Movement could be toward surface drainage where local geologic and hydrologic conditions permit leakage to the surface or to a surficial aquifer. (Woodard-USGS)

Water-Resources Investigations Report↗
USGS1969

Geology and regional metamorphism of some high-grade cordierite gneisses, Front Range, Colorado

Cordierite is common in regional metamorphic gneisses of Precambrian age in the central part of the Front Range. It occurs in discontinuous stratigraphic units that are structurally a minor component, except locally, of the thick succession of biotite gneisses that comprise the widespread Idaho Springs Formation. The rocks have mineral assemblages, that are characteristic of the sillimanite grade of metamorphism. The cordierite occurs in three principal rock types: (1) potassic feldspar-bearing cordierite-garnet-sillimanite-biotite gneiss, (2) cordierite-biotite gneiss, and (3) cordierite-gedrite-biotite gneiss; each type contains several characteristic mineral assemblages. The rock types are gradational and overlap in areal distribution, and mainly owe their diversity in mineralogy to differences in bulk chemical composition. The field relations are consistent with an interpretation that the diverse cordierite rocks were derived from original sedimentary rocks, largely pelitic sediments. The potassic feldspar-bearing cordierite-garnet gneisses were formed from shales that contained more MgO and FeO than the more abundant sedimentary facies that yielded sillimanitic biotite gneisses. Cordierite-gedrite-biotite gneisses contain much aluminum, iron, and magnesium and little sodium and potassium as compared to the other biotite gneisses; they have an extremely low content of minor elements. Although their chemical compositions are unlike those of known modern sediments, the cordierite-gedrite gneisses are considered also to have been derived from sedimentary rocks. The physical properties and chemical compositions of the mineral phases vary somewhat from one rock type to another. Biotite varies systematically in composition, and the changes are closely related to rock type and thus to bulk composition; the MgO/FeO ratios range from 1.7 in the more mafic cordierite-gedrite rocks to 0.49 in potassic feldspar-bearing cordierite-garnet gneisses. Cordierite is magnesium-rich and intermediate in the range of composition of all analyzed cordierites (Leake, 1960); its MgO/FeO ratio is higher in the gedrite-bearing gneisses than in the potassic feldspar-bearing gneisses. The garnets consist dominantly of the almandine and pyrope molecules, and range from 64 to 75 percent almandine and from 14 to 27 pyrope. These crystals are zoned; their rims are slightly more ferrous and less magnesian than their cores. Both monoclinic and triclinic alkali feldspars coexist in the potassic feldspar-bearing cordierite-garnet gneisses. The potassic feldspars contain from 18 to 27 weight percent NaAlSi 3 O 8 . Plagioclase (oligoclase-andesine) is uncommon in the rocks. Gedrite has an MgO/FeO ratio ranging from 1 to 1.2. Associated minor minerals include iron oxides, andalusite, spinel and its alteration product högbomite, and corundum. The mineral assemblages can be correlated imperfectly with episodes of deformation and metamorphism. Relict staurolite and associated garnet occur locally as remnants of an assemblage formed early in regional metamorphism, presumably early in the first period of deformation. The dominant assemblage biotite-cordierite-garnet-magnetite-plagioclase-potassic feldspar-quartz-sillimanite and associated assemblages having fewer phases, were formed during period one and period two deformations, the principal episodes of regional dynamothermal metamorphism in the central part of the Front Range. A minor assemblage andalusite-biotite-magnetite-plagioclase-quartz was formed later, possibly coincident with a third period of deformation, largely cataclastic in effects, which was more local than the earlier deformations and metamorphism. Phase equilibria studies of the assemblage biotite-cordierite-garnet-magnetite-plagioclase-potassic feldspar-quartz-sillimanite and associated assemblages are interpreted to indicate that the cordierite assemblages approach a state of chemical equilibrium. The scatter of points in a distribution diagram can be interpreted in terms of at least two sets of equilibrium conditions that prevailed during the major plastic deformations. Other discrepancies indicating departure from a homogeneous equilibrium can be explained as a result of mosaic equilibrium involving limited diffusion of iron and magnesium for short distances. The mineral assemblages and the compositions of the ferromagnesian minerals in the cordierite rocks of this region are dependent primarily on the bulk composition of the rocks and variations in the mineral species that comprise the rocks and, to a lesser degree, on the grade of metamorphism. Biotite and cordierite are markedly more magnesian in the more mafic cordierite-gedrite-biotite gneiss than in the potassic feldspar-bearing cordierite-garnet-sillimanite-biotite gneiss. Associated microcline gneiss and biotite-sillimanite gneiss that contains muscovite as a primary stable mineral provides a means to define the metamorphic grade in the area of study. It is concluded from analyses of the assemblages with respect to theoretical phase relations in the system SiO 2 -Al 2 O 3 -Na 2 O-K 2 O-H 2 O that at least some of the rocks in the Central City-Nederland area are above the sillimanite-potassic feldspar isograd as defined by Evans and Guidotti (1966). In rocks of appropriate composition, muscovite is a stable phase in assemblages containing potassic feldspar and sillimanite. The cordierite assemblages and associated rocks are inferred to have formed in an environment having a load pressure of 3–5 kilobars (fluid pressure equaled load pressure) and a temperature somewhat in excess of 620° C.

Colorado↗
USGS1992

Allochthonous impact-shocked rocks and superimposed deformations at the Beaverhead site in southwest Montana

Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age. The shocked rocks are present in the Cabin thrust plate—one of many in the Late Cretaceous Cordilleran Thrust belt—and hence are allochthonous, having been transported tens of kilometers from the west. They are considered to represent only a piece from the central uplift of an original complex crater at least 75 km in diameter. It is speculated that some of the considerable uplift and erosion inferred to have taken place in Late Proterozoic to early Paleozoic time in east-central Idaho (The Lemhi arch) may be related to the postulated impact event. Furthermore, quasi-circular magnetic and regional gravity anomalies (50 to 75 km diameter) centered south-southeast of Challis, Idaho, may mark the concealed scar of the original impact structure.

Idaho, Montana↗
USGS1988

Map showing radon potential of rocks and soils in Fairfax County, Virginia

Since 1984, indoor radon has gained national attention as a significant health hazard in the United States. Radon is a colorless, odorless, radioactive gas derived from uranium by radioactive decay. The U.S. Environmental Protection Agency (EPA) now projects that 5,000 to 20,000 lung-cancer deaths per year may be attributed to the long-term exposure to indoor radon and its radioactive decay products. Indoor radon has been previously recognized as a health hazard associated with uranium-bearing mill tailings or building materials, but it was not until December 1984 that some natural soils and rocks were found to be sources of indoor radon at levels comparable to those in uranium mines. It is now suspected that elevated indoor radon levels are far more widespread than initially though. The EPA considers 4 picoCuries of radon per liter of air (pCi/L) as the level (in a year-round measurement) at which actions ought to be taken to lower the concentration of indoor radon. All soils and rocks contain measurable amounts of uranium, which generate measurable amounts of radon. Certain soils and rocks, however, have a greater potential to cause indoor radon problems than others because (1) they have a higher uranium content and thus can generate higher levels of radon in soil gas (gas that occupies the pores of the soil), and (2) the permeability of the sol or rack is sufficiently high that radon-bearing soil gas can flow freely and move indoors through the foundation of the structure. This study was designed to demonstrate the correlation between the geologic environment and indoor radon levels and to demonstrate a method of assessment that could be used by other informed workers in areas of their interest. A parallel study by Gundersen and others (1988) of the radon potential of rocks and soils in Montgomery County, Md., used somewhat different methods of assessment because the data available for and assessment of Montgomery County differed.

Virginia↗
USGS1989

Characterization of coal-derived hydrocarbons and source-rock potential of coal beds, San Juan Basin, New Mexico and Colorado, U.S.A.

Coal beds are considered to be a major source of nonassociated gas in the Rocky Mountain basins of the United States. In the San Juan basin of northwestern New Mexico and southwestern Colorado, significant quantities of natural gas are being produced from coal beds of the Upper Cretaceous Fruitland Formation and from adjacent sandstone reservoirs. Analysis of gas samples from the various gas-producing intervals provided a means of determining their origin and of evaluating coal beds as source rocks. The rank of coal beds in the Fruitland Formation in the central part of the San Juan basin, where major gas production occurs, increases to the northeast and ranges from high-volatile B bituminous coal to medium-volatile bituminous coal ( R m values range from 0.70 to 1.45%). On the basis of chemical, isotopic and coal-rank data, the gases are interpreted to be thermogenic. Gases from the coal beds show little isotopic variation ( δ 13 C 1 values range −43.6 to −40.5 ppt), are chemically dry (C 1 /C 1–5 values are > 0.99), and contain significant amounts of CO 2 (as much as 6%). These gases are interpreted to have resulted from devolatilization of the humic-type bituminous coal that is composed mainly of vitrinite. The primary products of this process are CH 4 , CO 2 and H 2 O. The coal-generated, methane-rich gas is usually contained in the coal beds of the Fruitland Formation, and has not been expelled and has not migrated into the adjacent sandstone reservoirs. In addition, the coal-bed reservoirs produce a distinctive bicarbonate-type connate water and have higher reservoir pressures than adjacent sandstones. The combination of these factors indicates that coal beds are a closed reservoir system created by the gases, waters, and associated pressures in the micropore coal structure. In contrast, gases produced from overlying sandstones in the Fruitland Formation and underlying Pictured Cliffs Sandstone have a wider range of isotopic values ( δ 13 C 1 values range from −43.5 to −38.5 ppt), are chemically wetter (C 1 /C 1–5 values range from 0.85 to 0.95), and contain less CO 2 (< 2%). These gases are interpreted to have been derived from type III kerogen dispersed in marine shales of the underlying Lewis Shale and nonmarine shales of the Fruitland Formation. In the underlying Upper Cretaceous Dakota Sandstone and Tocito Sandstone Lentil of the Mancos Shale, another gas type is produced. This gas is associated with oil at intermediate stages of thermal maturity and is isotopically lighter and chemically wetter at the intermediate stage of thermal maturity as compared with gases derived from dispersed type III kerogen and coal; this gas type is interpreted to have been generated from type II kerogen. Organic matter contained in coal beds and carbonaceous shales of the Fruitland Formation has hydrogen indexes from Rock-Eval pyrolysis between 100 and 350, and atomic H:C ratios between 0.8 and 1.2. Oxygen indexes and atomic O:C values are less than 24 and 0.3, respectively. Extractable hydrocarbon yields are as high as 7,000 ppm. These values indicate that the coal beds and carbonaceous shales have good potential for the generation of liquid hydrocarbons. Voids in the coal filled with a fluorescent material that is probably bitumen is evidence that liquid hydrocarbon generation has taken place. Preliminary oil-source rock correlations based on gas chromatography and stable carbon isotope ratios of C 15+ hydrocarbons indicate that the coals and (or) carbonaceous shales in the Fruitland Formation may be the source of minor amounts of condensate produced from the coal beds at relatively low levelsof thermal maturity ( R m =0.7).

International Journal of Coal Geology↗
USGS1999

Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction

This study documents three localities in the Franciscan accretionary complex of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 Ma: Point Delgada-Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane). Vein assemblages of quartz, carbonate, sulfide minerals, and adularia were precipitated locally in highly fractured wall rock. Vitrinite reflectance (Rm) values and illite crystallinity decrease away from the zones of metalliferous veins, where peak wall-rock temperatures, as determined from Rm, were as high as 315??C. The ??18O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher ??18O fluids produced widespread quartz and calcite veins that are typical of the regional paleothermal regime. The widespread veins are by-products of heat conduction and diffuse fluid flow during zeolite and prehnite-pumpellyite-grade metamorphism, and we interpret their paleofluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks. Lower ??18O fluids, in contrast, evolved from relatively high temperature exchange between seawater (or meteoric water) and basaltic and/or sedimentary host rocks; focused flow of those fluids resulted in local deposition of the metalliferous veins. Heat sources for the three paleothermal anomalies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advected either from a site of ridge-trench interaction north of the Mendocino fracture zone or from a "slabless window" in the wake of the northward migrating Mendocino triple junction. A separate paradox involves the amount of Quaternary offset of Franciscan basement rocks near Shelter Cove by on-land faults that some regard as the main active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by A.D. 1906 surface rupture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-slip component of San Andreas offset at a rate of 3-4 cm/yr, then its activity began only 83-62 ka. This interpretation would also mean that a longer term trace of the San Andreas fault must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak heating of King Range strata north of the Mendocino triple junction. Conversely, shifting the fault to the east would be compatible with a slabless window heat source and long-distance northward translation of the King Range terrane after peak heating.

Geological Society of America Bulletin↗
USGS2013

Geologic model for the assessment of undiscovered hydrocarbons in Lower to Upper Cretaceous carbonate rocks of the Fredericksburg and Washita groups, U.S. Gulf Coast Region

As part of the assessment of undiscovered oil and gas resources in Jurassic and Cretaceous strata of the U.S. Gulf Coast in 2010, the U.S. Geological Survey assessed carbonate rocks of the Fredericksburg and Washita groups and their equivalent units underlying onshore lands and State waters. One conventional assessment unit extending from south Texas to the Florida panhandle was defined: the Fredericksburg-Buda Carbonate Platform-Reef Gas and Oil assessment unit. Assessed strata range in age from Early Cretaceous Albian to Late Cretaceous Cenomanian. The assessment was based on a geologic model that incorporated the Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System of the Gulf of Mexico Basin. The following factors were evaluated to define the assessment unit and estimate undiscovered oil and gas resources: potential source rocks, hydrocarbon migration, reservoir porosity and permeability, traps and seals, structural features, depositional framework, and potential for water washing of hydrocarbons near outcrop areas. Analysis of the production history of discovered reservoirs and well data within the assessment unit was also essential for estimating the numbers and sizes of undiscovered oil and gas reservoirs within the assessment unit. The downdip boundary of the assessment unit was drawn as an arbitrary line 10 miles downdip of the Lower Cretaceous shelf margin, to include potential reef-talus reservoirs, a facies described in the geologic model developed for the assessment. Updip boundaries of the assessment unit were drawn based on the updip extent of assessment unit carbonate reservoir rocks, basin margin fault zones, and (or) the presence of producing wells within the assessed interval. Using the U.S. Geological Survey methodology, mean undiscovered resources of 40 million barrels of oil, 622 billion cubic feet of gas, and 14 million barrels of natural gas liquids were estimated for the assessment unit.

U.S. Gulf Coast↗
USGS1982

Analytical determinations from samples taken in the Ten Mile West Roadless Area, Boise and Elmore counties, Idaho

A total of 584 stream-sediment and rock samples were collected from within or near the Ten Mile West Roadless Area, Boise and Elmore Counties, Idaho, as part of a geologic study aimed at appraising the mineral resource potential of the area. Emphasis was placed on stream-sediment samples because stream sediments represent the erosional products of a drainage system and offer a rapid means of locating anomalous concentrations of ore-forming elements. An unusually high proportion of the stream-sediment samples contained concentrations of silver and molybdenum. Unaltered rocks of the area contain normal quantities of the common rock-forming elements. Some exposures of altered or mineralized rocks and some quartz veins contain significant quantities of gold and silver.

Open-File Report↗
USGS1934

Gold-quartz veins south of Libby, Montana

The area described in this paper is one of rugged mountains, composed chiefly of folded and faulted sedimentary rocks of the Belt series. In the nearby region the beds are intruded by diorite sills and granitic stocks. Glacial drift, including silt deposited in a lake, covers much of the lower slopes of the mountains and partly fills the valleys. Incomplete records show a production of $200,000 in gold, mostly from the oxidized free-milling parts of quartz veins. Except a few in the Wallace formation the veins are found in sandstone and shale of the Prichard formation. They partly replace the country rock along fractures that commonly follow bedding planes but also cut across them at moderate angles. The veins average between 1 and 2 feet in thickness and reach a maximum of 6 feet. They are cut by faults of small displacement, with which the ore bodies are commonly associated. Quartz forms 95 percent or more of the veins. Of the remainder, one or more of the minerals pyrite, galena, sphalerite, and pyrrhotite are relatively abundant, and chalcopyrite, arsenopyrite, tetrahedrite, and scheelite are present sparingly. A little silver is present, and in some veins it exceeds the gold considerably. In many veins an original quartz filling has been slightly brecciated or sheared by movement parallel to the walls. Sulphides and gold have been introduced along the shear planes, partly replacing the quartz and giving the vein a "ribbon" structure. The gold is closely associated with the sphalerite, and silver is more plentiful in the veins that carry galena. As a rule oxidation has not extended far below the surface, and its chief product is limonite. Cerusite and anglesite are present in some veins, and a little malachite, azurite, pyromorphite, or manganese oxide occurs in a few. Native gold is present in both the oxidized and unoxidized ores. Although mining of these veins in the past may have been unprofitable, that result was due partly, at least, to difficulties of transportation and milling. With improvement in roads and methods, not to mention appreciation in the value of gold, the deposits are believed to have a promising future.

Montana↗
USGS1974

Map showing potential sources of gravel and crushed-rock aggregate in the greater Denver area, Front Range Urban Corridor, Colorado

Gravel and (or) crushed-rock aggregates are essential commodities for urban development, but supplies in many places are exhausted or otherwise eliminated by urban growth. Gravel resources may be exhausted by exploitation, covered by urban spread, or eliminated from production by zoning. this conflict between a growing need and a progressively reduced supply can be forestalled by informed land-use planning. Fundamental to intelligent decisions on land use is knowledge of the physical character, distribution, and quantity of the gravel resources of an area, and of the alternative resource of rock suitable for crushing. This map has been prepared to supply data basic to land-use planning in the Front Range Urban Corridor.

Colorado↗
USGS2026

Understanding the resource potential of natural hydrogen on Earth: Scientific gaps, uncertainties and recommendations

A comprehensive scientific research roadmap is essential to bridge knowledge gaps and deepen the understanding of key geological, geochemical, and geophysical aspects of natural hydrogen (H 2 ) as a potential new energy resource. This paper reviews major scientific uncertainties on natural H 2 , suggesting research priorities, as a guide for defining exploration strategies, techniques, and data interpretation. The uncertainties concern all phases of the natural H 2 cycle, from generation (source rocks) through migration (advection and diffusion) and accumulation (reservoir and cap rocks) to the application and interpretation of subsurface and surface geochemical and geophysical exploration techniques. Understanding H 2 sources and generation rates (the amount of H 2 generated by a given volume of rock over time) is crucial for determining whether a geological H 2 system operates as a short-term dynamic system with rapid H 2 production and release, or as a conventional gas system with long-term accumulations, analogous to petroleum reservoirs. Preliminary estimates for serpentinisation, radiolysis, and organic matter degradation suggest that H 2 generation is not inherently fast, especially for non-hydrothermal continental systems (crystalline basement of shields, ophiolites, peridotite massifs, sedimentary basins), and long-term accumulations, like those of fossil natural gas systems, represent the most likely scenario. The mechanisms of H 2 migration through geological formations require application of fundamental principles of fluid-flow physics, distinguishing advection and diffusion, as well as their forms (from gas-phase, bubble flows to aqueous solutions). Additional studies of H 2 accumulation and retention in subsurface reservoirs could improve understanding of mechanisms of H 2 migration by focusing on the rock fluid-bearing properties and the factors affecting H 2 preservation, such as the presence of cap rocks impermeable to H 2 , pressure conditions, residence times, and microbial or abiotic consumption. Advanced techniques, including reservoir modelling, flow simulations, 3D imaging (micro-CT) of H 2 -bearing rocks, and extraction and analysis of gas occluded in rocks, can provide insights into the stability and potential recoverability of H 2 accumulations. The interpretation of surface exploration techniques, including gas geochemistry, geophysics, and remote sensing, long employed in mineral and energy resource exploration, is now being adapted for natural H 2 studies, but challenges remain in the data interpretation. Distinguishing H 2 seepage due to geological degassing from H 2 produced near the surface by modern microbial processes or artificial sources, such as hammering or drilling for soil-gas sampling, drilling into aquifers, and corrosion in boreholes, is an essential step in exploration. The simple detection of H 2 in soils, even in morphological structures like sub-circular depressions or “fairy circles”, cannot be cursorily interpreted as a signal of natural H 2 seepage from a deep source. A holistic geochemical approach, including isotopic analyses of gases associated with H 2 , is recommended to distinguish among the variety of possible H 2 origins. Observations of H 2 in wells should be interrogated to rule out possible artifacts such as corrosion and drill bit metamorphism. The integration of multiple geophysical methods, including seismic, gravimetric, magnetic, and electro-magnetic surveys, is recommended to mitigate interpretation ambiguities regarding the structure of a subsurface H 2 system (source and reservoir rocks, including fluid and gas storage), due to the non-uniqueness of rock-specific physical properties.

Earth-Science Reviews↗
USGS2023

Doing the same thing over and over again and getting the same result: Assessing variance in wetland invertebrate assemblages

Past efforts to explain variation of invertebrate assemblages in freshwater wetlands have been less productive than anticipated. To explore why efforts are disappointing, we assembled large invertebrate data sets from North Dakota prairie potholes, California rock pools, and Georgia Carolina bay wetlands that addressed spatial (among wetlands) and temporal (among seasons and years) variation. We anticipated that these large data-set sizes would enable robust conclusions to be drawn, and each place had unique environmental conditions that might contribute to greater explanatory power. We used statistical techniques that partitioned variation in invertebrate assemblages into spatial and/or temporal components, and that also yielded a measure of the amount of unexplained variation; Permutational Multivariate Analysis of Variation and Principal Coordinates Analysis assessed whole assemblage variation, and Analysis of Variance or Analysis of Covariance assessed variation in taxon richness, total abundances, and abundances of wide-spread individual taxa. Across all locations, variation explained by spatial and temporal factors, and unexplained variation were of comparable magnitudes (i.e., similar R 2 values of ~ 50%). Review of other published studies indicate that this pattern is widespread. The 50% or more unexplained variation is typically ignored by researchers, who instead focus on explained fractions. We argue that, besides addressing explained spatial and temporal variation in invertebrate assemblages (e.g., control by hydrology, resources, predation), efforts to understand what contributes to currently unexplained variation, that is unrelated to local spatial or temporal controls (e.g., broad climatic and biogeographic patterns, organism physiology and behavior), will lead to a fuller comprehension of how invertebrates in freshwater wetlands are controlled.

Wetlands↗