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

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At least 37 records · Page 2Linked to original sources
USGS1968

Structural pattern in central uplifts of cryptoexplosion structures as typified by Sierra Madera

The pattern of deformation in central uplifts of Sierra Madera and other well-known cryptoexplosion structures indicates that inward as well as upward movement of strata formed the uplifts. This kind of movement is incompatible with structures not of impact origin with which they have been compared. The structural style of cryptoexplosion structures, together with features that suggest shock deformation, supports the belief that they are the eroded roots of impact craters.

Science↗
USGS2009

Pore-water chemistry from the ICDP-USGS coer hole in the Chesapeake Bay impact structure--Implications for paleohydrology, microbial habitat, and water resources

We investigated the groundwater system of the Chesapeake Bay impact structure by analyzing the pore-water chemistry in cores taken from a 1766-m-deep drill hole 10 km north of Cape Charles, Virginia. Pore water was extracted using high-speed centrifuges from over 100 cores sampled from a 1300 m section of the drill hole. The pore-water samples were analyzed for major cations and anions, stable isotopes of water and sulfate, dissolved and total carbon, and bioavailable iron. The results reveal a broad transition between fresh and saline water from 100 to 500 m depth in the post-impact sediment section, and an underlying syn-impact section that is almost entirely filled with brine. The presence of brine in the lowermost post-impact section and the trend in the dissolved chloride with depth suggest a transport process dominated by molecular diffusion and slow, compaction-driven, upward flow. Major ion results indicate residual effects of diagenesis from heating, and a pre-impact origin for the brine. High levels of dissolved organic carbon (6-95 mg/L) and the distribution of electron acceptors indicate an environment that may be favorable for microbial activity throughout the drilled section. The concentration and extent of the brine is much greater than had previously been observed, suggesting its occurrence may be common in the inner crater. However, groundwater flow conditions in the structure may reduce the salt-water-intrusion hazard associated with the brine.

Special Paper of the Geological Society of America↗
USGS2005

New surveys of the Chesapeake Bay impact structure suggest melt pockets and target-structure effect

We present high-resolution gravity and magnetic field survey results over the 85-km-diameter Chesapeake Bay impact structure. Whereas a continuous melt sheet is anticipated at a crater this size, shallow-source magnetic field anomalies of ???100 nT instead suggest that impact melt pooled in kilometer-scaled pockets surrounding the base of a central peak. A central anomaly of ???300 nT may represent additional melt or rock that underwent shock-induced remagnetization. Models predict that the total volume of the melt ranges from ???0.4 to 10 km3, a quantity that is several orders of magnitude smaller than expected for an impact structure this size. However, this volume is within predictions given a transient crater of diameter of 20-40 km for a target covered with water and sedimentary deposits such that melt fragments were widely dispersed at the time of impact. Gravity data delineate a gently sloping inner basin and a central peak via a contrast between crystalline and sedimentary rock. Both features are ovoid, oriented parallel to larger preimpact basement structures. Conceptual models suggest how lateral differences in rock strength due to these preimpact structures helped to shape the crater's morphology during transient-crater modification. ?? 2005 Geological Society of America.

Geology↗
USGS2007

Lithospheric structure across the Transantarctic Mountains constrained by an analysis of gravity and thermal structure

The Transantarctic Mountains demarcate the boundary between the highly extended lithosphere of the West Antarctic Rift System and the Proterozoic East Antarctic Craton. Although the last stage of relief development was in the Eocene, the TAM retain peak elevations in excess of 4500 m. This combination of old age and high relief are difficult to reconcile, and the mechanism(s) responsible for uplift and support of this mountain range remain elusive and controversial. Recent seismic studies provide key constraints on the crustal structure. Here we constrain the lithospheric structure across this boundary by forward modeling of the gravity based on a density structure that reflects the thermal structure. Our results show that the observed very-long wavelength (>500km) gravity anomaly can be modeled by a West Antarctic lithosphere ~60 km thick, and an East Antarctic lithosphere ~250 km thick. In addition, the gravity anomaly associated with the TAM can be modeled by including the thermal effects of heat producing elements concentrated in the crust.

Open-File Report↗
USGS2010

Shallow subsurface structure of the Wasatch fault, Provo segment, Utah, from integrated compressional and shear-wave seismic reflection profiles with implications for fault structure and development

Integrated vibroseis compressional and experimental hammer-source, shear-wave, seismic reflection profiles across the Provo segment of the Wasatch fault zone in Utah reveal near-surface and shallow bedrock structures caused by geologically recent deformation. Combining information from the seismic surveys, geologic mapping, terrain analysis, and previous seismic first-arrival modeling provides a well-constrained cross section of the upper ~500 m of the subsurface. Faults are mapped from the surface, through shallow, poorly consolidated deltaic sediments, and cutting through a rigid bedrock surface. The new seismic data are used to test hypotheses on changing fault orientation with depth, the number of subsidiary faults within the fault zone and the width of the fault zone, and the utility of integrating separate elastic methods to provide information on a complex structural zone. Although previous surface mapping has indicated only a few faults, the seismic section shows a wider and more complex deformation zone with both synthetic and antithetic normal faults. Our study demonstrates the usefulness of a combined shallow and deeper penetrating geophysical survey, integrated with detailed geologic mapping to constrain subsurface fault structure. Due to the complexity of the fault zone, accurate seismic velocity information is essential and was obtained from a first-break tomography model. The new constraints on fault geometry can be used to refine estimates of vertical versus lateral tectonic movements and to improve seismic hazard assessment along the Wasatch fault through an urban area. We suggest that earthquake-hazard assessments made without seismic reflection imaging may be biased by the previous mapping of too few faults. ?? 2010 Geological Society of America.

Geological Society of America Bulletin↗
USGS2016

Structure of the Hat Creek graben region: Implications for the structure of the Hat Creek graben and transfer of right-lateral shear from the Walker Lane north of Lassen Peak, northern California, from gravity and magnetic anomalies

Interpretation of magnetic and new gravity data provides constraints on the geometry of the Hat Creek Fault, the amount of right-lateral offset in the area between Mt. Shasta and Lassen Peak, and confirmation of the influence of pre-existing structure on Quaternary faulting. Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of both normal and reversed polarity, whereas a markedly smoother magnetic field occurs over the Klamath Mountains and its Paleogene cover. Although the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek Fault, which is one of the most prominent normal faults in the region and forms the eastern margin of the Hat Creek Valley, is marked by the eastern edge of a north-trending magnetic and gravity high 20-30 km long. Modeling of these anomalies indicates that the fault is a steeply dipping (~75-85°) structure. The spatial relationship of the fault as modeled by the potential-field data, the youngest strand of the fault, and relocated seismicity suggests that deformation continues to step westward across the valley, consistent with a component of right-lateral slip in an extensional environment. Filtered aeromagnetic data highlight a concealed magnetic body of Mesozoic or older age north of Hat Creek Valley. The body’s northwest margin strikes northeast and is linear over a distance of ~40 km. Within the resolution of the aeromagnetic data (1-2 km), we discern no right-lateral offset of this body. Furthermore, Quaternary faults change strike or appear to end, as if to avoid this concealed magnetic body and to pass along its southeast edge, suggesting that pre-existing crustal structure influenced younger faulting, as previously proposed based on gravity data.

California, Nevada↗
USGS1941

Geologic structure and occurrence of gas in part of southwestern New York. Part 2. Subsurface structure in part of southwestern New York and mode of occurrence of gas in the Medina group

Based on the records of several hundred deep wells, contour maps have been prepared showing the monoclinal structure of part of western New York, and isopach lines have been drawn showing the westward convergence of the rocks. The mode of occurrence of natural gas in the Medina group is briefly discussed. The location of the gas fields has not been determined by structural traps, but rather stratigraphy and lithology are the controlling factors in trapping the gas, which occurs in porous lenses and streaks of sandstone sealed within impermeable beds. This mode of occurrences of the Medina gas makes the search for new fields in western New York more hazardous than in most natural gas regions. As structure has not formed traps for the gas there is no surface guide to favorable sites for testing, and new fields are found by haphazard drilling. It would be helpful, however, when wells are sunk, to study the lithology of the gas-bearing zone by an examination of the drill cuttings and core samples of the sand and to have electrical logs made of the wells to obtain measurements of permeability and porosity. Such tests may indicate the direction of greatest porosity in which the sand is more likely to contain gas.

New York↗
USGS2005

Petrography, structure, age, and thermal history of granitic coastal plain basement in the Chesapeake Bay impact structure, USGS-NASA Langley core, Hampton, Virginia

The USGS-NASA Langley corehole at Hampton , Va., was drilled in 2000 and was the first corehole to reach coastal plain basement in the late Eocene Chesapeake Bay impact structure . The Langley core provided samples of granite that had been concealed by 626.3 meters (2,054.7 feet) of preimpact, synimpact, and postimpact sediments. The granite, here named the Langley Granite, is pale red, medium grained, massive, and homogeneous in composition and fabric. It has a peraluminous composition (alumina saturation index 1.1) and a seriate-inequigranular, hypidiomorphic, isotropic fabric. A pervasive secondary mineral assemblage of chlorite + albite + clinozoisite is consistent with either deuteric alteration or lower greenschist-facies metamorphism. Chlorite, the principal mafic mineral, occurs as tabular masses that suggest pseudomorphous replacement of biotite. The top of the granite is weathered but not saprolitized and is nonconformably overlain by Lower Cretaceous clastic sediments. A SHRIMP 206 Pb/ 238 U weighted average zircon age of 612±10 Ma (2σ) indicates Neoproterozoic crystallization of the Langley Granite. The 40 Ar/ 39 Ar ages of microcline and plagioclase are consistent with regional cooling and uplift after the late Paleozoic Alleghanian orogeny. Zircon and apatite fission-track cooling ages of 375±44 Ma and 184±32 Ma (2σ), respectively, indicate no discernible impact -related thermal disturbance at the Langley corehole location in the annular trough of the structure about 19 kilometers (12 miles) outside the margin of the central crater. Modeling the apatite fission-track data places upper limits on the impact -related heating at this location. For an impact -related thermal disturbance equivalent to a modeled thermal spike having a duration of 1 to 0.1 million years, temperatures in this part of the impact structure could not have been higher than about 100°C-120°C. Most fractures, faults, and veins in the Langley Granite contain lower greenschist-facies minerals and are inferred to predate the impact . No shock-metamorphosed minerals or other features clearly attributable to the impact were found in the granite. Studies of the granite provide a glimpse into the nature of crystalline terranes beneath the Atlantic Coastal Plain and Chesapeake Bay and provide limits on the geographic extent of impact -generated shock and thermal effects.

Chesapeake Bay↗
USGS2006

Chesapeake Bay impact structure: Morphology, crater fill, and relevance for impact structures on Mars

The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best-preserved "wet-target" craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile-rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ???85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the "inverted sombrero" morphology observed at some craters in layered targets. The distribution of crater-fill materials i n the CBIS is related to the morphology. Suevitic breccia, including pre-resurge fallback deposits, is found in the central crater. Impact-modified sediments, formed by fluidization and collapse of water-saturated sand and silt-clay, occur in the annular trough. Allogenic sediment-clast breccia, interpreted as ocean-resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat-floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar-sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles. ?? The Meteoritical Society, 2006.

Meteoritics and Planetary Science↗
USGS1977

Chemical structure of humic acids - Part 1, a generalized structural model

A new model is proposed for the structure of humic acids. In this model humic acid is pictured as being made up of a hierarchy of structural elements. At the lowest level in this hierarchy are simple phenolic, quinoid, and benzene carboxylic acid groups. These groups are bonded covalently into small particles. Particles of similar chemical structure are linked together by weak bonds to form "homogeneous" aggregates. Two or more different types of aggregates may be linked together to form mixed aggregates. Complexes of humic acid and clay minerals are also formed.

Journal of Research of the U.S. Geological Survey↗
USGS2002

Dipolar-dephasing 13C NMR studies of decomposed wood and coalified xylem tissue: Evidence for chemical structural changes associated with defunctionalization of lignin structural units during coalification

A series of decomposed and coalified gymnosperm woods was examined by conventional solid-state 13C nuclear magnetic resonance (NMR) and by dipolar-dephasing NMR techniques. The results of these NMR studies for a histologically related series of samples provide clues as to the nature of codification reactions that lead to the defunctionalization of lignin-derived aromatic structures. These reactions sequentially involve the following: (1) loss of methoxyl carbons from guaiacyl structural units with replacement by hydroxyls and increased condensation; (2) loss of hydroxyls or aryl ethers with replacement by hydrogen as rank increases from lignin to high-volatile bituminous coal; (3) loss of alkyl groups with continued replacement by hydrogen. The dipolar-dephasing data show that the early stages of coalification in samples examined (lignin to lignite) involve a decreasing degree of protonation on aromatic rings and suggest that condensation is significant during coalification at this early stage. An increasing degree of protonation on aromatic rings is observed as the rank of the sample increases from lignite to anthracite.

Energy & Fuels↗
USGS2003

CO2 hydrate: Synthesis, composition, structure, dissociation behavior, and a comparison to structure I CH4 hydrate

Structure I (sI) carbon dioxide (CO 2 ) hydrate exhibits markedly different dissociation behavior from sI methane (CH 4 ) hydrate in experiments in which equilibrated samples at 0.1 MPa are heated isobarically at 13 K/h from 210 K through the H 2 O melting point (273.15 K). The CO 2 hydrate samples release only about 3% of their gas content up to temperatures of 240 K, which is 22 K above the hydrate phase boundary. Up to 20% is released by 270 K, and the remaining CO 2 is released at 271.0 plusmn; 0.5 K, where the sample temperature is buffered until hydrate dissociation ceases. This reproducible buffering temperature for the dissociation reaction CO 2 ·nH 2 O = CO 2 (g) + nH 2 O(1 to s) is measurably distinct from the pure H 2 O melting point at 273.15 K, which is reached as gas evolution ceases. In contrast, when si CH 4 hydrate is heated at the same rate at 0.1 MPa, >95% of the gas is released within 25 K of the equilibrium temperature (193 K at 0.1 MPa). In conjunction with the dissociation study, a method for efficient and reproducible synthesis of pure polycrystalline CO 2 hydrate with suitable characteristics for material properties testing was developed, and the material was characterized. CO 2 hydrate was synthesized from CO 2 liquid and H 2 O solid and liquid reactants at pressures between 5 and 25 MPa and temperatures between 250 and 281 K. Scanning electron microscopy (SEM) examination indicates that the samples consist of dense crystalline hydrate and 50-300 μm diameter pores that are lined with euhedral cubic hydrate crystals. Deuterated hydrate samples made by this same procedure were analyzed by neutron diffraction at temperatures between 4 and 215 K; results confirm that complete conversion of water to hydrate has occurred and that the measured unit cell parameter and thermal expansion are consistent with previously reported values. On the basis of measured weight gain after synthesis and gas yields from the dissociation experiments, approximately all cages in the hydrate structure are filled such that n ≈ 5.75.

Journal of Physical Chemistry B↗
USGS2018

Crustal structure and quaternary acceleration of deformation rates in central Washington revealed by stream profile inversion, potential field geophysics, and structural geology of the Yakima folds

Post‐Miocene tectonic uplift along fault‐cored anticlines within central Washington produced the Yakima Fold Province, a region of active NNE‐SSW shortening in the Cascadian backarc. The relative timing and rate of deformation along individual structures is coarsely defined yet imperative for seismic hazard assessment. In this work, we use geomorphic and geophysical mapping, stream profile inversion, and balanced cross‐section methods to constrain fault geometries and slip rates in the Yakima Canyon region. We extract stream profiles from LiDAR data and analytically solve for the rate of relative rock uplift along several active fault‐cored anticlines. To constrain the fault geometries at depth and the long‐term magnitude of deformation, we constructed two line‐balanced cross sections across the folds with forward‐modeled magnetic and gravity anomaly data. Our stream profile results indicate an increase of incision rates in the Pleistocene, and we infer the increase is tectonically controlled. We estimate modern slip rates between 0.4 and 0.6 mm/year accommodated on reverse faults that core the Manastash Ridge, Umtanum Ridge, and Selah Butte anticlines and establish that these faults reactivate and invert older normal faults in basement rocks. Finally, we calculate the time required to accumulate sufficient strain energy for a large magnitude earthquake (M ≥ 7) along individual structures in the Yakima Fold Province. Results show that the Yakima folds likely accommodate large magnitude earthquakes and that it takes several hundred to several thousand years to accumulate sufficient strain energy for an M ≥ 7 earthquake.

Washington↗
USGS2012

Crustal structure of east central Oregon: Relation between Newberry Volcano and regional crustal structure

A 180-km-long seismic refraction transect from the eastern High Cascades, across Newberry Volcano, to the eastern High Lava Plains is used to investigate the subvolcanic crustal and upper mantle velocity structure there. Near-surface volcanic flows and sedimentary debris (1.6-4.7 km/s), ranging from 3 to 5 km in thickness, overlie subvolcanic Basin and Range structures. East and west of Newberry Volcano, the subvolcanic basement (5.6 km/s) has been downwarped, producing 5-km-deep basins. The midcrust (8- to 28-km depth) is characterized by velocities ranging from 6.1 to 6.5 km/s and varies laterally in thicknesses. The lower crust is characterized by an unusually high velocity (about 7.4 km/s), and its geometry mirrors the subvolcanic basement geometry. The Moho is located at a depth of 37 km and represents a transition to an upper mantle velocity of 8.1 km/s. The shallow subsurface (1.2 km) beneath Newberry Volcano is characterized by high-velocity 5.6 km/s, versus 4.1 km/s for the surrounding area) intrusions and appears to be located on a basement high. Beneath the seismic refraction array at Newberry Volcano, an absence of low-velocity anomalies suggests that large silicic magma chambers do not exist in the upper crust, but apparent high attenuation of the seismic wave field may be consistent with either partial melts in small volumes, elevated crustal temperatures, and/or poor geophone-recording site coupling.

Oregon↗
USGS1986

Crustal structure of Yunnan province, People's Republic of China, from seismic refraction profiles

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile but not on another at right-angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Yunnan Province↗
USGS2009

Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA

The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite-K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase-quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middle- to upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafic source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites.

Virginia↗
USGS1998

Structure contours of top of Laramie-Fox Hills aquifer in "Structure, outcrop, and subcrop of the bedrock aquifers along the western margin of Denver Basin, Colorado." Hydrologic Atlas 742

This digital geospatial data set consists of structure contours of the top of the Laramie-Fox Hills aquifer along the Front Range of Colorado. The U.S. Geological Survey developed this data set as part of a project described in the report, "Structure, Outcrop, and Subcrop of the Bedrock Aquifers Along the Western Margin of the Denver Basin, Colorado" (Robson and others, 1998).

Colorado↗
USGS1998

Structure contours of base of upper Arapahoe aquifer in "Structure, outcrop, and subcrop of the bedrock aquifers along the western margin of the Denver Basin, Colorado." Hydrologic Atlas 742

This digital geospatial data set consists of structure contours on the base of the upper member of the Arapahoe aquifer. The U.S. Geological Survey developed this data set as part of a project described in the report,"Structure, Outcrop, and Subcrop of the Bedrock Aquifers Along the Western Margin of the Denver Basin, Colorado" (Robson and others, 1998)

Colorado↗