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At least 1,351 records · Page 75Linked to original sources
USGS2022

A methodology to assess the historical environmental footprint of in-situ recovery (ISR) of uranium: A demonstration in the Goliad Sand in the Texas Coastal Plain, USA

In-situ recovery (ISR) has been the only technique used to extract uranium from sandstone-hosted uranium deposits in the Pliocene Goliad Sand in the Texas Coastal Plain. Water plays a crucial role throughout the ISR lifecycle of production and groundwater restoration yet neither the water use nor other environmental footprints have been well documented. The goal of this study is to examine historical records for all six ISR operations completed in the Goliad Sand to identify and quantify parameters that indicate the surface and aquifer disturbances, water use, and radon emissions. Overall, the average mine area was 0.00023 ± 0.00006 acres per pound (ac/lb) U 3 O 8 . The average mine pore volume was 48.9 ± 50 gal/lb U 3 O 8 with a minimum affected aquifer volume of 0.51 ± 0.08 cubic feet per pound (cu ft/lb) U 3 O 8. An average of 258 ± 40 gallons (gal) of fluid were disposed per pound (lb) U 3 O 8 , with an average of 169 ± 26 gal/lb U 3 O 8 attributed to restoration and 89 ± 36 gal/lb U 3 O 8 attributed to the uranium production phase. The average radon emitted was 1.06 × 10 −3 ± 7.4 × 10 −4 curies per pound (Ci/lb) U 3 O 8 . Goodness-of-fit ( R 2 ) values are ≥0.79 for linear regressions of the amount of uranium produced versus mine area, mine pore volumes, mine aquifer volumes, water pumped, and total water disposed. The R 2 value for radon emitted was 0.68. However, the water disposed only during the uranium production phase is more strongly correlated to the number of production days ( R 2 = 0.96) than to uranium production ( R 2 = 0.84), whereas the volume of water disposed during restoration is more strongly correlated to the “pore volume” ( R 2 = 0.97) than to uranium production ( R 2 = 0.90). Pore volume is an industry term used to describe the amount of fluid circulated through the aquifer during the uranium production period and stipulated in bond agreements in order to satisfy groundwater restoration requirements. Models constructed in this study can be used to estimate probable water use and the extent of surface and aquifer disturbances associated with ISR-amenable undiscovered uranium resources in the Goliad Sand. The historical perspective offered by the data compiled and correlations may prove useful to both industry and regulators.

Texas↗
USGS2021

Identifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources

The San Gabriel and Canton faults represent early stages in the development of the San Andreas fault system. However, questions of timing of initiation and magnitude of slip on these structures remain unresolved, with published estimates ranging from 42-75 km and likely starting in the Miocene. This uncertainty in slip history reflects an absence of appropriate piercing points. We attempt to better constrain the slip history on these faults by quantifying the changing proportions of source terranes contributing sediment to the Ventura Basin, California, through the Cenozoic, including refining data for a key piercing point. Ventura Basin sediments show an increase in detrital zircon U-Pb dates and mineral abundances associated with crystalline sources in the northern San Gabriel Mountains through time, which we interpret to record the basin’s northwest translation by dextral strike-slip faulting. In particular, an Oligocene unit mapped as part of the extra-regional Sespe Formation instead has greater affinity to the Vasquez Formation. Specifically, the presence of a unimodal population of ~1180 Ma zircon, high (57%) plagioclase content, and proximal alluvial fan facies indicate that the basin was adjacent to the San Gabriel anorthosite during deposition of the Vasquez Formation, requiring 35-60 km of slip on the San Gabriel-Canton fault system. Mixture modeling of detrital zircon data supported by automated mineralogy highlights the importance of this piercing point along the San Gabriel-Canton fault system and suggests that fault slip began during the late Oligocene to early Miocene, which is earlier than published models. These two lines of evidence disagree with recent models that estimate >60 km of offset, requiring a reappraisal of the slip history of an early strand of the San Andreas transform zone.

California↗
USGS2020

Conceptual framework and approach for conducting a geoenvironmental assessment of undiscovered uranium resources

This report presents a novel conceptual framework and approach for conducting a geologically based environmental assessment, or geoenvironmental assessment, of undiscovered uranium resources within an area likely to contain uranium deposits. The framework is based on a source-to-receptor model that prioritizes the most likely contaminant sources, contaminant pathways, and affected environmental media for three common uranium extraction methods—open pit or underground mining with milling and in situ recovery (ISR). Data on regional geology, hydrology, and climate, as well as historical uranium mining and milling records are used to estimate the probable amounts of waste rock, tailings, wastewater, surface land disturbance, and subsurface aquifer disturbance for likely mining methods. Constituents of concern that might take the form of leachates, dust, radon, and sediments formed by chemical and physical weathering are also identified in the geoenvironmental assessment. Finally, areas where constituents of concern are likely to occur and persist in air, land, surface water, and groundwater are indicated by the potential for dispersion of dust by wind, accumulation of radon because of air stagnation, dispersion of sediments and wastewater by runoff, and infiltration of wastewater or leachates with consideration of the likely mobility of contaminants in surface water and groundwater. The geoenvironmental assessment output can be summarized in the following primary products: (1) a descriptive geoenvironmental model; (2) maps and statistics of variables that indicate the potential for constituents of concern to occur and persist in air, land, surface water, and groundwater within a tract that is geologically permissive for the occurrence of uranium; and (3) tables providing estimated or indicated quantities of waste rock, tailings, wastewater, dust, and radon emissions that could be associated with undiscovered uranium resources, if extracted, for each permissive tract. The uranium geoenvironmental assessment could help natural resource managers to prioritize and (or) identify (1) important potential contaminant pathways, (2) management practices required depending on the types of constituents that could be of concern, (3) areas for response in the event of accidental release, and (4) future directions for study. Furthermore, indicators of rock and water volumes potentially associated with an undiscovered uranium deposit may be evaluated to make quantitative comparisons of water required for uranium production or potential waste products generated during uranium extraction from areas permissive for uranium resource occurrence throughout the United States.

Scientific Investigations Report↗
USGS2019

Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy

The dacite of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene tephra eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and phenocryst assemblage establish that certain conspicuous dissected pumice exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene Plinian eruption . Deposit thicknesses and pumice sizes support an eruption on the order of low Volcanic Explosivity Index (VEI) 5, atypically explosive for dominantly lava-producing Mount Rainier. Statistically permissible 40 Ar/ 39 Ar plateau ages of plagioclase phenocryst separates are 138 ± 20 ka and 101 ± 11 ka (2σ). A previously published result of 206 ± 11 ka is herein shown to result from a sample selection error. Zircon from the pumice yields a U-Th crystallization age of 147 ± 8 ka if the isochron is required to pass through the tephra U-Th isotopic composition . In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved 40 Ar/ 39 Ar ages of stratigraphically associated lavas high on Mount Rainier, and is the best estimate of the pumice's true eruption age. Inclusions of undegassed melt (glass) in the plagioclase separates are proposed as biasing apparent 40 Ar/ 39 Ar plateau ages to old values through coupling of undegassed magmatic excess Ar with radiogenic Ar that accumulated post-eruptively from relatively K-rich glass. U-Th ages record zircon growth prior to eruption, consistent with a possible complex history of advanced solidification followed by remobilization . The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate Glaciation (MIS 6) and Last Glacial Maximum (MIS 2). This eruption age also provides an important time marker for glacial and other sedimentary deposits in southern Puget Sound lowland that, excepting the Vashon Drift (MIS 2), generally lack reliable age determinations.

Washington↗
USGS2013

Morphology and stratal geometry of the Antarctic continental shelf: Insights from models

Reconstruction of past ice-sheet fluctuations from the stratigraphy of glaciated continental shelves requires understanding of the relationships among the stratal geometry, glacial and marine sedimentary processes, and ice dynamics. We investigate the formation of the morphology and the broad stratal geometry of topsets on the Antarctic continental shelf with numerical models. Our models assume that the stratal geometry and morphology are principally the results of time-integrated effects of glacial erosion and sedimentation related to the location of the seaward edge of the grounded ice. The location of the grounding line varies with time almost randomly across the shelf. With these simple assumptions, the models can successfully mimic salient features of the morphology and the stratal geometry. The models suggest that the current shelf has gradually evolved to its present geometry by many glacial advances and retreats of the grounding line to different locations across the shelf. The locations of the grounding line do not appear to be linearly correlated with either fluctuations in the 5 l s O record (which presumably represents changes in the global ice volume) or with the global sea-level curve, suggesting that either a more complex relationship exists or local effects dominate. The models suggest that erosion of preglacial sediments is confined to the inner shelf, and erosion decreases and deposition increases toward the shelf edge. Some of the deposited glacial sediments must be derived from continental erosion. The sediments probably undergo extensive transport and reworking obliterating much of the evidence for their original depositional environment. The flexural rigidity and the tectonic subsidence of the underlying lithosphere modify the bathymetry of the shelf, but probably have little effect on the stratal geometry. Our models provide several guidelines for the interpretation of unconformities, the nature of preserved topset deposits, and the significance of progradation versus aggradation of shelf sediments.

Book chapter↗
USGS2013

Spatial and temporal variations in landscape evolution: historic and longer-term sediment flux through global catchments

Sediment generation and transport through terrestrial catchments influence soil distribution, geochemical cycling of particulate and dissolved loads, and the character of the stratigraphic record of Earth history. To assess the spatiotemporal variation in landscape evolution, we compare global compilations of stream gauge–derived () and cosmogenic radionuclide (CRN)–derived (predominantly 10 Be; ) denudation of catchments (mm/yr) and sediment load of rivers (Mt/yr). Stream gauges measure suspended sediment loads of rivers during several to tens of years, whereas CRNs provide catchment-integrated denudation rates at 10 2 –10 5 -yr time scales. Stream gauge–derived and CRN-derived sediment loads in close proximity to one another (<500 km) exhibit broad similarity ( stream gauge samples; CRN samples). Nearly two-thirds of CRN-derived sediment loads exceed historic loads measured at the same locations (). Excessive longer-term sediment loads likely are a result of longer-term recurrence of large-magnitude sediment-transport events. Nearly 80% of sediment loads measured at approximately the same locations exhibit stream gauge loads that are within an order of magnitude of CRN loads, likely as a result of the buffering capacity of large flood plains. Catchments in which space for deposition exceeds sediment supply have greater buffering capacity. Superior locations in which to evaluate anthropogenic influences on landscape evolution might be buffered catchments, in which temporary storage of sediment in flood plains can provide stream gauge–based sediment loads and denudation rates that are applicable over longer periods than the durations of gauge measurements. The buffering capacity of catchments also has implications for interpreting the stratigraphic record; delayed sediment transfer might complicate the stratigraphic record of external forcings and catchment modification.

The Journal of Geology↗
USGS2005

Catastrophic meltwater discharge down the Hudson Valley: A potential trigger for the Intra-Allerød cold period

Glacial freshwater discharge to the Atlantic Ocean during deglaciation may have inhibited oceanic thermohaline circulation, and is often postulated to have driven climatic fluctuations. Yet attributing meltwater-discharge events to particular climate oscillations is problematic, because the location, timing, and amount of meltwater discharge are often poorly constrained. We present evidence from the Hudson Valley and the northeastern U.S. continental margin that establishes the timing of the catastrophic draining of Glacial Lake Iroquois, which breached the moraine dam at the Narrows in New York City, eroded glacial lake sediments in the Hudson Valley, and deposited large sediment lobes on the New York and New Jersey continental shelf ca. 13,350 yr B.P. Excess 14 C in Cariaco Basin sediments indicates a slowing in thermohaline circulation and heat transport to the North Atlantic at that time, and both marine and terrestrial paleoclimate proxy records around the North Atlantic show a short-lived (<400 yr) cold event (Intra-Allerød cold period) that began ca. 13,350 yr B.P. The meltwater discharge out the Hudson Valley may have played an important role in triggering the Intra-Allerød cold period by diminishing thermohaline circulation.

Geology↗
USGS1972

Reconnaissance geology of the Bi'r Al Badriyah quadrangle, Kingdom of Saudi Arabia

The Bi'r al Badriyah quadrangle covers an area of 2843 sq km in the extreme eastern part of the Precambrian Shield in central Saudi Arabia. The Precambrian rocks in the southeastern part of the area are unconformably overlain by limestone of Permian age, which occupies only a small part of the quadrangle. Three great sequences of Precambrian rocks are recognized and called, from oldest to youngest, the Halaban Group, the Bi'r Khountina Group, and the Murdama Group. From evidence within the quadrangle itself the three groups are seen to be separated by erosional unconformities, and the Halaban and Bi'r Khountina Groups are intruded by granitic and gabbroic plutonic rocks and a wide variety of dikes. Exposures north of the quadrangle show that the Murdama Group is also intruded by granitic rocks. The Halaban Group, consists of three formations called, from oldest to youngest, the Umm Mushraha Formation, the Jebal al Egfool Formation, and the Wadi al Jifr Formation. These rocks are variably metamorphosed, but characteristically they are at the epidote-albite amphibolite facies of regional metamorphism, are polymetamorphic, and rest unconformably on ancient granite gneiss. The Umm Mushraha Formation consists of amphibolite, schistose andesite, and greenstone associated with minor meta-agglomerate, meta-graywacke, and marble. The Jebal al Egfool Formation consists of sheared and metamorphosed volcanic rocks of intermediate composition, and the Wadi al Jifr Formation is made up of metamorphosed felsic volcanic rocks. Rocks of the Bi'r Khountina Group are rather similar to the Halaban Group in original composition, but they are separated from the Halaban by an angular unconformity and have distinctive formations of conglomerate (the Idsas Formation), marble (the Fawara Formation), and graywacke (the Abu Sawarir Formation) at the base. Most of the Bi'r Khountina Group consists of andesitic volcanic rocks in the Badriyah Formation. Mostly, the Bi'r Khountina Group is unmetamorphosed or feebly metamorphosed to the greenschist facies. Locally, as in the aureole of intrusive rocks, the Bi'r Khountina Group reaches higher metamorphic grades. The rocks of the Murdama Group unconformably overlie the Bi'r Khountina Group, and basal conglomerates of the Murdama contain fragments of distinctive rocks intrusive into the Bi'r Khountina. The Murdama Group is composed of the Z'relba Formation, Consisting of conglomerate and schistose conglomerate and the Abt Formation, a thick sequence of laminated, mainly fine-grained, sedimentary rocks. Metamorphism in .the Murdama increases toward the north, and along the north edge of the area the Murdama Group is represented by chlorite-sericite schists. Three major episodes of folding are recorded in the Precambrian rocks. The earliest episode folded the rocks of the Halaban Group along north-trending axes which are now truncated by the unconformity at the base of the Bi'r Khountina Group. After the rocks of the Bi'r Khountina and Murdama Groups were deposited, the three groups were folded along northwest-trending axes of regional scale. Later, possibly at a time of major faulting along a northwesterly direction, the rocks were again folded on north-trending axes. Samples of rock materials collected at 105 localities in the quadrangle were analyzed for 27 elements. Only one anomalous value was found: a low positive anomaly for lead at the Umm Mushraha gold mine. Threshold amounts of Ag, B, Cr, Cu, La, Mn, Mo, NI, Sc, Sn, Ti, V, and Zr were detected. The distribution of copper and molybdenum suggests that two plutons of biotite-hornblende granite in the central part of the quadrangle, and the rocks between them, should be examined for possible disseminated copper and molybdenum of the porphyry type. The distribution of scandium and detrital scheelite suggests that a zone adjacent to phyllonite in the northeastern quarter of the quadrangle be examined for tungsten associated with amphibolite.

Open-File Report↗
USGS1982

Distribution of gas-charged sediment in Norton Sound and Chirikov Basin.

Numerous zone of anomalous acoustic resonses caused by gas in the subsurface sediment layers have been detected on seismic reflection records from Norton Sound and Chirikov Basin. The frequency and distribution of these zones suggest that as much as 7000km2 of the northeastern Bering Sea may be underlain by gas-charged sediment. Much of the gas is of shallow biogenic origin, having been generated in buried peat deposits. Seismic velocity beneath a large gas seep south of Nome decreases in the interval from 250 to 440m below the seafloor, where thermogenic gases of deeper origin are migrating upward along a system of basin margin faults.-from Authors

Geologie en Mijnbouw↗
USGS1984

A new occurrence of telluride minerals in South Carolina

A study of drill cores from the Haile gold mine, Lancaster County, South Carolina, has revealed grains containing large amounts of Te with various combinations of Pb, Ag and Au in pyrite. These telluride minerals have so far not been identified. The nearby Brewer mine, on the basis of chemical evidence, also contains tellurides. The probable telluride localities in South Carolina are now expanded to three, significantly increasing the few reports of Te minerals from the Au deposits of the southeastern Piedmont, many of which are now considered to be volcanogenic. The occurrence of telluride minerals in gold ore from the Haile-Brewer area may help to explain the divergence in Au/Ag ratios reported in chemical analyses of drill core, ore samples and production records. Te, in addition, may be useful in geochemical exploration programmes in the SE Piedmont, including programmes using heavy mineral concentrates derived from stream alluvium. -R.S.M.

South Carolina↗
USGS2011

Strong ground motion in Port-au-Prince, Haiti, during the M7.0 12 January 2010 Haiti earthquake

No strong motion records are available for the 12 January 2010 M7.0 Haiti earthquake. We use aftershock recordings as well as detailed considerations of damage to estimate the severity and distribution of mainshock shaking in Port-au-Prince. Relative to ground motions at a hard - rock reference site, peak accelerations are amplified by a factor of approximately 2 at sites on low-lying deposits in central Port-au-Prince and by a factor of 2.5 - 3.5 on a steep foothill ridge in the southern Port-au-Prince metropolitan region. The observed amplification along the ridge cannot be explained by sediment - induced amplification , but is consistent with predicted topographic amplification by a steep, narrow ridge. Although damage was largely a consequence of poor construction , the damage pattern inferred from analysis of remote sensing imagery provides evidence for a correspondence between small-scale (0.1 - 1.0 km) topographic relief and high damage. Mainshock shaking intensity can be estimated crudely from a consideration of macroseismic effects . We further present detailed, quantitative analysis of the marks left on a tile floor by an industrial battery rack displaced during the mainshock, at the location where we observed the highest weak motion amplifications. Results of this analysis indicate that mainshock shaking was significantly higher at this location (~0.5 g , MMI VIII) relative to the shaking in parts of Port-au-Prince that experienced light damage. Our results further illustrate how observations of rigid body horizontal displacement during earthquakes can be used to estimate peak ground accelerations in the absence of instrumental data .

Port-au-Prince↗
USGS2014

Cenozoic mountain building on the northeastern Tibetan Plateau

Northeastern Tibetan Plateau growth illuminates the kinematics, geodynamics, and climatic consequences of large-scale orogenesis, yet only recently have data become available to outline the spatiotemporal pattern and rates of this growth. I review the tectonic history of range growth across the plateau margin north of the Kunlun fault (35°–40°N) and east of the Qaidam basin (98°–107°E), synthesizing records from fault-bounded mountain ranges and adjacent sedimentary basins. Deformation began in Eocene time shortly after India-Asia collision, but the northeastern orogen boundary has largely remained stationary since this time. Widespread middle Miocene–Holocene range growth is portrayed by accelerated deformation, uplift, erosion, and deposition across northeastern Tibet. The extent of deformation, however, only expanded ~150 km outward to the north and east and ~150 km laterally to the west. A middle Miocene reorganization of deformation characterized by shortening at various orientations heralds the onset of the modern kinematic regime where shortening is coupled to strike slip. This regime is responsible for the majority of Cenozoic crustal shortening and thickening and the development of the northeastern Tibetan Plateau.

Book chapter↗
USGS2012

Sixty thousand years of magmatic volatile history before the caldera-forming eruption of Mount Mazama, Crater Lake, Oregon

The well-documented eruptive history of Mount Mazama, Oregon, provides an excellent opportunity to use pre-eruptive volatile concentrations to study the growth of an explosive silicic magmatic system. Melt inclusions (MI) hosted in pyroxene and plagioclase crystals from eight dacitic–rhyodacitic eruptive deposits (71–7.7 ka) were analyzed to determine variations in volatile-element concentrations and changes in magma storage conditions leading up to and including the climactic eruption of Crater Lake caldera. Temperatures (Fe–Ti oxides) increased through the series of dacites, then decreased, and increased again through the rhyodacites (918–968 to ~950 to 845–895 °C). Oxygen fugacity began at nickel–nickel-oxide buffer (NNO) +0.8 (71 ka), dropped slightly to NNO +0.3, and then climbed to its highest value with the climactic eruption (7.7 ka) at NNO +1.1 log units. In parallel with oxidation state, maximum MI sulfur concentrations were high early in the eruptive sequence (~500 ppm), decreased (to ~200 ppm), and then increased again with the climactic eruption (~500 ppm). Maximum MI sulfur correlates with the Sr content (as a proxy for LREE, Ba, Rb, P 2 O 5 ) of recharge magmas, represented by basaltic andesitic to andesitic enclaves and similar-aged lavas. These results suggest that oxidized Sr-rich recharge magmas dominated early and late in the development of the pre-climactic dacite–rhyodacite system. Dissolved H 2 O concentrations in MI do not, however, correlate with these changes in dominant recharge magma, instead recording vapor solubility relations in the developing shallow magma storage and conduit region. Dissolved H 2 O concentrations form two populations through time: the first at 3–4.6 wt% (with a few extreme values up to 6.1 wt%) and the second at ≤2.4 wt%. CO 2 concentrations measured in a subset of these inclusions reach up to 240 ppm in early-erupted deposits (71 ka) and are below detection in climactic deposits (7.7 ka). Combined H 2 O and CO 2 concentrations and solubility models indicate a dominant storage region at 4–7 km (up to 12 km), with drier inclusions that diffusively re-equilibrated and/or were trapped at shallower depths. Boron and Cl (except in the climactic deposit) largely remained in the melt, suggesting vapor–melt partition coefficients and gas fractions were low. Modeled Li, F, and S vapor–melt partition coefficients are higher than those of B and Cl. The decrease in maximum MI CO 2 concentration following the earliest dacitic eruptions is interpreted to result from a broadening of the shallow storage region to greater than the diameter of subjacent feeders, so that greater proportions of reservoir magma were to the side of CO 2 -bearing vapor bubbles ascending vertically from the locus of recharge magma injection, thereby escaping recarbonation by streaming vapor bubbles. The Mazama melt inclusions provide a picture of a growing magma storage region, where chemical variations in melt and magma occur due to changes in the nature and supply rate of magma recharge, the timing of degassing, and the possible degree of equilibration with gases from below.

Oregon↗
USGS1991

Geologic controls on the formation and evolution of quaternary coastal deposits of the northern Gulf of Mexico

A study of the barrier islands and wetlands in the deltaic plain of Louisiana is presented. Its purpose was to document rapid changes and to learn more about the processes responsible and the geologic framework within which they operate. It included systematic collection and analysis of precision nearshore hydrographic data, high resolution seismic profiles, surface sediment samples, continuous vibracores, digital shoreline plots, records of storm overwash events, and analysis of tide gage records to quantify the rise in relative sea level. Results from these studies demonstrate that deltaic progradation, river channel switching, and subsequent rapid erosion accompanying the marine transgression are regular and predictable events along the Mississippi River delta plain and will likely continue in the future. Mitigation measures, such as shoreline nourishment and barrier restoration, that mimic the natural processes may slow the land loss.

Conference Paper↗
USGS1996

Alamo megabreccia: Record of a late devonian impact in southern Nevada

The Alamo breccia is probably the most voluminous known outcropping carbonate megabreccia. It occupies ~4000 km2 across 11 mountain ranges in southern Nevada, has an average thickness of ~70 m, and contains a volume of 250+ km3. The breccia is a single bed, of early Frasnian (early Late Devonian) age, that formed in the wake of a giant slide that deposited a lower chaotic debrite, containing clasts as large as 80 × 500 m, and an upper exquisitely graded turbidite. It is anomalously intercalated with cyclic shallow-water platform carbonates of the Guilmette Formation. The Alamo breccia is interpreted as a product of the Alamo event, a nearby marine impact of an extraterrestrial object, whereby impact-generated crustal shock waves and/or marine superwaves detached the upper ~60 m of platform along a horizontal surface. Loosened bedrock slid seaward across the platform, and some of it accumulated as the lower debrite. Rock-water exchange induced landward-propagated tsunami(s), whose uprush and/or backwash deposited the upper turbidite, partly above sea level. Evidence for impact includes shockedquartz grains, an iridium anomaly, and reworked conodonts, all found only within the breccia. Because the Alamo breccia is not known outside of Nevada, and because the early Frasnian time of the Alamo event is not noted for accelerated extinctions, being ~3 m.y. before the Frasnian-Famennian impact(s) and biotic crisis, the impact was probably only of moderate size.

Nevada↗
USGS2021

New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

British Columbia, Northwest Territories↗
USGS2024

Spread and frequency of explosive silicic volcanism of the Carpathian-Pannonian Region during Early Miocene: Clues from the SW Pannonian Basin and the Dinaridesion during Early Miocene: clues from the SW Pannonian Basin and the Dinarides

Explosive silicic volcanism of the Carpathian-Pannonian Region (CPR) is increasingly recognized as the primary source of tephra across the Alpine-Mediterranean region during the Early and Middle Miocene. However, the tephrostratigraphic framework for this period of volcanic activity is still incomplete. We present new multi-proxy data from Lower Miocene ignimbrites and tephra fallout deposits from the southwestern CPR and the Dinaride Lake System and integrate them into existing datasets to better resolve the regional extent and scale of these eruptions of the CPR. Volcanic glass geochemistry indicates distal fallout tuffs deposited in the Sinj Basin are correlative with the proximal Ostoros ignimbrites from the Bükkalja Volcanic Field, indicative of regionally extensive volcanism at 17.295 ± 0.028 Ma, based on CA-ID-TIMS U Pb zircon geochronology. Based on integrated tephrostratigraphic data, newly identified 17.064 ± 0.010 Ma massive rhyolitic ignimbrite deposits from the Kalnik Volcaniclastic Complex located in the southwestern CPR are correlative with the 17.062 ± 0.010 Ma Mangó massive ignimbrite found in the Bükkalja Volcanic Field located in the northern CPR. Based on these new observations of its potential areal distribution and estimated thicknesses, these two widespread ∼17.1 Ma ignimbrites represent intermediate to large caldera-forming ignimbrites, larger than previously suggested. Finally, volcanic glass geochemistry of fallout deposits from the Dinaridic Sinj and Livno-Tomislavgrad Basins have similar volcanic glass geochemistry as the rhyolitic pumices from the lowermost part of the Bogács ignimbrite unit of the Bükkalja Volcanic Field. However, high-precision geochronology indicates that these distal ashfalls were deposited at 16.9567 ± 0.0074 Ma, significantly predating the 16.824 ± 0.028 Ma emplacement of the fiamme-bearing part of the Bogács ignimbrite. These distinct ages suggest that the Bogács unit represents multiple eruptive events and indicating that further work is required to deconvolve this portion of the CPR volcanic record. Together, these data suggest that large volume CPR ignimbrite volcanism was more frequent and widespread than previously understood, enhancing the existing volcanic framework and history of the source region for this time period.

Journal of Volcanology and Geothermal Research↗
USGS2016

Continental Shelf Morphology and Stratigraphy Offshore San Onofre, CA: The Interplay Between Rates of Eustatic Change and Sediment Supply

New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from ~ 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure. (C) 2015 Elsevier B.V. All rights reserved.

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