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USGS1989

Geochemical signatures of possible deep-seated ore deposits in Tertiary volcanic centers, Arizona and New Mexico, U.S.A.

A reconnaissance geochemical survey of stream drainages within 21,000 km 2 of southeastern Arizona and southwestern New Mexico shows broad zones of low-level to moderate contrast anomalies, many associated with mid-Tertiary eruptive centers and Tertiary fault zones. Of these eruptive centers, few are known to contain metallic deposits, and most of those known are minor. This, however, may be more a function of shallow erosion level than an indication of the absence of mineralization, since hydrothermal alteration and Fe-Mn-oxide staining are widespread, and geochemical anomalies are pervasive over a larger part of the region than outcrop observations would predict. Accordingly, interpretations of the geochemical data use considerations of relative erosion levels, and inferred element zonalities, to focus on possible undiscovered deposits in the subsurface of base-, precious-, and rare-metal deposits of plutonic-volcanic association. In order to enhance the identification of specific deep targets, we use the empirically determined ratio: Ag+Mn+Pb+Zn+Ba Au+Mo+Cu+Bi+W "> Ag+Mn+Pb+Zn+BaAu+Mo+Cu+Bi+W This ratio is based on reported metal contents of nonmagnetic heavy-mineral samples from the drainage sediment, determined by emission spectrographic analysis. Before the ratio was computed for each sample site, the data were normalized to a previously estimated regional threshold value. A regional isopleth map was then prepared, using a cell-averaging computer routine, with contours drawn at the 25th, 50th, 75th, 80th, 90th, 95th and 99th percentiles of the computed data.

Journal of Geochemical Exploration↗

USGS2004

Comparison of ground-water flow model particle-tracking results and isotopic data in the Mojave River ground-water basin, southern California, USA

Flow-path and time-of-travel results for the Mojave River ground-water basin, southern California, calculated using the ground-water flow model MODFLOW and particle-tracking model MODPATH were similar to flow path and time-of-travel interpretations derived from delta-deuterium and carbon-14 data. Model and isotopic data both show short flow paths and young ground-water ages throughout the floodplain aquifer along most the Mojave River. Longer flow paths and older ground-water ages as great as 10,000 years before present were measured and simulated in the floodplain aquifer near the Mojave Valley. Model and isotopic data also show movement of water between the floodplain and regional aquifer and subsequent discharge of water from the river to dry lakes in some areas. It was not possible to simulate the isotopic composition of ground-water in the regional aquifer away from the front of the San Gabriel and San Bernardino Mountains - because recharge in these areas does not occur under the present-day climatic conditions used for calibration of the model.

Journal of Hydrology↗

USGS2019

Four major Holocene earthquakes on the Reelfoot fault recorded by sackungen in the New Madrid seismic zone, USA

Three sequences of well-documented, major ~M7+ earthquakes (1811-1812 CE, ~1450 CE, and ~900 CE) in the New Madrid seismic zone, USA, contribute significantly to seismic hazard in the region. However, it is unknown whether this <550 yr recurrence interval has been constant throughout the Holocene given limited geomorphic evidence of prior earthquakes. We extend the record of paleoearthquakes along the Reelfoot fault via investigation of ridge-top gravitational failure features, interpreted as sackungen. The sackungen occur in bluffs along the eastern margin of the Mississippi River floodplain and are concentrated near (<15 km) the southwest-dipping Reelfoot reverse fault. A paleoseismic trench excavated across sackungen at the Paw Paw site exposed four packages of colluvial sediment that postdate 30-11 ka Peoria loess. We interpret the colluvial packages to have been deposited following episodic failure of the sackungen as a result of strong ground motions from the following sequence of earthquakes: event 4, 1640 ± 1730 BCE; event 3, 340 ± 670 CE; event 2, 1430 ± 380 CE; and event 1, 1810 ± 50 CE (2-sigma). Event timing corresponds to previously documented earthquakes and represents the longest archive of paleoearthquakes on the Reelfoot fault. If the trenched sackungen record all major Reelfoot fault earthquakes, our observations in combination with prior investigations indicate a period of quiescence from at least 11 – 4.7 ka, followed by four major seismic events culminating in the 1811-1812 CE sequence. This clustered earthquake recurrence helps place bounds on seismic-hazard and geodynamic models in the New Madrid seismic zone.

Arkansas, Missouri↗

USGS1999

Overview of the Mars Pathfinder Mission: Launch through landing, surface operations, data sets, and science results

Mars Pathfinder successfully landed at Ares Vallis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Path‐finder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high‐resolution orbiter images and in inertial space from two‐way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and ∼2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Air‐borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by ∼3–7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near‐surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.

Journal of Geophysical Research E: Planets↗

USGS2006

2480 Ma mafic magmatism in the northern Black Hills, South Dakota: A new link connecting the Wyoming and Superior cratons

The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ???2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the northern Black Hills, a 1 km thick, layered sill (the Blue Draw metagabbro) that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift-basin sedimentation. Ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207Pb/206Pb upper-intercept age of 2480 ?? 6 Ma (all age errors ??2??), comparable to two-point 207Pb/206Pb errorchron ages obtained by Pb stepwise leaching of the same titanites. Nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 ?? 16 to 2284 ?? 20 Ma (i.e., differentially reset along U-Pb concordia), and hornblende from an associated metadiorite yields a partially reset date with oldest apparent-age increments ranging between 2076 ?? 16 and 2010 ?? 8 Ma. We interpret these data as indicating that an episode of gabbroic magmatism occurred at 2480 Ma, in response to earlier rifting of the eastern edge of the Wyoming craton. Layered mafic intrusions of similar thickness and identical age occur along a rifted belt in the southern Superior craton (Sudbury region, Ontario). Moreover, these mafic intrusions are spatially aligned using previous supercontinent restorations of the Wyoming and Superior cratons (Kenorland-Superia configurations). This new "piercing point" augments one previously inferred by spatial-temporal correlation of the Paleoproterozoic Huronian (southern Ontario) and Snowy Pass (southeastern Wyoming) supergroups. We propose that layered mafic intrusions extending from Nemo, South Dakota, to Sudbury, Ontario, delineate an axial rift zone along which Wyoming began to separate from Superior during initial fragmentation of the Neoarchean supercontinent at ???2480 Ma. ?? 2006 NRC Canada.

Canadian Journal of Earth Sciences↗

USGS1999

Distinguishing Grenvillian basement from pre-Taconian cover rocks in the Northern Appalachians

Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accurate structural interpretations. The Cavendish Formation in southeastern Vermont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimentary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex.

northern Appalachians↗

USGS1998

Recovery and validation of historical sediment quality data from coastal and estuarine areas: An integrated approach

A comprehensive database of sediment chemistry and environmental parameters has been compiled for Boston Harbor and Massachusetts Bay. This work illustrates methodologies for rescuing and validating sediment data from heterogeneous historical sources. It greatly expands spatial and temporal data coverage of estuarine and coastal sediments. The database contains about 3500 samples containing inorganic chemical, organic, texture and other environmental data dating from 1955 to 1994. Cooperation with local and federal agencies as well as universities was essential in locating and screening documents for the database. More than 80% of references utilized came from sources with limited distribution (gray literature). Task sharing was facilitated by a comprehensive and clearly defined data dictionary for sediments. It also served as a data entry template and flat file format for data processing and as a basis for interpretation and graphical illustration. Standard QA/QC protocols are usually inapplicable to historical sediment data. In this work outliers and data quality problems were identified by batch screening techniques that also provide visualizations of data relationships and geochemical affinities. No data were excluded, but qualifying comments warn users of problem data. For Boston Harbor, the proportion of irreparable or seriously questioned data was remarkably small (<5%), although concentration values for metals and organic contaminants spanned 3 orders of magnitude for many elements or compounds. Data from the historical database provide alternatives to dated cores for measuring changes in surficial sediment contamination level with time. The data indicate that spatial inhomogeneity in harbor environments can be large with respect to sediment-hosted contaminants. Boston Inner Harbor surficial sediments showed decreases in concentrations of Cu, Hg, and Zn of 40 to 60% over a 17-year period.A comprehensive database of sediment chemistry and environmental parameters has been compiled for Boston Harbor and Massachusetts Bay. This work illustrates methodologies for rescuing and validating sediment data from heterogeneous historical sources. It greatly expands spatial and temporal data coverage of estuarine and coastal sediments. The database contains about 3500 samples containing inorganic chemical, organic, texture and other environmental data dating from 1995 to 1994. Cooperation with local and federal agencies as well as universities was essential in locating and screening documents for the database. More than 80% of references utilized came from sources with limited distribution (gray Task sharing was facilitated by a comprehensive and clearly defined data dictionary for sediments. It also served as a data entry template and flat file format for data processing and as a basis for interpretation and graphical illustration. Standard QA/QC protocols are usually inapplicable to historical sediment data. In this work outliers and data quality problems were identified by batch screening techniques that also provide visualizations of data relationships and geochemical affinities. No data were excluded, but qualifying comments warn users of problem data. For Boston Harbor, the proportion of irreparable or seriously questioned data was remarkably small (<5%), although concentration values for metals and organic contaminants spanned 3 orders of magnitude for many elements or compounds. Data from the historical database provide alternatives to dated cores for measuring changes in surficial sediment contamination level with time. The data indicate that spatial inhomogeneity in harbor environments can be large with respect to sediment-hosted contaminants. Boston Inner Harbor surficial sediments showed decreases in concentrations Cu, Hg, and Zn of 40 to 60% over a 17-year period.

Massachusetts↗

USGS2000

Metal exposure in a benthic macroinvertebrate, Hydropsyche californica, related to mine drainage in the Sacramento River

A biomonitoring technique was employed to complement studies of metal transport in the upper Sacramento River affected by acid mine drainage. Metals (Al, Cd, Cu, Fe, Hg, Pb, and Zn) were determined in a resident invertebrate, Hydropsyche californica (Insecta: Trichoptera), and streambed sediments (<62 µm) to assess metal contamination within a 111-km section of the river downstream of the mining area. Metals in H. californica also were interpreted to be broadly indicative of metal exposure in fish. Total Hg was determined in the whole body of the insect, whereas Al, Cd, Cu, Fe, Pb, and Zn were additionally separated into operationally defined cytosolic (used as an indicator of exposure to bioavailable metal) and particulate fractions. Total concentrations of Cd, Cu, Hg, Pb, and Zn in sediments were consistent with documented upstream sources of acid mine drainage. Metal distribution patterns in H. californica and sediments were generally consistent for Cd, Cu, and Pb but inconsistent for Hg and Zn. Concentrations in H. californica indicated that bioavailable Cd, Cu, Pb, and Zn was transported at least 120 km downstream of the mine sources. Zinc in H. californica was elevated, but unlike sediments, did not decrease downstream. Mercury in H. californica was not elevated.

California↗

USGS1975

Applications of remote sensing to structural interpretations in the southern Appalachians

Remote sensing is the technology of studying distant objects by measuring and recording energy from one or more segments of the electromagnetic spectrum. Imaging sensors which operate from medium- and high-altitude aircraft or from spacecraft can provide a synoptic view of large areas and of surface phenomena not evident in the field. Image-acquiring systems and instruments have been designed to partially automate data collection and to reduce the time devoted to analysis, information extraction, and detection of changes of surface phenomena. Among these phenomena are the surface distribution of heat, moisture, snow, water, vegetation, and cultural features. When coupled with ancillary data, including field surveys, sensor data provide useful information for the recognition and mapping of regional structure, jointing patterns, drainage patterns, fault and fracture traces, and rock types. The recognition of several major linear surface features, two of which proved to be traces of previously unrecognized faults (the Canebrake and the Coeburn faults) in the Appalachian Plateaus, demonstrates the pragmatic application of aircraft and spacecraft remote sensing to geological investigations in the Appalachians.

Virginia, West Virginia↗

USGS2002

Crustal structure of central Lake Baikal: Insights into intracontinental rifting

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39–42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05–7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.

Lake Baikal↗

USGS2002

Global Omori law decay of triggered earthquakes: Large aftershocks outside the classical aftershock zone

[1] Triggered earthquakes can be large, damaging, and lethal as evidenced by the1999 shocks in Turkey and the 2001 earthquakes in El Salvador. In this study, earthquakes with M s ≥ 7.0 from the Harvard centroid moment tensor (CMT) catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected. About 61% of earthquakes that occurred near (defined as having shear stress change ∣Δτ∣ ≥ 0.01 MPa) the M s ≥ 7.0 shocks are associated with calculated shear stress increases, while ∼39% are associated with shear stress decreases. If earthquakes associated with calculated shear stress increases are interpreted as triggered, then such events make up at least 8% of the CMT catalog. Globally, these triggered earthquakes obey an Omori law rate decay that lasts between ∼7–11 years after the main shock. Earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid. Omori's law is one of the few time‐predictable patterns evident in the global occurrence of earthquakes. If large triggered earthquakes habitually obey Omori's law, then their hazard can be more readily assessed. The characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of M s ≥ 7.0. I show an example application to the M = 7.7 13 January 2001 El Salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than Coulomb stress change calculations.

Journal of Geophysical Research B: Solid Earth↗

USGS2022

Ancient winds, waves, and atmosphere in Gale Crater, Mars, inferred from sedimentary structures and wave modeling

Wave modeling and analysis of sedimentary structures were used to evaluate whether four examples of symmetrical, reversing, or straight-crested bedforms in Gale crater sandstones are preserved wave ripples; deposition by waves would demonstrate that the lake was not covered by ice at that time. Wave modeling indicates that regardless of atmospheric density, winds that exceeded the threshold of aeolian sand transport could have generated waves capable of producing nearshore wave ripples in most grain sizes of sand. Reversing 3-m-wavelength bedforms in the Kimberley formation are interpreted not as wave ripples but rather as large aeolian ripples that formed in an atmosphere approximately as thin as at present. These exhumed bedforms define many of the ridges at outcrops that appear striated in satellite images. At Kimberley these bedforms demonstrably underlie and therefore predate subaqueous beds, suggesting that a thin atmosphere existed at least temporarily before subaqueous deposition ceased in the crater. The other three candidate wave ripples (Square Top, Hunda, and Voe) are consistent with modeled waves, but other origins cannot be excluded. The predominance of flat-laminated (non-rippled) beds in the lacustrine Murray formation suggests that some aspect of the lake was not conducive to formation or preservation of recognizable wave ripples. Water depths may generally have been too deep, lakebed sediment may have been too fine-grained, the lake may have been smaller than modeled, or the lake may have been covered by ice. Plain Language Summary Wave modeling and analysis of sedimentary structures were used to evaluate whether ancient lake deposits in Gale crater contain ripples formed by waves on the surface of the lake. Deposition by waves would show that the lake was not covered by ice at that time. Modeling shows that regardless of atmospheric density, winds capable of moving sand on land would generally have been strong enough to form waves that would produce ripples near shore. Large bedforms in the Kimberley formation are interpreted as ripples formed by the wind in an atmosphere similar to that of Mars today. These bedforms underlie and are older than other beds deposited in water, thereby showing that a thin atmosphere existed at least temporarily before deposition in water ceased in the crater. Three other candidate wave ripples are consistent with modeled waves, but other origins are possible. Thick sequences of sedimentary rock in Gale crater are flat-laminated rather than rippled, suggesting that some aspect of the lake was not favorable for their formation or preservation. Much of the lake may have been too deep or ice-covered, or the lake may have been smaller than modeled or had sediment too fine to form easily observed ripples.

Journal of Geophysical Research: Planets↗

USGS2016

Uncertainty in tsunami sediment transport modeling

Erosion and deposition from tsunamis record information about tsunami hydrodynamics and size that can be interpreted to improve tsunami hazard assessment. We explore sources and methods for quantifying uncertainty in tsunami sediment transport modeling. Uncertainty varies with tsunami, study site, available input data, sediment grain size, and model. Although uncertainty has the potential to be large, published case studies indicate that both forward and inverse tsunami sediment transport models perform well enough to be useful for deciphering tsunami characteristics, including size, from deposits. New techniques for quantifying uncertainty, such as Ensemble Kalman Filtering inversion, and more rigorous reporting of uncertainties will advance the science of tsunami sediment transport modeling. Uncertainty may be decreased with additional laboratory studies that increase our understanding of the semi-empirical parameters and physics of tsunami sediment transport, standardized benchmark tests to assess model performance, and development of hybrid modeling approaches to exploit the strengths of forward and inverse models.

Journal of Disaster Research↗

USGS1978

Remote-sensing methods for monitoring surface coal mining in the northern Great Plains

Recent studies at a large surface coal mine in southern Montana confirm that remote sensing is both feasible and effective for gathering land-use and environmental data (spatial, dynamic, and seasonal) for large-scale surface mines in the northern Great Plains. The Western Energy Co.'s Rosebud mine near Colstrip, Mont., was selected as a test site because it typifies surface operations in the Powder River Basin of Montana and Wyoming and elsewhere in the northern Great Plains. Several basic interpretive and analytical remote-sensing techniques were used to identify and delineate various categories of surface-mining operations and concurrent stages of reclamation that characterize most, if not all, such mining operations. Color infrared and black-and-white aerial photographs and a black-and-white band 5 Landsat image were used to identify (1) high wall and bench areas, (2) ungraded spoils, (3) graded and recontoured areas, (4) revegetated recontoured areas, (5) natural and impounded surface water, and (6) miscellaneous areas. Over the lifespan of an extensive surface mine, cultural and natural processes and cumulative environmental effects can be monitored by capitalizing on the close correlation between enhanced satellite imagery, infrared and (or) black-and-white aerial photography, standard large-scale topographic maps (such as U.S. Geological Survey 7½-minute quadrangle maps), and results of onsite inspection of mining and reclamation by Federal or State agencies.

Montana↗

USGS2006

Geologic, hydrologic, and geochemical interpretations of mineral deposits as analogs for understanding transport of environmental contaminants

Base- and precious-metal mineral deposits comprise anomalous concentrations of metals and associated elements, which may be useful subjects for study as analogs for migration of environmental contaminants. In the geologic past, hydrothermal mineral deposits formed at the intersection of favorable geologic, hydrologic and geochemical gradients. In the present, weathering of these sulfide-rich deposits occurs as a result of the interplay between rates of oxygen supply versus rates of ground or surface-water flow. Transport and spatial dispersion of elements from a mineral deposit occurs as a function of competing rates of water flow versus rates of attenuation mechanisms such as adsorption, dilution, or (co)precipitation. In this paper we present several case studies from mineralized and altered sedimentary and crystalline aquifers in the western United States to illustrate the geologic control of ground-water flow and solute transport, and to demonstrate how this combined approach leads to a more complete understanding of the systems under study as well as facilitating some capability to predict major flow directions in aquifers.

Journal of Geochemical Exploration↗

USGS2023

Methane pore accessibility, densification, and accommodation by organic matter in the Niobrara Formation at wet-gas thermal maturity conditions

Petroleum within unconventional source-rock reservoirs is hosted in organic matter and mineral pore space as well as in voids and microfractures. Recent work has shown that for source-rock reservoirs in the dry gas window, significant portions of methane (CH 4 ), the main component of petroleum at elevated maturities, can be stored within fine (<5 nm) organic matter porosity. However, within reservoirs at lower thermal maturities (e.g., peak oil or wet-gas conditions), the distribution and behavior of CH 4 and the higher alkanes that comprise gas condensates across pore sizes is unclear, especially for pores with diameters <50 nm. Understanding CH 4 distribution within these settings provides insight for petroleum generation, movement, and recoverability, ultimately enabling increased accuracy of estimated ultimate recovery. Here wide Q-range total neutron scattering was used to evaluate perdeuterated methane (CD 4 ) behavior at reservoir pressures (200–750 bar) and temperature (60 °C) in a sample at the late oil/wet gas thermal maturity stage from the Late Cretaceous Niobrara Formation, an active petroleum producing formation within the Denver-Julesburg Basin, U.S. Neutron scattering data show that mesopores within the Niobrara Formation sample exhibit mass fractal scattering, similar to previously measured U.S. marine shale samples. In the presence of CD 4 , scattering intensities between Q = 0.02–0.1 Å −1 (corresponding to nominal pore diameters from 25 to 5 nm, respectively) decrease with increased pressure up to 750 bar where at least 80% of all pores with ~25 nm diameters are CD 4 accessible. In contrast, between Q = 0.1–1 Å −1 (corresponding to nominal pore diameters from 5 to 0.5 nm, respectively), scattering intensity initially increased at the lowest CD 4 pressure tested (200 bar) before decreasing with increasing pressure. These signal fluctuations with CD 4 pressure are interpreted to arise from the creation of pores with diameters <5 nm, likely through deformation of solid bitumen by supercritical CD 4 , and/or the incorporation of CD 4 within sample organic matter. This new porosity represents an increase of at least ~8% in available pore volume within the sample, although the majority of these pores do not persist following removal of CD 4 . Additionally, there is strong evidence for densification of CD 4 within the sample indicated by a shift in the CD 4 intermolecular scattering peak to higher Q-values compared to bulk CD 4 . These results provide insight into fluid properties within source-rock reservoirs at late oil/wet gas thermal maturities, especially as they relate to organic porosity interconnectivity, and are discussed with perspective toward pressure management of gas condensate wells.

Colorado, Kansas, Nebraska, Wyoming↗

USGS2010

Mineralogy and stratigraphy of phyllosilicate-bearing and dark mantling units in the greater Mawrth Vallis/west Arabia Terra area: Constraints on geological origin

Analyses of MRO/CRISM images of the greater Mawrth Vallis region of Mars affirm the presence of two primary phyllosilicate assemblages throughout a region ∼1000 × 1000 km. These two units consist of an Fe/Mg-phyllosilicate assemblage overlain by an Al-phyllosilicate and hydrated silica assemblage. The lower unit contains Fe/Mg-smectites, sometimes combined with one or more of these other Fe/Mg-phyllosilicates: serpentine, chlorite, biotite, and/or vermiculite. It is more than 100 m thick and finely layered at meter scales. The upper unit includes Al-smectite, kaolin group minerals, and hydrated silica. It is tens of meters thick and finely layered as well. A common phyllosilicate stratigraphy and morphology is observed throughout the greater region wherever erosional windows are present. This suggests that the geologic processes forming these units must have occurred on at least a regional scale. Sinuous ridges (interpreted to be inverted channels) and narrow channels cut into the upper clay-bearing unit suggesting that aqueous processes were prevalent after, and possibly during, the deposition of the layered units. We propose that layered units may have been deposited at Mawrth Vallis and then subsequently altered to form the hydrated units. The Fe/Mg-phyllosilicate assemblage is consistent with hydrothermal alteration or pedogenesis of mafic to ultramafic rocks. The Al-phyllosilicate/hydrated silica unit may have formed through alteration of felsic material or via leaching of basaltic material through pedogenic alteration or a mildly acidic environment. These phyllosilicate-bearing units are overlain by a darker, relatively unaltered, and indurated material that has probably experienced a complex geological history.

Journal of Geophysical Research↗

USGS2007

Fluid chemistry and evolution of hydrothermal fluids in an Archaean transcrustal fault zone network: The case of the Cadillac Tectonic Zone, Abitibi greenstone belt, Canada

Detailed fluid geochemistry studies on hydrothermal quartz veins from the Rouyn-Noranda and Val-d'Or areas along the transcrustal Cadillac Tectonic Zone (CTZ) indicate that unmineralized (with respect to gold) sections of the CTZ contained a distinct CO2-dominated, H2S-poor hydrothermal fluid. In contrast, both gold mineralized sections of the CTZ (e.g., at Orenada #2) and associated higher order shear zones have a H2O-CO2 ?? CH4-NaCl hydrothermal fluid. Their CO2/H2S ratios indicate H2S-rich compositions. The Br/Cl compositions in fluid inclusions trapped in these veins indicate that hydrothermal fluids have been equilibrated with the crust. Oxygen isotope ratios from hydrothermal quartz veins in the CTZ are consistently 2??? more enriched than those of associated higher order shear zones, which are interpreted to be a function of greater fluid/rock ratios in the CTZ and lower fluid/rock ratios, and more efficient equilibration of the hydrothermal fluid with the wall rock, in higher order shear zones. An implication from this study is that the lower metal endowment of the transcrustal CTZ, when compared with the higher metal endowment in higher order shear zones (ratio of about 1 : 1000), may be the result of the lack of significant amounts of H2O-H2S rich fluids in most of the CTZ. In contrast, gold mineralization in the higher order shear zones appear to be controlled by the high H2S activity of the aqueous fluids, because gold was likely transported in a bisulfide complex and was deposited during sulfidation reactions in the wall rock and phase separation in the quartz veins. ?? 2007 NRC Canada.

Canadian Journal of Earth Sciences↗