Current-influenced depositional provinces, continental margin off Cape Hatteras, identified by petrologic method

Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces.     

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area,Venus: Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/10⁶ km², it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).     

Precious metals in massive base metal sulfide deposits

Gold and silver are ubiquitous, sometimes minor but economically important metals in massive base metal sulfide ores. Their content, proportions and distribution in the ores depend on complex, interrelated factors of their source, mobilization, transport and deposition.Different types of these deposits are formed by similar seafloor hydrothermal systems operating, however, in widely differing tectono-stratigraphic environments which span a spectrum from ensimatic-oceanic, through continent-margin to ensialic-continental ones. Like those of the base metals, the proportions and distribution of the precious metals in the ores vary regionally with these changing depositional environments. This suggests that precious metal content of the sub-seafloor rocks in which the generative fluids circulate is one factor that governs the amounts and distribution in the ores. The lithology of these source-rocks is also important. Pillowed, tholeiitic basalts have high permeability, golddepleted crystalline pillow interiors and relatively gold-rich palagonitic rims, and are consequently particularly favorable sources.Mobilization of gold from the sub-seafloor rocks may require basalt-water, and/or carbonaceous sediment-water reactions to produce strongly reduced bisulfide, carbonyl or cyanide complexes that promote gold transport. Chloride complexing and transport are less important for gold but more so for silver and the base metals.Seafloor hydrothermal discharge at shallow depth is commonly accompanied by boiling, steamblast explosions in the vent and resulting deep penetration and mixing of cool, oxygenated seawater with rising hot, reduced metalliferous fluid. This results in deposition of both chloride- and isulfide-complexed gold at depth and centrally in the footwall stockwork or in copper ore in the base of the massive body. Chloride-complexed silver, stable to lower temperatures, is carried farther and deposited with higher-level and more distal, massive zinc-lead ores. Boiling in deep water, however, although possible, is rare. This fact minimizes deep fluid mixing and allows transport of lower temperaturestable, bisulfide-complexed gold to the seafloor and outward from the vent. Gold too, is then deposited with the shallower, distal, massive zinc-lead-silver ore. Late-stage changes in fluid Eh, salinity and activity of sulfur during evolution of the generative hydrothermal system, and by discharge through previously deposited, early stage sulfides around the vent also cause diagenetic remobilization of gold, moving it to shallower, more distal locations in the system. In combination, these relationships explain the three associations of gold in primary, in-situ massive sulfide deposits; in central, deep footwall stockwork mineralization with or without copper, in central copper ore in the base of the massive body and in shallow, peripheral pyritic zinclead-silver ore.Primary, in-situ ore near the vent is sometimes reworked by seafloor density flows which transport clasts of the primary sulfides down-slope, mix them with rock and sedimentary detritus and redeposit them to form secondary, transported ore. Gold, like iron and the base metals, is diluted during this clastic transport. But silver and barite may be enriched indicating transport in the density flows not only as clasts of primary ore but partly also m solution in the hydrothermal fluids that, in this case, must have lubricated the density flows.

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Zusammenfassung Gold- und Silbervorkommen in massiven Metallsulfid-Lagerstätten sind stets ökonomisch wichtige Metalle, auch wenn sie nur in geringen Konzentrationen vorliegen. Der Gehalt an diesen Metallen und ihre Verteilung innerhalb der Lagerstätte hängt von komplexen, sich gegenseitig beeinflussenden Faktoren wie Metallquelle, Art der Mobilisation, Transport und Fällung ab.Unterschiedliche Lagerstättentypen werden von ähnlichen hydrothermalen Systemen auf den Ozeanböden gebildet. Die tektonostratigraphischen Environments unterscheiden sich dabei allerdings beträchtlich; sie befinden sich in ensimatisch-ozeanischen, kontinentalrandlichen und ensialischkontinentalen Bereichen. Innerhalb dieser regional wechselnden Ablagerungsbedingungen variiert Konzentration und Verteilung der Edelmetalle in den Lagerstätten wie bei den einfachen Metallen. Dies bedeutet, daß der Gehalt an Edelmetallen der Gesteine, die den Meeresboden unterlagern und durch die die metallhaltigen Lösungen zirkulieren, ein Faktor ist, der Menge und Verteilung der Metalle in der Lagerstätte steuert. Ebenso ist die Lithologie dieser Gesteine von Bedeutung. Als besonders gut geeignete Quellen gelten kissenartige tholeitische Basalte mit hoher Permeabilität, goldarmen Kisseninneren und relativ goldreichem palagonitischem Rand.Um das Gold aus diesen Gesteinen mobilisieren zu können, bedarf es einer Reaktion zwischen Basalt und Wasser und/oder eines karbonatischen Sediments mit Wasser, um stark reduziertes Bisulfid, Carbonyl-oder Cyanidkomplexe zu bilden, die den Goldtransport ermöglichen. Chlorid-Komplexbildung und -Transport sind zwar wichtig für Silber und einfache Metalle, für Gold spielen sie nur eine untergeordnete Rolle.Der Austritt hydrothermaler Lösungen an Ozeanböden in geringer Tiefe wird in der Regel von Sieden und explosionsartigem Dampfaustritt begleitet und führt deshalb zu einem tiefen Eindringen und Durchmischen von kaltem, sauerstoffreichen Meereswasser mit den aufsteigenden heißen, reduzierten metallischen Lösungen. Daher kommt es zur Fällung von sowohl an Chloridkomplexe als auch an Bisulfidkomplexe gebundenem Gold. Diese Ausfällung findet in größerer Tiefe statt und zwar hauptsächlich im liegenden Stockwerk oder mit Kupfer zusammen an der Basis der massiven Lagerstätte. An Chloridkomplexe gebundenes Silber ist auch bei niedrigeren Temperaturen stabil, wird also weiter transportiert und in einem höheren Niveau in distal gelegenen Blei-Zink-Lagerstätten gefällt. In größeren Wassertiefen kommt es seltener zu dem beobachteten Sieden der austretenden Lösungen. Diese Tatsache reduziert das Durchmischen der Lösungen in größeren Tiefen und ermöglicht den Transport von Gold, das an Bisulfidkomplexe gebunden ist. In diesem Fall ist die Verbindung auch bei niedrigeren Temperaturen noch stabil also transportfähig und kann bis zum Meeresboden oder außerhalb des Schlotes in Lösung bleiben. Dabei kann das Gold zusammen mit Blei, Zink und Silber in mehr distalen Lagerstätten angereichert werden. Späte Änderungen in Eh, Salinität und Schwefelaktivität der Lösungen während der Entwicklung des hydrothermalen Systems, sowie der Austritt durch früher abgelagerte den Schlot umgebende Sulfide, können eine diagenetische Gold-Remobilisation auslösen. Auch dabei kann das Metall zu in geringer Tiefe liegenden, distalen Ablagerungsorten transportiert werden. Berücksichtigt man alle Faktoren, so erklären diese Verhältnisse die drei möglichen Goldvorkommen in primären, in-situ vorliegenden Sulfid-Lagerstätten: Mit Kupfer vergesellschaftet, allerdings nicht unbedingt, zentral im liegenden Stockwerk; an der Basis der Kupferlagerstätte und in geringer Tiefe in Verbindung mit peripheren Blei-Zink-Silber-Vorkommen.Primäre, in-situ neben Schloten vorkommende Lagerstätten werden in einigen Fällen von meeresbodennahen Masseströmen aufgearbeitet. Diese transportieren Sulfidkomponenten, die während des Transports mit Sediment und Gesteinsbruchstücken vermischt und schließlich als sekundäre sedimentäre Lagerstätte abgelagert werden. Durch diesen Transport und die Mischung der Klastika wird die Goldkonzentration in der späteren Lagerstätte stark reduziert. Silber und Barit können dagegen in Ausnahmefällen während des Transports angereichert werden, da diese Komponenten nicht nur als Sulfidbruchstücke transportiert werden, sondern auch in Lösung in den hydrothermalen Lösungen vorhanden sein können. Diese Lösungen dienen in solchen Fällen den Masseströmen als Gleithorizont.

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Résumé Dans les gisements de sulfures métalliques massifs, l'or et l'argent sont des métaux ubiquistes, parfois mineurs, mais toujours d'importance économique. Leur teneur et leur distribution dans les corps minéralisés dépendent de facteurs complexes, en relation les uns avec les autres, tels que: leur source, leur mobilité, leurs modalités de transport et de dépôt.A partir des mêmes systèmes hydrothermaux en action sur le fond de la mer, divers types de gisements peuvent être engendrés, selon leur environnement tectono-stratigraphique: océanique ensimatique, de marge continentale ou continental ensialique. Les teneurs et la répartition des métaux précieux, comme celle des autres métaux varient régionalement selon ces divers milieux. Ceci suggère que le contenu en métaux précieux dans les roches sous-jacentes au fond marin à travers lesquelles circulent les solutions minéralisantes est un facteur qui détermine leurs teneurs et leurs répartitions dans les minerais. La lithologie de ces roches-sources est également importante. Une source particulièrement significative est représentée par les coussins des basaltes tholéiitiques, très perméables, avec leur coeur pauvre en or et leur couronne palagonitique relativement riche.Le lessivage de l'or dans les roches situées sous le fond marin peut impliquer des réactions eau-basalte et/ou eausédiments carbonatés, réactions susceptibles d'engendrer les bisulfures très réduits et les complexes carbonés ou cyanurés qui permettent le transport de l'or. Le transport par complexes chlorurés joue un rôle subordoné dans le cas de l'or, mais important dans le cas de l'argent et des autres métaux.L'arrivée de solutions hydrothermales sur les fonds marins peu profonds est d'ordinaire accompagnée d'ébullitons et d'émissions explosives de vapeur, ce qui provoque la pénétration profonde d'eau de mer froide et oxygénée et son mélange avec les fluides métallifères chauds et réducteurs ascendants. Il en résulte le dépôt de complexes aurifères bisulfurés et chlorurés. Cette précipitation s'opère en profondeur, particulièrement dans les roches sous-jacentes ou dans le minerai de cuivre, à la base des corps minéralisés massifs. L'argent des complexes chlorurés, stables à plus basse température, est transporté plus loin et se dépose, en situation plus distale, dans les minerals massifs de Pb-Zn. Dans les mers profondes, l'ébullition, sans être impossible, est néanmoins un phénomène rare; cette circonstance minimise le mélange des fluides en profondeur et permet le transport de l'or jusqu'à la surface du fond et même loin des évents sous la forme de complexes bisulfurés stables à basse température. L'or est alors déposé en situation distale peu profonde avec les minerals massifs de Zn-Pb-Ag. Des modifications tardives d'Eh, de salinité et d'activité du soufre dans les solutions au cours de l'évolution du système hydrothermal, de même que le lessivage des sulfures déjà accumulés autour des évents entraînent une remobilisation diagénétique de l'or vers des situations distales d'eau peu profonde. La combinaison de ces divers facteurs permet d'expliquer les trois occurrences de l'or dans les dépôts in situ de sulfures massifs primaires: dans les parties centrales des masses sous-jacentes en association ou non avec le Cu, à la base des corps minéralisés en Cu, et à faible profondeur, en liaison avec les gisements périphériques de Pb-Zn-Ag.Les gisements primaires, formés in situ près des évents sont parfois remaniés par des courants de densité, qui emportent des clastes de sulfures, les mélangent aux débris sédimentaires et les redéposent sous forme de minerais secondaires. De tels transports provoquent la dilution de l'or, en même temps que celle du fer et des autres métaux. Par contre, l'argent et la barite peuvent subir un enrichissement car leur transport dans les courants de densité ne s'effectue pas seulement sous forme de clastes, mais également en solution dans des fludies hydrothermaux, lesquels, dans ce cas, contribuent à lubrifier le courant de densité.

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Synthetic Seismograms and Wide-angle Seismic Attributes from the Gaussian Beam and Reflectivity Methods for Models with Interfaces and Velocity Gradients

— The effects of interfaces and velocity gradients on wide-angle seismic attributes are investigated using synthetic seismograms. The seismic attributes considered include envelope amplitude, pulse instantaneous frequency, and arrival time of selected phases. For models with interfaces and homogeneous layers, head waves can propagate which have lower amplitudes, as well as frequency content, compared to the direct arrivals. For media with interfaces and velocity gradients, higher amplitude diving waves and interference waves can also occur. The Gaussian beam and reflectivity methods are used to compute synthetic seismograms for simple models with interfaces and gradients. From the results of these methods, seismic attributes are obtained and compared. It was found that both methods were able to simulate wide-angle seismic attributes for the simple models considered. The advantage of using the Gaussian beam method for seismic modeling and inversion is that it is fast and also asymptotically valid for laterally varying media.     

Modeling of Wide-Angle Seismic Attributes Using the Gaussian Beam Method

In this study, observed seismic attributes from shot gather 11 of the SAREX experiment are used to derive a preliminary velocity and attenuation model for the northern end of the profile in southern Alberta. Shot gather 11 was selected because of its prominent Pn arrivals and good signal to noise ratio. The 2-D Gaussian beam method was used to perform the modeling of the seismic attributes including travel times, peak envelope amplitudes and pulse instantaneous frequencies for selected phases. The preliminary model was obtained from the seismic attributes from shot gather 11 starting from prior tomographic results. The amplitudes and instantaneous frequencies were used to constrain the velocity and attenuation structure, with the amplitudes being more sensitive to the velocity gradients and the instantaneous frequencies more sensitive to the attenuation structure. The resulting velocity model has a velocity discontinuity between the upper and lower crust, and lower velocity gradients in the upper and lower crust compared to earlier studies. The attenuation model has Q _p ^-1 values between 0.011 and 0.004 in the upper crust, 0.0019 in the lower crust and a laterally variable Q _p ^-1 in the upper mantle. The Q _p ^-1 values are similar to those found in Archean terranes from other studies. Although the results from a single gather are non-unique, the initial model derived here provides a self-consistent starting point for a more complete seismic attribute inversion for the velocity and attenuation structure.     

Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial environments in Georgia Basin and Puget Lowland, British Columbia, Canada and Washington, USA

Models of late-glacial environmental change in coastal areas are commonly based on radiocarbon ages on marine shell and basal lake sediments, both of which may be compromised by reservoir effects. The magnitude of the oceanic reservoir age in the inland waters of the Georgia Basin and Puget Lowland of northwestern North America is inferred from radiocarbon ages on shell-wood pairs in Saanich Inlet and previously published estimates. The weighted mean oceanic reservoir correction in the early and mid Holocene is −720±90 yr, slightly smaller than, but not significantly different from, the modern value. The correction in late-glacial time is −950±50 yr. Valley-head sites yield higher reservoir values (−1200±130 yr) immediately after deglaciation. The magnitude of the gyttja reservoir effect is inferred from pairs of bulk gyttja and plant macrofossil ages from four lakes in the region. Incorporation of old carbon into basal gyttja yields ages from bulk samples that are initially about 600 yr too old. The reservoir age declines to less than 100 yr after the first millennium of lake development. When these corrections are accounted for, dates of deglaciation and late-glacial sea-level change in the study area are pushed forward in time by more than 500 yr.     

An analysis of failure mechanism constraints on pre-failure rock block deformation using TLS and roto-translation methods

Rockfall hazards increase the risk of train derailment along railway corridors in western Canada. In this study, repeated terrestrial laser scanning (TLS) datasets were collected every 2–3 months at three different sites along the Thompson and Fraser River corridors in British Columbia, referred to as the Goldpan, White Canyon, and Mile 109 sites. A total of 207 rockfall events occurring across all three sites between November 11, 2014 and October 18, 2016 were recorded in a database. For each of these rockfalls, pre-failure deformation was measured using a method of three-dimensional roto-translation block tracking. Each rockfall was classified by its deformation behaviour and further categorised based on failure mechanism, volume, lithology, and the roughness condition of the failure plane. Results reveal that detectable levels of deformation were measured in 33% of the total number of rockfall events using the present methods. Rotation deformation was most commonly observed in toppling failures with relatively steep joint orientations. Conversely, planar sliding blocks generally exhibited the least measurable deformation, with the majority not showing any precursory translation or rotation. It is postulated that overhanging rockfall configurations may suppress the expression of deformation in rockfall source blocks, though additional research is required to confirm this.     

Geochemistry of waters in the Valley of Ten Thousand Smokes region, Alaska

Meteoric waters from cold springs and streams outside of the 1912 eruptive deposits filling the Valley of Ten Thousand Smokes (VTTS) and in the upper parts of the two major rivers draining the 1912 deposits have similar chemical trends. Thermal springs issue in the mid-valley area along a 300-m lateral section of ash-flow tuff, and range in temperature from 21 to 29.8°C in early summer and from 15 to 17°C in mid-summer. Concentrations of major and minor chemical constituents in the thermal waters are nearly identical regardless of temperature. Waters in the downvalley parts of the rivers draining the 1912 deposits are mainly mixtures of cold meteoric waters and thermal waters of which the mid-valley thermal spring waters are representative. The weathering reactions of cold waters with the 1912 deposits appear to have stabilized and add only subordinate amounts of chemical constituents to the rivers relative to those contributed by the thermal waters. Isotopic data indicate that the mid-valley thermal spring waters are meteoric, but data is inconclusive regarding the heat source. The thermal waters could be either from a shallow part of a hydrothermal system beneath the 1912 vent region or from an incompletely cooled, welded tuff lens deep in the 1912 ash-flow sheet of the upper River Lethe area.Bicarbonate-sulfate waters resulting from interaction of near-surface waters and the cooling 1953–1968 southwest Trident plug issue from thermal springs south of Katmai Pass and near Mageik Creek, although the Mageik Creek spring waters are from a well-established, more deeply circulating hydrothermal system. Katmai caldera lake waters are a result of acid gases from vigorous drowned fumaroles dissolving in lake waters composed of snowmelt and precipitation.