Traveling screens as sampling gear for vertical distribution studies

A sampling technique has been developed for increasing the information gathered during routine monitoring of impingement at water intake structures. Samples from impingement catches on traveling screens were taken from the sluiceway of the Brunswick Steam Electric Plant near Southport, North Carolina during the screen wash process so as to divide the catch into vertical catch components. Each component represented 1.2 m of the water column. Results showed differences in day and night vertical distributions of impinged organisms according to the spatial preferences of each species. Impingement during daytime was lower than during night. Impingement of surface-oriented species during daytime occurred at mid-depth, midwater species near the bottom, and bottom species were seldom impinged. During the night surface-oriented species were impinged at the surface, midwater species at mid-depth, and bottom species near the bottom. Residue (animals and debris) which remained within the screen wash system from collection of the previous sample, and those organisms which became impinged on the screens during retrieval of the sample, were used to calculate the rate of “continuous impingement” on the traveling screens. These rates were compared to the impingement catch in order to determine biases in the vertical catch components. Comparison of these rates indicated that impingement estimates determined by routine monitoring methods may under-estimate true impingement rates for certain species. We have concluded that this sampling technique for monitoring impingement at intake structures will increase knowledge of the local biologic system while minimizing the cost of obtaining the information. The technique will also aid in pinpointing specific impingement problems which may be corrected through modifications to the design of the intake structure.     

Low pressure mineral-melt equilibria in natural anhydrous mafic systems

Contributions to Mineralogy and Petrology - Low pressure phase equilibria for natural mafic systems may be calculated utilizing empirically derived single component distribution coefficients and...     

Cretaceous and Paleogene geologic history of coastal Ecuador

Following a general review of the Cretaceous and Paleogene geology of Ecuador, we propose a geologic history from early Cretaceous through Oligocene time with emphasis on the coastal part of the country that is based on the interpretive interaction of three oceanic plates (the Bolívar, proto-Nazca, and Nazca) and the continental South American plate. The proposed history invokes large lateral displacements, the intervention of the previously unrecognized Bolívar plate, the development of three successive subduction zones, the formation of a distant oceanic island arc and its subsequent collision with the South American plate, the emplacement of a major olistostrome complex, and an intricate record of displacement on a currently inactive transform fault. The proposed history offers a viable solution to four major problems of the geology of coastal Ecuador.

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Zusammenfassung Aus der Kreide- und Paläogen-Geschichte Ecuadors läßt sich die Entwicklung der Küstenregion ableiten. Sie wird vom Zusammenwirken von drei ozeanischen Platten (Bolivar, Proto-Nazca und Nazca) und der kontinentalen Platte Südamerikas bestimmt. Die Entwicklung umfaßt ausgedehnte Seitenverschiebungen, das Auftreten einer bisher nicht erkannten Bolivar-Platte, die Anlage von drei aufeinanderfolgenden Subduktionszonen, die Bildung eines Inselbogens und die folgende Kollision mit der Südamerikanischen Platte, die Ablagerung eines großen Olisthostrom-Komplexes und die Verschiebungen entlang einer heute inaktiven Transform-Verwerfung. Damit sind eine Reihe von Problemen der Geologie des Küstenabschnittes Ecuadors gelöst.

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Résumé Passant en revue la géologie de l'Equateur au Crétacique et Paléogène, les auteurs proposent une histoire géologique allant depuis le Crétacique inférieur à l'Oligocène en mettant l'accent sur la région cÔtière, et en se basant sur l'interaction de trois plaques océaniques (bolivienne, proto-Nasca et Nasca) et de la plaque continentale de l'Amérique du Sud. L'histoire proposée fait appel à de grands déplacements, à l'intervention de la plaque bolivienne non reconnue antérieurement, au développement de trois zones de subduction successives, à la formation d'une guirlande d'Îles océaniques distante et sa collision subséquente avec la plaque de l'Amérique du Sud, à la mise en place d'un grand complexe olistostromique et à un déplacement sur une faille transformante inactive. L'histoire proposée apporte une solution viable à quatre problèmes de la géologie de la Région cÔtière de l'Equateur.

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. (, protoNazca Nazca) . : , , , , , Olistostrome . .

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Contribution No. 6 A, Departamento de Geología, Escuela Politécnica Nacional, Quito, Ecuador.     

Pathways of manganese in an open estuarine system

The distribution of Mn was examined in the bottom sediments and water column (suspended paniculate matter) of the Laurentian Trough. Gulf of St. Lawrence. A characteristic profile of Mn with depth in the sediment consisted of a Mn-enriched surface oxidized zone, less than 20 mm thick, and a Mn-depleted subsurface reducing zone. A subsurface Mn maximum occurred within the oxidized zone. Below this maximum the concentration dropped sharply to nearly constant residual levels in the reducing zone. The accumulating estuarine sediments are deficient in Mn compared to the river input of suspended matter and are definitely not the ultimate sink for manganese. Manganese escapes from the sediment by diffusion and resuspension, forming Mn-enriched, fine-grained particles which are flushed out in the estuarine circulation. 5.0 × 10⁹gyr^?1 of Mn, or 50% more than the river input of dissolved Mn. are exported to the open ocean. In spite of the efficient mobilization and export of Mn, the quantity exported is a small fraction (0.2%) of the total flux to the deep-sea sediments. This is related to the low levels of paniculate matter transported by the St. Lawrence River. The export phénomenon, however, is probably true of many coastal regions of muddy sediments and thus has interesting implications for the oceanic budget of Mn.     

Difference in petrophysical properties between foliated and dilatant fault rocks in deeply buried clastics: The case of the Grès d'Annot Formation,SW French Alps

This study describes normal fault zones formed in foreland arkosic turbidites (the Grès d'Annot Formation, SW French Alps) under deep diagenesis conditions (~200 °C) and highlights the occurrence of two markedly different fault‐rock types: (1) the foliated fault rocks of the Moutière‐Restefond area; and (2) the dilatant fault rocks of the Estrop area. The deformation of (1) is dominated by intra‐ and transgranular fracturing, pressure solution of quartz and feldspar grains and syn‐kinematic phyllosilicate precipitation resulting from feldspar alteration. The combination of these mechanisms results in a strongly anisotropic strain with intense shortening normal to the foliation (pressure solution) and extension parallel to the foliation (quartz‐ and calcite‐sealed extension veins). This deformation implies local mass transfer that may be achieved without (or with limited) volume change. The deformation of (2) is expressed as dilatant quartz‐sealed veins and breccia textures in which the main mechanisms are transgranular fracturing and quartz precipitation. Type (2) implies fault volume increase, isotropy of deformation and mass transfer at distances larger than in type (1). This study discusses the origins of (1) and (2) and shows that the permeability of (1) is anisotropic, with higher values than the host rocks parallel to the Y main deformation axis (i.e. perpendicular to the slip vector), whereas the permeability of (2) is isotropic and equivalent to that of the host rocks.     

An experimental study of the formation of metallic iron in chondrules

The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (P_H2 = 1.3 × 10⁻⁵ atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at P_H2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H₂) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.     

Petrographic and geochemical evidence for an allochthonous, possibly impact melt, origin of pseudotachylite from the Vredefort Dome, South Africa

Hypotheses proposed to explain the origin of pseudotachylite bodies formed during impact cratering include: (1) frictional heating, (2) shock loading, (3) decompression or (4) drainage of impact melt into target rocks. In order to differentiate among these processes, we conducted detailed geochemical and petrographic analysis of the matrices in pseudotachylitic veins and dikes and of their respective wall rocks. Our analyses indicate that the chemical compositions of matrices locally deviate significantly from their immediate wall rocks and that assimilation of wall rock has substantially modified the pseudotachylite matrix compositions in places. Variable magnitudes of assimilation can be explained by the surface area of wall rock or its fragments in contact with melt, as well as the initial temperature and cooling rate of the pseudotachylitic melt. Chemical trends observed can be explained either by admixture of an exotic melt component with immediate wall rock or by mixing of melts derived from local lithologies. Trends in the compositional deviation of centimetre to metre-wide pseudotachylite dikes from their immediate wall rocks are consistent with the presence of a primary melt component having granitoid composition akin to the average composition of Vredefort Granophyre dikes. Within veins, melt transport can be geochemically and petrographically traced for distances of centimetres to metres, with the direction of melt transport from larger pseudotachylite veins toward smaller ones and into apophyses. Sulphide and silicate mineralogy indicates that the initial temperature of pseudotachylitic melt must have been at least 1200-1700 °C. Collectively, these characteristics point to an allochthonous origin of pseudotachylitic melt. We advocate the possibility that impact melt from the initially superheated impact melt sheet contributed to the formation of pseudotachylite bodies at Vredefort.