Streambank fish-shelter structures help stabilize tributary streams in Wisconsin

 A significant proportion of stream sediment yield in North America comes from stream channel and bank erosion. One method used for stream stabilization is the bank installation of timber and stone fish-shelter structures, but there is little evidence for their potential effectiveness. Nine to nineteen years of precise survey data from Coon Creek, Wisconsin, however, show that fish structures enhance sediment deposition along the stream and may retard lateral migration of channels. Such structures have greater utility for sediment control when streams are eroding away a high bank and replacing it with a lower bank. Received: 18 October 1996 · Accepted: 4 February 1997     

Environmental and legal aspects of karst areas

 Environmental impacts on karst settings are common as they are more sensitive than those of other rock terrains. Regulatory procedures that are effective in other rock terrains are not necessarily applicable to karst settings. Development and exploitation by man that affect the karst hydrology regime can trigger catastrophic events and result in numerous legal actions where the effects of changes go beyond property boundaries. A great variety of regulations and examples of litigation exist for karst areas. Received: 25 October 1994 · Accepted: 17 October 1995     

Radiometric characteristics of heavy mineral deposits along the west coast of South Africa

During the last decade, exploration and mining of modern-Tertiary heavy mineral beach and raised beach sands along the west coast of South Africa has developed into a major industry. High resolution radiometric techniques have demonstrated their use as a quantitative indicator of total heavy mineral concentration (THM) and also have the ability to discriminate between sediments derived from different provenance terrains. Results indicate that it is possible to calculate the total heavy mineral concentration from high resolution radiometric measurements, but the ability to quantify the concentration of individual mineral fractions, such as ilmenite, requires further refinement of the method. Radiometric characteristics of the light mineral fraction made it possible to distinguish between sediments from mixed marine-aeolian palaeoplacers and active present beach placers. The effects of in situ alteration of the ilmenite fraction in the palaeoplacers were also reflected by subtle contrasts in the radiometric character of the deposits. Furthermore, the uniform radiometric character of the sample population indicates a common provenance for the heavy mineral suite and supports previous results which indicate the metamorphites of the mid-Proterozoic Namaqua Province as the primary source terrain. The high resolution radiometric techniques discussed offer a broad spectrum of applications in sedimentology and have the potential to aid heavy mineral exploration. With further refinement it can possibly be used quantitatively in grade control during mining and beneficiation of heavy mineral deposits. Received: 19 July 1996 / Accepted: 7 January 1997     

Automated AF-demagnetization on the 2G-Enterprises through-bore,cryogenic magnetometer

Since the early sixties, alternating field demagnetization (AFD) has been a standard laboratory technique for demagnetizing rocks to expose the multicomponent structure of their natural remanent magnetization (NRM). In the majority of AFD implementations, however, the procedure remains as labour-intensive as ever. The implementation that we have developed at the Australian Geological Survey Organisation, automates the procedure for AFD based on the static method, and results in significant productivity and efficiency gains without compromising data quality. A properly formulated procedure for static AFD may be the only method of retrieving higher-coercivity components of natural remanence in samples prone to developing gyroremanence at higher alternating fields (AFs). Our AFD environment comprises: a 2G-Enterprises through-bore, cryogenic magnetometer; 2G AF-coils and control equipment; and personal computer software, developed by us, to control all procedural aspects for a complete AFD of a sample including, importantly, a counteracting procedure to neutralize the effects of gyroremanence build-up at higher AFs. With our system, AFD of 8 samples/day, each of 20+ steps, requires only 20 min of user attention compared with a full day for conventional systems.     

Geotechnical Hazards Associated with Desert Environment

The aridity of the Arabian Peninsula's deserts ranges between arid to hyperarid with hot dry climate, scarce precipitation and sparse vegetation. These harsh environmental conditions enhance some geomorphologic processes more than others, cause specific geotechnical problems, and increase desertification.From west to east, the general physiography of Saudi Arabia shows the Red Sea coastal plains and the escarpment foothills called Tihama followed by the Arabian Shield mountains, the Arabian Shelf plateau and finally the Arabian Gulf coastal plains. Sand moves by wind either as drifting sand or migrating dunes in four major sand seas, over the Arabian Shelf, and in the inter-mountain valleys, in the Arabian Shield causing problems of erosion and deposition. Human activities in the deserts may cause more instability to the sand bodies, enlarging the magnitude of the problem. Fine silty soil particles also move by wind, depositing loess mainly in selected areas downwind in the Tihama. These loess deposits subside and may form earth fissures by the process of hydrocompaction upon wetting. The addition of water can be either natural through storms or man-made through human agricultural or civil activities. Extensive sabkhas exist along the coastal plains of both the Red Sea and Arabian Gulf. The sabkha soil may also heave by salt re-crystallization or collapse by wetting. The shallow groundwater brines present in sabkhas also attack and corrode civil structures. Urbanization and excessive groundwater pumping may also deplete the fresh groundwater resources and may cause subsidence, ground fissuring and surface faulting as observed in some locations in the Arabian Shield. Although the average annual precipitation is very low, rain usually falls in the form of torrential storms, collected by dry valley basins and causing floods to unprotected downstream areas on the coastal plains of the Red Sea.The desert environment, being a fragile echo system, needs to be treated with care. Intercommunications between different national and international agencies and education of the layman should help to keep the system balanced and reduce the resulting environmental hazards. In addition, any suggested remedial measures should be planned with nature and engineered with natural materials.     

Geochemical exploration for platinum-group elements in the Bushveld Complex, South Africa

Analyses of stream sediment and soil samples from the Bushveld Complex, South Africa have revealed enhanced precious metal concentrations, which can be related both to mining activities and the presence of hidden concentrations of platinum-group elements (PGEs) and gold. The economically important PGE deposits hosted by the Upper Critical Zone of the Rustenburg Layered Suite are revealed by a high PGE and Au content in the overlying soils. A second zone of elevated precious metal concentrations straddles the boundary between the Main and Upper Zones and has to date been traced for more than 100 km. This zone follows the igneous layering of the Rustenburg Layered Suite and is offset by the Brits Graben. It is therefore thought to be the reflection of a magmatic PGE-Au mineralisation. Received: 31 May 1996 / Accepted: 7 January 1997     

Multi-phase water-rhyolite interaction and ore fluid generation at Aljustrel, Portugal

Detailed study of the Aljustrel volcanic rocks, through petrography, mineral chemistry, whole-rock geochemistry, and previously published stable isotope data, show that the rocks experienced extreme metasomatism. The metasomatic phenomena presented are explained accurately by a sea-water convective model. The evolution of the sea-water-derived fluid as it progresses downwards through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn. Several lines of evidence (feldspar mineralogy, paragenetic relations between regional and ore-zone alteration, and stable isotope data) show that the temperature of the hydrothermal circulation increased through time, and that the water-to-rock ratio evolved from very high to low values. According to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the Aljustrel Volcanics. This implies, for both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained in the massive sulphide deposits of the Aljustrel area. With regard to Fe, about 1% was on average removed from the rocks, producing enough metal to account for all the Fe in the MS deposits and overlying chemical sediments. This study documents semi-quantitatively the provenance of the main metals in major massive sulphide deposits via sea water leaching of the immediate host rocks.

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Resumen (translated by E. Pascual) Un estudio detallado de las rocas volcánicas de Aljustrel, mediante petrografia, química mineral, geoquímica de roca total y datos de isótopos estables previamente publicados muestra que estas rocas han sufrido metasomatismo en grado extremo. Los fenómenos metasomáticos presentados se explican con precisión mediante un modelo convectivo con agua de mar. Es evidente la evolución del fluido derivado del agua de mar conforme progresa en profundidad a través de las rocas, desde un fluido frío, oxidante y con magnesio a otro reductor, sin magnesio y enriquecido en Si, Fe, Cu y Zn. Varias líneas de evidencia (mineralogía de los feldespatos, relaciones paragenéticas entre la alteración regional y de las zonas mineralizadas, isótopos estables) muestran que la temperatura y la circulación hidrotermal se incrementaron con el tiempo y que la relación agua-roca evolucionó de valores muy altos a bajos. De acuerdo con nuestros datos, de las rocas volcánicas de Aljustrel se han extraído unas 30 ppm de Cu y unas 100 ppm de Zn. Esto implica, para ambos metales, que la misma roca lixiviada ha producido los 2 Mt de Cu y los 6 Mt de Zn contenidos en los depósitos de sulfuros masivos del área de Aljustrel. En cuanto al Fe, una media de alrededor de 1% fue extraída de las rocas, produciendo metal suficiente para explicar todo el Fe de los depósitos de sulfuros masivos y de los sedimentos químicos suprayacentes. Este estudio documenta de forma semicuantitativa la procedencia de los principales metales en depósitos mayores de sulfuros masivos, mediante lixiviado por agua de mar de sus rocas huéspedes inmediatas.

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Received: 1 February 1997 / Accepted: 30 July 1997     

Flexible spectral methods for the generation of random fields with power-law semivariograms

Random field generators serve as a tool to model heterogeneous media for applications in hydrocarbon recovery and groundwater flow. Random fields with a power-law variogram structure, also termed fractional Brownian motion (fBm) fields, are of interest to study scale dependent heterogeneity effects on one-phase and two-phase flow. We show that such fields generated by the spectral method and the Inverse Fast Fourier Transform (IFFT) have an incorrect variogram structure and variance. To illustrate this we derive the prefactor of the fBm spectral density function, which is required to generate the fBm fields. We propose a new method to generate fBm fields that introduces weighting functions into the spectral method. It leads to a flexible and efficient algorithm. The flexibility permits an optimal choice of summation points (that is points in frequency space at which the weighting function is calculated) specific for the autocovariance structure of the field. As an illustration of the method, comparisons between estimated and expected statistics of fields with an exponential variogram and of fBm fields are presented. For power-law semivariograms, the proposed spectral method with a cylindrical distribution of the summation points gives optimal results.     

Seismic Hazard Studies in the U.K.: Source Specification Problems of Intraplate Seismicity

Although the U.K. is in an area of only low to moderate seismicity, the seismic hazard is sufficient to pose a threat to sensitive structures such as chemical plants and nuclear facilities. In quantifying the level of hazard by conventional probabilistic methodology, however, some problems arise in attempting to interpret earthquake data in terms of geological structure and faults. In the U.K., not only is it impossible to identify any demonstrably active faults, but also it is extremely difficult to discern any relationship between the pattern of seismicity and local or regional geological structure.This study discusses the use of two zonation approaches which complement each other in such a way that the general character and trend of seismicity is preserved. In one approach, the zonation is informed by the structural geology, where possible; geological zonation is avoided if it produces sources with heterogeneous seismicity. In the other approach, the record of past earthquakes is divided up into very small zones around individual epicentres or groups of epicentres, the size of each zone usually being proportional to the uncertainty in the epicentral determination of the appropriate event. This zonation preserves an observed tendency of some British earthquakes to repeat themselves. It is suggested that, in intraplate areas such as the U.K., it is often inappropriate to attempt to model individual fault sources. No faults in the U.K. are provably active. Because an earthquake of moderate size can occur on a very short fault segment, it is impractical to restrict fault modelling to major features. Even the two largest U.K. faults, suspected to be active, pose problems in attributing historical seismicity to them as distinct features.     

Mafic microgranular enclaves from the Xanthi pluton (Northern Greece): An example of mafic-felsic magma interaction

Summary Petrographic, geochemical and isotopic data are reported for the mafic microgranular enclaves occurring in a calc-alkaline Oligocene pluton outcropping near Xanthi town in northern Greece. The intrusion is composed of dominant granodiorites with minor tonalites and quartz diorites. Mafic enclaves occur in large swarms concentrated near the margins of the pluton. They have fine-grained porphyritic textures and contain the same minerals as their granodioritic and tonalitic host rocks (Pl, Qtz, Bt, Hb1, Kfs ± Ap, Zrn, Mt, Spn) but in totally different modal proportions. Chemical compositions range from mafic to intermediate and are characterized by higher abundance of Na₂O) and lower contents of CaO and Sr with respect to an average basaltic magma with a similar SiO₂ content. The initial Sr isotopic ratios, ranging from 0.7061 to 0.7063, are similar to those of the enclosing rocks (0.7062-0.7063). Field evidence, as well as petrographic and geochemical data, suggests that the analyzed enclaves derived from disruption of synplutonic intrusions of cogenetic mafic magma. The Xanthi pluton could represent ascent of a zoned magma body, with a silicic cap (granodiorite, tonalite, quartz diorite) having intruded first followed by the deeper more mafic levels as syn-intrusive dikes.

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Mafische mikrogranulare Enklaven aus dem Xanthi Pluton, Nord-Griechenland: Ein Beispiel von mafisch-felsischer Magma-Wechselwirkung

Zusammenfassung Petrographische, geochemische und Isotopen-Daten für mafische mikrogranulare Enklaven in einem kalk-alkalischen oligozänen Pluton, der in der Nähe der Stadt Xanthi in Nord-Griechenland aufgeschlossen ist, werden präsentiert. Die Intrusion besteht hauptsächlich aus Granodioriten mit untergeordneten Tonaliten und QuarzDioriten. Mafische Enklaven kommen in großen Schwärmen, besonders an den Rändern des Plutons vor. Sie zeigen feinkörnige prophyritische Texturen und enthalten diesselben Minerale wie ihre granodioritischen und tonalitischen Wirtsgesteine (P1, Qtz, Bt, Hb1, Kfs ± Zrn, Mt, Spn), aber in vollkommen verschiedenen modalen Verhältnissen. Chemische Zusammensetzungen schwanken von mafisch bis intermediär und werden durch höhere Gehalte an Na₂O und niedriger Gehalte von CaO und Sr, verglichen mit durchschnittlichem Basaltmagma mit ähnlichem SiO₂-Gehalt, gekennzeichnet. Die Strontium-Initiale schwanken von 0,7061 bis 0,7063, und sind denen der umgebenden Gesteine (0,7062-0,7063) ähnlich. Geländebefunde, wie auch petrographische und chemische Daten, zeigen, daß die analysierten Enklaven auf das Aufbrechen von synplutonischen Intrusionen cogenetischen mafischen Magmas zurückgehen. Der XanthiPluton könnte dem Aufstieg eines zonierten Magmenkörpers zuzuordnen sein, mit einem Si-reichen Dach (Granodiorite, Tonalite, Quarzdiorite), das zuerst intrudiert ist, gefolgt von den tiefer angesiedelten, mehr mafischen Gesteinen als syn-intrusive G:ange.

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With 7 Figures