Toe-of-slope of a Cretaceous carbonate platform in outcrop,seismic model and offshore seismic data (Apulia,Italy)

Synthetic seismic models of outcrops in the Early Cretaceous slope of a carbonate platform on the Gargano Promontory (southern Italy) were compared to an offshore seismic section south of the Promontory. Outcrops of the same age on the promontory have the same sequence stratigraphic characteristics as their offshore equivalent, and are the only areas where the transition from platform to basin of Early Cretaceous is exposed on land. Two adjacent outcrop areas were combined into one seismic-scale lithologic model with the aid of photo mosaics, measured sections, and biostratigraphic data. Velocity, density, and porosity measurements on spot samples were used to construct the impedance model. Seismic models were generated by vertical incidence and finite difference programs. The results indicate that the reflections in the seismic model are controlled by the impedance contrast between low porous intervals rich in debris from the platform and highly porous intervals of pelagic lime mudstone, nearly devoid of debris. Finite difference seismic display showed best resemblance with the real seismic data, especially by mapping a drowning unconformity.     

The discovery of the Borobia world-class stratiform magnesite deposit (Soria,Spain): a preliminary report

A new world-class stratiform magnesite deposit has been recently discovered near the village of Borobia in Soria, Spain. The mineralization consists of a 25-45 m massive to banded magnesite layer sandwiched in between a Lower Cambrian carbonate formation and an upper shale horizon. It forms an arcuate belt striking NW-SE to W-E, which outcrops for 7.3 km in two elevations separated by a central valley that is covered by a thin layer of Pleistocene sediments. The orebody is confined by two regionally important faults and is probably associated with the dolomitization and silicification of the Cambrian limestones. Geological ore resources are estimated to exceed 175 Mt with MgO contents varying from 36 to 47% by weight. At present, the owners, Magnesitas y Dolomías de Borobia, S.L., are at an advanced stage of planning for extraction. This report presents some results of an ongoing geological exploration programme and shows the general features of the geology and mineralogy of the Borobia deposit.     

Development and Application of 2D and 3D GRASS Modules for Simulation of Thermally Driven Slope Winds

The ability to manage and process fully three‐dimensional information has only recently been made available for a few Geographical Information Systems (GIS). An example of integrated and complementary use of 2D and 3D GRASS modules for the evaluation and representation of thermally induced slope winds over complex terrain is presented. The analytic solution provided by Prandtl (1942) to evaluate wind velocity and (potential) temperature anomaly induced by either diurnal heating or nocturnal cooling on a constant angle slope is adopted to evaluate wind and temperature profiles at any point over both idealised and real complex terrain. As these quantities depend on the slope angle of the ground and on the distance from the slope surface suitable procedures are introduced to determine the coordinate n of a point in the 3D volume measured along the direction locally normal to the terrain surface. A new GRASS module has been developed to evaluate this quantity and to generate a 3D raster file where each cell is assigned the value of the cell on the surface belonging to the normal vector. The application of the algorithm implemented in GRASS to an ideal valley and to a real valley close to the city of Trento in the Alps provides results in accordance with data reported in the literature. An extension of Prandtl's (1942) model to take into account humidity and evaporation processes on the soil is also proposed and implemented.     

Genesis of sulfide-rich chromite ores by the interaction between chromitite and pegmatitic olivine–norite dikes in the Potosí Mine (Moa-Baracoa ophiolitic massif, eastern Cuba)

The Potosí Mine is located in the Moa-Baracoa massif in the easternmost part of the Cuban Ophiolitic Belt. Chromite mineralization occurs within the mantle-crust transition zone. Two events of magma intrusion overprint the chromitite bodies: one gave rise to the crystallization of pegmatitic olivine-norite dikes, and the other produced pegmatitic gabbro dikes. Sulfide-poor chromite ores, brecciated chromite ores, and sulfide-rich chromite ores can be distinguished in the different chromitite bodies. Sulfide-poor ores represent more than 80 vol% of the chromitites. This type occurs far from the zones intruded by pegmatitic gabbro dikes and shows petrographic and chemical features similar to other chromitite bodies described in the Moa-Baracoa massif. Brecciated chromite ores occur within pegmatitic gabbro dikes. In this type, chromite crystals occur included within chromian diopside and plagioclase. These silicates often contain droplet-like sulfide aggregates. Sulfide-rich ores are spatially associated to the contacts between sulfide-poor chromite and pegmatitic olivine-norite dikes. These ores mainly consist of recrystallized (coarse) chromite with interstitial pyrrhotite, pentlandite, cubanite, and chalcopyrite. Chromite from sulfide-rich ores exhibits TiO₂, FeO, V₂O₃, MnO, and especially, Fe₂O₃ contents, higher than those of chromite from brecciated ores and much higher than those of chromite from sulfide-poor ores. The sulfide-rich ores are PGE-rich (up to 1,113 ppb of total PGE), and show nearly flat chondrite-normalized PGE patterns, slightly above 0.1 times chondritic values. Mineralogical and chemical data indicate that the chromite ores of the Potosí Mine were modified by the intrusions of olivine-norite and gabbro dikes. The interaction between pre-existing sulfide-poor chromite ores and the intruding volatile-rich silicate melts produced strong brecciation, partial dissolution, and recrystallization (coarsening) of chromite. The sulfide assemblage formed by fractionation of the immiscible sulfide melt segregated from the volatile-rich silicate melt that generated the pegmatitic olivine-norite. The segregation of the sulfide melt can be interpreted as the consequence of chemical interaction between intruding melts and the host chromite. The variable extent of this interaction produced chromite ores with variable sulfide ratios. The magmatic nature of the sulfide mineralization is supported by sulfur isotope data, which range from -0.4 to +0.9‰. Sulfide melt collected incompatible PGE (Rh, Pt, Pd) to produce the typical flat chondrite-normalized pattern of sulfide-rich chromite ores.     

Freshwater macroinvertebrate distribution in two basins with different salinity gradients (Guadalete and Guadaira river basins,south-western Spain)

This work reports the results of one year's sampling of aquatic macroinver-brates in various streams with different salinity gradients. The study area was the headwaters of the rivers Guadaíra and Guadalete, located in the same geographical area in south-western Spain. The most interesting feature of the Guadaíra basin is the natural salinity of its waters due to the abundance of gypsum in its headwaters. Lithologically, the headwaters of the Guadalete basin flow over marls, clay, sandstone and limestone. Salinity values in most streams do not reach 1 mS cm^–1. At least at the levels of salinity found in the Guadaíra basin headwaters, the existence of a well-structured community of macroinvertebrates can be claimed. Of the groups studied, Diptera and Coleoptera showed the highest species richness, being distributed preferentially at the ends of the conductivity spectrum. Groups including taxa inhabiting waters of low salinty and taxa inhabiting waters with high salinity could be reliable indicators of salinity.     

Eoalpine and mesoalpine tectonics in the Southern Alps

According to structural and stratigraphic data, eoalpine and mesoalpine tectonics can be clearly recognized in the Southern Alps and distinguished from the better known neoalpine deformation.The eoalpine phase (Late Cretaceous) was generated by N-S compression. It deformed basically the central-western Southern Alps producing overthrusts to the west and flower structures by sinistral transpression in the Giudicarie Belt. Deposition of the coeval flysch successions was controlled by the trend of this belt. This tectonic pattern persisted until Middle Eocene.The mesoalpine phase (Eocene) was generated mainly by a ENE-WSW compression and produced thrust geometries with N-S or NW-SE direction in the eastern part of the Southern Alps. The coeval Eocene Flysch also followed this trend, filling the foredeep basin. This deformation is considered to be the front of the Dinarids, which began to be deformed since Late Cretaceous until at least Early Oligocene.The neoalpine tectonics inherited the eoalpine and mesoalpine structures and produced the major part of the deformation accounting for the present structural framework of the Southern Alps.

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Zusammenfassung Die eo- und mesoalpidische Tektonik in den Südalpen ist strukturell und stratigraphisch erkennbar und von den bekannteren neoalpidischen Bewegungsbildern deutlich zu unterscheiden.Die eoalpidische Phase (Obere Kreide) entspricht einer N-S Kompression, die sich in den zentralen bis westlichen Südalpen bemerkbar macht. Im westlichen Bereich treten Überschiebungen auf, das Judikarische Gebiet wird hingegen durch die von linkslateralen Transpressionen bewirkten Verwerfungsbündel gekennzeichnet, die auch die altersgleichen Flysch-Ablagerungen kontrollieren. Diese tektonische Phase ist bis zum Eozän aktiv.Die mesoalpidische Phase (Eozän) ist auf eine ENE-WSW Kompression zurückzuführen. Sie verursachte in den östlichen Südalpen N-S bis NW-SE gerichtete Überschiebungsbilder. In gleichorientierten vorozeanischen Becken kommt der eozäne Flysch vor. Dieses Deformationsbild kennzeichnet schon in der Oberen Kreide die Dinariden-Front.Die neoalpidische Tektonik vererbte die eo-mesoalpidischen Gefüge-Elemente und verursachte im wesentlichen den gegenwärtig erkennbaren Gefügeplan der Südalpen.

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Résumé Dans les Alpes Méridionales, des données stratigraphiques et structurales permettent de reconnaître les tectoniques éoalpine et mésoalpine et de les distinguer des déformations néoalpines, mieux connues.La phase éoalpine (Crétacé supérieur) a été engendrée par une compression N-S. Celle-ci a affecté surtout la partie centre-occidentale des Alpes Méridionales en produisant des charriages à l'ouest et des structures de décrochement sénestres dans la chaîne des Giudicarie, dont l'orientation a déterminé la sédimentation du flysch concommittant. Cette tectonique s'est poursuivie jusqu' à l'Eocène.La phase mésoalpine a été engendrée surtout par une compression ENE-WSW; elle a produit, dans la partie est des Alpes Méridionales, des charriages de direction N-S à NW-SE. Le flysch éocène a suivi cet alignement structural, en remplissant le bassin de l'avant-fosse. Cette déformation est considérée comme le front des Dinarides, dont la formation a commencé dés le Crétacé supérieur.La tectonique néoalpine a hérité des structures éoalpines et mésoalpines et est responsable de la plus grande part de la structure actuelle des Alpes Méridionales.

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Surface ozone measurements in the Venezuelan tropical savannah

A one-year set of surface ozone measurements in a four-station network located in the Venezuelan savannah is reported. The diurnal ozone variation is typical of continental stations with a maximum in the afternoon, when vertical turbulent mixing is strongest. The annual O₃ average concentration, based on the monthly averages of daily maxima, was 17±2 ppb, which is in good agreement with values reported for similar latitudes. The boundary-layer ozone levels did not fall below 8 ppb, in contrast with previous sporadic measurements made in tropical latitudes. No evidence was found that mesoscale O₃ downdrafts in the ITCZ in the South American continent are an important source of surface ozone. Finally, it is suggested that the relatively high ozone levels observed at the end of the dry season are probably of photochemical origin.