Fluid systems and fracture development during syn-depositional fold growth: An example from the Pico del Aguila anticline,Sierras Exteriores,southern Pyrenees,Spain

This paper reports an integrated, spatio-temporal analysis of the fracture-controlled paleo-fluid system in the Pico del Aguila anticline, a N–S trending fold located in the Sierras Exteriores, the southern front of the Spanish Pyrenees. Eight fracture sets (joints or faults) are recognized throughout the fold and are separated into a fracture sequence that is defined using field relationships and the remarkable temporal constraints offered by the syn-tectonic sedimentary deposits. This fracture sequence records a complex Paleocene to Early Oligocene structural evolution, including map-view, clockwise rotation and tilting of the fold axis. The geochemical analysis of calcite cements from the different mineralized fracture/vein sets reveals a compartmentalized fluid system during most of fold development. This initial paleofluid system was later perturbed when bending-related fractures associated with foreland flexure and outer arc extension triggered small-scale, vertical fluid migration. Fractures developed in shallow strata facilitated downward migration of surficial fluids that controlled the paleo-fluid system in the Late Priabonian/Stampian continental deposits. The study of the Pico del Aguila anticline depicts for the first time the evolution of a fluid system in a shallow, syn-depositional compressional setting, and results further strengthen the statement that fluids migrate vertically across stratigraphic boundaries take place during fold hinge-related deformation.     

Gold-bearing hedenbergite skarns from the SW contact of the Andorra granite (Central Pyrenees, Spain)

Several varieties of skarn outcrop have been found to develop along the contact between the Andorra granite and the Devonian limestones. The skarns contain variable amounts of gold ranging up to 5 g/t, always associated with sulphides. The sulphides in the skarn include arsenopyrite and pyrrhotite with lesser amounts of chalcopyrite, galena, sphalerite and Bi-minerals. Geothermometric and geobarometric data indicate the skarns formed at about 2 kbar and temperatures ranging from 500 ° to 350 °C from CO₂-free polysaline brines at a slightly acidic pH and oxygen fugacity which decreases with time from the pyrite-pyrrhotite-magnetite towards the QFM buffer. Available data on gold solubility suggest that sufficient quantities of gold to form an ore deposit could have been transported as AuCl ₂ ^- at the high temperatures and salinities under which the skarns formed. Both gold deposition and sulphide precipitation could have occurred due to a decrease in temperature and/or oxygen fugacity.     

The Barranco de Arás flood of 7 August 1996 (Biescas, Central Pyrenees, Spain)

Geomorphic effects observed in the Barranco (creek) de Arás basin are used to characterize the flood. Sediment features allow to qualify the flood as essentially a water flow. Using the critical section method, the peak flood discharge is estimated to be between 400 and 600 m³ s⁻¹. Similar results were obtained using a paleohydraulic formula based on the size of the largest mobilized clasts. Using the rational method with available rainfall data, the discharge for a recurrence interval of 500 years is estimated to be between 150 and 200 m³ s⁻¹. These results agree with predictions obtained using curves of peak discharge versus basin area based on regional data. Several trenches dug on the fan showed that the size of boulders mobilized by the event is larger that those left by previous floods at the same place. When the estimated peak flood discharge is related to the basin area, values between 20 and 30 m³ s⁻¹ km⁻² are obtained, demonstrating that the Barranco de Arás flood was most unusual.     

On the determination of polymetamorphic mineral associations,and its application to the Bosost Area (Central Pyrenees)

A method to demonstrate the simultaneous development of different minerals is discussed. Minerals are contemporaneous when they have formed before and after a point in time. Points in time are supplied by phases of deformation. The relations between deformation and crystallization can be deduced from the internal and external structures of porphyroblasts and matrix.This method is applied to unravel the metamorphic history of the Bosost area. Four metamorphic zones succeed one another: the biotite-zone, stauroliteandalusite-cordierite-zone, andalusite-cordierite-zone and cordierite-sillimanitezone. This zonal distribution represents a progressive metamorphic series and each higher metamorphic zone has passed through one or more lower metamorphic zones. Even within one zone a further subdivision of the crystallization history can be made.The described mineral associations are the result of low pressure metamorphism. They can be assigned to a low pressure faciesgroup of the amphibolite facies. The geothermal gradient during metamorphism was approximately 15° C pro 100 meter.

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Zusammenfassung Eine Methode, um das gleichzeitige Wachstum verschiedener Minerale zu zeigen, wird hier näher erklärt. Minerale sind gleichzeitig, wenn sie vor und nach einem Zeitpunkt gebildet worden sind. Zeitpunkte werden durch Deformationsphasen geliefert. Die Verhältnisse zwischen Deformation und Kristallisation kann man aus dem Intern- und Externgefüge ablesen.Diese Methode wird angewandt, um die metamorphe Geschichte vomBosost-Gebiet zu entziffern. Vier metamorphe Zonen folgen aufeinander: Biotit-Zone, Staurolith-Andalusit-Cordierit-Zone, Andalusit-Cordierit-Zone und Cordierit-Sillimanit-Zone. Diese Zoneneinteilung stellt eine progressiv metamorphe Reihenfolge dar, wobei jede höher metamorphe Zone durch die weniger metamorphen Zonen gewandert ist. Auch innerhalb einer Zone kann noch eine weitere Unterteilung nach der Kristallisationsfolge gemacht werden.Die beschriebenen Mineralassoziationen sind unter niedrigen hydrostatischen Druckverhältnissen entstanden. Sie werden in der Amphibolit-Fazies, und zwar in einer niedrigen Druck-Faziesgruppe untergebracht. Der geothermale Gradient ist ungefähr 15° C pro 100 Meter gewesen.

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Résumé Une méthode pour démontrer la croissance simultanée de minéraux différents est discutée. Des minéraux sont contemporains quand ils sont formés avant et après un certain instant qui peut être déterminé par des phases de déformation. Les relations entre déformation et crystallisation peuvent être déterminées à l'aide des structures internes et externes des porphyroblastes et de la matrice.Cette méthode est appliqué afin de déchiffrer l'histoire métamorphique de la région de Bosost, où quatre zones métamorphiques se suivent dans le temps: zone à biotite, zone à staurotide, andalousite et cordierite, zone à andalousite et cordierite et zone à cordierite et sillimanite. Cette classification en zones représente une série à métamorphisme progressif et chaque zone plus métamorphique a passé par une ou plusieurs zones moins métamorphiques. Même dans certains zones métamorphiques une subdivision plus détaillée est possible.Les associations de minéraux décrites sont formées sous des conditions de basse pression. Elles peuvent être placées dans un groupe de facies à basse pression du facies amphibolite. Le gradient géothermique pendant le métamorphisme était environ 15° C par 100 mètres.

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Kilometre-scale sheath folds in the western Betics (south of Spain)

The three-dimensional exposures of the Sierra Blanca marbles and neighbouring rocks in the western Betic cordillera of the south of Spain show tight to isoclinal F₃ folds re-folded by a larger sheath-like structure. The relationships between both structures are as follows: (1) The mean axial surface for Sierra Blanca sheath fold can be defined as the great circle of best fit through the ß-axes of sub-cylindrical F₃ folding domains. (2) The asymmetric F₃-folds indicate a single mainly eastward normal sense of shear. (3) The mean attitude of the axial-plane crenulation foliation, Sc, is also sub-parallel to the mean axial surface for Sierra Blanca sheath fold. The regional context of the Sierra Blanca sheath fold is finally discussed, and a model of a heterogeneous high-strain extensional zone proposed.     

Larger foraminifera distribution and strontium isotope stratigraphy of the La Cova limestones (Coniacian–Santonian, “Serra del Montsec”, Pyrenees, NE Spain)

The Upper Cretaceous La Cova limestones (southern Pyrenees, Spain) host a rich and diverse larger foraminiferal fauna, which represents the first diversification of K-strategists after the mass extinction at the Cenomanian–Turonian boundary.The stratigraphic distribution of the main taxa of larger foraminifera defines two assemblages. The first assemblage is characterised by the first appearance of lacazinids (Pseudolacazina loeblichi) and meandropsinids (Eofallotia simplex), by the large agglutinated Montsechiana montsechiensis, and by several species of complex rotalids (Rotorbinella campaniola, Iberorotalia reicheli, Orbitokhatina wondersmitti and Calcarinella schaubi). The second assemblage is defined by the appearance of Lacazina pyrenaica, Palandrosina taxyae and Martiguesia cyclamminiformis.A late Coniacian-early Santonian age was so far accepted for the La Cova limestones, based on indirect correlation with deep-water facies bearing planktic foraminifers of the Dicarinella concavata zone. Strontium isotope stratigraphy, based on many samples of pristine biotic calcite of rudists and ostreids, indicates that the La Cova limestones span from the early Coniacian to the early-middle Santonian boundary. The first assemblage of larger foraminifera appears very close to the early-middle Coniacian boundary and reaches its full diversity by the middle Coniacian. The originations defining the second assemblage are dated as earliest Santonian: they represent important bioevents to define the Coniacian-Santonian boundary in the shallow-water facies of the South Pyrenean province.By means of the calibration of strontium isotope stratigraphy to the Geological Time Scale, the larger foraminiferal assemblages of the La Cova limestones can be correlated to the standard biozonal scheme of ammonites, planktonic foraminifers and calcareous nannoplankton. This correlation is a first step toward a larger foraminifera standard biozonation for Upper Cretaceous carbonate platform facies.     

Conical folds and apparent rotations in paleomagnetism (a case study in the Southern Pyrenees)

The so-called apparent rotation was defined as the angular deviation between a local paleomagnetic direction (after the standard bedding correction) and their corresponding paleomagnetic reference [J. Geophys. Res. 85 (1980) 3659]. In this paper, we make a theoretical exploration on this concept and we conclude that (depending on the number, sequence, orientation and magnitude of the deformation axes that have affected to the rock volume) the apparent rotation may be the addition of a vertical-axis rotation plus a spurious rotation. The later is an error whose origin is the inappropriate application of the bedding correction during the restoration (which does not fit the reverse sequence of deformations). Then, Apparent rot. (s.l.) (δ)=Spurious rot. (θ)+Vertical-axis rot. (β).Conical folds are complex geometries that cannot be restored by using the bedding correction. However, appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts. In this paper, we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection. The modelling has to be based on a good characterization of the geometry (fold axis orientation) and understanding of the kinematics of the fold. General modelling has also allowed us to predict the apparent rotation in conical synclines. Its magnitude depends on the semiapical angle and on the degree of development of the fold; the sense of the rotation (clockwise or counter-clockwise) will depend on the sense of rotation the fold axis.The western External Sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall (Ebro foreland basin) of the South Pyrenean sole thrust. In addition, a vertical-axis clockwise rotation up to 47° (32° in average) has been detected in the hagingwall. An apparent rotation up to 28° (20° in average) is observed in the footwall of the structure when a simple bedding correction is used. This deviation does not fit with the expected Ebro basin direction (reference) and is caused by the effect of the Riglos conical syncline, developed by the flexure of the foot wall ramp of the South Pyrenean sole thrust. The forward modelling carried out considering the geometry and kinematics (non significant rotations in the autochthonous foot wall) of this structure predicts very well the paleomagnetic observations in the field (geographic coordinates) with angular departures of only 5° (in average). The only application of the bedding correction would introduce errors (spurious rotations up to 21°, 12 in average) related to the conical geometry that would not allow the differentiation of these distinct structural units.     

Upper cretaceous biostratigraphy of the south-central Pyrenees (Lleida,Spain)

Abstract

Biostratigraphical data using larger foraminifera and planktonic foraminifera permitted us to establish the correlation between shallow platform sediments rich in larger foraminifera (Montsec and Serres Marginals thrust sheets) and deeper ones containing planktonic foraminifera (Boixols thrust sheet).

Consequently, the Santa Fe limestones containing Ovalveolina-Praealveolinaassemblage represent the Cenomanian. Early Turonian ( ‘IT~ archaeocretacea and P. helvetica zones) exist in both, Montsec and Boixols thrust sheets and it is constituted by Pithonella limestones. Late Turonian (M. schneegansi zone) is only present in Boixols thrust sheet (Reguard Fm.), the Montsec thrust sheet having an erosive hiatus.

Late Coniacian-Early Santonian (D. Concavata zone) is represented in the Montsec thrust sheet (Cova Limestones) and in the eastern part of the Boixols thrust sheet (St. Corneli Fm.) by two differents facies giving two different microfaunal assemblages; the firts one, characterized by Ophtalmidiidae s.l indicate a restricted lagoonal environment while the second one, characterized by diverses species of complex agglutinated, Fabulariidae, Meandropsinidae and Rotaliidae, represents an open shallow platform. In the Boixols thrust sheet (Anseroles Fm.) dominate the planktonic foraminifera and small benthic.

In the late Santonian (D. asyrnetrica zone) the sea reached as far as Serres Marginales thrust sheet where sediments (Tragó de Noguera unit) are terrigenous and deposited in a very shallow platform. In the Montsec thrust sheet (Montsec marls) the larger foraminifera indicate a platform deeper than that of the Serres Marginals thrust sheet. In the Boixols thrust sheet the sediments are deposited in an outer platform (Herbasavina Fm.) or turbiditic basin (Mascarell Mb.).

During Campanian times the transgresion reaches the maximum. In the Serres Marginals sediments are deposited in a restricted shallow environment rich in Meandropsinidae (Serres Limestones). In the Montsec thrust sheet they are deposited in a platform with patch reefs and shoals (Terradets limestones) and in the Boixols one in an outer platform, talus and/or basin.

During Early Maastrichtian times (C. falsostuarti zone) terrigenous materials arrived in the basin, the rate of sedimentation increased outstripping the subsidence rate and the retreat of the sea to the north. Late Maastrichtian (C. gansseri zone) is only present in the Boixols thrust sheet.     

The origin of fluids involved in the formation of gold-bearing skarns of the Andorra granite (Central Pyrenees,Spain): Sulphur isotope data

Summary Arsenopyrite-gold mineralization in the lower Paleozoic series of the eastern Pyrenees is of two types: disseminated in Hercynian metasediments, and veins cross-cutting the main foliation. In the central Pyrenees, gold-bearing arsenopyrite mineralizations have recently been discovered in skarns along the SW contact of the Andorra granite. The ore minerals are similar to those of the veins, with gold also being associated with arsenopyrite, bismuth and sulphides.The late Hercynian Andorra granite intruded Paleozoic metasediments of pre-Caradocian to Carboniferous age. Six types of skarns have developed at or close to the contact between the granite and Devonian limestones. Only three of these, the hedenbergite, pyrrhotite and arsenopyrite skarns, are sulphide-bearing. Gold grades in these skarns range from less than 0.8 g/t in pyrrhotite skarns to 7 g/t in arsenopyrite skarns.The ³⁴S of arsenopyrite from disseminated and vein mineralizations show values from 11 to 15, indicating a sulphur source of crustal origin. Vein sulphides are considered to have formed after a metamorphic remobilization of the sulphide rich disseminations in the Paleozoic series. Sulphur isotopic composition of skarn sulphides is variable: pyrrhotite from pyrrhotite skarns has a ³⁴S +, suggesting a sulphur of magmatic origin. Their gold grades are very low (>0.8 g/t). The heavier values of arsenopyrite from arsenopyrite skarns (³⁴S + 10) and of galena and sphalerite from hedenbergite skarns (³⁴S + 7) correlate with the higher gold grades (7 and 5 g/t, respectively). This is interpreted to reflect a contribution of wall-rock sulphur-rich fluids generated from the thermal metamorphism of arsenopyrite-bearing wall-rock metasediments (³⁴S + 12%o) during skarn formation or by assimilation of country rock by the granitic intrusion during its emplacement. The economic importance of these skarns appears to be related to the amount of crustal fluid involved in skarn formation.

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Lagerstättenbildende Fluide in den goldführenden Skarnen des Andorra-Granites, Zentralpyrenäen, Spanien: Schwefelisotopen-Daten

Zusammenfassung Arsenkies-Goldmineralisation in altpaläozoischen Serien der ostlichen Pyrenäen tritt in zwei Typen auf: Disseminiert in Hercynischen Metasedimenten, und in Gängen, die die Haupt-Foliation durchschlagen. In den Zentralpyrenäen wurden goldführende Arsenkies-Mineralisationen vor kurzem in Skarnen längs des SW-Kontaktes des Andorra-Granites entdeckt. Die Erzminerale sind ähnlich denen in den Gängen, wobei Gold mit Arsenkies, Wismut und Sulfiden assoziiert ist. Der spät-hercynische Andorra-Granit intrudierte paläozoische Metasedimente von Prä-Caradoc bis Karbon-Alter. Im Kontaktbereich Granit/Devonische Kalke kommen sechs Typen von Skarnen vor. Nur drei davon führen Sulflde, nämlich die Hedenbergit-, Magnetkies- und Arsenkiesskarne. Goldgehalte in diesen Skarnen schwanken von weniger als 0,8 g/t in Magnetkiesskarnen bis zu 7 g/t in Arsenkiesskarnen. Das ö34 S von Arsenopyrit in disseminierten und Gang-Mineralisationen schwankt von 11-15%o, und dies weist auf eine Schwefelquelle in der Kruste hin. Gangsulflde haben sich auch nach einer metamorphen Remobilisierung der Sulfid-reichen Disseminationen in den paläozoischen Serien gebildet. Die Schwefelisotopenzusammensetzung der Skarnsulfide schwankt: Magnetkies aus Magnetkiesskarnen hat ö34 S Werte von +4%o, dies weist auf magmatischen Ursprung hin. Ihre Goldgehalte sind sehr niedrig (<0,8 0,8 g/t). Die schwereren Isotopen von Arsenkies in Arsenkies-Skarnen ( ³⁴S+ 10%o) und von Bleiglanz und Zinkblende aus den Hedenbergit-Skarnen ( ³⁴S+ 7%o) lassen sich mit höheren Goldgehalten (7 und 5 g/t) korrelieren. Dies wird Fluiden zugeschrieben, die reich an Schwefel aus den Nebengesteinen sind und durch die thermale Metamorphose der Arsenkies-führenden Metasedimente (S + 12%o) während der Skarnbildung, oder durch Assimilation von Nebengestein durch den Granit wahrend seiner Platznahme entstanden sind. Die wirtschaftliche Bedeutung dieser Skarne scheint in einer direkten Beziehung zur Menge krustaler Fluide zu stehen.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids und Mineral Deposits, ETH Zürich, March 21–23, 1991.     

Cretaceous–Tertiary tectonic inversion of the Cotiella Basin (southern Pyrenees,Spain)

The Cotiella Nappe includes one of the most important Mesozoic basins of the southern Pyrenees, which was subsequently inverted during the Tertiary compression. The Late Cretaceous Cotiella Basin is here interpreted as the western sector of the Cretaceous Cotiella-Bóixols basin (100᎜ km wide), located in the central part of the southern Pyrenees. The present-day complex structure of the Cotiella Nappe is the result of the inversion process, linked to the emplacement of basement thrust sheets of the Axial zone. In its western sector, the Cotiella Nappe consists of several superimposed thrust sheets, with complex geometry, becoming simpler towards the east, with a single thrust surface and smaller displacements. The Cotiella-Bóixols basin underwent strong subsidence during the Early Cretaceous at its eastern sector, and its depocentre migrated westward during the Late Cretaceous. The reconstruction of the sedimentary basin to the pre-compressional stage shows that during the Mesozoic the Cotiella-Bóixols basin was located to the south of a basement high, which later became the Pyrenean Axial Zone. From a balanced cross section, it can be inferred that the Cotiella, north-verging extensional system was connected with the north-Pyrenean rift by means of a 10-km deep horizontal detachment. The compressional Tertiary detachment within the upper crust was shallower than the extensional detachment, and individualised four basement thrust sheets, which form the Axial Zone antiform.     

Synchronous detachment folds and coeval sedimentation in the Prepyrenean External Sierras (Spain): a case study for a tectonic origin of sequences and systems tracts

This paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Pyrenees, Spain). The initiation, duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is formed by up to 400 m of azoic marls deposited in outer ramp areas. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to be controlled by regional tectonic pulses. An increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls (i.e. transgressive systems tract). The overlying highstand systems tract developed following periods of diminished tectonism, with the consequent growth and progradation of shallow carbonate platform facies.     

High-grade metamorphic rocks and peridotites along the Leiza Fault (Western Pyrenees,Spain)

Acid and basic granulites, migmatites, and lherzolites outcrop along the Leiza Fault (Navarra, Spain) in the western extremity of the Pyrenean Belt. The protoliths of the acid granulites have granodioritic composition. Textural and mineralogical data suggest that the acid granulites evolved from a first, syn kinematic medium-pressure granulite-facies stage [garnet-biotite I-Kfsp] to a post-kinematic granulite-facies stage of lower pressure and higher temperature [garnet-biotite II-cordierite-Kfsp-(spinel?), kinzigites]. Basic granulites were formed from protoliths with composition of tholeiitic to alkaline basalts. Basic granulites exhibit millimeter size subidiomorphic garnets dispersed through the matrix and smaller coronitic garnets between opaques or orthopyroxenes and plagioclase. Thermo-barometric estimates for the peak of the granulite-facies metamorphism are c. 800°C and 8 kbar. The migmatites presumably represent a shallower level of metamorphism (andalusite/sillimanite-Kfsp). Scapolite in the basic granulites was formed during a post-granulitic metamorphic episode. The lherzolites have been intensely brecciated and serpentinized, which makes difficult the comparison of their evolution with that observed in the other rocks associated with the Leiza Fault. Acid and basic granulites, migmatites, and lherzolites along the Leiza Fault may be correlated with similar rocks outcropping elsewhere in the North-Pyrenean Zone and along the North-Pyrenean Fault as tectonic slices and massifs. By analogy with those rocks, the granulitefacies metamorphism observed in the rocks studied must be Hercynian in age. The Leiza Fault constitutes, therefore, the western continuation of the North-Pyrenean Fault, and the rocks studied (except perhaps the lherzolites) may be considered as remnants of an Hercynian metamorphic massif, dismembered as a consequence of the activity of the fault at the end of the Hercynian cycle and during the Alpine tectonometamorphic events.     

A riparian plant community from the upper Maastrichtian of the Pyrenees (Catalonia,NE Spain)

Angiosperms began to colonise riparian habitats very soon in their evolutionary history, probably already in the Aptian, but it is still poorly known when flowering plants finally dominated entirely these kind of communities as they do in the present. A new fossil plant locality (Molí del Baró-1) from the upper Maastrichtian of the Southern Pyrenees is described in which meandering river facies represent one of the first riparian communities formed only by angiosperms. The fossil assemblage consists of abundant leaves, seeds, logs and sporomorphs. Angiosperms remains dominate in all these cases and the leaf sample is mostly composed of a new eudicot willow-like species, Saliciphyllum gaetei sp. nov., the palm Sabalites longirhachis and an helophytic monocot. Pollen remains suggest that the later belonged to Typhaceae. Most of these plant remains were parautochthonous and deposited in a pond formed in the accretional part of a meander loop. The locality of Molí del Baró-1 represents an unique plant fossil assemblage in the uppermost Cretaceous of southern Europe. It clearly differs from those reported in other Maastrichtian localities of the Pyrenees (Fumanya and South Isona) and from the Campanian-Maastrichtian of Austria and Romania. In addition, it reflects a surprisingly modern physiognomy for a Late Cretaceous riverine plant assemblage that was built up with willow-like plants, palms and reeds.     

Environmental problems and geological implications derived from evaporite dissolution in the Barbastro salt anticline (NE Spain)

The halite-bearing Barbastro Formation crops out in the core of the Barbastro Anticline (Ebro Tertiary Basin). This anticline is traversed perpendicularly by some of the most important Pyrenean drainages such as the Cinca and Noguera-Ribagorzana Rivers. The terrace sequences of these fluvial systems have been used as markers to identify and assess dissolution-induced subsidence and salt tectonics. In the limbs of the anticline, terrace deposits underlain by detrital bedrock do not show any evidence of deformation and have a consistent thickness of less than 10 m. The deposits of certain terrace levels of the Noguera-Ribagorzana River and its tributary, the Lo Reguer Creek, are locally thickened filling basins generated by dissolution-induced synsedimentary subsidence up to several kilometers long and more than 100 m deep. Conversely, terraces of the Cinca River do not show anomalously high thicknesses, but local uplifts related to differential upward flow of the halite-bearing bedrock. Locally, a minimum uplift rate of 0.3 mm/year has been estimated from a 64-ka terrace tilted away from the valley. The subsidence hazards occur chiefly in areas where the ground receives artificial water recharge. Serviceability of some canals has been notoriously affected by evaporite karstification. The problem has been mitigated to acceptable levels by grouting. Numerous buildings of Ivars de Noguera are severely damaged by dissolution subsidence, and possibly, by hydrocompaction of gypsiferous silts. The pipe network has been replaced to ameliorate the subsidence risk. In the Cinca River valley, cavities with a total volume of about 180,500 m³ have been created by solution mining at depths greater than 500 m. No investigation methods are applied in the brine field to monitor the distribution and evolution of artificial voids. Substantial increase in salinity of the Cinca River is another evidence of subjacent evaporite dissolution.     

Stable isotope composition of F-Pb-Zn mineralization in the Valle de Tena (Spanish Central Pyrenees)

This study, dealing with the stable isotope (C, O, and S) geochemistry of F-Pb-Zn deposits, is part of an integrated metallogenetic study of the Valle de Tena (Central Pyrenees). The objective of this work is to place constraints on the origin of the mineralizing solutions and sources of sulphur and metals. Two types of mineralization have been identified: (1) stratabound ores linked to an emersion episode that occurred during the Namurian to Westfalian transition (Portalet mines), and (2) low-temperature hydrothermal veins related to post-Hercynian extensional tectonics (Yenefrito and Lanuza mines, and Tebarray occurrences). Stable isotopic study indicates that the source of mineralizing components was the local Upper Palaeozoic country rocks in the case of the stratabound Portalet deposit (biogenic sulphides and connate waters, modified by some meteoric input through a karst surface). In the case of vein deposits, stable isotopes point toward either modified connate water or marine water as the mineralizing fluid, with sulphur and metals derived from a mixture, in different proportions, of regional igneous and (meta)sedimentary country rocks. Received: 6 November 1995 / Accepted: 6 September 1996     

2 1/2 and 3 dimensional interpretation of magnetic anomalies in the central-southern Pyrenees (Spain)

Abstract

Available aeromagnetic data have been used to interprete magnetic anomalies using 2 1/2 and 3 D techniques in an area that includes parts of the Axial Zone and of the South Pyrenean Thrust Sheets in the Spanish Pyrenees. The interpretation has been made with the help of field magnetic susceptibility measurements. The dip of the Ebro Basin basement to the north has been inferred for part of the magnetic profile. Large magnetically disturbing bodies with the top at about sea level can be interpreted as a stack of Paleozoic basement thrusts or, alternatively, as an accumulation of Triassic volcanic rocks. Some smaller anomalies are clearly associated with outcropping or buried basaltic rocks (ophites). The disposition of major magnetic anomalies suggests that shear movement may have affected part of the studied area.     

U-Pb age and lead isotopic characterization of Au-bearing skarn related to the Andorra granite (central Pyrenees,Spain)

Auriferous skarns are associated with post- and late-kinematic Hercynian granites that intruded into Cambro-Ordovician to Devonian sediments of the central Pyrenees. We determined the age of the Andorra granite and the associated skarn at 305 ± 3 Ma by U-Pb dating titanite from the endo-skarn. The sulfur isotopic composition from sulfides in the skarn (Cardellach et al. 1992) shows a significant variation with isotopically light sulfur (³⁴S +3) in the barren skarns and heavy sulfur (³⁴S +11) in the gold-bearing skarns. Outwards, it increasingly resembles sulfur from arsenopyrite disseminations in the Cambro-Ordovician sediments. The lead isotopic composition from sulfides of the skarns is very homogeneous (²⁰⁶Pb/²⁰⁴Pb = 18.410, ²⁰⁷Pb/²⁰⁴Pb = 15.699, ²⁰⁸Pb/²⁰⁴Pb = 38.574) in contrast to the one in gold-bearing arsenopyrite veins and in arsenopyrite disseminations in the sediments (e.g. ²⁰⁶Pb/²⁰⁴Pb varies from 18.54 to 30.36). Combined, sulfur and lead isotope data indicate that the lead in the skarn is dominantly derived from the granite, whereas the sulfur is derived at variable portions from both the granite and the sediments.     

Neotectonic evolution of the Central Betic Cordilleras (Southern Spain)

Paleostress orientations were calculated from fault-slip data of 36 sites located along a traverse through the Central Betic Cordilleras (southern Spain). Heterogeneous fault sets, which are frequent in the area, have been divided into homogeneous subsets by cross-cutting relationships observed in the field and by a paleostress stratigraphy approach applied on each individual fault population. The state of stress was sorted according to main tectonic events and a new chronology is presented of the Miocene to Recent deformation in the central part of the Betic Cordilleras. The deviatoric stress tensors fall into four distinct groups that are regionally consistent and correlate with three Late Oligocene–Aquitanian to Recent major tectonic events in the Betic Cordilleras. The new chronology of the neotectonic evolution includes, from oldest to youngest, the following main tectonic phases:

(1) Late Oligocene–Aquitanian to Early Tortonian: σ₁ subhorizontal N–S, partly E–W directed, σ₃ subvertical; compressional structures (thrusting of nappes, large-scale folding) and strike-slip faulting in the Alborán Domain and the External Zone of the Betic Cordilleras;

(2) Early Tortonian to Pliocene–Pleistocene: σ₁ subvertical, σ₃ subhorizontal NW–SE, partly N–S directed or E–W-directed (radial extension); large-scale normal faulting in the Central Betic Cordilleras and in the oldest Neogene formations of the Granada Basin related to the gravitational collapse of the Betic Cordilleras and the exhumation of the intensely metamorphosed rock series of the Internal Zones, at the same time formation of the Alborán Basin and intramontane basins such as the Granada Basin;

(3) Pleistocene to Recent: (3a) σ₁ subvertical, σ₃ subhorizontal NE–SW with prominent normal faulting, but coevally; (3b) σ₁ subhorizontal NW directed, σ₃ NE–SW subhorizontal with strike-slip faulting. Extensional structures and strike-slip faulting are related to the ongoing convergence of the Eurasian and African Plates and coeval uplift of the Betic Cordilleras. Reactivation of pre-existing fractures and faults was frequently observed. Phase 3 is interpreted as periodic strike-slip and normal faulting events due to a permutation of the principal stress axes, mainly σ₁ and σ₂.

Origin and evolution of tufa mounds related to artesian karstic springs in Isona area (Pyrenees,NE Spain)

This work analyses a suite of relict tufa mounds generated by artesian karstic springs in Isona area (Spanish Pyrenees). Geological and geophysical data (seismics and vertical electrical resistivity soundings) indicate that the location of the discharge area in which the spring mounds formed was controlled by (1) a bulge in the axial zone of the anticline that affects the Areny-Montsec aquifer, with the consequent thinning of the overlying confining unit and (2) N-S and E-W trending extensional faults. These uncommon meteogene mounds occur in two stepped morphostratigraphic units that constitute the caprock of a mesa 9 km2 in area. The upper tufa complex is 47 m thick and has yielded several U/Th dates of >350 ka. The obtained U/Th ages for the mounds of lower tufa complex, 10 m thick, range from >350 to 214 ± 11 ka. The sedimentological analysis of the scarce exposures and electrical resistivity profiles show the same morphological and depositional components as those described in geothermal springs: (1) Cylindrical vents; (2) Pools fed by the vents and dammed by annular tufa barriers; (3) Tufa barriers (rimstones) constructed by overflowing waters through vertical accretion and progradation. These rimstones may have overhanging upflow sides. (4) Slope tufa facies with terracettes and microgours.     

Glaucony from the Cretaceous of the Sierra de Guadarrama (Central Spain) and its application in a sequence-stratigraphic context

Detailed sedimentological and geochemical analyses of Upper Cretaceous glaucony-bearing deposits from the middle portion of the Castro de Fuentidueña Formation, in Central Spain, enable identification of a multiphase history of glaucony accumulation. Despite its relatively high maturity (K₂O > 7%), glaucony from the transgressive subtidal sandstones has anomalously low concentration in the host rock (generally <10%), suggesting remobilization from a different source. The remarkable thickness of the glaucony-bearing horizons, concurrently with concentration of glaucony in laminae, small size, and high degree of roundness and sorting provide further evidence for an allochthonous origin of the green grains. In contrast, authigenic glaucony from the overlying offshore clays exhibits higher abundance (up to 45%) and lower maturity (K₂O < 7%) and is interpreted to have formed in situ. Allochthonous glaucony originally developed in more distal areas during a prolonged period of sediment starvation. The green grains were then swept away from their place of origin and concentrated into tidal bars and channels within the upper transgressive systems tract of the third-order depositional sequence. Maximum concentration of autochthonous glaucony is recorded at the transition from tidal-influenced to offshore deposits: this glaucony, which is relatively less evolved due to lower time available for maturation, is interpreted to reflect the turnaround from transgressive to highstand conditions, marking the condensed section of the depositional sequence. As postulated by previous sequence-stratigraphic models, this study documents that allochthonous glaucony can be widespread throughout the transgressive systems tract (TST), while the condensed section (CS) typically hosts autochthonous glaucony. Contrary to the existing literature, however, this study shows that glaucony from the TST may be even more mature than glaucony from the CS, if enough time for maturation during transgression is allowed before the ultimate concentration of the green grains. Identification of spatial and temporal characteristics of glaucony, thus, is critical to a reliable sequence-stratigraphic interpretation of the glaucony-bearing deposits.