Francesc Calvet

Outer ramp cycles in the Upper Muschelkalk of the Catalan Basin, northeast Spain

The Upper Muschelkalk (Triassic) of the Catalan Basin, eastern Spain, was deposited upon a carbonate ramp of homoclinal type located in an intracratonic setting. The Rasquera Unit in the Upper Muschelkalk mostly consists of outer ramp carbonates arranged in upward shallowing cycles. Five cycles from 1.5 to 12 m thick are recognised. Each cycle shows an upward coarsening of grain-size, an upward increase in bed thickness, and an upward change in fossil content, reflecting an upward shallowing of the environment. In the complete cycle, marlstone and shale pass up into marlstone with thin-bedded line mudstones containing pelagic bivalves. The succeeding thin-bedded and nodular limestones with shaley partings contain a more varied fauna and are bioturbated; they pass up into thick-bedded skeletal wackestones. Packstone-coquinas locally cap the cycles and have a diverse benthic fauna, as well as the alga Tubiphytes occurring as bioclasts, encrustations around skeletal grains and forming microbuildups. Although some of the thin beds were deposited during weak storms, there is little evidence of intense storm activity. Petrological, sedimentological and palaeontological criteria, and the geological setting, allow the recognition of distal, intermediate and proximal facies on a relatively deepwater carbonate ramp.

Layer parallel shortening in salt-detached folds: constraint on cross-section restoration

Abstract Cleavage-fissility perpendicular to bedding is a common feature in the external part of fold-and-thrust belts. Three techniques were used to determine the internal distortion in the frontal Southern Pyrenees: the analysis of strain markers such as burrows and rain drops, the measurement of fissility, and the measurement of anisotropy of magnetic susceptibility (AMS). The comparison of the three techniques showed a good fit although they differ in sensitivity to penetrative strain variations in the range of deformation values explored in the study case. On the regional scale, the values of layer parallel shortening (LPS) derived from the markers analysis are very constant and account for 16–23% of shortening. These values are two to three times larger than the shortening values calculated from the restoration of the macroscopic scale structures and indicate a good decoupling above the Cardona salt Formation. This study permitted an accurate restoration of the low-amplitude el Guix detached anticline.

Beachrocks from the island of La Palma (Canary Islands, Spain)

Beachrocks on La Palma Island developed on platform-forming lavas of the Cumbre Vieja volcano. Some of these lavas are related to the 1585 (Puerto Naos), 1677 and 1971 (Echentive) eruptions. Radiocarbon dating of the Charco Verde beachrock gives a conventional age of 33 330 8 490 BP, while that at Playa Chica beach gives a calibrated age of 14 940 8 525 BP. The beachrocks, up to 1.5 m thick and some tens of metres wide, consist of several decimetre-thick horizons dipping 2^15‡ seaward. Petrographically, they can be classified as rudstones and arenites, with volcanic clasts as their main component. The original porosity of the beachrocks was intergranular (and occasionally intragranular) and was partially occluded by cementation and locally by internal sediments. The main cements are fibrous aragonite and micrite high-magnesium calcite (HMC). Spar aragonite, peloidal HMC and microbotryoidal HMC are scarce. The elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters. The evolution of beach deposits, and especially the beachrocks in La Palma island, follows three stages: (1) beach deposition, (2) beachrock formation, and (3) beach retrogradation and/or erosion. The studied beachrocks prompt us to make some important considerations. (1) The mean tidal range in the Canary Islands has not varied over the last thousand years. (2) The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years. (3) Volcanic activity supplies the sediment that forms the beaches. (4) A dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day) favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches. (5) The presence of beachrocks in the La Palma beaches prevents the total disappearance of the beaches. @ 2003 Elsevier Science B.V. All rights reserved.

Fluid history related to the Alpine compression at the margin of the south-Pyrenean Foreland basin: the El Guix anticline

The El Guix anticline is the southernmost structure of the south-Pyrenean fold-and-thrust belt (NE Spain). Compressional activity in the area represents the latest stages of Alpine compressional tectonics and aVects the Upper Eocene‐Oligocene fluvio-lacustrine deposits overlying an evaporite sequence. The evolution pattern of the fractures containing calcite cement consists of three stages of microfractures which reflect the evolution of the structure and the relationships between fluids and thrust development. In each fracture, deformation started with a network of discontinuous microfractures which acted as traps for local meteoric fluids (stage 1). During the second stage, dilatant thrust faults serving as the conduit for both evolved meteoric ascending and local meteoric fluids were developed. Migration of fluids through the thrusts was multiepisodic. After the precipitation of cements in microfracture stage 2, the thrusts were practically occluded by calcite cements, acting as fluid barriers and dividing the structure into diVerent hydrological compartments. The last stage of microfracture (stage 3) is attributed to a younger phase, where the circulation of fluids concerned only meteoric fluids and the previously formed fractures acted as barriers to fluid circulation. Meteoric fluids, with high Fe/Mn and Fe/Mg ratios, without Na and Sr, enriched with 13C and with low 87Sr/86Sr with respect to their host rock were widely distributed in the structure throughout all its evolution, within a relatively open palaeohydrological system. Evolved meteoric fluids, with lower Fe/Mn and Fe/Mg ratios, with Na and Sr, depleted in 13C and with high 87Sr/86Sr with respect to their host rock were only present during thrust faults development (stage 2) within a relatively closed palaeohydrological system. The underlying evaporites acted as the lower boundary of the aquifer. Comparison with older thrust fronts of the same system reveals that the Pyrenean fold-and-thrust belt and its deformed southern foreland basin were compartmentalised hydrologically in time and space. During the early Eocene, when the thrust front aVected soft-sediment in the Ainsa basin, the thrust faults were dominated by a medium scale fluid flow. The fluids in the basin were basically formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters were also present. During the middle Eocene, coeval with the Gavarnie thrust emplacement, the thrust fault was dominated by a medium scale fluid flow. The fluid was basically a hypersaline Sr-rich brine stored within Triassic redbeds. No evidence of a significant input of either surface or metamorphic fluids during thrusting was found. During the same period, in the crystalline basement of the central Pyrenees the thrust faults were dominated by a large scale fluid flow mainly derived from the underlying silicate rocks

Sediment dewatering and pore fluid migration along thrust faults in a foreland basin inferred from isotopic and elemental geochemical analyses (Eocene southern Pyrenees, Spain)

Abstract The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migration of imbricated thrust-folds. Isotopic and elemental geochemistry of syn-kinematic veins (calcite and celestite) and their marly host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of their evolution and circulation during the early deformation of the basin-fill. The isotopic composition of sulfur and the 87 Sr 86 Sr ratios of calcites and celestites from the veins in the footwall syncline show that the original fluid had the isotopic composition of Eocene seawater. The different 87 Sr 86 Sr ratio in veins from the thrust-fault zones compared with the same ratio in the marly host-rock of the footwall syncline indicates that the thrust-fault zones acted as conduits for advective fluids. The relatively high 87 Sr 86 Sr ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from deeper sources or from the meteoric weathering of the emerged part of the belt. δ18O and δ13C values of calcites show that the isotopic composition of the calcite-cements in veins was controlled by the isotopic composition of the marly host sediment. Depletion of both δ18O and δ13C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a formation water composition. The distribution of δ18O and δ13C values of the marly host-rock and calcite cements in veins of the four outcrops probably resulted from differences in the meteoric water influences. The hydrogeological regime at the toe of the submarine thrust system was dominated by tectonically-induced dewatering of the foreland basin sediments. The thrust-fault zones were the channelizing paths for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.

Fluid migration during Eocene thrust emplacement in the south Pyrenean foreland basin (Spain): an integrated structural, mineralogical and geochemical approach

Abstract In the frontal part of the south Pyrenean Eocene thrust-fault system, syn-kinematic fluid flow during the early compressional deformation of the foreland basin marls is evidenced macroscopically by the abundance of calcite shear veins within the thrust-fault zones and folds. The geometry and distribution of the veins are indicative of the mechanisms and kinematics of fluid-deformation relationships, and give assessment of the fluid migration paths. The crack-seal mechanism of formation of the shear veins attests to the episodic nature of fault-slip and associated fluid flow in fractures. The distribution of the veins suggests that the main source of fluid was the dewatering of the overpressured, poorly permeable marls from the thrust footwalls, probably related to both (i) vertical compaction due to burial under thrust sheets and (ii) tectonic horizontal shortening. These fluids drained upwards towards the thrust-fault zones, in which they migrated laterally towards the thrust front due to the anisotropy of the fracture permeability in these zones. The geochemistry of the vein-filling minerals and their comparison with the geochemistry and mineralogy of the host marls are indicative of the fluid types, fluid origins, fluid-sediment interactions, and fluid migration paths. The δ34S and 87Sr/86Sr ratio of the host marl calcite and of the calcite and celestite in the veins away from the thrust-fault zones indicate that the original water trapped interstitially in the marls was Eocene seawater. The elemental composition (Ca, Sr, Mg, Mn, and Fe), δ18O, and δ13C of the same samples reveal a change of the pore-water composition from marine to formation water during the early burial stage. Fluid-inclusion analyses of the celestite in the veins reveal the presence of a hot, saline ascending fluid restricted to these discontinuities, where it was mixed with the local formation water. These two types of fluids drained towards the thrust-fault zones where they acquired a higher 87Sr/86Sr ratio, probably related to local fluid-sediment reactions. Indeed, dickite precipitated during cleavage formation in the most intensely strained part of the fault zones, and its formation was probably mainly controlled by stress. δ18O depletion in the calcite from the structurally highest/innermost thrust-fault zones suggests also the influence of meteoric water derived from the emerged part of the belt in these structures. The earlier fluid regime in the Ainsa basin was an intergranular (porous) flow regime (compactional flow) allowing for a pervasive isotopic, and elemental exchange of the marls prior to vein formation. With the onset of compressional deformation, channelized flow along tectonic slip surfaces became dominant.

Hydrogeochemistry and diagenesis of Miocene sandstones at Montjuı̈c, Barcelona (Spain)

Abstract Massive silicification of Miocene sandstones cropping out at the Montjuic mountain in Barcelona has been analyzed in order to constrain paleoflow systems, which may have contributed to the diagenetic reactions. The section consists of five units with alternating sandstone–marl units. The principal diagenetic features are observed in strongly silicified sandstone units. Alunite mineral precipitation indicates presence of saline fluids and low pH. Salinity is most probably derived from marine seawater and low pH may be due to oxidation of pyrite. A quantitative thermodynamic modeling is applied to characterize the percolating fluids and to constrain the hydrodynamic system.

Proposition d'un modele de silicification superficielle des gres neogenes de Montjuic, Barcelone (Espagne); parageneses minerales, environments geochimiques et circulation des fluides

The Montjuic hill is part of the Neogene horst and graben system of the Catalan Coastal Ranges at the northwestern edge of the Valencia Trough. It is located to the SE of Barcelona City and consists of a 200 m thick strongly silicified detrital succession (mainly conglomerate and sandstone units alternating with lutitic units) of Miocene age. The geological constraints of this area (young age, shallow depositional environment and no evidence of burial processes) ensure that authigenic minerals formed during silicification have not been modified by further diagenetic processes and allow to constrain the age and nature of the silicification. Silicification has strongly increased the hardness of the original sediment. Textural effects of mechanical compaction are rare, testifying that burial processes had no effect on diagenesis and pointing towards an early and/or shallow cementation. Two main diagenetic facies with characteristic associations of authigenic minerals can be identified, namely: (1) non silicified facies are present in ochre-coloured, fine-grained sandstones with high clay and carbonate content. In these facies, cementation is scarce and generally forms minor feldspar overgrowths around detrital K-feldspar as well as layers or nodules of calcite spar cement mainly filling interparticle porosity; (2) silicified facies are red, purple-coloured and characterized by the presence of opal, microquartz and quartz overgrowths as well as other minor authigenic minerals such as Ti and Fe oxides and alunite. Particularly, alunite and opal appear often at the boundary of the silicified/non silicified facies, coming with the development of bleached facies and are replaced by silica. In this paper, a detailed sampling of the silicification fronts has been made, in order to establish the main silicification pathways. In the sampled zone the non silicified sandstones are mainly made up of quartz, K-feldspar, muscovite, phyllite fragments and bioclasts and cemented by thin K-feldspar overgrowths and decimetric concretions of intergranular calcite spar with spherical and tabular shapes. Sandstones contain some pyrite pseudomorphs and 20 to 30% of clay minerals, essentially illite-mica. Samples collected perpendicular to the silicification fronts reveal significant textural, compositional and petrographical transformations, namely: (1) The color of the sample varies strongly from ochre in the non silicified facies to white and red in the bleached weakly silicified front and finally to red, purple and grey in the massively silicified facies; (2) The siliciclastic framework of Montjuic sandstones remains stable during the silicification, only detrital feldspars are partially altered into illite, and biotites are completely altered. The detrital carbonate components disappear quickly towards silicified facies; (3) Within the silicification front, either bleached or not, authigenic minerals show quite important variability. Calcite disappears progressively. The first silicification stage is built by incipient quartz overgrowths, then microquartz develops towards the massively silicified facies. Alunite and opal are usually present in samples collected in this silicification boundary; (4) In the massively silicified facies quartz overgrowths and microquartz take up almost all the intergranular volume of sandstones. Clay content is strongly reduced to 5-10% (mainly illite), so the primary clay-carbonate matrix has been replaced and/or transformed to microquartz. Iron oxides appear around feldspar and phyllite fragments. Because of the geological constraints Montjuic sandstones silicification was a surface/sub-surface phenomenon. Therefore, silicification occurred at relatively low temperature and pressure conditions. Partly, silica may have an internal origin (supplied by clay and feldspar hydrolysis). Supposing that diagenetic transformations inside sandstones are made at steady state conditions it is necessary to consider a strong external supply of silica. The presence of alunite points to acidic fluids with pH between 1,5 and 4. In these conditions, quartz solubility is unaffected, but the aluminium becomes mobile, thus aluminosilicate minerals (like feldspars) are hydrolyzed and clay minerals are transformed into opal CT. A feasible process which may have contributed to the acidification is the oxidation of the pyrite and organic matter present in the original sediments, testified by numerous pyrite ghosts in the non silicified and silicified sandstones. Silicification occurred in an oxidizing environment where sulfides were oxidized and iron oxides precipitated, explaining the colour of these materials. At the basin scale, different models can be considered: (a) a topographic driven flow that moved groundwater from the horst towards the basin; (b) a thermoconvective driven flow that moved phreatic and formation waters along the main faults of the graben or (c) a compaction driven flow that also moved formation waters. Only shallow systems driven by topographic flows can explain the oxidizing nature of the silicification solutions of Montjuic. Conclusions. The Montjuic sandstone silicification is remarkable in several aspects. (1) The lack of compaction and the oxidizing nature of the silicification indicate that this diagenesis was induced by subsurface groundwater, in shallow environments. (2) Silicification is pervasive in medium and coarse-grained sandstones and conglomerates. On the contrary, silicification is restricted to fracture zones in fine-grained sandstones.

Fifth-order cyclicity and organic matter contents relationship (Lower Eocene, Pyrenees)

The Upper Limestone Member of the Corones Formation of the Spanish Pyrenees consists of various units (Lower and the Upper Foraminifera Units, Shale Unit, Cherty-ostracode Unit, Ostracode Unit and Chara-ostracode Unit) and offers strong facies and lateral thickness (20 to 80 m) variations. Detailed facies analyses, fifth-order cycles and organic geochemical determinations in the central domain of the Corones platform carbonates (Cherty-ostracode Unit), lower Eocene in age, were carried out to establish a case of close relationship between variations in organic matter productivity and cyclicity with annual period. The Cherty-ostracode Unit displays a continuous and pervasive fifth-order cyclicity, represented by 5 cycles. Each cycle consists of a lower part (mollusc facies) and an upper part (laminated ostracode facies). The calculated fifth-order cycle period ranges from about 17,000 to 28,000 years, which falls within the Milankovitch Band. Variations in organic matter content related to these carbonate cycles have been established. The lower mollusc facies members show a low organic carbon content and Hydrogen Index (HI) below 0.6% in weight and 261, respectively. By contrast, the upper laminated ostracode facies members show high organic carbon contents (up to 2% in weight) and high HI (between 164 and 373), and are also characterized by important silicification processes (the content in chert is up to 30%). The organic geochemistry resulting from these organic rich levels reflects a contribution of algal marine input.

Las mineralizaciones de Pb-Zn-Ba en el Muschelkalk inferior de los Catalánides

The Catalan Triassic is divided into the three characteristie facies of the Germanic Triassic: Buntsandstein, Muschelkalk (Loxver, Middle and Upper Muschelkalk) and Keuper. The base fractures condition the Mesozoic sedimentation, whereby the Catalanid are divided into sedimentary domains, which are separated by transversal faults to the mountain range (ANADON et aL, 1979) (fig. 1). The carbonatic facies from the Catalanid Lower Muschelkalk, Anisien Age, corresponds itself with a ramp depositional environment, in which the paleokarstic levels are frequent, the importance of which development, number of Ihe afected paleokarstic surfaces and materials vanes in accordance with the location of the different domains (MARZO et al., 1983). In this context you can find numerous traces of Pb-Zn-Ba stratabound mineralizations (SAGRISTA et al., 1980, and MATA, 1982) associated with paleokarstic surfaces or algal-mats leveIs (fig. 2). me associated karst mineralizations are singenetic with the karstic filling, Anisiense age. Ihe algal-mats, rich in organic matter, present a high level of Pb-Zn-Ba, Uds elements themselves accumulated into the algal facies while this build up. As fon the origin of this deposits a model named supergen singenetic by AMSTUTZ (1962) is proposed, in `which Ihe minerals themselves deposited at selisame time which the sediments and the mineralizants elements are supergen source. The main contribution of the elements comes from the continent lixiviation (strong anomalies in Pb, Zn, Ba, Cu in the Paleozoic Age) (CASAS, 1979 and FONT, 1983).