South Pyrenean Foreland Research Articles

Decoupled flexure in the South Pyrenean Foreland

Complex subsidence patterns during the mid‐late Eocene of the easternmost South Pyrenean foreland are well explained by a decoupled flexure model in which upper crustal buckling is accommodated by ductile flow in the mid‐lower crust while, simultaneously and independently, the lower lithosphere flexes above a ductile asthenosphere. The resulting predicted subsidence values agree with observation to well within the estimated uncertainties. The resulting lithospheric effective elastic thickness (EET) estimate (10±3 km) is low but similar to values obtained in the South Pyrenean foreland by other authors. The separate EET obtained for the upper crust (1.5±0.8 km) also agrees with tentative values given by other workers.

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 affects 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 different 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 affected 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 as a result of metamorphic devolatilisation reactions. During the Oligocene, the most external part of the fold-and-thrust belt developed on top of a salt detachment horizon. The thrust front affected cemented continental materials of late Eocene–Oligocene age. At this moment, the thrusts were conduits for meteoric fluids arriving from the surface and also for evolved meteoric fluids migrating a short distance upwards after being in contact with the underlying evaporitic beds.

The South Pyrenean Eocene carbonate megabreccias revisited: new interpretation based on evidence from the Pamplona Basin

The South Pyrenean Foreland Basin contains numerous units of Eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland trough, sheds new light on the origin and regional significance of these South Pyrenean Eocene carbonate megabreccias (SPECMs). The number of the SPECM units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed. The SPECM units appear to occur as time–stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megabreccias were derived from a carbonate-platform system hosted by the foreland basin's southern (passive) margin. The episodic instability and mass wasting were triggered by phases of structural steepening (forebulge uplift) accompanied by high-magnitude earthquakes, with the former causing platform emergence, increased load stresses and excess pore-water pressure in the carbonate ramp. The SPECM deposits were emplaced by cohesive debris flows evolving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differentiated, bipartite debrite and turbidite in the medial part; and (3) an incomplete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the SPECM units, and the original terms `megaturbidite' and `seismoturbidite' thus seem to be inappropriate for these deposits.

Coral reef diagenesis records pore-fluid evolution and paleohydrology of a siliciclastic basin margin succession (Eocene South Pyrenean foreland basin, northeastern Spain)

An integrated field, petrographic, and geochemical study has determined the fluid-rock interaction history in part of an Eocene reef complex on the margins of the siliciclastic-dominated Catalan sector of the South Pyrenean foreland basin. Results show that at least four distinct fluid systems influenced the basin margin, and demonstrate the sensitivity of reef rocks as paleohydrological archives in siliciclastic environments. The earliest calcite cements precipitated from meteoric waters at shallow burial depths, and mineralogical stabilization of reef carbonate was mostly completed during this episode. Textural and isotope trends typical of paleo-exposure surfaces are lacking, and trace element results suggest that meteoric waters were transmitted laterally and/or upward into the reef via delta-lobe sandstone paleoaquifers. The distribution of late cements is highly variable and fracture-fed, demonstrating the importance of deformation in controlling fluid flow subsequent to reef lithification. Isotopic and elemental compositions of burial calcites, plus their paragenetic association with barite and dolomite, show that fluids of marine parentage were expelled from compacting basinal marls into the basin margin paleoaquifers. The Calders reef also records an episode of extrabasinal fluid circulation, probably associated with faulting of the basin margin. Ferroan saddle dolomite contains fluid inclusions with elevated homogenization temperatures, indicating thermal disequilibrium with the surrounding strata. Hydrothermal activity was possibly related to Neogene extensional tectonics. The final cement stage in the reef records the influence of telogenetic meteoric water, possibly reflecting the present-day basin margin hydrology.

Abstract: Anatomy and Evolution of the Tectonically-Control led Ainsa Slope System. Eocene, South-Pyrenean Foreland Basin, NE Spain 

Abstract: Anatomy and Evolution of the Tectonically-Control led Ainsa Slope System. Eocene, South-Pyrenean Foreland Basin, NE Spain 

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 δ 34 S and 87 Sr/ 86 Sr 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), δ 18 O, and δ 13 C 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 87 Sr/ 86 Sr 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. δ 18 O 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.

The origin of barren bodies in the Subiza potash deposit, Navarra, Spain; implications for sylvite formation

ABSTRACT Barren are meter-size lens-like features in sylvite beds where sylvite is absent or depleted. They are commonly found when mining potash deposits. Most barren bodies are believed to be formed by selective dissolution of sylvite, due to the circulation of diagenetic or later fluids. Contrary to this commonly favored postdepositional interpretation, barren bodies of the Subiza mine, Navarra, Spain are thought to be the result of synsedimentary processes. This potash deposit contains a 100 m thick Upper Eocene succession of alternating claystone and evaporites (sulfate, halite, and sylvite). The evaporites accumulated in an elongated basin forming one of the depocenters of the 250 km long South Pyrenean foreland basin. Along the margin of the basin, slope instability, promoted probably by tectonism, created mass wasting, forming mounds 0.5-2 m high and tens of meters in extension. As evaporation progressed, two stratified brines formed. Halite precipitated at the air-brine interface and sank to the bottom of the basin with terrigenous clays. Sylvite, however, precipitated from the lower brine. The mounds extended into the upper brine, thus sylvite did not precipitate over these upper zones. With progressive deposition the lower brine covered the mounds, the sylvite beds overlapping the mounds. The mineralogical and petrographical features of the barren bodies and their surroundings are continuous. Analyses of primary fluid inclusions from the halites of the barren bodies show a Mg-K-Cl rich composition. This corresponds to a primary brine and is unlike the Na-Cl rich brines expected from replacement processes. The thickness of the sylvite-forming brine (< 1 m) inferred from the mounds is inconsistent with the thickness required to form centimeter-thick beds of sylvite in a closed basin. Alternatively, the existence of a more restricted subbasin, open to a halite-forming basin similar to that observed in saline ponds, is proposed. Numerical simulations of the evaporation processes under these conditions give results that match the mineral associations observed and the solute content of the fluid inclusions.

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

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 87Sr 86Sr 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 87Sr 86Sr 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 87Sr 86Sr 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. δ 18O and δ 13C 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 δ 18O and δ 13C 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 δ 18O and δ 13C 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.

Growth of the South Pyrenean orogenic wedge

A six‐step reconstruction of the South Pyrenean foreland fold‐and‐thrust belt in Spain delineates the topographic slope, basal décollement angle, internal deformation, and thrust‐front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene. Style of thrust‐front advance, dip of the basal décollement, slope of the upper surface, and internal deformation are decoupled and not simply related. Internal deformation increased, decreased, and maintained surface slope angle at different stages. From the onset to the cessation of deformation, the basal décollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time. Taper angle of the Pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate, the relative rate of creation of structural relief in the rear versus the front of the wedge, the extent of deposition of eroded material within the deforming wedge, and the taper of the pretectonic stratigraphic wedge.

Proximal, Supply-Controlled Sequences Versus Distal, Base Level-Controlled Sequences in Alluvial-Fan and Fan-Delta Complexes (Eocene, South Pyrenean Foreland Basin, NE Spain): ABSTRACT

Proximal, Supply-Controlled Sequences Versus Distal, Base Level-Controlled Sequences in Alluvial-Fan and Fan-Delta Complexes (Eocene, South Pyrenean Foreland Basin, NE Spain): ABSTRACT

Regional Diagenesis of the Upper Eocene Pre-Evaporite Reef Complex, South Pyrenean Foreland Basin, Northeast Spain

Regional Diagenesis of the Upper Eocene Pre-Evaporite Reef Complex, South Pyrenean Foreland Basin, Northeast Spain

A carbonate complex in an active foreland basin: the Paleogene of the Sierra de Port del Comte and the Sierra del Cadi (Southern Pyrenees)

The Sierra de Port del Comte and the Sierra del Cadi consist of Paleogene (Paleocene to middle Eocene) neritic carbonates which record the evolution of the Eastern South Pyrenean foreland basin. Based on facies interpretation and on biostratigraphic data (Al-veolina, Nummulites, Assilina and planktonic foraminiferal zones) 7 depositional sequences can be differentiated. The Navarri Sequence (Thanetian), the Ager Sequence (late Thanetian-middle Ilerdian), the Llimiana Sequence (middle Ilerdian), the Odén Sequence (late Ilerdian-middle Cuisian), the Cadi Sequence (middle Cuisian) and the Bagá-Josa Sequence (late Cuisian-early Lutetian). These sequences reflect global changes of the sea-level. The structural evolution of the analyzed time interval is characterized by low tectonic activity during the Paleocene, increased rates of subsidence during the early Eocene and the incorporation into overthrusting of the South Pyrenean nappes during the middle Eocene.