South Pyrenean Foreland Basin Research Articles

Unraveling the sediment routing systems evolution of the south Pyrenean foreland basin during the lower to middle Paleogene period

Orogenesis plays a crucial role in creating, modifying, and disrupting sediment routing fairways. Reconstructing past source to sink systems is a critical step to unravel the geological history, mandatory to disentangle tectonic and climatic forcings, and to understand the perturbations that affected the overall system. The Paleocene-Eocene period is a debated early stage of the early Pyrenean orogeny, characterized by the piggy-back style tectonic partitioning of sub-basins as the deformation front propagates towards the southern foreland. An important issue is to understand how sedimentary systems reacted and reorganized in response to this dynamic scenario. and whether the stratigraphic and sediment provenance results align with the envisioned scenario.In this paper we aim at contributing to the paleogeographic, sedimentologic and tectono-sedimentary evolution of the South-Pyrenean foreland basin by reviewing the chronostratigraphic framework of the basin infill in its south-central sector (the Ager sub-basin in the Serres Marginals thrust sheet). We built five new magnetostratigraphic sections, which together encompass most of the Paleogene record, aimed to best reconcile magnetostratigraphic data with the defined biostratigraphic framework of the region including marine Shallow Benthic Zones (SBZ biozones) and continental mammalian localities (MP levels). Detailed trends in subsidence show the development and evolution of the foreland depozones, from forebulge to foredeep and wedge-top setting, relative to the successive emplacement of the Montsec and Serres Marginals thrust sheets.A correlation with the eastern portion of the foreland basin (Lower-Middle Pedraforca and Cadí Thrust Sheets) and adjacent northern Graus-Tremp basin (Montsec Thrust Sheet) was feasible and seeks to contribute to the tectono-sedimentary evolution of the South-Pyrenean foreland in the light of a source to sink approach. Our proposal includes a new paleogeographic evolution of the area in a series of paleogeographic maps from Late Thanetian to Late Cuisian times.

Water discharge variations control fluvial stratigraphic architecture in the Middle Eocene Escanilla formation, Spain

Ancient fluvial deposits typically display repetitive changes in their depositional architecture such as alternating intervals of coarse-grained highly amalgamated (HA), laterally-stacked, channel bodies, and finer-grained less amalgamated (LA), vertically-stacked, channels encased in floodplain deposits. Such patterns are usually ascribed to slower, respectively higher, rates of base level rise (accommodation). However, “upstream” factors such as water discharge and sediment flux also play a potential role in determining stratigraphic architecture, yet this possibility has never been tested despite the recent advances in the field of palaeohydraulic reconstructions from fluvial accumulations. Here, we chronicle riverbed gradient evolution within three Middle Eocene (~ 40 Ma) fluvial HA-LA sequences in the Escanilla Formation in the south-Pyrenean foreland basin. This work documents, for the first time in a fossil fluvial system, how the ancient riverbed systematically evolved from lower slopes in coarser-grained HA intervals, and higher slopes in finer-grained LA intervals, suggesting that bed slope changes were determined primarily by climate-controlled water discharge variations rather than base level changes as often hypothesized. This highlights the important connection between climate and landscape evolution and has fundamental implications for our ability to reconstruct ancient hydroclimates from the interpretation of fluvial sedimentary sequences.

Fluvio-deltaic record of increased sediment transport during the Middle Eocene Climatic Optimum (MECO), Southern Pyrenees, Spain

Abstract. The early Cenozoic marine sedimentary record is punctuated by several brief episodes (<200 kyr) of abrupt global warming, called hyperthermals, that have disturbed ocean life and water physicochemistry. Moreover, recent studies of fluvial–deltaic systems, for instance at the Palaeocene–Eocene Thermal Maximum, revealed that these hyperthermals also impacted the hydrologic cycle, triggering an increase in erosion and sediment transport at the Earth's surface. Contrary to the early Cenozoic hyperthermals, the Middle Eocene Climatic Optimum (MECO), lasting from 40.5 to 40.0 Ma, constitutes an event of gradual warming that left a highly variable carbon isotope signature and for which little data exist about its impact on Earth surface systems. In the South Pyrenean foreland basin (SPFB), an episode of prominent deltaic progradation (Belsué–Atarés and Escanilla formations) in the middle Bartonian has been usually associated with increased Pyrenean tectonic activity, but recent magnetostratigraphic data suggest a possible coincidence between the progradation and the MECO warming period. To test this hypothesis, we measured the stable-isotope composition of carbonates (δ13Ccarb and δ18Ocarb) and organic matter (δ13Corg) of 257 samples in two sections of SPFB fluvial–deltaic successions covering the different phases of the MECO and already dated with magnetostratigraphy. We find a negative shift in δ18Ocarb and an unclear signal in δ13Ccarb around the transition from magnetic chron C18r to chron C17r (middle Bartonian). These results allow, by correlation with reference sections in the Atlantic and Tethys, the MECO to be identified and its coincident relationship with the Belsué–Atarès fluvial–deltaic progradation to be documented. Despite its long duration and a more gradual temperature rise, the MECO in the South Pyrenean foreland basin may have led, like lower Cenozoic hyperthermals, to an increase in erosion and sediment transport that is manifested in the sedimentary record. The new data support the hypothesis of a more important hydrological response to the MECO than previously thought in mid-latitude environments, including those around the Tethys.

Interplay of Multiple Sediment Routing Systems Revealed by Combined Sandstone Petrography and Heavy Mineral Analysis (HMA) in the South Pyrenean Foreland Basin

Combined sandstone petrography and heavy mineral analysis allow to decipher different sediment routing systems that could not be resolved by one method alone in the South Pyrenean foreland basin. We apply this approach to deltaic and alluvial deposits of the southern part of the Jaca basin, and in the time equivalent systems of the nearby Ainsa and Ebro basins, in order to unravel the evolution of source areas and the fluvial drainage from the Eocene to the Miocene. Our study allows the identification of four petrofacies and five heavy-mineral suites, which evidence the interplay of distinct routing systems, controlled by the emergence of tectonic structures. Two distinct axially-fed systems from the east coexisted in the fluvial Campodarbe Formation of the southern Jaca basin that were progressively replaced from east to west by transverse-fed systems sourced from northern source areas. In the late stages of evolution, the Ebro autochthonous basin and the Jaca piggy-back basin received detritus from source areas directly north of the basin from the Axial Zone and from the Basque Pyrenees. Coupling sandstone petrography with heavy mineral provenance analysis allows challenging the existing model of the South Pyrenean sediment dispersal, highlighting the relevance of this approach in source-to-sink studies.

Thermal record of the building of an orogen in the retro‐foreland basin: Insight from basement and detrital thermochronology in the eastern Pyrenees and the north Pyrenean basin (France)

AbstractAn understanding of the evolution of foreland basins improves our knowledge of how mountain belts have grown and helps us to decipher events which may not be preserved in the orogen. The infilling of the north Pyrenean retro‐foreland basin (Aquitaine Basin, France) during the main exhumation of the Pyrenees and its corresponding thermal history have not been fully investigated. We applied apatite fission track (AFT) and (U‐Th‐Sm)/He (AHe) methods coupled with inverse thermal modelling on both the detrital Eocene (47 to 33 Ma) syn‐orogenic Palassou conglomerates of the eastern part of the Aquitaine Basin and basement samples from the North Pyrenean Zone and the Axial Zone of the Pyrenees. Apatite crystals were separated from granitic cobbles found in the conglomerates. AFT ages for detrital samples range from 27 ± 2 to 43 ± 4 Ma, and AHe ages from 13 ± 1 to 76 ± 5 Ma. For in situ massifs AFT ages range from 35 ± 2 to 90 ± 17 Ma and AHe ages from 39 ± 2 and 80 ± 5 Ma. AFT ages for detrital samples are close to deposition ages, whereas AHe ages are older and younger than deposition ages and show a partial thermal resetting due to burial. A detailed analysis of the ages obtained and thermal histories derived from modelling shows that ages reflect (a) exhumation from 70 to 55 Ma revealed by a long stay in the partial retention zone (PRZ), (b) a Palaeocene–Eocene cooling in the Pyrenees, (c) a post‐depositional episode of moderate heating of the sediments in the basin represented by partially reset young AHe and AFT ages compared to deposition ages and (d) an early to mid‐Miocene final exhumation of the basin deposits as evidenced by young AHe ages and geological constrains. These results reflect a common event with the south Pyrenean foreland basin that is characterized by high piedmont aggradation from the late Eocene to the Miocene. The aggradation of sediments is possibly connected with well‐known high elevation low relief surfaces in the core of the Pyrenees and followed by a Miocene exhumation event that is already observed on the southern flank. However, the timing of the aggradation and exhumation events could be different between the north and the south. Erosion occurred most probably during the early to mid‐Miocene in the north and during the late Miocene–early Pliocene in the south.

Forward seismic modeling as a guide improving detailed seismic interpretation of deltaic systems: Example of the Eocene Sobrarbe delta outcrop (South-Pyrenean foreland basin, Spain), as a reference to the analogous subsurface Albian-Cenomanian Torok-Nanushuk Delta of the Colville Basin (NPRA, USA)

Abstract A forward seismic model of the Middle Eocene Sobrarbe delta outcrop (South-Pyrenees, Spain) has been made to investigate its seismic signature. The Sobrarbe delta complex corresponds to a syntectonic sedimentary wedge locally thicker than 1000 m. This 15 km-long, highly-progradational and moderately aggradational sedimentary system presents sigmoid and oblique clinoforms, characterized by slopes ranging from 0.2° to 5.7°. Along the clinoforms, the classical deltaic depositional environments grade progressively from one to the other: alluvial plain, delta plain, delta front and prodelta. Locally, specific carbonate deposits occur, corresponding to transgressive pulses. The transition from delta front to prodelta is locally marked by large slide scars affecting clinoform morphology. A detailed outcrop transect of the Sobrarbe delta was converted to an acoustic impedance model by assigning petrophysical properties to each geological facies. This model was then used to create synthetic seismic lines characterized by various input wavelets. The resulting various peak frequency synthetic seismic lines have been interpreted, and the seismic interpretation compared to the outcrop description to calibrate the seismic interpretation and assess the “limits” of that interpretation. This work shows that the “seismic stratigraphy” methodology is consistent and allows the correct interpretation of most seismic sequences. However, composite reflections from rapid lateral facies transitions and higher-frequency sequence boundaries can clearly lead to seismic interpretation mistakes. The seismic facies of seven facies associations have been described, providing guidelines to recognize them on actual seismic lines. The synthetic seismic image of four selected systems tracts comprising good reservoir bodies has been analyzed to highlight recognition criteria which can be used to interpret analog deltaic systems in the subsurface, and therefore enhance the prediction and characterization of the reservoir bodies that they hold. These guidelines were systematically applied to a direct subsurface analog: the Torok-Nanushuk Delta (Colville Basin, NPRA, USA).

Syn‐orogenic fluid flow in the Jaca basin (south Pyrenean fold and thrust belt) from fracture and vein analyses

AbstractThis study aims at understanding the origin and nature of syn‐orogenic fluid flow in the Jaca basin from the South Pyrenean fold and thrust‐belt, as recorded in calcite and quartz veins of the Sierras Interiores (Spain) and the turbiditic basin, which cover upper Cretaceous to Late Eocene syntectonic deposits. The fracture network consists of a classical pattern of transverse and longitudinal fractures related to Layer Parallel Shortening (LPS) and folding respectively. Veins filled equally about the third of fractures in the carbonate shelf and turbidites. Carbon and oxygen isotopes of calcite veins mostly indicate precipitation from isotopically buffered water, consistent with high water‐rock interaction. In the Sierras Interiores, petrographical observations and fluid inclusion microthermometry are consistent with two distinct stages of precipitation. The first stage is characterized by relatively low Th and low salinities (155–205 °C and 0.5–3.2 wt% eq. NaCl). The second stage, which was characterized both by the formation of mode‐I joints and by mode‐I reactivation of pre‐existing veins, shows higher Th and salinities (215–270 °C and 2.2–5.7 wt% eq. NaCl). Waters recorded in the second stage are interpreted to have interacted with underlying Triassic evaporites and flowed along major thrusts before vein precipitation, which are locally in thermal disequilibrium with host‐rocks. We suggest the transition from a rather closed hydrological system during the first stage of vein formation, interpreted to have occurred during Eaux‐chaudes thrusting (upper Lutetian‐Bartonian), to a more open hydrological system during the second stage, which likely occurred during Gavarnie thrusting (Priabonian‐early Rupelian). Finally, we also document the migration in space and time of hydrothermal pulses along the South Pyrenean Foreland Basin, related to the westward propagation of major thrusts during the Pyrenean orogeny.

Lithium saltworks of the Salar de Atacama: A model for MgSO4-free ancient potash deposits

Potash basins similar to those in the geological past no longer exist at present. There are only hypersaline lakes such as those in the Tibetan Plateau where carnallite [KCl·MgCl2·6H2O] precipitates. Given that the purpose of common saltworks is to obtain NaCl, the halitic stage is not exceeded. By contrast, saltworks extracting lithium by solar evaporation reach more advanced stages of brine evolution and saline precipitation. In the Salar de Atacama saltworks, with a relative humidity commonly below 30% (monthly average), salts precipitate following a sequence of halite [NaCl], sylvite [KCl], carnallite and bischofite [MgCl2·6H2O] and display a useful model of evolution similar to that which gave rise to MgSO4-free potash deposits in the past. Prismatic-acicular gypsum [CaSO4·2H2O], the main accessory mineral, is transformed into a less hydrated calcium sulfate, probably anhydrite [CaSO4], during carnallite precipitation. Mineral crystal size and annual sedimentary thickness of each paragenesis are compared with those in ancient potash deposits. The behavior of sylvite and carnallite as cements is observed at the onset of their precipitation in ponds where both minerals coexist with previous halite. Millimeter cyclicity due to day-night thermal variations is observed in these saltworks, whereas cm- and mm-cyclicities have been interpreted in ancient sites as annual (regular) and intra-annual (irregular), respectively. Conditions similar to those existing at the Salar de Atacama were necessary to preserve carnallite, which is commonly the most evolved mineral in the ancient deposits. The development of potash deposits in the South Pyrenean Foreland Basin and the Rhine Graben, near the Eocene-Oligocene boundary, is attributed to global aridity and cooling and to the basin restriction associated with a decrease in the ocean level caused by the spread of the Antarctic ice cap.

The Bartonian-Priabonian marine record of the eastern South Pyrenean foreland basin (NE Spain): a new calibration of the larger foraminifers and calcareous nannofossil biozonation

This study presents a combined biostratigraphic (calcareous nannofossils, larger foraminifers) and magnetostratigraphic study of the Middle and Late Eocene marine units of the Igualada area, eastern Ebro Basin. The studied sections of Santa Maria de Miralles and La Tossa encompass the complete marine succession of the Santa Maria Group, where assemblages rich in larger foraminifers have been studied since the early 1950's. A total of 224 paleomagnetic sites and 62 biostratigraphic samples were collected along a 1350m-thick section that ranges from chron C20n to chron C16n (~43Ma to ~36Ma). The resulting magnetostratigraphy-based chronology challenges existing chronostratigraphic interpretations of these units and results in a new calibration of the biostratigraphic zonations. The base of calcareous nannofossil Zone NP19-20 is pinned down to an older age than its presently accepted attribution, whereas the time span assigned to Zone NP18 is significantly reduced. A revised calibration of larger foraminifers indicates that Zone SBZ18, formerly assigned exclusively to the late Bartonian, extends its range to the earlymost Priabonian, the Bartonian stage being almost entirely represented by Zone SBZ17. A division of Zone SBZ18 into two subzones is proposed.

Large scale facies change in the middle Eocene South-Pyrenean foreland basin: The role of tectonics and prelude to Cenozoic ice-ages

The present study reports a sedimentological analysis of the Guara Limestone Formation deposited during the Lutetian in the Sierras Exteriores, in the South-Pyrenean foreland basin. We provide a detailed facies analysis of the carbonates to precise the paleoenvironmental context during their deposition. We show that those limestones are mainly composed of shallow-water foraminifers and were deposited in relative shallow-water environments (<120m) during the whole Lutetian (SBZ 13 to SBZ 16). The Guara Limestone Formation represents the last occurrence of carbonate platform in the South-Pyrenean foreland basin and disappeared definitely at the Lutetian to Bartonian transition. The demise of carbonate producers at the end of the Lutetian could be related to an increase of continental erosion, due to tectonic and/or climatic forcing. We illustrate that in the Jaca basin, this event correlates with a marked increase in subsidence rate. However, this deformation event is local and the carbonate systems in the Pyrenean foreland resisted to many deformation events during the whole basin history before. Paleobathymetric reconstructions in the Jaca basin, where shallow marine sections outcrop, suggest an increase of the amplitude of high-frequency sea-level cycles. This increase is contemporaneous with several climatic evidences, which suggest the appearance of early ice-sheets near the Lutetian–Bartonian boundary. The demise of carbonate producers seems, therefore, to be the result of a major environmental shift in the basin accompanying increased subsidence rates, switching from low nutrient oligotrophic conditions – favourable for shallow water benthic foraminifers – to eutrophic conditions due to the increase of erosion and terrigenous nutrient input associated with higher-frequency sea-level changes and river destabilization.

Nummulitic banks in the upper Lutetian ‘Buil level’, Ainsa Basin, South Central Pyrenean Zone: the impact of internal waves

AbstractNummulites,a particularly abundant and diverse genus of larger benthonic foraminifera, formed huge accumulations (banks) during the Eocene, which are often good hydrocarbon reservoirs, especially in North Africa. Despite their economical interest, these accumulations are not well‐understood and their origin is still under discussion. Reasons for this debate are the absence of livingNummulitesaccumulations and the high‐variability of facies, including the size, shape and extension of the banks, which reflect the array of processes controlling sediment production and accumulation. The nummulitic banks near Santa María de Buil, in the Ainsa Basin (South Pyrenean Foreland Basin) are composed of recurrent facies associations within mappable bed units bounded by physical surfaces. The depositional processes that produced theNummulites deshayesiaccumulations are interpreted considering the shape of the banks, the type of bounding surfaces, the distribution of sedimentary textures,Nummulitestest shapes and the associated skeletal components within the banks. This integrative analysis indicates that nummulitic banks accumulated from mass flows, with very poor sediment sorting. Textural and compositional differences among banks suggest that globoseNummulitesthrived in the shallower part of the mesophotic zone with abundant nummulithoclasts, whereas flat nummulitic forms thrived in deeper mesophotic, clay‐dominated settings. Interbedded with nummulitic banks, coral biostromes and coral mounds bearingOperculina,DiscocyclinaandAsterocyclina, representin situaccumulation near the base of the photic zone. Internal waves (waves that propagate along the pycnocline) are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. In contrast to surface waves (both fair‐weather and storm), whose impact is strongest near the sea surface and decreases with bathymetry, the impact of internal waves is usually strongest in the mid‐shelf region and weaker in shallow water. This explains the compositional character of the nummulitic banks.

Closing and continentalization of the South Pyrenean foreland basin (NE Spain): magnetochronological constraints

ABSTRACTThis paper presents new magnetostratigraphic results from a 1100‐m‐thick composite section across the marine to continental sediments of the central part of the SE margin of the Ebro basin (NE Spain). Integration with existing marine and continental biochronological data allows a robust correlation with the geomagnetic polarity time scale. The resulting absolute chronology ranges from 36.3 to 31.1 Ma (Priabonian to Rupelian), and yields an interpolated age of ∼36.0 Ma (within chron C16n.2n) for the youngest marine sediments of the eastern Ebro basin. This age is in concordance with a reinterpretation of earlier magnetostratigraphic data from the western South Pyrenean foreland basin, and indicates that continentalization of the basin occurred as a rapid and isochronous event. The basin continentalization, determined by the seaway closure that resulted from the uplift of the western Pyrenees, was probably coincident with a mid‐amplitude eustatic sea level low with a maximum at 36.2 Ma. The base level drop that followed the basin closure and desiccation does not appear associated to a significant sedimentary hiatus along the margins, suggesting a late Eocene shallow marine basin that rapidly refilled and raised its base level after the seaway closing. Rapid basin filling following continentalization predates the phase of rapid exhumation of the Central Pyrenean Axial Zone from 35.0 to 32.0 Ma, determined from the thermochronology data. It is possible then that sediment aggradation at the front of the fold‐and‐thrust belt could have contributed to a decrease in the taper angle, triggering growth of the inner orogenic wedge through break‐back thrusting and underplating. Contrasting sedimentation trends between the western and eastern sectors of the South Pyrenean foreland indicate that basin closing preferentially affected those areas subjected to sediment bypass towards the ocean domain. As a result, sediment ponding after basin closure is responsible for a two‐fold increase of sedimentation rates in the western sector, while changes of sedimentation rates are undetected in the more restricted scenario of the eastern Ebro basin.

A model for the diagenetic formation of sandstone-hosted copper deposits in Tertiary sedimentary rocks, Aragón (NE Spain): S/C ratios and sulphur isotope systematics

Numerous small, noneconomic copper deposits are associated with syntectonic sandstones of Tertiary age (Campodarbe and Uncastillo Formations) in the South Pyrenean foreland basin (SPFB). Sulphides, native metal, and their oxidation products occur within elongate lenses in fluvial sandstones and conglomerates. In all cases, the geometry of the ore is highly consistent with the primary depositional and biogenic structures of the host sandstones. Copper ores replace wood debris and diagenetic pyrite or, more frequently, appear as intergranular cement replacing previous diagenetic calcite cement. Sulphide sulphur and organic carbon distribution shows that pyrite-rich sandstones are the copper ore precursor, and that mineralizing processes provoked the depletion of both reduced S and organic C as a consequence of interaction with an oxidized Cu-bearing fluid. On the other hand, low δ 34S values are consistent with bacteriogenic derivation of sulphur after Pyrenean evaporites. Petrographic studies, sulphide sulphur and organic carbon concentrations, and δ 34S values suggest that Cu-rich fluids were introduced into reduced sediments during diagenesis. Mineralizing processes took place in two different periods: the first one related to the filling of the Jaca Basin during erosion of the Pyrenean basement and the second one related to the emplacement of a thrust ramp which initiated the erosion of the previously formed cupriferous sandstones.

ABSTRACT: Outbank Trigger Mechanisms of the Eocene Carbonate Debris Sheets in the South Central Pyrenean Foreland Basin

From Early Ypresian to Late Lutetian, the western part of the South-central Pyrenean (Jaca) Basin stayed as an underfilled foreland, with deep terrigenous turbidites and carbonate debris sheets in their basin trough, and carbonate platforms in their craton margin. The carbonate debris sheets provenance, from the southern foreland platforms, are well stablished by their own composition and geometry. The terrigenous turbidites come laterally from the eastern part of the South-central (Tremp-Graus) Basin; a piggy back basin over the south-central structural unit. The basin architecture consists of three backstepped stratigraphic sequences. Each lasts 4,5 to 5 MY and they are well defined by the stratigraphic structure of their correlative carbonate platforms. The carbonate platforms have a ramp type geometry, with macroforaminiferal limestones in their inner shallow facies, and nodular marnocalcareous facies and debrites in their outer deep facies. Two trigger mechanisms could be invoked to induce the catastrophic failure of the carbonate margin: 1) The platform margin flexure linked to the tectonic subsidence rise in the basin trough; 2) The intersection of the frontal advance of a piggy back blind thrust sequence, developed laterally to the South-central structural unit, with the carbonate platform margin. Outcrop based evidences of the influence of both mecanisms in the origin of the SouthPyrenean carbonate margin collapse, are found. Moreover, the differences in the structure and composition of the carbonate debris sheets bring an exceptional picture of the close relationship between subsidence, surface deformation and carbonate margin collapse.

Sedimentary response to thrusting and fold growing on the SE margin of the Ebro basin (Paleogene, NE Spain)

Abstract The Catalan Coastal ranges are the alpine structural unit parallel to the Mediterranean coast in the NE of the Iberian Peninsula. They shape the southeastern margin of the Ebro Foreland basin (the latest stage of the South Pyrenean Foreland trough). The Paleogene sedimentary evolution of this basin margin is closely related to the tectonic evolution of the adjoining Catalan Coastal ranges, a transpressive chain characterized mainly by contractional structures (NNW-verging folds and thrusts). The syntectonic condition of the Paleogene deposits of the Ebro basin is evidenced by facies directly related to the growth of specific structures, thinning, fanning and truncation of the sedimentary units towards the growing structures, as well as to abrupt changes in the composition of the coarse-grained sediments caused by tectonic activity or landslides in the catchment area. A tectonosedimentary evolution of approximately 17 Ma has been deduced: (a) folds and thrusts involving Mesozoic rocks were developed during Ypresian (resulting in the deposition of Mesozoic cover derived breccias); (b) large-scale fold growth mostly from Lutetian to early Bartonian (leading to progressive unconformities, fanning and growth strata); and (c) out-of-sequence basement-involving thrusting during Lutetian and Bartonian (development of large alluvial fans). These events did not take place simultaneously along the 30-km long basin margin studied. There is a clear NE to SW migration of the deformation, as observed along the whole Catalan Coastal ranges and the South Pyrenean Foreland basin.

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&amp;nbsp;

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

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.