4. INTRODUCTION TO THE REGIONAL GEOLOGY OF THE PYRENEES
-
Published:January 01, 2017
- Standard View
- Open the PDF for in another window
-
CiteCitation
2017. "4. INTRODUCTION TO THE REGIONAL GEOLOGY OF THE PYRENEES", Propagation of Environmental Signals within Source-to-Sink Stratigraphy, Julian Clark, Cai Puigdefàbregas, Sébastien Castelltort, Andrea Fildani
Download citation file:
- Share
4.1. TECTONIC FRAMEWORK AND STRUCTURAL EVOLUTION OF THE PYRENEES
The Pyrenees extend over 400km between France and Spain and were formed from the collision of the Iberian Plate into southwest Eurasian plate during the Late Cretaceous to Miocene (Fig. 4.1). The Pyrenean deformation associated with this orogeny extends from Provence in southern France to the Cantabrian ranges of Northern Spain over a distance of 1500km. The collision involved subduction of the Iberian Plate beneath the European Plate resulting in a doubly vergent asymmetric mountain belt. The Pyrenean orogeny is an early phase of the more widespread Alpine Orogeny that resulted in the closure of the Tethys Ocean due to the collision of the African and Indian plates into Eurasia, and the formation of a series of mountain belts extending from the Cantabrian Mountains to the Himalayas, which includes the Alps, Atlas, Carpathians, Hellenides and Zagros mountains. This mountain building episode initiated in the late Mesozoic, peaked in the Early Cenozoic, and in some areas continues to the present day.
PLATE MOTIONS
The Iberian plate formed from island arc accretion during the Late Precambrian (650-550 Ma) Cadomian Orogeny (also known as the Avalonian or Pan-African Orogeny). This microplate, together with several other small continental microplates, became involved in the suturing of Pangea from the collision of Gondwana, Laurentia and Baltica during the Variscan Orogeny (also known as Hercyninan) that occurred from the Late Devonian to Early Permian (380-280 Ma), resulting in regional metamorphism and widespread granitization (Matte, 1986).
Collapse of the Variscan Orogeny in the Permian, and regional extension caused by the propagation of Tethyan seafloor spreading into the western Mediterranean in the Triassic and early Jurassic, lead to phases of rifting which culminated in the Mid Jurassic continental break up of Pangea along the axes of former Variscan sutures (Ziegler et al., 1999). Mid-Jurassic ocean spreading in the central Atlantic and break-up of Pangea created a sinistral transtensional system between Africa and Europe forming oceanic pull apart basins in an east–west zone linking the Tethys and Atlantic oceans (Schettino & Turco 2011; Vissers and Meijer, 2012; Verges & Fernandez 2012; Ford et al., 2016). A second rifting phase from the Aptian to Cenomanian resulted in hyperextension and exhumation of subcontintental mantle, and the opening of deep marine basins along the whole Pyrenean margin (Debroas 1987, 1990; Lagabrielle & Bodinier 2008; Jammes et al. 2009; Lagabrielle et al. 2010; Clerc et al. 2012; Tugend et al. 2015; Ford et al, 2016). Though Iberian plate motion during this time is controversial, the opening of the Bay of Biscay is often related to the 35° counter-clockwise rotation of the Iberian plate with respect to Eurasia (Puigdefàbregas & Souquet 1986; Verges & Garcia-Senz 2001; Gong et al. 2008; Jammes et al., 2009; Vissers and Meijer, 2012). These extensional events made the pre-convergence crustal template complex, highly non-cylindrical, and appear to control much of the later tectonic evolution.
Separation of Africa and South America was followed by Late Cretaceous seafloor spreading in the southern Atlantic and Indian oceans, causing the northward motion of the African plate and the onset of the Pyrenean Orogeny. The collision was diachronous, commencing in the Late Cretaceous, propagating westwards. The deformation initially resulted in the inversion of the Mesozoic extensional basins, and was followed by continental collision and partial subduction of the Iberian plate beneath the European plate (Choukroune and ECORS Team, 1989; Martín-Chivelet et al., 2002; Capote et al., 2002). As a result, the crust was thickened by the development of deep-seated thrusts. In the western and central Mediterranean, since no old oceanic crust was involved in this subduction, the Alpine orogeny resulted in very little magmatism (Ziegler et al., 1999).
Crustal loading of the Iberian-Eurasian plate boundary resulted in flexural subsidence of the previously attenuated crust forming deep, narrow foreland basins on the Iberian and Eurasian plates. Sediment routing systems evolved in response to fold-thrust belt and Axial Zone exhumation redistributing sediment into the foreland basin. In the Pyrenees, a pause in compression (lasting 10–15 My) occurred during the Paleocene, attributed to intense compression focused in the Western Alps and ophiolite obduction over the Arabian margin (Rosenbaum et al., 2002). In the Early Eocene, compression resumed and continued through the Oligocene and into the Mid Miocene (Rosenbaum et al., 2002; Capote et al., 2002; Alonso-Zarza et al., 2002). Peak exhumation rates were reached in the Oligocene (Fitzgerald et al., 1999), contemporaneous with topographic damming of the western Pyrenees and establishment of an internally drained foreland basin (Costa et al., 2010). In the Neogene, convergence between Europe and Africa shifted to the southern margin of the Iberian plate.
STRUCTURAL ZONES OF THE PYRENEES
The structure of the Pyrenees is well constrained through extensive field studies, subsurface seismic and well data from oil and gas exploration in the area, and the ECORS (Etude Continentale et Océanique par Réflexion et Réfraction Sismique) deep seismic reflection profile across the Pyrenees (ECORS Pyrenees team 1988; Choukroune et al., 1989; Muñoz, 1992). The ECORS profile provided imaging of the structure at depth, showing the subduction of the Iberian lithospheric mantle and lower crust beneath the European Plate. The profile also showed the geometry of the crustal scale imbricate thrusts forming an antiformal stack in the Axial Zone and the corresponding flexure of the lithosphere.
The Pyrenees can be divided into 5 structural zones (e.g. Vergés et al., 1995). The largely Paleozoic-age rocks of the Axial Zone are flanked to the north and south by thrust and fold belts involving Mesozoic-Cenozoic foreland basin sediments, which are in turn flanked by the relatively undeformed present-day foreland basins; the Aquitaine Basin in France, and the Ebro Basin in Spain (Fig. 4.1). The North Pyrenean Zone, north of the Axial Zone, is a narrow (25–35 km wide) retro-wedge fold-thrust belt, characterized by steep north vergent reverse faults exhuming Hercynian basement and highly deformed and partially metamorphosed Mesozoic strata (Ford et al., 2016). The retro-wedge fold-thrust belt is bound to the north by the North Pyrenean Frontal Thrust and to south by the North Pyrenean Fault, the interpreted suture between the Iberian and European plates (Mouthereau et al., 2014; Ford et al., 2016). The South Pyrenean pro-wedge fold-thrust belt to the south of the Axial Zone is characterized by low angle south vergent thrust faults translating Mesozoic and Cenozoic basin fill above a décollement of Triassic evaporates (Puigdefàbregas et al., 1992). The South Pyrenean fold-thrust belts ends in the Sierras Marginales, where it is thrust over the Ebro peripheral foreland basin. The Ebro basin is the undeformed region south of the Pyrenees extending from the south Pyrenean fold-thrust belt to the Catalan Coastal Range and Iberian Range.
The southern thrust and fold belt is the focus for this field trip, and is characterized by thin-skinned tectonics (Choukroune et al., 1989) with the most intense shortening and deformation occurring in the early Middle Eocene to Oligocene (Puigdefàbregas et al., 1992; Vergés et al., 1998, 2002). This zone can be divided laterally into an eastern, central and western part with different structural styles. The eastern part comprises the Pedraforca and Cadi thrust sheets involving both basement and cover rocks. In the central part, known as the South Central Unit (Séguret 1972), thrust sheets comprising Mesozoic and syntectonic Paleogene sediments are overthrust on autochthonous Paleogene rocks in the Ebro Basin. These thrust sheets include (in order of emplacement) the Cotiella- Bóixols (late Cretaceous), Peña Montañesa-Montsec (Paleocene-late Ypresian) and Sierras Exteriores-Serres Marginals (Lutetian-Oligocene) (Símo & Puigdefàbregas 1985; Símo 1986), and together they form a salient structure that extended further south than thrusts in the eastern and western southern fold and thrust belt (Muñoz et al., 2013). The location of this salient structure is related to the distribution of weak Upper Triassic evaporites which provides the decollement. These deposits are thinner west of the Ainsa basin (Soto et al., 2002), and as a result the thrusts west of here show less translation. Consequently, the Ainsa area comprises a zone of oblique structures (Ainsa Oblique Zone) formed to accommodate the 44.5km of differential displacement and involving up to 60° of clockwise vertical axis rotation (Muñoz et al., 2013). Total shortening in the central part of the Pyrenees is approximately 150km (Muñoz 1992, Beaumont et al., 2000), and the timing of maximum translation was 55-28 Ma (Burbank et al., 1992) with maximum tectonic subsidence in the Mid Lutetian (44–45Ma) (Vergés et al., 1998).
The axial zone comprises 3 main thrust units forming an antiformal stack. These are the Rialp, Orri and Nogueres thrust sheets (Figs. 4.2 & 4.3). These also show decollement at the Triassic level and their imbricate geometry is also affected by the depositional distribution of the evaporites (Muñoz et al., 2013).
4.2. SOUTH CENTRAL PYRENEES FORELAND BASIN
The Eocene foreland basin extended 200km from Tremp to Pamplona, and had a fully marine connection to the Atlantic. To the south an extensive shallow-marine ramp accumulated carbonate and clastics shoaling towards the subaerial Iberian margin. The northern margin was represented by the Gavarnie thrust and the uplift of the Axial Zone. The main depocenter of the basin is estimated to be 30–40 km wide. The first order control on subsidence was flexural loading due to the stacking of thrusts in the axial zone in the Eocene (Mutti et al., 1988). Depocenter geometry and overall paleogeography were strongly influenced by ramp and flat geometry of the thrust units. The main sediment source was the emerging uplands of the axial zone, in addition to sediment sourced from the southern margin.
The basin can be divided into 4 basins (Tremp-Graus, Ager, Ainsa and Jaca), characterized by different structural and stratigraphic styles, but they are never the less part of the same foreland basin (the Tremp-Pamplona Basin or South Central Pyrenees Foreland Basin). The entire syntectonic foreland basin succession commences with Upper Cretaceous turbidites and marls, and includes late Cretaceous deltaics, Paleocene red-beds, Eocene Alveolina Limestone, Eocene clastics and carbonates and Oligocene conglomerates. In the Eocene, the eastern sector of the basin filled with primarily alluvial, fluvial, tidal and near-shore facies, represented by the Montanyana and Ager groups (Nijman & Nio, 1975; Nijman 1998). The Montsec thrust separated the Tremp Basin from the narrow elongate Ager Basin (Fig. 4.3).
At the western edge of the Tremp-Graus sector along the ridge separating the Ésera valley from the La Fueva valley, large-scale submarine erosional surfaces, produced by mass failure of the shelf edge and upper slope, can be observed cutting into deposits of prograding deltaic systems. The position of the shelf edge appears to match closely that of the lateral ramp of the Montsec thrust unit, which is represented by the Atiart thrust, thus suggesting a strong structural control on this major paleogeographic element. This resulted in significant deepening of the basin and the development of expanded mud-rich stratigraphy represented by the overbank wedges (sensuMutti & Normark, 1987) in the Ainsa Basin. Within these slope mudstone deposits numerous intervals of sand-rich turbidites and associated mud-rich debris flow deposits can be found which were clearly deposited in channelized environments. In the Ainsa Basin the channels show evidence of being influenced by structures controlling the bathymetry of the basin, and changes in this structural control observed in the stratigraphy document the timing of these syn-depositional structures (e.g. the Boltaña anticline).
Within the Ainsa Basin, evidence of significant flow bypass can be observed in the turbidite channels, which corresponds to the very large volume of sediments found 30–50km further west into the Jaca Basin where they represent the deposition in unconfined lobe deposits. These deposits form the Hecho Group (Mutti et al., 1972) and develop a stratigraphic thickness of 4500m (Remacha & Fernández, 2003). Down dip from the lobes, extensive mud-rich basin plain facies are found in the Pamplona region, and toward the western end of the basin these strata are buried by thrust sheets. Contemporaneous deep-water deposits from the related Basque Basin are found in the coastal regions of the Basque country (Pujalte et al., 1997; Payros et al., 2007). In both the Basque Country outcrops and the Jaca Basin distinctive large-volume event beds can be observed (Soler & Puigdefàbregas, 1970; Mutti et al., 1972; Labaume et al., 1987; Payros et al., 1999). These comprise resedimented carbonate and clastic material, and characteristically show a lower portion comprising mass-transport deposits and an upper turbidite portion (graded bed with thick fine-grained mud cap). These large-scale event beds can reach thicknesses of 250m and have been termed Mega-Turbidites (Séguret et al., 1984; Labaume et al., 1987), or South Pyrenean Eocene Carbonate Megabreccias (SPECMs) (Payros et al., 1999). Eight of these have been correlated in the Jaca Basin (numbered MT1–MT8) and provide useful marker beds (Remacha et al., 2003).
In the Late middle Eocene-Oligocene deformation in the axial zone caused an increase in the rate of uplift and erosion of the axial zone units (Sinclair et al., 2005; Metcalf et al., 2009). This resulted in a depocenter shift, regional unconformity and the delivery of coarse-grained conglomerate facies into the basin. Marine sedimentation ended in the Priabonian at 36.2Ma (Costa et al., 2010) as a consequence of the Atlantic seaway closure that resulted from the uplift of the western Pyrenees (and which was probably coincident with a mid-amplitude eustatic sea level low) with deposition of evaporites in the central sector that later became an important detachment level for the late-stage thrusting of the Serres Marginals (central and eastern units). A phase of rapid exhumation of the Central Pyrenean Axial Zone from 35.0 to 32.0 Ma determined from thermochronology data followed this continentalization. It has been suggested that alluvial sediment aggradation at the front of the fold-and-thrust belt could have contributed to a decrease in the taper angle of the orogenic wedge (e.g., Viacamp panorama), triggering uplift of the axial zone by out-of-sequence back thrusting and underplating in order to recover critical taper (Costa et al., 2010).
4.3. STRATIGRAPHIC UNITS AND DEPOSITIONAL SYSTEMS
The following text is an explanation of the stratigraphic names used in the cross-section, map and legend of figure 4.4, 4.5 & 4.6.
Alveolina limestone: shallow marine carbonates rich in Alveolina. Well-developed on the southern basin margin (Llimiana, Àger, Mediano, Guara), thins significantly northwards to the basin axis (Figols section) and thickens again to the northern margin (Serradúy) where local reefs may develop Merli, Iscles, Berganuy). The unit may be partly eroded along the northern outcrops in the Broto-Jaca sector.
Metils: marl to carbonate sequence rich with echinoids and sponges, capped by a basin-wide flooding event. The unit corresponds to Millaris-Metils in Ainsa and Broto, Riguala-La Puebla at the Isábena section, and La Baronía sandstone in the Àger basin.
Yeba (Roda): marly unit, gently westward deepening from Tremp to Boltaña. Includes the Roda tidal sandstone in the Isábena section, and thin sandy layers in the deeper Boltaña and Yeba areas. The sequence is capped by a thin algal carbonate bed with Nummulites and Alveolina (Morillo key bed) that can be traced westwards from the Isábena down to the slope incisions near Atiart, and seen again in the area of Yeba. The equivalents in Tremp are the Figols and Porredó sandstone, and in Ager it is represented by the La Passarela marls and tidal sandstones, and L’Ametlla deltaic sandstones.
Castigaleu, Corçà and Santa Marina: fluvial sequences in the Àger basin (Corçà). Fluvio-deltaic, often tidal dominated in the Tremp-Graus basin (Castigaleu). Shallow marine, often large-scale cross-bedded, sandy or bioclastic sequences at the Mediano and Boltaña anticlines (Santa Marina). Thin turbidite beds are seen at the base of the sequence west of the Ésera valley. The sequence is capped by a basin-wide transgression (Santo Cristo) with Nummulites-rich beds (Ésera section), or oyster beds and tidal sands in the Isábena and Ribagorzana sections (Chiriveta beds). Within the Boltaña anticline the transgression is represented by the Ascaso Nummulites carbonate bed above the Santa Marina package. Both Castigaleu and Santa Marina are deeply incised by the regional unconformity at the base of the Hecho Group. The unconformity includes the Atiart canyon incision at La Fueva.
Castissent: fluvial formation with two thick fluvial sandstone units (not always coinciding in a given section), within a section of floodplain mudstones. The Castissent sandstone sits in the Tremp-Graus basin but has a composition that rather compares to that of Corçà from the Àger basin. Two sequences (C1 and C2) are distinguished within the more shallow-marine area west from the Ésera section. C1 fills the Atiart incision and C2 fills the Pocino incision. The Castissent sandstone is capped by the Rolespe transgression, typically including tidal Nummulites-rich and bioturbated sandstones (Ésera), or oyster beds (Isábena). The Castissent sandstone is also represented within the Boltaña anticline section as tidal dominated sandstone, sandwiched between the Ascaso bed below and the Rolespe transgression above (Nummulites bed, bridge to Jánovas).
Fosado, Arro and Tobacor: deep water channel sandstones (Fosado and Arro), and slope marls of changing thickness in relation to growth structures. Emerging thrusts related to the Montsec lateral ramp and Peña Montañesa structure, caused slope instability and created an active topography on the sea bed that controlled sand deposition. The Fosado and Arro channels grade down basin to lobe sandstones (Tobacor), presently outcropping around Ordesa and Añisclo tops of Mondaruego, Tobacor and Plana Canal.
Fornosa and Silves: a distinct and continuous muddy interval is mapped above the Castissent sequence, comprising the Rolespe transgression, the marls above, and a distinct Nummulite-rich carbonate (Fornosa beds), also seen in the Ésera valley (Torre de Obato). Rolespe beds are outcropping down the Ésera present day river bed near Ventas de Santa Lucía. The shallow marine Fornosa from the Ésera section gradually turns to non-marine muddy section to the East (Montanyana), always above the Castissent sandstone. Within the Boltaña anticline, the muddy section includes two nummulitic carbonate beds (Silves).
Güell: conglomerate unit sharply deposited above the Fornosa beds. Massive and thicker conglomerates are seen in the cliffs west from Roda (Isábena valley) and in the Ésera section. These conglomerates quickly grade to the west and south-west to shallow marine conditions with well-developed pebble beach sequences (including bored pebbles). In the Montanyana area, the Güell unit consists of braided gravelly streams with transport directions suggesting an East or NE area of supply. The Güell sequence is generally capped by a flooding surface with Nummulites in the Ésera section, locally with well-developed reefs (Perarrúa Castle) and oyster beds in the Isábena section.
Lascorz incision: in the shallow marine and slope area, the Güell sequence is deeply incised by the Lascorz submarine erosion surface (canyon incision), traceable from Fuendevino to Lascorz and Charo. The incision cuts into the Güell and down to the previous Fornosa and Castissent. The Lascorz incision surface correlates with the Besians incision in the Ésera valley, where the two canyon margins are exposed. The canyon is mud filled in Besians and Lascorz and includes thick turbidite sandstones and pebbly-muddy debrites at Charo.
Caballera: tidal dominated sandstones between the Perarrúa castle reef below and the Monclús flooding above. Extends over the Isábena and Ésera valleys, and connects to conglomerate fans to the north (Campanué) and dramatically thickens to slope marls at La Fueva. At Monclús, the Caballera section is cut by the Monclús canyon incision. Banastón channels and Torla lobes correspond to the lowstand of the Caballera sequence.
Capella: fluvial formation with frequent tidal indications within the channels. Grades to open marine and slope conditions to the west. Bounded between the Monclús incision below and the Pano tidal sandstone on top, the Capella unit connects to the Ainsa and Morillo turbidite channels along the Monclús canyon. The Capella unit becomes more fluvial dominated to the east (Montanyana), with local thickening east of the Luzás fault. To the north, Capella connects to the conglomerate fans of the Campanué system. In the slope area, the Capella unit is cut by the Formigales canyon incision.
Pano and Grustán: thick tidal dominated sandstone (Pano) extending above Capella, still cut by the uppermost margins of the Formigales incision. Probably feeding the Guaso slope channels. The Grustán carbonate platform, locally with reef development, extends from Graus to Samitier in the Ainsa area. Pano and Grustán cannot be traced eastward because of the unconformity at the base of Escanilla.
Campanué conglomerate system: constitutes the feeder system from the northern basin margin during time deposition of Güell, Caballera and Capella, and receding at Pano time. The Campanué system progrades to the SW from alluvial fans to coastal environments (Campanué is not represented in the legend but indicated on the map with the conglomerate sign).
Gerbe and Banastón: deep water channel sandstones and pebbly debrites containing abundant shells and Nummulites from the shallower environments. Thick marl section overlies the sandstone bodies. The clasts (pebbles) from the Banastón debrites are sourced from the Güell pebble beaches through the Lascorz-Besians canyon. Contrastingly, shell fragments are nearly absent in the Gerbe sandstones. At Sierra de Griebal they show transport directions from the south, from the area ahead of the Montsec thrust lateral ramp. The Gerbe and Banastón sandstones grade from channel to lobe to the NW with maximum thickness of Banastón on both flanks of the Boltaña anticline (Morillo Sanpietro, Comiello and across the Chate valley). West of the Boltaña anticline they are mapped as Torla lobes.
Ainsa and Morillo: deep water channel sandstones also including pebbly debrites, probably fed from the Capella sequence through the Monclús canyon incision. Thick marly sections, often slumped, overlie the different Ainsa and Morillo channel bodies. Channel orientations show indications of initial growth of the Boltaña anticline. West of the anticline they are mapped as Broto lobes.
Guaso: the last group of turbidite channels within the Ainsa basin, probably fed from the Pano sequence through the Formigales canyon. Pebbly debrites and re-deposited Nummulites or shell fragments are rare. West of the Boltaña anticline they are mapped as Cotefablo lobes.
Guara: carbonate platform developing at the outer passive basin margin during Lutetian, initially more restricted (Guara 1) and more open platform above (Guara 2). These platforms are deepening to the north, grade to marl sections (Paules) and develop north-facing carbonate slopes in the area of Castellazo, with carbonate debrites lapping on the platform edges. These debrites can be mapped to laterally correspond to the upper part of Ainsa and particularly above the Morillo turbidite channels (MT4).
Carbonate debrites (MT3, MT4, MT5 and MT8): formed ahead of the north facing slopes of the Guara carbonates. MT1, local monomictic debrite near Fanlo including exclusively carbonate and marly fragments from the incised Metils and Millaris Formations. MT3 extends from the flank of the Boltaña anticline as far as 80 km to the west, with changes in thickness and clast size, eventually including huge carbonate blocks (Torla and Villanúa). MT3 contains abundant Cuisian re-deposited forams together with Lutetian specimens (Assilina spira). MT3 is also mapped to onlap the Castissent and Silves beds (Cuisian in age), on the west flank of the Boltaña anticline. The turbidites from the Jaca basin above MT3 are necessarily of Lutetian age or younger. MT4 overlies the Morillo turbidites in the Ainsa basin, and therefore can be used to correlate the Cotefablo turbidites above MT4 to Guaso. MT5, mapped just below the Bergua lobes, contains benthic forams of SBZ15 corresponding to the assemblage from Grustán beds. Finally, MT8, only present in the area of Jaca, corresponds to SBZ 16 and sits below the Jaca lobes.
Sobrarbe 1: prograding delta sequence unconformably overlying the Guaso sequence below. The direction of progradation shown by the clinoform orientation is to the NW. Large-scale slump scars facing NW are seen on the pro-delta slopes (road to Arcusa). Turbidite bodies are observed at the base of the prograding wedges (La Capana, Los Gorgos). The basal unconformity (sequence boundary) is considered to correspond to the Bergua Lobes in the Broto-Jaca basin. To the SE, Sobrarbe 1 grades the Escanilla fluvial formation.
Sobrarbe 2: renewed deltaic progradation to the NW from the Escanilla fluvial formation. The Mondot conglomerate corresponds to the sequence boundary above Sobrarbe 1, immediately overlain by the transgressive Nummulites bed of Buil. Sobrarbe 2 corresponds to the Jaca turbidite lobes and Larrés (Arguis) marl.
Sabiñánigo sandstone: only present in the Jaca basin, constitutes a deltaic unit that progrades to SW above the Larrés marl, with thicker and coarser sands ahead of the Oturia thrust. The Sabiñánigo sandstone is capped by a sharp transgression. In the Ainsa, this basin probably corresponds to the Olsón braided stream conglomerate within the Escanilla formation.
Atarés: a renewed delta progradation above the top-Sabiñánigo transgression, fed by the Campodarbe fluvial systems.
Escanilla - Campodarbe: A thick fluvial section from the Upper Lutetian to late Eocene. The thickest section is in Olsón (Buil syncline). In this area, changing vertical patterns from braided to meandering may be in relation to relative sea level changes at the corresponding coastal settings. Three main events allow a tentative subdivision in Olsón: the Mondot conglomerate, the Olsón conglomerate, and higher up, the fresh water carbonate beds of Frontiñán. Farther east, where the effects of seal level changes cannot be distinguished, braided channels predominate (Lascuarre and Viacamp). Sources for Escanilla have probably been from the Pyrenean paleo-valleys to the north and northeast, and from the southern foreland.
Pessonada – Cajigar: Conglomerate unit sourced from Mesozoic carbonates. Individual beds are reversely graded. Basal depositional shear and pebble imbrications show transport directions to the WSW. Thickness is up to 200 m at Pessonada and Gurp, but much thinner in Cajigar. Some of the beds are locally derived from a very specific source (from Vallcarga Formation). The Pessonada conglomerate fills a paleo-valley between the Bóixols structure to the south and Serra del Boumort to the north. The valley incision is also seen at Serra de Gurp, and from Claramunt to Espills, Escarlà and Colls in the Ribagorzana valley. As magneto-stratigraphy dates the upper Pessonada as Upper Lutetian, the basal valley incision could have started much earlier, suggesting a correlation to Güell and Capella.
Ermita – Cornudella: Thinner bedded and muddier than Pessonada, the Ermita beds are also reversely graded and show the same current directions. Ermita overlies Pessonada but in the northern side directly overlies the Mesozoic reliefs (Hortoneda) They include lacustrine carbonate beds and thin coal seams at Sossís, with vertebrate remnants Upper Eocene in age. At Cajigar, the Ermita unit is represented by the Cornudella beds, also including lacustrine carbonates and thin coal seams.
Montsor – Sis: Thick package of stacked conglomerate fan sequences (up to 450m in Montsor), architecturally organized in three mappable units. They are sourced from Paleozoic, and Mesozoic rocks. In the area of Montsor they are of polymictic composition, but laterally interfinger with local monomictic fans from the Mesozoic carbonate reliefs. Both braided stream and debris flow are processes responsible for transport and deposition. Directions are to the south and southwest in the Montsor area and to the west at Roc de Santa. At Serra de Sis they correspond to the main body of the unit above the Cornudella beds.
Antist-Graus-Guarga: The Antist unit unconformably overlies Montsor and develops to a thick conglomerate package north of the Morreres backthrust at Senterada and Perbes. The unit is sourced mainly from Triassic and Devonian from the Nogueres Zone. Paleo-valley morphology is eventually preserved in the landscape (Creu de Perbes) around the paleo-reliefs of Serra de Peracalç, and Serres de Sant Gervàs and Aulet. The Antist is Oligocene in age and was probably the feeder for the Viacamp conglomerates, correlating to Graus and Colungo in the Ainsa area with contribution from other sources (Serra de Sis). West of the Boltaña anticline, Oligocene fluvial sequences are mapped within the Guarga basin.
CORRELATING EVENTS ACROSS THE BOLTAÑA ANTICLINE:
A lower group of turbidites (Fosado, Arro Gerbe, Banastón and maybe Ainsa) have been deposited prior to the initial anticline folding. Correlative turbidite units (Tobacor, Torla and lower part of Broto) consisting largely of lobe deposits, expanded into the Jaca basin west of the anticline. An intermediate group of turbidites (Morillo and Guaso) have been affected by the initial growth and rotation of the Boltaña anticline, but turbidites still managed to reach the Jaca basin through relatively narrow pathways inferred to have existed north of Boltaña. These units are correlative to the Broto and Cotefablo lobes, which expanded to Jaca basin.
The Ainsa basin may have been eventually ponded. Dark shale horizons may indicate water stratification at this stage. A deep erosive incision developed above the Boltaña anticline between Morcat and Campodarbe by the time of base-Sobrarbe unconformity, and Bergua turbidites expanded to the Jaca basin. The delta sequences of Sobrarbe 1 and 2 overfilled the Ainsa basin, and fluvio-deltaic sequences prograded to the west.