The Pyrenean orogen is a fold-and-thrust belt that resulted from the collision between the Iberian and the European plates from Late Cretaceous to Miocene times. The Pyrenean convergence was accommodated by the development of an upper crustal doubly-vergent orogenic prism and the subduction of the Iberian lower crust under the European plate (Beaumont et al., 2000; Muñoz, 1992; Teixell, 1998; Teixell et al., 2016). The Pyrenean orogen is flanked by the Ebro and Aquitanian foreland basins to the south and the north, respectively (Fig. 1A). The southern side of the Pyrenean orogen consists of a basement-involved thrust stack formed by three main imbricate thrust sheets known as Lakora–Eaux-Chaudes, Gavarnie, and Guarga thrusts (Fig. 1B). This thrust stack includes a Palaeozoic basement characterized by the occurrence of granitoids and metamorphic rocks (Axial Zone), and a cover imbricate thrust system with preorogenic Mesozoic rocks and synorogenic assemblage of Late Santonian to Early Miocene clastic and carbonate rocks (Fig. 1B). These synorogenic rocks constitute the South Pyrenean basin, which during Early and Middle Eocene times concentrated fluvio-deltaic sedimentation in the east (Tremp-Graus and Ager basins, Fig. 1A) passing to the west to coastal/slope to deep-marine sedimentation (Ainsa and Jaca basins). These fluvio-deltaic systems were fed from the Palaeozoic basement (Axial Zone) and from the Mesozoic rocks that constituted the South-Central Pyrenean Unit (Séguret, 1972).

Thrust activity, and associated flexural subsidence, highly controlled the Jaca basin infill. It firstly consists of turbidites (Lower–Middle Eocene flysch, Hecho Group) and coastal, non-marine deposits (Upper Eocene–Lower Oligocene molasse) during the overfilled stage (Barnolas and Teixell, 1994; Labaume et al., 1985; Mutti et al., 1988; Puigdefàbregas, 1975; Teixell, 1996). The 3800 m-thick analysed sedimentary succession crops out in the northern Jaca basin (Fig. 1A). The lower part of the section is located in the footwall of the Oturia thrust (north of Jaca town; Fig. 2), while the upper part is located in the northern limb of the Yebra de Basa anticline (Fig. 2).

The succession starts with the Banastón turbidite systems (Lutetian according to Labaume et al., 1985; Oms et al., 2003) from the upper Hecho Group (Fig. 3), that are built up by a succession of sandstones and mudstones including carbonate megaturbidites (labelled as MT-5, MT-6 and MT-7 by Labaume et al., 1987). They correspond to turbiditic sheet-like lobes, changing into basin plain facies more to the west (Remacha and Fernández, 2003; Remacha et al., 2005). As for the preceding turbiditic systems (i.e. Cotefablo and Broto of the Lower Hecho Group), the Banastón system was fed by a single entry point located in the southeastern end of the foredeep (Remacha et al., 1998b), mainly sourced from erosion of the eastern Pyrenees (Caja et al., 2010; Puigdefàbregas et al., 1992; Teixell, 1998). This first unit is overlain by the Jaca turbidite systems (Fig. 2; Fig. 3) subdivided into the Jaca-1 to Jaca-4 depositional sequences (Remacha and Picart, 1991; Remacha et al., 2003). Turbidites of Jaca-1 to Jaca-3 are representative of the deep-marine stage, whereas the occurrence of channelized turbidites in the Jaca-4 sequence (the Rapitán system of Remacha et al., 1995) depicts a landward stepping trend leading to a molassic stage (Remacha et al., 2005). This facies change is also coincident with a shift of the direction of the palaeocurrents with transport direction from the north (Remacha and Picart, 1991). The Rapitán system (Lutetian–Bartonian transition) evolves further up to the Larrés Marls (Fig. 2; Fig. 3), which represent the delta slope sediments (Puigdefàbregas, 1975) of the platform deposits of the Sabiñánigo Sandstone delta (Middle Bartonian). The Sabiñánigo Sandstone shows a wide variety of facies that are mainly associated with outer shelf and delta front environments, with palaeocurrents from the east and southeast, and in the upper part from the north (Remacha and Picart, 1991; Oms and Remacha, 1992) The upper part of the Sabiñánigo Sandstone (Figs. 2 and 3) is mostly transgressive, passing upward into the Atarés delta and the Santa Orosia alluvial fan (Figs. 2 and 3). The Atarés delta (Priabonian according to Hogan and Burbank, 1996) shows a cyclic delta front stacking pattern that ends up with marked alluvial influence. The palaeocurrents indicate sediment transport from the east and the southeast (Oms and Remacha, 1992; Oms, 1994). The overlying Santa Orosia fan (Priabonian) is the first important irruption of conglomeratic facies alternating with red-coloured palaeosoils, with palaeocurrents from the north and the northeast (Puigdefàbregas, 1975). The lower part of the fan is characterised by marine influence (Oms and Remacha, 1992), while in the upper part continental facies are found (Fig. 3). Therefore, the continental Santa Orosia fan belongs to the Campodarbe formation in the sense of Puigdefàbregas (1975), and marks the establishment of generalized terrestrial environments in much of the Jaca basin. The continentalization of the basin is evidenced by the subsequent irruption of other alluvial fans such as the Peña Oroel and the Canciás fans (Fig. 2). The analysed stratigraphic section ends with the overlying Canciás alluvial fan that shows thick conglomeratic levels, interrupted by lacustrine limestone beds and yellow-to-grey shales.

A total of 103 sandstones and conglomerates samples were collected and examined in thin sections under the polarizing microscope. Sixty-five representative samples were selected for a detailed quantitative study based on point counting analysis in order to establish the detrital modes. Textural features as size and sorting were set for all the samples following the criteria of Beard and Weyl (1973) that allow us to establish the degree of sorting depending on each grain size using thin section comparators. All thin sections were stained using Na-cobaltinitrite (Chayes, 1952) for accurate identification of feldspar and alizarine red-S staining was applied for distinction of carbonate composition, such as dolomite and calcite.

Quantification of detrital modes was performed by petrographic analysis of thin sections using the Gazzi–Dickinson point counting method (Dickinson, 1970; Gazzi, 1966; Zuffa, 1985). Grain size effects are minimized, as crystals and other sand size (> 0.0625 mm) grains that occur in larger rock fragments are classified by the type of crystal below the cross-hair (Ingersoll et al., 1984), as well as the type of rock fragment. The distance between two counting points was larger than the coarsest grain fraction in all the samples (Van der Plas and Tobi, 1965). A total of 500 points were counted for each thin section (following Dryden, 1931) and fifty-six petrographic classes were considered, referring to framework grains (forty-five classes), matrix, cement and porosity (twenty one classes). Framework grains were grouped into the four main categories defined by Zuffa (1980): non-carbonate extrabasinal (NCE), non-carbonate intrabasinal (NCI), carbonate extrabasinal (CE) and carbonate intrabasinal (CI).

Quantification of the main clast lithologies in the conglomeratic levels of the Santa Orosia and Canciás alluvial systems were performed on outcrops. Four sampling locations were established for the Santa Orosia conglomerates, and one for the Canciás system. Clast counting was set at the same conglomerate bed whose matrix had been sampled for sandstone petrography. Procedure for clast counting was done following the method of Howard (1993) consisting in counting four closely spaced subsets of 100 clasts each, obtaining a total of 400 clasts for each studied sampling location.

A composite stratigraphic section was constructed across all the studied systems (Fig. 2), where all the samples are situated (Fig. 3).

All the samples are grain supported with a very low matrix ratio (less than 1% in most sandstone). Turbidite sandstones show angular to subangular grains with low sphericity and sorting ranges from poorly to well sorted, depending on the grain size. Grains from deltaic and alluvial samples are subrounded to angular and show a highly variable sphericity. Samples from deltas are very well to well sorted, while sorting of alluvial fan samples ranges from moderately to well sorted.

Ternary diagrams (Fig. 3) are used in order to classify and identify the main compositional changes of the studied systems. The modal composition of sandstones is here represented according to the criteria of Zuffa (1980) (NCE–CE–CI) and Dickinson et al. (1983) (QFL). Supplementary diagrams have been used according to the most distinctive components of each sedimentary system. For deep-marine environments, a ternary diagram comparing the main lithic grains (F + PRF: feldspar and plutonic rock fragments; SndSilt + VRF: sandstone rock fragments and volcanic rock fragments; MRF: metamorphic rock fragments) allow us to identify compositional trends. Transitional to continental environments were represented in a triangular diagram comparing hybrid-sandstone fragments (hybrid sand), plutonic and volcanic fragments (PRF + VRF) and metamorphic fragments (MRF). The term “hybrid-sandstone fragments” is here used in the sense of Zuffa (1980), and is referred to those sandstone fragments that contain intrabasinal and extrabasinal grains.

A first-order compositional classification has been obtained for all the studied samples by representing the relative content on NCE, CE and CI components.

4.1 Modal sandstone composition: turbiditic systems

4.2 Modal sandstone composition: deltaic and alluvial systems

4.3 Modal composition of pebbles (conglomerates)

Proportion of carbonate clasts ranges from 10 to 30% with a wide variety of limestones. In the conglomerate bodies, grey mudstone clasts are the most abundant, although black mudstone, grey packstones and grainstones are also common. Bioclasts such as nummullites, alveolines or rudists occur within limestone clasts. Orange and white mudstone-wackestone clasts are less abundant. Limestone clasts with flint concretions are also present (Fig. 4). The coarsest pebbles correspond to hybrid sandstones and in some cases to limestones. The main trend that can be identified in the Santa Orosia fan is an increase of hybrid sandstone and plutonic clasts and a decrease in limestone clasts, from the base to the top of the system. In contrast, the Canciás fan shows a decrease in hybrid-sandstone fragments and higher contents of limestone clasts (Fig. 4).

Although detrital grains of the sedimentary systems of the Jaca basin can be sourced from a wide variety of rocks, some characteristic features allow us to recognize their most likely provenances. The quartz content decreases from base to top along the entire turbiditic succession (Banastón and Jaca systems) and transitional marine to alluvial environments (Sabiñánigo Sandstone to Canciás fan). Its origin can be diverse, as it appears in a wide kind of varieties. Well-rounded quartz indicates recycling from terrigenous grains of the lowstand wedge prograding complex deltas (Fontana et al., 1989) or from Cretaceous or Palaeocene sandstone formations. The presence of inherited quartz overgrowths in detrital quartz grains represents the cementing of grains from a previous sedimentary cycle (Sanderson, 1984). This kind of detrital quartz suggests recycling of Palaeozoic (Carboniferous or Permian–Triassic rocks) or Upper Cretaceous (Marboré sandstone) rocks, that crop out in the western part of the Pyrenees (north of the Jaca and Ainsa basins). Fine-grained polycrystalline quartz can be associated with medium/high grade metamorphic rocks such as schists and quartzites from the Palaeozoic basement. In contrast, some euhedral quartz grains with abundant evaporitic inclusions (halite and anhydrite) derive from the Triassic Keuper facies (Marfil, 1970).

K-feldspar and plagioclase are attributed to the crystalline rocks from the Palaeozoic basement such as granitoids or metamorphic rocks. Lithic clasts like shales, schists, plutonic and radiolarite fragments are also indicative of a contribution from the Palaeozoic basement.

Hybrid sandstone and siltstone rock fragments are interpreted as mainly supplied from erosion from the Hecho Group turbidites, as they present both intrabasinal and extrabasinal components and appear to widely suggest a large source area containing this kind of petrofacies. Features such as lamination and gradation found in sandstone pebbles support also the attribution to a turbiditic source area.

Subvolcanic rock fragments are associated with erosion from Triassic tholeiitic dolerites (ophites), cropping out in Keuper materials that even can be part of diapiric structures such as the Naval or Tolva diapirs, located in the South-Central Pyrenean Unit (Séguret, 1972). The provenance of subvolcanic fragments is also interpreted as fed from the North Pyrenean zone, where Triassic dolerites are widely found.

The bioclast content of the terrigenous grains indicates the presence of Cretaceous and Palaeocene/Eocene limestones in the source area. Microcodium grains are attributed to the erosion of Palaeocene limestones of the South-Central Pyrenean Unit. Some wackestones fragments with pithonellid tests occur in Cenomanian limestones, while dolostone and dolomitized limestones clasts are interpreted as supplied from the Palaeocene carbonate materials. Some dolostone fragments with sucrosic texture are associated with erosion from Jurassic dolostones involved in the main thrust sheets of the South-Central Pyrenean Unit. Single dolomite grains are related to weathering of both dolostone formations.

The changes observed in the petrofacies of the studied sedimentary systems of the Jaca basin indicate variations in source areas during the sedimentation of the last turbidites of the Hecho Group and the overlying deltas and conglomeratic fans.

Calclithites are the most representative petrofacies for the lower part of Banastón turbidites, although the increasing trend on intrabasinal components towards the upper part indicates a transition to hybrid arenites. Our results are in accordance with those of Fontana et al. (1989) for the Banastón system located in the hanging wall of the Oturia thrust (Fig. 2)). The increase of carbonate intrabasinal components to the uppermost part of each turbiditic system has been associated with episodes of carbonate platform development evidencing abatement in siliciclastic input related to reduced tectonic activity (Caja et al., 2010; Fontana et al., 1989).

Our petrographical results for the Banastón and Jaca turbiditic systems differ significantly from those obtained by Gupta and Pickering (2008) in the Jaca basin. This difference is due to their poor recognition of CE grains, feldspar varieties (K-feldspar vs. plagioclase) and some lithic grains such as shales, schists and volcanic fragments in the Banastón and Jaca turbidites.

Palaeocurrent analysis for the Banastón turbidites indicates a sediment supply coming from the east (Remacha and Fernández, 2003), consistent with the sediment composition. High proportions of terrigenous carbonate grains indicate a major contribution from Mesozoic and Palaeocene platforms from the South-Central Pyrenean Unit. The high amounts of feldspar and plutonic fragments are indicative of the exhumation of Palaeozoic granitoids. The Jaca-1 turbidites show a composition similar to that of the former system, with similar paleocurrent directions (Fig. 3). In contrast, an important change is recorded in the Jaca-4 system (Rapitán turbidites), evidenced by a variation in composition, facies and palaeocurrent directions that point to a new sediment routing system coming from the north (Remacha et al., 1995). This change is shown by an increase of sandstone and subvolcanic rock fragments together with higher P/K ratios (Fig. 3C). The change is interpreted as the result of a new tectonic pulse that caused the emergence of new source areas to the north. This tectonic pulse is represented by the activity of the Lakora–Eaux-Chaudes, whose age (Early–Middle Eocene) is consistent with the depositional age of the Rapitán turbidites (Jolivet et al., 2007; Labaume et al., 1985; Meresse, 2010; Oms et al., 2003; Teixell, 1996; Teixell et al., 2016). The fragments of Triassic subvolcanic rocks (ophites) currently cropping out in the leading edge of the Lakora thrust sheet (e.g., Bedous area) are among the most distinctive grains of the Rapitán system. Thus, the first record of a northern source area in the Jaca basin can be attributed to the uplift of new terrains that created a strongly emergent, northern thrust margin of the basin during the deposition of the upper Hecho Group turbidites.

P/K ratios for the Banastón turbidites obtained in this work are consistent with those obtained by Fontana et al., 1989 for the Banastón System in the Fiscal area (eastern Jaca basin). The increase in the P/K ratio in Jaca-4 turbidites is interpreted as related to the creation of new emerged areas, where fresher rocks were exhumed. In contrast, Banastón and Jaca-1 turbidites were derived from relatively more mature source areas that supplied higher amounts of K-feldspar. Thus, the change in the nature of source rocks and its residence/exposure time due to their distinct tectonic evolution is here interpreted as the main cause of the P/K ratio changes, and not as the degree of chemical weathering due to climate characteristics.

As for the former turbiditic systems, compositional ternary diagrams allow us to discriminate between the first deltaic systems of the basin (Fig. 3F). The Sabiñánigo Sandstone and Atarés deltas are characterized by the high amounts of carbonatic extrabasinal fragments and detrital monocrystalline dolomite grains. These kinds of grains can be attributed to the erosion of the Mesozoic cover rocks of the South-Central Pyrenean Unit, indicating supply of eastern source areas. Nonetheless, the presence of hybrid-sandstone fragments in the Sabiñánigo Sandstone is interpreted as early erosion of the turbiditic basin, indicating a contribution of a northeastern source area. Paleocurrent directions support this interpretation for both the Sabiñánigo and Atarés deltas (Remacha and Picart, 1991). The Atarés delta exhibits higher proportions of metamorphic rock fragments than the Sabiñánigo Sandstone, which shows significant contributions of plutonic and subvolcanic rock fragments.

The overlying alluvial systems of Santa Orosia and Canciás are characterized by a northern input, evidenced by palaeocurrents and facies architecture (Puigdefàbregas, 1975). This input is recorded through the high amounts of hybrid-sandstone rock fragments (Fig. 3F), sourced from the turbidite sandstones of the Hecho Group that originally overlaid the uplifting Axial Zone. This sharp petrological change implies the contribution of Hecho Group turbidites as part of a new source area and points to a major paleogeographic reorganization, here interpreted as generated by the onset of the basement-involved Gavarnie thrust (Roigé et al., 2013). The activity of the Gavarnie thrust has been set to the Late Eocene–Early Oligocene according to structural data from Teixell (1996) and thermochronological data from Jolivet et al. (2009) and Meresse (2010), who obtained exhumation ages around 37–28 Ma. According to these data, the emplacement of the Gavarnie thrust was contemporaneous with the Santa Orosia fan, dated as Priabonian (Canudo and Molina, 1988; Hogan and Burbank, 1996).

The involvement of recycled turbidites of the Hecho Group increases from the base to the top of the Santa Orosia fan. In contrast, the Canciás fan records a minor contribution of this recycling as well as an increase on limestone and dolostone rock fragments that indicates major contributions of Palaeocene/Eocene carbonates that are interpreted as indicative of erosion of deeper levels in the source area (Fig. 4).

Sandstone compositions allow us to link the main paleogeographic changes that controlled the infill of the Jaca basin during the transition from deep-marine to terrestrial environments (Middle Lutetian to Priabonian). The petrological analysis shows that the most representative petrofacies of the Banastón turbidite systems (Middle Lutetian) are calclithites and hybrid arenites, whereas in the youngest turbidites (Lutetian to Bartonian transition) of the Jaca Basin are lithic arenites and hybrid arenites. The proportion of carbonate intrabasinal components depicts a similar increasing upwards trend in each turbiditic system. The Banastón and the lower Jaca turbidite systems were fed from an eastern source, which dominated during most of the sedimentation of the Hecho Group in the Jaca Basin. In contrast, the upper part of the Jaca turbidite Systems (Rapitán turbidites) records a change in the source area, being the first depositional system clearly fed from the north. The content increase of subvolcanic rock and sandstone fragments is the main feature associated with the shift of sediment input from the north. This change is interpreted as the result of a new tectonic pulse associated with the Eaux-Chaudes/Lakora thrusts that acted at the northern margin of the foreland basin during sedimentation of the Jaca turbidites.

The petrofacies of the Sabiñánigo Sandstone and Atarés deltas and of the overlying Santa Orosia alluvial systems are mainly calclithites and lithic arenites. The provenance of the Sabiñánigo Sandstone sediments is interpreted from both eastern and northeastern source areas, this last source predominating in the upper part. In contrast, the overlying Atarés delta complex records only sediment input from the east, characterized by a higher proportion of metamorphic fragments and single dolomite grains. Moreover, the Santa Orosia and Canciás alluvial systems, sited in the uppermost part of the studied section, record an important compositional change evidenced by the high content of recycled hybrid sandstones, which are absent from the former Atarés delta. These hybrid sandstones were sourced from the turbidites of the Hecho Group, which originally overlaid the uplifting Axial Zone. This recycling is interpreted as related to the onset of the basement-involved Gavarnie thrust during Priabonian times.

The change of the eastern source for the northern source acted as a major control during the continentalization of the basin. This shift in source location was gradual, and reflects a competition between the east-derived and north-derived systems that constitute the infill of the Jaca basin, highly controlled by the thrust activity of the Pyrenean prowedge.