Salt sheet extrusion and emplacement within the South-Central Pyrenean fold-and-thrust belt: the Les Avellanes Diapir case-of-study

ABSTRACT A detailed geological map of the Les Avellanes salt Diapir (South-Central Pyrenees, Spain) that includes both the diapir body and adjacent areas is presented to understand the diapir evolution and geometry. Structural, stratigraphical, and sedimentary data north and south of the diapir is used to infer the timing of its emplacement. The northern diapir boundary is characterized by a set of extensional faults oblique to the main Pyrenean trend, while the southern boundary is an extrusive salt sheet that overlays the late Eocene-early Oligocene sequence in three adjacent sub-basins. Salt extrusion occurred due to synorogenic folding. The topography created as salt extruded trapped the arrival of external sediments from the north, blocking the transport pathways southward. Low sedimentation rates southwards allowed for the lateral salt extrusion, advancing southwards from the feeder. The salt sheet emplacement was postdated by Oligocene conglomerates, indicating that the salt extrusion was a relatively quick event.


Introduction
The study of salt diapirism in the South-Central Pyrenees has primarily focused on the internal regions of the South Pyrenean fold-and-thrust belt.During the Lower Cretaceous rifting stage, pre-orogenic diapirs developed in those regions (e.g.Cotiella Basin, Lopez-Mir et al., 2015;2016, Sopeira Basin and Sant Gervàs Basin, Saura et al., 2016and Organyà Basin, Casini et al., 2023) which were subsequently inverted during the Upper Cretaceous-Oligocene contraction (Beaumont et al., 2000;García-Senz, 2002;Martinez-Peña & Casas-Sainz, 2003;Ramos et al., 2020).However, the South Pyrenean external areas also contain several diapirs associated with syn-orogenic sedimentation and the southwards propagation of the Pyrenean deformation (Burrel & Teixell, 2021;Cofrade et al., 2023;Salvany, 1999;Santolaria et al., 2014;Santolaria et al., 2022b).These diapirs offer quality exposures for a direct field approach and provide insights into the relationship between tectonosedimentary processes and diapirism, thus shedding light on how salt interacts with mountain building processes (Duffy et al., 2018;Rowan et al., 2019;Santolaria et al., 2022a).Among these diapirs, the Les Avellanes Diapir, located next to the frontal Pyrenean thrust within the external South-Central Pyrenean region, shows one of the most extensive exposures and is an excellent field example that illustrates salt-sediment interaction during tectonic compression.
One approach to unravel the combination of tectonics, sedimentation, and diapirism is to map the diapir body as well as the adjacent areas using structural and sedimentary data (e.g.Sivas Basin, Turkey, Callot et al., 2016;Tazoult Salt Wall, Morocco, Martín-Martín et al., 2017;Flinders Range, Australia, Rowan et al., 2019;Vidal-Royo et al., 2021) as used in this work in the Southern Pyrenees.Geological maps from the Institut Cartogràfic i Geològic de Catalunya (ICGC, https://www.icgc.cat) in the eastern South Pyrenean area (1:25000 scale) (ICGC Figuerola de Meià sheet, 2007;ICGC Àger sheet, 2008; ICGC Os de Balaguer sheet, 2010 and ICGC Camarasa sheet, 2014), and the Mapa Geológico de España (1:50000 scale) (Os de Balaguer sheet by Teixell & Barnolas, 1996; Artesa de Segre sheet by Saula et al., 2000), as well as maps published in other geological reports (i.e.Burrel & Teixell, 2021;Lopez-Mir et al., 2016;Muñoz et al., 2018), have created an extensive cartographic database.However, geological mapping must be appropriate to the scale of the features portrayed to illustrate the range of geometries and structures produced during their geological evolution (Borradaile, 2015).Since diapirs are structures that can exhibit severe structural and stratigraphic changes in their surroundings regions (Roca et al., 2021;Rowan et al., 2019), it is necessary to improve current maps to reach the optimal scale required to observe saltsediment interactions accurately.
In this regard, the Les Avellanes Diapir is located in a region of intense deformation where the pre-orogenic and syn-orogenic host sequence outcrops along compressive structures, constraining the diapir's evolution.At the SW boundary of the Les Avellanes Diapir, the salt body covers the syn-orogenic late Eocene-early Oligocene sequence which records the extrusion of the salt as an extrusive (subaerial) advancing salt sheet (Cofrade et al., 2023).This contribution presents a detailed geological map of the Les Avellanes Diapir (1:32000) that showcases the complete extension of the allochthonous salt-bedrock contact and the internal architecture of this diapir.The map combines structural, sedimentological, and stratigraphic observations to deduce the diapir geometry and evolution.
Specifically, the South-Central Pyrenean fold-andthrust belt is formed of three main thrust sheets branching southwards from a sole thrust running along the Upper Triassic detachment and emplaced over the Eocene-Oligocene evaporites of the Ebro Foreland Basin (Figure 1C) (Muñoz, 1992;Vergés et al., 1995).These are, from north to south, the Bóixols-Cotiella Thrust Sheet, the Montsec-Peña Montañesa Thrust Sheet, and the Serres Marginals Thrust Sheet, emplaced during the late Cretaceous, the Paleogene-Eocene, and the late Eocene-Oligocene, respectively (Beaumont et al., 2000;Cruset et al., 2020;Garcés et al., 2020;Muñoz et al., 2013;Muñoz-López et al., 2022;Teixell & Muñoz, 2000).These three thrust sheets form an orogenic salient flanked by highly oblique tectonic structures (N-S to E-W) (Figure 1B).Differential tectonic transport displacement alongstrike caused the gradual formation of the orogenic curvature (Garcés et al., 2020;Muñoz et al., 2013;Sussman et al., 2004).The larger tectonic transport of these thrust sheets was caused by the superposition of the two predominant décollement levels in the Pyrenees: the Upper-Middle Triassic evaporites and the upper Eocene-Oligocene evaporites in the Ebro Foreland Basin (Muñoz et al., 2013;2018;Santolaria et al., 2022b) The Les Avellanes salt Diapir is located at the frontal part of the thrust salient in the Serres Marginals Thrust Sheet.Here, the distribution and mobilization of the Triassic evaporites strongly controlled the mountain-building process, impacting the location, type, and geometry of contractional structures.In the Serres Marginals, Triassic exposures are associated with thrusts, detached folds (Salvany, 1999), and salt diapirs (García-Senz, 2002;McClay et al., 2004;Santolaria et al., 2014).Triassic outcrops are mainly formed of gypsum, carbonates, and mudrocks and salt is not exposed at the surface (Calvet et al., 2004;López-Gómez et al., 2019;Salvany & Bastida, 2004).However, exploration wells and gravimetric maps reveal the presence of subsurface Middle to Upper Triassic salt (Lanaja, 1987, e.g.Monzón-1 well log; Santolaria et al., 2014;Camara & Flinch, 2017;Ayala et al., 2021).These Triassic sequence was sedimented an epicontinental, restrictive marine-coastal evaporitic environment within a polyphase rifting that spans from the late Permian to Middle-Late Triassic associated with the fragmentation of the supercontinent Pangea and the opening of the North Atlantic Ocean (Ortí et al., 2017).In this sequence, there are two main salt intervals, middle Triassic (Ladinian, middle Muschelkalk facies, M2) and upper Triassic (Carnian to Norian, lower and middle Keuper facies, K1 and K2) in age respectively (Figure 2) (Calvet et al., 2004;Camara & Flinch, 2017;Klimowitz & Torrescusa, 1990;Ortí, 1987;Ortí et al., 1996).These salt intervals also contain interbedded sulphates and mudrocks, which are more abundant in the upper part of the Triassic sequence, in the K1-K2 salt interval (Jurado, 1990;Ortí et al., 1996).These two salt intervals are separated by a late Ladinian carbonate layer that corresponds to the upper Muschelkalk facies (M3).Dolerites are also found within the salt sequence, intruding into the Muschelkalk and Keuper intervals as sills and/ or dykes during or slightly after salt deposition (López-Gómez et al., 2019).During diapirism, the mobilization of the Triassic salt caused the detachment and rupture of the intrasalt Muschelkalk (M3) layer.Resulting fragments of the intrasalt carbonate strata, called stringers, behaved as competent bodies in the host salt (Cofrade et al., 2023).These stringers experienced intense deformation while being transported within the salt flow and accumulated at the surface due to salt dissolution.Thus, major Triassic exposures in the area, including the Les Avellanes Diapir body, are interpreted as caprock-like deposits formed by a caprock matrix constituted by dissolution residues (gypsum and mudrocks), that embeds decameter-thick and laterally extensive carbonate stringers and dolerite bodies (Cofrade et al., 2023).Dolerite bodies were also probably transported within the salt flow and accumulated at the surface as salt dissolved.
The Les Avellanes Diapir is surrounded by a southward wedging sedimentary host sequence.This sequence is formed from the erosion and condensed deposition occurring at the southernmost tip of the Pyrenean basin (former location of the Serres Marginals Thrusts Sheet), detached along the Triassic salt and tectonically emplaced over the Ebro Foreland Basin (Garcés et al., 2020;Muñoz et al., 2018).This host sequence spans from the Upper Triassic to the Oligocene and can be subdivided into a pre-orogenic and a syn-orogenic interval (Figure 2).
The pre-orogenic sequence is constituted of an alternation of white marls and dolostones (Norian in age), which is the top of the Keuper facies, K3; followed by well-bedded dolostones and limestones of the Isábena/Imón Fm. (Rhaetian in age).These units are overlain by a marine Jurassic succession (Pocoví, 1978) formed of lower Lias evaporites and breccias, upper Lias marls with interbedded limestones, and Dogger-Malm platform carbonates.
The syn-orogenic sequence in the studied area began with the sedimentation of the Upper Cretaceous sediments, consisting of a basal unit of sandstones and conglomerates from the Adraén Fm. (Santonian in age), which unconformably overlays the Jurassic rocks (Figure 2) (Pocoví, 1978;Ullastre & Masriera, 2004).The marine limestones and calcareous sandstones from the Calcària de les Serres Fm. (Maastrichtian in age) overlie the Adraén Fm.The Upper Cretaceous gradually evolves into a continental siliciclastic-calcareous alternation named the Tremp Group (also known as Garumnian facies), which represents a marine regression that occurred during the Cretaceous-Paleocene boundary (Gómez-Gras et al., 2016).This episode was followed by a new transgression during the early Eocene (early Ypresian times) (Pujalte et al., 2009), represented by the marine Alveolina Limestones characterized by their content of Alveolina (Pocoví, 1978).The basal horizon of this unit is easily recognizable in the field, being useful for regional correlations.South of the diapir, the Alveolina Limestones gradually change into more siliciclastic, shallower facies during the Lutetian, which are almost missing elsewhere in the Serres Marginals.After that, late Eocene-early Oligocene mixed clastic and evaporitic facies covered the structural paleorelief produced by the uplift and ongoing formation of tectonic structures in the Serres Marginals, therefore recording the orogenic deformation at the external Pyrenean wedge (Teixell & Muñoz, 2000) and at the foreland basin (Ramirez-Perez et al., 2023;Santolaria et al., 2022b).Oligocene deposition postdating the contractional structures, buried the fold-and-thrust belt under more than 700 m of conglomerates in the Les Avellanes Diapir area (Fillon et al., 2013).Finally, the Pleistocene erosion sculpted the present relief.

Methods
The geological map of the Les Avellanes Diapir covers an area of approximately 200 km 2 .It is presented at a scale of 1:32000 (Main Map, Figure 3) but the cartography work was done at a detailed scale of 1:10.000.The map was created through several months of fieldwork and is also based on the existing geological maps (ICGC Figuerola de Meià sheet, 2007; ICGC Àger sheet, 2008; ICGC Os de Balaguer sheet, 2010 and ICGC Camarasa sheet, 2014, and Os de Balaguer sheet by Teixell & Barnolas, 1996; Artesa de Segre sheet by Saula et al., 2000).The topographic base comprised of the 1:25000 and 1:5000 maps and orthophotographic aerial images (25 cm pixel maximum resolution) from the ICGC (series 25 cm (OF-25C) v4r0, 2022 flight), as well as a high-resolution DEM (ICGC, HD-DEM, 2 × 2 m (MET-2) v2.0 (2016-2017), https://icgc.cat).Structural data (up to 3000 dip directions/dip angles from bedding surfaces, faults, and foliations) was collected directly on the field with a compass-clinometer, georeferenced with FieldMove software, and digitalized alongside surfaces and stratigraphic contacts with ArcGIS software.Fault symbology is simplified based on stratigraphic constraints to avoid the complexity caused by the inversion, tilting, and/or reactivations during the intense deformation history of the area.The map is accompanied by a stratigraphic key, which is divided into pre-and syn-orogenic lithostratigraphic units and the diapir units, as well as a stratigraphical sketch summarizing the relationships between these units in the area.Quaternary deposits have not been included on the map.

Structural domains of the Les Avellanes Diapir adjacent areas
The Les Avellanes Diapir has an irregular shape produced by the combination of tectonic structures and diapirism, resulting in different structural domains.These are described below regarding their location relative to the diapir boundaries (Main Map and Figure 3).
Along the northern boundary of the diapir, the contact between the diapir body and the host sedimentary sequence runs along the trace of two sets of faults, NW-SE and SW-NE, oblique to the main compressional structures in this area (W-E).The north-central part of the Les Avellanes Diapir exposure is located in the intersection of both sets of faults.Towards the NE and NW areas, these faults extend outwards and cut through the folded Jurassic to Eocene sequence, resulting in grabens (Main Map, Figure 3).Accordingly, these faults are evaluated as extensional, regardless of the relationship observed along the diapir contact, which resulted from the diapir emplacement.They were formed during the Eocene-Oligocene since Oligocene conglomerates filled the grabens and postdate the faults at the NE and NW areas.
Adjacent to the western boundary of the Les Avellanes Diapir, the structure is characterized by a set of NW-SE trending folds affecting the salt-detached Jurassic to Oligocene Serres Marginals sequence.The fold wavelength gradually decreases southwards as the detached sequence becomes thinner.Broad synclines are tectonically imbricated, separated by thrusts and back-thrusts with the Keuper facies outcropping along the base of the hangingwall (e.g.along the Os de Balaguer back-thrust, Figure 3 and Main Map).In addition, the Ypresian Alveolina Limestones exhibit rotational offlap to onlap growth strata architectures close to the fold limbs (Cofrade et al., 2023), indicating that folds grew during the early Eocene.Some anticlines, such as the Canelles Anticline, are box-shaped with wide hinges and Triassic rocks cropping out in the eroded cores (Figure 3, Main Map).Furthermore, the Canelles Anticline is intersected by oblique extensional faults similar to those at the northern diapir boundary.These faults displace stepping blocks downwards at the diapir contact (between Tartareu and Vilamajor villages, Main Map).
The eastern to south-eastern boundary of the Les Avellanes Diapir is dominated by the Montroig Thrust, which has a W-E strike and continuously outcrops eastwards up to the Segre River (Figure 3).This thrust was emplaced out of sequence, after the formation of the Serres Marginals Thrust (Teixell & Muñoz, 2000).The hangingwall of the Montroig Thrust, with upper Triassic rocks at its base, exposes a complete Jurassic sequence up to 800 m thick unconformably overlaid by the Upper Cretaceous units.Both of these units are truncated by NW-SE extensional faults, mainly dipping towards the NE.The footwall of the Montroig Thrust corresponds to the eastern termination of the Vilanova-Sant Llorenç unit, which is a detached tectonic unit emplaced over the Ebro Foreland Basin.This unit is characterized by a late Eocene to early Oligocene thin and incomplete stratigraphic sequence relative to the northern Serres Marginals cover, deformed by an imbricated system of shortwavelength folds and thrusts (Main Map).The thin stratigraphy in this unit is related to a condensed deposition and the presence of internal unconformities found at the southern termination of the Pyrenean Basin.

Stratigraphical, sedimentological, and petrological constraints on diapir emplacement
The Tartareu, Os de Balaguer, and Montroig subbasins show a similar late Eocene to Oligocene sedimentary record that constrains the salt sheet emplacement.These sub-basins are adjacent to the diapir body and occupy structural lows formed by folding or faulting.Each section can be divided into two main intervals separated by erosional unconformities (Figure 4): The lower interval is characterized by evaporitic facies that are stratigraphically enclosed by late Eocene-early Oligocene alluvial conglomerates.Evaporitic units (Figure 5A) lay on top of conglomerates at the core of the Os de Balaguer and the Tartareu sub-basins, and in the Montroig Thrust footwall.These units overlap the previous stratigraphy postdating tectonic deformation (Main Map) (Figure 4, sections 3 and 4).The enclosing alluvial conglomerates rapidly evolve laterally from massive, proximal facies to distal facies, where they are intercalated with finebedded sandstones and mudrocks (Figure 4, sections 1, 2, and 5 and Figure 5B).Clast composition, mainly limestones from the Jurassic, Upper Cretaceous, Paleocene, and lower Eocene units, indicates that   3 and in the Main Map).In all studied locations the salt sheet extrusion covers a conglomeratic/ breccia deposit (debrites) derived from the erosion of the diapir and emplaced under it.This supports the extension and emplacement of the diapir body as a salt sheet that advanced southwards, originated along the northern boundary where the salt migrated towards the surface.
conglomerates were formed by the erosion of the Serres Marginals reliefs without any major contribution from the Pyrenean hinterland.Triassic clasts are also present, indicating the erosion of the diapir, which was exposed during the progressive filling of these sub-basins (Figure 5C).The deposit is organized in channel-like bodies with cross-bedding at the base interbedded with red mudrocks (see also Figure 4).(C).Close view from the facies described in Figure 5B.These conglomerates present a polymictic composition mainly derived from the erosion of the Serres Marginals units, contrasting with the northern conglomeratic facies.Pink arrows point towards Muschelkalk clasts, green arrows to Calcàrea de les Serres clasts, and orange arrows to Alveolina Limestones clasts (see Figure 5F).(D) Depositional breccias and conglomerates (debrites) exposed along the Montroig sub-basin (See also Figure 4).The deposit is typically organized in inverse graded beds with erosional bases.Paleocurrent senses in addition of compositions support that these breccias resulted from the erosion, mass-transport, and sedimentation of the diapir body rocks (Ghassemi & Roustaei, 2021).This deposit is overridden by the salt and thus lies beneath the diapir body (megabreccia) in all the studied sections (Figure 4).(E) Close-up view of the facies described in Figure 5D.These breccias are monomictic, composed of fragments derived from the Muschelkalk carbonate stringers, and clast-supported.(F) Oligocene conglomerates at the northern diapir boundary, postdating the Les Avellanes lateral extrusion.There is a prevalence of metamorphic and igneous rock fragments derived from the Pyrenean Axial Zone.(G) Banded gypsum facies located along the northern diapir exposure.(H) Megabreccia facies.Blocks of deformed, laminated gypsum floating in a fine matrix with gypsum nodules.This matrix contains a high percentage of mudrocks and carbonate milimeter-sized inclusions which gives a 'dirty' aspect.
The upper interval is characterized by breccias and conglomerates sourced exclusively from the erosion of the diapir units, since they are made of gypsum, Muschelkalk carbonates, and dolerite clasts (Figure 5D and E).The internal architecture of this interval shows an upwards transition from normal-graded to inverse-graded beds, with a level of significantly bigger blocks, up to 60 cm in diameter, of dolerites and Muschelkalk carbonates with gypsum clasts, accumulated atop.The base of this interval is an unconformity caused by the erosion of the previous sequence, except in the Tartareu sub-basin (Figure 4, section 1) where these breccias are interbedded with the sub-basin infill (lower interval).In all studied locations this upper breccia interval is directly overlain by the diapir body (Figure 4).
Along the northwestern and northeastern diapir boundaries, Oligocene conglomerates and sandstones overlap the diapir body and preserve the extensional faults that bound the northern contact (in Vilamajor village and close to the Pantà de Camarasa, Main Map).A similar relationship is observed in a section of the southern boundary, between Os de Balaguer and Vilanova de la Sal villages.However, these three locations are the only ones where this relationship occurs, as generally the diapir is emplaced on top of the clastic facies.On the other hand, clast composition is well-differentiated in northern and southern areas, since the northern conglomerates contain abundant fragments of metamorphic and plutonic rocks transported from the Axial Zone (Figure 5F).
The Muschelkalk stringers are formed of two clearly different stratigraphic intervals: (1) a basal alternation of laminated limestones and marls overlaid by, (2) well-bedded, tabular limestones with minor intercalated thin levels of marls, which allow to establish the stratigraphic polarity of these stringers.Across the diapir exposure, the presence and disposition of the M3 stringers vary from north to south (Main map).In the northern region, the abundance of stringers is lower in comparison to the central and southern parts where they are relatively accumulated.In the northern area, they are located towards the NW, between Tartareu and Vilamajor villages, and the NE, close to Santa Linya (Main Map), where they are vertically emplaced, subparallel to the diapir contact.Conversely, in the central part where they are concentrated, around the Les Avellanes village (Main Map), they are horizontally stacked or overturned.Along the southern boundary, they are imbricated and primarily show a southwards vergence.
Likewise, the caprock matrix embedding the stringers also exhibits different facies from north to south.In the north, the caprock matrix facies are predominantly composed of banded gypsum with minor content of mudrocks and carbonates (Figure 5G).However, towards the south, along with the increasing number of outcropping M3 stringers, the gypsum is significantly more altered and mixed with accumulations of mudrocks and carbonates.These caprock matrix facies present deformation bands with mylonitic fabrics (named foliated gypsum, Main Map) suggesting the combination of brittle and ductile deformation related to the diapir evolution.In addition, in the Os de Balaguer sub-basin and in the Montroig sub-basin, the base of the diapir body is characterized by a megabreccia featuring meter to decameter blocks of gypsum and Muschelkalk stringers surrounded by a fine matrix, partially cemented with sulphates (Figure 5H).This megabreccia is emplaced over the late Eocene-early Oligocene sedimentary units (Figure 4) and shows a characteristic sequence where ductile shear deformation (foliations and lineations) is preferentially concentrated along its base.The megabreccia was interpreted as a dissolution breccia, or a debrite-like deposit created by the disaggregation of the caprock at the frontal lobe of the salt sheet and subsequently overrode by the spreading salt (Cofrade et al., 2023).The deformation was probably produced during the forward advance of the salt sheet as an intrasalt basal shear-band (Cofrade et al., 2023).

Discussion
The structural relationships between the diapir and the main tectonic elements in the area, as well as the stratigraphical, sedimentological, and petrological observations, allow us to differentiate between the two parts that form the Les Avellanes Diapir geometry, the extrusive salt sheet and the columnar feeder diapir, as well as the timing of its emplacement (Main Map).

The Les Avellanes Diapir geometry
The extensional faults, oblique to the contractive structures (Figure 3), form the northern boundary of the diapir.Late Eocene-Oligocene sediments fill the associated grabens and ultimately postdate the extensional deformation, which is dated to the late Eocene-Oligocene.These faults are associated with the collapse of the overburden caused by the migration of the salt towards the diapir stem, and therefore also constrain diapirism to the late Eocene-Oligocene.Thus, the northern part of the diapir exposure is interpreted as the Les Avellanes Diapir feeder.The mobilization of the salt in the study area was probably triggered by the onset of synorogenic deformation, which started along the Serres Marginals during early Ypresian times (Muñoz, 1992;Muñoz, 2017), and is recorded by the existence of growth strata architectures in the Alveolina Limestones.The subsequent erosion of the elevated salt-cored contractive structures exposed the salt, initiating diapirism.This is constrained by the regional erosive surface at the base of the late Eocene-Oligocene series, that records the incision of the drainage network during the onset of orogenic deformation in the Serres Marginals.
The late Eocene-early Oligocene facies below the megabreccia in the Os de Balaguer sub-basin (Figure 4, sections 2, 3 and 4) are interpreted as the sedimentological record of the advance of an extrusive salt sheet laterally spreading by gravity from the feeder (Cofrade et al., 2023).The same facies, overlain by the megabreccia, are recorded in the Tartareu and Montroig sub-basins (Figure 4), and therefore, the salt sheet basal contact can be interpreted by the stratigraphic correlation of these facies along the diapir boundary.The trace and location of this contact suggest that a salt sheet advanced southward covering younger stratigraphy, although the precise extension of the coverage is not recorded.
The two parts of the Les Avellanes Diapir geometry, the diapir feeder in the north and the salt sheet in the south, are also supported by the structural configuration of the M3 stringers and the composition of the diapir body since the internal architecture of a salt structure reflects its evolution (Jackson & Hudec, 2017;Talbot & Aftabi, 2004;Talbot & Jackson, 1987).On the one hand, the arrangement formed by the stringers within the diapir body is the product of different salt flow kinematics during different stages of diapirism and can be preserved in the dissolution residue of an extruding diapir (Alsop et al., 2015;Amri et al., 2020;Jackson & Hudec, 2017;Talbot & Jackson, 1987).The stringers in the Les Avellanes diapir are subvertical along the northern contact and horizontally stacked in the central and southern areas.Therefore, their configuration reflects the vertical flow that characterizes a diapir stem (Sarkarinejad et al., 2018) and the subhorizontal flow during the advance of the salt sheet, respectively (Ghassemi & Roustaei, 2021;Jackson & Hudec, 2017).On the other hand, the residue left after the salt dissolution that surrounds the stringers (Cofrade et al., 2023) is made of gypsum with different proportions of embedded mudrocks and other insoluble components, and gradually transforms from north to south.In the northern area, where the feeder is inferred to be, the dominant lithology is gypsum (gypsum caprock, Main Map).However, towards the south, the content of mudrocks and carbonate within the gypsum (mixed caprock, Main Map) increases.The increasing percentage of mudrocks in the caprock matrix is in agreement with a higher degree of alteration, which is usually present at the frontal part of an extrusive salt sheet versus its feeding area, as the salt sheet further transforms during its advance (Nekouei & Zarei, 2016;Závada et al., 2021).Nevertheless, the gradual increase in mudrocks and carbonates towards the salt sheet frontal area may reflect lithological variations in the depositional Middle to Upper Triassic salt sequence.Diapirs show a concentrical internal zonation inherited from the stratigraphical architecture of the salt source sequence where lower, older units are in the core of the structure surrounded by younger, upper units.As salt extrudes laterally to form a salt sheet/namakier (Ghassemi & Roustaei, 2021)., this zonation rotates, so the older units occupy the core of the allochthonous sheet and the younger upper units tend to be located along its front (Dooley et al., 2015;Talbot & Aftabi, 2004;Talbot & Pohjola, 2009).Therefore, since the upper Keuper interval (K2-K3) contains more mudrocks interbeds, an enrichment of this lithology within the caprock matrix is expected along the frontal part of the salt sheet.Thus, given the sedimentological and stratigraphical relations observed along the southern boundary of the Les Avellanes Diapir, as well as the internal architecture of the diapir body, the southern part of the diapir exposure is interpreted as an extrusive salt sheet/ namakier that extruded from a feeder diapir.

Relative timing of the salt sheet emplacement
The sedimentological and stratigraphical relationships between the salt extrusion and the deposition of the Oligocene conglomerates constrain the timing for the emplacement of the salt sheet.The different clast composition observed in the Oligocene conglomerates north and south of the diapir is explained by the existence of a barrier formed by the dynamic bulge over the feeder, deflecting the southwards arrival of sediments sourced in the Axial Zone.The isolation of the subbasins from the regional depositional pathways (NE to SW, Garcés et al., 2020), reduced the sedimentation rate in the southern sub-basins during the salt extrusion, so the salt sheet advance was registered in these areas.Oligocene conglomerates postdate the feeder (NW and NE areas) and part of the southern boundary (east of the Os de Balaguer village).These conglomerates also postdate the Pyrenean structures and thus, the salt sheet extrusion is dated to the late Eocene-early Oligocene.

Concluding remarks
Mapped structural features, together with the stratigraphical, sedimentological, and petrological correlations support the hypothesis that the Les Avellanes Diapir geometry resulted from the combination of two different salt structures, an extrusive salt sheet, and a feeder diapir located along the northern diapir exposure from where the salt spread laterally and advanced southwards, covering the southern low areas.
The Triassic salt migrated and accumulated in the Serres Marginals during the Eocene-Oligocene due to the Pyrenean deformation.The detached anticline hinges collapsed, as recorded in the system of conjugate extensional faults oblique to the contractive structures, facilitating the expulsion of the Triassic salt towards the surface in the northern area, thus interpreted as the feeder.A salt sheet emplacement is supported by the sedimentological and stratigraphical interpretation of the facies overlaid by the diapir body, as well as the internal architecture of the diapir body, so the salt sheet basal contact has been traced accordingly in the Main Map.Its emplacement occurred relatively fast during the late Eocene-early Oligocene and was later postdated by the Oligocene sedimentation.

Software
FieldMove® (Petroleum Experts) application was used for field-collected data and sketching, running in a high-performance tablet device with a GPS system integrated with a typical resolution of + -2 m.Orthoimages (PNOA-IGN Plan Nacional de Ortofotografía Aérea -Instituto Geográfico Nacional, http://ign.es and ICGC 25 cm/px resolution, https:// www.icgc.cat),DEM (LiDAR-based, 2 × 2 m resolution, IGCG, https://www.icgc.cat),and geological and topographic maps were projected in the same device and used during fieldwork (MAGNA 1:50000, http://info.igme.es;and IGCG 1:25000 series, https:// www.icgc.cat).Data were then plotted and transformed into georeferenced vector files using the 3D software MOVE® (Petroleum Experts), then imported into ArcGIS Pro software where the Main Map was constructed, also importing the mentioned cartographic database to fine-tuning the geological contacts and to improve accuracy.Symbology was designed according to a standard geological library and adapted when necessary.

Figure 2 .
Figure 2. Sketch representing the stratigraphy of the Les Avellanes Diapir and adjacent areas, not-to-scale.Geometries are oversimplified to understand stratigraphic relationships.The thicknesses of the units are approximations (see sources from Cofrade et al., 2023).

Figure 3 .
Figure 3. Structural map of the Les Avellanes Diapir area and surroundings with Triassic exposures.1. Tartareu sub-basin, 2. Os de Balaguer sub-basin, and 3. Montroig sub-basin.See also the location of the Main Map (red square).

Figure 4 .
Figure 4. Stratigraphical sections and correlation (see their location in the map at the right bottom corner of the figure.Subbasins locations are in Figure3and in the Main Map).In all studied locations the salt sheet extrusion covers a conglomeratic/ breccia deposit (debrites) derived from the erosion of the diapir and emplaced under it.This supports the extension and emplacement of the diapir body as a salt sheet that advanced southwards, originated along the northern boundary where the salt migrated towards the surface.

Figure 5 .
Figure 5. (A).Tabular gypsum beds at the Tartareu sub-basin with minor intercalation of mudrocks.Gypsum nodules are also observed.White arrow shows stratigraphic polarity.(B) Oligocene conglomerates at the southern diapir boundary (Os de Balaguer sub-basin).The deposit is organized in channel-like bodies with cross-bedding at the base interbedded with red mudrocks (see also Figure4).(C).Close view from the facies described in Figure5B.These conglomerates present a polymictic composition mainly derived from the erosion of the Serres Marginals units, contrasting with the northern conglomeratic facies.Pink arrows point towards Muschelkalk clasts, green arrows to Calcàrea de les Serres clasts, and orange arrows to Alveolina Limestones clasts (see Figure5F).(D) Depositional breccias and conglomerates (debrites) exposed along the Montroig sub-basin (See also Figure4).The deposit is typically organized in inverse graded beds with erosional bases.Paleocurrent senses in addition of compositions support that these breccias resulted from the erosion, mass-transport, and sedimentation of the diapir body rocks(Ghassemi & Roustaei, 2021).This deposit is overridden by the salt and thus lies beneath the diapir body (megabreccia) in all the studied sections (Figure4).(E) Close-up view of the facies described in Figure5D.These breccias are monomictic, composed of fragments derived from the Muschelkalk carbonate stringers, and clast-supported.(F) Oligocene conglomerates at the northern diapir boundary, postdating the Les Avellanes lateral extrusion.There is a prevalence of metamorphic and igneous rock fragments derived from the Pyrenean Axial Zone.(G) Banded gypsum facies located along the northern diapir exposure.(H) Megabreccia facies.Blocks of deformed, laminated gypsum floating in a fine matrix with gypsum nodules.This matrix contains a high percentage of mudrocks and carbonate milimeter-sized inclusions which gives a 'dirty' aspect.