The Basque – Cantabrian Pyrenees: report of data analysis

This report presents the analysis of a data set used in the PhD thesis of J. Miró to propose and test new extensional concepts for the Basque – Cantabrian Pyrenees [e.g. multistage and polyphase (Péron-Pinvidic and Manatschal, 2009)] and their role on the subsequent inversion of these basins. This report is part of a PhD that is in the framework of the OROGEN Project (www.orogen-project.com). The aim of the OROGEN Project is to make all data sets used and produced available for the community, the reason why this report is part of this special volume.

Datasets are fundamental to propose, discuss, improve, and test new tectonic concepts and models. In the case of the Basque – Cantabrian Pyrenees, many datasets have been published during the last decades supporting the most recent interpretations. These datasets include: magnetic data (Pedreira et al., 2007); gravity data (Pedrera et al., 2017; García-Senz et al., 2019); velocity models (Pedreira et al., 2003); seismic refraction (Ruiz et al., 2017); thermochronology data (Fillon et al., 2016), and magnetic susceptibility data (Soto et al., 2007; Soto et al., 2008). Some studies using reflection seismic data have been published recently (e.g.Carola et al., 2015; Pedrera et al., 2017) but a continuous seismic line crossing the entire Basque – Cantabrian Pyrenees has not been assembled and published yet. In this report we present a composite reflection seismic line crossing the whole Basque – Cantabrian Pyrenees from the Ebro Basin in the south to the offshore Landes High in the north. This composite line combined with available drill hole data from the area and tomographic data provides a dataset that forms the foundation for new interpretations and models of the area as well as for their calibration. To localize and help the understanding of the data presented (i.e. seismic lines and drill holes) a thematic map of the Basque – Cantabrian Pyrenees has been compiled based on the revision of previous geological maps, as well as field observations. The aim is to analyse the tectono-sedimentary evolution of the area in order to propose a new, simple and clear legend to understand the tectono-stratigraphic evolution of the area. Although this map is mainly built on existing data and does not pretend to be new, the compilation as presented in the map defines and describes the distribution of tectono-stratigraphic units in a new way and forms the foundation to support a new set of models and ideas presented in this volume and other upcoming publications.

The Pyrenees, geologically speaking, constitute the westernmost branch of the Alpine orogenic system that connects from the Pyrenees through the Alps and the Carpathians to the Himalayas (Carola, 2014). Within the Pyrenees, the stratigraphic record and the structural style changes significantly along strike (Carola et al., 2013) and as a result, different physiographic units can be identified along the mountain belt. This fact has been captured in the literature with a wide range of names, both geographical and geological, being used to refer to the same areas (Fig. 1) (see Barnolas and Pujalte, 2004 for an extended revision of terminology). In this work, the area located between the Pamplona System [(Lescoutre and Manatschal 2020 (in this volume)] to the east and the Asturian Massif (also referred to as Cantabrian Mountains) to the west, will be named as the Basque – Cantabrian Pyrenees (Fig. 1). The traditional term Basque – Cantabrian Basin will be used to refer to the Mesozoic extensional basins that were formed between the Iberia, Ebro and Eurasian plate boundaries in this domain.

The Basque – Cantabrian Pyrenees are part of the doubly verging Pyrenean Orogen, which resulted from the Late Cretaceous to Cenozoic collision between the Iberia and Eurasian plates (Puigdefàbregas et al., 1992; Muñoz, 2002; Vergés et al., 2002) (Fig. 2a). The Alpine orogeny inverted the Mesozoic basins developed in the Bay of Biscay– Pyrenean realm, where a segmented hyperextended system formed the limit between the Iberian and European plates during the Late Jurassic to the Early Cretaceous. The transtensional and extensional evolution led to extreme crustal thinning (< 10 km) and locally to mantle exhumation in some of those basins (e.g. Basque – Cantabrian Basin and Mauleón Basin), whereas the Bay of Biscay further to the west reached the formation of oceanic crust (Sibuet et al., 2004; Tavani et al., 2018). However, the so-called Basque – Cantabrian Basin comprises, in fact, up to three Mesozoic sub-basins (Fig. 2a): (1) the Basque Cantabrian Basin sensu strictu, a NW– SE elongated basin with a more than 12 km thick succession of Upper Jurassic to Cenomanian sediments forming, at present, the Bilbao Anticlinorium (Lotze, 1960; Brinkmann and Lögters, 1968; Pujalte et al., 1989; García-Mondéjar et al., 1996; Barnolas and Pujalte, 2004; García-Mondéjar et al., 2005); (2) the Cabuerniga Basin, a triangular shape basin with at least 3 km of Upper Jurassic to Lower Cretaceous sediments located to the north-west of the Basque – Cantabrian Pyrenees (Pujalte, 1979, 1981; García-Mondéjar, 1982; Espina, 1994); and (3) the Polientes Basin, located to the south-west of the area with a sedimentary succession of more than 4 km of sediments, Upper Jurassic to Lower Cretaceous in age (Garcia de Cortazar and Pujalte, 1982; Pujalte, 1982; Espina, 1996). The formation and reactivation of the Basque – Cantabrian Basin is the subject of a long debate between different schools, as detailed below.

Three main extensional rift basin models can be invoked to explain the formation of the North Iberian rift system: the classical low-β basin (i.e. North Sea– type basins), the strike-slip transtensional basin (i.e. San Andreas system) and the high-β basin model (i.e. hyperextended Iberian margin type system) (Fig. 2b).

In the Basque-Cantabrian Pyrenees all the previous models have been proposed to explain the formation and evolution of the extensional system. Quintana et al. (2015), Pedrera et al. (2017) and García-Senz et al. (2019) proposed that the basins were controlled by a main south-dipping normal fault located to the north. Consequently, they interpreted the Mesozoic succession in the Bilbao Anticlinorium area as the result of an extensional rollover related to the major fault. A second group of authors (e.g.García-Mondéjar, 1989; García-Mondéjar et al., 1996, 2018) suggested a transtensional strike-slip model to explain the Mesozoic depocenter of the Basque – Cantabrian Basin. Moreover, Cámara (1997, 2017) introduced salt tectonics in the same kinematic model to explain the presence of diapirs and salt walls. The latter model proposed a sort of salt-controlled mini basins instead of basement-controlled half-graben basins. A third interpretation, inspired by the recent publications addressing the Newfoundland and West Iberia margins (Péron-Pinvidic and Manatschal, 2009) and the Arzacq-Mauleon Basin (Jammes et al., 2009; Lagabrielle et al., 2010), explains the Mesozoic evolution of the Bay of Biscay by a combination of the classical extensional model and the hyperextended model (Roca et al., 2011). Following this interpretation, high-angle normal faults controlled the deformation in the brittle parts of the crust and mantle while deformation was distributed and decoupled in the ductile levels. In areas where crust was notably thinner due to high lithospheric extension, the basin structure was asymmetric and controlled by extensional detachment faults dipping to the north and cutting the entire residual < 10 km thick brittle crust (e.g. Basque – Cantabrian Basin).

For the subsequent reactivation of the depocenters located in the Basque – Cantabrian Basin, different models have been proposed, being grouped in: (1) thin-skinned models, (2) basement-involved thin-skinned models and (3) thick-skinned strike-slip models.

The first thin-skinned model (Hernaiz, 1994 and reference therein; Serrano et al., 1994) was inspired on the south-western part of the Basque – Cantabrian Pyrenees related to the frontal imbricates (Folded Band) and the Burgalesa Platform (Fig. 3). This model argued for Jurassic to Lower Cretaceous sediments detached from the Paleozoic basement along the Triassic evaporites and transported several kilometres to the south. A more recent study proposed a thin-skin reactivation for the entire Basque – Cantabrian Basin, suggesting a large transport of the Mesozoic succession to the south on top of the Upper Triassic evaporites (Muñoz, 2019).

The second group of models assumed the involvement of the basement even in the frontal imbricates. Some authors argued for relatively thin basement thrust sheets, suggesting a basement-involved thin-skinned deformation (Espina, 1996; Quintana et al., 2015). These authors recognized the presence of Triassic evaporites, which formed salt accumulations, such as diapirs, and acting as a partial decoupling horizon. Opposed to this interpretation, Pedrera et al., 2017 and García-Senz et al., 2019 suggested the classical thick-skinned deformation style, where thrusts are cutting the entire upper crust and some of them even the lower crust, implying a minor role of the salt during compressional reactivation. These models assumed that the compressional phase was controlled by the reactivation of inherited extensional structures describing a complex set of basement blocks, defining the architecture observed on the sedimentary cover.

The third set of models explained the contractional evolution of the Basque – Cantabrian Pyrenees as a reactivation of the extensional faults bounding the half-grabens with a significant component of strike-slip deformation, either with an important role of the Upper Triassic salt (Tavani et al., 2013; Cámara, 2017) or a minor influence of salt (García-Mondéjar et al., 1996; García-Mondéjar et al., 2018).

The composite seismic line presented in this report includes nine seismic lines provided by the Spanish Geological Survey (IGME) (see Appendix 1 for details): BB-1, PR-42, PR-42-P, S-82-14, VA-4, BR-9, BR-61, BR85-25 and 615. They were acquired between the 70’s and the 90’s during different hydrocarbon exploration campaigns after the first oil discoveries in the Basque – Cantabrian Pyrenees, such as the Ayoluengo oil field in 1963. Nine drill holes drilled during the same period for hydrocarbon exploration purposes and also provided by IGME have been projected to the composite seismic line (see Appendix 2 for more information): Rojas NE-1, Navajo-1, Trespaderne-1, La Hoz-2, Zuazo-1, Ubidea-1, Arratia-1, Cormoran-1 and Vizcaya B-1. In order to project the boreholes that are in depth in a seismic line that is in time, a simple time-depth conversion to the seismic section has been done by using a generic velocity of 4800 m/s.

A compilation of several geological maps as well as maps from other thesis and publications have been compiled to present a new global thematic map of the entire Basque – Cantabrian Pyrenees (see Appendix 3 for the references of all the maps used). The simple but consistent organization of the units allows to document the tectono-stratigraphic evolution of the area from Triassic to present-day. The Figure 4 shows the polygons of the areas covered by the maps compiled in this report and the location of the drill hole data as well as seismic data presented.

The extensional models proposed to explain the formation of the Basque – Cantabrian Basin summarized above need to be further discussed in the light of the new ideas developed on the formation and evolution of rifted margins. Therefore, new thematic maps need to be developed analysing the tectono-stratigraphic sequences in order to accompany the new interpretations and concepts. Many studies in the literature have addressed and described the stratigraphy observed in the Basque – Cantabrian Pyrenees and the structures affecting the area (Triassic: García-Mondéjar et al., 1986; Espina, 1997; Gómez et al., 2007. Jurassic: Quesada et al., 1991, 1993; Aurell et al., 2003; Quesada et al., 2005. Early Cretaceous: Pujalte, 1979, 1981, 1982; García-Mondéjar, 1989; García-Mondéjar et al., 1996; Hernández et al., 1999; Bodego et al., 2015. Late Cretaceous: Pujalte et al., 1989; Rodriguez-Lázaro et al., 1998; Pujalte et al., 2000; Castañares et al., 2001; López-Horgue et al., 2014. Cenozoic: Riba and Jurado, 1992; Quintà and Tavani, 2012, among many others) and the geological knowledge coming out has been captured in multiple geological maps (e.g.Carola et al., 2015; Ábalos, 2016).

In this section, the tectonic evolution of the area and its stratigraphic response is summarized (Fig. 5). Therefore, a re-organization of units is proposed that reflects and allows to address further interpretations based on the recent developed concepts of rifted margins pointed out before (i.e. multistage and polyphase evolution). As a result, the thematic map presented in this work aims to be the basis for the upcoming studies addressing the formation and reactivation of the Basque – Cantabrian Pyrenees, as well as surrounding areas in the OROGEN project. Some of these interpretations that are based on the proposed legend of this report can be found in this volume [(Lescoutre and Manatschal, 2020 (in this volume) and Cadenas et al. (in this volume)] or they are related to ongoing investigations (Lescoutre, 2019). This map, therefore, represents a modification of existing maps combined with the revision of existing data sets as well as more than 1200 new field observations (e.g. dip-data measurements, lineation and contact observations).

The presented map (Appendix 4) is a graphic representation at 1:300 000 scale covering an area of 33 300 km² limited to the north by the Cantabrian Sea, to the south by the Ebro and Duero Tertiary Basins and to the east and west by Paleozoic massifs (Basque Massif and Asturian Massif, respectively). The organization of the legend has been defined based on the main tectonic phases recognized in the study area and its stratigraphic response (Fig. 5), as it is explained hereafter, rather than by geological ages (classical geological maps).

The oldest unit (i.e. basement, pre-rift 1) groups all the rocks from Paleozoic up to Permian. As the focus of the map is the Alpine cycle (i.e. Mesozoic to present-day), the previous events are not addressed in detail. The overlying Permo-Mesozoic to Cenozoic succession has been subdivided into syn-rift, post-rift and syn-compressional sequences, which can be further subdivided. The first sequence is the Lower Triassic extensional phase (i.e. Syn-rift 1), characterized by continental clastic sediments (Robles and Pujalte, 2004) and the late Triassic evaporites (i.e. Post-rift 1) (Serrano and Martínez del Olmo, 2004), which have been identified as a separate sequence due to its importance for the subsequent tectonic evolution. During Early to Middle Jurassic, a period of relative tectonic quiescence governed the northern Iberian domain (i.e. Post-rift 2). This phase is characterized by large carbonate platforms and marly successions defined as a unit itself in the map (Aurell et al., 2003; Quesada et al., 2005). The Late Jurassic to Barremian is characterized in the northern Iberian domain by a large-scale transtensional episode (Rat, 1988; Espina, 1994; Robles et al., 1996), resulting in a syn-rift sequence localized in small, but kilometre-deep, depocenters (i.e. Syn-rift 2) (e.g. Asturian Basin, Cadenas et al., 2018). The previous event contrasts with the next Aptian to Middle Cenomanian event that is linked to the major extensional phase structuring the Basque-Cantabrian Pyrenees. The sediments linked to the major extensional event were deposited leading to widely distributed depocenters that are either overlapping or forming away from the previous Late Jurassic to Barremian depocenters (Robles et al., 1988; García-Mondéjar et al., 1996; Lescoutre, 2019). The Aptian to Middle Albian extensional period is comprised in two units: the Aptian to Middle Albian succession (i.e. Syn-rift 3) and the Upper Albian to Middle Cenomanian (i.e. Syn-rift 4). The latter is unconformably overlying the previous sediments (Peropadre et al., 2012) and it is related to a phase of mantle exhumation (Tugend et al., 2014; Pedrera et al., 2017). A post-rift (i.e. Post-rift 3) sequence recording thermal subsidence of Late Cenomanian to Early Santonian age is observed and manifested by shallow carbonate platforms established in the margins of the main depocenters, whereas a marly sedimentation was occurring basinwards (Floquet, 2004). Onset of convergence occurred at Middle Santonian time and coincided with a reestablishment of clastic sedimentation (i.e. Syn-compression 1). Finally, the Cenozoic period (i.e. Syn-compression 2) is known as the major compressional phase (mostly Eocene to Miocene) (Puigdefàbregas et al., 1992; Pedrera et al., 2017; Martín-Chivelet et al., 2019).

A SSW– NNE composite seismic section from the Ebro Basin in the south to the Landes High offshore in the north is presented in Figure 6. It provides a first order view of the architecture and nature of depocenters composing the Basque – Cantabrian Pyrenees on its central segment. Moreover, results of refraction seismic surveys (Fernández-Viejo et al., 2000; Pedreira et al., 2003; Ruiz et al., 2017), and the source that is the origin of the gravimetric and magnetic anomalies (Pedreira et al., 2007) have been projected in the section to have a complete crustal scale section crossing the entire Basque – Cantabrian Pyrenees.

In the section, the top basement interface corresponds to a major change in the reflectivity and can therefore be described as a main reflection across the composite line. The top basement is flat underneath the Duero Basin at about 3800 m depth, as confirmed by the drill hole Rojas NE-1 (Figs. 6 and 7), projected 12 km from the south. Beneath the Tesla Mountains and all along the imbricates related to the Sierra de Cantabrian Frontal Thrust, this interface appears slightly dipping to the north, being at 4000 m depth in the Navajo-1 drill hole (Figs. 6 and 7). The top basement keeps dipping to the north about 5° degrees until the diapir alignment of Villasana de Mena – Orduña – Murgia, where the dip attitude changes, showing northward steeply dipping reflections. From that point to the north, the depocenter of the Basque-Cantabrian Basin s.s. thickens significantly, coinciding with the northward increase of the dipping of top basement. In the North Biscay Anticlinorium, top basement is flat and located at 4000 m, whereas at the shoreline, this contact is at 3300 m depth, as confirmed by the Cormoran-1 drill hole (Figs. 6 and 7). In the Landes High, top basement was drilled at 2800 m below sea level at the Biscaya B-1 well (Figs. 6 and 7).

In contrast to the top basement, the sedimentary cover is more deformed, which clearly argues for a strong decoupling between sediments and basement. Nevertheless, the sediments in the Ebro Basin are flat lying on top of the basement until the Sierra de Cantabria Frontal Thrust, where a panel of reflectors is dipping to the north representing the hanging wall of such frontal thrusts (Fig. 6). The Tesla Mountains area is deformed by thrusts and related folds and therefore the response in the seismic line is not clear (Appendix 4 and Fig. 6). However, some folded reflectors resulting from the compressional reactivation can be identified. As exposed at surface (Appendix 4), the complete stratigraphic succession in this area (i.e. Jurassic to Cenozoic) is thin and represented by few kilometres of sediments in total. To the north, a gentle syncline (i.e. Villarcayo Syncline) is recognized and made of a thick syn-compressional sedimentary pile as evidenced by the Trespaderne-1 well that drilled more than 2000 m of Late Cretaceous to Cenozoic sediments in its core (Figs. 6 and 7). The northern limb of the Villarcayo Syncline is conditioned by salt structures, such as the Salinas de Rosio– Salinas de Añana salt diapirs, which are the surface expression of a salt inflated area all along the northern limb of this syncline (Appendix 4). This is supported by the La Hoz-2 drill hole, which penetrates more than 1500 m of continuous evaporites (Fig. 7). Immediately to the north, the reflectors of a thick Mesozoic succession are dipping to the south on top of the north dipping basement. This tilted panel defines the southern limb of the Bilbao Anticlinorium (Appendix 4 and Fig. 6). The surface geology as well as the Ubidea-1 and Arratia-1 drill holes show a thick sedimentary cover of Late Jurassic to Middle Albian sediments forming the Bilbao Anticlinorium (Appendix 4 and Figs. 6 and 7). North of it, the quality of the seismic lines is not as good as further south due to the intense deformation in the area. However, some reflectors are showing the northern limb of the Biscay Sinclinorium and some sub-horizontal trends in the northern Biscay anticlinorium area are observed (Fig. 6). Finally, close to the shoreline, the presence of more recent seismic lines and the Cormoran-1 drill hole reflects a still deformed sedimentary cover by presenting some repetitions of the stratigraphy on top of Paleozoic basement (Fig. 7). The Biscay B-1 drill hole penetrated a thick and flat non deformed Late Cretaceous to Tertiary succession deposited on top of a thin Late Albian to Middle Cenomanian sequence, which is directly on top of the basement at 2800 m in the Landes High (Figs. 6 and 7).

From a crustal perspective, a normal, about 30 km thick Iberian crust is observed to the south beneath the Duero Basin but slightly tilted to the north underneath the frontal thrust and southern Basque – Cantabrian Pyrenees. A constant trend is observed until the northern edge of the Alavesa Platform, where the top basement increases the northward tilting as mentioned above, whereas the base of the crust follows a similar tendency slightly dipping to the north, suggesting a crustal thinning underneath the Bilbao Anticlinorium. However, in the same position, but at shallower level, a high magnetic and gravimetric anomaly is reported (Pedreira et al., 2007). From that anomaly to the north, the European crust is present with a Moho depicted at around 30 km whereas the top basement remains at deeper levels as the thick sediments reported in the Bilbao anticlinorium are on top. Therefore, a thin European crust is assumed to the south whereas to the north its thickness increases up to a normal, about 30 km thick crust beneath the Cormoran-1 and Viscaya B-1 boreholes (Fig. 6). Geological cross sections partially based on the previous observations have been published (Muñoz, 2019) suggesting an interpretation of the Basque – Cantabrian Pyrenees with a strong decoupling between basement and sedimentary cover.

The goal of this data report is to compile and present an analysis of a data set consisting of a composite seismic line crossing the Basque – Cantabrian Pyrenees with nine drill holes projected into the seismic transect. A new thematic map of the Basque – Cantabrian Pyrenees at 1:300 000 scale is also provided to better understand the tectonostratigraphic evolution of the area with a simple but robust organisation. This data is presented in order to provide the base for new interpretations on the formation and reactivation of the Basque – Cantabrian Pyrenees. Thus, this data report will be important and can be used to understand how the stacking of the different Mesozoic events recognized in the Basque – Cantabrian Pyrenees are organized and may built the foundation to test new concepts such as multistage and polyphase rift evolutions. Attempts to apply these concepts have been published already in examples presented in this volume.

This work represents the data-set that has been complied in the PhD of Jordi Miró and that belongs to the project CGL2017-85532-P of the Ministerio de Economía y Competitividad, the GEOMODELS Research Institute and the Grup de Geodinàmica i Anàlisi de Conques (2014SGR-467). We also thank reviewers Jesús Garcia-Senz and Ruth Soto and editors Olivier Lacombe and Stefano Tavani for helping to improve an earlier version of the report. The Instituto Geologico y Minero de España (I.G.M.E.) is thanked for providing seismic sections and well logs and Midland Valley for providing the academic license of the used Move software.