Miocene syntectonic fluvial and lacustrine sedimentation linked to a fault-parallel buttress syncline in the Nigüella sector (NW Iberian Range)
Sedimentación fluvial y lacustre sintectónica ligada a un sinclinal de “buttressing” paralelo a una falla en el sector de Nigüella (NW Cordillera Ibérica)

N. Santos Bueno^1,3, C. Arenas Abad^1,3, A. Gil Imaz^2,3

¹Stratigraphy division. Department of Earth Sciences. University of Zaragoza. 50009 Zaragoza, Spain. Email: santosbuenonerea@gmail.com, carenas@unizar.es; ORCID ID: http://orcid.org/0000-0002-2876-9554, http://orcid.org/0000-0002-4212-0524

²Geodynamics (Structural Geology) division. Department of Earth Sciences. University of Zaragoza. 50009 Zaragoza, Spain. ORCID ID: http://orcid.org/0000-0001-6110-1081

³Institute for Research on Environmental Sciences of Aragón (IUCA) and Geotransfer group, University of Zaragoza. 50009 Zaragoza, Spain

IntroductionTOP

There are a number of small intra-mountain basins developed during the Alpine orogeny in the Iberian Range that are filled with alluvial, fluvial and lacustrine deposits of Paleogene and/or Neogene age (e.g., Vera, 2004) (Fig. 1). Their sedimentary evolution and tectonic context are testimony of the relief and climate evolution, as well as of the hydrological characteristics of the sedimentary systems. In contrast to the Calatayud, Montalbán, Aliaga or Teruel basins (Fig. 1B), which have been the focus of numerous and detailed geological studies (Gabaldón et al., 1991; Sanz Rubio, 1999; Ezquerro, 2017 and references therein), the Palaeogene and/or Neogene outcrops between the Moncayo Massif and the Sierra de Vicort have received little attention from a stratigraphic and tectonic point of view. As a matter of the fact, the only published studies are the geological maps on a 1:50,000 and 1:200,000 scale (Hernández-Samaniego et al., 1972; Aragonés Valls et al., 1978; Ramírez del Pozo et al., 1978; Hernández-Samaniego et al., 1978; Olivé Davó et al., 1983; Robador Moreno et al., 2006).

In this region between the Moncayo Massif and the Sierra de Vicort, the Neogene deposits (age provided by Ramírez del Pozo et al., 1978) consist of conglomerates, sandstones and mudstones formed in alluvial and fluvial systems that were related to carbonate deposits (limestones and marls) that formed in fluvial and lacustrine environments. Limestones include a great variety of sedimentary facies, such as tufa, oncolites and stromatolites, as well as thick bioclastic limestones (Ramírez del Pozo et al., 1978; Santos Bueno, 2015). Although tufas are not common in the Pre-Quaternary record, in this region these deposits are thick and well preserved, and therefore represent a significant source of information concerning climate and drainage arrangement in the past.

Different ductile and brittle deformation structures affecting the Neogene strata suggest a complex tectonic evolution. Actually, the available geologic maps suggest that some pre-Neogene tectonic structures conditioned the deformation style during the Alpine orogeny, influencing the arrangement of sedimentation areas, surface drainage and lithofacies distribution during the Cenozoic.

The aim of this work is to discuss the sedimentary evolution (depositional context and fill geometry) of a small area with Neogene fluvial and lacustrine deposits (Fig. 1C) and its relation to allogenic factors, in particular the effects of Alpine compressional tectonics affecting a major, previously extensional fault. The study demonstrates that the depositional fill context is similar to models for formerly extensional asymmetrical basins (i.e., semi-graben basins), and represents an example of combined tectonics, climate changes and drainage pattern evolution.

Geological settingTOP

The study area is located in the northwestern sector of the Iberian Range, to the south of the Moncayo Massif (Fig. 1A, B), in the southernmost end of a NNW-SSE trending kilometric-scale Cenozoic syncline, the La Cabota syncline, that folds part of the Cenozoic sequence, as described by Ramírez del Pozo et al. (1978) (Figure 1C). In this area, the stratigraphic sequence includes Lower Cambrian and Lower Devonian sandstones and mudstones, Triassic clastic rocks, carbonates and evaporites (Buntsandstein, Muschelkalk and Keuper Facies, and Imón Formation), Jurassic carbonate breccias (Cortes de Tajuña Formation) and evaporites (Lécera Formation), and Cenozoic conglomerates, sandstones, mudstones and limestones (Fig. 2A). These units have been affected by different tectonic phases during the Hercinian and Alpine orogenies. The Mesozoic and Cenozoic sequence has been affected by a dominant NW-SE trending folding and fracturing, which are disharmonic in relation to the Paleozoic basement.

The Cenozoic rocks lie unconformably on pre-Cenozoic rocks. The total thickness is approximately 350 m. Two units are differentiated: 1) conglomerates, mudstones and rare sandstones with interbedded limestones and marls, of Burdigalian-Upper Vindobonian and Pontian age, at the lower part, and 2) limestones (oncolitic limestones, micritic and pelmicritic limestones) and marls of Upper Vindobonian-Pontian age, at the upper part. These ages are based on the presence of Cypria curvata LEN., Hydrobia and Limnaea (Ramírez del Pozo et al., 1978). The studied area comprises a small extent to the north of the locality of Nigüella (Fig. 2A). Despite the age uncertainly of these fossils, in this work it is considered that the studied record is of Miocene age, given the lack of other dating criteria at this moment.

Material and MethodsTOP

A detailed geological mapping at 1/10,000 scale was made to determine the spatial distribution of the studied Miocene sequence, in the southern part of the La Cabota syncline (Figs. 1C and 2). Three stratigraphic sections that cover the full Neogene record were measured and correlated based on cartographic, lithologic and textural criteria. Indeed, the continuity of beds in the field allowed to know the spacial distribution of the several lithological units that compose the Miocene record. A total of thirty nine rock samples were taken. Fifteen polished sections and twenty thin sections of different facies were studied with a binocular microscope and a petrographic microscope for textural and structural characterization. In addition, five samples were studied in Scanning Electron Microscope of the University of Zaragoza (Carl Zeiss MERLIN™ FESEM). Nomenclature used for detrital facies follows Miall (1978, 2006)’s code and for carbonate rocks is based on Vázquez Urbez (2008) and Arenas-Abad et al. (2010)’s code. Texture of carbonate rocks follows classification by Dunham (1962), with modifications by Embry & Klovan (1971), and in the case of sandstones the Pettijohn et al. (1973) classification. The structural study included both a structural analysis from field data, coming from bedding planes and hinge lines, and several detailed geological cross sections in order to characterize the structural geometry at a regional scale and visualize the tectosedimentary relationships.

The structure of the Nigüella sectorTOP

The Miocene studied units unconformably lie on both Triassic and lower Jurassic strata (Figs. 2A, 3, 4). The dominant NNW-SSE Alpine structural trend in the studied area is characterized by two kilometric-scale structures: 1) the Nigüella fault, and 2) the Nigüella syncline, subparallel to this fault (Fig. 2A). The fault put into contact Triassic rocks, in the footwall, with Cenozoic rocks, in the hanging-wall (Fig. 2A, 3). Towards its northern edge, this fault changes to a N-S trend with dominant dextral-normal component (Figs. 2A, 3A). As shown by cross section in figure 3A, the lower Jurassic strata are involved in a large horse in the vicinity of this sector. The Nigüella syncline, involving different Miocene units, runs with a mean axis orientation 26, 306 and an axial surface orientation 128,84N (Fig. 2B), with dips ranging between 19°, in its central part, up to 70°, in its northwestern part (Figs. 2, 3). Two other NW-SE trending southwest-vergent folds, affecting upper Triassic and lower Jurassic strata, represent the southern prolongation of the Nigüella syncline (Fig. 2A).

StratigraphyTOP

In the southernmost end of the La Cabota syncline (Fig. 1C), where the study area is located (Nigüella syncline in figure 2A), the Miocene succession varies, from north to south, from 77 m to 120 m in thickness. The stratigraphic sections measured in the studied Miocene sequence (S1, S2 and S3) allowed to distinguish three stratigraphic genetic units that have been correlated based on cartographic and stratigraphic criteria (i.e., the sedimentary evolution of the sequences and the presence of sharp lithological changes). Units 1 and 2 are separated through an angular unconformity in the north and a correlative conformity towards the south. In contrast, the boundary between units 2 and 3 is a sharp lithological change throughout the studied area (Figs. 2A and 5).

Unit 1 crops out to the west and south of the studied area. It lies unconformably on the Upper Triassic and Lower Jurassic rocks. The boundary with Unit 2 is an angular unconformity in the west (e.g., section 1) and a correlative conformity in the south (e.g., section 3). The vertical sedimentary evolution is fining upward. In section 1, this unit is formed of 29 m of monomict grey conglomerates at the base, mostly consisting of carbonate clasts from upper Triassic and Jurassic rocks and also phytoclasts and oncoids among the clasts, followed by phytoclast and oncoid packstones and rudstones, and then bioclast and oncoid wackestones. In section 3, Unit 1 consists of monomict grey and light brown conglomerates and ochre, orange and red mudstones, with rare interbedded grey limestones. This unit encompasses calcretes. The monomict conglomerates are formed primarily of carbonate Upper Triassic and Jurassic-derived carbonate clasts, varying from rounded to angular in shape.

Unit 2 crops out throughout the studied area. It lies on the Mesozoic substrate in the northeast (e.g., section 2) and on Unit 1 in the rest of the study area. The upper boundary with Unit 3 is a sharp lithological change, i.e. from clastic deposits to dominant limestones. The evolution of Unit 2 is cyclic, fining and then coarsening upward. In section 1, Unit 2 is 62 m thick and is formed of polymict conglomerates, consisting of clasts formed of Jurassic limestones, Triassic sandstones, dolostones and limestones, and Paleozoic-derived sandstones and quaztarenites, up to 0.9 m long. The unit also includes rare mudstones and phytoclast limestone interbeds. In section 2, Unit 2 is 47 m thick and the outcroping portions show mostly conglomerates and minor phytoclast limestones. In section 3, Unit 2 is ≈ 50 m thick and includes monomict and polymict conglomerates that alternate with ochre and orange mudstones, marls and minor phytoclast, oncoid and bioclast limestones.

Unit 3 crops out throughout the study area and consists of two differentiated portions. The lower one with phytoclastic and oncolitic limestones, 25 to 35 m thick, and the upper one with fine-grained bioclastic, micritic limestones with variable presence of marly interbeds, up to 16 m thick. The total thickness of this unit increases southward, from 30 m in sections 1 and 2, to 45 m in section 3 (Fig. 5).

SedimentologyTOP

Diverse clastic (allochtonous sediment) and carbonate (authochtonous sediment) lithofacies are distinguished based on lithology, texture, microstructure and nature of components. Table 1 shows the main characteristics and sedimentological interpretation of the different facies and includes references to works that deal with similar facies and depositional environments. Coarse clastic facies consist of monomict and polymict, clast-supported conglomerates that deposited by dominant aqueous processes. Monomict conglomerates (noted as Gm1 and Gh1) consist of dominant carbonate clasts (dolostones and limestones) derived from Upper Triassic and Jurassic rocks, with less abundant siliceous clasts (sandstones, quaztites and rare siltstones) from Lower Triassic and Permian rocks (Fig. 6A, B). Polymict conglomerates (noted as Gm2 and Gt2) are formed of mixed carbonate and siliceous clasts from Paleozoic and Mesozoic rocks (Cambrian, Permian, Triassic and Jurassic)(Fig. 6C, D). Polymict conglomerates that include intraclasts (mainly tufa-derived clasts) are named Gm3. Textural features along with sedimentary structures of the coarse-grained sediments allow to distinguish:

Sandstones are formed of siliceous and carbonate grains. They are rare and form lenticular bodies up to 1 m thick, either structureless (Sm) or with trough cross-stratification (St), always associated with facies Gt2 (Fig. 6E, F). Siliciclastic mudstones (Fm), which associate with the conglomerates (Fig. 6B, E, F), resulted from fine-size sediment deposition during flooding on inactive sectors of the alluvial and fluvial systems (i.e., floodplains) (Porter & Gallois, 2008).

Carbonate facies are varied (Fig. 6G to M). Three groups are present:

Facies associationsTOP

The different facies are associated vertically into simple sequences (i.e., facies associations, FA) that represent cycles of variation of water level and/or flow conditions (e.g., hydrodynamics), and the migration of active parts of the fluvial system (e.g., by channel fill) as a result of progradation, retrogradation or lateral accretion processes. Seven main FA have been recognized (Fig. 8). Three FA correspond to deposition in proximal and middle areas of alluvial fans (FA A1, A2 and A3). These FA have been observed in the southern and northern studied areas. FA A1 formed as a result of flash floods that deposited coarse gravel (facies Gm1) in the form of lobes over floodplain areas with fines (facies Fm) producing “blankets”; the progradation of gravel sheets or lobes produced coarsening upward evolution and crude horizontal stratification (Arenas, 1993; López-Gómez & Arche, 1997). In contrast, FA A2 represents gravel deposition from gently channelled and sheet flows that were filled with poorly sorted and disorganized gravels. Then, these channels quickly waned up to become inactive areas with episodic deposition of facies Fm. Similar associations have been described in the upper Permian of the Iberian Ranges (López-Gómez & Arche, 1997) and the Middle Miocene of the Ebro Basin (Vázquez-Urbez et al., 2013). Shallow lakes could develop in these inactive areas, and be sites for lime mud deposition (facies Lp at the top of FA A2). These carbonate deposits could develop in middle-distal alluvial areas and in inactive areas between alluvial fans (Platt, 1989; Sacristán-Horcajada et al., 2016). In both alluvial fan FA A1 and A2, dry conditions would have allowed the development of calcretes during the cease of alluvial accretion (Wright & Tucker, 1991; Alonso-Zarza, 2003; Sacristán-Horcajada et al., 2016).

FA A3 formed by coarse polymict gravel deposition in gently channelled and sheet flows that quickly waned in proximal and middle alluvial sectors, then becoming sites for fine deposition (facies Fm). Oncoid and phytoclast facies that follow up in the sequence give evidence of shallow carbonate fluvial deposition. This type of carbonate deposition on top of coarse gravel deposits is common in Quaternary fluvial systems of the Iberian Range (Vázquez-Urbez et al., 2012). The incomplete FA A3, lacking the carbonate part, is common in Unit 2 in the northern studied area.

FA B is a fluvial association that formed by polymict gravel and rare sand deposition in shallow channels and longitudinal and transverse bars (facies Gm2a, Gm2c, Sm, St) of a braided fluvial system whose floodplain broadened southwards (i.e., with increasing extent of facies Fm in the southern section 3, Fig. 5). Accordingly, the sequence shows varying thickness of fines and changing size and shape of clasts depending on the location through the system. Siliceous and carbonate clasts with moderate to good roundness suggest long and/or energic transport. This FA is common in middle and middle-distal sectors of fluvial systems (Arenas et al., 1989; Miall, 2006).

FA C has been defined in the northern and southern studied area. It represents fine-size gravel deposition channels of distal sectors of a fluvial system that included mixed extraclast-intraclast sediment (facies Gm3) and finally was replaced with carbonate deposition in shallow channels (facies Lph, Lo and occasionally Ls). These areas later became site for lime mud deposition in shallow lakes (facies Lm). Therefore, it corresponds to a change from clastic to carbonate deposition with final expansion of standing water bodies. Similar associations have been described in the Oligocene fluvial deposits of Mallorca (Arenas et al., 2007).

Two FA represent dominant carbonate deposition, either lacustrine or palustrine-fluvial-lacustrine (FA D1 and FA D2). The lower part of FA D1 represents formation and deposition of oncoids and phytoclasts (facies Lo, Lph) in shallow, low-sinuosity channels and pools with extensive tufaceous palustrine areas, where hydrophilous plants would be dominant (Lst). The environment would be consistent with a low-gradient fluvio-lacustrine context characterized by having water with high Ca and HCO^-₃ content (e.g., as described by Ordóñez & García del Cura, 1983; Pedley, 1990; Pentecost, 2005; Arenas-Abad et al., 2010). This context evolves upwards to marls (M), which represent rapid expansion of lacustrine conditions consequent to increased water inputs. Afterwards extensive lime mud deposition in shallow quiet conditions took place (facies Lm and Lh; i.e., top of FA D1 and D2 in Fig. 8). In brief, FA D1 records the expansion of carbonate lacustrine sedimentation over fluvial and fluvio-lacustrine carbonate settings. Similar sequences have been found in the late fill stage of the Cenozoic Calatayud Basin (Sanz-Rubio, 1999) and Ebro Basin (Arenas et al., 2000; Vázquez-Urbez et al., 2013). FA D2 corresponds to offshore dominant carbonate lacustrine deposition producing micritic limestones with ostracods that typically occur in still and permanent lakes (cf., Platt, 1989; Cabrera et al., 2002, 2011; Gierlowski-Kordesch, 2010; Vázquez-Urbez et al., 2013).

DiscussionTOP

Sedimentary evolution, paleogeography and structural constraints on sedimentary featuresTOP

Sedimentological data of the studied Neogene succesion indicate a sedimentary context consisting of alluvial and fluvial environments related to carbonate fluvial-lacustrine-palustrine and lacustrine environments. The occurrence and distribution of such environments through space and time (Fig. 9) and the location of the main depocentre in the study area (i.e., towards the southeast and parallel to the main syncline axial trace; Figs. 2 and 5) were closely related to the local tectonic evolution. A cross section normal to the Nigüella syncline shows that the maximum thickness of the detrital deposits (Units 1 and 2) is found in the vicinity of its hinge zone and the thickness decreases outward the syncline axis (Figs. 2 and 5). This fact suggests a strong tectonic control on the sedimentary thickness and the surface drainage arrangement. Several evolutionary stages have been sketched (Fig. 9).

Depositional Stage 1 (Unit 1) recorded alluvial fan deposition (FAs A1 and A2 in Fig. 8; Fig. 9A) that stemmed from eastern and southeastern close uplands, less commonly from western uplands, which surrounded the study area. Moreover, a distal south-flowing fluvial system occupied part of the northern area, coinciding and finally being replaced with a carbonate fluvial system flowing southeastward. Shallow carbonate lakes developed locally between the carbonate channels (FA C), and beyond the toe of the alluvial fans, mostly on mud-dominated areas (FA A2). Suspended sediments transported beyond the fan toe, along with dissolved ions, could be pooled in shallow areas, where the fine grains settled out and the water evaporation caused increased chemical concentration and then calcite precipitation occurred (mudstones and micritic limestones at the top of FA A2).

The overall evolution of Unit 1 in the southern area is fining-upward (Fig. 5). This evolution is linked to a general retrogradation of the sedimentary system. Most clastic sediment originated from Upper Triassic and Jurassic carbonate rocks, producing dominant monomict clastic lithofacies. The development of calcretes in Unit 1 suggests interruptions in sediment accretion that were linked to drought conditions (e.g., Alonso-Zarza, 2003) before deposition of Unit 2. The fining upward sequences in Unit 1 (Fig. 5) represent retrogradation of the alluvial fans, followed by the development of small and shallow lakes, afterwards affected by long periods of drought (cf., Sacristán-Horcajada et al., 2016).

Depositional Stage 2 (Unit 2) was dominated by fluvial and less common alluvial deposition (FA A1, A3 and B; Fig. 8). The most common situation (Fig. 9B) corresponded to a large braided fluvial system flowing southeastward, evolving from proximal-middle sectors in the north to middle-distal sectors in the south (i.e., trunk system), with small alluvial fans from the east and northeast uplands (i.e., marginal fans). Polymict gravels dominated throughout, which denotes the enlargement of the sediment source areas by addition of older rock outcrops. It is worth noting that sandstones are rare in this system, which denotes lack of maturity in the transport process (or in other words, short transport distance). This stage includes carbonate deposits approximately in the middle part of the unit that formed in shallow channels and pools, i.e., a fluvial and fluvio-lacustrine setting in which hydrophilous plants, phytoclasts and oncoids were common. This situation coincided with a retrogradation of the trunk fluvial system (Fig. 9C). From this moment onward there is no record of marginal alluvial fan deposition.

The evolution of Unit 2 is cyclic (i.e., fining upward and then coarsening upward), which is attributable to a regrogradation-progradation cycle of the sedimentary system. The sediment source area for all these fluvial and alluvial deposits were mainly the upper Triassic and Jurassic rocks in the south, and the lower and middle Triassic and Paleozoic rocks located towards the North (outside the studied area). Unit 2 represents a sharp palaeogeographic change respect to Unit 1. In the southern studied area, the lower half of Unit 2 includes both coarsening and fining-upward sequences (Fig. 5, section 3), with an increase in roundness of the clasts, an increase in fluvial versus alluvial processes (e.g., channelled flows), and a decrease of calcrete formation in the uppermost deposits. The upper part of Unit 2 corresponds to a general progradation with extensive floodplains in the south. Together, these facts suggest an overall progradation of the fluvial system, providing the area with more water availability, i.e. due to more water availability, likely related to more humid conditions, since the end of deposition of Unit 1.

Unit 3 is dominated by carbonate deposits (Figs. 5, 9; FA D1 and D2, Fig. 8). Two distinct consecutive depositional stages are differentiated: one with phytoclastic and oncolitic limestones and marls (Stage 3, Fig. 9D) and another one with micritic, bioclastic limestones (Stage 4, Fig. 9E). A fluvio-lacustrine environment with Ca- and HCO₃^--rich water is proposed for Stage 3.

In the Depositional stage 3 (lower part of Unit 3) shallow channels and ponded palustrine areas among them would be sites for hydrophilous plants to thrive; there, the submerged parts of plants would be coated by calcite (facies Lst). Breakage of these calcite-coated plants would cause formation of phytoclasts that would be deposited in nearby zones (facies Lph). Oncoids —mostly formed around plant stems fragments (already calcite-coated and non coated stems) and rare molluscs— and phytoclasts would form and accumulate in shallow, low-sinuosity channels (e.g., as described by Vázquez-Urbez et al., 2013). Other examples of oncoidal and associated carbonate deposits are also reported to occur mainly in fluvial and fluvial–lacustrine systems (Leinfelder & Hartkopf-Fröder, 1990; Zamarreño et al., 1997; Hernández Gómez, 2000; Meléndez & Gómez-Fernández, 2000; Arenas et al., 2007; Astibia et al., 2012). The general channel flow in the studied area would run south-southwestwards, entering a shallow lake with marl and lime mud deposition. Marly deposits (e.g., Fig.5, sections 2 and 3) represent periods of lake expansion, with thickest accumulation of marls in the south.

Depositional Stage 4 (upper part of Unit 3) is recorded by dominant micritic limestones (Lm) (Figs. 5, 6H), which formed by extensive lime mud lacustrine sedimentation in calm conditions (Fig. 9E). During this stage, the lacustrine deposition area expanded, as inferred from distribution of facies Lm outcrops. Outcrops in the studied area do not provide evidence of deposits corresponding to lake margin (e.g., bioturbated or palustrine facies) or surrounding fluvial channels (e.g., phytoclastic facies). Thus, the outcropping upper limestones of Unit 3 deposited in a relatively deeper, still and permanent water body compared to ponds in previous stages, and likely corresponding to close-lake conditions. The absence of wave structures and rarity of bioturbation features, along with the preservation of fossils and horizontal lamination vouch for such still and relatively deep context (e.g., Cohen et al., 1997; Gierlowski-Kordesch, 2010; Vázquez-Urbez et al., 2013).

Therefore, the overall sedimentary system in this area evolved from dominant alluvial-fluvial to dominant lacustrine carbonate environments through time. This evolution is consistent with the basin expansion, in particular of the lacustrine area westward (i.e., toward the passive margin; Figs. 2, 9), and is also consistent with the decrease in tectonic activity (compressive regime) through time, as discussed below. The decrease in tectonic activity, and thus in relief rejuvenation in the source areas, through the studied interval, i.e. after Unit 2 deposition, could favour fluvial incision uphill, downcutting the Mesozoic rocks, and then the outflow of ground water hosted in Jurassic carbonates (i.e., springs), thus forming the source of Ca- and HCO₃^--saturated streams. This circumstance would explain the increase in carbonate deposition, in particular tufa, through time (i.e., in Unit 3).

The characteristics of the dominant lacustrine deposition (micritic limestones with lamination, lacking bioturbation and current activity) suggests a hydrologically-closed lake, at least at the end of the deposition (i.e., last depositional stage of Unit 3, Fig. 9E). Carbonate lake expansion would have been favoured by increasing water inputs into the basin, likely during a period of more humid climate conditions that may have coincided with the basin closure. The study of nearby fluvial and lacustrine outcrops and comparison with other close time-equivalent lacustrine records will help validate or reject this hypothesis.

Tecto-sedimentary evolution of the Nigüella sector in the Iberian RangeTOP

The proposed sedimentary evolution during the Miocene occurred under a compressive regime which is consistent with some observed geometrical features including: 1) the paralellism of the main cenozoic syncline fold axis (Nigüella syncline) and the main-fault cartographic trace (Nigüella fault) respect to the NW-SE trending southwest-vergent folds affecting upper Triassic and lower Jurassic strata, and 2) the close relationship between the spatial distribution of thickness variation of the studied units with respect to the hinge zone of the Nigüella syncline, and the related sedimentary environments. These facts suggest that sedimentation was controlled by accomodation space created during the syncline development on the hanging-wall Nigüella fault, as a result of the Miocene compression.

The Nigüella fault acted first under extensive regime as a normal fault, and then under compression producing a large fault-parallel buttress fault (the Nigüella syncline). The extensional history during early Jurassic times is deduced from regional data obtained from some other large-scale brittle structures linked to the deposition of the Cortes de Tajuña Formation in neighbouring areas of the Moncayo Massif (San Román & Aurell, 1992). Concerning the study zone, such extensional regime of the fault is compatible with the presence of extensional horses affecting the lower Jurassic strata (Figs. 2, 3A). On the contrary, the Nigüella fault’s kinematics during the Cenozoic can be unambiguously deduced from the presence of hanging-wall southwest-vergent folds that are coaxial with respect to the main Mesozoic fault (Figs. 2, 3). In addition, in the northwesternmost studied area, the trace of the master fault bifurcates, and a secondary dextral strike-slip fault occurs.

In order to explain the tecto-sedimentary evolution of the Nigüella sector it is necessary to consider it within the Mesozoic framework of the NW Iberian Range. According to the above-mentioned evidence, the present-day configuration of the Neogene strata in this sector is consistent with two main tectonic stages (Fig. 10): 1) a lower Jurassic extensional stage responsible for a half-graben basin geometry, and 2) a Cenozoic shortening stage responsible for the formation of a fault-parallel buttress syncline.

The first tectonic stage is linked to the syn-sedimentary extensional tectonics related to the deposition of the Cortes de Tajuña Formation, Hettangian in age (Goy et al., 1976) (Fig. 10A), which has been recognized broadly in other sectors of the Iberian Range (San Roman & Aurell, 1992). This tectonic stage is well represented in the Nigüella area through both thickness variation of such Mesozoic unit in relation to the master Nigüella fault (Figs, 2, 3), and the presence of extensional structures (i.e., horses like-structures) affecting the lower Jurassic units (Figs. 2, 3A).

The second tectonic stage recorded in the studied area involves the deformation of the previous extensional structures. During the Cenozoic compression, the NW-SE Mesozoic extensional master fault was folded and a hanging-wall syncline basin formed as a result of butressing of the NE block against the verticalized fault. Similar examples have been described in different geological contexts (Butler, 1989; O’Dea & Lister, 1985; Simón, 2004; among others). Deposition of the studied Neogene succession took place in such a compressive scenario. Two Neogene deformation episodes can be differentiated. During the first one, deposition of predominantly clastic units (Units 1 and 2) took place, e.g. coinciding with deposits dated as Early Miocene from the fossils cited above, according to Ramírez del Pozo et al. (1978) (Fig. 10B). This first deformation pulse is not seen extensively through geometric devices, but can be inferred from the sedimentary evolution of the studied sequence. For instance, the change from fining-upward evolution in Unit 1 to Unit 2 represents a sharp grain-size jump that is accompanied by erosion (Fig. 5). Then, the cyclic fining-to-coarsening-upward evolution of Unit 2 corresponds to an overall increase in the intensity of tectonic activity, which resulted in a fault verticalization (Fig.10B). The second episode would represent waning tectonics, with the deposition of the predominantly carbonate sequence (Unit 3), likely during the end of compression later in the Miocene (Fig. 10C). In this stage a slight increase of the fault dip took place. The progressive decrease in tectonic intensity favoured lake expansion and resulted in a reduction of strata dip through time. The sedimentary features suit well other depositional fill contexts of extensional asymmetrical basins, such as those described in the Quaternary African east rift (Cohen et al., 1997) or Upper Triassic-Lower Jurassic rift basins (Olsen et al., 1990).

The proposed model explains the evolution of a portion of an intra-mountain basin in the northwestern sector of the Iberian Range, and shows the strong relationship between tectonics (formerly extensive and then sin-sedimentary compressive), drainage pattern evolution, sedimentation processes and climate changes. The results may help interpret the evolution of other complex basins whose sedimentary record is related to changes of relief and climate.

ConclusionTOP

The sedimentary and structural study of a Cenozoic (likely Miocene) record in the northwestern Iberian Range and its relationship with the Mesozoic substrate allows the following considerations and conclusions:

ACKNOWLEDGEMENTSTOP

This work was financed by Project CGL2013-42867 of the Spanish Government and European Regional Funds. This work is a contribution of the Geotransfer research group (Aragón Government, FEDER and University of Zaragoza). Our gratitude to Dr. G. Pardo Tirapu for wise advise, and to reviewers Dr. I. Armenteros Armenteros and E. Arribas Mocoroa for pertinent corrections. We thank the Servicio de Preparación de Rocas y Materiales Duros (Servicio General de Apoyo a la Investigación-SAI) of the University of Zaragoza for their technical support. This contribution presents preliminary results of a IUCA research contract held by N. Santos Bueno.

ReferencesTOP