Stratigraphy and sedimentology of distal-alluvial and lacustrine deposits of the western-central Ebro Basin (NE Iberia) reflecting the onset of the middle Miocene Climatic Optimum

Stratigraphic and sedimentological study of distal alluvial and lacustrine deposits in the Plana de la Negra-Sancho Abarca area (western-central Ebro Basin, NE Iberia) within the early and middle Miocene allows five main lithofacies to be characterized and mapped within two tectosedimentary units, construction of a sedimentary facies model and discussion on allogenic controls on sedimentation. In this area, the boundary between tectosedimentary units T5 and T6 appears to be conformable and is marked by the change from dominant clastics to carbonates. Correlation of the studied outcrops with nearby sections that already had magnetostratigraphic and biostratigraphic data allows the studied succession to be dated from C5Dr to C5Cn (Burdigalian-Langhian), placing the boundary T5/T6 at ca. 16.1-16.05Ma. Seven vertical facies sequences document deposition of distal alluvial clastics and palustrine and lacustrine carbonates. Sandstones and mudstones represent low-sinuosity channels and lateral and terminal splays by unconfined flows runnig across the alluvial plain, associated to the Pyrenean-derived Luna fluvial system. The carbonates contain charophytes, ostracods, bivalves and gastropods, indicating deposition in 2-4m deep lakes. Laminated carbonate facies record reworking of shore carbonates and the influx fine-siliciclastic sediment offshore. Abundant bioturbation and desiccation features indicate episodic submergence and subaerial exposure. Four main episodes of alluvial and associated palustrine/lacustrine facies belt shifts are identified. Alluvial deposition in the studied T5 unit is related to low lake level conditions, rather than to a Pyrenean uplift. The maximum extent of the freshwater carbonates occur at the base of unit T6. This is consistent with conditions of increasing humidity of the Middle Miocene Climatic Optimum.


INTRODUCTION
Lacustrine deposits are shown to be excellent indicators of tectonic and climatic change De Wet et al., 1998;Gierlowski-Kordesch, 2010;Kelts and Talbot, 1990). Given the small size of lake systems compared to ocean basins, and their intimate connection with coeval drainage/catchment areas, lake basins are very sensitive to climate and tectonic changes at different time scales (Bohacs et al., 2000;Vázquez-Urbez et al., 2013). Catchment area tectonics and lithology have a direct characteristics and facies evolution (Carroll and Bohacs, 1999;Davis et al., 2008). At the same time, the type and amount of sediment in the lake basin is also controlled by climate, which ultimately drives lake level variations through changing evaporation and precipitation (Alonso-Zarza et al., 2012;De Wet et al., 1998;Platt and Wright, 1991).
The Ebro basin is one of the largest cenozoic intermontane basins in the Iberian Peninsula that developed during the Paleogene and Neogene, as a result of the alpine collision of the Iberian and Eurasian plates. The Oligocene-Miocene continental deposits of the Ebro Basin (Fig. 1A) lacustrine systems Muñoz et al., 2002;Valero et al., 2014 originated in the marginal alluvial systems, and carbonate and evaporitic deposits and minor coals (Cabrera and Sáez, 1987) formed in the lacustrine systems. Carbonate deposits, mostly limestones and marlstones, consist of varied palustrine and lacustrine facies that were coeval with alluvial plain deposits. The carbonates crop out across morphological uplands in the basin centre that are known locally as "sierras" and "muelas" or "planas". One of these uplands, the Plana de la Negra-Sancho Abarca, within the Bardenas Reales area (Fig. 1A, B) is the focus of this study. A series of recent works in this area yielded reliable new magnetostratigraphic, biostratigraphic and paleoecologic data (Larrasoaña et al., 2006;Martínez-García et al., 2014;Murelaga et al., 1997;Ruiz-Sánchez et al., 2012Suarez-Hernando, 2017). These works have provided a chronostratigraphic framework and allow the comparison of environmental and climatic conditions over time. Navarro-Jiménez et al. (2011) presented a stratigraphic study of the area, establishing the boundary between two tectosedimentary units (T5 and T6) at 16.3Ma, on the basis of magnetostratigraphic results obtained by Larrasoaña et al. (2006). Detailed stratigraphic analysis and correlation with neighbouring areas that have precise datings (i.e. those of Pérez-Rivarés, 2016;Suarez-Hernando, 2017) remains to aiming to assess the evolution of facies and environmental conditions through time.
This work focuses on stratigraphic and sedimentological analysis of the distal alluvial and lacustrine-palustrine deposits of the Plana de la Negra-Sancho Abarca area in order to: i) provide a precise stratigraphic framework with absolute chronology for the Miocene of this region and evolution of the sedimentary facies environments through time, taking into account the available palaeoecological data. A discussion on the wider environmental controls of tectonics and climate on sedimentation is presented, with emphasis on the middle Miocene Climate Optimum (MMCO; cf. Zachos et al., 2001 among others).

GEOLOGICAL SETTING
The study area is located in the western-central sector of the Ebro Basin, in NE Iberian Peninsula (Fig. 1A). The Ebro Basin formed during the Paleogene as a consequence of the convergence of the Iberian and Eurasian plates. It represents the last evolutionary stage of the south Pyrenean foreland basin (Muñoz et al., 2002;Riba et al., 1983). The to the north, the Catalan Coastal Ranges to the southeast the outlines of the basin and its sources of terrigenous sediments.
During Paleocene to middle Eocene times the basin recorded successive phases of marine and continental sedimentation. By the late Eocene, the basin had lost connection with the ocean (Costa et al., 2010) and became a large endorheic depression which was hydrologically isolated at least until middle Miocene times (Muñoz et al., 2002;Riba et al., 1983). The basin's current structure and boundaries were acquired between late Oligocene and youngest south Pyrenean and Iberian thrust systems in the Marginal and External Sierras and Sierra de Cantabria, and Cameros-Demanda, respectively (Fig. 1A). From the the Mediterranean Sea, and since then the basin has been subject to erosion (Arasa and Cabrera, 2018;Babault et al. 2006;García-Castellanos et al., 2003;Vázquez-Urbez et al., 2003.
The upper Oligocene to middle Miocene is represented mudstones, grading into lacustrine carbonate and sulphate deposits (limestones, marls and gypsum) in the basin centre ( Fig. 2) (Pardo et al., 2004).
Unit T5 consists of mudstones with sandstone and limestone intercalations. The unit crops out extensively at the foothills of the Plana de la Negra-Sancho Abarca upland. Unit T6 is made up of thick limestone succesions with marlstone and mudstone interbeds. The boundary between units T5 and T6 is conformable and marked by an abrupt change from a mudstone succession with interbedded sandstone and limestone strata to a succession dominated by lacustrine limestones (Larena-Martin, 2019). The T5/ T6 boundary is generally conformable throughout the basin B A FIGURE 1. A) Geological setting of the studied area (Plana de la Negra-Sancho Abarca, blue frame included in Bardenas Reales) in the westerncentral sector of the Ebro Basin. The Luna and Huesca fluvial systems, as named by Hirst and Nichols (1986). B) Enlarged map showing the location of the studied area, with sections measured herein, as well as location of sections from other authors that are integrated in this study. Map adapted from Muñoz et al. (2002) and Pérez-Rivarés et al. (2018). centre, with diachrony up to 0.28ka (see discussion by Pérez-Rivarés et al., 2018), and locally is unconformable, i.e. at the basin margins in the southwesternmost and southeast sectors of the Ebro Basin. This boundary is not known at the northern central basin margin (i.e. the age of rocks unit T5 is unknown) (Pardo et al., 2004;Pérez-Rivarés, 2016).
Lithostratigraphically, the studied deposits belong to the Tudela Formation (Astibia et al., 1995;Crusafont et al., 1966;Larrasoaña et al., 2006;Riba, 1955), which represents the early to middle Miocene in the Navarra-La Rioja NW sector of the Ebro Basin, and unconformably overlies the Oligocene to lower Miocene Lerín Formation, which is equivalent to the Zaragoza Formation in the basin centre (Fig. 2).

The Tudela Formation passes laterally through complex
Ujué, Uncastillo and Alfaro Formations at the northern and southern basin margins (Castiella, 1977;Soler and Puigdefábregas, 1970). Towards the basin centre and to the east and southeast of the study area, the Tudela Formation passes laterally into the carbonate-and evaporite-dominated series of the Alcubierre and Zaragoza Formations (Fig. 2) (Muñoz et al., 2002).
The Tudela Formation has been interpreted as representing distal alluvial deposits sourced from the Pyrenean and Iberian ranges, to the north and south stable freshwater lake across laterally extensive palustrine fringes (Larrasoaña et al., 2006).

METHODOLOGY AND TECHNIQUES
Two stratigraphic sections were measured and drawn up on a 1:100 scale. Correlation between these sections and throughout the studied area is based on cartographic (detailed mapping of units and key surfaces), lithological and textural criteria (e.g. sequential sedimentary evolution through time and presence of sharp lithological changes through the sequences) (see Fig. 3A). Following the methodology of the tectosedimentary analysis (Pardo et al., 1989), stratigraphic genetic units named TectoSedimentary Units (TSUs) have been characterised and correlated with nearby sections which had been dated based on magnetostratigraphic and paleontological studies. Sedimentary structures including cross bedding and lamination, and channel orientations were used to deduce palaeocurrent directions in the clastic deposits.
sediments, using a manocalcimeter (Geoservices, France) based on the method of Scheibler, at the stratigraphy laboratory of the University of Zaragoza (Spain).

Based on the two measured sections and stratigraphic
Miocene succession in the Plana de la Negra-Sancho i.e. mappable facies associations, cf. Bates and Jackson, 1985), which are traceable as laterally continuous units through the study area (Figs. 3A, B; 4).
Lithofacies 1 consists of grey and red mudstones and marls, with interbedded sandstones, limestone and rare gypsum beds. This lithofacies makes the older T5 deposits in the studied area, which are 13m thick at the base of the Sancho Abarca section (SA). Lithofacies 2, 3 and 4 are present in both measured sections. Lithofacies 2 is formed of grey limestones and marls with interbedded yellowish to ochre mudstone and thin and laterally discontinuous ochre sandstone strata. This lithofacies forms the base of the succession in section CO and is up to 37m thick in section SA.
Lithofacies 3 is composed of thick red and orange mudstones with interbedded reddish and ochre sandstone strata and very ocassional grey to ochre limestone beds. This unit is 60m thick at SA section and 102m thick at CO section.    Lithofacies 4 consists of grey limestones with interbedded grey marls and less abundant grey mudstones. The lower part of this lithofacies is a 20m thick interval dominated by marls in section SA, while in section CO is a 22m thick interval dominated by mudstones and marls and marlstones. The rest of the lithofacies is formed by thick deposits of limestones, 25m thick in section SA and 30m thick in section CO.
Lithofacies 5 is only present in the southern portion of the studied area (section SA). It occurs as a 37m thick succession of grey and ochre mudstones with interbedded grey limestones and less common grey marls (in section SA).
The main datum for correlation between the two studied sections was the T5/T6 boundary and a distinct limestone bed described as surface B (Fig. 4). This boundary lies within Lithofacies 4, being set at the base of the dominant limestone succesion. Thus, in the Plana de la Negra-Sancho Abarca area, the T5/T6 boundary is recognized by a sharp transition from dominant mudstones and marls to a limestone-dominated succession (Figs. 4;5). In the Montes de Castejón, some 20km to the east of Plana de la Negra-Sancho Abarca area, the T5/T6 boundary is marked by a change from a heterolithic succession dominated by ochre and red mudstones and minor overlying gypsum to a prominent limestone-dominated sequence (Arenas, 1993).
Further southeast, in the central sector of the basin, this boundary is seen as a sharp transition from evaporitic to carbonate deposits (Pérez, 1989) and, towards the northern and eastern sectors, it is found within a carbonate succession (Arenas, 1993;Arenas and Pardo, 2000).

Correlation with adjacent areas
The two studied stratigraphic sections were correlated with laterally-equivalent deposits exposed in three previously described sections, using the T5/T6 boundary surface as a regional marker, together with the vertical sedimentary trends (Fig. 6). The sections are: Pico del Fraile (Larrasoaña et al., 2006) and Loma Negra (Martínez-García et al., 2014) in the Bardenas Reales area, and Castillo de Sora (Pérez-Rivarés, 2016) in the Montes de Castejón area (see Fig. 1B for location of all sections). These sections show a similar arrangement of facies as compared to the studied sections herein, and provide biostratigraphic and magnetostratigraphic data, which then permit accurate dating of the two studied sections.
A magnetostratigraphic correlation has been performed between: i) section Pico del Fraile (Larrasoaña et al., 2006), approximately 3km southwest of the studied area; ii) the uppermost 85m of section Sancho Abarca, as measured by Larrasoaña et al. (2006), and iii) the Castillo de Sora section, as measured by Pérez-Rivarés (2016), The boundary T5/T6, main reference surface for correlation ( Fig. 6), was dated by Pérez-Rivarés (2016) through magnetostratigraphy in several sections of the central sector of the Ebro Basin. In the Castillo de Sora section the boundary was set at 16,099Ma. This author, by using the Larrasoaña et al. (2006) magnetostratigraphy from section Pico del Fraile, set the boundary T5/T6 at 16,062Ma in Pico del Fraile. In both cases, corresponding to chron C5Cn.1n (Burdigalian, early Miocene).
Correlation with the available magnetostratigraphic data and published micromammal ages (as exposed above) allows dating of the studied deposits as Ramblian (MN3) to middle Aragonian (MN4), i.e. from C5Dr to C5Br (Fig. 6).  (Larrasoaña et al., 2006) Loma Negra (LN)   (Table I for symbols). A) Tabular limestone and minor marl strata of lithofacies 4 in Sancho Abarca section (unit T6). Note the varied thickness and extensive lateral continuity of strata. B) Yellowish and pinkish fine detrital deposits (mudstones, facies Fm, and sandstones, facies Sr, Sh) passing upward to minor marl strata (facies Mm) and light grey and cream limestone (facies Lmo and Lmb). C) Tabular limestone strata in Sancho Abarca section. The thickest bed consists of laminated limestones (facies Llbi). D) Detail of C) showing laminae consisting of mudstone and bioclast and intraclast wackestone-packstone. E) Dark and light gray, angular and subrounded, poorly sorted intraclasts, along with smaller shell fragments of bivalves, typical of massive intraclastic limestones (facies Lmi), which also contain small quarz grains. F) Bioclastic limestone with empty subvertical traces that correspond to root bioturbation (Lbt). G) Bioclastic limestone with root bioturbation (facies Lbt) and desiccation craks (Lbtd). Note the presence of gastropods and small cracks. . E) Bioclastic limestone with circumgranular cracks that are filled with calcitic cement, denoting formation of nodules by dessication and pedogenesis (facies Lbtd). F) Bioclastic limestone with circumgranular cracks, nodules and breccias, and spar calcite cement, possible "pseudomicrokarst" as described by Plaziat and Freytet (1978), (facies Lbtd). Miocene alluvial and lacustrine deposits, Ebro Basin, NE Iberia 12 Carbonate facies have been named according to texture and the most abundant allochem components and/or the most relevant sedimentary features. Clastic and mixed and facies take their names from the sedimentary structures they exhibit. Table I (see appendix I) summarizes the principal characteristics, abundance and sedimentological interpretation of each facies. The table also provides references to similar facies described and interpreted in the sedimentological literature.

Northern sector
Carbonates in the Corretroncos section are dominated by massive packstones containing intraclasts, peloids and bioclasts and massive wackestones-packstones containing intraclasts and bioclasts (facies Lmip and Lmi); these bioclastic limestones may present evidence of bioturbation (facies Lbt), as well as desiccation cracks (Lbtd)
G e o l o g i c a A c t a , 1 8 . 7 , 1 -2  The following clastic alluvial facies are present in the south: massive mudstones (facies Fm..; Figs. 10A, B, D; 11D) and a range of sandstones. These are mainly horizontally and cross-laminated, and more rarely trough Fig. 10A, E) and massive (facies Sm; Figs. 10C, D; 11B, C).

Facies associations
The sedimentary facies described in Table I are stacked in simple, centimetre-to metre-thick sequences, named Facies Associations (FA). These FA represent the superposition of deposits developed in laterally related subenvironments, and its vertical evolution is interpreted processes, avulsion, etc., corresponding with progradation/ contraction of the coeval lacustrine-palustrine system. low sinuosity and high-energy channels, with development of small to medium-scale sandy dunes that would generate  (Miall, 1996;Porter and Gallois, 2008).
FA2 represents distal alluvial deposition followed by lacustrine and palustrine deposition, thus recording expansion of the lake area through time. The lower half siliciclastic mud and sand sedimentation, and affected by to lateral and terminal splays (Nichols and Fisher, 2007). produce thin interbedded massive tabular sand bodies (Brierley, 1996;Fisher et al., 2007;Paredes et al., 2007). expansion, with increasing water level (Mm), then lowering lake level allowing lime mud deposition with bioclasts (Lmb). Commonly, the tops of these limestone strata are affected by root bioturbation and desiccation cracks (Lbtg), which are typical features of palustrine conditions and subaerial exposure (e.g. Alonso-Zarza, 2003;Freytet and Plaziat, 1982).
FA3 is characterized by the upward transition from marls into lime mud facies with abundant charophyte, gyrogonites and thalli. These facies (Lmc) indicate stabilization of lake level to allow the development of extensive charophyte meadows in the photic zone and in still conditions (cf. Sáez et al., 2007a;Soulié-Märsche et al., 2010). The upper part of this facies sequence represents shallow areas vegetated by hydrophilous plants and affected by lake level variations, causing bioturbation of the lime mud by roots, nodulization, desiccation cracks (facies Lbt and Lbtg; Scott and Smitt, 2015) and, in some cases, pseudomicrokarst and brecciation (Alonso-Zarza et al., 1992;Plaziat and Freytet, 1978).
FA4 formed in a lacustrine environment that shared characteristics with that represented by the lower part of FA6. In this case, the shallow areas would record deposition during calm or low energy periods, alternating with high energy events, e.g. by currents that rework and transport bioclasts (Clausing and Boy, 2000;Glenn and Kelts, 1991 sediment transported by currents originated from episodic Table I). These laminated limestones are occasionally bioturbated by benthonic organisms (Fig.  9B). This biogenic structure denotes the probably shallow conditions (Scott and Smith, 2015), with occurrence of episodic current activity. The upper part of FA4 represents shallowing conditions as interpreted in FA3.
FA5 begins with massive, occasionally laminated, marls (facies Mm y Ml) that would form in calm offshore zones, in which the absence of currents and bioturbation would allow primary lamination to be preserved (cf. Gierlowski-Kordesch, 2010). The upward passage from marls to laminated limestones (facies Llbi) indicates the occurrence of currents that transported intraclasts and bioclasts from the shallow areas. The decreasing in the size of bioclasts and the upward increase of micrite content within the laminae suggest deposition from discrete events associated with density currents (Fig. 9C) (Sáez et al., 2007b). Shallow lacustrine areas would be loci for minor gastropods and bivalves (facies Lmb), leading to the formation of bioclastic limestones, e.g. as described by Platt (1989Platt ( , 1995, Platt and Pujalte (1994) and Quijada (2014) in the early Cretaceous lacustrine systems of the western and eastern Cameros basins, in north Spain. The upper part of FA5 exhibits characteristics to the top of FA3, with indications of emergence and plant colonization in palustrine conditions. study area. It has similarities with FA7 in the lower part, with marls (facies Mm and Ml) and bioclastic limestones (Lmb ; Table I), which formed in calm offshore lake zones. Laminated limestones containing intraclasts and ooids (Llio; Table I) indicate water level agitation by currents (e.g. during fair weather) that would generate intraclasts and favor the formation of ooids, depositing them in close offshore areas (cf. Quijada et al., 2013). In marginal areas bioclasts, intraclasts and small quartz grains would produce wackestone-packstones (Lmi; Table I).
FA7 represents shallowing from lacustrine to palustrine siliciclastic sediments and lime mud from within the water column, following water inputs. The overlying limestones with abundant ostracods, charophyte gyrogonites and thalli, and fragments of gastropods and bivalves (facies Lmo and Lmb; Table I) indicate deposition in an open lacustrine area (cf. Platt and Wright, 1991;Vázquez-Urbez et al., 2013). In marginal and/or shallow zones, colonization of would cause nodulization and the formation of desiccation cracks (Figs. 7G; 8E, F), e.g. as described in the Bembridge Limestone (Late Eocene) in southern England (Armenteros and Daley, 1998) and in the modern day Florida Everglades (Platt and Wright, 1992).

Sedimentary facies model
A sedimentary facies model has been constructed for the study area taking into account the facies and facies associations and their spatial distributions (Fig. 13). Particular attention has been paid to studies investigating the depositional environment colonized by charophytes (González-Pardos, 2012) and ostracods (Martínez-García et al., 2014) in the Miocene of the Ebro Basin, in areas close to this study. Charophytes are good indicators of environmental conditions in lakes (e.g. water composition, depth, light and dynamics). The chemical composition of water is the most important parameter constraining charophyte development; water chemistry and pH largely determine the distribution of different species. Charophyptes thrive in alkaline and oligotrophic lakes and disappear where phosphorus and nitrogen concentrations are high (Martín-Closas and Diéguez, 1998).
The most common genuses in the studied sections are Chara and Lychnothamnus (González-Pardos, 2012). At present, Lychnothamnus barbatus is found at depths of 2 to 4m in cool-water, low energy lakes (Soulié-Märche et al., 2010), as in the cases of some modern mid latitude lakes (e.g. Zürich Lake, Kelts and Hsü, 1978). These characteristics can be attributed to the Miocene lake in the studied area.
Ostracods are the second most abundant organisms in the studied sections. In nearby areas the basal sections, show an abundance of Paralimnocythere sp. in marls and limestones, suggesting deposition in shallow stagnant ponds in distal alluvial environments, with warm and oligohaline fresh water (Martínez-García et al., 2014). The occurrence of sandstone layers containing Cyclocypris levis suggests relatively higher energy (Martínez-García et al., 2014). The abundance of Pseudocandona parallela in marls and limestones of the uppermost parts of sections in adjacent areas, allowed this authors to infer a stable lake with a higher water column and cooler freshwater to mesohaline water.
The depositional environment is interpreted as a lowgradient and shallow lacustrine system, probably between 2 to 4m in depth (Fig. 13), and with slighly cool water. The studied area was part of a larger lacustrine system with similar physiographic characteristics that extended tens of kilometres through the central Ebro Basin (cf. . Carbonate deposition would take place in the littoral and sublittoral zones and marls would be deposition at greater depth than gastropod and bivalve limestone facies. Periodic brecciation and reworking by events transporting siliciclastics and/or intraclast and ooids offshore. The lacustrine environments would be surrounded by discontinuous palustrine fringes with abundant hydrophilous vegetation, associated with the lake margin and with distal alluvial areas. Migration of these fringes would be directly related to changes in lake water level, with expansion when the lake level was low. In this context with would take place. Similar interpretations have been put forward for many palustrine limestone sequences in the Mesozoic and Cenozoic of Europe and North America (Alonso-Zarza, 2003;Armenteros, 1986; Armenteros  l o g i c a A c t a , 1 8 . 7 , 1 -2  Miocene alluvial and lacustrine deposits, Ebro Basin, NE Iberia 18 and Daley, 1998;Gierlowski-Kordesch, 2010;Platt, 1989Platt, , 1995Wright, 1990; among others).
Palaeocurrent analysis in unit T5 suggests water and (Pyrenean Range; see Fig. 1), i.e. The distal part of this basin marginal distributary system intersected by rare mostly low-sinuosity channels, with small and shallow ephemeral ponds, where freshwater to oligohaline conditions allowed deposition of the ostracod limestone facies (e.g. Paralimnocythere sp.). These facies would be subject to desiccation and pedogenesis within palustrine zones which were periodically exposed as well.
A similar lacustrine-palustrine facies model was proposed for the Lower Cretaceous of the Serranía de Cuenca, Spain (Gierlowki-Kordesch et al., 1991) and for the freshwater lakes of the lower and middle Miocene of the central sector of the Ebro Basin .

DISCUSSION
Concerning the boundary T5/T6 established herein through stratigraphic criteria and dated by correlation with other nearby areas having magnetostratiphic studies, the age proposed for this boundary in the Sancho Abarca-Plana de la Negra area (16.05 to 16.06Ma) is consistent with the small diachrony (ca 30ka) of this boundary throughout the Ebro Basin centre (Pérez-Rivarés et al., 2018).
Sedimentological analysis of the studied sections and stratigraphic correlation with three nearby sections (Fig.  6) have allowed reconstruction of the palaeogeographic evolution for units T5 and T6 (cf. Muñoz et al., 2002;Pardo et al., 2004). This evolution prompts a discussion on the allogenic factors that controlled the sedimentary evolution through space and time.

Palaeogeographic evolution
The palaeogeographic evolution of the study area can be described through four successive stages (Figs. 6; 14). macrosequences and, though incomplete (as the base of the unit is not present in this area), the overall evolution is Arenas, 1993). The interval of Unit T6 recorded in Sancho Abarca in turn describes a trend from carbonate-rich to marly-rich facies.
In unit T5, the alluvial deposits are more abundant in the northern portion of the studied area (CO section). To the east, in the Montes de Castejón area (section Castillo de Sora), the top of unit T5 is formed by evaporitic deposits (i.e. nodular gypsum levels within marls and limestones),  These evaporitic deposits overlie distal alluvial and palustrine deposits. To the west, in the Sancho Abarca and Pico del Fraile sections, shallow lacustrine carbonate sedimentation dominated at the end of deposition of unit T5. Northwards, these deposits grade laterally into distal alluvial deposits with root bioturbation traces (e.g. in the Corretroncos section). In contrast, unit T6 deposition Stage 1 (T5/1) is represented by the lower to middle portion in each of the sections (Fig. 6). Floodplain mudstones, with sandstones representing sheet-like most of the upper portion of this stage, with rare lacustrine deposition (marls and limestones). Lacustrine deposition is, however, more common in the lower portion of sections CO, PF and SA, represented by intervals of limestones and marls (e.g. between metres 20 and 30 in the SA section; Fig.  6). Overall, the upper part of this stage corresponds to an ponded areas surrounded by palustrine fringes developed, as represented in Figure 14A. The greater thickness of the mudstone deposits in the north and the paleocurrent directions measured at Correctroncos section, both suggest sediment provenance from the north. This is consistent with paleocurrents oriented from the north in the distal Pérez-Rivarés et al., 2018), where sediment supply was derived from the Pyrenean margin (see Figs. 1; 2). Paleocurrents from the northwest, however, would suggest overspilling i.e. overbank deposits; Fig. 10B, D).
Stage 2 spans the upper 20m of unit T5 (T5/2) and records a gradual change from the laterally extensive deposits of T6/1 (Fig. 14B). This evolution is interpreted sedimentation in the area of Plana de la Negra-Sancho diminished. Further to the east, in the western part of the Montes de Castejón, e.g. at Castillo de Sora (see Figs. 1B; 6), gypsum facies within marls and limestones occurred at the end of this stage (coindicing with the top of unit T5). These overprint occurring in an area representing a north-south oriented low-relief barrier. This area corresponded to exposed fringes of previous saline carbonate depositional areas in the Montes de Castejón , while outer freshwater-carbonate fringes, as formed in the Plana de la Negra-Sancho Abarca area, could be affected by desiccation and pedogenetic processes . There are not enough data to prove whether the lacustrine area in Plana de la Negra-Sancho Abarca and Montes de Castejón corresponded to a single water body.
T6/1 is formed mainly of limestones in the Loma Negra, Pico del Fraile and Castillo de Sora sections (Fig. 6), Root traces and nodulisation are more abundant in the north, at Loma Negra and Corretroncos sections than in the south, at Pico del Fraile and Sancho Abarca sections (Stage 3, Fig.  14C). This facies distribution suggests an initial deepening and expansion of the lake area followed by shallowing and expansion of palustrine areas at the end of the stage, i.e. the development of a palustrine fringe across the studied area, present in the south and southeastern portions of the study area (upper part of section SA). The presence of interbedded thin limestone beds suggest the occurrence of small shallow lakes and/or the expansion phases northward of a larger lake on a distal alluvial plain. No palaeocurrent data could be collected from the deposits in this stage. On the basis of mineral composition, crystallochemical parameters of illites and the palaeogeographic evolution, Arenas et al. (1993) and  inferred that the deposition of the lacustrine record of unit T6 in the southeast (e.g. Montes de Castejón and Sierra de Alcubierre; see Fig. 1A for location) was linked to sediment supply from the Pyrenean margin, which most likely was the situation in the study area.

Allogenic factors: tectonics and climate
The evolution of the Ebro Basin during the Miocene was mainly controlled by compressional tectonic activity at the Pyrenean and Iberian bounding margins (Riba et al., 1983). Tectonics conditioned the topography and extent of the catchment areas, as well as variations in subsidence and sediment supply to the central zones of the basin until the middle Miocene. From early Miocene times onwards, the Iberian Range acted as an almost passive basin boundary, of the basin (Muñoz et al., 2002). The vertical evolution and spatial distribution of lacustrine deposits record climatic variations operating at different scales during the Miocene Vázquez-Urbez et al., 2013;Valero et al., 2014).

Tectonic Control
At the Pyrenean margin, sedimentation of unit T5 proved to be related to tectonic activity of the Gavarnie thrust unit. Dating indicates that sedimentation of units T5 and T6 was contemporaneous with the emplacement of the Garvarnie unit (Fig. 1A), during the latest phases of movement of the Guarga thrust sheet (Arenas et al., 2001;Millán et al., 1995). In this context, around the boundary between units T4 and T5, during the Aquitanian and lower Burdigalian (lower Miocene), extensive progradation of uplift at the Pyrenean range. As a result, progradation of two i.e. the Huesca and Luna systems, which span deposition of units T4 and system, which developed in association with the Guarga thrust sheet emplacement, was the main source to the study area ( Fig. 1; Arenas et al., 2001).
Unit T5 in the Luna system has a complex sequential without reaching the end of the unit), directy related to the tectonic deformation. However, in the south, the area of middle of the unit (Arenas, 1993). From these facts, it was suggested that the sedimentary evolution of this unit in the basin centre was mostly controlled by climate, except for the Pyrenean uplift (Arenas, 1993). This relative change occurs stratigraphically below the base of the studied sections herein. The small incursions of distal alluvial deposits southward by the end of deposition of unit T5 were rather related to low lake-level stages. Likely this situation was coeval with a general scenario of increase in humidity by the end of unit T5, though modulated by short periods of increase in aridity, and overall decreasing tectonic activity (Arenas, 1993).
The carbonate and siliciclastic deposits in the lower unit (stage 1, T5/1) correspond to phases of alluvial incursion favoured by a low lake level, rather than caused by a tectonic pulse, as suggested in nearby areas of the basin (Arenas, 1993) (Fig. 14A). In contrast, the uppermost unit T5 (T5/2) appears to record the overall decrease of alluvial inputs onto the basin centre, and the initial expansion of shallow carbonate lakes across the study area, with saline mud Fig. 6). During stage 2 (T5/2), evaporite deposits became dominant throughout the basin centre. Therefore, the tectonic activity does not seem to have exerted a great control on this basin area (Pardo et al., 2004).
Stage T6/1 began with a general expansion of the lacustrine system. Basin margins were broadly similar to the present (cf. Fig. 1A). The carbonate lacustrine system extended southward onlapping against the southern basin margin formed by the Iberian Range, where deformation ceased or markedly disminished during late Burdigalian-Langhian times (Pérez, 1989). There are no known alluvial deposits dated as equivalent to units T6 and T7 in the Pyrenean margin (Arenas, 1993). In the basin centre, the sequential evolution of unit T6 cannot be attributed to tectonic control, Milankovitch cyclicity in T6 lacustrine deposits in the Sierra eccentricity cycles (Pérez-Rivarés, 2016).
It is worth noting that the boundary between units T6 and T7 elsewhere in the basin centre is a sharp lithological change, locally with erosional features, indicative of progradation of Pyrenean alluvial systems throughout the basin centre, likely coeval to renewed Pyrenean uplift (Pardo et al., 2004;Vázquez-Urbez et al., 2013). Locally, on the southern Iberian margin, this boundary is an unconformity within proximal alluvial deposits (Fig. 1A;Pérez Rivarés, 2016). However, humidity and tectonic uplift (Arenas, 1993).

Climate control
Climate controls many lacustrine systems, as is the case of the Miocene Ebro basin; however, discrimination Carroll and Bohacs, 1999;Meléndez et al., 2009;Tanner, 2000). Within the context of overall decreasing compressive tectonic activity at the central and western-central basin margins, the upward sequence from distal alluvial deposits to lacustrine carbonate sediments recorded by units T5 and T6 in the study area is consistent with a transition to a more humid climate during the deposition of the studied succession. This interpretation is supported by studies of charophytes (González-Pardos, 2012), ostracods (Martínez-García et al., 2014) and micromammals (Larrasoaña et al., 2006;Murelaga, 2000;Murelaga et al., 2002;Ruiz-Sánchez et al., 2013) in the nearby Pico del Fraile and Loma Negra sections (Figs. 6; 15). This climatic evolution is also consistent with sedimentological interpretations and carbon and oxygen stable isotope results obtained from lacustrine carbonates elsewhere in the Ebro Basin by  and Vázquez-Urbez et al. (2013). Moreover, this climatic evolution has also been detected in other Neogene lacustrine systems of the Iberian Peninsula (Calvo et al., 1993). In the Madrid Basin, for instance, the upper sequence of the Intermediate Unit represents an expansion of the freshwater lacustrine areas interpreted in terms of increasing humidity (Alonso-Zarza and Calvo, 2002), as is the case of unit T6 in the Ebro Basin.  The studied interval in the Plana de la Negra-Sancho Abarca area was partly sedimented during the period of warmer conditions referred to as the MMCO (Suarez-Hernando, 2017). The MMCO is dated from the marine sedimentary record between 17 and 14.5Ma (Zachos et al., 2001(Zachos et al., , 2008 and is roughly characterised by an abrupt and persistent increase in temperature and humidity, within a period of progressive cooling throughout the Neogene (Böhme, 2003).
This climatic history is broadly consistent with micromammal, charophyte and ostracod data obtained of lower to middle Miocene strata in the studied area (Fig.  15). Rodent data in the Bardenas Reales area (see Fig. 1 for location) record the establishment of warm conditions at 19Ma (Suarez-Hernando, 2017). Similar support for warming conditions was provided from fossil site PF1, at the base of the Pico del Fraile section, dated at 17.5Ma (Fig. 6), corresponding with an age below the lower and middle Burdigalian boundary. Younger rodent assemblages in these sections also indicate warm conditions, with a Suarez-Hernando, 2017).
Thus, in the studied continental sediments of the Ebro basin, a warming trend starting ca. 2 million years earlier than in marine environments, that is, between 20 and 18Ma, has been detected by rodent assemblages. However, the thermal maximum is recorded from 17.4Ma onward, coinciding with the increase in fauna related to tropical forest and wetter ecosystems, such as the glirid Vasseuromys (Suarez-Hernando, 2017) and the decrease in cricetids, coinciding with northward migration of the glirid key taxon Peridyromys murinus (Daams et al., 1997). Böhme (2003) described a similar pattern based on ectothermic vertebrates of central Europe and placed the onset of the warm and humid optimum between 18 and 16.5Ma.
Ostracods collected from the top of the outcropping T6/1 unit in the Loma Negra and Sancho Abarca sections by Martínez-García et al. (2014) include abundant specimens of Pseudocandona parallela. This species is characteristic of a stable lacustrine environment, ephemeral ponds present within the T5 alluvial plain. These conditions are recorded between 16.1 and 15.5Ma, coinciding with the maximum development of limestones within the Tudela Formation (Larrasoaña et al., 2006), which partly correlates to unit T6, and this interval records the maximum expansion of the lacustrine conditions in the Ebro Basin during the Neogene . These changes in water temperature and the presence of the glirid Vasseuromys in these deposits could indicate climate with strong seasonal variations (Martínez-García et al., 2014;Suarez-Hernando, 2017), which appears to be typical of the MMCO, together with an increase of humid ecosystems (Böhme, 2003;Diester-Haass et al., 2009). studies, e.g. based on stable isotopes.

REMARKS AND CONCLUSIONS
Stratigraphic and sedimentologic study of deposits in the T5 and T6 tectosedimentary units in the Plana de la Negra-Sancho Abarca area (western central sector of the Ebro Basin) has allowed i) construction of a sedimentary facies model, ii) analysis of sedimentary environments through space and time, and iii) an assessment of climatic and tectonic controls on sedimentation. The main highlights of this study are: Five lithofacies have been characterized and mapped. The boundary between tectosedimentary units T5 and T6 has been established as a regional marker which represents the change from clastic to carbonate sedimentation. 17 sedimentary facies comprising carbonate, mixed and siliciclastic deposits have been recognised. These facies are stacked within 7 typical facies sequences (Facies Associations, FA) corresponding with deposition in three distinct environmental settings: distal alluvial, palustrine and lacustrine. A sedimentary facies model for this succession envisages a laterally extensive alluvial plain intersected by low sinuosity bedload channels with lateral and terminal splays, and associated ephemeral freshwater ponds developed on the alluvial plain. Sedimentation of clastic at the Pyrenean (northern) basin margin. Further South, a large freshwater carbonate lake was developed, with fauna was surrounded by wide palustrine fringes that underwent intermittent subaerial exposure.
Correlation with magnetostratigraphically and biostratigraphically dated sections nearby has allowed assignment of the Plana de la Negra-Sancho Abarca succession to the Burdigalian to Langhian (early Miocene to middle Miocene), corresponding to C5Dr to C5Br. The boundary between units T5 and T6 dated at 16.06-16.05Ma.
Unit T5/1 records deposition on a laterally extensive palustrine ponds. This stage occurred during an overall retreat of alluvial inputs, with alluvial incursions within the lacustrine deposits favoured by low lake level conditions. Stage T5/2 records a gradual transition from distal alluvial to lacustrine-palustrine environments. D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 0 . 1 8 . 7 Miocene alluvial and lacustrine deposits, Ebro Basin, NE Iberia 22 Unit T6/1 is dominated by lacustrine and palustrine carbonates across the area and is interpreted to have been deposited during the maximum expansion of the lacustrine system associated with increasing humidity. T6/2 crops out only in the south and southeast of the area inputs from the north.
Regardless the contractional deformation in the Pyrenees (Guarga thrust unit) during the early to middle Miocene, the sedimentary evolution of the studied succession rather in central areas of the Ebro Basin.
The lacustrine expansion represented by the beginning of unit T6 is ascribed to a regional increase in humidity,

ACKNOWLEDGMENTS
of the Spanish Government and European Regional Funds. This work is a contribution of the Geotransfer Research Group (Aragón Government, FEDER and the University of Zaragoza), and is also a contribution to the Group of the Basque Research Group System IT-930-16. We thank the Servicio de Preparación de Rocas y Materiales Duros (Servicio General de Apoyo a la Investigación-SAI) of the University of Zaragoza and the SGIKER of the  l o g i c a A c t a , 1 8 . 7 , 1 -2  Miocene alluvial and lacustrine deposits, Ebro Basin, NE Iberia