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New insights on stratigraphy and sedimentology of the Oncala Group (eastern Cameros Basin): implications for the paleogeographic reconstruction of NE Iberia at Berriasian times/ Nuevas aportaciones sobre la estratigrafia y sedimentologia del Grupo Oncala (Cuenca de Cameros oriental): implicaciones para la reconstruccion paleogeografica del NE de Iberia durante el Berriasiense.

1. Introduction

Berriasian successions in the Basque-Cantabrian, Iberian, and Pyrenean Basins include continental, coastal, and open marine facies. Prior paleogeographic reconstructions of NE Iberia for Berriasian times placed a northern shoreline in the southern margin of the Basque-Cantabrian Basin, and a southern shoreline in the southeasternmost area of the Iberian Basin (Salas et al., 2001; Aurell et al., 2003; Mas et al., 2004; and references therein). The Cameros Basin, which occupied an intermediate position between these areas, has been traditionally interpreted as a continental basin with few sporadic marine incursions (e.g. Mas et al., 1993; Mas et al., 2004). Specifically, the Berriasian Oncala Group of the Cameros Basin has been previously interpreted as developed in an endorheic playa-lake system, which consisted of continental sandy-muddy flats passing eastwards to saline lakes, in the lower part, and as deposits of deep carbonate lakes laterally related with fluvial deposits in the upper part (Gomez-Fernandez, 1992; Gomez-Fernandez and Melendez, 1994a; Melendez and Gomez-Fernandez, 2000). However, more recent studies on the Berriasian succession of the Cameros Basin indicate that siliciclastic tidal flats and coastal, shallow, carbonate-sulphate water bodies (Quijada et al., 2010b; 2013; 2014) were developed in the basin; an interpretation that implies significant changes for the paleogeographic reconstructions of NE Iberia at Berriasian times.

In order to address the paleogeographic implications of these new data about the Oncala Group (Berriasian in age), a stratigraphical and sedimentological analysis is performed in this study to interpret the depositional systems developed in the Cameros Basin at Berriasian times, the vertical and lateral facies evolution, and the paleogeographic reconstruction of the basin. The new information about the Oncala Group is compared with published data of adjacent basins (Basque-Cantabrian and southern Iberian Basins) to establish possible connections of the Cameros Basin with marine waters, and to revise the paleogeographic reconstructions of NE Iberia at Berriasian times.

2. Geological setting

As a result of the opening of the North Atlantic Ocean, an intraplate rifting took place in Iberia from the late Jurassic to the Lower Cretaceous (e.g., Mas and Salas, 2002; Mas et al., 2004), and a series of extensional basins were developed in Iberia, which include the Iberian Mesozoic Rift System, the Basque-Cantabrian Basin, and the Pyrenean Basin (Fig. 1). The Cameros Basin was the northwesternmost basin of the Iberian Mesozoic Rift System (Mas et al., 1993; Guimera et al., 1995), and recorded the highest subsidence and accumulation rates of this system, with more than 6000 m of vertical thickness of sediments from the Tithonian to the early Albian (Mas et al., 1993; 2002; 2011; Arribas et al., 2003). The infill of the basin corresponds to a large cycle or super-sequence, divided in eight depositional sequences (Fig. 2) bounded by stratigraphic unconformities (Mas et al., 1993; 2002; Arribas et al., 2003). These depositional sequences consist of continental and transitional deposits (Mas et al., 1993; 2002; Quijada et al., 2010b; 2014; Suarez-Gonzalez et al., 2010; 2014).

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The Oncala Group, which is the aim of this study, corresponds to the third depositional sequence of the basin (Fig. 2), and was deposited in the eastern sector of the Cameros Basin (Mas et al., 1993; Gomez-Fernandez and Melendez, 1994b), which limits to the West with the Demanda Range (Fig. 3A). Based on data of charophyte and ostracod assemblages, its age is considered as Berriasian (Salomon, 1982a; Schudack and Schudack, 2009). The Oncala Group overlies the Tera Group (Fig. 2), which corresponds to the first and second depositional sequences, and is overlain by the Urbion Group, which corresponds to fourth to seventh depositional sequences (Mas et al., 1993; 2002; Gomez-Fernandez and Melendez, 1994b). The Oncala Group is one of the thickest units of the Cameros Basin comprising up to 2500 m of sediments in depocentral areas of the basin, and contains both siliciclastic and carbonate-evaporitic deposits, which are laterally related. Western areas contain mostly siliciclastic deposits, and change gradually to carbonates and evaporites to the East (Fig. 3B). Various sedimentological interpretations of these deposits have been given by different authors. Salomon (1982a) and Guiraud (1983) interpreted western siliciclastic deposits as developed in fluvial systems, and eastern carbonate-evaporitic deposits as formed in sabkhas. Gomez-Fernandez (1992) and Melendez and Gomez-Fernandez (2000) proposed that the lower sediments of the Oncala Group were deposited in a continental playa-lake system with sandy-muddy flats in western areas and saline lakes in eastern areas, while the upper sediments were deposited in deep carbonate lakes passing to fluvial systems westwards. Recent studies state that the siliciclastic succession of central areas of the Oncala Group deposited in broad tidal flats (Quijada et al., 2010b; 2014), and the upper carbonate-evaporitic deposits were developed in shallow, carbonate-sulphate water bodies and their peripheral mudflats (Quijada et al., 2013). Moreover, the latter interpretation is consistent with studies on the ostracod assemblages of the Oncala Group, which indicate mixed fresh and brackish water conditions with marine incursions (Schudack and Schudack, 2009).

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3. Methods

Geological mapping of the Oncala Group, including its limits, lithostratigraphic units, and tectonic structures, was performed using field observations, aerial photographs, and satellite images. All the obtained cartographic information was georeferenced using the ArcGIS program, the map shown in Figure 3B having been created with this software.

Four selected stratigraphic sections (Yanguas, Magana, Aguilar, and Cervera sections) were logged in detail from base to top of the Oncala Group in the central and eastern areas of this unit (Figs. 3B, 4), collecting data on bed thickness, lithology, sediment grain size, sedimentary structures, paleocurrents, and fossil content. The sections were logged in the locations where the successions show the best outcropping conditions. Bed thickness measurements were performed at the decimetre scale, and observations at the centimetre and millimetre scale. In addition, 16 individual outcrops and partial stratigraphic sections of the Oncala Group have been studied. Data of the westernmost area of the Oncala Group are based on Gomez-Fernandez (1992), as well as on our own observations.

A total of 566 rock samples were collected for laboratory studies. For each sample, a polished and uncovered thin section was prepared to 30 pm thickness for petrographic analysis under transmitted-light microscopy. A half of each thin section was stained with Alizarin Red S andpotassium ferricyanide (Dickson, 1966).

In order to analyse the relationships of the Oncala Group with other Berriasian deposits in Iberia an exhaustive bibliographic study has been carried out, including the compilation of stratigraphic and sedimentological information from scientific articles, 1:50000 geological maps edited by the Spanish Geological Survey (I.G.M.E.) and the Catalonian Geological Survey (I.G.C.), and synthetic stratigraphic sections of oil exploration wells, included in Lanaja and Navarro (1987). The information obtained from this bibliographic study has been summarized in a map created with the ArcGIS program by adding the points which show the location of the outcrops and wells that contain Berriasian rocks (Fig. 5). Chronostratigraphic information of the used 1:50000 maps has been revised on the basis of the data published in more recent publications. The added points have been coloured according to their sedimentary environment, as stated in the existing literature.

4. Stratigraphic framework

The Oncala Group was originally defined by Tischer (1966) mainly for the lower carbonate units deposited in the Cameros Basin (Fig.6). This definition was also used by Clemente (2010), who also included the carbonate deposits of the Tera Group (see Fig. 2) located in southern and southeastern areas of the Demanda Range, and the Leza Formation within this group. Gomez-Fernandez (1992), Mas et al. (1993), and Gomez-Fernandez and Melendez (1994b) redefined the Oncala Group to include only the sediments of the third depositional sequence of eastern Cameros Basin. The latter definition is widely used nowadays, and it is used in the present study. The Oncala Group as defined by Gomez-Fernandez (1992), Mas et al. (1993), and Gomez-Fernandez and Melendez (1994b), corresponds to Cycles IIC and IID proposed by Salomon (1982a; 1982b), and Cyclothem II by Guiraud and Seguret (1985) (Fig. 6). Several authors include the Leza Formation (Fig. 2), which crops out at the northern margin of the Cameros Basin, within the Oncala Group (Hernandez-Samaniego et al., 1990; Doublet, 2004; Clemente, 2010), but other authors (Tischer, 1966; Guiraud, 1983; Alonso and Mas, 1990; Mas et al., 1993; MartinClosas andAlonso, 1998; Suarez-Gonzalez et al., 2010) interpret the Leza Formation as part of the Enciso Group (see Fig. 2). Suarez-Gonzalez et al. (this volume) discuss this controversy in depth, and show that the Leza Formation is not part of the Oncala Group, but is included in the Enciso Group.

The Oncala Group contains both siliciclastic and carbonate-evaporitic deposits, which are laterally related, and is characterized by very gradual lateral and vertical changes (Figs. 3B, 4, 6, 7). Siliciclastic deposits are mostly present in western areas of the basin, and they show gradual lateral changes from coarser-grained facies in westernmost areas of the basin to finer-grained sediments eastwards. Carbonate-evaporitic deposits occur in eastern areas of the basin, and they are progressively more extensive towards the upper part of the unit, until they occupy most of the basin (Figs. 3B, 4, 6).

Attending to the lithostratigraphic characteristics, several subdivisions of the Oncala Group have been proposed by different authors (Fig. 6). Salomon (1982a; 1982b) divided these deposits in two phases, and defined seven different facies. Guiraud and Seguret (1985) divided the Oncala Group into two phases almost identical to those of Salomon (1982a; 1982b), but these authors defined only three formations. The lower phase was subdivided in a predominantly siliciclastic Huerteles Formation and a predominantly carbonate Aguilar Formation. The Valdeprado Formation was defined for the gypsiferous laminated carbonates of the upper phase.

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Gomez-Fernandez (1992) and Gomez-Fernandez and Melendez (1994b) subdivided the Oncala Group into two alloformations separated by an unconformity: the Huerteles Alloformation and Valdeprado Alloformation. Both alloformations contain laterally related siliciclastic and carbonate-evaporitic deposits. The lower Huerteles Alloformation is not present in the westernmost area of the basin, whereas the upper Valdeprado Alloformation is present in the entire eastern Cameros Basin.

5. Sediments of the Oncala Group

Sediments of the Oncala Group are studied in detail in this work. In order to analyse the lateral and vertical changes that occur within this unit, different areas of the basin are described separately.

5.1. Western area of the Oncala Group

Deposits of the western area of the Oncala Group, which are mainly made up of siliciclastic facies, crop out in the area of the town of Montenegro de Cameros (Fig. 4, Mo stratigraphic section), comprising around 650 m of sediments (Gomez-Fernandez, 1992; Gomez-Fernandez and Melendez, 1994b). In this study, sediments of the western area of the Oncala Group are divided in three main intervals based on their facies associations.

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The lowermost interval (interval B of Fig. 4) consists of sandstone bodies, and less abundant conglomerate bodies, interbedded with red lutites (Gomez-Fernandez, 1992; Melendez and Gomez-Fernandez, 2000). According to these authors, sandstone bodies are up to 6 m thick, and they display erosive bases and trough cross-stratification. The conglomerate bodies are approximately 4 m thick, and display erosive bases. They consist of up to 50 cm-thick tabular massive bodies. The red lutites are massive, and commonly show pedogenic features, such as nodulization and rhizoliths. Sandstone meander loop bodies are also present in the upper part of this interval.

The next interval (interval C of Mo, Fig. 4) contains mainly sandstone meander loop bodies, which are made up of lateral accretion units, and lutites, interbedded with black peloidal carbonates and black shales (Gomez-Fernandez, 1992; Mas et al., 2011). Carbonates are commonly laminated, and contain ostracods, charophytes, bivalves, gastropods, and vertebrate and plant remains. The proportion of carbonates relative to sandstones increases upwards the succession.

The third interval (interval D of Mo, Fig. 4) consists exclusively of black carbonates with abundant desiccation mudcracks and black shales (Mas et al., 2011).

5.2. Central area of the Oncala Group

The deposits of the central area of the Oncala Group are studied in detail in the stratigraphic sections of Yanguas and Magana (Fig. 4, Yn and Ma stratigraphic sections). The central area of the Oncala Group comprises up to 2000 m of sediments, and contains both siliciclastic and carbonate-evaporitic deposits. Four main intervals are recognized in the central area of the Oncala Group.

Interval A of the stratigraphic section of Magana (Fig. 4) is made up of alternating laminae of carbonate mudstone and calcite and quartz pseudomorphs after gypsum, interbedded with shales and some sandstones, which are more common upwards. Interval A of the stratigraphic section of Yanguas consists of laterally extensive heterolithic layers and meander loop bodies, interbedded with some carbonate layers.

Interval B of sections Yn and Ma (Fig. 4) contains mostly siliciclastic deposits (analysed in detail by Quijada et al. 2010; 2014), which consist of laterally extensive heterolithic layers and less abundant, meander loop bodies (Fig. 8A, B). Laterally extensive layers (tens to hundreds of metres wide) are made up of interlaminated siliciclastic mudstone and sandstone, displaying lenticular, wavy, and flaser stratification, and ubiquitous desiccation mudcracks and vertebrate footprints. Meander loop bodies are tabular, with widths of up to 70 m and thicknesses of1-3 m. They are formed by one or more adjoined point bar bodies limited by reactivation surfaces (Fig. 8A). Point bar bodies fine upwards, and display inclined heterolithic stratification (IHS) and flaser, wavy and lenticular bedding (Fig. 8C). Desiccation mudcracks and vertebrate footprints are common in the upper part of the point bars. Fossil content is scarce in both laterally extensive layers and meander loop bodies, and it includes fragments of bones, ostracods, and rare charophytes. Predominant paleocurrents indicate transport directions to East and Southeast, but occasionally they may display bidirectional pattern. These siliciclastic deposits pass eastwards to, and are interbedded with, generally massive, carbonate mudstones with centimetre-size, calcite and quartz pseudomorphs after lenticular gypsum (Fig. 4, interval B of Ma, Figs. 8D, E).

Interval C of sections Yn and Ma (Fig. 4) consists of interbedded parallel-laminated, carbonate-evaporitic deposits and siliciclastic deposits (Figs. 7A, 9A, 9B), which include meander loop bodies, laterally extensive heterolithic layers, and black shales. The parallel-laminated carbonate-evaporitic deposits consist of an alternation of laminae of carbonate mudstone and (sub-) millimetre-size calcite and quartz pseudomorphs after gypsum (Quijada et al., 2013). Pseudomorphs after gypsum are interpreted as originally lenticular gypsum crystals and detrital gypsum. Fossil content is limited to ostracods, rare charophytes, and stromatolites, and some carbonate layers contain peloids. Siliciclastic layers are more common to the West, and they are progressively less abundant eastwards and upwards, eventually disappearing in the upper part of this interval (Fig. 4).

Interval D of sections Yn and Ma (Fig. 4) consists also of parallel-laminated, carbonate-evaporitic deposits, but they display lower proportion of evaporites, abundant ripples, desiccation mudcracks (Fig. 9C), centimetre-size pseudomorphs after gypsum, and a higher fossil content, which includes ostracods and gastropods. Some laterally extensive sandstone layers interbedded with laminated carbonate-evaporitic deposits are present to the West in the Yanguas stratigraphic section.

5.3. Eastern area of the Oncala Group

The deposits of the eastern area of the basin are studied in the stratigraphic section of Aguilar (Ag, Fig. 4), which contains the thickest sedimentary infill of the Oncala Group, comprising up to 2500 m of sediments of mainly carbonate-evaporitic deposits. Four main intervals are recognized in this area (Ato D of Ag section, Fig. 4).

Interval A (Fig. 4) is made up of an alternation of laminae of carbonate mudstone and gypsum, which commonly has been replaced by calcite and quartz (Fig. 9D, E). Evaporitic layers were originally made up of displacive and detrital gypsum, and rare selenite crystals. Desiccation mudcracks filled with pseudomorphs after lenticular gypsum are present at the top of some carbonate laminae. Fossil content includes ostracods, and rare charophytes. The amount of evaporitic laminae and its thicknesses increase towards the eastern areas of the basin. These carbonate-evaporitic laminae are interbedded with shales, commonly displaying desiccation mudcracks, and carbonate breccias (Fig. 9D), which consist of millimetre to 10 centimetre-size, rectangular fragments of carbonate mudstone with pseudomorphs after gypsum, floating in a pseudosparitic calcite matrix. The breccias are interpreted as the result of tectonic deformation affecting alternating carbonate and sulphate laminae (Quijada et al., 2012; 2013).

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Interval B (Fig. 4) contains the same facies as interval A, but the amount of carbonate breccias is much larger, and laterally extensive sandstone layers, although rare, are also present. The amount of evaporitic laminae and breccias and its thicknesses increase also towards the eastern areas of the basin in this interval (Fig. 7B).

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Interval C (Fig. 4) consists of parallel-laminated, carbonate-evaporitic deposits interbedded with shales, identical to those in the central area of the basin (interval C of Yn and Ma in Fig. 4). These carbonate-evaporitic deposits are distinguishable from those of the intervals A and B in the Aguilar section due to more conspicuous, continuous, parallel lamination, comparatively less amount of pseudomorphs after gypsum, and scarcity of carbonate breccias (Fig. 4). Sporadic desiccation mudcracks are present in interval C. Fossil content includes ostracods, stromatolites, and according to Gomez-Fernandez (1992) and Melendez and Gomez-Fernandez (2000), very scarce foraminifera.

Interval D (Fig. 4) is very similar to that of the central area because it consists of parallel-laminated, carbonateevaporitic deposits with lower proportion of evaporites, ripples, centimetre-size pseudomorphs after gypsum, desiccation mudcracks, rare tepees, ostracods, gastropods, and bivalves.

5.4. Easternmost area of the Oncala Group

The easternmost area of the Oncala Group is studied in the stratigraphic section of Cervera, which comprises 720 m of sediments (Ce, Fig. 4), and despite the differences in thickness, shows many parallelisms with the adjacent area of Aguilar (Ag, Fig. 4). Four main stratigraphic intervals (A to D of Ce stratigraphic section, Fig. 4) are recognized in this area.

Interval A (Fig. 4) is made up of alternating laminae of carbonate mudstone and gypsum (Fig. 9F). Desiccation mudcracks are common at the top of the carbonate laminae, in contrast with deposits of the interval A in the adjacent Aguilar stratigraphic section (Fig. 4). Evaporitic laminae, now made up of secondary gypsum, were probably originally made up of lenticular gypsum crystals, detrital gypsum, and less abundant selenite gypsum crystals (Quijada et al., 2010a). Sulphur isotopic composition of the secondary gypsum has values ranging from 17.8 to 20.3[[per thousand].sub.V.CDT], and the mean value is 18.2[[per thousand].sub.V.CDT] (Alonso-Azcarate et al., 2006).

Interval B (Fig. 4) is characterized by the presence of sandstone-mudstone layers and carbonate breccias in addition to alternating laminae of carbonate mudstone and secondary gypsum. Sandstone-mudstone layers are up to 6 m thick and laterally very extensive (hundreds of metres wide), and show flat bases and tops. They display lenticular, wavy and flaser stratification. Desiccation mudcracks are abundant at the top of the siliciclastic and carbonate mudstones. Paleocurrents indicate transport directions to the South.

Interval C (Fig. 4) is made up of parallel-laminated, carbonate-evaporitic deposits, with abundant stromatolites and rare peloidal carbonate layers, made up of alternating laminae of peloid packstone and grainstone. The uppermost interval D (Fig. 4) is also made up of parallel-laminated, carbonate-evaporitic deposits with lower proportion of evaporites, centimetre-size pseudomorphs after gypsum, desiccation mudcracks, and larger fossil content.

6. Discussion

6.1. Sedimentological interpretation

Attending to lithological characteristics, the deposits of the Oncala Group can be classified in two main groups: siliciclastic deposits, which occur mostly in western areas of the basin, and carbonate-evaporitic deposits, which are more abundant eastwards. Firstly, the sedimentological characteristics of the siliciclastic and carbonate-evaporitic deposits are discussed separately; then, in order to make a paleogeographical interpretation of the Oncala Group, the lateral relationships of both types of deposits are analysed.

Siliciclastic deposits

Siliciclastic deposits of the Oncala Group occur in western to central areas of the basin, and show very gradual lateral and vertical facies changes. Westernmost areas of the basin (Mo section, Figs. 3B, 4) contain conglomerate and trough cross-stratified sandstone bodies in the lower part of the succession (interval B of Mo, Fig. 4), which were formed in braided fluvial systems, according to Gomez-Fernandez (1992) and Melendez and Gomez-Fernandez (2000). These conglomerate and sandstone bodies progressively change upwards to sandstone meander loop bodies (interval C of Mo, Fig. 4), which were probably deposited in meandering fluvial systems (Gomez-Fernandez, 1992; Melendez and Gomez-Fernandez, 2000).

Towards central areas of the basin, the fluvial deposits pass to laterally extensive heterolithic layers and meander loop bodies (intervals A to C of Yn and Ma, Fig. 4). Contrarily to previous interpretations of the deposits of central areas of the basin as continental sandy-muddy flat deposits (Gomez-Fernandez, 1992; Melendez and Gomez-Fernandez, 2000), recent studies demonstrate that they were formed in broad, low-gradient, inter- to supratidal flats, traversed by shallow, meandering channels (Quijada et al., 2014). The criteria that led to this interpretation are: the presence of meander loop bodies displaying low angle, lateral accretion units and IHS (Fig. 8A); alternation of sandstone and mudstone laminae that form lenticular, wavy and flaser bedding (Fig. 8C); rhythmic variations in the type of bedding and the thicknesses of the sandstone-mudstone couplets; abundant evidence of subaerial exposure at the top of numerous, consecutive laminae; predominance of non-channelled facies over meander loop bodies (Fig. 8B); and the fine grain size of the sediments and large amount of mudstone.

Carbonate-evaporitic deposits

The carbonate-evaporitic deposits can be divided in a lower and an upper group, attending to facies differences.

The lower carbonate-evaporitic deposits (stratigraphic intervals A and B of Ma, Ag, and Ce, Figs. 4, 9D, 9E), which contain large amounts of evaporites, carbonate breccias, and shales with desiccation mudcracks, are interpreted as formed in shallow, carbonate-sulphate water bodies and saline mudflats (Gomez-Fernandez, 1992; Gomez-Fernandez and Melendez, 1994a; Quijada, 2009). The sulphur isotope composition of the gypsum of the easternmost area of the basin (17.8 to 20.3[[per thousand].sub.V.CDT]) led Alonso-Azcarate et al. (2006) to interpret that the main source of sulphate was probably the recycling of Triassic evaporites, which have [[delta].sup.34]S compositions around 14.5[per thousand], and that the sulphur isotope composition was modified by bacterial sulphate reduction in the water bodies. Nonetheless, they also suggested that Berriasian seawater could have been an additional source of sulphate in the Cameros Basin. However, recent seawater sulphur isotope curves for the Cretaceous indicate that lowermost Cretaceous seawater had values of around 20[per thousand] (Paytan et al., 2004; Paytan and Gray, 2012). This value matches the composition of the gypsum of the Oncala Group, suggesting that it is more plausible that the source of sulphate was the Berriasian seawater. This possibility is reinforced by the fact that these carbonate-evaporitic deposits are laterally related with tidal flat deposits, and by the large amount of sulphate that precipitated in the Oncala Group. Although seawater input was recurrent, the water bodies were somehow confined, so water loss exceeded inflow, and consequently, salinity increased to gypsum supersaturation point (cf. Warren, 2006; Orti, 2010).

The upper carbonate-evaporitic deposits (intervals C and D of Yn, Ma, Ag, and Ce, Fig. 4), which consist of parallel-laminated alternating carbonate mudstone and pseudomorphs after gypsum, contain slightly less proportion of pseudomorphs after sulphates relative to carbonates than the lower carbonate-evaporitic deposits, and show more conspicuous lamination (Figs. 9A, B). Contrarily to previous interpretations that ascribed a deep lacustrine origin to the upper carbonate-evaporitic deposits (Gomez-Fernandez, 1992; Melendez and Gomez-Fernandez, 2000), recent studies indicate that they accumulated in shallow, perennial, carbonate-sulphate water bodies (Quijada et al., 2013). This interpretation is based on the presence of sporadic desiccation mudcracks, which are very abundant in some stratigraphic intervals (Fig. 9C); presence of rippled carbonates; interbedding of parallellaminated carbonate-sulphate deposits with carbonate displaying centimetre-size pseudomorphs after gypsum, tepees, and desiccation cracks; absence of marked slope gradients in the basin; presence of the same laminated carbonate facies along tens of kilometres; and direct interbedding and lateral change with siliciclastic sandymuddy flat deposits (fig. 7A). Salinities in the upper carbonate-sulphate water bodies were likely slightly lower than during deposition of the lower carbonate-evaporitic deposits, because sulphate deposits are less abundant.

Both the lower and upper carbonate-evaporitic deposits show less abundant pseudomorphs after gypsum in the areas adjacent to the siliciclastic deposits (Fig. 7B), which probably indicates lower salinities. This could be related with more freshwater input in these areas, probably coming from the siliciclastic fluvial system located to the west (Mo section, Fig. 4), and thus, relative smaller influence of marine waters.

Paleogeographic reconstruction of the Oncala Group

The presence of the same facies associations along tens of kilometres in the Oncala Group, the very gradual, lateral and vertical facies changes that characterize this unit (Fig. 4) and the presence of facies deposited only in shallow water environments, suggest that the sediments of the Oncala Group were developed in very broad, low-gradient, shallow areas. These broad shallow areas were occupied by rivers and floodplains in the westernmost area of the basin, tidal flats in western and central areas, and carbonate-sulphate water bodies in central and eastern areas (Fig. 10A). These depositional systems were laterally related, as shows the gradual, lateral and vertical facies changes between the siliciclastic and the carbonate-evaporitic deposits of the Oncala Group (Figs. 4, 10A). Both siliciclastic tidal flats and carbonate-evaporitic water bodies had marine connection, as indicate respectively the influence of tidal currents, and the large sulphate input and sulphur isotope compositions. Moreover, the ostracod assemblages of the Oncala Group indicate mixed fresh and brackish water conditions with marine incursions (Schudack and Schudack, 2009). The evidence of marine influence in these two systems suggests that a broad, coastal, protected, shallow embayment was developed in the eastern sector of the Cameros Basin during Berriasian times. According to the facies distribution, the basin received freshwater and siliciclastic input from westernmost areas. The siliciclastic source was probably the Demanda Massif (Figs. 3A, 10A), which began to behave as a paleogeographic high since the Late Jurassic (Alonso et al., 1986-1987; Benito and Mas, 2006). Abundant freshwater discharges from rivers into the tidal siliciclastic system may have impeded the presence of marine fossils in the Oncala Group, although additional causes, such as high rates of sedimentation, high suspended-sediment concentrations, or strongly acidic waters, cannot be excluded (Quijada et al., 2014).

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Some modern sedimentary settings present similarities with the paleoenvironments of the Oncala Group. An example could be the Bombetoka Bay in Madagascar, a large tidal embayment where freshwater from the Betsiboka River mixes with salty water from the sea (Raharmahefa and Kusky, 2010). The large freshwater discharges and strong river dynamics cause the prevalence of freshwater over the marine influences in the Betsiboka Estuary. The Upper San Francisco Estuary/ Sacramento Delta in California is another example of an inland tidal delta located in a protected bay (Brown and Pasternack, 2004) in which fresh to brackish conditions occur (Wells and Goman, 1995). However, these two modern examples do not contain carbonate-evaporitic deposits adjacent to the siliciclastic tidal system, as occurs in the Oncala Group, probably because of lower evaporation/water input rates in these modern environments. The eastern area of the Indus Delta is an example of a tide-dominated siliciclastic system laterally related with a vast evaporitic mudflat that receives seawater input (Inam et al., 2007; Dalrymple and Choi, 2007). These modern coastal evaporitic mudflats occupy a protected area, as it is interpreted for the carbonate-evaporitic system of the Oncala Group.

Moreover, the broad, shallow, carbonate-sulphate water bodies interpreted for the Oncala Group can be compared with ancient, evaporitic epicontinental seaways that spread out across large continental areas (see Warren, 2006,2010), such as those developed in the Northern Delaware Basin margin, USA, at Permian times (Elliott and Warren, 1989), and in the Arbuckle Group, Oklahoma (USA), during the Ordovician (John and Eby, 1978). These seaways caused covering of huge restricted areas by extremely shallow water sheets, much of them only tens of centimetres deep. Extensive evaporite deposits and muddy carbonates, mostly peloid mudstones and packstones, were deposited in these evaporitic seaways. The laminated carbonate mudstone and sulphate deposits of the Oncala Group show some resemblances to these ancient deposits, in that they were developed in broad, shallow, confined areas with high salinities, and muddy/ peloidal carbonates and evaporites were deposited (Quijada et al., 2013).

6.2. Lateral and vertical evolution

In addition to the identification of the four main stratigraphic intervals (Figs. 4, 6), the analysis of the lateral and vertical facies changes of the Oncala Group allows the recognition of very gradual retrogradational and progradational trends of the siliciclastic deposits and the carbonate-evaporitic deposits. Stratigraphic interval A (Figs. 4, 6) is characterized by the gradual progradation of siliciclastic sediments over carbonate-evaporitic deposits (Figs. 4, 6). The maximum progradation of the siliciclastic sediments marks the start of the stratigraphic interval B (Figs. 3B, 4, 6). A general, gradual retrogradational trend is then observed in this interval.

The important retrogradation of the siliciclastic facies that occurred at the base of the interval C triggered a westward migration of the siliciclastic deposits, and the concurrent expansion of carbonate-evaporitic deposits over most of the basin (interval C, Fig. 4, 6). Moreover, the carbonate-evaporitic deposits of interval C contain smaller amounts of evaporites, which likely indicate shorter periods of confined conditions in the carbonatesulphate water bodies than in the lower intervals A and B. In the central areas of the basin (Yn, Figs. 3B, 4, 6, 7A), siliciclastic facies of the lower part of the interval C are interbedded with the carbonate-evaporitic deposits. The siliciclastic content gradually decreases upwards while the content of carbonate-evaporitic deposits increases until they occupy the entire basin in the upper part of interval C (Figs. 3B, 4, 6). This suggests that the retrogradational trend recorded in interval B continued throughout interval C.

Interval D (Fig. 4) is characterized in all the stratigraphic sections by laminated carbonate-evaporitic deposits containing larger fossil content, lower proportion of evaporites, black shales, abundant mudcracks, ripples, centimetre-size pseudomorphs after gypsum, and tepees. This demonstrates more abundant freshwater and siliciclastic discharges into the water bodies than in the underlying sediments, indicative of a progradational trend.

Gomez-Fernandez (1992) interpreted that the succession of the westernmost area of the basin (Mo, Fig. 4) was laterally related only with the upper carbonate-evaporitic deposits (i.e. intervals C and D of this study). However, this author describes a gradual vertical change in the stratigraphic log of Montenegro from deposits of braided rivers to meandering rivers interbedded with rare carbonate layers, and finally to black carbonate deposits. We suggest that this retrogradational trend is rather related to the retrogradational trend recorded in the stratigraphic intervals B and C (Fig. 4). The occurrence of the carbonate layers is likely related with the important retrogradation that marks the beginning of the interval C.

[FIGURE 10 OMITTED]

Another important issue is the sharp change in thickness, from 2500 m to 700 m of sediments, between the sections of Aguilar and Cervera (Ag and Ce in Fig. 4), i.e. a difference of 1800 m between areas that are less than 10 km apart. An important fault and several tectonic structures separate now these two areas (Fig. 3B). It is suggested that this fracture zone was active during deposition of the Oncala Group, as was previously proposed by Gomez-Fernandez and Melendez (1994a), and it was afterwards reactivated during the Alpine Orogeny. Moreover, in the easternmost area of the basin the same retrogradational and progradational trends as in the adjacent area are recognized, which suggests that the four stratigraphic intervals of the Oncala Group were also recorded in this area of reduced thickness. These considerations indicate that the syn-sedimentary fracture zone was active during the deposition of the entire Oncala succession. The lower accommodation rate in the area of Cervera relatively to the area of Aguilar, however, did not cause major facies changes between both areas, but is responsible for more common subaerial exposure in the area of Cervera (Fig. 4).

6.3. Lithostratigraphic revision of the Oncala Group

The lithostratigraphic units proposed by other authors for the Oncala Group are revised considering the updated stratigraphic data (Fig. 6). Although Gomez-Fernandez (1992) and Gomez-Fernandez and Melendez (1994b) based their subdivisions of the Oncala Group on the presence of an unconformity (see section 4 and Fig. 6), we have not recognized any unconformity within the Oncala Group, but gradual vertical and lateral facies changes, as discussed above (Figs. 3B, 4, 6). As a consequence, we consider that subdivisions based on lithostratigraphic considerations, similar to those proposed by Guiraud and Seguret (1985, see section 4 and Fig. 6), are more suitable. In agreement with Guiraud and Seguret (1985), the western siliciclastic deposits of the Oncala Group are grouped as the Huerteles Formation; the lower carbonate-evaporitic deposits of intervals A and B, as the Aguilar del Rio Alhama Formation; and the upper carbonate-evaporitic deposits of intervals C and D, as the Valdeprado Formation (Fig. 6). Nevertheless, in contrast to the original definition of Guiraud and Seguret (1985), we interpret that the Valdeprado Formation is also laterally related with siliciclastic deposits towards the west (Figs. 3B, 4, 6, 7A). We also establish that the lower limit of the Valdeprado Formation in the central areas of the basin (Yn, Fig. 4) is marked by the presence of parallel-laminated carbonate-deposits of stratigraphic interval C, instead of by the disappearance of sandstone bodies, as was interpreted by Guiraud and Seguret (1985), Gomez-Fernandez (1992), and GomezFernandez and Melendez (1994b). Although Guiraud and Seguret (1985) named the lower carbonate-evaporitic deposits Aguilar Formation, we have enlarged this name to Aguilar del Rio Alhama Formation to avoid misunderstanding with the Tithonian-Berriasian Aguilar Formation of the Basque-Cantabrian Basin (Hernandez et al., 1999).

6.4. Comparison with other Berriasian successions of NE Iberia

Once established that the Berriasian deposits of the Cameros Basin were formed in coastal environments, it remains the question of the provenance of the marine waters. To answer that question the deposits of the Oncala Group are compared with those of the neighbouring Basque-Cantabrian, Iberian, and Pyrenean Basins, all of which containing open marine sediments.

Berriasian deposits of the Basque-Cantabrian Basin display different facies associations from western to eastern areas (summarized in Fig. 5). The Tithonian-Berriasian succession of the Cantabrian graben, in the western Basque-Cantabrian Basin (Fig. 5), is made up of continental to shallow-marine deposits (Pujalte, 1982; Pujalte et al., 2004; and references therein). Continental deposits, including fluvial sandstone and mudstone and lacustrine-palustrine carbonates and evaporites, are more extensive in the lowermost part of the succession, being best developed in southwesternmost areas (Garcia de Cortazar and Pujalte, 1982; Lanaja and Navarro, 1987; Pujalte et al., 2004; and references therein). Sediments of the middle part of the succession, which consist of siliciclastic and carbonate mudstones and less abundant sandstones, are interpreted as formed in shallow brackish lagoons (Pujalte, 1982; Garcia de Cortazar and Pujalte, 1982). The overlying sediments, made up of sandstones, sandy limestones, and mudstones, are interpreted as deposited in broad tidal flats and channels, and they form a transgressive-regressive cycle (Pujalte, 1982; Garcia de Cortazar and Pujalte, 1982). Continental deposits then prograde over the tidal flat sediments in the uppermost part of the Berriasian succession.

In the Bilbao anticlinorium, in the central area of the Basque-Cantabrian Basin (Fig. 5), Lower Cretaceous thick successions containing interbedded black shales, evaporites (anhydrite and gypsum, and also halite in Cegama-1 and Aitzgorri-1 wells), and minor sandstones and limestones, have been drilled in oil exploration wells (Lanaja and Navarro, 1987; Abalos et al., 2008; Iribar and Abalos, 2011). Unfortunately, the low biostratigraphic resolution of the fossils in these deposits makes them difficult to date. Thus, Sanchez-Ferrer (1991) tentatively attributed them a Late Jurassic-Valanginian age, whereas Abalos et al. (2008) lithologically correlated them with the earliest sediments of the Villaro Formation, whose age is also uncertain (Berriasian for Ramirez del Pozo, 1969; upper Berriasian-early Valanginian for Garcia-Garmilla, 1989; upper Valanginian for Pujalte et al., 2004). Whatever the case, sulphur isotope compositions of the sulphates range between 17.4 and 23.7[[per thousand].sub.V.CDT] (mean value = 19.6[[per thousand].sub.V.CDT], Abalos et al., 2008; Iribar and Abalos, 2011), which is very similar to the composition of the gypsum of the Oncala Group (mean value = 18.2[[per thousand].sub.V.CDT], Alonso-Azcarate et al., 2006). In the northern part of the Bilbao region (Fig. 5), oil exploration wells discovered shallow marine limestones in a stratigraphic position equivalent to the described Berriasian deposits of the Basque-Cantabrian Basin. This suggests that a connection with the open sea existed in this northern area (Rosales et al., 2002b).

Eastern areas of the Basque-Cantabrian Basin (Fig. 5) contain black limestones with pellets, serpulids, gastropods, and bivalves, interpreted as deposited in restricted marine environments (Pujalte, 1982). These sediments thin eastwards, eventually disappearing in the Basque massifs, which were paleogeographic highs at that moment (Soler and Jose, 1972; Rosales et al., 2002a).

Berriasian deposits of the Iberian Basin are present in the South Iberian sub-basin, Maestrat sub-basin, and Catalonian Coastal Ranges, and display also important lateral changes (Aurell et al., 1994; Badenas et al., 2004; Mas and Salas, 2002; Mas et al., 2004; and references therein). In the western areas of the Iberian Basin (Fig. 5), siliciclastic tidal flats incised by meandering tidal channels were developed (Mas et al., 1984; Aurell et al., 1994; Cobos et al., 2010). They pass southwards to carbonate mudstones to wackestones formed in a subtidal lagoon. In eastern areas of the Iberian Basin (Fig. 5), carbonate tidal flats and fringing oolitic-bioclastic shoals were developed (Salas, 1989; Badenas et al., 2004). The lateral relationship between the carbonate and siliciclastic tidal flats is gradual, and both grade southeastwards (Fig. 5) into hemipelagic Calpionella limestones (Salas, 1989; Aurell et al., 1994). Moreover, thinner (up to 70 mthick), Tithonian-Berriasian, marine-influenced siliciclastic mudstones and sandstones (Ipas et al., 2007) have been described in the Iberian Basin in the province of Zaragoza (Villanueva de Huerva and Aguilon sectors, Fig. 5).

Berriasian deposits of the Pyrenees are present in the Organya Basin and Figueres-Montgri area (Fig. 5), and consist of a succession of marls, limestones, silty sandstones, and carbonaceous silty limestones. This succession shows a shallowing-up evolution, from pelagic deposits at the base to inner ramp sediments in the middle part, and finally, to lagoonal deposits in the upper part (Berastegui et al, 2002; Robador and Garcia-Senz, 2004).

The sedimentary record of Berriasian times in Iberia shows that open marine areas were located in northernmost Basque-Cantabrian Basin, southeasternmost Iberian Basin, and the Pyrenees. The Oncala Group display strong similarities with the deposits of the Basque-Cantabrian Basin, such as the presence of broad, brackish shallow areas, or the occurrence of siliciclastic sediments (frequently tidal) in western areas of the basin and carbonate facies in eastern areas. Moreover, evaporites may have also been formed in the Bilbao region of the Basque-Cantabrian Basin at Berriasian times. Although their age is uncertain, the similar sulphur isotope compositions and sedimentological features of the evaporitic deposits of both basins make us think that at least part of the evaporitic deposits of the Basque-Cantabrian Basin could be correlatable with those in the Oncala Group. If this hypothesis is right, the strong similarities in the sulphur isotope compositions of the sulphates of both basins would suggest that the sulphate source was the same for both areas. Furthermore, these [[delta].sup.34]S compositions between 17.4 and 23.7[per thousand] are similar to the sulphur isotope composition of lowermost Cretaceous seawater (cf. Paytan et al., 2004; Paytan and Gray, 2012), and strongly different from [[delta].sup.34]S compositions around 14[per thousand] of the Triassic evaporites of the Basque-Cantabrian and Cameros Basins (Alonso-Azcarate et al., 2006; Iribar and Abalos, 2011), which were also proposed as a sulphate source. Thus, the sulphur isotope compositions indicate that the more plausible sulphate source in both the Basque-Cantabrian and Cameros Basin was probably Lower Cretaceous seawater. All these similarities plus the proximity of both basins suggest that the Cameros Basin was connected with coastal and marine areas of the Basque-Cantabrian Basin during Berriasian times (Fig. 10B). This paleogeographic affinity of the sediments of the Cameros area with Boreal marine areas of the BasqueCantabrian Basin is not exclusive of the Oncala Group; in fact, during the late Kimmeridgian the Boreal shoreline was much closer to the Cameros area than the Tethys shoreline (Benito et al., 2005). Unfortunately, the presence of the Tertiary Ebro Basin between the Cameros Basin and the Basque Cantabrian Basin impedes establishing the exact location of the connection between these basins during Berriasian times.

An attempt of correlating the progradational-retrogradational trends observed in the Oncala Group and in the Berriasian sediments of the Cantabrian graben has been carried out. In both basins a general retrogradational trend is observed in most of the succession. In the Cantabrian graben continental deposits in the lower part pass gradually to tidal flat deposits (Pujalte, 1982; Garcia de Cortazar and Pujalte, 1982; Pujalte et al., 2004; and references therein). In the Oncala Group the siliciclastic system migrated progressively landwards and carbonatesulphate water bodies developed over most of the basin. It is interpreted that both retrogradational trends could be related to the same relative sea-level rise episode, which can be tentatively correlated with the transgressive cycle that occurred from lower to middle Berriasian in Boreal and Tethyan European basins (Hardenbol et al., 1998; Ogg et al., 2008; Ogg et al., 2012). Berriasian successions in both basins recorded a progradational trend in the uppermost part, which caused the progradation of fluvial sediments over tidal deposits in the Cantabrian graben (Pujalte, 1982; Pujalte et al., 2004), and more abundant siliciclastic and freshwater discharges in the Oncala Group. This progradational trend could be related with the regressive cycle that occurred during the upper Berriasian in the European basins (Hardenbol et al., 1998; Ogg et al., 2008; Ogg et al., 2012).

Apart from the connection with the Basque-Cantabrian Basin, a southeast connection of the Cameros Basin with the Iberian Basin at Berriasian times cannot be excluded (Fig. 10B). The development of tidal flats in the Iberian Basin, and the presence of basins with marine-influenced deposits in an intermediate position between the South Iberian and Maestrat Basins and the Cameros Basin, such as the Villanueva de Huerva and Aguilon sectors (Fig. 5), lead to interpret that a connection of the Cameros Basin and the Tethys Sea could have existed. The relationship between the Cameros and Pyrenean Basins cannot be established because a major stratigraphic gap exists in the Berriasian sedimentary record of the Pyrenean Basin. However, it is documented that a paleogeographic high, the so-called Ebro Massif, separated both basins (e.g. Ziegler, 1988; Salas et al., 2001; Mas et al., 2004).

7. Conclusions

The sedimentological analysis of the Oncala Group indicates that the eastern sector of the Cameros Basin during Berriasian times was occupied by a fluvial system in the westernmost area; broad, siliciclastic tidal flats in the western to central areas; and extensive, coastal, shallow, carbonate-sulphate water bodies, which received marine seawater input, in the central to eastern areas. This new interpretation differs from previous paleoenvironmental reconstructions, which ascribed an exclusively continental origin to these deposits.

The analysis of the lateral and vertical evolution of the Oncala Group allows the recognition of very gradual progradational and retrogradational trends of the siliciclastic deposits and the carbonate-evaporitic deposits. The Oncala Group shows a general retrogradational trend in most of the succession, which caused the gradual migration of the siliciclastic deposits westwards and the progressive spreading of the carbonate-evaporitic deposits over most of the basin. This interpretation implies that the development of carbonate-evaporitic deposits over large areas of the basin in the middle to upper part of the Oncala Group is not related with an unconformity within the unit, as was previously interpreted, but with gradual, vertical and lateral facies changes.

The sharp change in thicknesses between the easternmost area of the basin, close to the town of Cervera del Rio Alhama (up to 720 m of sediments), and the adjacent area, close to the town of Aguilar del Rio Alhama (up to 2500 m of sediments), is probably related with a syn-sedimentary fracture zone that was active during the deposition of the entire Oncala Group.

The development of broad tidal flats and coastal, shallow water bodies in the Oncala Group suggest that a wide, shallow, coastal embayment was developed in the Cameros Basin at Berriasian times. This new interpretation makes necessary to revise the paleogeographic reconstructions of this area of Iberia during Berriasian times. Comparison of the deposits of the Oncala Group with Berriasian deposits of adjacent basins strongly suggests that the Cameros Basin was connected with transitional and marine areas of the Basque-Cantabrian Basin, in which similar facies associations were deposited and a similar vertical evolution is evident. Additionally, a connection with the southern Iberian Basins cannot be excluded.

http://dx.doi.org/10.5209/rev_JIGE.2013.v39.n2.42503

Acknowledgements

This work was funded by the Spanish DIGICYT projects CGL2008-01648/BTE and CGL2011-22709, the "Sedimentary Basin Analysis" Research Group of the Complutense University of Madrid-Madrid Community and a Spanish Department of Education FPU scholarship. The authors are very grateful to B. Tessier and V. Pujalte for their thoughtful and thorough reviews, and the editors J. Lopez-Gomez and J. Martin-Chivelet for their kind support. We thank S. Lugli and F. Orti for helpful discussion about the sulphur isotope compositions, S. Sacristan for providing information about the location of Berriasian outcrops in western Cameros Basin and help during field work, and A. Baza for help in the preparation of thin sections. We thank also the staff of the IGEO and the Department of Stratigraphy of the Complutense University of Madrid for their technical support, especially to B. Moral, G. Herrero, and J.C. Salamanca for preparation of thin sections, V. Lopez for help with GIS, and L. Donadeo for bibliographic support.

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I.E. Quijada (1,2) *, P. Suarez-Gonzalez (1,2), M.I. Benito (1,2), R. Mas (1,2)

(1) Departamento de Estratigrafia, Fac. Ciencias Geologicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

(2) Instituto de Geociencias IGEO (CSIC, UCM), C/ Jose Antonio Novais 12, 28040 Madrid, Spain equijada@geo.ucm.es; pablosuarez@geo.ucm.es; maribel@geo.ucm.es; ramonmas@geo.ucm.es

* corresponding author

Received: 04/05/2013 /Accepted: 02/08/2013
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Title Annotation:texto en ingles
Author:Quijada, I.E.; Suarez-Gonzalez, P.; Benito, M.I.; Mas, R.
Publication:Journal of Iberian Geology
Date:Jul 1, 2013
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