1 Introduction

Marine sedimentary environments, especially those located in coastal areas, are very sensitive to fluctuations in sea-level associated with eustatic and tectonic factors. In a context of global warming with a potential sea-level rise, there is increasing interest in similar situations that occurred in the past, as recorded in sedimentary rocks. The Cenomanian (Late Cretaceous, from 100.5 to 93.9 Ma according to Cohen et al., 2013 updated), especially the late Cenomanian, was characterized by a high sea-level as well as by a perturbation in the carbon cycle (Erba, 2004; Jarvis et al., 2011; Kuypers et al., 2002; Pogge von Strandmann et al., 2013; Schlanger et al., 1987). These were related to mainly palaeoceanographic and palaeoclimatic changes that manifested themselves as a global warming event with oxygen-depleted conditions in many basins (Aguado et al., 2016; Bornemann et al., 2008; Huber et al., 2002; Monteiro et al., 2012; Norris et al., 2002; Pogge von Strandmann et al., 2013; Tsandev & Slomp, 2009). This event has been recorded at the South-Iberian Palaeomargin of the Tethys Ocean (e.g. Reolid et al., 2016a; Rodríguez-Tovar et al., 2020). At this palaeomargin, currently represented by the External Zones of the Betic Cordillera, organic-rich deposits typical of anoxic events in pelagic facies have been identified (O’Dogherty et al., 2001; Sánchez-Quiñónez et al., 2010; Rodríguez-Tovar et al., 2009, 2020; Reolid et al., 2016a). However, most of the works that analyse the Cenomanian of the South-Iberian Palaeomargin focus on distal facies with hemipelagic to pelagic sedimentation, whereas the areas representing shallow marine environments close to emerged lands are poorly studied (Arias et al., 1979, 1982). Other examples described of the Cenomanian flooding in shallow environments comes from Basque-Cantabrian Basin in North Spain (Ladrón de Guevara et al., 2023), Aquitaine Basin in western France (e.g., Andrieu et al., 2015), west Moroccan margin (e.g., Gertsch et al., 2010), and Western Saharan Atlas of Algeria (e.g., Benadla et al., 2018), among other basins. The aim of this work is to study the effects of the Cenomanian transgression at coastal environments, paying special attention to changes in facies from a continental to a marine environment and the establishment of marine conditions in the Sierra de Montearagón-Carcelén (Prebetic, Betic Cordillera) from the stratigraphic analysis combined with ichnological and isotopic analyses.

2 Geological setting

This work focuses on the carbonate deposits of the Sierra de Montearagón-Carcelén (Fig. 1), located around 8 km to the east of the city of Albacete (Spain). From a geological point of view, the area under study corresponds to the External Prebetic, the northernmost part of the External Zones of the Betic Cordillera. The South-Iberian Palaeomargin, located at the westernmost end of the Tethys, is represented by the External Zones of the Betic Cordillera; it evolved during the Mesozoic as a transtensive margin where the sedimentation was controlled by eustatic-climatic and tectono-sedimentary factors (e.g. Vera, 2001). The different phases of fragmentation during the Jurassic and Cretaceous, with the regional and global sea-level changes and the changes in the sedimentation rate, resulted in stratigraphic discontinuities in the sedimentary successions of the South-Iberian Palaeomargin (García-Hernández et al., 1980, 1989; Vera, 2001).

Fig. 1
figure 1

Geological setting. A Geology of the Betic Cordillera. B Geological sketch of the studied area surrounding the Chinchilla de Montearagón town (Albacete province). Five outcrops are also indicated: 1 Castillo, 2 Fuente de la Raya, 3 El Tejar, 4 El Morrón, 5 Mirador

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The External Zones of the Betic Cordillera comprise deposits that accumulated at the South-Iberian Palaeomargin, in palaeogeographical domains individualized throughout the Mesozoic with different structural, stratigraphic, or palaeogeographical features: the Subbetic and the Prebetic (Azema et al., 1979; García-Hernández et al., 1979, 1982; Vera, 1979; Vera et al., 2004). From a stratigraphic and palaeogeographical point of view, the Subbetic was located oceanward and dominated by pelagic sedimentation from the late Pliensbachian (Vera, 2001). The Prebetic was an area adjacent to the emerged land (García-Hernández et al., 1979).

The Cretaceous history recorded in the sedimentary succession of the Prebetic corresponds to the evolution of a palaeomargin with shallow marine environments, with some continental episodes and even with intervals of erosion (e.g. Castro et al., 2008; García-Hernández et al., 1982; Linares-Girela, 1976). However, some sectors show hemipelagic sedimentation during the latest Cretaceous (e.g. Chacón and Chivelet, 2003). The Prebetic is subdivided into the External and Internal Prebetic. The former is located to the north and characterized by thinner sedimentary successions, longer hiatuses, and structural organization in snapes, whereas the Internal Prebetic presents a thicker sedimentary succession with less extended hiatuses and a predominance of folds (Azema et al., 1979; García-Hernández et al., 1979, 1980).

In the Chinchilla area (Sierra de Montearagón-Carcelén), the External Prebetic is composed of a ~ 240 m-thick stratigraphic succession that includes the Upper Jurassic and Cretaceous. The Lower Cretaceous crops out better in the southern part where the thickness increases and there are fewer stratigraphic gaps. The Lower Cretaceous begins with 20 m of clays and sands, with local records of algal pisolithic calcarenites interpreted as deposited in a continental environment during the early Barremian. These deposits lie unconformably over the Upper Jurassic limestones (e.g. Arias et al., 1979; Linares-Girela, 1976; Olóriz et al., 2003). Above these materials, there are limestones with charophytes (~ 80 m thick) that rapidly pinch out to the east. These materials indicate brackish water environments for the middle and late Barremian (Arias et al., 1979). Next in the stratigraphic succession are sands and sandy dolostones (65 m thick) characterized by cross-lamination and with fossil remains, interpreted as a very shallow marine environment during the Aptian. Above these deposits is an interval with red shales and around a hundred metres of sands with cross-lamination and cross bedding. This is the Utrillas sand facies, which ranges from uppermost Aptian to lower Cenomanian and represents fluvio-deltaic environments (Arias et al., 1979; IGME, 1981) (Fig. 2).

Fig. 2
figure 2

A General stratigraphic column of the Cretaceous of Chinchilla de Montearagón. Colours of materials are as in Fig. 1B. B Calcarenitic bar over the green marls of the Chera Formation. C Sands with cross stratification of the Utrillas Formation

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Above these deposits is the carbonate sedimentary succession under study (~ 70 m thick), which is composed of calcarenites with cross bedding at the base, and marly limestones and limestones with rhizoliths (Fig. 2). These deposits are middle and upper Cenomanian. This work analyses the development of a shallow marine platform during the middle-late Cenomanian above the fluvial deposits of the Utrillas sand facies.

For this study five outcrops (Fig. 1) were selected: Castillo (38°55′ 15″ N, 1º43′ 45″ W), Fuente de la Raya (38° 55′ 27″ N, 1°43′ 02″ W), El Tejar (38° 55′ 53″ N, 1°43′ 02″ W), El Morrón (38° 56′ 16″ N, 1°42′ 58″ W), and Mirador (38°56′ 3″ N, 1° 40′ 45″ W). Little or no deformation was found, only sporadic normal faults (035-065º/60ºSE) with a throw of less than one metre associated with joints that can become penetrative in marly beds. The most representative observable structures are sinistral strike-slip faults (217°/82°NW, Fig. 1), with striae plunging 20° north-eastward and an estimated slip of tens of metres, thus slightly uplifting the promontory of Chinchilla Castle with respect to the main Cretaceous outcrop (Fig. 1). These structures were included for the estimations of thicknesses and facies distribution.

3 Methods

The work consists of a bed-by-bed study of the stratigraphic column in the west face of Chinchilla Hill, the Castillo section (Fig. 3). The studied interval is 68 m thick. Samples were described from a stratigraphic, sedimentological, and ichnological point of view. The other outcrops were used for complementary observations, mainly focusing on the lower part of the sedimentary succession (Fuente de la Raya, El Tejar, El Morrón, and Mirador; Figs. 1 and 3). These outcrops were especially important for observing lateral changes in facies and thickness. The study also focused on the boundary surface between the fluvial sands and the carbonate platform deposits, paying special attention to the sedimentary structures below and above this surface, as well as to the presence of trace fossils in the surface (burrows and borings).

Fig. 3
figure 3

Stratigraphic successions of the reference Castillo section and Mirador section. Sedimentary sequences are observed in the alternance of limestones with rhizoliths and laminated marly limestones and marls, developed over the calcarenite bar. Note the indication of the levels sampled for geochemistry and microfacies in the Castillo section

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A total of 40 thin sections were prepared and analysed for microfacies characterization using a Leica M205C microscope, allowing the limestones to be classified by texture. Eight sieved samples were prepared from the marl and marly limestone layers for the micropalaeontological study. About 200 g of dried rock was reduced to small fragments and dispersed in water. When the rock disaggregated, each sample was rinsed in a column of standard stainless-steel sieves (openings of 1 mm, 500 μm, 200 μm, 100 μm and 53 μm), with a jet of water at the top. Analysis of the residues revealed no microfossils in the different fractions.

The mineralogy of the green marls was analysed by X-ray diffraction (XRD) from unoriented powders (whole rock analysis) to determine their mineralogical composition. The < 2 μm fraction of the crust was extracted by centrifugation and oriented aggregates were prepared by sedimentation on glass slides to determine the clay minerals present in the samples. To obtain the diffraction patterns, a PANalytical Empyrean diffractometer was used at the Centro de Instrumentación Científico-Técnica (CICT) of the University of Jaén (Spain), equipped with an X'Celerator solid-state linear detector and θ/θ goniometer, using CuKα radiation, 45 kV voltage and 40 mA current. The XRD patterns were acquired from 4° to 64° 2θ for the whole rock samples and from 4° to 32°° 2θ for the < 2 μm fraction. In both cases, a step increment of 0.01° 2θ and a counting time of 10 s/step was used. The recording was made with XPowderX software (Martin, 2017).

The geochemical analyses were exclusively of the stable isotopes of the Castillo stratigraphic section. A total of 44 samples were analysed for δ13C and δ18O ratios. For the δ13C and δ18O analyses of bulk carbonate, the samples were reacted, after roasting, at 73ºC in an automated carbonate reaction system (Kiel III Carbonate Device) coupled directly to the inlet of a Thermo Fisher MAT-252 gas ratio mass spectrometer, at the Scientific and Technological Centre of the University of Barcelona (Spain). The isotope ratios are described in per mil notation relative to the Vienna Pee Dee Belemnite (VPDB) standard. The values were calibrated using the international standard NBS-18 (δ13CVPDB = − 5.10‰ and δ18OVPDB = − 23.20‰) and the internal laboratory standard RC-1 (δ13CVPDB =  + 2.83‰ and δ18OVPDB = − 2.08‰), traceable to the international standard NBS-19. Analytical precision was monitored by daily analysis of NBS powdered carbonate standards (+ 1.95‰ VPDB). The measured precision was maintained above 0.06‰ for δ18O and 0.03‰ for δ13C.

Two polished slabs with rhizoliths were prepared and scanned at the Universidad de Jaén using a Bruker XR-microfluorescence M4 Tornado equipped with a rhodium target X-ray tube with a high voltage of 50 kV, a current of 600 µA and a pressure of 20 mbar. The spotsize of the X-ray optics was 25 µm. The geochemical compositional maps obtained for each element were represented by a range of colour intensity to indicate the relative concentration of each element.

4 Results

4.1 Lithofacies

The sedimentary rocks under study overlie a siliciclastic succession of fluvio-deltaic facies (Utrillas sand facies; Arias et al., 1979) mainly composed of lithofacies of sandstones with cross bedding (Fig. 2) and less common clay-rich levels from the Albian to lower Cenomanian. The carbonate succession corresponds to the middle and upper Cenomanian. The stratigraphic interval where the shift from fluvio-deltaic sandstones to the carbonate platform occurs, is composed of the following (Fig. 4):

  1. (1)

    A stratigraphic interval of green marls lithofacies 0.4 m to 10 m thick, which is recorded directly above the sandstones. The upper part is constituted by sandy marls (Fig. 4A);

  2. (2)

    A lithofacies of limestone bed rich in irregular dolomitic concretions, probably rhizoliths, with a variable thickness ranging from 20 to 110 cm (Fig. 4) and mudstone microfacies with patches of saccaroid dolomite crystals;

  3. (3)

    A second level of green to yellow marls lithofacies of variable thickness (< 80 cm) that laterally disappears (Figs. 4 and 5);

  4. (4)

    A thick calcarenite bar of variable thickness from 2 to 11 m, which shows large-scale cross-bedding and features common marine trace fossils (Figs. 2B, 3–5).

Fig. 4
figure 4

Distinct views of the base of the studied sedimentary succession at the Fuente de la Raya section, with indication of the different levels and the location of the base of the calcarenitic bar rich in trace fossils of the Glossifungites ichnofacies. A. General view of the different intervals within the green marls of the Chera Formation below the calcarenitic bar. B and C Views of the irregular morphology of the limestone with concretions and the last green marl level locally separated by irregular (erosive) surface. D Base of the calcarenitic bar with abundant green pebbles coming from the erosion of underlying green marls

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Fig. 5
figure 5

Calcarenitic bar in the base of the marine sedimentary succession. A and B Calcarenitic bar deposited over the green marls of the Chera Formation in the Castillo section. C Calcarenitic bar over the green marls in the El Tejar section showing large-scale cross-bedding in the lower part. D Detail of the cross bedding in the lower part of the calcarenite bar in the El Tejar section

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According to the XRD analyses, the green marls below the calcarenite bar are mainly composed by quartz (77%) and clay minerals (18%), and secondarily by orthoclase and dolomite. The analyses of the < 2 µm fraction mineralogy indicate a composition by illite (86%) and kaolinite (14%).

The calcarenite bar and the underlying materials lie slightly unconformably on the green marls, and the limestone with dolomitic concretions pinches out laterally. The calcarenite bar is 11 m thick in the Fuente de la Raya sect., 8.5 m thick in the Castillo sect., 7.7 m thick in the El Tejar sect., 4.5 m thick in the El Mirador section, and 2.1 m thick in the El Morrón section. The base of the calcarenite bar is a surface rich in burrows and borings (Figs. 4A, B). The lower centimetres of the calcarenite lithofacies are locally rich in soft green pebbles (up to 2 cm in diameter) of the same composition as the underlying green marls (Fig. 4C). The large-scale cross-bedding ranges from 1 to 4.5 m hight with the largest sedimentary structures located in the Castillo section, with decreasing size to the north in El Morrón and the base of the El Tejar sections < 2 m hight).

Above the thick calcarenite bar there are numerous sequences of metric scale composed of a lower part with lithofacies of laminated marly limestone (sometimes massive) followed by a more carbonate upper part rich in rhizoliths (Fig. 6). A total of 12 sedimentary sequences ending with lithofacies of limestones with rhizoliths are identified in the Castillo section (Figs. 3 and 6). Commonly, the lithofacies of limestone with rhizoliths (Fig. 7) is thicker than the lower laminated part. The thickest limestone bar rich in rhizoliths (level CW-28) reaches a thickness of 4.5 m. The total thickness of this interval is 46 m in the Castillo section (Fig. 3). The presence of fossil invertebrates is limited to scarce remains of isolated ostreids located in the lower part of the sedimentary sequences. The analyses of sieved samples from the studied outcrops were barren for microfossils.

Fig. 6
figure 6

Sedimentary sequences with laminated marly-limestone lithofacies in the lower part and limestones with rhizoliths in the upper part. Examples from the Castillo section

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Fig. 7
figure 7

A–C Details of the lithofacies of limestones with rhizoliths from Castillo section, that compose the upper part of the depositional sequences

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In addition, the Castillo section has some limestone beds composed of dense accumulations of disarticulated bivalve shells (Fig. 8). These deposits constitute an interval that is 1 m thick. The lower part of the beds present disarticulated shells without a dominant orientation, whereas the disarticulated valves (< 2 cm) mainly appear in a convex-up orientation in the top of the bed and show a high degree of cementation. This record of bivalve-rich limestones constitutes an interruption in the above-described succession of sedimentary sequences with a laminated lower part and rhizolites in the upper part. These facies are observed exclusively in the Castillo section.

Fig. 8
figure 8

Details of accumulations of disarticulated bivalve shells from Castillo section

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Above the studied stratigraphic interval there are around 20 m where the sedimentary sequences are recrystallized (partially dolomitized). This occurs mainly in the beds with rhizoliths, giving them an appearance of rauhwacke. The upper part of the sedimentary succession in the Sierra de Montearagón-Carcelén is recrystallized in all the outcrops under study, and the original sedimentary structures and textures are not preserved. The rauhwackes (or cornieules) are brecciaed in appearance, with high degrees of recrystallization and a cavernous morphology.

The lateral continuity of the sedimentary sequences, and especially the banks of limestones with rhizoliths, can be followed for hundreds of metres, and in the case of the calcarenite bar at the base, the continuity is greater than 4.5 km. The calcarenite bar can be followed in the landscape and also in satellite images due to its thickness and the absence of strong deformation in the Sierra de Montearagón-Carcelén. However, the thickness of both the calcarenite bar and of the rest of the sedimentary succession decreases progressively from the south in the Castillo Sect. (68 m) towards the north, with a thickness of around 12 m in the El Morrón section.

4.2 Microfacies

The microfacies of the calcarenite bar is a packstone to grainstone of peloids and peloidal lumps, with abundant quartz grains and porosity. Bioclasts are scarce and correspond to mollusc fragments (Fig. 9A). In addition, there are pebble-size and coarse-sand size grains reworked in the lower part of the calcarenite bar.

Fig. 9
figure 9

Microfacies from Castillo section. A Packstone-grainstone of peloids and peloidal lumps from the basal calcarenitic bar. B Laminated mudstone with alternance of non-porous laminae and porous laminae with different fenestral richness from the laminated marly-limestone lithofacies. C Mudstone with high fenestral porosity in the lithofacies of limestones with rhizoliths. D Grainstone of bivalve shells and peloids, where the shells are disarticulated and oriented convex up, from the bivalve-rich limestone lithofacies

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In the sedimentary sequences above the calcarenite limestone the microfacies are mudstones to wackestones of peloids, with thin lamination in the lower part of the sedimentary sequences (laminated marly limestones, Fig. 9B), whereas towards the top the lamination disappears, and the fenestral porosity increases with the limestones with rhizoliths (Fig. 9C). Bioclasts are very scarce that is congruent with the absent of sieved samples from more marly layers. Sand-size quartz grains are scarce but locally more abundant.

The bivalve-rich limestones present a packstone-grainstone of bivalve shells with a matrix composed of peloids, small lumps, and incipient ooids (Fig. 9D). There is shelter porosity associated with the bivalve shells as well as sparitic cements that developed under the shells. Some layers are characterized by the dissolution of the bivalve shells, which results in mouldic porosity or infilling by sparitic calcite.

4.3 Trace fossils

Three different ichnoassemblages may be differentiated in the stratigraphic succession studied: (1) the trace fossils at the boundary between the green marls and the calcarenite bar; (2) the trace fossils in the calcarenite bar; (3) the trace fossils in the laminated limestones and limestones with rhizoliths.

The lower part of the studied stratigraphic succession, the calcarenite bar, presents a high content in trace fossils (Fig. 10). The base of the calcarenite bar, the contact with the green marls, is a surface densely occupied by the following ichnospecies: Gastrochaenolites torpedo Kelly & Bromley, 1984, Gastrochaenolites ornatus Kelly & Bromley, 1984, and Glossifungites saxicava Łomnicki 1886. These ichnofossils penetrate the green marls as convex hyporeliefs. Burrows are passively infilled by the overlying calcarenitic sediment, including small soft pebbles of green marl. Because the soft sediment of the green marl displays low resistance to erosion compared with the calcarenite, the base of the calcarenite bar shows very well-exposed trace fossils in the Fuente de la Raya and El Tejar outcrops. These trace fossils correspond to the Glossifungites ichnofacies (Fig. 10).

Fig. 10
figure 10

Glossifungites ichnofacies in the contact between the green marl and the calcarenitic bar that shows the basal surface of the calcarenite with abundant burrows and borings that originally penetrated in the green marls. A Lower surface of the calcarenitic bar with dense accumulation of trace fossils in the El Tejar section. B Detail of the trace fossils, almost exclusively Gastrochaenolites in the Fuente de la Raya section. C Patches dominated by Glossifungites saxicava in the El Tejar section. D. Gastrochaenolites torpedo (and scarce Glossifungites saxicava) penetrating in the green marls in the Fuente de la Raya section. E and F. Views of the Glossifungites ichnofacies in the contact with the green marls with local abundance of Glossifungites saxicava and well-preserved bioglyphs of Gastrochaenolites ornatus

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Gastrochaenolites at the base of the calcarenite occur as clavate borings with the aperture narrower than the chamber, which may vary from spherical to tear-drop-like structures with an average diameter of around 3.5 cm (the thickest up to 5.2 cm) and a length of around 5 cm (the largest specimens up to 9 cm). In the case of Gastrochaenolites ornatus, the wall presents sculptures such as bioglyphs arranged in a circular pattern (Fig. 10F). The smaller Gastrochaenolites (< 3.5 cm long and < 2 cm wide) are characterized by a smooth surface and are assigned to G. torpedo (Uchman et al., 2018). The ichnogenus Gastrochaenolites is interpreted as a dwelling trace fossil assigned to endolithic, suspension-feeding bivalves that usually bore in shallow hardgrounds or rockgrounds (Bromley, 2004; Kelly & Bromley, 1984; Nieto et al., 2018; Wilson, 2007). However, some authors have described Gastrochaenolites as bivalve burrows in firmgrounds (but unlithified substrates; Buatois & Mángano, 2011; Carmona et al., 2007; Pemberton et al., 1992).

Glossifungites saxicava takes the form of an oblique, unbranched, tongue-shaped burrow with its medial part narrower than the lateral part. The apertural portion of G. saxicava is also narrower than the distal portion. The size is commonly < 2.5 cm in width and 4 cm in length. Some specimens show bioglyphs like those described by Belaústegui et al. (2016). The inclination of the oblique Glossifungites commonly dips to the north in the La Raya section. There are some patches in the lower surface of the calcarenite bar dominated by Gastrochaenolites and others by Glossifungites, with Gastrochaenolites ornatus being the most abundant trace fossil. Cross-cutting relationships between described ichnotaxa were not observed.

Over the Glossifungites ichnofacies, restricted to the boundary between green marls and the calcarenite bar, there is a new ichnofacies. The calcarenite presents a trace fossil assemblage dominated by Rosselia socialis Dahmer 1937 and secondarily Ophiomorpha nodosa Lundgren 1981, and very large specimens of Ophiomorpha cf. irregulaire Frey, Howard and Pryor 1978 (Fig. 11). The calcarenite bar thus probably corresponds to the Rosselia ichnofacies (McEachern & Bann, 2020). Rosselia socialis appears as a pseudospherical burrow resembling a bulb (6 to 18 cm in diameter), with onion-like concentric laminae (Fig. 11) and with a central, cylindrical, pencil-thick tube that is poorly preserved in most cases. The Ophiomorpha nodosa specimens are characterized by horizontal tubular galleries with a thick pelleted mud lining with a maximum size of 3 cm in diameter (commonly around 1.5 cm) and a vertical shaft < 3 cm long. In addition, the El Tejar section includes Ophiomorpha cf. irregulaire. These are thick (5 cm in diameter) and long (around 40 cm in length) subhorizontal burrows mainly of meandering shape without branching, lying parallel to the bedding, as described by Bromley and Ekdale (1998) and Nagy et al. (2016) with a characteristic wall-lining with flame-like mud pellets of Boyd et al. (2012) and Leaman et al. (2015), only observed in cross section of some specimens.

Fig. 11
figure 11

Traces fossils of the Rosselia ichnofacies in the calcarenitic bar. A and B Abundant Rosselia with variable size ranging from 8 to 24 cm in diameter from the Castillo section. C Large specimens of Ophiomorpha irregulaire in El Tejar section. D. Transversal cross section of Ophiomorpha from Fuente de la Raya section

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Invertebrate trace fossils are very scarce in the sedimentary sequences that developed above the calcarenite bar, and only Planolites-like trace fossils are recorded, rarely, in the laminated marly limestone lithofacies. The main trace fossils in this part of the section are related to plant roots forming rhizoliths towards the top of the sedimentary sequences (Figs. 6 and 7). The rhizoliths show greater carbonate cementation than the matrix, as well as a different colour in freshly cut and polished slabs. The geochemical compositional maps of the rhizoliths show them to contain a greater concentration of Mg, as well as some trace elements (Sr, Pb, Mo, and U) compared to the matrix between the rhizoliths, which is comparatively rich in Ca and secondarily Fe and P (Fig. 12).

Fig. 12
figure 12

Geochemical maps of rhizoliths from Castillo section elaborated on a polished slab. Note that in natural colour of rhizoliths (grey patches) is different that surrounding sediment. Geochemical maps correspond to Ca, Mg, Fe, P, and Sr with higher intensity of colour corresponding to areas with higher content of the respective element

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4.4 Isotope geochemistry

Stable isotopes were analysed in the Castillo section (Fig. 13). The δ13C values of bulk carbonate show an average of − 0.42 ‰ (ranging from 1.20 to − 4.02 ‰). The δ13C values show some abrupt fluctuations, with a significant negative carbon-isotope excursion (CIE) of − 2.44 ‰ in level CW-25, coinciding with the lithofacies of bivalve-rich limestones, and a second negative CIE of − 4.47 ‰ in level CW-38, which corresponds to a limestone with rhizoliths in the upper part of the studied stratigraphic interval.

Fig. 13
figure 13

Castillo section with δ13C and δ18O curves

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The δ18O values of bulk rock present an average value of -0.07 ‰ (ranging from 0.79 to − 3.98 ‰). There are two negative excursions located between levels CW-23 and CW-25 (bivalve-rich limestones) and in level CW-38, in this case coinciding with the negative CIE.

To test the potential effect of diagenesis on the isotopic signal, the δ13C vs δ18O cross-plot was calculated. The coefficient of determination R2 is 0.414 that is meaning low relation between δ13C and δ18O and the effect of diagenesis is negligible.

5 Interpretation

The integrated analysis of stratigraphic, sedimentological, ichnological, and geochemical data make it possible to characterize the palaeoenvironmental evolution of the Cenomanian succession in the External Prebetic of the Sierra de Montearagón-Carcelén.

The outcrops under study evidence the development of a shallow carbonate platform above the fluvial to fluvio-deltaic sands of the Utrillas sand facies (Arias et al., 1979). However, recently some works have suggested an aeolian origin for the Utrillas sand facies related to arid (erg desert) conditions (Rodríguez-López et al., 2009), but these authors did not study the Utrillas sands in the External Prebetic.

The shift from fluvial or aeolian sands to the carbonate platform is mediated through green marls (metric scale) with a calcareous layer rich in irregular dolomitic concretions that resemble rhizoliths. The green marls are similar to those described in the Iberian Range by Vilas et al. (1982), Meléndez (1983), and Sopeña et al. (2004) and assigned to the Chera Formation. This formation overlies the Utrillas Group, which includes the Utrillas Formation and the highly terrigenous deposits of the Aras de Alpuente Formation (Meléndez, 1983). The content of benthic foraminifera of the Chera Formation in the Iberian Range (Orbitolina corbarica, O. duranddelgai, Ovoalveolina maccagnoae, among others; Fourcade & García, 1982) indicates a lower Cenomanian age and represents a very littoral environment (Sopeña et al., 2004). The analyses of the sieved samples from the studied outcrops revealed no microfossils. In the case of the green marls, the dominance of quartz and illite, as purely detrital components, points to high rates of erosion of continental source areas. The kaolinite forms in humid warm lowland regions with intense weathering (see review in Coimbra et al., 2021). Alteration of feldspar in immature sandstones can also contribute to high kaolinite content (e.g. Galán & Ferrell, 2013). However, the dominance of illite (86%) over kaolinite (14%) < 2 µm fraction, indicates the prevalence of arid conditions during the deposition of the green marls (e.g., Adatte et al., 2002; Deconinck et al., 2003; Dinis et al., 2020).

The beginning of the marine sequence is represented by the calcarenite bar that is recorded for more than 4 km in the Sierra de Montearagón-Carcelén (e.g., 4.5 km from Castillo section to Mirador section). The green marls preluded the start of the widespread thick succession of marine carbonates that are probably correlative with the Alatoz Formation and the Villa de Ves Formation of the Cenomanian (Sopeña et al., 2004; Vera et al., 2004). However, there are not works about the presence of Glossifungites ichnofacies in these formations.

The top of the green marls was an omission surface densely colonized by infaunal forms that produced abundant galleries infilled by calcarenitic sediment from the overlying calcarenite bar (Fig. 14). The boundary between the green marls and the calcarenites shows a high abundance of Gastrochaenolites and Glossifungites, trace fossils typical of indurated sea bottoms (firmground; Buatois & Encinas, 2006; Buatois & Mángano, 2011; Marred et al., 2022). This Glossifungites ichnofacies represents an ichnoassemblage of vertical and U-shaped burrows that occur in firmgrounds in shallow marine settings (Marred et al., 2022; McEachern et al., 2007a). The ichnofacies thus indicates sediment starvation and the subsequent early cementation of the bottom and colonization by burrowers and borers such as pholadid bivalves (Gastrochaenolites; Carmona et al., 2007; Donovan, 2013) and potentially amphipods (Glossifungites; Belaústegui et al., 2016). Cross-cutting relationships between described ichnotaxa were not observed indicating trace makers coexisted at the same time in the sea bottom that constitutes an omission surface.

Fig. 14
figure 14

Palaeoenvironmental reconstruction of the installation of the carbonate platform (calcarenite bar) over the omission surface developed on top of the green marls with Glossifungites ichnofacies

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According to Pemberton (1998), the Glossifungites ichnofacies develops following a regression and just after the subsequent transgression. These types of surfaces are typical of transgressive surfaces as described in the literature (Savrda, 1991; Pemberton et al., 2004; Pemberton & McEachern, 2005; Buatois & Encinas, 2006; McEachern et al., 2007b; Marred et al., 2022). Passive burrow fills reflect colonization by suspension-feeders (e.g., Gastrochaenolites) or passive predators (e.g., Glossifungites), which construct open domiciles that are subsequently filled by calcarenite. The record of soft green pebbles in the first few centimetres of the calcarenite bar and within the sediment infilling the trace fossils indicates the erosion of the top of the green marls during the marine flooding. This erosive process is also consistent with the slight unconformity between the green marls and the calcarenite bar. The base of the calcarenite bar thus indicates a major flooding event in the External Prebetic during the Cenomanian. This event is evidenced in other areas of the Iberian Range and Prebetic by the record of the Alatoz Formation above the green marls of the Chera Formation (Babinot et al., 1991; Sopeña et al., 2004; Vera et al., 2004). The transgression could be correlated with the mid-Cenomanian Event described in the Umbria Marche Basin of Italy (Coccioni & Galeotti, 2003), Aquitaine Basin of France (Andrieu et al., 2015), Western Morocco (Gertsch et al., 2010) and Demerara Rise (Friedrich et al., 2009), among others.

The large-scale cross-bedding that appears in the calcarenite bar indicates high-energy conditions and the presence of sand-waves at the bottom of a relatively shallow marine environment. The abundance of Rosselia trace fossils, domichnia trace fossils typical of high-energy environments (Reolid et al., 2016b), is congruent with the presence of large-scale cross-bedding. In addition, the Ophiomorpha record points to poorly consolidated bottom sediment (loose ground) dominated by sand-sized sediment as well as the relatively sudden deposition of sandy beds (Nagy et al., 2016; Reolid et al., 2016b; Sendra et al., 2020). The high abundance of Rosselia suggests the Rosselia ichnofacies proposed by McEachern and Bann (2020) for sandy delta-front settings. In the studied sector, the calcarenite bar is interpreted as a flooding event with high-energy features resulting in the development of sand-waves, as confirmed by the record of cross-bedding and large-scale cross-bedding structures. In this sense, the Rosselia ichnofacies would here be associated with shallow environments and loose ground in high-energy conditions. This is congruent with the record of Ophiomorpha, that is a substrate-controlled trace fossil registered in sandy deposits (Ekdale, 1992) and high-energy environments (Gibert & Martinell, 1999) in changing and unstable bottoms (Pervesler & Uchman, 2004).

The large-scale cross-bedding points to the development of sandwaves reaching some around 3 m (probably higher) in the Castillo section, and decreasing in size to the north. The abundance of Rosselia is decreasing in the same sense as well as the thickness of the calcarenite bar.

After the deposition of the high-energy calcarenite, the stratigraphic succession turns to repetitive sedimentary sequences of micritic laminated limestones and thick banks (0.4 to 4.5 m) of limestones with rhizoliths, interpreted as subtidal and intertidal mangrove environments respectively. The presence of high fenestral porosity is consistent with these shallow environments, but mainly the record of dense framework constituted by the rhizoliths. The repetition of these sedimentary sequences finishing with limestones with rhizoliths points to shallowing-upwards sequences in very shallow environments, generally with low energy considering the absence of sedimentary structures such as cross-lamination and cross-bedding. This is the first record of extensive mangrove swamp environments after the Cenomanian transgression at the east and southeast Iberian palaeomargins. Babinot et al. (1991) reported various shallow environments during the highstand sea-level, including lagoons and rudist buildups in the Iberian Range.

Microscopy analyses of the calcite forming the rhizoliths show a homogeneous texture, suggesting that calcite precipitation occurred in a stable within-sediment environment around the plant roots. There is no petrified root texture preserved such as septal alveolar tissue. The organic matter of the root decomposed in semi-open, oxic conditions, preventing root preservation by recrystallization and replacement (Sun et al., 2020). Carbonate precipitation occurred early during diagenesis, forming carbonate concretions that grew during root decomposition. The carbonate forming the rhizoliths was comparatively enriched in Mg, as well as in elements such as Sr, Pb and U. Strontium enrichment of rhizoliths is indicative of calcite precipitation during diagenesis (Banner, 1995; Kinsman, 1969). According to Lorens (1981), a comparatively high calcite precipitation rate favours the incorporation of Sr. In the case of Pb, this could substitute the Ca from calcite from sediment pore-water, also during early diagenesis. However, the processes in question are poorly known (Callagon et al., 2014). Uranium accumulates most effectively in the upper few centimetres below the seafloor (McManus et al., 2005; Morford et al., 2009), and additional mobilization of U is enabled by humic and fulvic acids, which facilitate the reductive dissolution of U in sediments (Luo & Guo, 2009). U could be incorporated into the carbonate forming rhizoliths in the root voids during diagenesis. The differences between the composition of calcite forming the matrix and the rhizoliths could thus be related to the high calcite precipitation rate within the voids that originated after the root organic matter had decayed. The high precipitation rate of the calcite concretions favoured the incorporation of elements with a divalent charge in the calcite structure irrespective of differences in the ionic radius.

Locally, the record of bivalve-rich limestones with preferent convex-up disarticulated valves in the top of the bed, and the absence of fine sediment (micrite), confirm the existence of high-energy episodes probably related to storms. The isotopic values in the bivalve-rich limestones are different from those in the rest of the section, with negative excursions of δ13C and δ18O. These isotopic fluctuations could be interpreted as related to a climatic event but could also be due to the different isotopic composition of bivalve shells and synsedimentary sparitic cements compared with the predominantly micritic sediment of the tidal shallowing sequences.

Together with high porosity, the top of the formation features large voids that can even be visited. These caves show the characteristics of hypogenic caves (Klimchouk, 2009, 2017), such as closed rooms, large crystals in all the walls, and narrow conduits that act as feeders or outlets (Bakalowicz et al., 1987). Among other factors, the origin of this type of cave is associated with the generation of reactive fluids loaded with CO2 and/or SO2 from the oxidation of levels with organic matter and sulphides (e.g. D’Angeli et al., 2019). Levels rich in carbonaceous material and pyrites have been identified near the outcrops under study in the Albian-lower Cenomanian formations of the External Prebetic (Gómez-Alday et al., 2004). These levels, which are present below the studied stratigraphic section, could have provided the necessary CO2- and SO2-bearing fluids (Gómez-Alday et al., 2014), which could have risen through the fault system to produce this type of cave.

The lateral continuity of the limestone with rhizoliths is hundreds of metres, and for the calcarenite bar it is more than 4.5 km. However, the thickness of the complete sedimentary succession and the calcarenite bar decreases north of the Castillo and Fuente de la Raya Sects. (68 m) towards the El Morrón section (< 15 m). These major fluctuations in the thickness of the sedimentary succession are related to the tilting of the Jurassic basement under the control of normal faults. Specifically, they would be compatible with a regional listric fault system, approximately E-W oriented, that produces a synsedimentary roll-over controlling the differential subsidence, with a higher sedimentation rate located to the south at the Castillo and Fuente de la Raya sections. These listric faults could have conditioned the palaeogeography of the so-called Gulf of Albacete (Arias et al., 1982) during the Late Cretaceous. The current relief, with promontories in Cretaceous materials, is compatible with the tectonic inversion of these previously normal faults.

6 Conclusions

Stratigraphic, sedimentological, and ichnological analyses of the Cenomanian sedimentary rocks of the External Prebetic in the Sierra de Montearagón-Carcelén have allowed us to characterize the marine flooding and subsequent development of an internal carbonate platform.

The transgressive surface presents a characteristic Glossifungites ichnofacies that indicates sediment starvation and erosion, as well as colonization of a firmground by various infaunal trace-makers (Gastrochaenolites and Glossifungites). The marine transgression occurred initially on a high-energy shallow carbonate platform, as recorded by a thick calcarenite bar with large-scale cross-bedding related to the migration of sand waves. The ichnoassemblage is dominated by Rosselia and secondarily Ophiomorpha, confirming the presence of an unstable sandy bottom under high-energy conditions.

After the high-energy facies associated with the flooding, the stratigraphic succession shows low-energy, very shallow environments ranging from subtidal to intertidal. There was an extended very shallow sector close to the newly emerged land, with successive shallowing-upwards sedimentary sequences finishing with limestones with rhizoliths interpreted as mangrove environments. The record of bivalve-rich limestones representing shell lags of disarticulated valves confirms the occurrence of high-energy events in this environment, probably related to a climatic perturbation, as inferred from negative excursions of δ13C and δ18O. This shallow, low-energy environment persisted in time because of the subsidence in the sector, with the development of tectonically controlled depocenters located in the south of the Sierra de Montearagón-Carcelén. The differential subsidence, evidenced by the increased thickness in the south of the sector, indicates the activity of listric faults that controlled the depocenters.

This is the first description of the Cenomanian transgression in this sector of the External Prebetic and the first record of the very extensive mangrove swamp that developed close to the land that emerged. The transgression could be correlated with the mid-Cenomanian Event described in other regions such as Umbria Marche, Aquitaine Basin and Western Morocco.