Sharp-based , mixed carbonate-siliciclastic shallow-marine deposits ( upper Miocene , Betic Cordillera , Spain ) : the record of ancient transgressive shelf ridges ?

Isolated sharp-based sedimentary bodies in shelf settings can develop via the reworking of regressive deposits during transgressions. An example of these are shelf ridges, formed under a wide range of processes, and widely studied due to their high reservoir potential. However, there is still a lack of examples in mixed (carbonate-siliciclastic) successions. This study presents an outcrop example from the Upper Miocene of the Betic Cordillera (Spain), with the aim to propose a model for the development of transgressive sharp-based mixed carbonatesiliciclastic deposits, and to provide criteria to differentiate those from their regressive counterparts. The studied succession is ca. 300 m-thick, and shows a cyclic alternation of coarse and fine-grained mixed deposits. Depositional cycles start with siliciclastic-dominated offshore to offshore transition deposits, progressively replaced by lower shoreface deposits. These are abruptly truncated by sharp erosive contacts bioturbated by passively-infilled large burrows; their ichnological features allow assignation to the Glossifungites ichnofacies. These contacts are interpreted as ravinement surfaces. They are overlain by mixed carbonatesiliciclastic barforms, rich in skeletal fragments and extraclasts, and displaying large-scale cross bedding. These form several m-thick and hundreds of m-long depositional elements interpreted as mixed shelf ridges. These ridges formed in a fine-grained, shallow-water shelf, which occasionally received coarse siliciclastic sediment supply via gravity flows, but had a coeval offshore carbonate factory, which provided the skeletal fragments. The sharp-based, coarser-grained nature and lithological break at the base of these mixed carbonate-clastic deposits could lead to their misinterpretation as forced-regressive wedges. However, the nature of their lower contact, combined with the reworked offshore skeletal fragments, and their stacking pattern are consistent with these mixed units forming during transgression. Other studies in relatively time-equivalent deposits have demonstrated the existence of coeval regressive, coarser siliciclastic-dominated shoreline systems in relatively close localities. These evidences a complex basin configuration in the area during the upper Miocene, with the development of local depocentres and relatively narrow corridors or seaways in the Mediterranean-Atlantic connection, which could have favoured shelf reworking processes, but also promoted the development of diverse stacking patterns, reflecting the differential interaction between active tectonics and sedimentation across the region.


Grey structureless marlstones (FA1-Offshore)
This facies association is composed by whitish grey, structureless to faintly laminated marlstones (Fig. 4A, B). Despite the lack of structures, subtle grain-size changes occur within mm-scale beds, and bedding contacts are roughly parallel where visible. Beds are mm to cmthick, but packages can reach several meters in thickness (Fig. 6). Thin section analysis reveals these deposits are dominated by quartz grains and planktonic foraminifera, floating inside the muddy matrix that occupies more than 15% of the rock (Fig. 5A). Other studies have also described sponge spicules and radiolarian in the same deposits (Soria, 1993). Large accumulations of well-preserved bivalves are locally observed in these deposits in the lower part of the section (Fig. 6). Bioturbation is absent to low (BI 0-2). Regional mapping reveals they form laterally extensive units, which can be followed for several km (Figs. 2,3,7C).
Scattered thin-bedded (up to 10 cm-thick), normally-graded muddy sandstone beds are observed within the mudstone successions, some with erosive bases and rippled tops, and up to moderately bioturbated (BI 0-3).
Interpretation -The dominant fine-grained nature of these deposits, combined with the fossil content and relatively low bioturbation, suggests they accumulated in a relatively distal offshore setting, below storm wave base, with occasional siliciclastic input by low-density turbidity currents and hemipelagic suspension settling.

Heterolithic sandstone/marlstone packages (FA2 -Offshore transition)
This facies association is composed by grey laminated sandy marlstones and sandstone/marlstone heterolithic packages, interbedded with 5 to 40 cm-thick isolated fine to coarse-grained sandstone beds (Fig. 4D). These beds are tabular or lens shaped, with erosive and/or deformed bases (e.g. load casts), normal grading and rippled tops, hummocky-cross stratification and common soft-sediment deformation (Fig. 4C, E), and locally abundant organic matter, bioclasts and extraclasts (mainly quartz). Thin section analysis reveals they are dominated by quartz grains, with minor metamorphic rock fragments and planktonic and benthic foraminifera (Fig. 5B). Tool marks (mainly flutes) and foresets show paleocurrents ranging to the SW-NW (Fig. 6). Sandstone beds can be up to moderately bioturbated (BI 0-3), with vertical or horizontal traces at the top surface (Fig. 8A). Packages range from 8 to 40 m in thickness (Fig. 6). The top of these packages can be gradational to overlying lower shoreface deposits (FA3) or be abruptly truncated by ravinement (FA4) deposits (Fig. 6).

Wavy-laminated sandy mudstones to muddy sandstones (FA-3 -Lower shoreface)
This facies association is composed by grey laminated sandy mudstones to muddy sandstones, with wavy bedding and symmetrical ripple cross-lamination (Fig. 4F, G), and isolated cm-thick beds with low-angle, hummocky and tangential/sigmoidal cross stratification and soft sediment deformation. Paleocurrents from cross-stratification foresets, where observed, point dominantly towards the S-SE. Packages are 3 to 19 m-thick, and tend to stack forming coarsening-up successions (Fig. 6). They generally display a gradational lower contact from underlying offshore transition deposits (FA2), and are conformably overlain by condensed deposits (FA7) or abruptly truncated by ravinement (FA4) or channel-fill (FA7) deposits (Fig.   6).
Interpretation -The sandy but fine-grained and thin-bedded nature of the deposits, common presence of symmetrical wave ripples, and only occasional appearance of thick sandstone beds with larger-scale combined-flow structures suggests these deposits accumulated in a dominantly low-energy lower shoreface setting (Yang et al., 2005).

Structureless bioclastic calcarenites (FA4 -Ravinement deposits)
This facies association is composed by yellow medium to very coarse-grained, structureless bioclastic calcarenites (Fig. 4H). Beds are 60 to 250 cm-thick and moderately to highly bioturbated (BI 3-5) (Fig. 6). They have a prominent sharp, erosive highly bioturbated base, with vertical, sub-vertical and oblique J-shaped burrows and shallow cylindrical rounded structures, as well as circular sections and horizontal, branched, forms. Most traces can be assigned to Thalassinoides, but with local presence of Rhizocorallium, Skolithos and Bergaueria. Burrows are undeformed and characterized by sharp contacts, showing, in some cases a penetration depth up to around 20 cm into the underlying deposits, and are passively infilled by mixed carbonate-clastic sediment (Fig. 8B, C). This includes abundant skeletal fragments (dominantly from bivalves and bryozoans, and minor red algae and echinoids), organic matter, coal fragments and extraclasts (quartz and volcanic-rock fragments), in a relatively poorly-sorted organization. Normally-graded bed tops occur. They are commonly found abruptly truncating offshore transition (FA2) or lower shoreface (FA3) deposits, and overlain by mixed bar (FA5) or condensed (FA6) deposits (Figs. 6, 7).
Interpretation -The ichnological features found at the base of these deposits allow assignation to the Glossifungites ichnofacies, developed into compacted, semi-lithified substrates (Seilacher, 1967). This firmground ichnofacies has been used extensively in the identification of omission surfaces and the identification and interpretation of transgressive surfaces (MacEachern et al., 1992;Bann et al., 2004;Rodríguez-Tovar et al., 2007). The contacts are therefore interpreted as transgressive surfaces, although evidence is not conclusive to associate them to either wave or tidal ravinement (see Cattaneo and Steel, 2003). The poorlysorted and bioclastic-rich deposits immediately overlying these surfaces are consequently interpreted as ravinement deposits (Zecchin et al., 2019), resulting from the remobilization of a coeval carbonate factory, mixed with the erosion of underlying offshore transition (FA2) and lower shoreface (FA3) deposits. However, the reworking of forced-regressive poorly-sorted sandstone wedges as well as the entrainment of immature extraclasts from intrabasinal basement highs cannot be ruled out.

Sigmoidal cross-bedded bioclastic calcarenites (FA5 -Mixed bars)
This facies association is composed by yellow fine to coarse-grained, cross-bedded bioclastic calcarenites ( Interpretation -These deposits are interpreted as mixed siliclastic-carbonate barforms, resulting from the reworking of a coeval carbonate factory, together with the underlying offshore transition (FA2) and lower shoreface (FA3), but also ravinement deposits (FA4), possibly accumulated preferentially in some areas of the seabed, favouring a higher reworking by shelf currents.

Highly bioturbated, concretionary sandstones (FA6 -Condensed deposits)
This facies association is composed by grey-yellow, intensely bioturbated sandstones (BI 5-6; Interpretation -The high bioturbation index of these deposits, with multiple generation of traces, together with presence of glauconitic grains and their concretionary/cemented nature are consistent with condensed deposits. These represent a considerable span of time recorded by only relatively thin layers, and form under low energy, low sedimentation rate conditions, associated with regional flooding events.

Erosive-based, bioclastic pebbly sandstones (FA7 -Channel-fill)
This facies association is composed by bioclastic, cross-bedded pebbly sandstones, contained in a concave-up erosive base, and forming a 5 m-thick package, (Fig. 4L). The package is slightly fining-up, and contains a mix of skeletal fragments (dominantly bivalves, but also bryozoans and red algae), organic matter and large (up to several cm-long) angular extraclasts (quartz and volcanic fragments), more concentrated towards the base (Fig. 4M, 4). This facies association has only been recognized in the upper part of the studied section, abruptly truncating lower shoreface deposits (FA3), and overlain by mixed bars (FA5) (Fig. 6).
Interpretation -The highly erosive, concave-up basal surface, together with the coarser nature and larger presence of landward material, mixed with reworked skeletal fragments, is consistent with these deposits being interpreted as subaqueous channel/gulley fills, possibly containing a regressive surface of marine erosion at the base (Fig. 6).
These are overlain by poorly-to moderately-sorted mixed carbonate-clastic units, rich in skeletal fragments and extraclasts (mainly quartz and volcanic fragments), and displaying large-scale sigmoidal cross bedding (FA5mixed bars, Table 1, Fig. 4I, J, K). These deposits show a fining-and thinning-up arrangement, often capped by highly-cemented and concretionary bioturbated sandstones, with high ichnodiversity (FA6condensed deposits, Table 1, Fig. 4N), interpreted as containing maximum flooding surfaces.
Interpretation -The vertical sequence of offshore deposits (FA1), progressively replaced by offshore transition (FA2), and in some cases, passing to lower shoreface deposits (FA3), is consistent with a progradational stacking, and interpreted to record the regressive phase of a distal siliciclastic-dominated shoreline system (Fig. 9). However, in some cycles, these regressive trend is abruptly truncated by a sharp, highly bioturbated contact, interpreted as a transgressive ravinement surface (TRS). Above this, structureless (FA4) and sigmoidal crossbedded (FA5) skeletal-rich bioclastic calcarenites, forming mixed carbonate-clastic shelf ridges, commonly stack in a retrogradational fining, thinning-up trend, consistent with a transgressive phase (Fig. 9C). The cycles often culminate in either a sharp top or in a thin, highly bioturbated package (FA7), interpreted as a condensed section containing a maximum flooding surface (MFS). This surface marks the boundary between cycles, as it is often overlain by the offshore (FA1) or offshore transition deposits (FA2) of the next cycle.

Fine-grained siliciclastic shelf and the origin of the remobilized carbonate factory
The studied succession is interpreted to have deposited in a relatively shallow-water shelf (Fig.   9A), dominantly above storm-wave base, as suggested by the evidence of combined-flow structures (i.e. hummocky cross stratification) in sandstone beds within the more distal, finergrained packages. The fine-grained nature of the coarsening and thickening up successions of offshore (FA1), offshore transition (FA2) to lower shoreface (FA3) deposits (Fig. 6) suggests there was a coeval north-westward prograding shoreline system, although the shelf was only receiving occasional coarse-grained siliciclastic sediment supply via gravity flows (e.g. hyperpycnal flows). However, these dominantly fine-grained shelf deposits are abruptly truncated by mixed carbonate-siliciclastic units, through sharp, highly bioturbated transgressive ravinement surfaces (MacEachern et al., 1992;Bann et al., 2004). These mixed deposits are remarkably different from the underlying shelf deposits, with coarse-grained, bioclastic calcarenites (FA4, FA5, Table 1) with skeletal fragments. Because those skeletal fragments are only recognized in the mixed clastic-carbonate units (see Fig. 5), this implies the presence of a coeval carbonate factory, located in either (i) a more distal position or (ii) a lateral position within the shelf.
The occurrence of a carbonate factory with bryomol-type skeletal association interpreted from the bioclasts observed in the mixed deposits (mainly bryozoans and bivalves, and minor red algae and echinoids), would indicate non-tropical, temperate-type shallow-water conditions (Betzler et al., 1997). Because of the relative dominance of siliciclastic material of the studied  (Brandano and Corda, 2002). This biota association is therefore characteristic of deeper depositional environtments (i.e., aphotic zone in outer-middle ramp, Brandano and Corda, 2002) than other skeletal associations, like branching red algae-dominant (i.e., oligophotic zone -middle ramp) identified in other time-equivalent successions in the nearby Tabernas Basin (García-García et al., 2006b). The scenario where the carbonate factory is located in distal offshore positions relative to the equivalent shoreline supplying the siliciclastic fraction can occur quite commonly in mixed carbonate-siliciclastic shallow-marine systems (Schwarz et al., 2018; see also Reijmer, 2021).

Depositional model for the development of transgressive mixed carbonate-clastic shelf ridges
This study proposes an evolutionary model for the development and preservation of sharpbased, mixed carbonate-clastic transgressive shelf ridges (Fig. 10). During regressive periods, the normal progradation of a relatively distal shoreline resulted in a dominantly fine-grained shelf, formed by coarsening-up successions of marlstone-dominated offshore (FA1) to offshore transition (FA2) deposits, and local preservation of lower shoreface muddy sandstone deposits (FA3) (Fig. 10A). This shelf was only receiving coarse-grained siliciclastic sediment (extraclasts) and organic debris occasionally via forced regressions and/or gravity flows (e.g. hyperpycnal flows), which underwent storm reworking during or shortly after deposition, and resulted in discrete cm-thick sandstone beds within offshore transition deposits (Myrow et al., 2002;Pattison et al., 2007;Lamb et al., 2008;Jelby et al., 2020) (Fig. 10B). After some time (enough to create a firm or compacted substrate), offshore transition to lower shoreface deposits were partially removed during transgression, with the development of an erosive and highly bioturbated ravinement surface (Fig. 10C), due to the undeformed and sharp nature of the burrows (Fig. 8B, C) and their association to Glossifungites ichnofacies. This ravinement surface was followed by deposition of a relatively poorly-sorted assemblage of mixed deposits (FA4), dominated by skeletal fragments resulting from the remobilization of a more distal offshore or alongshore carbonate factory (Fig. 10D). The uneven accumulation of these mixed deposits on the seabed possibly resulted in areas that favoured higher reworking via shelf (most likely storm-wave) processes and nucleation of laterally extensive shelf ridges, with the development of sigmoidal cross-bedded barforms (FA5) (Fig. 10E). These can locally show bidirectional accretion orientations (N-S), but dominantly pointing southward, at a high angle with respect to the dominantly westward orientation of unidirectional paleocurrents recorded from gravity flow deposits (Fig. 9B). Continued transgression resulted in regional flooding, increased water depth and decrease of reworking processes and deposition, leading to lower sedimentation rates and the development of highly biortubated, condensed deposits (FA7), containing a maximum flooding surface, and locally preserved above the shelf ridges (Fig.   10F). Finally, the next phase of advancement of the regressive shoreline system led to progressive deposition of fine-grained sediments in offshore and offshore-transition settings, resulting in the burial and effective preservation of the underlying mixed carbonate-clastic shelf ridges (Fig. 10G).

Poorly-sorted versus well-sorted sand ridges
Several of the mixed carbonate-siliciclastic deposits in the studied section are relatively poorly sorted and contain abundant extraclasts (mainly quartz and lithic fragments) and terrestrial organic matter fragments (Fig. 5). This contrasts with conventional transgressive shelf ridges, mostly composed of well-sorted sandstones (Cattaneo and Steel, 2003), particularly those undergoing long-term reworking/remolding during their migration through the shelf (Snedden and Dalrymple 1999). The absence of an efficient segregation of heterolithic grains in the studied mixed shelf ridges is consistent with high-energy conditions induced by persistent storm-wave action. This is more characteristic around the shoreface zone than in more distal offshore settings (van Heteren et al., 2011;Rossi et al., 2017), where tidally-modulated segregation commonly occurs (Chiarella et al., 2012). The textural nature of the studied shelf ridges, more poorly-sorted and coarser-grained than conventional tidal-dominated offshore ridges, would therefore suggest they developed around the shoreface zone, where sediment reworking by storm waves was common. The abundant extrabasinal detrital material derived from the high-energy storm reworking of (i) regressive poorly-sorted sandstones and (ii) sediment gravity flow deposits, as extraclasts and terrestrial organic debris are commonly observed even in isolated sandstone beds within lower shoreface and offshore transition deposits. Additionally, well-developed burrowed ravinement basal surfaces and relatively short ridges (with single cross-bedding sets, and not forming compound bars) are more characteristic of gentle slopes (Nnafie et al., 2014) and shallower-water settings (i.e., shoreface).
Simulations of sand ridges with morphodynamic models conclude that the morphology and activity of sand ridges are controlled by the rate of sea-level rise, depth and coastal-shelf slope (Nnafie et al., 2014). Following those models, the shelf ridges studied here, with more common examples of single than compound barforms, would have been enhanced during low rates of sea-level rise on gentle coastal to inner shelf slopes. Marine transgressions represent common scenarios for the development of mixed carbonate-siliciclastic shelves (García-García et al., 2006;Fontana et al., 2015;Salocchi et al., 2017), where the interplay of high-energy currents removing carbonate factories and coming from detrital input drowning emerged areas encourage the mixing of carbonate and siliciclastic grains (Longhitano et al., 2014).

Implications for other studies
Several studies have proposed the sharp-based, coarser-grained nature and significant lithological break at the base of shallow-marine deposits as criteria to interpret them as detached forced-regressive wedges (e.g. Hunt and Tucker, 1992;Ainsworth et al., 2000;Fitzsimmons and Johnson, 2000;Posamentier and Morris 2000;García-García et al., 2011).
However, the bioturbated ravinement bases of the mixed carbonate-siliciclastic deposits studied here, the presence of skeletal fragments from an offshore carbonate factory, significantly different from the underlying offshore transition to lower shoreface siliciclastic deposits, and the fining, thinning-up stacking of the deposits are consistent with these mixed units being interpreted as transgressive deposits (Fig. 9C). Therefore, this study emphasizes the importance of a careful analysis of the geometry and ichnology of sharp basal contacts in shallow-marine deposits, potential differential composition across their boundaries, and their stacking pattern, as key criteria to differentiate transgressive sharp-based mixed carbonatesiliciclastic deposits from their regressive counterparts.

The influence of basin configuration in the upper Tortonian
One of the most characteristic features of the studied succession is the repetition of offshore/shoreface siliciclastic-and shelf mixed-lithofacies into 8 cycles (C1-8, see  (Betzler et al., 2006;Martín et al., 2014). This configuration resulted in intensification of bottom currents and favoured shelf reworking processes, as seen in this study and also in overlying deposits (García-García et al., 2009), and in the nearby Rifian corridor (Capella et al., 2017;de Weger et al., 2020;Beelen et al., 2021;Miguez-Salas et al., 2021. But it also promoted the development of local sediment entry points and variable stacking patterns, reflecting a differential interaction between active tectonics and sedimentation across the region (e.g. Ándric et al., 2018).

Conclusions
This study analyses and discusses the origin and development of sharp-based, mixed carbonate-siliciclastic deposits in a shallow-marine succession from the Upper Miocene of the Betic Cordillera (Spain). The studied succession (ca. 300 m-thick) shows a recurrent alternation of coarse and fine-grained mixed carbonate-siliciclastic deposits, arranged in 8 depositional cycles (C1-8), starting with fine-grained dominated offshore/offshore transition deposits, progressively replaced by sandy lower shoreface deposits. These are abruptly truncated by sharp, highly bioturbated contacts (Glossifungites ichnofacies), passively infilled by poorly-sorted, coarser bioclastic deposits and interpreted as ravinement surfaces. They are overlain by mixed carbonate-siliciclastic sigmoidal barforms, rich in skeletal fragments and extraclasts, forming several m-thick and hundreds of m-long depositional elements, interpreted as mixed carbonate-clastic shelf ridges, and capped by condensed deposits containing maximum flooding surfaces. These ridges formed in a shelf which received occasional coarse siliciclastic supply via sediment gravity flows, but with a coeval offshore carbonate factory, eroded and remobilized during transgressions. These sharp-based mixed carbonate-clastic deposits could be tentatively misinterpreted as forced-regressive wedges in other studies.
However, this work provides criteria to distinguish them, including the nature of their lower contact, presence of reworked skeletal fragments and their stacking pattern, which are consistent with their interpretation as transgressive deposits. When put in context with other studies in relatively time-equivalent regressive and more siliciclastic-dominated successions nearby, this evidences a complex configuration of the Mediterranean-Atlantic connection during the upper Miocene, with sea corridors increasing currents and shelf reworking processes, and local sediment supplies and depocentres resulting in laterally variable stacking patterns, and reflecting differential and complex tectono-sedimentary interactions.     Mudstone-prone facies (FA1, offshore); quartz grains (Qz) and planktonic foraminifera (PF) floating inside the muddy matrix (Mx), that occupies more than 15% of the rock whole. B: Sandstone levels of heterolithic facies (FA2, offshore transition); densely packed framework formed dominantly by quartz (Qz) with minor metamorphic rocks clasts (MR) and planktonic (PF) and benthic (BF) foraminifera. C, D, E: Mixed carbonate-siliciclastic units facies (FA5, mixed bars); quartz grains (Qz) and carbonate rocks fragments (CR), with skeletal fragments including bryozoans (Bz) and bivalves (Bv), with minor planktonic (PF) and benthic (BF) foraminifera, as well as glauconitic grains (Gt). F: Detail of a hand specimen of carbonatesiliciclastic units facies (FA5, mixed bars), showing the relative abundance of bryozoans (Bz).    FA2). B-C) Vertical, and oblique burrows, as well as circular sections, passively infilled by mixed carbonate-clastic sediments (ravinement deposits, FA4), and penetrating a few cm into the light sandy siltstone deposits below (lower shoreface, FA3). D) Frequent bioturbation in mixed carbonate-clastic cross beds (mixed bars, FA5), with dominant Planolites and well-developed Thalassinoides structures. E) Vertical shaft of probable Ophiomorpha (pellets along the wall can be envisaged) (mixed bars, FA5). F) Bichordites/Scolicia traces showing cross-cutting relationships and similar infilling material than the host mixed carbonate-clastic sediment (mixed bars, FA5). G) Several traces of Scolicia showing cross-cutting relationships in the upper surface of a bioclastic sandstone bed (condensed section, FA6).