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 through regressions associated with abrupt lowering of relative sea level, but also via the reworking of regressive deposits during transgressions. An example of these are shelf ridges, observed in modern and ancient shelves, and formed under a wide range of mixed processes. However, despite they have been widely studied partly due to their high reservoir potential, 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 depositional and sequence stratigraphic model for the development of transgressive sharp-based mixed deposits, and to provide criteria to differentiate those from their regressive counterparts. The studied succession is ca. 300 m-thick, and shows an alternation of coarse and fine-grained mixed carbonate-siliciclastic deposits. A detailed facies and stratigraphic analysis allows grouping the deposits in several depositional cycles, starting with mudstones (FA1-offshore) or sandy mudstones interbedded with HCS sandstones (FA2-offshore transition), progressively replaced by coarsening-up, wavy-bedded muddy sandstones (FA3-lower shoreface). These deposits are abruptly truncated by sharp erosive contacts bioturbated by large burrows, passively infilled by overlying coarser bioclastic sediments (FA4-ravinement deposits); burrows are sharp-walled and undeformed, and their ichnological features allow assignation to the Glossifungites ichnofacies. These contacts are therefore interpreted as ravinement surfaces. They are overlain by mixed carbonate-siliciclastic units, rich in skeletal fragments and extraclasts, and displaying large-scale sigmoidal cross bedding forming accreting barforms (FA5-mixed bars). The bioclastic sediments overlying the ravinement surfaces combined with these cross-bedded calcarenites form several m-thick and 100’s of m-long depositional elements, interpreted as mixed carbonate-clastic shelf ridges. They are capped by thin, highly-cemented and bioturbated sandstones containing maximum flooding surfaces (FA7-condensed deposits). These shelf ridges formed in a shallow-water setting, relatively starved of siliciclastic sediment supply, but with a coeval offshore carbonate factory, which was remobilized and provided skeletal fragments during transgressions. The high amount of extraclasts, organic matter and carbonaceous debris derives from the erosion of older regressive deposits, basement topography or supplied locally via sediment gravity flows. 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 bioturbated and ravinement nature of their lower contact, combined with the reworked skeletal fragments from an offshore carbonate factory, significantly different from the underlying regressive fine-grained offshore to lower shoreface successions, and the fining, thinning-up stacking of the deposits 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 areas relatively close to the studied locality. These studies evidence an actively developing tectonically-driven 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 counterintuitive stacking patterns, reflecting the differential interaction between active tectonics and sedimentation across the region.