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

Abstract

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 carbonate–siliciclastic deposits, and to provide criteria to differentiate these 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 carbonate–siliciclastic 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 evidence 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.