High frequency cyclicity in a vertical alternation of Gilbert-type deltas and carbonate bioconstructions in the late Tortonian, Tabernas Basin, Southern Spain

The deltaic succession in the Tabernas Basin represents the first deposition of marine sediments deposited during the late Tortonian transgression that linked the Mediterranean Sea to the Atlantic Ocean through the basins of the Betic Cordillera. The 100 m-thick succession consists of seven 5 to 20 m-thick vertically stacked wave-dominated Gilbert-type deltas. Towards the top of each unit, coral reef patches overlie the delta plain and are in turn overlain by calcarenites formed by accumulation of branching red algae colonies. Locally, the calcarenite deposits fill microkarst depressions developed on the reefal carbonates during emergence. Unlike the corals, the red branching algae colonies grew at a few metres depth under low energy conditions below wave base. The calcarenites derived from branching red algae colonies formed when the delta plain was drowned, creating the new accommodation space needed for the progradation of the delta forming of the overlying unit.The repeated alternation of coral reef carbonates (tropical carbonates) and carbonates with bryomol skeletal assemblages (temperate carbonates) in the deltaic succession is linked to relative sea-level changes reflecting the sum of glacio-eustatic sea-level fluctuations and tectonic subsidence. Each delta unit is represented by a transgressive systems tract, built by the stacking of distributary-mouth bars capped by reef patches, a maximum flooding surface represented by branching red algal calcarenites and a highstand systems tract consisting of progradational wave-dominated Gilbert-type delta. Because of the general increase in accommodation space controlled by glacio-eustatic sea-level rise and tectonic subsidence, beach topset strata and distributary mouth bars were preserved in the cases studied. Subsidence rates exceeded the glacio-eustatic sea-level lows, preventing the development of normal lowstand systems tracts in the delta stack.