Barite septarian concretions in the Albian (Cretaceous) of the Subbetic (Southern Spain)

Abstract This works analyses the Albian barite septarian concretions of the Fardes Formation located in the Geopark of Granada within the Subbetic (Betic Cordillera, SE Spain) from a stratigraphic, textural, mineralogical and geochemical point of view. The early diagenetic conditions that favoured the development of the concretions and their septaria are interpreted, taking into account the importance of the organic-rich clay facies (black shales) in which they are found as well as the interruptions in the sedimentation rate. The barite concretions formed in different stages: (1) Sedimentation of black shales containing Ba of organic origin and deposited in low oxygen environments; (2) diffusion of seawater interstitial solutions containing Ba 2+ mainly from organic matter and sulfate originating from the decomposition of organic matter by bacterial reduction; (3) crystallization of barite on nucleation sites and rapid growth of concretion at the water–sediment interface when the depth of the barite front stabilized at a few meters depth due to a very low or no sedimentation rate; (4) rapid growth of the concretions and sediment load favoured the formation of septarian cracks that were filled first with Sr-rich barite crystals and finally by calcite. The association of barite concretions and stratigraphic discontinuities can be very useful for the recognition of depositional hiatuses in thick monotonous clayey or marly sequences.