Slope breccias colonized by bivalves and serpulids during the Middle Jurassic (Subbetic,SE Spain)

The Middle Jurassic of the Intermediate Domain, in the northern Subbetic (External zones of the Betic Cordillera, SE, Spain), reveals the transition from hemipelagic limestones (Baños Formation) to overlying shallow-water oolitic limestones (Jabalcuz Formation). Near La Guardia (Jaén), the basal part of the succession that records this transition comprises clast-supported calcareous breccias 24 m thick. Transverse sections reveal a fan shape close to a normal fault. The upper parts of some breccia beds were colonized by bivalve patches that locally evolve to bivalve-serpulid bioherms. The bioherms are massive, with low relief and a metre-scale lenticular shape. Two main parts have been differentiated: a bivalve bioherm and a serpulid bioherm. The bivalve bioherm consists of densely packed mytiloids in life position with secondary serpulids, where the matrix is a grainstone of bioclasts, peloids, and ooids. Laterally, there are accumulations of reworked bivalves. The serpulid bioherm is made up of serpulid aggregates in a bioclast wackestone. On an unstable slope with abundant breccias, bioherm development took place in favourable periods of low sedimentation rate. The pioneer colonisers of the sea-bottom were mytiloid bivalves in relatively high-energy waters. Shell comissures were aligned parallel to the dip of the slope as an adaptation to feeding from the plankton and seston inputs associated with palaeocurrents. The intercalation of reworked bivalve beds likely indicates brief periods of higher-energy storm events. The serpulid bloom in the upper part of the buildups was probably related to a stage of low-energy conditions (as evidenced by the delicacy of these structures), which is congruent with the wackestone texture. The relief provided by the bivalves favoured the development of the serpulid bioherm phase in an elevated and advantageous position for suspension-feeding. The dense packing in these bioherms offered clear advantages in this unstable palaeoenvironment: (1) dense populations are less susceptible than isolated individuals to overgrowth by other sessile organisms or predator attacks; and (2) the interconnection among individuals enhances stability and resistance against high-energy events, and increases the growth potential of future generations. The interruption of bioherm growth was probably related to the deposition of new breccia beds and reworked oolitic limestones.