Lower Toarcian (Jurassic) Brachiopod-rich carbonate facies of the Gran Sasso range (Central Apennines, Italy)

Several stratigraphic studies have proved that Early Toarcian carbonate platforms and basins suffered a sharp sedimentary and paleoecologic evolution worlwide reflecting the onset of dramatic environmental perturbations in the atmosphere/hydrosphere system. An upper Liassic slope carbonate succession cropping out in the Gran Sasso range (Abruzzi, central Italy) and pertaining to the transition between the Latium-Abruzzi carbonate platform and the Umbria-Marche basin has been studied in this light. According to our sedimentologic, biostratigraphic and palaeoecologic analyses on a reworked biotic facies, a prominent Brachiopod-rich assemblage with the remarkable occurrence of Cyanobacteria, Crinoids and Echinoids thrived during the Early Toarcian (Serpentinum Zone) in open shelf/proximal ramp setting. A microbially-induced sedimentation could develop also in deeper-water settings since automicrite partly infilled Brachiopod shells after their basinward reworking. This points out the spreading of deteriorated environmental conditions in the oceanic waters which supported r-competitor forms to thrive in connection with the Early Toarcian Oceanic Anoxic Event. The related environmental disruption induced a sharp drop in the sediment productivity of the shallow-water carbonate factory as well as a marked starvation of the basin. Biostratigraphic study has allowed us to correlate the recovered association to the Lower Toarcian Soaresirhynchia Beds. The type-species S. bouchardi has been recovered for the first time in Italy. This species can be considered as a disaster taxon thriving after the Early Toarcian Oceanic Anoxic Event.