Toarcian ammonitico rosso facies from the South Iberian Paleomargin (Betic Cordillera, southern Spain): paleoenvironmental reconstruction

The Toarcian ammonitico rosso facies were widespread in the Mediterranean Tethys (between 15 and 30 degrees N latitude) in the North Gondwana Paleomargin (Apulian promontory and North African Margin) and southern Iberian Paleomargin (Betic Cordillera). These facies were associated with epi-oceanic slopes of a sedimentary swell-trough system related to the extensional phase of continental rifting. In the Median Subbetic (southern Iberian Paleomargin), ammonitico rosso facies show a progressive change through the Toarcian on the hemipelagic swells after the fragmentation of a carbonate platform. During the latest Pliensbachian to the Bifrons Zone (middle Toarcian), sedimentation was dominated by epi-oceanic limestone and marl with a high influence of neighboring shallow-water environments represented by common turbidite-tempestite beds (with foraminifera and ooids). Microfossils and trace fossils provide no evidence of oxygen-restricted conditions. In the Gradata Zone (middle Toarcian), the ammonitico rosso facies appear (red nodular limestone and marly-limestone rich in the trace fossils Phycodes, Planolites, Thalassinoides, and Chondrites). Progressively more pelagic conditions and a restricted influence of emergent lands and carbonate platforms are reflected by the reduced input of turbidite-tempestite beds and increase of ammonitellas and radiolaria. A sea-level fall affected the hemipelagic swell during the middle-late Toarcian and favored sediment-winnowing by currents, with subsequent nodulation. The combined action of burrowing, compaction, and dissolution controlled nodulation, which ranges from diffuse nodules to sharp-edged nodules. The sedimentation rate conditioned the time available for nodule growth, the migration of the Ca2+ and HCO3- precipitation horizon, and the degree of nodulation (from horizons with diffuse-edged nodules to semi-continuous to continuous layers formed by the coalescence of sharp-edged nodules).