René Rodrigues

Lower to Middle Toarcian from the Coimbra region (Lusitanian Basin, Portugal): sequence stratigraphy, calcareous nannofossils and stable-isotope evolution

This multidisciplinary study (facies analysis, sequence stratigraphy, calcareous nannofossils and carbon stable isotopes) focuses on the Lower to Middle Toarcian succession cropping out in the Coimbra region (Northern sector of the Lusitanian Basin), Portugal. Deposited on homoclinal carbonate ramp, the sampled hemipelagic series, consisting of marl/limestone alternations can be subdivided into three third-order depositional sequences (ST1, ST2 and ST3) characterised by different vertical facies arrangements and palaeontological contents. The sequence boundaries lie within the Polymorphum Zone, around the Polymorphum/Levisoni Zone boundary and in the Bifrons Subzone (Bifrons Zone). The calcareous nannofossils provide a continuos succession of age-significant assemblages, and a useful set of nannobiohorizons that include the LO of Calcivascularis jansae , the LOs of Biscutum grande and Biscutum finchii , and the FOs of Carinolithus cantaluppii , Carinolithus superbus , Discorhabdus striatus and Discorhabdus ignotus . The evolution of the ð13C agrees with the sequential developments of the series because the positive excursions roughly coincide with transgressive depositional phases, whereas a negative trend is observed during regressive phases.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.