The Upper Bajocian and Bathonian in the Cordillera de Domeyko, North-Chilean Precordillera : sedimentological and biostratigraphical results.

The Jurassic ammonites of the Cordillera de Domeyko have been studied in several monographies. The classic memoirs of Steinmann (1881), Tornquist (1898), Burckhardt (1903) and Stehn (1923) are particularly relevant. However, the biostratigraphic problems concerning stage boundaries arisen only in recent dates. New sedimentological and biostratigraphical data have been obtained in relation to the Bajocian/Bathonian of this region. Several sections have been studied in two separate areas : Caracoles and Sierra de Varas. More than one thousand ammonites were collected in situ from these localities. Facies analysis shows that sedimentation during Late Bajocian and Bathonian occurs in a marine environment on a carbonate platform with abundant siliciclastic supply. Facies distribution was controlled by structural segmentation of the platform and processes of differential subsidence. The stratigraphical-sedimentological results together with taphonomical-palaeoecological observations are a diagnostic tool for sequence analysis. The successive ammonite assemblages which were found have allowed the establishment of four biostratigraphic units, spanning the interval from Upper Bajocian to Upper Bathonian. The sedimentological and biostratigraphical results indicate the existence of a regional discontinuity at the base of the Bathonian