The values of the ratio 87Sr / 86Sr in 16 belemnites samples from three sections of the upper part of the
Gavilán Fm. of the Subbetic chain are analyzed. Elemental and C and O stable isotopes analysis were also
made focused to test the preservation of primary marine calcite. The results of this study confirm the
chronostratigraphy significance of Sr Isotope Stratigraphy from Pliensbachian belemnites. The interpretation
of the results elucidates the age of the hardground or condensed level that supplies the belemnites rostra
after adjusting the 87Sr / 86Sr ratio to pre-existing curves