Sedimentological and paleoecological aspects of the Aptian transgressive event of Sierra del Rosario, Durango, northeast Mexico

Results of detailed microfacies analysis of units studied in a Barremian–Aptian stratigraphic sequence from Durango State of northeast Mexico, indicate primarily carbonate series representative of the transgressive episode that drowned the Cupido Platform during the late early to late Aptian. Formally established lithostratigraphic units of the studied section include from bottom to top: (a) the Cupido Formation (Barremian–lower Aptian): a unit of resistant thick- to massive-bedded limestones, representative of a shallow carbonate-platform facies; and (b) the La Peña Formation (middle–upper Aptian): a unit of intercalated marls, marly limestones, and shales, indicative of a deepening environment. Typical Aptian index ammonites dominate the macrofauna of the deepening unit, allowing a biostratigraphic analysis and consequently a correlation of this transgressive episode on the basis of the biochronology of these macrofossils. Vertical variations in the carbon/carbonate contents and in the relative abundance of benthic and planktonic foraminifera and radiolaria throughout the section, allow characterization of the sedimentological regimes during Barremian–Aptian times in this area of Mexico. The data suggest that intervals of C-rich levels may be related to periods of bottom dysoxic–anoxic conditions as a result of combined effects by eustatic, paleoceanographic, and paleoecological changes.