Abstract At the Curití Quarry, Eastern Cordillera of Colombia, a 23.35 m section includes a 6.3 m interval of organic-rich marlstones, calcareous clay-shales and clay-shales devoid of benthic fossils and bioturbation, at the base of the Paja Formation. It overlies carbonate ramp deposits of the Rosablanca Formation, with the topmost layer containing reworked and phosphatized assemblages of middle Barremian to lowest Aptian ammonites of the genera Pulchellia , Gerhardtia , Toxancycloceras , Karsteniceras and Prodeshayesites . High-resolution analyses of the section, including total inorganic carbon (TIC, 0.09–50 wt%), total organic carbon (TOC, up to 8.4%) and stable carbon isotope measurements (δ 13 C org between −22.05‰ and −20.47‰) allow the determination of the stratigraphic relationship between the organic-rich level within the Paja Formation and Oceanic Anoxic Event-1a (OAE-1a). The range of values and the overall pattern of the δ 13 C org curve are comparable with the Lower Aptian interval C7. Thus, the organic-rich shale interval of the Paja Formation at the Curití section, showing characteristics of oxygen-depleted conditions, was deposited after OAE-1a, which is known to occur between isotopic levels C3 and C6. Supplementary material: TIC, TOC and Stable Carbon Isotope data at Curiti Section are available at http://www.geolsoc.org.uk/SUP18634