The C-isotope stratigraphy of mixed siliciclastic-carbonate fossiliferous strata cropping out to in the southwestern flank of the Sierra Nevada de Santa Marta is reported. A pronounced positive C-isotope excursion of up to 7‰ suggests a lower Aptian (ca 123 Ma) depositional age. The mixed carbonate-siliciclastic successions concordantly overlay basaltic volcanics from the Guatapuri Formation. We relate the published 125 Ma K-Ar ages of such volcanics and the depositional age of the studied successions to an extensional event, followed by rapid tectonic stabilization and platform deposition. The pronounced positive C-isotope excursion is a global feature displayed by marine sediments deposited during a global oceanic anoxic event (OAE1). As the OAE1 would have favored the preservation and deposition of organic matter in the global oceans substrates, the studied sedimentary successions may be considered as a potential hydrocarbon-generating unit. The proposed age ultimately precludes the correlation of the studied successions with other hydrocarbon generating units in northern Colombia (e.g. The Cogollo Group and the La Luna Formation). The inferred depositional age and the occurrence of the OAE1 may also be considered when planning future oil exploration programs in the northern Colombian basins. This work highlights the use of stable C isotope chemostratigraphy for identification of cretaceous oil source rocks in NW South America and the Caribbean.


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