C- And Sr- Isotope Age Constraining Oceanic Anoxia In Northern Colombia: Implications For Dating The Cretaceous Caribbean Oil System

Silva-Tamayo, Juan Carlos¹; Bayona, German; Sial, Alcides; Cardona, Agustin; and Pardo, Andres
¹[email protected]

The Cogollo Group consists of mixed siliciclastic-carbonate fossiliferous strata that includes potential reservoir units as well as source rocks along the Cesar-Rancheria Basin in northern Colombia. Despite its potential, this unit has been poorly dated with biostratigaphic methods. Here we use C- and Sr- isotope chemostratigraphy for constraining the age of the Cogollo Group along the Cesar-Rancheria Basin.

The C- isotope chemostratigraphy of carbonates cropping out along the southern part of the Cesar Rancheria Basin shows a pronounced positive C-isotope excursion of up to 7‰. These carbonates display ⁸⁷Sr/⁸⁶Sr values between 0.7075 and 0.7076, suggesting a lower Aptian (ca 123 Ma) depositional age. The pronounced positive C-isotope excursion is a global feature displayed by marine sediments deposited during a global oceanic anoxic event (OAE1a). As the OAE1a would have favored the preservation and deposition of organic matter in the global oceans substrates, the studied inner carbonate platform sedimentary successions may be considered as potential hydrocarbon-generating rocks, besides la Luna Formation, along the Cesar-Rancheria Basin. The occurrence of the OAE1a in the Cogollo Group may thus be considered when planning future oil exploration programs along the Cesar-Rancheria basin.

A similar, yet less pronounced, C-isotope anomaly (ca 3‰) is displayed by carbonates from the Kisima Member of the Palanz Formation along the Cosinas Anticline, Lower Guajira Basin. As the Cogollo Group, the Palanz Formation overlies pre-Cretaceous volcanics associated to extensional settings. This positive C-isotope excursion together with ⁸⁷Sr/⁸⁶Sr values between 0.7073 and 0.7074 suggest that the carbonate platform and related coralline reefs deposited during the Hauterivian OAE (Weissert Event).

The different depositional ages of the lower Cretaceous carbonate records along the Cesar-Rancheria and Baja Guajira Basins suggest a diachronic development of the carbonate platform preserving the Cretaceous OAEs after the breakup of Pangea and the formation of the proto-Caribbean in northern South America. This work highlights the use of stable C and Sr isotope chemostratigraphy for identification of potential Cretaceous oil generating rocks in NW South America and the Caribbean.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013