Dating the Chengue/Arroyo de Piedra formation of the northern San Jacinto foldbelt: Results of the application of in situ U-Pb carbonate geochronology in NW Colombia

The Cretaceous to Recent stratigraphic units preserved in the Lower Magdalena Valley basin and Sinú-San Jacinto fold belt of NW Colombia have been commonly dated through biostratigraphic analyses of samples recovered in outcrops and drill holes. Yet, the obtained relative age ranges of most of the stratigraphic units are wide and, in most cases, require a more accurate definition. While sediment reworking is one of the main causes of such wide age ranges, depositional environments which prevent good preservation of diagnostic microfauna and microflora and even the type of collected samples are additional causes. In this study we are providing absolute age measurements from Eocene carbonates of the Chengue/Arroyo de Piedra Formation, which outcrop in the northernmost San Jacinto fold belt of NW Colombia, and which have also been studied in cores recovered in exploratory wells. For such measurements we successfully carried out in situ U-Pb carbonate geochronology analyses. These analyses were performed in two samples (biomicrites) collected in the Arroyo de Piedra stratigraphic section, located in the eastern flank of the Luruaco Anticlinorium, yielding ages of 40.8 ± 1.1 Ma (2σ) and 40.5 ± 1.8 Ma (2σ), which correspond to a middle Eocene, lower Bartonian age. Sedimentological observations in both outcrops and cores, integrated with available sedimentary provenance data show that these carbonates were deposited in not very wide, shallow marine carbonate platforms, which also received clastic input from the northern Central Cordillera, the Santa Marta massif and from related paleo-highs. The analyzed carbonates represent the Mid-Eocene climatic optimum that has been identified in other basins of the world. Our results not only increase the knowledge of the stratigraphy and petroleum systems of the area, but they also encourage the application of this technique in other areas of the Sinú-San Jacinto fold belt and in other basins with mixed carbonate and siliciclastic deposition.