USING EOCENE–MIOCENE STRATIGRAPHY TO SHED LIGHT ON MAGDALENA RIVER INITIATION IN THE NORTHERN ANDES

The Magdalena River is the longest river in Colombia, flowing >1,500 km between the Central and Eastern Cordilleras. Discourse over the initiation of the Magdalena River has been ongoing for decades; either a proto-Magdalena River formed from the partitioning of the Magdalena and Llanos basins in the Paleogene, or it occurred consequently to a later episode of Eastern Cordilleran uplift in the Miocene, driving drainage reorganization throughout the Magdalena Basin. Here we evaluate the initiation of the Magdalena River by studying the provenance of the sedimentary record in the Middle Magdalena Valley. We present ~2.6 km of Eocene–Miocene strata within the Caceres Syncline along with new detrital zircon U-Pb geochronology. Potential source regions have unique zircon U-Pb age spectra, where igneous and metamorphic rocks of the Central Cordillera are dominated by zircons <500 Ma, and the Eastern Cordillera is composed primarily of Cretaceous–Paleogene sedimentary rocks dominated by >900 Ma zircons. Adventitiously, the distinct age spectra can be recognized in the rock record. Our detrital zircon analyses of the Caceres Syncline reveal a shift in provenance from distal Central Cordilleran sources to proximal Eastern Cordilleran sources by ~16 Ma which indicates uplift of the Eastern Cordillera. The shift in provenance is accompanied by a change in paleocurrents from a general southeastern to northerly flow, as well as an increase in grain size and channel amalgamation. Together, these observations could indicate Magdalena River initiation in the mid-Miocene.