EVOLVING TRANSVERSE TO LONGITUDINAL FLUVIAL SYSTEMS DURING EOCENE TO PLIOCENE GROWTH OF THE COLOMBIAN ANDES

Abstract

The Magdalena Valley situated between the Central and Eastern Cordilleras of Colombia hosts one of the largest rivers in the Andes, the Magdalena River. Drainage configurations throughout the Cenozoic remain contested, including when the Magdalena River established its north-direct modern path. Within the Magdalena Valley, up to 7 km of siliciclastic sedimentary rocks archive exhumation and unroofing during northern Andean uplift. We present a measured section of Eocene to Pliocene strata in the Middle Magdalena Valley, with εNd from mudstones and detrital zircon U-Pb analyses from sandstones to document shifts in drainage configuration through time. Eocene strata support initial east-directed transverse river systems tapping high topography in the Central Cordillera. By the mid-Eocene, paleocurrents indicate topographic growth of the Eastern Cordillera, yet detrital zircon U-Pb geochronology does not require derivation from the Eastern Cordillera until the Oligocene, reflecting a progressive unroofing sequence complicated by sedimentary recycling. In the mid-Miocene, distal sources from the magmatic arc and the Panama-Chocó Block are introduced, reflecting drainage connections through trans-Andean fluvial passages within a large fluvial system. The Pliocene records diminishing distal sources and a shift toward detrital zircon U-Pb age signatures that mirror age spectra from the modern Magdalena River. We suggest a proto-Magdalena River formed by the mid-Miocene (~14 Ma) and the drainage configuration responsible for the modern Magdalena River was emplaced by the Pliocene (4 Ma).