LONGITUDINAL SEDIMENT MIXING IN THE MAGDALENA RIVER OF THE COLOMBIAN ANDES

Intermountain longitudinal rivers, which flow internal and parallel to the orogenic belt, offer an important counterpart to transverse river systems, altering sediment flux, erosional efficiency, and sediment mixing; despite this, these rivers are underrepresented in the literature. The Magdalena River drainage basin is one of the longest longitudinal fluvial system in the Andes Mountains, flowing >1500 km northward between the Central Cordillera and Eastern Cordillera of Colombia. Tributary rivers feeding this system initiate from diverse and unique sediment sources located within the Eastern, Central, and Western cordilleras of the northern Andes. Here we evaluate sediment mixing and dilution in the modern Magdalena River basin, as an analog for sediment mixing in ancient longitudinal systems. We present detrital zircon U-Pb analyses on modern river sands from multiple locations along the trunk stream, as well as its tributaries that span distinct source regions. In general, Eastern Cordillera tributaries contain abundant ca. 2000-900 Ma zircons, reflecting the prevalence of Cretaceous sandstones exposed in their headwaters, which contain zircons that were originally derived from Precambrian cratonic sources. Samples from catchments draining the Central Cordillera are more heterogeneous but are dominated by <300 Ma age modes. Samples in the Magdalena trunk stream reflect progressive input, and subsequent dilution, of discrete tributary point sources. This is expressed in multi-dimensional scaling space, where the Magdalena samples fluctuate between distinct Eastern versus Central Cordillera sources, depending on the source of the closest major upstream tributary.