Mixing of Source Populations Recorded in Detrital Zircon U-Pb Age Spectra of Modern River Sands

Detrital zircon U-Pb geochronology is widely used in reconstructing sedimentary provenance, sediment dispersal pathways, and tectonic histories. These applications implicitly assume that sample age distributions from sedimentary basins closely match the zircon age populations within the drainage catchments of the eroding source region. However, recent studies question this largely untested assumption. Here we compare detrital zircon data from unconsolidated sand samples from two modern rivers in the Colombian foreland with data from Mesozoic-Cenozoic sedimentary strata exposed in their respective Andean catchments (similar to 1500 km(2)). We forward model the expected detrital zircon age distribution from each mountainous catchment by integrating age data from all exposed sedimentary units into two modeled age distributions: one assuming equal contributions from each formation and one in which each formation's contribution is proportional to its exposure area within the catchment. Multiple statistical methods show that the area-proportional models most closely match the modern river data. We further test the ability to estimate the contributing source areas by iteratively solving for formation exposure areas to minimize the misfit between the model and the modern river data. The results show a robust correlation between modeled and observed source areas when considered at the epoch level and a qualitative correlation when considered at the formation level. We conclude that detrital zircon age populations of the Colombian foreland basin fill accurately reflect mixing of their Andean sources, in proportions roughly equal to their exposed areas within each catchment. We further conclude that the approximate relative exposure areas of contributing sources can be estimated from detrital zircon analyses. These results highlight the need for consideration of variations in the exposure area rather than solely the presence or absence of competing sediment sources in detrital zircon provenance studies.