INVESTIGATING THE POTENTIAL ROLE OF HYDRODYNAMIC SORTING ON DETRITAL ZIRCON AGE SIGNATURES

Sediment provenance methods such as detrital zircon geochronology rely on the assumption that the suite of zircons preserved in sedimentary rocks are representative of the catchment source area. Emerging concerns question the extent to which depositional processes bias sediment provenance interpretations derived from detrital zircon geochronology. Here we show spatial and vertical variations in detrital zircon age spectra, sedimentary structures, and grain size in the Permian Cutler Group along a source-proximal to source-distal transect and evaluate these data using paleohydraulic reconstruction. Detrital zircon age spectra are consistent with source material from the Uncompahgre Uplift, but presence and magnitude of the two dominant age modes vary along the transect. To assess the role of sorting in the varying provenance signal, we reconstruct the paleo-flow depth, slope, velocity and discharge of the ancient fluvial system and compare these calculations to modern river systems. We find that the reconstructed upper reaches of the Permian river system compare favorably to pebble/cobble-bedded systems while the distal reaches compare to sand-bedded rivers. Applying a critical threshold criterion to assess sorting in upper reaches, we find that the ancient Cutler fluvial system had sufficient competency to transport all zircon in suspension, arguing against sorting as a major influence on detrital zircon geochronologic provenance results in these samples. In the sand-bedded part of the system we use settling equivalence calculations, which similarly indicate zircons would not be sorted along the transect we measured. Both results strengthen confidence in the interpretation that observed variations in detrital zircon age spectra reflect mixing of different sediment sources from the Uncompahgre Uplift during transport around and across uplifting salt walls in the Paradox Basin. The integrated paleohydraulic and detrital zircon provenance experimental design provide a richer assessment of ancient fluvial systems that can be applied with typical field methods.