EXPLORING THE EFFECTS OF SEDIMENT MIXING AND HYDRAULIC SORTING ON DETRITAL ZIRCON SIGNAL PROPAGATION: EXAMPLES FROM THE EOCENE CASTISSENT FORMATION, SOUTH CENTRAL PYRENEES, SPAIN

Detrital zircon (DZ) U-Pb provenance signatures are modified during sediment transport by mixing of different source areas with different zircon fertility, and by variable amounts of sediment storage/recycling on the floodplain. It has been hypothesized that hydraulic sorting of specific zircon grainsizes may have a substantial impact on DZ age spectra if there exists a relationship between grainsize and U-Pb age. This study examines the effects of these processes on the downstream propagation of DZ U-Pb spectra in an ancient fluvial network, and quantifies the variability in DZ U-Pb spectra within a single formation. The Eocene Castissent formation (Fm), is a fluvial sandstone with E-W paleoflow deposited in the Tremp-Graus wedgetop basin in the South-Central Pyrenees. We collected a downstream transect along the Castissent Fm paleodrainage in the Tremp-Graus Basin, and the deep marine equivalents in the Ainsa-Jaca Basin (W of the Tremp-Graus Basin). Additional samples were collected of contemporaneous deposits from the Ager basin to explore the possibility of a southern river input.

This study examines three aspects of DZ signal propagation: 1) We quantify the mixing proportions of the axial derived sediment with the transverse derived sediments. 2) We draw paleogeographic interpretations about the sediment routing network and the interconnectivity of these deposits. 3) We explore the effects of hydraulic sorting on the DZ U-Pb age spectra by collecting a transect across a single channel form in outcrop capturing a spread in bulk grain sizes. DZ U-Pb Results indicate the Castissent is a two-component mixture of an axially fed river system mixing with compositionally distinct sediment entering from transverse fans. The axial source DZ U-Pb signature is progressively diluted by the transverse DZ U-Pb signature in the direction of the paleoflow. Preliminary results indicate negligible effects of hydraulic sorting on the variability in age component proportion within the DZ age spectra for the Castissent Fm. Paleogeographic implications can be drawn from these new data. These data support a fluvial connection between the Castissent Fm and the Corçà Fm (Ager Basin) indicating the Corçà paleoriver flowed onto the wedgetop supplying sediment to the Castissent paleoriver.