CONSTRAINING TIMING AND RATES OF RIFT-RELATED DEPOSITION VIA AR40/AR39 GEOCHRONOLOGY

Active continental rifts display the initial phase of tectonic plate break up. Rift basin sediments can record this phase; however, basin fill can be fueled by either geomorphic processes like fluvial deposition or tectonic processes such as fault exhumation and alluvial fan deposition. The Dry Union Formation, in central Colorado, is described as river gravel deposits and can provide valuable information to assess processes of basin formation and sources of sedimentation in the Rio Grande Rift (RGR). In this study, we apply Ar⁴⁰/Ar³⁹ geochronology to refine the depositional timeline of the Dry Union Fm., located near Poncha Pass, Colorado. Ar⁴⁰/Ar³⁹ dating is a critical radiometric technique for determining precise ages of minerals. The Dry Union Fm. is comprised of half-graben basin-fill material that includes poorly sorted sandstone and conglomerate with minor layers of siltstone. Our objectives include constraining the formation's age, calculating sedimentation rates, correlating stratigraphic sections, and providing a foundation for a provenance study by collecting and dating volcanic ash deposits within the formation. We collected nine Ar⁴⁰/Ar³⁹ samples, including one near the base of the Dry Union Fm., enabling precise constraints on its upper boundary. Previous studies produced limited Ar⁴⁰/Ar³⁹ dates of 15.6 Ma and 9.3 Ma from ash layers within the formation. These dates, alongside mammalian vertebrate fossils attributed to the Pliocene, suggest the Dry Union Fm. spans the middle Miocene to Pliocene. However, we anticipate Ar⁴⁰/Ar³⁹ ages between 20 Ma and 5 Ma, which will challenge the assigned Pliocene age, and extend initial deposition to an earlier time. These results will enhance our understanding of the formation’s temporal framework, sedimentation history, and stratigraphic correlations, and show critical context for understanding basin formation and sedimentation associated with active continental rifting.