HETEROGENEITY IN THE UNCOMPAHGRE UPLIFT REFLECTED IN PROXIMAL-DISTAL SEDIMENT MIXING IN THE PARADOX BASIN

The Uncompahgre Uplift is one of several basement-cored uplifts thought to exhume primarily metamorphic rocks of the Yavapai (1.7-1.8 Ga) terrane during the evolution of the Ancestral Rocky Mountains (ARM). The timing of exhumation is primarily constrained by ages of synorogenic strata in the adjacent Paradox Basin. However, there has been no direct link identified between the uplift and basin. Previous detrital zircon U-Pb geochronology from Paradox Basin alluvial fans that are proximal to the Uncompahgre Uplift record a unimodal 1.4-1.5 Ga signature which was unidentified in the adjacent northern uplift. In contrast, fluvial deposits in the medial-distal Paradox Basin include a bimodal mixture of 1.4-1.5 Ga zircons along with 1.7-1.8 Ga zircons. While the 1.7-1.8 Ga signature is easily explained by exhumation of Yavapai terrane crust in the Uncompahgre Uplift, the origin of the 1.4-1.5 Ga zircons was unclear. We address the origin of these enigmatic 1.4-1.5 Ga zircons by conducting zircon U-Pb analyses of basement samples collected from an east-west traverse through Unaweep Canyon which transects the northern Uncompahgre Uplift. Analyses of the meta-igneous and meta-sedimentary basement rocks show a dominance of middle Proterozoic ages of 1.7 Ga equivalent to Yavapai terrane, with secondary contributions of 1.4-1.5 Ga ages, demonstrating that the northern Uncompahgre Uplift is actually comprised of 1.4-1.8 Ga rocks. We conclude that the unimodal detrital zircon signature from the proximal Paradox Basin alluvial fans was sourced from an extremely limited catchment dominated by 1.4-1.5 Ga meta-igneous rocks. In contrast, the medial-distal fluvial strata tapped a wider, potentially axially flowing, catchment and show both the 1.7-1.8 Ga and 1.4-1.5 Ga zircon ages reflecting the broader uplift. This provides the first direct connection between the Uncompahgre Uplift and Paradox Basin fill.