USING PETROGRAPHIC ANALYSIS TO DETERMINE THE COMPOSITION OF BASIN FILL SOURCES WITHIN THE RIO GRANDE RIFT

Rifting and fault growth play important roles in basin connectivity and drainage pathways. The basins and basin-fill in the northern Rio Grande Rift (RGR) preserve a record of unique interactions between horsts and grabens and river evolution. Despite abundant research on active faulting in the RGR, there is a lack of understanding about the provenance of the basin fill. The Poncha Block in Colorado separates the upper Arkansas River Valley from the San Luis Valley and is blanketed in the Dry Union Fm., mapped as fluvial gravels. Determining where the sediment comes from and obtaining a better understanding of the depositional environment in which the sediments were deposited will provide insight into the paleo-geographical history of the area. We collected twenty petrographic samples within the Dry Union Fm., across the uplifted Poncha Block, and at different levels within the stratigraphic sequence. The Gazzi-Dickinson point-counting method was used to determine the composition of source rocks and identify source changes in both time and space. Petrographic analysis shows several sources for the sediments including (i) local volcanic rocks, most-likely associated with the 39-mile and San Juan volcanic fields, (ii) regional basement rock, which shows some alteration, and (iii) nearby Paleozoic sedimentary rocks. Further provenance analysis comes from new detrital zircon U-Pb geochronology from samples collected in the Dry Union Fm. These ages show similar sources to the point count data, but also reveal a population of 500 Ma ages not easily accounted for. These 500 Ma zircons may be sourced from the McClure Mountain Complex to the southeast, suggesting a more complex river history, or may represent ages of alteration. Identifying the provenance of the basin-fill will contribute to our understanding of the exhumation history of the Poncha Block and aid in interpretations for the evolution of the Arkansas River.