INSIGHTS INTO PALEOGEOGRAPHY AND SEDIMENTATION DURING EARLY STAGES OF THE OLIGOCENE IGNIMBRITE FLARE-UP USING DETRITAL ZIRCON U-PB ANALYSES ON SEDIMENTARY UNITS IN THE SOUTHERN ROCKY MOUNTAINS: TELLURIDE CONGLOMETATE AND BLANCO BASIN FORMATION
The Telluride and Blanco Basin Formations flank the western and southeastern slopes of the San Juan Mountains and stratigraphically pre-date the main pulse of Oligocene volcanism, but our analyses shows that both have similar maximum depositional ages (~33 Ma) that overlap in age with early San Juan volcanism. We suggest these units record uplift of the underlying Rocky Mountain Erosion Surface associated with early stages of the Ignimbrite flare-up, as also supported by the presence of volcanic clasts in the upper portion of the unroofing sequence preserved within the Telluride. Paleoflow directions and thicknesses of the Telluride was deposited by a large, high-energy WNW-flowing braided river system. Major caldera eruptions at 28.4 Ma (San Juan and Uncompahgre) and 27.6 Ma (Silverton) may have just predated or been synchronous with Telluride deposition as shown by 33-28 Ma youngest grains. The Blanco Basin depositional system (post-33 Ma) reflects southerly paleoflow from an emerging San Juan uplift. Early Oligocene depositional paleoslopes have been subsequently tectonically modified by uplift of the Mount Wilson block and Neogene uplift of the San Juans such that age constraints on these units also help constrain the timing and magnitude of Neogene uplift components.