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

Sedimentary units above the Rocky Mountain Erosion Surface, including the Telluride Conglomerate, Blanco Basin and McDermott Formations in Colorado and El Rito, Ritito, and Galisteo Formations in New Mexico, have been dated using U-Pb detrital zircon analyses. Maximum depositional ages of these units help to constrain erosion and sedimentation patterns in the southern Rockies. The Telluride, Blanco Basin, and Ritito Formations have returned maximum ages of 33-28 Ma, 33 Ma, and 26 Ma, respectively, which reassign the Telluride and Blanco Basin units from their former "Eocene" designation to post-early Oligocene in age, and confirms the Oligocene age of the Ritito. Detrital zircon analyses for the El Rito and Galisteo Formations do not have significant Cenozoic grain populations and hence do not yield new constraints on their depositional age. All analyses produced age distributions that help in indicating provenance of the units.

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.