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2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 72-10
Presentation Time: 4:20 PM

EARLY OLIGOCENE UPLIFT OF THE ROCKY MOUNTAINS BASED ON U-PB DATING OF FLUVIAL CONGLOMERATES FROM SOUTHWESTERN WYOMING AND NORTHWESTERN COLORADO


ASLAN, Andres1, NICHOLS, Doug1 and BORAAS, Marisa2, (1)Department of Physical and Environmental Science, Colorado Mesa University, 1100 North Avenue, Grand Junction, CO 81501, (2)Dept. of Geosciences, Colorado State University, 400 University Ave, Fort Collins, CO 80523, aaslan@coloradomesa.edu

New U-Pb detrital-zircon ages from fluvial conglomerates in Wyoming and Colorado support multi-staged uplift of the Rocky Mountains, including a mid-Tertiary (early Oligocene) event. The newly dated conglomerates overlie regional erosional surfaces (e.g., Gilbert Peak, Green Mt, Rocky Mt surfaces), cover a significant portion of the central Rockies, and record up to several hundred meters of erosion of basin-fill strata.

Maximum depositional ages (MDAs) calculated from the U-Pb ages show that the conglomeratic units have similar ages (34-28 Ma). In southwestern Wyoming, the Gilbert Peak erosion surface bevels upland areas of the Uinta Mountains as well as basin-fill strata along the north flank of the uplift. The overlying Bishop Conglomerate yields MDAs of ca. 34 Ma. Stratigraphic reconstructions across the Rock Springs Uplift coupled with the new age estimates for the Bishop Conglomerate indicate that ~400-500 m of erosion occurred ca. 47-34 Ma, probably due to late Eocene reactivation of the Rock Springs Uplift.

In northwestern Colorado, post-Laramide conglomeratic units include the Bishop Conglomerate and the basal unit of the Browns Park Fm., which yield MDAs of ca. 30-28 Ma. These units also overlie regional surfaces that record significant ca. 45-30 Ma erosion of Laramide ranges and adjacent basin-fill strata. Similar fluvial conglomeratic units elsewhere in the Rocky Mountain region overlie the Rocky Mt erosion surface, and include the Telluride Conglomerate (31.5-28 Ma) of southwestern Colorado and the Tallahassee Creek and Castle Rock Conglomerates (<37 Ma) of south-central Colorado.

The broad similarity in the ages of these fluvial conglomerates and their associated erosion surfaces, their broad areal extent, and the magnitude of erosion associated with these fluvial systems (hundreds of meters), support broad-wavelength, post-Laramide (Oligocene) uplift in the Rocky Mountain region.

Session No. 72

T188. Inside or Out? Investigations into Driving Forces in Fluvial Systems
Sunday, 1 November 2015: 1:30 PM-5:30 PM
Room 340 (Baltimore Convention Center)
Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p.220
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