INTEGRATION AND EVOLUTION OF THE FREMONT RIVER INTO THE GREATER COLORADO RIVER SYSTEM

The Fremont River drainage basin sits on the NW edge of the Colorado Plateau and covers nearly 2000 km² in south-central Utah. After confluence with Muddy Creek, the Fremont becomes the Dirty-Devil River, a major tributary of the Colorado. Oligocene to Pliocene age volcanic rocks cover the W 1/3 of the basin while flat lying to steeply folded Permian to Tertiary age sedimentary rocks comprise the E 2/3 of the basin. The volcanic rocks are part of the Marysvale volcanic field and comprise thick sheets of felsic to intermediate tuffs, lava flows and volcanic breccias. The most extensive of these units range in age from 26–23 Ma and thin to the E. These rocks were likely erupted contemporaneously, or just after emplacement of the Henry Mountain porphyries (recent U/Pb ages of 28–25 Ma; Murray et al., 2016 GEOLOGY). Remnant patches of ~5 Ma basalt occur throughout the W part of the basin. In several locations the basalts preserve clear flow directions or have relationships to regional structures that indicate the landscape character during eruption. Basalts from Elsies Nipple, and Abes, Red and Parker Knolls generally flow easterly, while basalts near Hogan Pass and East Tidwell Canyon indicate flow along, or structural control by, the strike of the Thousand Lakes Fault. In all locations the amount of incision around the basalts is minor. This indicates that at ~5 Ma the W 1/3 of the Fremont River drainage basin was likely tilted to the E, as it presently is, and the Thousand Lakes fault was a significant structural feature of the landscape. Quaternary deposits in the E 2/3rd of the basin almost always contain volcanic rocks, either from Boulder or Thousand Lakes Mountains or from the Awapa or Fish Lake Plateaus. Volcanic clasts in Quaternary deposits are both physically and chemically more resistant to erosion than the local sedimentary bedrock. The strength of the volcanic clasts combined with the large boulder sizes ultimately promotes boulder armoring of pediments or straths and lead to wholesale topographic inversion. ³He exposure ages of multiple boulders from many of these deposits range from 90 to 1400 ka, with individual deposit treads sitting 30 to 280 m above modern floodplains. These ages and incision depths suggest maximum incision rates of 0.20 to 0.43 m ka^-1 for the Fremont River and tributaries over the last ~1 Ma.