RATES OF FLUVIAL INCISION AND LANDSCAPE EVOLUTION IN THE FREMONT RIVER BASIN, NORTHWESTERN COLORADO PLATEAU

In the Fremont River basin (northwestern Colorado Plateau) there are tens to hundreds of individual volcanic clast armored surfaces throughout the landscape. These armored surfaces are former valley floors that now stand 20 to 300 m above the local drainages. All of these armored surfaces are capped with coarse basaltic-andesite boulder deposits derived from the high (>3400 m) volcanic plateaus of Fish Lake, Boulder or Thousand Lakes Mountains. Using 3He exposure age dating we determined the exposure ages of multiple boulders from several of these surfaces. We interpret these deposits to be proximal debris-flows and therefore assume deposition was rapid and do not include a correction for cosmogenic inheritance due to exhumation or transport. We do include a correction for non-cosmogenic (nucleogenic) 3He produced in the basaltic-andesites since crystallization (approximately 25 Ma). This component is typically less that 3% of the total 3He inventory and this correction has only a very minor effect on the exposure ages. Maximum boulder exposure ages of these armored surfaces range from 100 ka to 900 ka and represent minimum ages for the deposition of these surfaces. Using the minimum ages we calculate maximum incision rates around these surfaces of 0.20 to 0.43 m/kyr for the Fremont River and several of its tributaries. In addition to the exposure ages we estimate long term exhumation rates for the Fremont River basin from well-dated dikes and laccoliths that were emplaced at quantifiable depths. Data from diabase dikes in the northern part of the basin and the Henry Mountain laccoliths in the eastern part of the basin suggest long-term exhumation rates of 0.23 to 0.25 km/Ma.