ESTIMATING BEDROCK INCISION RATES USING 10BE AND 26AL ON STRATH TERRACE GRAVELS, BULLFROG CREEK, UTAH

Our understanding of the incisional history of the Colorado River remains incomplete, and recent estimates of long-term incision rates near Glen Canyon differ substantially. Here we investigate recent bedrock incision along Bullfrog Creek, a tributary of the Colorado at Glen Canyon, which drains the western slopes of the Henry Mountains. Prominent sequences of strath terraces exist along lower Bullfrog Creek canyon. Correlating absolute ages and elevation differences of these terraces yields incision rates between terrace levels, and provides insight into the nature of strath terrace development in this region. Mapping and correlation of the relative terrace ages suggests a minimum of seven distinct episodes of terrace formation along Bullfrog Creek. We collected topographic survey data using an integrated differential GPS, a laser rangefinder, and a total station. We surveyed terrace surface elevations, strath elevations, terrace slopes, lithologic contacts, sediment sample locations, and a longitudinal profile of the modern channel. The measured terrace levels along Bullfrog Creek have sub-parallel downstream slopes of about 0.01, and differ significantly from the slope of the modern channel, which may be influenced by the backwater created by Lake Powell. An ongoing portion of this study involves determining absolute terrace ages to constrain bedrock incision rates of Bullfrog Creek using cosmogenic ¹⁰Be and ²⁶Al. We collected surface samples for 12 distinct terraces, and obtained 4 additional depth profile samples to constrain the average nuclide inheritance. To further constrain exposure ages, we sampled a distinct and locally derived sandstone, the Summerville Formation. Our samples currently await measurement at the PRIME Lab at Purdue University. In lieu of these data, we use a published erosion rate of 0.5 m/ka for Glen Canyon and estimate the downstream sampled terrace ages to be 19 ka, 39 ka, and 57 ka. Through our spatial characterization and age estimations of the terraces that line Bullfrog Creek, we will be able to estimate bedrock incision rates of Bullfrog Creek and help constrain the incisional history of the Colorado River near Glen Canyon.