QGG DONALD R. COATES AWARD: RESOLVING DIFFERENTIAL INCISION RATES TO TEST LANDSCAPE EVOLUTION MODELS: INTEGRATING NEW 40AR/39AR DETRITIAL SANIDINE DATING WITH PUBLISHED DATA ON LAVA CREEK B ASH TERRACES IN THE ROCKY MOUNTAIN REGION

The Yellowstone Lava Creek B (LCB) tephra eruption (630 ka) covered the Rocky Mountain region with ash and provides a snapshot of late-Pleistocene river incision history. This eruption took place during the MIS 16-15 transition when rivers were aggrading near the end of a major glacial cycle. We focus on terrace straths to estimate rates and patterns of bedrock incision. Terraces containing and overlain by Lava Creek B ash deposits were likely preserved within 10s of ka after the eruption (before the next incision cycle) and hence provide near-direct (but still maximum) incision rates. We also consider ‘cryptic’ ash deposits where channel sands and gravels contain 630 ka detrital sanidine grains; these provide maximum depositional ages and minimum incision rates. Numerous terrace flights from numerous regional rivers, and for the same timeframe, show a robust incision signal of differential long term average incision rates that can help resolve debates about landscape evolution. As also highlighted in a prior compilation, the data shows the entire region to be differentially incising: 0-50 m/Ma in Kansas, 50-100 m/Ma on flanks of the Rockies, and 100-150 m/Ma throughout the Rockies. Previous workers have suggested that increased incision in the Rocky Mountains may be in response to Quaternary glacial episodes. Our alternative hypotheses suggests tectonically driven surface uplift as a driver of differential incision rates. This seems to be supported by the variable, non systematic changes in incision rates observed along the river profiles and by similar rates extending back more than 1 Ma. Our data focuses attention on highest rates (>200 m/Ma) within ~ 100-km-long reaches: 1) above Lees Ferry knickpoint on the Colorado River, 2) in the upstream-diverging terraces in the Animas River, 3) within a knickpoint in the Black Canyon of the Gunnison River, and 4) on the flanks of the Jemez Mountains. These may have diverse explanations that include: differential rock uplift across faults, mantle-driven epeirogenic uplift, uplift due to young magmatism, and knickpoint migration in actively adjusting rivers.