MAPPING SURFICIAL GEOLOGY AND TRACKING RIVER INCISION IN BEARS EARS NATIONAL MONUMENT

Bears Ears National Monument (BENM) in southeastern Utah is a landscape rich with cultural and geological resources. Despite its beauty and the controversy surrounding its recent Monument status, surprisingly little geological research has taken place in the Monument. In an effort to shed more light on the geological and archaeological resources in BENM, we present new mapping and interpretations of river terraces preserved along the San Juan River near Bluff, UT. Developing a higher-resolution understanding of this watercourse that has played a large role in shaping the spectacular landscape of BENM may have regional implications for understanding the late Cenozoic evolution of the Colorado Plateau and how glacially-fed rivers have responded to glacial-interglacial cycles. We produced a detailed surficial geologic map (at a 1:12,000 scale) of the area around the confluence of Comb Wash and the San Juan River using field mapping techniques, aerial imagery, and digital elevation data. We assigned map units by geomorphic position, the height above the modern river, and the sedimentology of the deposits. The highest terrace within the study area sits 140 m above river level (ARL) and has been dated to ~1.36 Ma (Wolkowinsky and Granger, 2003), yielding a long-term incision rate of ~110 m/ Ma. A flight of at least 6 more terrace generations marches down to river level. A prominent strath terrace at 32 m ARL can be correlated to one downstream dated to ~62 ka (Steelquist et al., 2017), yielding an incision rate of ~507 m/Ma. This disparity in incision rates requires that river incision has accelerated in the middle-late Pleistocene. Ongoing geochronology work is aimed at clarifying the timing of that acceleration so that it may be explained in the context of down-cutting on the Colorado River, uplift of the Colorado Plateau, and/or increasing influence of glacial-interglacial cyclicity in the San Juan Mountains.