MICROCLIMATIC CONTROLS ON SLOPE FORMS AND PROCESSES, NORTHEASTERN ARIZONA

Relationships between small-scale climatic changes and geomorphic response can be difficult to define due to uncertainties in timing and magnitude of both climate change and landscape response. In some localities, topography induces microclimatic conditions where nearby geomorphic systems experience minor but consistently different temperature and moisture conditions. By studying slopes with similar lithology and topography but different microclimates, it may be possible to make links between geomorphology and climate that are useful in interpreting past landscape dynamics. Near Blue Gap on the Colorado Plateau in NE Arizona, small (~0.5 km²), narrow, eastward-draining basins along an escarpment expose massive sandstone and mudstone of the Jurassic Morrison formation, and Cretaceous rocks including bedded Dakota sandstone and Mancos shale, locally capped by Mesa Verde Group sandstones or bouldery landslide deposits derived from them. On three south-facing and two north-facing side slopes, we collected data on slope angle, aspect, weathering products and debris cover, vegetation, and bedrock strength. Slope angles are bimodal at ~40° and near 90°, and geomorphic relations suggest active replacement of slopes by cliffs. Basin-floor and gully deposits provide clues to dominant processes transporting material from slopes. Air, surface, and soil temperature and soil moisture are being recorded over the seasonal cycle at four locations to quantify microclimatic differences on north- and south-facing slopes. Preliminary results suggest that south-facing slopes are steeper, with more exposed bedrock, higher cliffs and more evidence for active sapping and rockfall processes than directly adjacent north-facing slopes. North-facing slopes exhibit a greater degree of vegetative cover and in situ weathering by granular disintegration. Rock strength estimated by Schmidt hammer rebound tends to be lower, and the thickness of weathered mantle and debris greater, on north- and east-facing slopes. Soil moisture may control weathering of Jurassic sandstones, which is shown by previous studies to occur largely by hydration and expansion of clay cements. Temperature differences may affect spalling in summer and freeze-thaw action in winter, but the efficacy of these processes in the Blue Gap area remains unknown.