ASPECT AND MICROCLIMATIC INFLUENCES ON HILLSLOPE GEOMORPHOLOGY, NORTHEASTERN ARIZONA

Strong across-canyon asymmetry indicates that slopes on sandstones and shales of the the Dakota, Morrison and Bluff Formations on the Colorado Plateau near Blue Gap, Arizona are particularly sensitive topographically induced microclimate, and by extension, to Quaternary climate change. The south-facing slopes of small (~0.5 km²) east-draining canyons are 1 - 3° steeper, with significantly more exposed unweathered bedrock and less vegetation and colluvium than north-facing slopes. Large cliffs on south-facing canyon slopes account for 29% of the canyon relief, but low north-facing cliffs only account for 2.5% of the relief. Slope asymmetry is largely independent of rock type and can be attributed to differences in aspect-controlled microclimate and slope processes. Temperature and soil moisture were recorded on opposite sideslopes for one year beginning September, 2002. Air, surface and subsurface (10 cm depth) temperatures were ~1.8°C, ~3.6°C and ~5.6°C warmer respectively on south-facing slopes than on north-facing slopes, consistent with topography-based insolation modeling. Morrison Formation sandstones are almost exclusively cemented by smectite and kaolinite clay. Weathering occurs primarily by clay hydration that results in progressive reduction of bulk density and grain contacts and increased porosity. Sufficient moisture for clay hydration existed on north-facing slopes all year, but south-facing slopes were too dry most of the year. Slope angles are controlled more by rock strength and weathering than vegetation control of erosion. South-facing slopes are steeper because microclimate and weathering feedbacks produce more erosion-resistant unweathered bedrock slopes. Cliff area reflects longer-term slope evolution and accounts for most of the aspect-related asymmetry. Cliff initiation appears to occur only where unweathered bedrock slopes are persistent. Small north-facing cliffs are active at present but are probably mostly a Holocene phenomenon since few are > 3 m high. Enhanced weathering and debris formation during the Pleistocene was likely sufficient to erase most north-facing cliffs. High south-facing cliffs (up to 70 m) indicate that the aspect-related microclimate has allowed unweathered bedrock and cliffs to persist throughout the late Quaternary.