ASPECT-RELATED MICROCLIMATIC INFLUENCES ON PHYSICAL WEATHERING AND HILLSLOPE GEOMORPHOLOGY, BLACK MESA AREA, NORTHEASTERN ARIZONA

We documented clear asymmetry in slope form and processes in three small (0.5 km²) basins in northeastern Arizona. The eastward-draining basins are formed mostly in flat-lying sandstones and mudstones of the Salt Wash Member of the Morrison Formation. Field measurements and 3-m DEM data indicate that south-facing slopes have much more cliff area (29%) than north-facing slopes (2.5%), and are 1-3° steeper excluding cliffs. We propose that these differences are the result of aspect-controlled microclimate. Temperature and soil moisture data were collected over one year. Annual average temperatures measured in air, at the surface and at 10 cm depth on south-facing slopes were 1.4-5.6°C warmer than nearby north-facing slopes, which experienced 10-50% more freezing days. Soil moisture was significantly lower on south-facing slopes throughout the year, especially during the warmer months.

The Salt Wash Member is cemented by smectite clays and is highly porous, thus weathers readily by clay hydration and frost action, creating an in-situ disintegration mantle that often exhibits a “popcorn” surface. The mantle varies from 0 to >1 m thick, but is thicker and more extensive on north-facing slopes, which also have more vegetation cover and colluvium. A series of feedbacks suppresses cliff formation on north-facing slopes. Greater moisture for clay hydration and more freezing cycles readily disintegrate the bedrock surface, allowing greater infiltration and less erosion by runoff, promoting further weathering. Where erosion exceeds weathering, as is more common on south-facing slopes, unweathered bedrock surfaces generate runoff, further enhancing erosion and stripping of adjacent weathered mantle. Thus the slope may steepen over time until failure occurs on sub-vertical fractures, creating a cliff. These cliff-forming processes are apparent on all slope aspects, but predominantly on south-facing slopes where weathering is limited by warmer, drier conditions. Modern north-facing cliffs are small (<3 m) and may be largely of Holocene age. Small cliffs formed in previous warm periods may have been erased during cooler, wetter periods in the Pleistocene. South-facing cliffs reach up to 70 m in height, however, indicating that cliff formation was favored on these aspects for much of the late Pleistocene.