Influences of Topoclimate, Soil-Forming Processes, and Vegetation Distribution on the Evolution of Hillslopes in the Sevilleta Wildlife Refuge, Central New Mexico

In a semiarid landscape characterized by both largely soil-mantled slopes and slopes with both exposed bedrock and soil mantles, field-based studies show that lithologic variation strongly affect soil-forming processes, which in turn strongly influence slope form and evolution. In an east-west oriented valley on the Los Pinos Mountains piedmont, New Mexico, microclimate variations related to slope orientation favor asymmetry in vegetation distribution, which in turn affects soil-forming processes and ultimately variation in slope form. North-facing slopes are steeper with a thicker soil mantle and support a juniper savannah, while south-facing slopes support a creosote scrubland, have thinner soils and extensive areas of exposed bedrock. Hillslopes are associated with two different materials: (1) Late (?) Quaternary gravelly alluvium deposited on a bedrock pediment; (2) Upper Paleozoic sedimentary rocks including dolomite and gypsum-rich limestone of the Yeso formation, and red mudstones and sandstones of the Abo formation. To understand the processes driving fine-scale slope form, soils were examined along a north-south oriented full toposequence associated with the Palo Duro Wash and its tributaries. Additional soils were examined at backslope positions, primarily also facing north or south, on slopes with lower relief. Where exposed, bedrock forms resistant outcrops which act as local base level controls. The south-facing slope's soils exhibit weak soil development, and buried bedrock is also minimally weathered. The north-facing slope's soils are, however, more strongly developed, as shown by significant carbonate accumulation and reddening, and weathering of the upper 40 cm and 14 cm respectively, of the buried limestone and Abo formation. Field investigations indicate that the alluvial deposits provide the primary source of hillslope colluvium, whereas bedrock contributes far less colluvium. Laboratory studies (particle size, carbonate, and clay mineralogy) will help elucidate the roles of rocktype, climate, biota and weathering and pedogenesis in hillslope evolution.