ASPECT RELATED DIFFERENCES ON SLOPES OF THE WEAKLY CEMENTED TESUQUE GROUP SANDSTONES: SPATIAL VARIABILITY AND THRESHOLD RESPONSES

In the northern hemisphere, aspect as a topographic variation creates differences in microclimate that specifically leads to drier, warmer southern aspect slopes and moister cooler northern aspect slopes. These differences affect soil and slope forming processes. A recent study of slopes in northeast Arizona has shown that thinner soil mantles on southern aspect slopes are attributable to limited weathering and vegetation cover and sensitive rock types. Weakly cemented sedimentary rocks are sensitive to erosion, especially in warmer and drier climates. This study evaluates soils on slopes in northern New Mexico which have also developed in weakly cemented sedimentary rocks and have similar vegetation cover, but a slightly cooler and moister climate than that of northeast Arizona. In contrast to the studies of the southern aspect slopes in NE AZ, the study area slopes maintain soil mantles comparable to those on the adjacent northern aspect slopes. Other southern aspect slopes have, however, thin discontinuous soil mantles and significant areas of exposed bedrock that resemble such slopes in NE AZ. Three toposequences were completed, one on a northwest-facing slope (NW-A) and two on southwest-facing slopes (SW-A, and SW-B). NW-A and SW-A have similar soil profiles, while SW-B has thinner soils and bedrock exposure, with the exception of the thick cumulate back slope soil. The valley fill surface below NW-A and SW-B is largely unincised; the valley fill below SW-B is deeply incised and has evidence of rapid cut-and-fill cycles. Vegetation cover is highest on NW-A, lowest on SW-B. Throughout the study basin vegetation cover is highest on north- and northwest-facing slopes and lowest on south-facing slopes. XRF analysis revealed little evidence of chemical weathering and likely presence of eolian dust contributions to A horizons. GIS analysis and visual inspection of aerial photographs show that the surrounding basin is varied in regards to slope gradients: steeper slopes in the study basin are propagating from south to north and west to east, or upstream. This evidence combined with other geomorphic research shows that four major processes affect soil development and slope form in the study basin: 1) weathering processes; 2) slope processes; 3) basin processes; and 4) pulse and threshold responses.