POST LAST GLACIAL MAXIMUM HILLSLOPE EVOLUTION IN THE SOUTHEAST SAN JUAN MOUNTAINS, CO, USA

The spatial and temporal variability of pedogenic and hillslope processes that influence basin morphology and sediment supply in subalpine-alpine catchments are poorly characterized. Yet soils can provide important insight into our understanding of landscape evolution in these environments, including information about the episodicity and timing of erosion, deposition and periods of relative stability. Here, we present field data from two tributary basins on Cornwall Mountain adjacent to the main axial valley of the Upper Conejos River basin, a majority tributary of the Rio Grande in Southeast San Juan Mountains in an effort to elucidate the post-glacial history of hillslope processes in this region of the Southwestern US.

Sawmill Gulch (SMG) and Robinson Gulch (RG), are located between 3,011-3,352 m and 3,048-3,744 m and drain 2.35 and 3.83 km², respectively. The surficial geology and geomorphology of both basins and their alluvial fans were mapped and described. A total of 54 soil pits were dug, described (Birkeland 1999) and sampled (~350) on identified map units. SMG is characterized by a broad, u-shaped valley containing glacial deposits and landforms whereas RG contained no valley glacier and instead comprises periglacial landforms (i.e. patterned ground) and a narrow, v-shaped valley flanked with steep talus slopes. Basal radiocarbon ages from weakly developed hillslope surface soils in RG range from ~763-2000 years BP in contrast to C14 dates from buried hillslope soils in SMG ranging from ~ 1881-3100 years BP. However, well-developed buried soils in terrace and bogs units above 3,231 m in SMG and RG date to ~7200-10600 years BP. Multiple buried soils, Bt horizons and soil reddening across map units indicates periods of sediment aggradation punctuated by long periods of storage on hillslopes throughout both basins. The oldest alluvial fan units at SMG’s outlet date to ~9621 and ~8500 years BP. Voluminous debris flow (RG) and sheet wash (SMG) fans found at basin outlets temporally correlate to local paleoclimatic shifts. Overall these data suggest that Post LGM hillslope erosion is not spatially or temporally uniform either within or between even very proximal (~1 km) basins with different geomorphic histories.