28 Quaternary Geology and Geomorphology (Posters)

Sunday, 4 November 2012
9:00 AM - 6:30 PM
Hall B

POST GLACIAL EVOLUTION OF SUBALPINE HILLSLOPES AND SOILS, SE SAN JUAN MOUNTAINS, CO, USA

Paper number: 28-10
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Previous research conducted in the Conejos River basin suggests that some hillslopes in this subalpine landscape have been relatively stable since 9837 ± 72 cal. years B.P.; based on radio carbon dates and the presence of well developed soil profiles on lower elevation portions of valley interfluves (Layzell et al., 2012). Alluvial fans and fluvial terraces in the axial valley adjacent to these slopes, however, were aggrading episodically throughout the Holocene; (Johnson, 2011; Layzell et al., 2012). These data suggests that sediment involved in valley bottom aggradation is likely derived from spatially distinct hillslope sources during temporally distinct climatic events. However, the processes and spatial and temporal variables controlling from where and when sediment is originating are unknown. Here we use soil development as a proxy for hillslope stability in order to test the role of landscape position on erosion in subalpine environments.

The geomorphic features and deposits of Sawmill Gulch, a tributary drainage basin of the Conejos River, were mapped and divided into 14 mapping units based on surface geomorphology, deposit type, vegetation and slope. We hand-dug a total of 28 soil pits (a minimum of 2 per mapping unit) in the basin. Soil was sampled from each soil horizon in every pit. To determine the variability of parent material with depth in terms of rock type, roundness, mineralogy, and degree of weathering a clast count of 30 rocks was performed in each pit at 5 cm down and 10-20 cm across, depending on the abundance of clasts in the pit face. Soils varied predictably as a function of landscape unit which varied at small spatial scales (50m or less); with some map units exhibiting well developed soils with strong structure and color. These data suggest that erosion since deglaciation has been limited to certain types of slopes and deposits. For example, shallow forested (conifer) moraine features most commonly exhibit the weakest soil development suggesting that these positions are least stable over Holocene time scales. An abundance of charcoal was found in soil pits throughout Sawmill Gulch. Future work will attempt to determine the key characteristics and processes that result in preferential erosion of certain landscape positions over others.