A SUBALPINE SOIL CHRONOSEQUENCE IN POST-GLACIAL DEPOSITS OF THE SAN JUAN MOUNTAINS, SOUTHERN COLORADO

In the subalpine landscapes of the southern Rocky Mountains, the stratigraphy and soils of post-Last Glacial Maximum (LGM) surficial deposits are not well characterized. Glacial, fluvial, alluvial and colluvial sediments provide a post-glacial record of the transport of sediment from hillslopes to valley bottoms. However, without age control, understanding the evolution of these landscapes is difficult. Typically, soils can be used to correlate, provide relative age control, and provide insights into episodic deposition of such deposits, but in this setting, differences in parent material and the relatively young age of the deposits make the feasibility of such correlations uncertain. Here, using a chronosequence of 41 soils developed in a variety of post-LGM landforms and deposits (glacial, fluvial, alluvial, colluvial) ranging in elevation from ~3600 -2600 meters, we examine the spatial and temporal variability of soil characteristics in this subalpine landscape. Soils developing in deposits ranging in age from ~1,000 to ~15, 000 years were examined, described, and sampled in the field. Various laboratory analyses including particle size, pH, Fe extractions, and total organic carbon were completed. Ratios of oxalate/dithionite Fe extractions exhibit a robust trend with age for all soils. In general, there is a qualitatively predictable progression of soil morphology with time regardless of parent material, yet, quantifiable trends in certain properties such as reddening or clay content are not always evident. Eolian inputs are also apparent in the A horizons of all soils. Variation in eolian deposition and parent material sedimentology likely led to the observed spatial variability in soils of similar age that are located at different elevations and formed in different deposits in the field area.