A SOIL STRATIGRAPHIC ANALYSIS: LINKING TRIBUTARY BASINS AND ALLUVIAL FANS IN THE SE SAN JUAN MOUNTAINS, CO, USA

Soil erosion directly influences the rates and timing of valley bottom processes like alluvial fan aggradation, but the spatial and temporal variability of soil development/erosion in tributary basins are rarely known leading to assumptions about their contributions to the axial system. Here, we present soil geomorphic data from two ~2-4km² watersheds, Sawmill Gulch (SMG) and Robinson Gulch (RG), and from alluvial fans that issue from these basins onto the axial valley of the Upper Conejos River. These basins differ in morphology – SMG contains glacial landforms and RG does not – and in the size of alluvial fans issuing from their outlet – the RG fan complex is 4 times larger in volume and 3 times larger in area than SMG. These basins are located ~1 km apart and are underlain by similar volcanic clastic rock types. We employed a unilateral mapping scheme that focused on the dominant landforms and associated components - slope gradient and shape, vegetation, ground cover, lithology - to map the surficial geology. A total of 54 soil pits were dug, described, and sampled with a minimum of three pits per mapping unit in the basins and on alluvial fans.

C¹⁴ dates combined with soil forming properties indicate three distinct periods- PH1 (11.5-7.0 ka years B.P.); H2 (6.5 - 3.5 ka years B.P.); H3 3.5 ka years B.P. – present) of sediment mobilization throughout SMG and RG that coincide with alluvial fan aggradation and correspond with climatic cooling events and periods of high frequency climatic change. PH1 time was marked by abundant sediment erosion and deposition in these basins. PH1 soils - 7.5YR or redder hues, strong ped structure and overall clay or silty clay textures- are generally buried at depths >50 cm. H2 aged soils - 7.5YR hues, moderate ped structure, and silty loam to sandy clay textures- are the dominate surface unit in the headwaters of SMG and RG and are found commonly as buried soils in the valley bottoms of both basins. H3 aged soils -10YR hues, weak ped structure and silty or sandy loam textures- dominate the surface soils of colluvial slopes, alluvial fans, and debris flows throughout SMG and RG. Our work demonstrates that sediment has been mobilized in spatially distinct areas in these tributary basins throughout the Holocene and that sediment mobilization is driven, at least in part, by climate fluctuations since the LGM.