USING SOIL DEVELOPMENT, SEDIMENTARY FACIES, AND LUMINESCENCE CHRONOLOGY TO INTERPRET DEBRIS-FLOW ALLUVIAL FAN DYNAMICS IN THE STRATIGRAPHIC RECORD

Debris-flow alluvial fans are ubiquitous landforms and depositional environments used in geologic investigations to interpret past environmental, climatic, and tectonic settings. The active Pioneer alluvial fan, which emanates from the Sangre de Cristo Mountains in south-central Colorado, offers a unique opportunity to document an approximately 60-meter long, 16-meter-high, gravel quarry exposure of debris flow alluvial fan strata. The exposure of this active alluvial fan provides multiple buried soils, sedimentary facies amenable for luminescence age dating, and facilitates comparison between stratal and geomorphic signatures. We find that soil development and facies representative of surface formation spatially coincide within discrete vertical intervals of the exposure. Luminescence ages span from ca. 5 ka near the surface to ca. 100 ka at the bottom of the exposure. We apply an age model constrained by these luminescence ages that reveal fan depositional cycles, periods of surface formation, and/or depositional quiescence. We show that relatively well-developed soils record periods of depositional stasis of ~20-30 ka duration, while relatively poorly developed soils record stasis of ~5-15 ka duration. These data provide the basis for a conceptual model for interpreting fan stratigraphy and sedimentary processes in addition to elucidating soil-formation timescales. A greater understanding of the alluvial fan stratigraphic record within a modern active fan environment where surface processes may be documented and temporally constrained, offers a unique tool for interpreting sediment transport and system-wide alluvial fan dynamics when only the sedimentary record is available.