MAPPING THE HISTORY OF ALLUVIAL FAN DEPOSITION WITH ANALYSIS OF HIGH-RESOLUTION TOPOGRAPHIC DATA

Sequences of alluvial fans reveal a history of deposition into sedimentary basins and often record displacement from active faulting. Periodic transport events and frequent avulsions produce rough surfaces on active alluvial fans. Following fan abandonment, new transport processes dominate. In the absence of incision into fan surfaces, these post-abandonment processes typically act to smooth initially rough topography. Past work and observations from the San Luis Basin of Colorado highlight mapped fan units that exhibit measurable decreases in surface roughness with increasing age. We interpret the decrease in roughness from active to older surfaces in the context of a linear diffusion model for surface evolution. We present a quasi-analytic solution for the evolution of topography that relies on spectral decomposition of fan profiles and allows for direct evaluation of morphologic ages from topography. Predicted morphologic ages highlight the dependence of the decay rate of relief on topographic wavelength. Recognizing this we establish practical bounds on the ability of our diffusion model to resolve different morphologic ages given the characteristic wavelengths of the landforms of interest and the level of noise present in the data. We test this approach on a suite of alluvial fans surrounding Blanca Peak in the Sangre de Cristo Mountains, CO, imaged by airborne laser swath mapping. The relative chronology of fan surface morphologic ages agrees with independent field investigations of surface texture, soil development, and cross-cutting relationships. Importantly, model-derived morphologic ages also make specific predictions about the relative timing of establishment of major fan surfaces and those formed by minor avulsions. Drawing on geochronology of a subset of fan surfaces, we calibrate the efficiency of diffusive transport on fans in our study area and calibrate our morphologic ages against absolute dating. This scalable method provides a useful tool for establishing fan chronologies based solely on geologic maps and high-resolution topographic data.