Construction and modification of debris-flow alluvial fans as captured in the geomorphic and sedimentary record: examples from the western Sangre de Cristo Mountains, south-central Colorado

Abstract

Alluvial fans and their deposits in the stratigraphic record are key in unraveling intricacies of landscape, tectonic, and climatic dynamics, though integrative geomorphologic and sedimentologic studies that comprehensively evaluate processes which build and modify fans are lacking. Therefore, a gap within the current body of literature exists concerning the sedimentological signature of depositional and surface-modifying processes on alluvial fans. This dissertation presents the sedimentological characteristics, both surficial and in the sedimentary record, of processes that build and modify alluvial fans while revisiting the contemporary concept of what defines an active surface. Detailed analysis of a suite of Quaternary active debris-flow alluvial fans on the western range front of the Sangre de Cristo Mountains in south-central Colorado was made using integrative sedimentological and geomorphic analysis, facies and soils mapping, along with infrared stimulated luminescence (IRSL) geochronology to document timing of fan construction and modification processes recorded in the alluvial fan deposits. Analysis of surface geomorphology, facies assemblages, and particle-size distributions of matrix from various facies of exposed alluvial fan deposits were also applied. These data show a clear distinction between sedimentary facies that represent processes of the primary depositional lobe surfaces versus those operating during periods of non-primary deposition, dominant on abandoned lobes. Primary processes on depositional lobes are debris flow and hyperconcentrated flow with minimal secondary modifying processes. Overland flow, input of eolian material, pedogenesis, and rock and mineral weathering are the main secondary modifying processes that govern abandoned lobes. Addition of wind-blown material, a secondary modifying process, plays a significant part in the sedimentary processes that operate on alluvial fan surfaces, ultimately influencing fan smoothing by mobilizing material derived from primary processes.