LATE PLEISTOCENE RECORD OF SURFACE-MODIFYING PROCESSES ON INACTIVE LOBES OF THE PIONEER DEBRIS-FLOW ALLUVIAL FAN, SAN LUIS VALLEY, COLORADO

Active alluvial fans and fan deposits in the rock record are often used as indicators of landscape dynamics, yet the sedimentological signature of constructive and surface-modifying processes is often overlooked. Using integrative sedimentological and geomorphic analysis, we applied facies and soils mapping along with infrared stimulated luminescence (IRSL) geochronology to document timing of fan construction and modification processes recorded in deposits of the Pioneer fan. The Pioneer fan is located on the extensively glaciated western range front of the Sangre de Cristo Mountains in the San Luis Valley of southern Colorado. Constructive fan processes are mainly debris-flow deposition on the active lobe, with progressive lateral lobe switching due to autogenic topographic compensation. Debris-flow activity on adjacent inactive fan surfaces is minimal, allowing modifying processes to dominate the inactive fan surfaces. Both constructive and modifying processes are recorded in a ~10-meter vertical quarry exposure at the fan toe, comprising intervals of active lobe deposition and inactive lobe modification, respectively. Matrix- to clast-supported gravel (Gms-Gmsc facies) represent primary debris-flow deposition (active lobe). Surface reworking after lobe switching by sheet wash is characterized by massive gravel (Gm), horizontal gravel (Gh), low-angle cross-stratified sand (Sl), and horizontally-stratified sand (Sh). Modification periods with strong eolian influence are represented by massive sand (Sm), while surficial water modification of eolian deposits is shown by Sh and Sl facies. Three buried soils are exposed in the quarry, each defining a period of surface stability with IRSL ages of ~115 ka and ~57 ka for the oldest two. Associated with each soil is a facies assemblage that originates with constructive Gms and Gmsc facies (debris flow) and transitions upward to surface-modifying Gm, Gh, Sl, Sh facies (sheet wash), capped by eolian-influenced Sm, Sl, and Sh facies. On the Pioneer fan, and likely other debris-flow fans, eolian-influenced facies are associated with periods of surface stability shown by soil formation. This suggests that eolian deposition, water reworking of eolian sediment, and pedogenesis are important processes on the inactive surfaces of debris-flow alluvial fans.