HOLOCENE STRATIGRAPHY AND GEOCHRONOLOGY OF SMALL ALLUVIAL FANS IN THE PONDEROSA PINE FOREST ECOSYSTEM OF NORTHERN ARIZONA

A number of Holocene alluvial geochronologies and paleoclimatic models have been constructed for semi-arid, mid to low elevation (1000-1700m) zones of the southern Colorado Plateau, but similar models have not been developed for the high elevation (~2125m) ponderosa pine forest of the same region. Present within much of the limestone lithology of the high-elevation ponderosa pine forest of northern Arizona are small drainages (<5km2), many producing single alluvial fans. Alluvial fan stratigraphy and radiocarbon chronology provide insight into the cause and timing of erosion from the fan’s corresponding drainage basin. Five alluvial fans were chosen for this study based on the presence of at least 1m subsurface exposure, fan location within limestone lithology, and drainage basin size <5km2. Hand-auger cores were taken at 10cm increments below the deepest exposure of each fan to extend sample depths. Thirteen AMS radiocarbon dates and one standard, bulk radiocarbon date were used to construct fan geochronologies. Two auger cores from separate fans reached bedrock, and record stratigraphic sequences spanning the entire Holocene. Fan aggradation from these two, long cores initiated at the Pleistocene-Holocene boundary and was marked by the highest sedimentation rates of the study (>100cm/1000yr). The lowest recorded sedimentation rates (<10cm/1000yr) and possibly a depositional hiatus correspond with the dry middle Holocene (8-4 ka B.P.) as documented from multiple lake core records in the same area. Lastly, fan aggradation during the past 4000-years was characterized by moderate sedimentation rates (~50cm/1000yr) and approximately 1m of cumulic soil development. Preliminary analyses suggest hillslope erosion and subsequent alluvial fan aggradation is a result of climate change and possibly fire. A better understanding of long-term, alluvial trends in the ponderosa pine forest of northern Arizona will aid in land management decisions concerning forest restoration policy in light of predicted future climate change scenarios.