T64 Quaternary Geology and Its Applications: In Honor of David M. Mickelson

Sunday, 9 October 2011
1:30 PM - 5:30 PM
Room L100A-C

LATE HOLOCENE RELATIONS BETWEEN WILDFIRES AND ALLUVIAL FAN FORMATION IN THE SOUTHERN UINTA BASIN, NORTHEASTERN UTAH

Paper number: T64-13
Program: Technical Sessions
Time: 4:40 PM – 5:00 PM

Authors

Abstract

Forest fires have long been recognized as potentially effective geomorphic agents by promoting slope erosion, debris flows, and floods. Sites where fire-related sediment accumulates, such as alluvial fans, provide an ideal setting for studying the millennial-scale effects of wildfire regimes on landscape formation; one such ideal location is on the southern margin of the Uinta Basin in northeastern Utah. Here, streams incise the lacustrine Eocene Uinta Formation; in places where the exposed bedrock is sandy and more resistant to erosion than overlying and underlying silty horizons, relatively deep main-stem canyons have been formed. Short, steep tributary valleys empty into the main-stem streams, and are depositing alluvial fans along several stream reaches on the South Unit of the Ashley National Forest. Erosion by the main-stem streams has exposed the stratigraphy of the distal margins of over three dozen of these fans. Debris flow and hyperconcentrated flow deposits comprise the majority of the sediments exposed in the fans, often with burn horizons and wildfire-related charcoal fragments underlying individual flow deposits.

The exposure of the distal margins of these fans provides the opportunity to investigate the relationships between wildfire occurrence and fan-forming debris and hyperconcentrated flows. Fire-related deposits comprise roughly 30 – 40% of the late Holocene fan deposits. A total of 62 radiocarbon dates from coarse angular charcoal fragments from 30 alluvial fans record the occurrence of fires over the past 4000 years. Frequent fires cluster into discrete time intervals at 2400 to 1900 cal yr BP, 1500 to 1100 cal yr BP, and 700 to 300 cal yr BP. These time periods with numerous fire deposits are separated by time intervals with few or no dated wildfire deposits. The non-random clustering of dates suggests that climate variability exerts control on fire regime (frequency of fires), which is then reflected in debris flow frequency. The periods of frequent fires documented by these data corresponds to periods of smaller than modern bankfull floods previously documented in the adjacent Uinta Mountains (Carson et al., 2007), suggesting a common climatic control on both wildfire regimes in the Uinta Basin and flood magnitudes in the Uinta Mountains.