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

Streams on the southern margin of the Uinta Basin in northeastern Utah 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 onto the floodplain surface of Left and Right Forks of Indian Canyon and Sowers Canyon 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 45 radiocarbon dates from coarse angular charcoal fragments from 14 fans cluster into discrete time intervals at 2400 to 2100 ¹⁴C yr BP, 1500 to 1200 ¹⁴C yr BP, and 1000 to 300 ¹⁴C 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.