RELATIONSHIPS BETWEEN WILDFIRES AND
ALLUVIAL FAN FORMATION IN THE UINTA
BASIN, NORTHEASTERN UTAH
CARSON, Eric C., HANLY, Brittany R., and WILSON, Desiree S., Geology
Department, San Jacinto College, 5800 Uvalde Road, Houston, TX 77049,
eric.carson@sjcd.edu
Streams on the southern margin of the Uinta Basin in northeastern Utah incise into the
lacustrine Eocene Uinta Formation. In places where the bedrock is resistant to erosion,
steep tributary valleys have formed, and are currently depositing alluvial fans onto the
floodplain surface of the mainstem valleys. On the South Unit of the Ashley National
Forest, we identified 36 fans in Indian Canyon and Sowers Canyon whose distal margins
have been incised by the mainstem streams. Description of the sediments exposed in
three of the fans indicates that debris flow and hyperconcentrated flow deposits are the
dominant fan-forming process in this setting.
Commonly, debris flow deposits are found immediately overlying burn horizons and
wildfire-related charcoal fragments, suggesting a strong causal relationship between fire
frequency and fan formation. Radiocarbon dating of charcoal deposits from three fans
provides chronologic control on the timing and rates of sediment deposition. Radiocarbon
dates of 1930 ± 40 14C yr BP (BETA-232604) and 460 ± 40 14C yr BP (BETA-232603)
retrieved from charcoal-stained alluvial sediment in fan LIC-4 suggests that over 5.5 m of
debris flow sediment has accumulated on the fan in the past ~2000 calendar years, and as
much as 4 m of sediment accumulated in the past ~500 calendar years. Similar
radiocarbon dates of 1570 ± 40 14C yr BP (BETA-232607) from RIC-1 and 280 ± 40 14C
yr BP (BETA-232609) from RIC-2 indicate that as much as 2.7 m of sediment has
accumulated on RIC-1 in the past ~1440 calendar years and as much as 2.9 m of
sediment has accumulated on RIC-2 in the past 380 calendar years. These data are
significant for land-management in this setting, considering that the current development
plan for natural gas resources on National Forest land is preferentially locating drilling
platforms on alluvial fan surfaces. A total of 20 radiocarbon dates have been accumulated
from five fans; over half of these dates cluster into a time period from 1000 to 300 14C yr
BP. This suggests that climate variability exerts control on fire regime, which is then
reflected in debris flow frequency. The period of frequent fires documented by these data
corresponds to a period of smaller than modern bankfull floods previously documented in
the adjacent Uinta Mountains (Carson et al., 2007).