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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
5. (September), Pages 776-791
Fire-Related Sedimentation Events on Alluvial Fans, Yellowstone National
Park, U.S.A.
Grant A. Meyer (1), Stephen G. Wells (2)
ABSTRACT
We document initiation and flow processes, deposit facies, and geomorphic
effects of forest-fire-related sedimentation on small alluvial fans in
Yellowstone National Park. Brief, intense convective-storm precipitation
on steep basins burned in the 1988 fires produced sedimentation events
involving a variety of depositional processes on fans. Over the course
of all documented events, flows on fans progressed from higher to lower
sediment concentration. Events were often dominated by either debris flows
or relatively sediment-poor streamflow processes; in some events, however,
flows ranging a over wide spectrum of sediment concentration produced significant
fan deposits. Debris flows were generated by progressive sediment bulking
involving pervasive surface runoff and rill erosion on st ep upper basin
slopes, followed by deep incision as flows progressed down channels. Debris-flow
deposits show a marked decline in thickness and coarse gravel content downfan,
often with extensive distal gravel-poor facies. We recognized a relatively
minor percentage of noncohesive debris-flow and hyperconcentrated-flow
facies, with sorting and stratification intermediate between muddy debris-flow
and streamflow facies; these were deposited where dilute flows bulked with
coarse sediment by eroding channel alluvium or earlier deposits of the
event. Below incised fan channels, streamflows expanded as sheetfloods,
which prograded lower fans with distally fining deposits. Basins > 3 km2
typically produced streamflow events on fans, but sediment texture and
availability on slopes and in channels are primary factors determining
flow processes on fans of smaller basins. Burned soil surfaces provided
abundant silt and clay for debris-flow generation, but because soil surface
sediment was stripped and/or compacted over time, the lack of available
fines resulted in dominance of streamflow processes in later events.
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