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Alluvial fan deposits as recorders of fire and geomorphic response


, : Alluvial fan deposits as recorders of fire and geomorphic response. Abstracts with Programs - Geological Society of America 31(7): 312

Alluvial fans have significant potential to provide Holocene event-stratigraphic records of forest fires and their geomorphic effects in mountain landscapes, where small fans of steep tributary basins are ubiquitous. Intense, widespread burns often result directly in fan deposition, but in some cases lack of storms or rapid revegetation by herbaceous plants may preclude a response. Fire-related debris flows generated through rilling of burned soil surfaces contain much coarse angular charcoal, especially in finer-grained facies where charcoal is less comminuted. Although reworking of older charcoal is of concern for C-14 dating, multiple AMS dates on single fragments indicate that charcoal produced in the related fire predominates. Debris flows may also occur after fire through loss of root strength and evapotranspiration, leading to failure of colluvium during saturation. A causal link to fire is more difficult to establish, but their deposits typically contain charred logs. High-energy hyperconcentrated flows and streamflows from burned basins often deposit little charcoal on fans, as it remains suspended while higher density particles drop out. Thus, a substantial proportion of fire-related fan deposits cannot be recognized as such. Nonetheless, in the ponderosa pine environment of the South Fork Payette River, Idaho, one small fan contains charcoal-rich sandy sheetflood deposits with evidence of 8-20 fires between 6495 and 5965 yr BP ( approximately 7420-6770 cal BP); we dated six events. The recurrence interval for fire-related sedimentation is thus 33-80 yr for this period, suggesting a regime of frequent low-moderate intensity fires. Burned soil surfaces with charred litter are commonly preserved in fan stratigraphy and provide direct, datable evidence of fire; bioturbation suggests a significant time lag before deposition of overlying sediments. Increased magnetic susceptibility from incorporation of heated soil may fingerprint fire-related fan deposits in some environments, but in NE Yellowstone, susceptibility variations are dominated by particle-size control of magnetite content. Comparison to more continuous charcoal records in lake sediments should help to assess the completeness of fire records from multiple alluvial fans.

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