Sedimentology and geomorphology of quaternary alluvial fans with implications to growth strata, Lost River Range, Idaho

Abstract

Small-radius (<2 km), steep (8-17°), Holocene debris flow dominant alluvial fans are depositing on top of large-radius (~5 km), shallow (2-3°) inactive Pleistocene sheetflood dominant alluvial fans along the western flank of the Lost River Range, Idaho. Channel ways, with and without backfilling, have developed within the large sheetflood dominant alluvial fans. The sheetflood dominant alluvial fans are being dissected. Three wedge shaped gravel packages were identified by field mapping and measured sections in four alluvial fans along the active extensional Lost River fault. These alluvial fan deposits have all of the characteristics of growth strata (progressive unconformities) observed in extensional tectonic settings. The ages of the surfaces were calculated from carbonate coat thicknesses on clasts in the soil and are between 42,000 ± 18,000 yr and the present. Each of the four alluvial fans has different stratal patterns even though they are all situated on the active Mackay fault segment. This lateral variability has implications to rock record interpretations. The younger fan depositing on top of the older fan exhibits the interaction of neighboring alluvial fans, and there is a changing of the sediment transport process over time on an individual alluvial fan. Complexly interacting controls on alluvial fan development include: 1) temporal change in the locus of maximum displacement on the Mackay fault segment, and 2) changes in Pleistocene and Holocene discharge.