SEDIMENTARY RECORD OF CALDERA-FORMING ERUPTIONS, EOCENE CHALLIS VOLCANIC FIELD, IDAHO

Large explosive eruptions and base-level change by synsedimentary faul ting controlled fluvial geometry in deposits 35 km down-stream from th e Van Horn Peak cauldron complex, Idaho. Eruptions varied the volume a nd size of sediment load, The major impact of large eruptions was a ma rked increase in the proportion of sand-sized pyroclastic material (fr om 42% to 93%), Smaller eruptions, in contrast, increased only the was hload component. Following major eruptions, the river developed into a multichannel system characterized by wide (75+ m) and shallow (approx imate to 0.5 m) channels, The sedimentary record of this system is a s heet sandstone with width:thickness ratios ranging from 42:1 to 83:1 i n the best-exposed sandstone body, The sheets comprise 86% sandstone w ith low-amplitude stratification. Most of this is planar-laminated san dstone with tabular geometry, This laminated sheet assemblage is an un usual sedimentary accumulation for perennial rivers. The assemblage re presents accumulation under a combination of base-level rise associate d with faulting and increased sandy sediment loads produced by eruptio ns. These factors created an environment conducive to rapid aggradatio n. Declining sandy sediment loads, together with influx of fine ash (m ud-sized) sediment caused the river to change to a single-channel syst em characterized by more stable and deep channels, The sedimentary rec ord of this system is the mudrock-sandstone assemblage that has 46% ov erbank deposits with a significant mudrock fraction, Channel sandstone bodies in this assemblage are smaller sheets with width:thickness rat ios ranging from 8:1 to 25:1.