Abstract

Large explosive eruptions and base-level change by synsedimentary faulting controlled fluvial geometry in deposits 35 km downstream from the Van Horn Peak cauldron complex, Idaho. Eruptions varied the volume and size of sediment load. The major impact of large eruptions was a marked increase in the proportion of sand-sized pyroclastic material (from 42% to 93%). Smaller eruptions, in contrast, increased only the washload component.

Following major eruptions, the river developed into a multichannel system characterized by wide (75+m) and shallow (≈0.5 m) channels. The sedimentary record of this system is a sheet sandstone with width:thickness ratios ranging from 42:1 to 83:1 in the best-exposed sandstone body. The sheets comprise 86% sandstone with low-amplitude stratification. Most of this is planar-laminated sandstone with tabular geometry. This laminated sheet assemblage is an unusual sedimentary accumulation for perennial rivers. The assemblage represents accumulation under a combination of base-level rise associated with faulting and increased sandy sediment loads produced by eruptions. These factors created an environment conducive to rapid aggradation.

Declining sandy sediment loads, together with influx of fine ash (mud-sized) sediment caused the river to change to a single-channel system characterized by more stable and deep channels. The sedimentary record of this system is the mudrock-sandstone assemblage that has 46% overbank deposits with a significant mudrock fraction. Channel sandstone bodies in this assemblage are smaller sheets with width:thickness ratios ranging from 8:1 to 25:1.

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