Abstract

Lacustrine deposits in the Eocene Challis volcanic field accumulated in a deep lake bordered by steep ranges. Volcanic and nonvolcanic sediment was transported into the lake via a major dispersal system from the north. High-density turbidity currents transported this sediment throughout the lake system. Paleozoic rocks in the adjacent ranges were secondary sediment sources. This sediment was trapped along the shoreline, and only a small fraction was transported offshore by turbidity currents. The frequency of turbidity currents varied with changes in eruption style. Thus, turbidite accumulation patterns are a faithful record of changes in volcanic activity and associated landscape processes. Effusive volcanism that characterized the early activity in the Challis volcanic field produced little sand-size sediment and had little impact on lacustrine sedimentation patterns. Relatively slow settling from suspension characterized lake sedimentation during this time; turbidity currents were infrequent. The resulting accumulation pattern is a single sheet of turbidite sandstone encased in thick-bedded mudrock. Initial eruptions at the Van Horn Peak cauldron complex altered sedimentation patterns dramatically by increasing the frequency of turbidity currents. A turbidite ramp prograded into the study area, marking a major increase in sedimentation rate. Sediment composition and turbidite accumulation patterns record changes in landscape processes. Syneruption deposits are stacked turbidite sheets. Early turbidites contain mostly sediment derived from the fresh pyroclastic material. The proportion of sediment from pre-eruption sources increases upward through the stacked sheets, recording a change in sediment sources at the end of the syneruption period. Ramp growth continued through this part of the period, although at a much slower rate.

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