Abstract

Volcaniclastic rocks in the Mount Dutton Formation form coalescing aprons of lahar deposits around several volcanoes. The aprons accumulated across a featureless alluvial plain in an arid to semi-arid climate. Under these conditions, debris flows dominated apron construction, building large aprons extending as much as 27 km from the flank of the source volcano. Three distinct but gradational lithofacies assemblages within the aprons show a proximal to distal change from thickly bedded, coarse lahar deposits to thinly bedded, fine-grained ones. This lithofacies change represents a change from channelized flow on the proximal apron to unchannelized flow on the medial apron. Unconfined flow on the medial apron, together with lower slope, caused lahars to deposit the bulk of their sediment load. Most deposition in distal areas resulted from small, dilute lahars. Large lahars capable of transporting coarse sediment into distal areas were rare.

The aprons also show a down-apron decrease in abundance of debris-flow deposits, corresponding to an increase in hyperconcentrated flow deposits. This trend represents reworking of more proximal deposits by floods that did not form debris flows, rather than flow transformation from debris flow to hyperconcentrated flow. Arid climate and unchannelized flow prevented the flow transformation that is a common feature of volcaniclastic deposits in the Pacific Northwest, where glacially dissected valleys with permanent streams control flow of lahars away from the volcanoes.

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