Depositional features of March 1982 Mount St Helens sediment flows

Abstract

On 19 March 1982, 2 yr after initial eruption, Mount St Helens erupted explosively, resulting in the deposition of new sediment flows in the Toutle River drainages (Fig. 1). We conducted field observations of these flows during the period 29 March–5 April 1982. By this time, the rivers, rapidly re-establishing pre-eruption channel levels, had down-cut through the new flows, allowing us to examine the internal structure of the flows before they could be extensively modified by reconstruction projects. The flows were characterized by a distinct three unit sequence. A basal massive to graded layer (unit 1) consists of large clasts in grain to grain contact. It is overlain by a distinctly finer-grained stratified unit (unit 2) of similar thickness. The top unit (unit 3) contains very large matrix-supported clasts and transported log debris (Fig. 2). This sequence has not previously been documented where its origin could clearly be ascribed to a single catastrophic depositional event. The sequence provides a useful model for interpreting similar deposits in the rock record.