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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 464

Last Page: 464

Title: Sedimentary Sequence from 1980 and 1982 Mount St. Helens Sediment Flows: A Model for Older Volcaniclastic Deposits: ABSTRACT

Author(s): William J. Fritz, Sylvia Harrison

Article Type: Meeting abstract


Sediment flows resulting from May 18, 1980, and March 19, 1982, eruptions of Mount St. Helens produced a three-unit sequence that provides a model for interpreting similar deposits in the rock record. After the 1982 eruption, rivers rapidly reestablished pre-eruption channel levels and downcut through the new flows, allowing examination of the internal structures before extensive modification by reconstruction projects.

The depositional sequence consists of a basal graded to massive layer (Unit 1) of large clasts in grain-to-grain contact 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. Grain size and total thickness of the sequence varies from 3.5 m (11 ft) thick, 40 cm (16 in.) boulders, and coarse sand in proximal flows above Camp Baker to 0.5 m (1.5 ft) thick, coarse sand, and silt in distal flows on the lower Cowlitz River. This sequence resembles some fluvial deposits, but clearly formed during extremely rapid deposition related to mudflows. We have noted similar Tertiary volcaniclastic sediments and believe Mount St. Helens sediment flows provide a model for interp eting volcaniclastic deposits often considered dominated by fluvial processes.


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