Poorly Sorted Terrace Deposits of the Cispus Valley: Glacial Drift or Mount Adams Lahar?

Pope, Isaac, Centralia College, isaac.pope@student.centralia.edu (Poster)

Incising through folded Oligocene basaltic andesite bedrock, the Cispus River meanders along the western slopes of Mount Adams before flowing westward towards Mayfield Lake near Randle, Washington. A veneer of tephra dated 1.2 ka from Mount St. Helens covers the Cispus River terraces and adjoining hills, attesting to recent manifestation of volcanism in the region. Despite this valley being classified as high-risk for lahars from Mount Adams, little research has been done to identify any previous lahars along the drainage. Examination of the Cispus River terraces reveals imbricated, poorly sorted, reverse graded deposits typical of debris flows. These deposits are found along multiple terrace levels and are sedimentologically similar, while clast lithologies indicate a provenance to the east. The 30-meter elevation difference between the lowest terrace and the surface of the highest terrace denotes the minimum flow depth; it is here estimated that the lahar was 0.3 cubic kilometers. Clasts within the debris flow deposits are often andesites and volcaniclastics distinct from the local basaltic andesite bedrock. Coupled with the eastward dipping imbrication, the clast lithologies suggest a provenance to the east near Mount Adams. While the pumiceous tephra (1.2 ka) that blankets the region provides the minimum age, the poorly sorted terrace deposits have been previously dated as approximately 20 ka, which is based on the interpretation that the deposits are glacial drift. If these deposits are instead laharic in origin, however, the lack of overlying till or drift suggests a younger, late-glacial age of the lahar. This age correlates nicely to the latest series of Mount Adams eruptions from 40 ka to 10 ka during which time the present edifice of Mount Adams was produced. This lahar may be among the largest and oldest lahars yet identified from Mount Adams.


Isaac Pope is a young undergraduate student with an insatiable fascination of geoscience. In addition to his field work, Isaac has studied numerous books ranging from graduate to professional level on a variety of geological and mathematical disciplines which contributed to him beginning his college studies at the age of fourteen. With publications in peer-reviewed journals, he has not only conducted much university-level research on both geological and mathematical topics, but he is also greatly interested in education stemming from his desire to share the wonder of science and mathematics with others.

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