64-8 Using Micromorphology to Discover Evidence of Fault Activity in the Palouse Loess

Session: Emerging Voices in Soil and Paleosol Science (Posters)

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Paleoseismic trenches are useful tools to determine prehistoric fault activity and seismic hazard. Trench studies rely on visibly disrupted features such as bent, offset, or disrupted rock and sediment layers. In homogeneous sediments, lack of evidence of disruption in the field may potentially be observed using micromorphological studies. The north flank of the Horse Heaven Hills anticline in southern Washington and northern Oregon is seismically active, but the basalt bedrock is unconformably overlain by homogenous loess deposits, making it difficult to observe sediment seismic disruption in the field. In this study we use soil micromorphology at a seismic trench site on the Wallula fault system at the Schubert’s Quarry paleoseismic trench site (SQ) in the Horse Heaven Hills, 2 miles south of Umapine, Oregon, to determine whether evidence of seismic disruption is visible in thin sections. We compared thin sections from the SQ site to thin sections from the Rulo site, a nearby undisturbed Palouse loess site, using standard petrographic microscope techniques. The void spaces and organic matter in SQ thin sections are linearly oriented, which is not the case in the undisturbed sediments from Rulo. Additionally, the SQ sediments are more abundant in lithic fragments, and contain fractured mineral grains. Except for a disrupted tephra layer, this evidence of seismic disturbance was not visible at the field scale. Studying the micromorphology of loess paleosols is a cost effective and productive way to discover evidence of fault disruption in soils of the Palouse loess.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7668

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