MT. MAZAMA TEPHRA IDENTIFICATION USING MICROPROBE GEOCHEMICAL ANALYSIS AT SALTESE FLATS, EASTERN SPOKANE COUNTY, WASHINGTON

Saltese Flats, in eastern Spokane County, is a wetland marsh area underlain by extensive peat deposits. A 1958 study evaluating peat resources reported a significant tephra (volcanic ash) horizon from a series of soil bore holes. The tephra was not described, and a volcanic source was not correlated. In 2019, a Spokane County wetlands restoration project at the site afforded direct examination of the tephra, via excavation. This study presents the results of trench mapping, sampling, and tephra characterization from the Saltese site along with the potential unit correlation using glass shard geochemistry. A temporary trench exposed a 71 cm layer of fine, light-grey tephra at a depth of 1.9 m. Dark brown, fibrous peat occurs above the tephra. A 14 cm layer of fine, blue sand occurs at the base of the tephra with black peat to the bottom at 4 m. The upper and lower contacts of the tephra are sharp, not gradational or interbedded with peat. A bore hole transect profile from the 1958 study shows the ash as a continuous layer across the basin, and the thick sequence of peat underlain by lake sediments indicates a low-energy lacustrine environment such as a shallow pond or marsh. SEM imaging and petrographic examination indicate that the fine (<150 μm) tephra is composed of glass shards (~90%) with rare microlites of lath-shaped feldspar, and possible amphibole, iron-titanium oxides, and pyroxene. Microprobe analysis of glass shard chemistry from each sample was performed at the Washington State University GeoAnalytical Laboratory. Major element oxide results for each sample (25 shards analyzed per sample) were consistent suggesting minimal alteration. SiO₂ compositions range from 72.78-72.97%, with Na₂O and K₂O compositions ranging from 5.39-5.53%, and 2.74-2.79%, respectively. Utilizing statistical comparison to the Washington State University Pacific Northwest tephra database (1716 records), the Saltese Flat tephra matches (0.98 similarity coefficient) to the Mt. Mazama climactic eruption at 7700 cybp.