DIFFERENTIATION OF TWO QUATERNARY MT HOOD LAHARS BY CLAY MINERALOGICAL, QUANTITATIVE X-RAY DIFFRACTION AND PARTICLE SIZE ANALYSES

Multiple analytical methods have successfully differentiated two similar Quaternary lahar deposits on the northern flank of Mount Hood and in downstream valleys. These methods include X-ray diffraction (XRD) of clay mineral content, quantitative XRD of bulk mineral content, and laser diffraction particle size analysis (LDPSA). The best differentiators are clay and bulk mineralogy. QXRD results of bulk samples indicate that the older lahar (older than 425 Ka) contains 15-23% plagioclase feldspar and 50-80% total dioctahedral clay (predominantly halloysite), whereas the younger lahar (40 to 80 Ka) contains 34-53% plagioclase feldspar and 24-32% total dioctahedral clay (predominantly expandable smectitic clay). At most sites, the older lahar is relatively enriched in clay and sand-sized grains as compared to the more silt-rich younger lahar. These preliminary results will prove valuable in determining the spatial variability of lahars and may be used in remote sensing mapping. Previous work by the USGS Cascade Volcano Observatory (CVO), as part of their hazard assessment program, has identified at least two distinct Quaternary lahars based on field mapping and limiting K-Ar dates. The field area encompasses roughly 200km^2 from the northern flanks of Mt. Hood through the Hood River valley to across the Columbia River. This work provides the impetus for our current study because of speculation raised during mapping the lahars. Since it is apparent that field identifiers are not the most reliable indication of specific units, this study was undertaken to produce quantitative data that can then be applied to more accurate mapping and hazard analysis.