Technical Note: Gray Clay (Malpass Clay) in the West Eugene Wetlands and the Willamette Valley is Weathered and Redeposited Ash from Mount Mazama.

The climactic eruption of Mount Mazama approximately 7,600 years ago distributed air fall tuff throughout what is now eastern and offshore Oregon. This distinctive tephrostratigraphic marker had been thought to be absent across the Willamette Valley. X-ray diffraction, examination of pyrogenic mineral suites, and new high resolution radiometric dating (40AR/39AR) of Malpass clay from tuffaceous marker beds suggest that it is the missing ash from Mount Mazama. Over time and weathering, the ash has transformed into a
smectitic gray imogolite and clay. Analysis of this widespread gray clay provided a radiocarbon date of 6,850 ± 70 years before present. Radiometric dating of the clay provided the ages of 7,987 ± 102, 8,451 ± 57, and 8,540 ± 48 years before present. These dates are comparable with that of the climactic eruption of Mount Mazama.