MICRO-X-RAY FLUORESCENCE REVEALS EVIDENCE OF THE SEDIMENTARY PROCESSES THAT CONTROL DEPOSITION: THE RECORD OF EXTREME EVENTS FROM A SMALL OREGON LAKE

Cascadia forearc lakes are being used increasingly for their paleoseismic potential. Lake sediments, however, also record other types of disturbances, such as extreme floods. Lower Squaw Lake, OR, located ~100 km inland of the coast near the California/Oregon border, provides an opportunity to test the hypothesis that southern Cascadia lakes contain earthquake deposits that can be uniquely identified in the sedimentary record. The unique geologic setting makes this possible: X-ray diffraction spectra (XRD) identified calcium amphibolite as the dominant component of watershed bedrock whereas lake bedrock is composed of calcium-free, potassium muscovite schist. Micro-X-ray fluorescence (XRF) supports the use of calcium and potassium as provenance indicators, and were used, along with sediment density (estimated from the strong correlation between coherent:incoherent scattering and computed tomography density), to determine if these provenance indicators can be used to differentiate between flood and earthquake deposits for the historic sequence of known events (including the ~M9 1700 Cascadia and ~M7 1873 intraplate earthquakes). Distinctive patterns emerge when potassium and calcium are plotted against one-another: endmembers begin to deviate from the initial background ratio, then vary as density increases and decreases, then eventually return to the initial starting ratio. Earthquake deposits are complex sequences with separate schist and watershed components, both of which have an upward increase in % organics. The watershed component is not a turbidite but settled directly out of the water column. Flood deposits are graded units of variable composition, with an erosive base and thin clay cap (and no grading with respect to organics). Using XRF in this way allows cryptic portions of deposit sequences to be identified, improves estimates of interevent sedimentation, and provides evidence of the underlying sedimentary processes controlling deposition.