understanding deposit complexity
Just as lidar and remote sensing by satellite and sonar provide a lens we can use to see our world in more detail than we had ever imagined possible, micro X-Ray Fluorescence and CT (computed tomography) density allow us to see millimeter-scale details in sedimentary sequences. The complexity of the deposits attributed to earthquake ground motions observed in my dissertation research was puzzling. Could I use the mm-scale CT density and XRF elemental data to differentiate between the sedimentary processes they represent?
Cascadia lakes have recently explored for their paleoseismic potential, but a relationship to earthquakes remains untested. Ground motions are not predicted to be strong enough to cause slope failures for inland locations where most people live, yet similarities between disturbances in lake sediments and earthquake deposits from the marine record have been documented. Here the historic record of disturbances from lower Acorn woman Lake, Oregon (~150 km from the Cascadia deformation front), was compared to the historical sequence of events, including floods, the ~M9 1700 CE Cascadia subduction earthquake, and the ~M7 1873 CE Brookings earthquake. A test of the hypothesis that deposits attributed to earthquakes are composed of lake-margin sediment (schist), and deposits attributed to extreme floods are composed of watershed-sourced sediment (metavolcanics and amphibolites) was not supported by the data: Deposits attributed to extreme floods are turbidites composed primarily of lake-margin sediment with a mixed composition and have a thin, fine-grained inorganic deposit tail, and deposits attributed to earthquakes were more complex sequences composed of watershed-sourced sediment sourced from the delta or landslide deposits. To describe and interpret the deposits, downcore XRF data of the two elements identifying watershed and lake-margin endmembers were plotted against one-another resulting in “XRF loops” of deposit evolution. The results suggest that earthquake deposits are unique in that they are composed of watershed-sourced sediment with long tails that increase in organic content upward.