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XRF reflects the sedimentological processes influencing flood and earthquake deposits preserved in southern Cascadia lakes

ABSTRACT:

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 Squaw Lakes, Oregon (~100 km from the coast) was compared to the historic sequence of events, including floods, the ~M9 1700 AD Cascadia earthquake, and the ~M7 1873 AD intraplate earthquake. A test of the hypothesis that deposits attributed to earthquakes are composed of lake-margin sediment (muscovite 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 hyperpycnites composed primarily of lake-margin sediment with an inorganic deposit tail, and deposits attributed to earthquakes were more complex sequences. 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 hysteresis loops” of deposit evolution. The results suggest that earthquake deposits are formed from two units, one composed of lake-margin schist including a short organic tail, followed by a unit composed of fine-grained, well-sorted sediment sourced from the watershed with a long organic tail. These characteristics suggest an initial, shaking-induced slope failure, followed by settling of fine sediment released from the lakes’ delta as a result of liquefaction and/or settling.