Sediments From Lower Squaw Lake, OR, Contain Evidence of the 1700 AD Cascadia and 1873 AD Intraplate Earthquakes and Suggest a New Method for the Precise Dating of Earthquake Deposits

Date: 4/25/2019

Time: 08:30 AM

Room: Cascade II

Accurate dating of earthquake-triggered deposits is essential. This is especially important for Cascadia paleoseismology, given the most recent Cascadia earthquake occurred in 1700 AD. Here we present results from an investigation of the historic record of sedimentary disturbances from Lower Squaw Lake, OR, in which we find that earthquake-triggered deposits can be differentiated from flood deposits because they are complex deposits with long, organic-rich tails. The deposits have two parts: a lower unit composed of lake-margin sediment, and an upper unit composed of dense, fine-grained silt with a watershed composition. Both units have organic-rich tails composed of short-lived plants, including diatoms and algae, and degraded organic matter and fine inorganic particles. The fine-grained, watershed-sourced sediment is not a turbidite because the composition is very well-sorted and the basal contact shows evidence of “loading,” also observed in ash-fall deposits. We suspect the organic-rich tail results from flocculation: mixing of negatively charged algal matter with Ca+2 ions and fine-particles (as described by Avnimelech et al., 1982) is likely to cause aggregation, flocculation and rapid settling of water column organics and trapped fine inorganic particles to occur as a result of liquefaction of the lake’s delta. This suggests an opportunity for precise dating of earthquake deposits. Radiocarbon dating of tail material from the deposit assumed to result from the 1700 AD earthquake produced an age of 110 +/-25 RCY. Calibration produces multiple intersections with the radiocarbon production curve, however the position on the curve can be limited based on other radiocarbon data and the position of the earthquake deposit from 1873 AD above it. We suggest a strategy using bulk tail material for replicate radiocarbon ages, along with samples selected to limit the possible positions on the curve, to accurately identify and date earthquake deposits.

Presenting Author: Ann E. Morey

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