Megathrust paleogeodesy at the Cascadia subduction zone
Final Report Abstract
The study shows that fossil foraminiferal assemblages are precise indicators of relative sealevel that can be used to reconstruct coseismic land-level changes along subduction-zone coastlines such as the U.S. Pacific Northwest. We also show that the reliability of quantitative sea-level estimates can be influenced by taphonomic processes associated with early diagenesis, but the reliability is less influenced by inter-annual and seasonal variations in foraminiferal distributions or by small-scale and infaunal foraminiferal variability. The quantitative reconstruction of past land-level changes using a foraminiferal-based transfer function implies variable amounts of coseismic subsidence during the seven earthquake cycles at Cascadia, with five larger and three smaller earthquakes during the past ~3500 years. Comparison of coseismic subsidence estimates for the same contacts between two nearby cores shows that most reconstructions are reproducible. However, in some cases taphonomic alterations of fossil assemblages may have a strong influence on the accuracy of coseismic subsidence estimates. However, the Bandon marsh experiment implies a delayed re-colonization of foraminifera in a suddenly flooded, former marsh, which is an analog to marsh flooding following coseismic subsidence during an earthquake. The delay shows that estimates of the amount of coseismic subsidence using foraminiferal-based transfer functions may underestimate the amount of coseismic subsidence if significant post-seismic uplift occurs.
Publications
- Annual and seasonal distribution of intertidal foraminifera and stable carbon isotope geochemistry, Bandon Marsh, Oregon, USA. Journal of Foraminiferal Research (2015) 45 (2): 146-155.
Milker, Y., Horton, B.P., Vane, C.H., Engelhart, S.E., Nelson, A.R., Witter, R.C., Khan, N.S., Bridgeland, W.T.
(See online at https://doi.org/10.2113/gsjfr.45.2.146) - Variability of intertidal foraminiferal assemblages in a salt marsh, Oregon, USA. Marine Micropaleontology, Volume 118, June 2015, Pages 1-16
Milker, Y., Horton, B.P., Nelson, A.R., Engelhart, S.E., Witter, R.C.
(See online at https://doi.org/10.1016/j.marmicro.2015.04.004)
