SEVERE WINTER STORM EFFECTS REVEALED IN COASTAL BARRIERSTRATIGRAPHY, SOUTHWEST WASHINGTON, USA

摘要

Unlike many coastlines, which are erosional, large portions of the southwest Washingtonrncoastline have been progradational for several thousand years. A record of seaward progradation isrnpreserved in sediments as highly reflective, seaward dipping subsurface layers. These layers, visualizedrnusing ground penetrating radar (GPR), are interpreted as beachface and upper shoreface deposits (Jol, etrnal., 1998). High-resolution GPR records from prograded coastal barriers reveal patterns of coastal changernwith individual subsurface layers representing interannual to annual shoreface locations, depending on thernrate and persistence of progradation. In Ocean Shores, WA, the rate of progradation has been sufficientlyrnhigh to preserve an annual signal resulting from the winnowing of beach sands by winter storms. This annualrnsignal is detectable in the subsurface using 200 MHz antennae. At a lower frequency of 100 MHz, only thernmost prominent of the higher frequency reflecting horizons are detected. Anomalous 100 MHz reflectionsrnrepresent winters with particularly severe coastal storms and coincide with strong El Ni?o and La Ni?arnevents according to the Japan Meterological Society El Ni?o Southern Oscillation Index. Although both ElrnNi?o and La Ni?a events appear to leave heavy mineral lags that can be detected using GPR, they appearrnto have different affects on coastal progradation and aggradation. A comparison of aggradation ratesrncompiled from the 200 MHz GPR record reveals slightly increased rates of aggradation following El Ni?ornevents compared to La Ni?a events. The increase is hypothesized to result from differences in wave energyrnand direction associated with the two ENSO phases.