Optical dating of tsunami-laid sand from an Oregon coastal lake

Optical ages for five samples of tsunami-laid sand from an Oregon coastal l ake were determined using an infrared optical-dating method on K-feldspar s eparates and, as a test of accuracy, compared to ages determined by AMS C-1 4 dating or detrital plant fragments found in the same beds. Two optical ag es were about 20% younger than calibrated C-14 ages of about 3.1 and 4.3 ka . Correction of the optical ages using measured anomalous fading rates brin gs them into agreement with the C-14 ages. The approach used holds signific ant promise for improving the accuracy of infrared optical-dating methods. Luminescence data for the other three samples result in optical age limits much greater than the C-14 ages. These data provide a textbook demonstratio n of the correlation between scatter in the luminescence intensity of indiv idual sample aliquots and their normalization values that is expected when the samples contain sand grains not adequately exposed to daylight just pri or to or during deposition and burial. Thus, the data for these three sampl es suggest that the tsunamis eroded young and old sand deposits before drop ping the sand in the lake. (C) 2001 Elsevier Science Ltd. All rights reserv ed.