Optical dating studies of Quaternary organic-rich sediments from southwestern British Columbia and northwestern Washington State

Abstract

The suitability of optical dating using 1.4 eV (infrared) excitation for determining the time of deposition, or compaction, of organic-rich sediments and peat is assessed with measurements on seven samples from six different lithostratigraphic units. One is of zero age, two have associated 14C ages, three are known to have been deposited during an interglaciation, and one is ~1 Ma old. The samples yield satisfactory optical ages ranging from 0 to over 100 ka. We conclude that the Muir Point Formation (southern Vancouver Island) and the Whidbey Formation (northwestern Washington State) were both deposited during δ18O stage 5, as previously deduced from other evidence. The age obtained from the ~1 Ma sample was significantly too low. The optical dating method is simpler and more precise than thermoluminescence dating, and is recommended for future work.