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

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 14 C 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 δ 18 O 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.