DATING SEDIMENTS OLDER THAN 100 KA IN THE SEATTLE-TACOMA URBAN CORRIDOR: A TEST FOR INFRARED STIMULATED LUMINESCENCE AND THERMOLUMINESCENCE ON FINE GRAIN DEPOSITS

As part of a six-year project to map the Quaternary framework for the central Puget Lowland, more than forty samples have been collected for dating using infrared stimulated luminescence and thermoluminescence. During the past five years samples for dating were from approximately twenty different localities, including representative samples from previously dated type sections of the Olympia beds, Vashon Drift (fluvial and deltaic deposits), Double Bluff Drift and Whidbey Formation. Although some prior workers have speculated that samples saturate out at 100-150 ka for IRSL, we have used analytical procedures to minimize this problem, and the results show good correlation with independent stratigraphic relationships out to about 250 ka. Following the work of Berger and Easterbrook and subsequent recommendations of Berger and Easterbrook (1993, Can. J. Earth Sci. 30, 1815-1828), we dated the fine-grain, polymineral component of the sample. The silt was run using blue TL and IRSL emissions. Samples were exposed to 4N HCl and 35% H2O2 before being separated in the lab to obtain the 4-11 micron size particles. A special form of the partial bleach method was used in which a heavy yellow filter (CS-3-67) covered the samples before exposure to a light source for bleaching tests. Comparisions were made of the natural signal acquired during burial with the artificial signals acquired from exposure of the sample to a beta source. In our studies we used the additive dose, multi-aliquot approach. We also tested for sublinearity responses in the samples, as this is a common complication in older samples. Elemental concentrations of K, U, and Th were measured on a low-level gamma spectrometer along with the addition of a cosmic ray component. These concentrations were then calculated together for a Dose Rate. Dose Rates for all deposits averaged 2.3 Grays/ka and these relatively low values might be one of the reasons that older ages can be obtained. Thermoluminescence ages that were in good agreement with magnetostratigraphic data marking the Blake event (110-120 ka) were recorded. Subsequent measurement of the same sample using infrared stimulated luminescence gave the same age results. Other ages ranging from 150 ka to 250 ka were also found to be reliable and to fit in well with current stratigraphy.