FURTHER CONSTRAINTS ON AGE DATING OF LATE-PLEISTOCENE AND HOLOCENE COASTAL DUNES AND SPODOSOL CHRONOSEQUENCES FROM THE CENTRAL OREGON COAST BASED ON RECONNAISSANCE THERMOLUMINESCENCE DATING

Preliminary dating of stabilized coastal dunes in the Oregon Dunes National Recreation Area (ODNRA) of the central Oregon coast indicated a late-Pleistocene (24-70 ka) origin of the dunes (Beckstrand, 2000). Additional thermoluminescence (TL) dating of 9 new dunal locations within the 80 km long ODNRA both tests and extends the initial dating effort. Specifically, a very-old date from the southernmost end of the Coos Bay dune sheet (JN1 >65,000±4,000 TLYBP) confirms early dunal advances indicated by a previous 70 ka date from Mercer Lake in the Florence Dune sheet. The JN1 site south of Coos Bay is located at an elevation of 100 m and a distance of 6 km inland from the modern coast. The next oldest date is from the southern end of the Florence Dune Sheet (SP1 45,300±3100 TLYBP) where a 3 m thick Bt horizon implied an old dune age within 1.4 km of the modern coast. On the other end of the age spectrum 2 new TL dates confirm the age of Holocene dune advance in the Florence Dune Sheet (CG2 7,300±600 TLYBP, and MS1 6,400±700 TLYBP). Significantly, the CG2 site is at the basal Holocene contact near the modern coast in Florence whereas the MS1 sample was taken below a dunal topsoil (terminated dune advance) located 4 km east of the CG2 site. These two dates constrain the duration of the initial Holocene dune advance of >4 km to 1,000 years (7.3--6.4 ka) for the central Florence dune sheet. In the Coos Dune Sheet a stabilized tree-island dunal site dates to about 5,300 TLYBP (6 m depth subsurface) at a distance of about 0.75 km from the modern coast. These dates suggest a rapid onshore advance of the Holocene dunes immediately following the dramatic decline in eustatic sea-level rise after 7 ka. All of the TL-dated dune sites have been profiled for topsoil properties of soil color, thickness, texture, structure, and cementation (see Beckstrand this session). The new TL dates confirm the importance of cementation of the B horizon in constraining relative dune age. Increasing clay content, apparent consolidation, and penetrometer readings of the preserved Bw and Bt horizons correlate well with increasing dunal TL dates in the ODNRA. Future work will focus on extending the ODNRA soil chronosequencing to other West Coast dune fields with calibration from additional reconnaissance TL dating.