MAGNETOSTRATIGRAPHY, OPTICAL DATING, AND VERTICAL DEFORMATION OF PLEISTOCENE DEPOSITS IN THE SOUTH CENTRAL PUGET LOWLAND

Recently acquired optical dates from Pleistocene deposits in the Puget Lowland, Washington, provide information significant to the interpretation of magnetostratigraphic data from the area. Over seventy localities in fine-grained lacustrine sediments, mostly at sea level, have been collected for paleomagnetic analysis, and, remarkably, transitional directions were discovered at eight localities. These transitional directions could potentially serve as useful stratigraphic markers throughout the region. A reversed-to-transitional polarity record (bottom to top) was found in a thick section of silt near the eastern end of the Tacoma Narrows Bridge, and a more complete reversed-to-normal polarity record was found in another thick section at Wingehaven Park near the northern tip of Vashon Island. Initially, these polarity transitions were inferred to be the Brunhes-Matuyama transition at ~780 ka because of a 1.1 Ma fission-track date from reversed-polarity sediments just north of Tacoma; TL and IRSL work on samples from this locality show that they are saturated and most likely have an age >500 ka. TL and IRSL dates for deposits exposed on the eastern side of Tacoma Narrows, however, are on the order of 101 to 135 ka, consistent with the age of the Blake event (~110-120 ka). Age determinations for samples from Wingehaven Park indicate saturation of the IRSL curves and a minimum age of 295 ka; we believe the polarity transition recorded here is that at the Brunhes-Matuyama boundary. Reversed-polarity and transitional directions are almost always found at sites associated with the Seattle uplift indicating exposure of older stratigraphic levels in this area. Normal-polarity sites also occur within the Seattle uplift, and are almost exclusively found to the north and south of the uplift, with the exception of Tacoma Narrows, and have TL and IRSL ages ranging from ~65 to ~285 ka. Magnetostratigraphy and optical dating have proved a useful combination in helping to understand the Pleistocene stratigraphy, and thus structure, of the Puget Lowland.