2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 1
Presentation Time: 10:00 AM


DETHIER, D.P.1, SARNA-WOJCICKI, A.M.2 and FLECK, R.J.2, (1)Dept. Geosciences, Williams College, Williamstown, MA 01267, (2)US Geol Survey, Menlo Park, CA 94025, ddethier@williams.edu

A new 40Ar/39Ar age and chemical analyses of dacitic pumice from pumice-rich alluvium at Blowers Bluff, central Whidbey Island, show that the interglacial deposits at that site have an age of about 120 ka. Pumice is found as thin beds, lenses as thick as 80 cm and bedded pumiceous alluvium in a 15 m-thick sequence of deposits that lies beneath two tills and middle Wisconsinan glaciomarine deposits at Blowers Bluff. Plagioclase separated from the pumice contained minor excess argon and gave a plateau age of 128 ± 9 ka and an isochron age of 120 ± 50 ka. The 40Ar/39Ar age is similar to a TL age estimate reported by Berger and Easterbrook (1993) from adjacent deposits. These ages suggest that the pumice deposits at Blowers Bluff are part of the Whidbey Formation, a widespread, mainly subsurface unit in the northern Puget lowland. The glass chemistry of the pumice does not match any known source, but Glacier Peak and Mt. Baker are likely candidates. During Whidbey Fm time, deep troughs of the modern northern Puget Sound area must have been filled with fluvial and lacustrine deposits, allowing volcanic-rich sediment to reach what is now Whidbey island. Erosion of thick glacial and interglacial deposits occurred during ice retreat when a west-flowing lobe of Vashon ice carved what is now Penn Cove. Fragments and local cm-thick layers of an undated pumice crop out in latest Wisconsinan (Everson) glaciomarine deposits exposed in gravel pits on Ebey's Prairie, 10 to 15 km WSW of Blowers Bluff. Chemistry of the pumice suggests a Cascade source, but does not correlate with pumice chemistry from Blowers Bluff. Balanus fragments in the glaciomarine gravel gave 14C ages > 40 ka, suggesting that they were reworked from stratigraphically and topographically lower deposits. The undated pumice could have been reworked from older deposits during erosion of Penn Cove and adjacent troughs or record an eruption at about 13 ka, when glaciomarine deposits were graded to a local relative sea level of ~70 m.