Hagerman's Thesis Abstract

Age and Petrology of Tumtum Mountain, southwestern Washington

Katie Hagerman

Tumtum Mountain is a conical volcano located in a downdropped fault block on the Chelatchie Prairie of southwestern Washington, about 20 miles southwest of Mount St. Helens. The Tumtum Mountain Dacite is little studied and poorly understood, due mostly to the unavailability of outcrop and contacts between the Tumtum Mountain Dacite and the Amboy Drift. It has been postulated that Tumtum Mountain is post-glacial, based on mapping, but has been fission track dated as pre-glacial (Vance and Logan, 1988). The author studied one outcrop, retrieving nine, one-inch diameter, oriented cores from the site. Using these nine cores, the author studied the petrographical and petrological characteristics of the Tumtum Mountain Dacite along with the daciteÕs relative age and relationship to the Amboy Drift. Variable paleomagnetic directions from different parts of from the single outcrop suggest that the outcrop is not in place and may have moved through landsliding from further up the mountain. Thus, the contact between the dacite and the Amboy Drift is not a reliable indicator of relative age. Petrographic and petrochemical analyses indicate that the dacite is homogeneous, and contains plagioclase, hornblende, and oxide phenocrysts. This indicates that preexisting crystals were incorporated into the lava prior to eruption, calling into question the 12.5 My zircon fission track date.

University of Puget Sound