HISTORY OF PLEISTOCENE AND HOLOCENE VOLCANICLASTIC SEDIMENTATION IN THE COWLITZ RIVER VALLEY, SOUTHWESTERN WASHINGTON STATE

Sedimentary deposits in the Cowlitz River valley upstream of Castle Rock, WA, have preserved a complex history of volcanically-induced sedimentation from Mount St. Helens (MSH). Sand- and gravel-sized pumice clasts preserved in valley fill near the confluence of the Toutle and Cowlitz rivers provide a good temporal framework of deposition. Near the confluence, ca 46,500 to 37,600 year old MSH Cy and Cw tephras from the Ape Canyon eruptive stage are preserved 48 m above the modern Cowlitz channel in a 0.5 m thick cross-stratified granule-rich sand and 1 – 2 m thick sandy gravel, respectively. The granule-rich sand, a product of tractional processes, is overlain by the thick sandy gravel, deposited by a sizeable lahar (debris flow). The lahar deposit is capped by crudely laminated, 2.5 m thick micaceous Missoula flood silt, constraining the age of the lower pumiceous deposits between ca 46,500 and the youngest of the Missoula floods, or ca 12,500 radiocarbon yrs. B.P. More than 3 km upstream of the Toutle – Cowlitz confluence, gravel-sized pumice clasts from MSH sets Mp (18,230 years B.P.) and Jy (13,855 - 12,500 years B.P.) are preserved in gravelly fill 5 – 20 m above the modern channel. Set Mp is preserved in strata that cap a 0.9 m thick, normally graded, granule-rich sand, while set Jy tephra lie in strata that cap a 2 m thick sandy gravel with a laharic origin. The location of these deposits in relation to the confluence is evidence for at least one large sedimentation event sometime after 12,500 years B.P, during which large volumes of loose tephra were remobilized and transported into backwater areas of the Toutle – Cowlitz confluence. Deposition during the Swift Creek stage was unlikely at this elevation because global sea level was approximately 125 m lower than present at that time, thus deposition would have occurred far below the current base level. The landscape position of several gravel-rich deposits, across from or adjacent to the Cowlitz – Toutle confluence, suggests that this portion of the valley has tended to serve as a temporary sediment sink for debris flows from MSH. Tephra correlation, unique stratigraphic “markers” (Missoula flood silt), and landscape position help to constrain the ages and timing of these events.