QUATERNARY STRATIGRAPHIC FRAMEWORK FOR THE WILLAMETTE VALLEY, OREGON

We propose a modified late Quaternary stratigraphic framework for the central and southern parts of the Willamette Valley, Oregon, based on geologic mapping, examination of stratigraphic exposures and well logs, and new radiometric age and tephrochronology. This framework consists of six episodes of distinct depositional environments: (1) Since at least 400 ka, large and thick (locally greater than 120 m) gravel fans have formed where major Cascade Range tributaries to the Willamette River enter the valley, and have forced the northward-flowing mainstem Willamette to the western margin of the valley. Locally exposed in the fan stratigraphy are debris flow deposits from a ca. 75 ka eruption of Mt. Jefferson, and a 430-400 ka obsidian-rich lahar that came down the McKenzie or Middle Fork Willamette Rivers. (2) Between about 30 and 22 ka (radiocarbon yr), widespread sand and gravel transport by braided river systems resulted in 5-20 m of sand and gravel deposition on the fans and across much of the valley bottom. (3) A period of local surface stability and soil formation between about 22 and 15 ka (radiocarbon yr), indicating a somewhat incised river system, ended when (4) much of the Willamette Valley was repeatedly backflooded by cataclysmic releases of ice-dammed Glacial Lake Missoula into the Columbia River drainage between 15 and 12.5 ka (radiocarbon yr). These floods left up to 35 m of gravel, sand, silt, and clay that are locally mappable to an altitude of 105 m. (5) Subsequent to deposition of Missoula Flood sediment in the valley, but prior to about 12.0 ka (radiocarbon yr), there was renewed aggradation along the major Cascade Range tributaries, resulting in several-kilometer-wide swaths of 3-to-15 m thick gravel deposits within locally incised Missoula flood deposits. (6) The Willamette River and its major tributaries have subsequently incised into these gravels, forming multilevel Holocene floodplains composed of gravel, sand, and silt.