VOLCANIC DEBRIS FLOWS AND IMPLICATIONS FOR COASTAL EVOLUTION: SUBSURFACE (GPR) IMAGERY OF A LATE HOLOCENE DELTA AT PUGET SOUND, WASHINGTON

The stratigraphic framework and sedimentary characteristics of a late Holocene delta indicate rapid deposition from sediment-rich floods that probably originated as volcanic debris flows (lahars) from Mount Rainier, Washington. The non-cohesive, sandy flows traveled ~ 70 km downstream and reached southern Puget Sound, where they strongly influenced delta evolution in the formerly glaciated region. Data from ground-penetrating radar (GPR) and vibracores were used to examine the near-surface stratigraphy of the Nisqually River delta, located only a few kilometers from the epicenters of damaging earthquakes in 1949 and 2001. The main objective was to determine when large debris flows occurred, and how the delta has responded to episodically large pulses of sediment. Five GPR facies were recognized based on reflection characteristics and sedimentology of deposits. Laterally extensive layers of pumice- and charcoal-rich sand form a stacked sequence of sheet-like deposits that extend ~ 3 km across the entire width of the delta. These andesitic sand deposits are 1-4 m thick, locally massive and contain reverse-graded beds, suggesting rapid emplacement from sediment-rich floods. Intrusive, sand-filled dikes and sills indicate liquefaction caused by ground shaking. Two flat-lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries; one was marked by a well developed paleosol containing seeds that yielded a radiocarbon date of 2370-2350 cal yr B.P., which is time-correlative with a known debris flow upstream (National Lahar). Buried paleochannels record the former location of rivers that probably were filled by debris flows during one or more episodes of eruption. Wood fragments in the channel-fill deposits yielded dates of 794-515 cal yr B.P., which also correlate in time to lahar deposits observed in upstream areas (Electron Mudflow). These and other data support a conceptual model in which the delta front: 1] advances into Puget Sound during relatively brief, eruptive phases on Mount Rainier, when sediment supply is high, and 2] retreats during longer, quiescent periods when sediment supply is relatively low and the delta is reworked by tidal currents.