Turbidite megabeds in an oceanic rift valley recording jokulhlaups of latepleistocene glacial lakes of the western United States

Escanaba Trough is the southernmost segment of the Gorda Ridge and is fille d by sandy turbidites locally exceeding 500 m in thickness. New results fro m Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) C-14 dates and revised petrographic evaluation of the sediment provenance, combined with high-resolution seismic-reflection p rofiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining-upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and De ep Sea Drilling Program (DSDP) Site 35. Six AMS C-14 ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates excee ded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition o f one similar to 60-m interval of sand. Petrography of the sand beds is con sistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High-resolution acoustic stratigraphy shows that the turb idites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100-km-l ong pathway from the Columbia River to the west flank of the Gorda Ridge. T he age of the upper fining-upward unit of sandy turbidites appears to corre spond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or jokulhlaups, from the glacial lakes probably continued fl owing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.