LOCATION, LOCATION, LOCATION: CASCADIA TURBIDITE STRATIGRAPHY AT THE INTERFACE BETWEEN PALEOSEISMIC AND PALEOCLIMATE RECORDS

Paleoseismic turbidites along the Cascadia margin are spatially restricted, more so than one might expect. Excellent high-resolution records are found in channel and canyon systems, and in basins that have existing mapped slope failures. Canyon systems, whether relict of otherwise, dominate the Holocene sedimentary record by virtue of greater surface area excited by seismic loadings. Weaker records are found in basins with few previous open landslide scars, and between canyon systems along the frontal thrust. Paired adjacent basins in central and N. Oregon show clearly the importance of previously failed surfaces in supplying coarse material for paleoseismic records. Several areas show no evidence of paleoseismic records at all, and record late degalcial paleoflooding associated with the Columbia River/Astoria-Willapa Canyon system. Several such areas include the lower slope slightly above the abyssal plain at the Willapa Canyon mouth, and on the upper slope near Astoria Canyon. Physiography works against recording paleoseismic events in favor of climate records in these locales. Trinidad (Trinity) Canyon is a highly unusual system. The upper canyon includes a series of sub-parallel to radial rill and gully features that are apparently not active, or are only weakly active. These structures appear to be inherited from drainage related to underlying Pleistocene normal faulting and landsliding of the eastern limb of the mid-slope terrace. The primary sedimentary pathway skirts the northern limit of the gullied basin and is fed from the Klamath River. The upper slope reach has late quaternary levees that feed the lower canyon, which then empties into an abyssal plunge pool that contains a very high-resolution record. The Trinidad paleoseismic record extends only to ~ 3500 BP, at earlier times the record becomes a very high-frequency sequence that does not reflect only earthquakes. Similarly, between ~ 500 and 800 BP, a high-frequency record that does not correlate to nearby land and marine paleoseismic sites is observed. We suggest that the Trinidad stratigraphy may reflect episodes of alpine glaciation in the Trinity mountains during these periods, in addition to earthquakes, feeding the Klamath River and hence Trinidad Canyon. Active Alpine glaciation is unique to this system, and is a sediment source unavailable to other S. Cascadia systems in post-Mazama times.