Variation of Modern Turbidite Systems Along the Subduction Zone Margin of Cascadia Basin

Nelson, C. H., U.S. Geological Survey And Dept Of Oceanography, Texas A&M University, College Station, TX 77843, Chris Goldfinger, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331

Abstract

On the Washington margin, multiple canyon sources funnel turbidites into the Cascadia deep-sea channel. Astoria Canyon feeds the Astoria fan that fills the subduction zone trench off Oregon. Rogue Canyon mouth feeds a small base-of-slope apron, whereas similar nearby Trinidad and Eel canyon mouths feed into plunge pools, radial bedform fields for 15-25 km and Eel Channel into a turbidite-rich lobe.

All systems contain 12 or more correlative Holocene turbidite events based on marker beds of Mazama Ash or the Holocene/Pleistocene boundary. Turbidite events are lost progressively downstream in the first 100 km of Astoria Channel but continue for hundreds of kilometers in Cascadia Channel. The number of Holocene turbidite events progressively increases toward the Mendocino triple junction at the southern end of Cascadia Basin where 1 per 452 yr, 1 per 226 yr and in 1 per 65 yr occur, respectively, in the Trinidad, Eel, and Mendicino turbidite systems. The synchroneity of turbidite events in the northern two thirds of Cascadia Basin (1 per 655 yr) and the increasing frequency of events toward the triple junction can only be explained by tectonic generation of Holocene events. The greater frequency, thickness and grain-size of beds in the late Pleistocene deposits suggests the addtion of sediment supply and of sedimentologiclly triggered events at lowstand times.

In sum, these correlative turbidite events along 1000 km of the tectonically active Cascadia margin show variation in distribution and stratal architecture that need to be considered for reservoir modeling in these variable turbidite systems.