NELSON, C. H.¹, C. GOLDFINGER², S. C.WOLF¹, and G. DUNHILL^1
1USGS, Menlo Park, CA
²Oregon State University, Corvallis, OR

Abstract: Cascadia Basin Channel Pathways And Late Holocene Turbidite Event History

Swath bathymetry, sidescan sonar imagery, high-resolution seismic profiles, and AMS radiocarbon ages of turbidites in channel pathways of Cascadia Basin have been used to outline a variable Holocene turbidity-current history in different channels. Cascadia Channel shows an approximate 600 yr periodicity among 13 turbidite events since deposition of the first turbidite with Mt. Mazama glass about 7500 y BP. The youngest event occurred about 300 years ago, yet in the nearby Astoria Channel off northern Oregon, no turbidite events younger than about 4300 y BP are evident even though imagery shows that the channel pathway is open, and sediment input to Astoria Canyon is equal to that of the Washington canyons feeding Cascadia Channel. In contrast, a parallel channel along the Oregon slope base is blocked by slumping and active faulting; also, the southern half of Astoria Channel is filled, apparently because gradients were disrupted by the 100 km-long Heceta slide which occurred about 100,000 years ago and by the 25 km-long 44 N slide which moved about 11,000 years ago. On the California margin, the mouth of Trinidad Canyon opens into a 100 m-deep hole followed downstream by a 13 km long field of sediment waves that have maximum heights of 40 m and lengths of 3,000 m. Eel Canyon mouth also debouches into a sediment wave field (maximum 80 m wave height and 4,000 m wave length) that extends 24 km downstream to enter a 37 km long, 2.5 km wide channel-levee complex. Mendocino Channel meanders along the base of the Gorda Escarpment at the southern edge of Cascadia basin and contains 4 turbidite beds deposited between 100- 250 years ago based on 210 Pb activity and AMS radiocarbon ages. Thus, during the late Holocene there is at least an order of magnitude difference in turbidity-current activity between the northern (600 y events) and southern channels in Cascadia Basin (<60 y events).

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas