Variation of Modern Turbidite Systems Along the Subduction Zone Margin of Cascadia Basin and Implications for Turbidite Reservoir Beds
Published:December 01, 2000
C. Hans Nelson, Chris Goldfinger, Joel E. Johnson, Gita Dunhill, 2000. "Variation of Modern Turbidite Systems Along the Subduction Zone Margin of Cascadia Basin and Implications for Turbidite Reservoir Beds", Deep-Water Reservoirs of the World, Paul Weimer
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Cascadia Basin contains a variety of turbidite systems located from Vancouver Island, Canada to Cape Mendocino California, USA. These systems have been studied with multibeam bathymetry, sidescan sonar, high-resolution seismic profiles, and piston cores. On the Washington margin, multiple canyon sources funnel turbidites into Cascadia Channel, a single high-relief deep-sea channel, that extends across Cascadia Basin and cuts through Blanco Fracture Zone. Astoria Canyon feeds Astoria Fan, a submarine fan with channel splays and depositional lobes which fill the subduction zone trench off Oregon. Both of these large turbidite systems (1000 km length) prograde mainly southward parallel to the margin in northern Cascadia Basin. In south Cascadia Basin, small turbidite systems (5-50 km) prograde perpendicular to the margin. Rogue Canyon feeds a small (<5 km) base-of-slope apron. Trinidad and Eel canyons feed into plunge pools and sediment wave fields that extend tens of km radially out from the canyon mouth. A channel-levee complex drains the Eel sediment waves and feeds into a sandrich lobe. Mendicino Channel, a connecting channel-levee complex without distal lobes, traverses the base of Mendocino Escarpment at the triple junction. Turbidite systems from the Rogue River north contain 13 correlative post-Mazama turbidite events based on the first occurrence of Mazama Ash (MA) at about 7530 calendar yr BP. Another 12,300 calendar yr datum, at approximately the Pleistocene/Holocene boundary (H/P), is found throughout Cascadia Basin. Based on these datums, turbidite events appear to be triggered by seismic events on average every 600 years in northern Cascadia Basin and progressively more often toward the Mendocino Triple Junction (i.e in Trinidad pool every 492 yr, in Eel lobe every 246 yr and in Mendocino Channel every 40-65 yr)
The correlation of turbidite events can be used to compare bedding continuity within systems and between different systems to provide important implications for turbidite reservoir characteristics. The progressive loss of post MA turbidites down the proximal 150 km of Astoria Channel suggests that during this time, downfan continuity in turbidite beds is less in fan channels compared to Cascadia Channel where all 13 post-MA beds are continuous throughout the deep-sea channel. In contrast, both deep-sea and fan channels exhibit cut and fill in proximal regions, sediment bypassing and down channel dropout of beds during the Pleistocene. As a result, high sand:shale ratios (1:1 to 3:1) are found in distal fan lobes during the Pleistocene whereas low ratios are found during the Holocene. Good lateral bedding continuity is found throughout the Rogue apron that is undisrupted by channels. Turbidite events are twice as common in plunge pools compared to the downstream sediment waves, which suggests a loss of bedding continuity in sediment waves that is analogous to that in channel levees. However, in the case of the Eel system, when the pool and waves are drained by a channel-levee complex, the highest frequency of turbidite beds and sand:shale ratios (1.8:1) are found in the distal lobe. Sand:Shale ratios and frequency of events suggest that during the Pleistocene, sediment erosion and bypassing took place in the pools compared to the infilling of the Holocene. The greatest Holocene infilling rate takes place in Mendocino Channel where turbidite events occur every few decades and sand:shale ratios are 2.5:1.