Abstract 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.

Abstract This paper analyzes recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California active margins of western Northern America. We compare the age, frequency, and recurrence time intervals of turbidites using two methods: (1) radiometric dating ( 14 C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The two approaches complement each other, and when used together provide a better age framework than 14 C ages alone. Comparison of hemipelagic sediment thickness in several cores from the same site is used to evaluate the erosiveness of turbidity currents and improve the correlation of turbidites and consequent paleoseismic history based only on less complete and unrefined data sets of 14 C turbidite ages along the continental margin. Chronology of hemipelagic sediment thickness provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~ 550 yr in northern Cascadia Basin and ~ 200 yr along northern California margin. This difference in frequency of turbidites in a subduction zone compared to a transform-fault margin suggests significant differences in earthquake activity that compare favorably with independent paleoseismic indicators.

Abstract The volume and interplay of mass-transport (MTD) and turbidite-system deposits varies on different continental margins depending on local and external controls such as active-margin or passive-margin tectonic setting and climatic and/or sea-level change. Erosion and breaching of local grabens at the shelf edge of the southern Bering Sea produce giant, gullied canyons and MTD sheets that dominate the basin-floor deposition and disrupt development of turbidite systems. In contrast, external controls of great earthquakes (> 8 M ) along the Pacific active tectonic continental margins of Cascadia and northern California cause seismic strengthening of the sediment, which results in minor MTDs compared to turbidite-system deposits. Messinian desiccation of the Mediterranean Sea caused a deeply eroded Ebro subaerial canyon and an unstable central segment with an MTD sheet, whereas other stable Ebro margin segments have only turbidite systems. In the northern Gulf of Mexico, the delta-fed Mississippi Fan and intraslope mini-basins contain MTDs and turbidites that are equally intermixed from the largest scales with MTD sheets hundreds of kilometers long to the smallest scales with beds centimeters thick. In the Antarctic Wilkes Land margin, global climate cooling caused a late Oligocene to middle Miocene time of temperate continental ice sheets that resulted in massive deposition of MTDs on the margin, whereas later polar ice sheets favored development of turbidite systems. Our case studies provide the following new insights: (1) MTDs can dominate entire margins, dominate segments of a margin, be equally mixed with turbidites, or dominate a margin during some geologic times and not others; (2) on active tectonic margins with great earthquakes, the maximum run-out distances of MTD sheets across abyssal-basin floors are an order of magnitude less (~ 100 km) than on passive-margin settings (~ 1000 km), and the volumes of MTDs are limited on the abyssal sea floor along active margins; (3) where the most precise radiocarbon ages are available, major MTD episodes of deposition are correlated with the most rapid falls or rises of sea level; (4) gullied canyons feeding MTD sheets have irregular and steep axial gradients (5-9°), whereas canyons feeding turbidite systems have a regular graded profile and less steep gradients (1 to 5°). Our examples of MTD and turbidite systems provide analogues to help interpret ancient systems.