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.

Abstract Three case studies are used to exemplify the wide variety of controlling factors that combine to influence the development of modern turbidite systems, and how these vary with location and time. For example, Cascadia Basin in the Pacific Ocean off western North America, which is underlain by the Cascadia Subduction Zone, exhibits the dominant tectonic control of earthquake triggering for turbidity currents, the increased sediment-supply effects of the Mt. Mazama catastrophic volcanic eruption in 7626 yr B.P., the glacial climatic and sea-level lowstand control on rapid turbidite–system growth rates, and the recent anthropogenic control that reduces sediment supply rates. Lake Baikal in Russia shows how the rift-basin tectonic setting controls the number and type of sediment input points, the amount of sediment supply, and the consequent types of turbidite systems developed along different margins of the Baikal basin. Pleistocene glacial climatic changes, without changes in lake base level, causes increased sediment input and the rapid growth rate of Baikal turbidite systems that is three to five times greater than that during the Holocene interglacial climate. The Ebro turbidite systems in the northwest Mediterranean Sea exhibit control of system types by the Messinian salinity-crisis lowstand, of channel locations by oceanographic current patterns, and of sediment-supply increase by glacial climatic changes as well as recent decrease by anthropogenic changes. Both active-margin and passive-margin settings have some common controls such as climatic and sea-level changes, and develop similar types of turbidite systems such as base-of-slope aprons, submarine fans, and deep-sea or axial channels. Each margin also has specific local controlling factors, for example the volcanic events in Cascadia Basin, glacial climatic without erosional base-level control in Lake Baikal, and the Messinian extreme lowstand in the Mediterranean Sea. Comparison of modern turbidite systems points out new insights on external controls such as importance of: (1) earthquakes for triggering turbidity currents on active tectonic margins, (2) equal or greater Pleistocene climatic control compared to lowered base level for sediment supply, (3) direct glacial sediment input that results in doubled proximal channel size, (4) greatly reduced deposition rates in drained compared to ponded turbidite basins, (5) importance of ocean currents on location of turbidite systems and channel development, and (6) anthropogenic effects from river damming during the last century that sometimes reduces present sediment supply to turbidite systems by orders of magnitude. External Controls on Deep-Water Depositional Systems SEPM Special Publication No. 92 (CD version), Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-200-8, p. 57–76.

Abstract The Mississippi Fan is a Plio-Pleistocene deposit that occupies much of the eastern Gulf of Mexico Basin. Sidescan sonar imagery, high-resolution seismic profiles, and short cores indicate a three-stage progression in the evolution of the fan surface since the last lowstand of sea level: (1) deposition of turbidites and debrites on the distal fan sourced from the shelf edge during the initial stage of the Holocene transgression, (2) failures in the Mississippi Canyon and along the adjacent continental slope that buried large areas of the surface of the proximal Mississippi Fan, including the channel that had previously supplied sediment to the distal fan, and (3) continued reworking of the surface of the fan and the overlying hemipelagic cover by bottom currents up to the present. Because not all of this progression in depositional styles is recorded in any one place on the fan, both an accurate chronology and a regional synthesis of stratigraphy and processes are needed to decipher these three stages that have shaped the fan surface and to link them to the external conditions that contributed to their formation.

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 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.

The climactic eruption of Mount Mazama and the resulting sedimentation may have been the most significant convulsive sedimentary event in North America during Holocene time. A collapse caldera 1,200 m deep and 10 km in diameter was formed in Mount Mazama, and its floor was covered by hundreds of meters of wall-collapse debris. Wind-blown pyroclastic ash extended 2,000 km northeast from Mount Mazama and covered more than 1,000,000 km 2 of the continent. On the Pacific Ocean floor, Mazama ash was transported westward 600 to 700 km along deep-sea channels by turbidity currents. The initial single-vent phase of the climactic eruption, a Plinian column, emptied over half of the magma erupted. Debris from this phase accumulated as a pumice deposit 10 m thick at the rim to 50 cm thick as much as 100 km from the vent. This deposit created a mid-Holocene stratigraphic marker over the continent and the continental margin of western North America. A ring-vent phase followed as a second part of the climactic eruption and produced highly mobile pyroclastic flows. These flows covered the mountain for at least 14 km from the vent, continued down the valleys nearly 60 km, and deposited as much as 100 m of pumiceous ignimbrite. After the caldera collapsed as a result of the eruption of more than 50 km 3 of magma, heat of the climactic eruption apparently created phreatic explosion craters along the ring fracture zone of the caldera floor. Initially, explosion debris and sheetwash of pyroclastics off highlands seems mainly to have filled the local craters with bedded volcaniclastics. This basal, generally flat-lying unit, was quickly covered by wedges of chaotically bedded debris flow and avalanche-type deposits that thin inward from the caldera walls. These deposits may have formed in response to seismic activity associated with postcaldera volcanism that apparently began soon after the caldera collapsed. The lower two units of non-lacustrine beds (50 to 60 m) make up the majority of the postcaldera sedimentary deposits and seem to have deposited rapidly after the climactic eruption. Twenty to 25 m of lacustrine sediment has been accumulating more slowly over the subaerial debris during the past 6,900 yr. Some Mazama ash probably was transported by rivers to the sea immediately after the climactic eruption because significant amounts of this ash appear in mid-Holocene turbidites of Cascadia Basin. The presence of Mazama ash mixed with Columbia River sand in texturally and compositionally graded turbidites shows that Mazama ash periodically was moved by sediment-gravity flows down the canyons and through channels to deposition sites in the Astoria Fan and the Cascadia Channel. The coarsest and thickest tuffaceous turbidites were deposited on channel floors, and the ash-rich suspension flows that overtopped the levees were deposited as thin-bedded turbidites in interchannel areas. Study of the Mount Mazama climactic eruption shows that such an event in the Cascade Mountains has the potential to: (a) cause major destruction within 100 km of the vent, (b) severely affect biota as far as 2,000 km downwind, and (c) disrupt commercial river and marine transportation or natural sedimentation as far as several hundred kilometers in the opposite direction from wind-blown debris. Present geologic characteristics on the Crater Lake caldera floor suggest that geologic hazards from a significant volcanic event appear to be minimal for the next few thousand years.