Full article available in PDF version.

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.

Abstract Because the shelfedge bridges shallow and deep ocean environments, sedimentary processes characteristic of each of these provinces interact at the shelfbreak to influence sediment transport in the benthic boundary layer. Processes at the shelfedge and mechanics of sediment transport have been inferred from data gathered in many regional shipboard sampling and surveying investigations. Grouping these processes into two major categories—geologic factors and oceanic factors—aids in conceptualizing the complex system of sediment dynamics at the shelfedge. Geologic factors include tectonism, sediment supply, sediment size, shelf width, depth of the break below sealevel, gradient of the upper slope, and bathymetric irregularities. Oceanic factors include fronts between water masses, boundary currents, meteorologically-induced currents, tides, internal waves, and surface waves. Although any of these factors may operate simultaneously on any continental margin, their relative importance varies with time and space; i.e., one, two, or several of these factors may dominate shelfedge sediment transport on a given continental margin or at any given time. Few investigators have actually measured the water and sediment motions in the benthic boundary layer at the shelfedge. Regional sediment-transport data are of limited value as long as the various factors of the forcing mechanisms have not been properly studied and correlated with the flow field and sediment activity at the bottom. Sophisticated instruments deployed for long periods of time are necessary to acquire data adequate for an assessment of the forcing mechanisms that control sediment transport. The few existing measurements of this type support the concept that shelfedge processes differ with place and time among continental margins and on any given continental margin. It follows that caution should be exercised when one attempts to generalize about the shelfedge transport system.

ABSTRACT Pacific-style continental margins, such as that of western North America, are marked by large contrasts in the type of shelfedge sedimentary deposits and the processes that form them. The Pacific shelves of the United States are generally much narrower than the Atlantic shelves, and the source areas exhibit more relief. The greater relief of Pacific coast source terranes results in a relatively high rate of sedimentation in humid areas and fluctuating (areally and seasonally) sedimentation patterns and rates in semiarid areas. Sediment shed from the adjacent landmass is discharged, generally seasonally, onto the Pacific Continental Shelf at point sources. Many of the sediment sources of the northwestern United States and southern Alaska feed directly onto swell- and storm-dominated shelves. On such narrow unprotected shelves, sediment has a short residence time in submarine deltaic deposits before being remobilized and dispersed to outer-shelf and upper-slope environments. Through study of high-resolution seismic-reflection profiles, we have identified four principal types of shelfedge deposits: (1) starved, (2) draped, (3) prograded, and (4) upbuilt and outbuilt. Each type of shelfedge deposit results from a characteristic balance between sedimentation rate and distributive energy (waves and currents) and is, therefore, characterized by distinctive seismic facies and bedding patterns. A special type, the cut-and-fill shelfedge, and a composite type consisting of two or more of the main depositional styles supplement the four principal types of shelfedge. Incorporated within each of these facies, especially on the upper slope, are chaotic deposits formed by slumps or slides, which are common along technically active margins.

Abstract Analyses of high-resolution seismic profiles have revealed the presence of a well-defined, massive submarine slide located at the north end of the Kayak Trough in the northern Gulf of Alaska. This slide is about 18 km long and 15 km wide, has a volume of about 5.9 km 3 , and has moved down a 1 ° slope. Sediment from the upper 2 m of the slide consists of low-strength, greenish-gray clayey silt. Morphologically this slide is a classic example, with a well-preserved pull-apart scarp in the headward regions, a well-developed toe, disrupted internal bedding, and hummocky surface topography. The age of the slide is unknown, but its clearly defined morphology in an area of very high sedimentation (7.5 to 15 m/1,000 yr) suggests that it is extremely young. The slide may have been generated by intense storm activity or earthquake-triggered, prolonged ground shaking. Laboratory and shipboard observations indicate that the slide sediments are very weak (peak shear strength of 0.02 kg/cm 2 ) with high water content. Other areas of the Gulf of Alaska are known to have thick accumulations of similar sediment, although a comparable slide has not been observed. The characteristics of this slide are indicative of problems that will have to be surmounted if this or similar areas in the Gulf of Alaska are to be used for pipeline emplacement or platform siting for petroleum production. High seismic risk makes the problems even more severe.