The surface area of Cascadia Basin is ~170,000 km2, extending from the base of the Cascadia accretionary prism westward to the edge of sediment onlap onto juvenile basement of Juan de Fuca Ridge (Fig. 1). To the east, the muddy slope apron of the Cascadia subduction margin is underlain by deformed sequences of accreted abyssal-floor strata (Carson et al., 1974; Barnard, 1978; Davis and Hyndman, 1989; Hyndman et al., 1990). Near-shore bathymetry becomes more complicated toward the north because of Pleistocene glacial erosion. A large marine re-entrant (Strait of Juan de Fuca) connects inland waters of western Washington and British Columbia to the Pacific Ocean. The Strait of Georgia (Fig. 1), a steep and narrow passage between Vancouver Island and mainland British Columbia, merges into the eastern end of the Strait of Juan de Fuca. West of the Cascadia subduction front, the total thickness of deep-marine strata reaches a maximum of more than 3 km (Carlson and Nelson, 1987). The basin floor widens and slopes to the south. The average seafloor gradient is 1:1000, and the maximum water depth is 2930 m. The abyssal floor is highlighted by low-relief channel-levee complexes and two submarine fans (EEZ-SCAN 84 Scientific Staff, 1986).

Physical Oceanography

The water column above Cascadia Basin is affected by several currents that influence regional patterns of sediment dispersal. As the North Pacific Current approaches North America from the central Pacific, it splits into the north-flowing Alaskan Current (a counter-clockwise gyre) and the south-directed California Current (e.g., Kirwan et al., 1978; Huyer, 1983; Lynn and Simpson, 1987; Shipboard Scientific Party, 1997d). The latitude of this separation migrates seasonally from ~50°N in summer to 43°N in winter and also drifts on decadal time scales. Generally, these surface currents are weak (4-8 cm/s). During winter, the Davidson Current also flows northward over the Oregon/Washington margin at 14-40 cm/s and merges with the eastern edge of the Alaska gyre. Less is known about bottom-water circulation within the region. A north-directed undercurrent moves at speeds up to 15 cm/s above the upper continental slope (Reid and Halpern, 1976; Halpern et al., 1978). Sluggish thermohaline currents in deeper water evidently transport suspended sediment toward the south (Stokke et al., 1977). A near-bottom nepheloid layer is well developed throughout the region; concentrations of suspended sediment are higher in submarine canyons (Baker, 1976; Stokke et al., 1977; Baker and Hickey, 1986).

Depositional Character of Cascadia Basin

Average sedimentation rates on the Cascadia abyssal floor have been ~170 cm/k.y. during Pleistocene glacial intervals, whereas Holocene rates range from 2 to 10 cm/k.y. (Griggs et al., 1969). Submarine canyons function as the principal conduits for sediment transport onto the abyssal floor. Gravity-controlled processes (turbidity currents, debris flows, and slumps) have been more active during lowstands of sea level. During the Holocene highstand, canyons trap mostly fine-grained sediments and help focus sluggish downslope movement of the bottom nepheloid layer (Carlson and Nelson, 1969; Stokke et al., 1977; Baker and Hickey, 1986). The accretionary margin of southern Vancouver Island is incised by a long array of submarine canyons (Herzer, 1978; Davis and Hyndman, 1989). Another anastomosing network of canyons (Barkley, Nitinat, and Juan de Fuca) cuts the outer continental shelf near the termination of the Strait of Juan de Fuca (Fig. 1). Quinault Canyon begins on the outer continental shelf of Washington and hooks south across the middle forearc (Carson et al., 1986; Hickey et al., 1986; Thorbjarnarson et al., 1986). Farther south, Willapa and Astoria Canyons (Fig. 1) connect directly to the Columbia River mouth during lowstands of sea level (Carlson and Nelson, 1969; Baker, 1976; Barnard, 1978).

The floor of Cascadia Basin contains several important channel-levee complexes. Juan de Fuca Channel (Fig. 1), the westernmost example, originates in the northern portion of the basin (Carson, 1973). Vancouver Valley begins near 50°N, downslope of Vancouver Island, and merges with Juan de Fuca Channel near 46°N. These two channels have created low-relief hummocks and swales near the Leg 168 transect area, and their maximum depth is about 70 m. Channels emanating from the mouth of the Barkley-Nitinat Canyon network branch off to the west and south. Nitinat Valley intersects Vancouver Valley just north of the Leg 168 study area (Fig. 1). Cascadia Channel, the most prominent of the regional system, reaches a total length of over 2000 km (Griggs and Kulm, 1970). Beginning at the mouth of Juan de Fuca Canyon, this channel trends parallel to the base-of-slope toward its confluence with Willapa Channel (Karl et al., 1989). From there, it curves southwest between Nitinat and Astoria Fans, then turns south to join Vancouver Valley and several distributaries of Astoria Fan (Fig. 1). Cascadia Channel eventually intersects the Blanco Fracture Zone and passes west through Cascadia Gap onto the Tufts Abyssal Plain (Griggs and Kulm, 1970).

Nitinat Fan (Fig. 1), with an apex near the mouth of Barkley Canyon, is the most conspicuous depositional system of northern Cascadia Basin (Carson, 1973; Stokke et al., 1977). This elongate fan is bordered by Vancouver Valley to the west, Cascadia Channel to the east, and the fringes of Astoria Fan to the south. Astoria Fan (Fig. 1) radiates asymmetrically from the mouths of Astoria and Willapa Canyons. The Astoria system has prograded southward for more than 200 km between Cascadia Channel and the base-of-slope (Carlson and Nelson, 1969; Nelson et al., 1970; Nelson, 1976), but its influence on sedimentation within the Leg 168 study area, to the north, probably has been minimal.

Stratigraphy of Leg 168 Drilling Sites

Shipboard scientists during Leg 168 grouped ten drilling sites into three transects (Fig. 2). Sediment-basement relations range from outcrops of basalt near the spreading axis to deep burial by 600 m of sediment at Site 1027. With two exceptions (Sites 1030 and 1031), the stratigraphic successions include three lithostratigraphic units and display a crude upward-coarsening and upward-thickening trend (Fig. 3). Facies boundaries are time transgressive, so temporal correlations among sites have been made using nannofossil datums (Shipboard Scientific Party, 1997b, 1997e, 1997a; Su et al., Chap. 4, this volume). Within the context of our study, the designation of muds as "hemipelagic" refers to their texture (silty clay) and composition (mixture of biogenous and terrigeneous constituents). Rigorous discrimination between the physical processes of deposition (i.e., muddy turbidity current vs. vertical settling) requires analysis of grain fabric by such methods as scanning electron microscopy (e.g., Giambalvo et al., in press). The hemipelagic deposits are typically structureless, with scattered clay-rich bands, silt laminae, zones of bioturbation, concentrations of calcareous nannofossils, and pyrite nodules. Subunit IA is composed of hemipelagic mud and coarser interbeds that range from medium-fine sand to sandy silt and silt. Most of the sand beds contain sedimentary structures that are diagnostic of turbidites: sharp to erosional bases, normal size grading, plane-parallel laminae, ripple cross-laminae, and wavy to convolute laminae. The deepest discrete sand layer defines the base of Subunit IA. Subunit IB contains thin interbeds of silt and hemipelagic mud, whereas Unit II is composed entirely of hemipelagic mud. The first recovery of basalt rubble defines the base of Unit II at each site.

The Hydrothermal Transition Transect includes Sites 1023, 1024, and 1025 (Fig. 2). This region, 20-35 km east of the Juan de Fuca Ridge, is characterized by a change from open-basement hydrothermal circulation to a sediment-covered basement. Basement ages inferred from magnetic anomalies range from 0.860 to 1.237 Ma. The wedge of sediment increases in thickness from 97 to 192 m toward the west above a relatively smooth basement surface (Fig. 3). Seismic reflection profiles also show small-scale levees between Sites 1025 and 1024 that are associated with the Juan de Fuca Channel (Shipboard Scientific Party, 1997b).

The Buried Basement Transect, 40-75 km east of the ridge axis, includes Sites 1028 through 1032 (Shipboard Scientific Party, 1997a). Basement ages range from 1.615 to 2.621 Ma (Fig. 2). For the most part, this region displays a relatively smooth basement surface and progressive eastward increases in sediment thickness. Sites 1030 and 1031, however, are situated directly above a basement high that rises to within 41.3 m of the seafloor (Fig. 3). Sediments from Sites 1030 and 1031 consist of hemipelagic mud, carbonate-rich mud, and rare beds of silt to sandy silt; these strata were not subdivided into lithostratigraphic units. Analyses of physical properties and fluid chemistry indicate that the sediments at Sites 1030 and 1031 are undercompacted and affected by upward flow of pore fluid (Shipboard Scientific Party, 1997a).

The Rough Basement Transect is located ~100 km east of the ridge axis (Fig. 2), in an area that contains linear basement ridges and troughs (Shipboard Scientific Party, 1997e). Basement relief is typically 300-500 m, and some basement highs form seafloor outcrops. Site 1026 is located over a ridge, whereas Site 1027 is located above a basement valley (Fig. 3). Basement ages are 3.511 and 3.586 Ma, respectively (Fig. 2). Seismic reflection profiles near Site 1027 include a transparent interval roughly 0.2 s (two-way traveltime) below the seafloor reflector; this interval corresponds to a zone of no core recovery (87-145 mbsf) and is probably composed of thick sand layers (Fig. 3). Recovered sand layers reach thicknesses up to 7 m. Two types of debris-flow deposits were also cored from this subunit; the first type is very poorly sorted muddy sand with a high proportion of primary matrix, and the second contains contorted mixtures of either muddy sand with mud clasts or clayey silt with mud clasts.

Depositional sites on the ridge flank have experienced substantial temporal changes in turbidite influx as dictated by thermal subsidence of the lithosphere, tectonic and volcanic modification of basement structure, and the tendency of sediment-gravity flows to funnel between or deflect around bathymetric obstructions. The three-dimensional morphology of the seafloor changed dramatically over the past 3.5 m.y. as juvenile lithosphere subsided and moved away from the spreading ridge and basement lows filled with sediment. Gradual smoothing out of the seafloor, in turn, affected the behavior of unconfined sheet flows and channel-levee complexes. This dynamic link between sedimentation and geomorphology helps account for the large spatial and temporal differences in total accumulation rates and turbidite recurrence intervals (Shipboard Scientific Party, 1997b, 1997e, 1997a).

Deposition at Site 1027 began soon after basement formed at 3.586 Ma. Initially, this valley filled slowly by suspension fallout, but as basement obstructions to the east were buried, the first silty turbidity currents encroached at ~1.6 Ma. By 1.1 Ma, sandy sediments had filled the valley and turbidites began to lap onto the nearby ridge beneath Site 1026. Farther west, the basement high beneath Sites 1030 and 1031 continued to block transport of turbidity currents to the Hydrothermal Transition sites until ~0.45 Ma (Shipboard Scientific Party, 1997b). The most recent episode of sedimentation (<90 ka) has included westward progradation of channel-levee deposits within the Hydrothermal Transition Transect area.