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For a description of this area, see Oregon Margin
The MBARI survey of the central Oregon margin extends from about 43°50’N to 45°10’N along the continental slope and includes Hydrate Ridge (Clague et al, 2001). A contiguous, shallow-water survey of Heceta Bank, collected by NOAA (Embley et al., 2001), is also included on this CD.
The central Oregon margin is characterized by the collision between two tectonic plates. The oceanic Juan de Fuca plate is subducting obliquely beneath the North American plate at about 4.5 cm/yr. The sediments on the subducting plate, consisting largely of coarse-grained turbidites—sequences of underwater landslide deposits—are accreted to the margin of the over-riding North American plate (MacKay et al., 1992, MacKay, 1995) and deformed by the compression between the plates. Within this survey, every ridge or hill with closed contours has high acoustic backscatter, indicative of carbonate rocks precipitated out of fluids squeezed from the sediments and emerging at cold seeps. Backscatter amplitude on the ridges northeast of Hydrate Ridge is usually higher than at any point on Hydrate Ridge and is also less patchy. Of these ridges, the summit regions of all but two are shallower than 500 meters depth and therefore outside the stability field for methane hydrate (methane molecules trapped in a cage of water, forming an ice-like solid), so the high backscatter for these regions must be caused by carbonate pavements or active chemosynthetic communities. At two deeper sites, as at Hydrate Ridge, the high backscatter is probably caused by a combination of carbonate and methane hydrate on the surface or shallowly buried. At both of these deeper sites, the areas of high backscatter are more patchy than at the shallower sites.
The region mapped includes Hydrate Ridge, a site of methane hydrates and cold seeps. Hydrate Ridge is the location of Ocean Drilling Program hole 892 and of numerous submersible dives and extensive geophysical surveys to map and collect samples. The MBARI-collected high-resolution data for Hydrate Ridge were presented in Clague et al. (2001), from which the following summary was paraphrased. Hydrate Ridge is bounded between the Daisy Bank fault to the north and the Alvin Canyon Fault to the south. The Oregon margin, and Hydrate Ridge in particular, is a site of active fluid expulsion that results in communities of chemosynthetic organisms (dominated by clams and tube worms) (e.g., Kulm et al., 1986) and carbonate precipitates (Ritger et al., 1987). A continuous bottom-simulating reflector (BSR) occurs in seismic reflection profiles of the sediments along much of the lower slope of the Oregon margin (Trehu et al., 1995) and is interpreted to be caused by free methane gas trapped beneath a zone containing some methane hydrate (ODP Leg 146 Scientific Party, 1993). More recently, focus has broadened to include studies of methane emissions into the water column, and methane hydrates near or at the sea floor (Bohrmann et al., 1998). Many of these studies have been conducted on Hydrate Ridge.
The bathymetric data show that the northern hill rises to 588 meters and the southern hill to 773 meters. Moderate-to-high backscatter areas—interpreted to indicate patchy, discontinuous chemosynthetic communities, carbonates precipitated at low pressures, or methane hydrates—cover about 26 km2 of the summit area of the northern hill. Similar moderate-to-high backscatter areas comprise about 4.9 km2 of the southern hill, but only about 0.095 km2 of high-backscatter, nearly-continuous carbonate and hydrate crop out on the western edge of the summit. A site on the southern hill where interbedded carbonate and hydrate was previously recovered is within the region with only patchy outcrop. The occurrence of extensive carbonate and hydrate deposits associated with the summits of hills suggests that the fluid flow responsible for forming the carbonates and hydrates is focused through topographic highs rather than along faults.
The NOAA multibeam survey of Heceta Bank was described by Embley et al. (2001). The following summary is derived from that study. Heceta Bank, on the edge of the central Oregon continental shelf, reveals outcroppings of differentially-eroded, jointed, and folded mudstones, siltstones, and sandstones of Miocene and Pliocene age. Most of the bank was eroded above sea level during low sea level stands. Its outer edge is defined by a change to low-backscatter muds of the upper slope and by a wave cut terrace formed during lowered sea level. The deeper-water late Miocene outcrops are poorly stratified compared to the strongly layered Pliocene strata that form distinct strike ridges on the high-resolution bathymetry. These ridges appear to be hogbacks like those in outcropping beds of folded rocks in continental exposures. Jointing is present in both types of bedrock, but in the late Miocene rocks is generally more pervasive. Direct observations show that extensive cobble and boulder pavements correspond to the high-backscatter zones. Pockmarks up to 200 meters in diameter are found farther offshore (between 200 and 450 meters depth) in several areas within the mud zone along the northern and western seaward slopes of the bank. Carbonate slabs, bacterial mats, and mollusks indicative of cold seeps were observed during dives in several of these features.
Perspective view from the north