114 research outputs found
Relationship between carbonate deposits and fluid venting: Oregon accretionary prism
Active fluid venting and its surface manifestations (unique animals and carbonates) occur over the accretionary prism in the Cascadia subduction zone located off central Oregon. A large variety of authigenic carbonate deposits and unique carbonate structures have been observed from submersibles and remotely operated vehicles and recovered with aid of submersibles and bottom trawls from the outermost continental shelf and lower continental slope. The carbonate deposits range from relatively thin crusts and slabs to irregular edifices and well-formed circular chimneys that rise from 1 to 2 m above the seafloor. Mineralogically, the carbonate cement consists of aragonite, calcite, Mg-calcite, or dolomite with varying amounts of detrital constituents. Stable carbon and oxygen isotope data identify four distinct subgroups of methane-derived carbonates from several different vent sites and different fluid source zones. Subgroup I represents one vent site on the lower slope and is characterized by oxygen isotope values ranging from +6.8â° to +4.7â° PDB. Subgroup II represents another vent site about 1 km away and exhibits oxygen values of +3.4â° to +4.9â° PDB. Carbon isotopic values range from â40.96 to â30.23â° versus â44.26 to â53.44â° PDB, respectively, for the two vents. An irregular edifice from the outer shelf has the same isotopic composition as subgroup II. A companion study shows that the expelled fluids contain largely biogenic methane and methane-derived dissolved carbonate; a shallow fluid source zone (<1 km) is indicated. The isotopic carbon values of the subgroup I and II carbonates are consistent with the carbon composition of the expelled fluids and apparently represent a historical record of the composition of these fluids. In subgroup III, strong 18O enrichment and heavier carbon values characterize the dolomitic chimneys from the outer continental shelf. Cemented sandstones from a âwindowâ in the accretionary complex of the lower slope (subgroup IV) are characterized by extreme ÎŽ18O (â5.9 to â5.98 â°) and moderate ÎŽ13C (â18.7 to â12.67â°)-depleted carbonates. This âlightâ oxygen isotope composition most likely originated from the upward migration of warm hydrothermal fluids along the main dĂ©collement, which tapped the warm subducting basaltic slab, during the early stages of formation of the accreted complex. Well-defined plumbing tubes within some carbonate chimneys on the shelf infer a single well-defined subsurface conduit with a fairly energetic fluid flow. The majority of the chimneys probably formed above the seafloor as long as the rate of carbonate precipitation exceeded the rate of detrital input during their formation. We calculate a minimum of one conduit for each 35 m2 at one vent site on the shelf. A less energetic flow is suggested by the chaotic plumbing network of an irregular edifice and by the widespread occurrence of the carbonate slabs and crusts at numerous vent sites
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Oblique strike-slip faulting of the central Cascadia submarine forearc
At least nine WNW trending left-lateral strike-slip faults have been mapped on the Oregon-Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations. The faults range in length from 33 to 115 km and cross much of the continental slop. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with little or no offset by the frontal thrust. Left-lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene and late Pleistocene. Offset of surficial features ranges from 120 to 900 m, and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip. Near their western tips, fault ages range from 300 ka to 650 ka, yielding late Pleistocene-Holocene slip rates of 5.5 ± 2 to 8.5 ± 2 mm/yr. The geometry and slip direction of these faults implies clockwise rotation of fault-bounded blocks about vertical axes within the Cascadia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc. The basement-involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate-coupling forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.Copyrighted by American Geophysical Union
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Listric normal faulting on the Cascadia continental margin
Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E-W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low-angle (0.1°-2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction-related compression
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Sediments of Yaquina Bay, Oregon
Three realms of deposition, Marine, Fluviatile, and Marine-
Fluviatile, are recognized in Yaquina Bay, Oregon, on the basis of
sediment texture and mineralogy. The Marine Realm extends 1.5
miles into the entrance of the estuary and is typified by normal
marine salinity and vigorous tidal action. Sediments of this realm
are similar to those of the adjacent beach and coastal dune sands
and consist of well-sorted, subangular to subrounded, fine to medium
sand. The immature arkosic sands in this realm are distinguished
by the marine suite of heavy minerals which include abundant
pyroxenes, primarily hypersthene and diopside, and such metamorphic
minerals as kyanite, sillimanite, and staurolite. The
Fluviatile Realm occurs at the fresh-water head of the estuary and
reaches to a point 6 miles from the entrance, where brackish water
conditions prevail. The poorly sorted, angular to subangular sediments
of this realm range in grain size from silt to coarse sand.
They are somewhat more arkosic than the sands of the Marine
Realm and are represented by the fluviatile suite of heavy minerals.
This assemblage includes such diagnostic minerals as biotite and
muscovite, and hematite and limonite. Diopside is absent, hypersthene
is restricted, and there is a marked decrease in the abundance
of garnet and the number of metamorphic species, compared
with the Marine Realm. The Marine-Fluviatile Realm lies between
the Fluviatile and Marine Realms and contains admixtures of sediments
of the other two realms.
The chief sources of Recent sediments in the Yaquina Bay
area are the Tertiary rocks of the central Oregon Coast Range, the
Pleistocene marine terrace sands and estuarine deposits near the
bay mouth, and the Recent transitory beach and dune sands that
flank the bay entrance.
Marine sand from the adjacent ocean beaches is transported
into the estuary by strong tidal currents to Oneatta Point 6 miles
from the entrance. Nearby coastal dune sands are blown into the
tidal channel near the mouth of the estuary and onto the southwestern
shore of Southbeach Tidal Flat by strong onshore winds. Suspended
sediments are contributed by the Yaquina River during periods
of high runoff.
The type of estuarine system is dependent upon seasonal and
annual climatic conditions. Generally, from June to October the
system is well-mixed, but it may alternate between a well-mixed to partly-mixed system from November to May. Precipitation recorded
at Newport apparently reflects the type of estuarine system
present during each month of the year for any given year.
Deposition in Yaquina Bay appears to be largely seasonal.
Maximum deposition probably occurs in the winter and early spring
when river runoff is highest, the littoral drift is from south to
north, and the highest velocity winds are from the southwest. At
this time, the partly-mixed estuarine system is effective in transporting
drifting beach sands into the entrance of the estuary. During
the summer, deposition is slight because of the low runoff,
southward littoral drift, and northwest winds. The well-mixed estuarine
system inhibits the transportation of sediments into the
estuary.
Known areas of shoaling occur on the bar, in the main channel,
and in the turning basin. The shoaled areas have maintained
a fairly constant position from 1950 to 1961. Estimated average rate
of deposition in the dredged channel is 9.1 inches per year. Marine
sand is the principal shoaling material. As a result of jetty construction
in 1888, and through subsequent additions, extensive
deposition has occurred on the southern ocean beach behind the
south jetty. An average estimate of Z74 cubic yards of material
accumulated annually during the past 73 years
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