Geologic cross section across the continental margin of southwestern Washington

Snavely, Parke Detweiler; Wagner, H.C.

doi:10.3133/ofr82459

Cited by 17 publications

(21 citation statements)

References 5 publications

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“…North of Willapa Bay at the latitude of Grays Harbor, the west edge of the Siletz terrane is thinned and has a lower-angled contact with the accretionary prism (Plate 9). This is consistent with deformation of the Crescent Formation basalt along the coast where it is exposed in seaward vergent overturned anticlines in the hanging walls of landward dipping thrust faults [e.g., Snavely and Wagner, 1982 The Siletz terrane acts as a backstop against which accreted rocks are thrust, and this contact may have seismogenic potential. We have provided a 3-D map of the contact zone at depth that shows a fairly steep angle in northern Oregon and southernmost Washington.…”

Section: Discussionsupporting

confidence: 69%

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Parsons¹,

Wells²,

Fisher³

et al. 1999

J. Geophys. Res.

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Section: Discussionsupporting

confidence: 69%

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Parsons¹,

Wells²,

Fisher³

et al. 1999

J. Geophys. Res.

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“…Both seismic profiles and independent evidence from commercial drilling that included the requirement for heavy drilling mud weights indicate that the MBF is permeable, overpressured, and mobile (Palmer & Langley, 1989;McNeill et al, 1998). The abundant triangular-shaped diapirs visible in seismic profiles on the Washington continental shelf are also largely rooted in the Mélange and Broken formation (Figures 3b, 5a and 5b;Snavely & Wagner, 1982;Wagner & Batatian, 1986;Schlische, 1991;McNeill et al, 1998). Examination of Washington coastal fossil outcrops show that the MBF has been associated with methane venting at least since the Miocene (Campbell, 1992;Campbell et al, 2006).…”

Section: Washington Margin Geologymentioning

confidence: 98%

Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

et al. 2019

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A recent compilation of methane plumes detected offshore Washington State includes 1,772 individual bubble streams issuing from 491 discrete vent sites. The majority of these plume sites form a narrow 10‐km‐wide band located shallower than 250‐m water depth, with most sites located near the 175‐m‐deep continental shelf break that tracks the head scarps of large submarine canyons. Archive multichannel seismic profiles over the Cascadia shelf and uppermost margin that were co‐located within a few hundred meters with active emission sites show that methane bubble streams arise from listric/normal faults and triangular‐shaped regions of disturbed seismic reflectors that intersect the seafloor and extend several kilometers into the subsurface. Geological processes were evaluated for producing the narrow emission site depths including nonuniform distribution of methane within the Cascadia accretionary sediment wedge and horizontal transfer of groundwater from onshore subaerial sources. A model of enhanced sediment permeability arising from a contrasting response between the inner and outer portions of the accretionary wedge deformation during a megathrust earthquake cycle appears the most likely mechanism. This faulting is generated during extension of the overriding plate during megathrust earthquake cycles, with semicontinuous permeability enhancement of the fluid pathways from excitation by contemporary incident seismic waves.

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“…, 1998; Pazzaglia & Brandon, 2001; Stewart et al , 2003). Biostratigraphic data from offshore wells also indicate that the shelf and slope sediment accumulation rate has been relatively constant during this time period (Snavely & Wagner, 1982). The steady state, tectono‐stratigraphic system experienced continuous subsidence of the forearc basin to provide accommodation, and the tectonic hinge‐line has been located approximately at the coast over this time period (Brandon et al.…”

Section: Regional Geotectonic Settingmentioning

confidence: 99%

“…The formation is poorly exposed up to a few kilometres inland, where it is mostly concealed by dense vegetation and thick Pleistocene glacial drift (Rau, 1973). Offshore, extensive deposits of late Cenozoic rocks equivalent to the Quinault Formation are known from exploratory wells and acoustical surveys of the present Washington shelf (Rau, 1970; Snavely & Wagner, 1982; Wagner et al. , 1986).…”

Section: Quinault Stratigraphy Structure and Depositional Environmentsmentioning

confidence: 99%

Signatures of storms, oceanic floods and forearc tectonism in marine shelf strata of the Quinault Formation (Pliocene), Washington, USA

Campbell¹,

Nesbitt

Bourgeois

2006

Sedimentology

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Marine shelf strata of the Quinault Formation reflect the influences of storm–flood processes and convergent margin tectonism on sedimentation and palaeocommunity distributions in an active forearc basin of Early Pliocene age, western Washington, USA. The sedimentologic, ichnologic and invertebrate megafaunal character of coastal sea cliff exposures in the Pratt Cliff–Duck Creek area, Quinault Indian Nation, reveal five different sedimentary facies – scoured, Rosselia, bioturbated, mixed and Acharax. These facies document the shifting interplay and intensities among storms, waves and river‐flood plumes during transgression in inner to mid‐shelf settings. Storm sedimentation on the inner shelf is recorded north of Pratt Cliff by amalgamated, proximal tempestites of the scoured facies, which grade up‐section to thick deposits of hummocky cross‐stratified sandstone, indicative of strong wave influences. These hummocky beds alternate, in metre‐scale packages, with banded mudstone and siltstone that have distinctive sedimentologic and ichnofaunal characteristics (Rosselia facies). In particular the mudstone and siltstone occur as 1–15 cm‐thick, rhythmic, parallel beds that are laterally continuous, internally homogeneous to faintly laminated, and thus similar in nature to fine‐grained, oceanic flood deposits reported from shelf settings offshore the modern Eel River, northern California. The Quinault flood deposits are dominated by the ubiquitous trace fossil Rosselia socialis, comprising vertical, mud‐packed, flaring burrows with a sand‐filled central shaft which has been inferred as the feeding‐dwelling structure of a vermiform invertebrate adapted to high sedimentation rates in inner‐shelf settings. Fairweather conditions in between the higher energy periods of storms, waves and floods are recorded north of Pratt Cliff by the mixed facies, which is interpreted as representing the sand and mud zone of the inner‐ to mid‐shelf transition. Quieter, deeper, mid‐shelf, fairweather settings are typified by the bioturbated facies south of Pratt Cliff, where lower sedimentation rates and lower physical energies produced extensively bioturbated deposits of sandy siltstone punctuated, in places, by isolated sandy beds of distal tempestites. Quinault strata also chronicle stratigraphic signatures of subduction of the Juan de Fuca plate beneath western Washington during the Pliocene. For example, the imprint of geochemically unusual authigenic carbonates and a chemosynthetic palaeocommunity (Acharax facies) have been interpreted as a methane seep on the Quinault seafloor. Furthermore, a mobile rockground epifauna of pholadid bivalves became established on abundant, dark mudstone cobbles and pebbles sourced from the Hoh Assemblage, a Miocene accretionary prism that was actively deforming as well as interacting with Quinault forearc sediments during the Pliocene. Hoh mudstone clasts were supplied to the Quinault shelf via seafloor‐piercing diapirs and eroding mélange shear zones, exposures of which today occur in fault c...

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Geologic cross section across the continental margin of southwestern Washington

Abstract: The geologic cross section A-A" is located across the continental margin of southwestern Washington near latitude 46°50'N. The section extends westward from longitude 123°30' in the Coast Range and terminates on the continental shelf near longitude 124°40' (fig. 1).

Cited by 17 publications

References 5 publications

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

Signatures of storms, oceanic floods and forearc tectonism in marine shelf strata of the Quinault Formation (Pliocene), Washington, USA

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