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123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 122°20'
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Themes visible in the above map: Regional Geology (REG_GEO) MAPPED FAULTS, LATITUDE/LONGITUDE, COASTLINE and LAND.

Eastern Juan de Fuca Strait- Regional Geology Map

Samuel Y. Johnson
U.S. Geological Survey, Golden, CO USA

David C. Mosher*
Geological Survey of Canada, Sydney, BC CANADA
present address: Geological Survey of Canada, Dartmouth, NS, CANADA

Introduction
This schematic map shows the regional geology of the eastern Juan de Fuca Strait study area (Figs. 1, and map above). This part of western Washington State and southwestern British Columbia lies within the structurally complex continental margin of the Pacific Northwest. Oblique convergence of the Juan de Fuca plate below North America (Fig. 2), along with larger-scale shearing of the Pacific Plate against North America (Fig. 3), provide the driving force for modern crustal faulting and deformation (e.g., Wang, 1996; Wells and others, 1998; Stanley and others, 1999; Wang, 2000). Within this setting, the eastern Juan de Fuca Strait region is part of a segmented, linear topographic depression that extends south from Canada's Strait of Georgia to Washington's Puget Lowland. This trough is bounded by Vancouver Island and the Olympic Mountains to the west and the Cascade Range volcanic arc to the east.

The eastern Juan de Fuca Strait region overlies a fundamental northwest-trending crustal boundary between basement rocks of the pre-Tertiary Cascades province to the northeast and the Eocene Coast Range province to the southwest. The province southwest of this boundary is part of a forearc sliver consisting of Eocene volcanic basement and overlying sedimentary rocks. This forearc sliver is moving northward relative to the Cascade Range, and is buttressed to the north by pre-Tertiary basement exposed in the San Juan Islands and Vancouver Island. The southern Whidbey Island fault and the Leech River fault represent parts of the crustal boundary (Map above).

Rock Units
Pre-Tertiary
Within the eastern Juan de Fuca Strait region, pre-Tertiary bedrock consisting of the Mesozoic Fidalgo ophiolite and tectonically mixed, variably metamorphosed, sedimentary, volcanic, and plutonic rock is exposed in the Skagit delta, on northern Whidbey Island and Fidalgo Island, and in the San Juan Islands (Whetten and others, 1980; 1988; Brandon and others, 1988; Tabor, 1994). Pre-Tertiary bedrock consisting of the Mesozoic Leech River Complex and Paleozoic to Mesozoic volcanic and plutonic or metamorphic rocks which underlie southern Vancouver Island north of the Leech River fault (Roddick and others, 1979; Muller, 1983; Yorath et al., 1999). These pre-Tertiary rocks comprise several distinct crustal terranes with allochthonous and(or) exotic origins (e.g., Tabor, 1994). Final assembly of these crustal terranes occurred by earliest Paleogene, after which they formed the framework of the Pacific Northwest continental margin. Pre-Tertiary rocks are generally non-reflective and represent acoustic basement.

Eocene Metchosin and Crescent Formations
On southern Vancouver Island, the basement rock of the Coast Range province consists of lower to lower middle Eocene marine basalt of the Metchosin Formation and its plutonic root, the Sooke Gabbro (Massey, 1986). Similar rocks exposed on the northeastern Olympic Peninsula are known as the Crescent Formation (Wells and others, 1984; Snavely, 1987; Babcock and others, 1992). In the Olympic Mountains at the northern end of the Washington Coast Range, dramatic late Miocene and younger uplift in the Olympics has tilted the Crescent Formation and overlying rocks into a steep east-plunging anticline that is structurally underlain by Eocene and younger subduction-zone deposits (Tabor and Cady, 1978; Brandon and Calderwood, 1990). The upper part of the Crescent/Metchosin Formation is commonly charcaterised on offshore seismic-reflection data by variable to high amplitude, discontinuous, subparallel to low-angle reflections (e.g., Johnson and others, 1996). The lower part of this unit is generally non-reflective.

Tertiary sedimentary rocks
Tertiary sedimentary rocks crop out on the northeastern Olympic Peninsula and east of Whidbey Island in the Skagit River delta. Units on the Olympic Peninsula overlie the Crescent Formation and include marine deposits of the middle Eocene sandstone of Scow Bay, the upper Eocene Aldwell and Lyre Formations, the upper Eocene to lower Oligocene Quimper Sandstone and Marrowstone Shale and the Oligocene Twin Rivers Group (Armentrout and Berta, 1977; Rau and Johnson, 1999). Units that occur in the Cascade Range foothills include the Eocene, non-marine Chuckanut Formation (Johnson, 1984; Evans and Ristow, 1994) and the upper Eocene to lower Oligocene marine to marginal marine rocks of Bulson Creek (Marcus, 1980). Age-equivalent rocks on southern Vancouver Island include the Eocene to Oligocene clastic sediments of the Carmanah Group, including the sandstones and conglomerates of the Sooke Formation. These Tertiary sedimentary rocks are cut by numerous faults and are gently to steeply folded (Whetten and others, 1988). On offshore industry seismic-reflection profiles, Tertiary strata are characterized by relatively continuous, high amplitude, parallel to sub-parallel, moderate frequency reflections.

Quaternary sediments
During the Pleistocene, the eastern Juan de Fuca Strait region was occupied several times by lobes of the continental ice sheet. As a result, Pleistocene deposits of the region comprise a stratigraphically complex basin fill of glacial and interglacial deposits that are locally as thick as 1,100 m (see Quaternary Thickness maps). Blunt and others (1987) and Easterbrook (1994) described six distinct glacial drift units, three of which are exposed on Whidbey Island and at least two on Vancouver Island (Armstrong and Clague, 1977). These glacial drift units comprise till, outwash, and glaciomarine deposits of the Double Bluff and Westlynn Drifts (~250-120 ka), Possession, Semiahoo, and Dashwood Drifts (~80-60 ka), and the Vashon Drift (~18-15 ka). Associated interglacial strata deposited in fluvial and deltaic environments include the Whidbey and Muir Point Formations (~120-80 ka) and the Olympia beds and Cowichan Head Formation (~60-18 ka).

On offshore seismic-reflection profiles, Pleistocene strata (excluding latest Pleistocene post-glacial deposits) form a distinct seismic unit, bounded below by pre-Tertiary or Tertiary basement and above by typically flat-lying latest Pleistocene to Holocene deposits that commonly fill in erosional or depositional relief. On both industry and higher-resolution seismic-reflection data, Pleistocene strata display highly variable amplitudes, are discontinuous, and have parallel, divergent, and hummocky reflections. Internal truncation, onlap, and offlap of reflections are all common. Given the physiography of the eastern Juan de Fuca Strait, it is likely that much of the Pleistocene strata as seen on seismic profiles consist of recessional glaciomarine drift. Present day seafloor morphology is governed largely by these drift deposits (see Relief map).

Seismic-reflection data show that post-glacial latest Pleistocene and Holocene sediments continue to accumulate in local basins bounded by Pleistocene bathymetric highs. Holocene basin-fill typically yields variable-amplitude, parallel, and continuous reflections. East of Whidbey Island, these sediments are inferred to be clay and silt derived from the Skagit River. In the eastern Juan de Fuca Strait, where there are no major sources of sediment, the latest Pleistocene to Holocene basin fill probably consists largely of reworked glacial recessional deposits.

Active and potentially active faults
Recent studies indicate the eastern Juan de Fuca Strait region contains several active or potentially active faults. These include the southern Whidbey Island fault, the Leech River fault and the Devils Mountain fault (MacLeod and others, 1977; Gower and others, 1985; Johnson and others, 1996, 2000, in revision).

Leech River fault
The Leech River fault cuts across southern Vancouver Island and represents a portion of the crustal boundary between basement rocks of the pre-Tertiary Cascades province to the northeast and the Eocene Coast Range province to the southwest (Clowes and others, 1987). During the Eocene, northward movement of the Coast Range province was in part accommodated by southward directed thrusting on this structure. Although this fault forms a significant topographic lineament, offset since the Eocene has not been demonstrated.

Southern Whidbey Island fault
The southern Whidbey Island fault is an unexposed fault that was first postulated by Gower and others (1985) on the basis of magnetic and gravity anomalies, evidence for displacement of Quaternary strata in boreholes, and minor faulting exposed in upper Quaternary sediments. Subsequently, Johnson and others (1996) provided considerable new information to show that the structure represents another part of the boundary between the pre-Tertiary Cascades province to the northeast and the Eocene Coast Range province, and has a long-lived history associated with continental margin rifting, strike-slip faulting, and transpressional deformation. Deformation of Quaternary sediments in outcrops and on seismic-reflection profiles indicates the fault has been recently active.


Devils Mountain fault and associated structures
The Devils Mountain fault (Hobbs and Pecora, 1941; Tabor, 1994) cuts across the northern portion of the eastern Juan de Fuca Strait region and forms the northern boundary of the Tertiary to Quaternary Everett basin (Johnson and others, 1996). This fault is associated with an alignment of aeromagnetic anomalies that extend more than 100 kilometres from the Cascade Range foothills to Vancouver Island. In the Cascade Range foothills, the fault forms a prominent ~30-km-long topographic lineament. Farther west, the Devils Mountain fault trace extends through Quaternary deposits of the Skagit River delta and Whidbey Island and into the eastern Juan de Fuca Strait. Johnson and others (2000) used information from seismic-reflection profiles, outcrops, and subsurface well logs to show that the Devil's Mountain fault is a complex structural zone that has been active in the Quaternary.


In the eastern Juan de Fuca Strait region, the west-trending Devils Mountain fault is bounded to the north by northwest-trending faults and folds and on the south by faults that cut across northern Whidbey Island (Johnson and others, 2000) and also deform Quaternary strata. The map geometry suggests the Devils Mountain fault and these en echelon structures are components of an oblique-slip transpressional deformation zone (e.g., Harding and others, 1983; Christie-Blick and Biddle, 1985).

References
Armentrout, J.M., and Berta, A., 1977, Eocene-Oligocene foraminiferal sequence from the northeast Olympic Peninsula, Washington: Journal of Foraminiferal Research, v. 7, p. 216-233.

Armstrong, J.E., and Clague, J.J. 1977. Two major Wisconsin lithostratigraphic units in southwest British Columbia. Canadian Journal of Earth Science. 14: 1471-1480.

Babcock, R.S., Burmester, R.F., Engebretson, E.C., and Warnock, A., 1992, A rifted origin for the Crescent basalts and related rocks in the northern Coast Range volcanic province, Washington and British Columbia: Journal of Geophysical Research, p. 6,799-6,821.

Blunt, D.J., Easterbrook, D.J., and Rutter, N.W., 1987, Chronology of Pleistocene sediments in the Puget Lowland, Washington: Washington Division of Geology and Earth Resources Bulletin, v. 77, p. 321-353.

Brandon, M.T., Cowan, D.S., and Vance, J.A., 1988. The Late Cretaceous San Juan thrust system. San Juan Islands, Washington: Geological Society of America Special Paper 221, 81 p.

Brandon, M.T., and Calderwood, A.R., 1990, High-pressure metamorphism and uplift of the Olympic subduction complex: Geology, v. 18, p. 1252-1255.

Christie-Blick, Nicholas, and Biddle, K.T.,1985, Deformation and basin formation along strike-slip faults, in Biddle, K.T., and Christie-Blick, Nicholas (eds.), Strike-slip deformation, basin formation, and sedimentation: Society of Economic Paleontologists and Mineralogists, Special Publication 37, p. 1-34.

Clowes, R.M., Brandon, M.T., Green, A.G., Yorath, C.J., Sutherland Brown, A., Kanesewich, E.R., and Spencer, C., 1987, LITHOPROBE - southern Vancouver Island -- Cenozoic subduction complex imaged by deep seismic reflections: Canadian Journal of Earth Sciences, v. 24, p. 31-51.

Easterbrook, D.J., 1994, Chronology of pre-late Wisconsin Pleistocene sediments in the Puget Lowland, Washington, in Lasmanis, R., and Cheney, E.S., eds., Regional geology of Washington State: Washington Division of Geology and Earth Resources Bulletin 80, 191-206.

Evans, J.E., and Ristow, J., Jr., 1994, Depositional history of the southeastern outcrop belt of the Chuckanut Formation: implications for the Darrington-Devils Mountain and Straight Creek fault zones, Washington (U.S.A.): Canadian Journal of Earth Sciences, v. 31, p. 1727-1743.

Gower, H.D., Yount, J.C., and Crosson, R.S., 1985, Seismotectonic map of the Puget Sound region, Washington: U.S. Geological Survey Map I-1613, scale 1:250,000.

Harding, T.P., Gregory, R.F., and Stephens, L.H., 1983, Convergent wrench fault and positive flower structure, Ardmore Basin, Oklahoma, in Bally, A.W., ed., Seismic expression of structural styles: American Association of Petroleum Geologists Studies in Geology Series no. 15, v. 3, p. 4.2-13-17.

Hobbs, S.W., and Pecora, W.T., 1941, Nickel-gold deposit near Mount Vernon, Skagit County, Washington: U.S. Geological Survey Bulletin 931-D, p. 57-78.

Johnson, S.Y., 1984, Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, northwest Washington: Canadian Journal of Earth Sciences, v. 21, p. 92-106.

Johnson, S.Y., Dadisman, S.V., Mosher, D.C., Blakely, R.J., and Childs, J.R., 2000, Late Quaternary tectonics of the Devils Mountain Fault and related structures, Northern Puget Lowland: Geological Society of America Abstracts with Programs, v. 32, p. A21-22.

Johnson, S.Y., Dadisman, S.V., Mosher, D.C., Blakely, R.J., and Childs, J.R., in revision, Active tectonics of the Devils Mountain fault and related structures, northern Puget Lowland and eastern Juan de Fuca Strait region, Pacific Northwest: U.S. Geological Survey Professional Paper.

Johnson, S.Y., Potter, C.J., Armentrout, J.M., Miller, J.J., Finn, C., and Weaver, C.S., 1996, The southern Whidbey Island fault, an active structure in the Puget Lowland, Washington: Geological Society of America Bulletin, v. 108, p. 334-354 and oversize insert.

MacLeod, N.S., Tiffin, D.L., Snavely, P.D., Jr., and Currie, R.G., 1977, Geologic interpretation of magnetic and gravity anomalies in the Strait of Juan de Fuca, U.S.-Canada: Canadian Journal of Earth Sciences, v. 14, p. 223-238.

Marcus, K.L., 1980, Eocene-Oligocene sedimentation and deformation in the northern Puget Sound area, Washington: Northwest Science, v. 9, p. 52-58.

Massey, N.W.D., 1986, Metchosin Igneous Complex, southern Vancouver Island: Ophiolite stratigraphy developed in an emergent island seting; Geology, v 14, p. 602-605.

Muller, J.E., 1983, Geology, Victoria: Geological Survey of Canada Map 1553A, scale 1:100,000.

Rau, W.W., and Johnson, S.Y., 1999, Well stratigraphy and correlations, western Washington and northwest Oregon: U.S. Geological Survey Map I-2621, 3 oversized sheets/charts, 31 p.

Roddick, J.A., Muller, J.E., and Okulitch, A.V., 1979, Fraser River, British Columbia-Washington: Geological Survey of Canada Map 1386A, Sheet 92, scale 1:1,000,000.

Snavely, P.D., Jr., 1987, Tertiary geologic framework, neotectonics, and petroleum potential of the Oregon-Washington continental margin, in Scholl, E.W., Grantz, A., and Vedder, J.G., eds., Geology and resource potential of the continental margin of western North America and adjacent ocean basins, Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 6, p. 305-335.

Stanley, W.D., Villasenor, A., and Benz, H., 1999, Subduction and crustal dynamics of western Washington -- a tectonic model for earthquake hazard evaluation: U.S. Geological Survey Open-File Report 99-311, 90 p.

Tabor, R.W., 1994, Late Mesozoic and possible early Tertiary accretion in western Washington state - The Helena-Haystack melange and the Darrington-Devils Mountain fault zone: Geological Society of America Bulletin, v. 106, p. 217-232.

Tabor, R.W., and Cady, W.M., 1978, Geologic Map of the Olympic Peninsula, Washington: U.S. Geological Survey Map I-994, scale 1:125,000.

Wang, K., 1996, Simplified analysis of horizontal stresses in a buttressed forearc sliver at an oblique subduction zone: Geophysical Research Letters, v. 23, p. 2012-2024.

Wang, K., 2000, Stress-strain "paradox", plate coupling, and forearc seismicity at the Cascadia and Nankai subduction zones, Tectonophysics, v. 319, p. 321-338.

Wells, R.E., Engebretson, D.C., Snavely, P.D., Jr., and Coe, R.S., 1984, Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington: Tectonics, v. 3, p. 274-294.

Wells, R.E., Weaver, C.S., and Blakely, R.J., 1998, Fore-arc migration in Cascadia and its neotectonic significance: Geology, v. 26, p. 759-762.

Whetten, J.T., Carroll, P.I., Gower, H.D., Brown, E.H., and Pessl, F., Jr., 1988, Bedrock geologic map of the Port Townsend 30- by 60-minute quadrangle, Puget Sound Region, Washington: U.S. Geological Survey Map I-1198-G, scale 1:100,000.

Whetten, J.T., Zartman, R.E., Blakely, R.J., and Jones, D.L., 1980, Allochtonous Jurassic ophiolite in northwest Washington: Geological Society of America Bulletin, v. 91, p. 359-368.

Yorath, C J; Sutherland-Brown, A; Massey, N. W. D., 1999, LITHOPROBE, southern Vancouver Island, British Columbia: geology; Geological Survey of Canada Bulletin no. 498, 145 pages.

 

Reference citation:
Johnson, S.Y., and Mosher, D.C., 2000. The eastern Juan de Fuca Strait - regional geology map, in: Mosher, D.C. and Johnson, S.Y. (Eds.), Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics of the eastern Juan de Fuca Strait; a digital geological and geophysical atlas. Geological Survey of Canada Open File Report 3931