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).
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