Mile 13.1, STOP 1. Lake Crescent Overlook. Roadcut in Crescent Formation, Tcb unit
of Tabor and Cady (1978a). Flows of black pillow basalt striking approximately east-west
and dipping steeply (~850) north; dense to highly vesicular; contains microphenocrysts
of clinopyroxene [Fe, Mg, Ca, Na, SiO2, and Al] and calcic to soda plagioclase [(Ca,
Na) (Al,Si) AlSi2O8]. A submarine flow at Crescent Lake, just below the contact
with the overlying Aldwell Formation yielded an 40Ar/39Ar date of 52.9±4.6 Ma while
the base of the submarine Crescent Formation flows on Hurricane Ridge Road yielded
an 40Ar/39Ar date of 45.4±0.6 Ma. These two dates suggest the Crescent Formation,
while mapped as a single unit between these two locations, had more than one eruptive
center (Babcock et al., 1994). There is disagreement among investigators as to whether
the chemistry of the basalt justifies separating the formation into lower and upper
members. Glassley (1974) and Muller (1980) maintain that the chemistry points to
two members – a lower mid-ocean ridge basalt (MORB) and upper oceanic island basalt
(OIB) member. Cady (1975) and Babcock et al, 1994) argue there is no clear difference
in chemistry between the upper and lower members. More work needs to be done to
resolve this issue.
Mile 82.5, STOP 2. Beach 4. Wavecut outcrops north of end of trail from parking
lot. Hoh rock assemblage of Rau (1975) consists of massive to thick-bedded graywacke
(informal term for coarse-grained sandstone with poorly sorted subangular to angular
quartz, feldspar, and rock fragments all mixed together in a clayey matrix); thin
to medium bedded siltstone and sandstone; and coarse- to very coarse-grained graywacke,
grit, and conglomerate. The rocks, mapped as Hoh lithic assemblage by Tabor and
Cady (1978a), include thick-bedded coarse-grained lithic and feldspatholithic sandstone,
with angular locally well-sorted grains and minor thin-bedded siltstone and sandstone.
Ripple drift cross-laminations, groove, flute, and flame casts are present in thicker
beds as well as crossbedding and channels.
The Coastal OCS outcrops at this stop consist primarily of thick- to thin-bedded
turbidite sandstone (graywacke) and siltstone. At the end of the trail are steep,
east-dipping sandstones. Their orientation (right-side up or overturned) can be
determined from primary sedimentary structures. (Examine the outcrop and determine
the orientation of bedding.) The beach here is underlain by a Pleistocene wave-cut
surface. Notice the numerous paddock clam borings. Just north of the end of the
trail is well-exposed angular unconformity of Pleistocene gravels on the steeply-dipping
turbidite deposits. A short distance farther north the beds have been deformed into
a series of folds (fold train). The folds have northeast-striking axial planes and
are truncated at the base of the outctrop by a low angle thrust fault. The geometry
of the folds and other structural criteria indicate they formed by a combination
of flexural slip and flexural flow.
Mile 86.3, STOP 3. Ruby Beach. Wavecut cliffs and seastacks north of end of trail
from parking lot. Hoh rock assemblage of Rau (1975) made up of graywacke sandstone,
mélange rocks of intensely sheared claystone and siltstone containing blocks of indurated
siltstone and graywacke sandstone and altered volcanic rocks, and undifferentiated
volcanic rocks with very large blocks within mélange rocks. (The term mélange refers
to a body of rock, large enough to be mapped, that is characterized by a lack of
internal continuity of contacts or strata and by the inclusion of fragments and blocks
of all sizes, both exotic and native, embedded in a fragmental matrix of fine-grained
material.) The outcrops were mapped by Tabor and Cady (1978a) as the same Hoh lithic
assemblage unit as at Beach 4, except for a small offshore outcrop of the Lyre Formation.
The rocks at Ruby Beach don’t look anything like those at Beach 4 and were more
properly mapped by Rau, at a scale of 1:62,500, as mélange. The Tabor and Cady
(1978a) map is half the scale (1:125,000) of Rau’s map, justifying using the same
unit designation as at Beach 4. The description of the unit, however, could have
been improved by mentioning the presence of mélange.
Several different origins for Hoh mélange have been proposed including gravity
tectonism, shear zones between large structural blocks, and diapirism. Earlier studies
by Rau (1973, 1975, 1979) and Rau and Grocock (1974) identified both shear-zone and
diapiric mélanges in the Coastal OSC. Orange (1990) and Orange et al. (1993) have
done detailed investigations of Hoh mélanges that demonstrate the complexity of geologic
conditions under which they form. Studying middle Miocene mélanges in the Coastal
OSC, Orange (1990) developed criteria to distinguish shear-zone and diapiric mélanges.
Diapiric mélanges have radial scaly foliation that is well-developed at their margins
and poorly-developed in the centers, opposite fold vergent directions from margin
to margin, rare exotic clasts. The clasts range in shape and orientation from elongate
with a strong preferred orientation at the margins to angular and mostly random orientation
in the center. Shear-zone mélanges have a strongly developed and pervasive scaly
foliation that maintains the same orientation across outcrops, uniform fold vergent
directions, exotic clasts. The clasts are mostly elongate and have a strongly developed
long-axis preferred orientation.
The cliff at this stop has the appearance of a shear-zone mélange. Large eye-shaped
structural blocks bounded by faults and high angle faults accommodated tectonic shortening.
Mile 19.1, Stop 4. Hurricane Hill Trail. The Hurricane Hill trail crosses three
stratigraphic units of Tabor and Cady (1978a). Approximately the first 0.7 km (2000
ft) of the trail traverses the Tnm unit of the Needles Gray-Wolf lithic assemblage,
which they describe as a micaceous sandstone, with less than 60% siltstone and slate.
The angular, medium-grained, lithic to feldspathic sandstone is poorly sorted. Calcite
and slate chips are common. It is thin to very thick bedded with small crossbeds,
and rare graded beds, ripple marks, and load casts. Slate is micaceous and highly
fissile; it grades to siltstone.
Leaving the trail head there’s a very low topographic dip in it, a short distance
on you will see the first outcrops of the Needles Gray-Wolf lithic assemblage. In
this area it is thinly-to thick bedded graywacke sandstone, siltstone and slate.
The most pervasive structure in these outcrops is a pencil cleavage, slivers of
rock (pencil-like) formed by the intersections of two or more cleavages or, more
typically, the intersection of cleavage (a planar fabric created by the rock tendency
to split in a particular direction) and bedding. The pencil cleavage may reflect
an intermediate stage in the development of slaty cleavage (a foliation defined by
elongate domains of quartz or feldspar aggregates separated by anastomosing mica-rich
laminae) and, therefore, occur only in weakly metamorphosed rocks, like those on
this trail. Tabor and Cady (1978b) found pencil cleavage in the western and northeastern
parts (where we are now) of the eastern core lying in bedding. In the central part
of the core pencils generally do not lie in bedding but are formed by two cleavages
and are perpendicular to fold axes. Because they found pencils to be the most consistent
structural element in the core they used pencil orientations to divide it into two
large structural domains that they subdivided into 19 subdomains. In the field the
boundary between the two main domains, what they called Domain East and Domain West,
is identified by opposing dips and plunges. In Domain East the planar structures
dip west to southwest and pencils plunge west. In Domain West planar structures
dip east and northeast and pencils plunge east. The boundary which winds roughly
north-northwest across the core, passing about 8 km east of Mount Olympus, separates
the west verging structures in the Olympic Mountains from the east verging structures.
Hurricane Hill Trail falls within Tabor and Cady’s Domain East, Subdomain 1. Their
contour diagrams of Subdomain 1 data show bedding mostly striking northwest and dipping
steeply northeast, cleavage striking west-northwest and dipping steeply north-northeast,
steeply plunging pencils trending south-southeast to south, and steep to moderately
steep plunging fold axes trending northeast to northwest.
The first sign on the trail is titled “Folded Rock”. The fold opposite the sign
is a shear fold - a fold in which shearing or slipping takes place along closely
spaced planes parallel to the fold’s axial surface, also called a similarfold. The
axis of this well-developed fold trends northeast. Notice the steeply dipping fault
that cuts across the axial plane, creating a small apparent offset of bedding. A
short distance farther on the trail very thick graywacke sandstone beds are present.
They are devoid of the cleavage that is so prominent in the thinly-bedded layers.
The map by Tabor and Cady (1978a) shows the Hurricane Ridge fault crossing the trail
where its grade changes steeply up on the west side of the saddle located about 2,000
feet from the trail head. When you cross the fault the trail is on the Blue Mountain
unit. Tabor and Cady describe it as sandstone and argillite (a compact rock derived
from claystone, siltstone, or shale that has undergone a somewhat higher degree of
induration but is clearly less laminated than shale and without its fissility, and
that lacks the cleavage distinctive of slate) - very fine to medium-grained lithic
sandstone, volcanic rich; fair to poorly sorted and angular with thin to thick beds.
The rocks in the area of the Hurricane Ridge fault are more highly deformed, probably
reflecting a wide zone of deformation associated with displacement along the master
fault. The map pattern of the fault (Tabor and Cady, 1978) shows it is nearly vertical
here. Bedding orientations change significantly over short distances and there are
a significant number of faults with widely varying orientations, many with low dips.
The trail sign “Wind the Sculptor” is west of the fault on the Blue Mountain unit.
Higher up (about 4,500 feet from the trail head) the trail crosses the contact with
the Crescent Formation, which caps Hurricane Hill.
Mile 16.0, Stop 5. Hurricane Ridge Road (Mile Marker 15.9). This is an excellent
location to see the Hurricane Ridge fault, the contact of the Blue Mountain unit
and the Needles Gray-Wolf lithic assemblage of the core rocks, the same units we
saw at Stop 4. The geologic map by Tabor and Cady (1978a) shows the road passes over
fault at about mile marker 16.0. The roadcuts west and east on this mile marker
have steeply dipping bed dipping south and north, right side up and overturned, and
highly disrupted by imbricate faults. Graded bedding and cross laminations are present
in some of the thin greywacke sandstones. On the south side of the mile marker a
large elongate (~ 1 meter) block of graywacke is surrounded by thin beds of slate,
siltstone and sandstone, that is like the exotic blocks found in tectonic mélange.
There are several well-developed faults in these exposures. The fault closest to
mile marker 16 may be the master fault of the fault zone or what Tabor and Cady (1978b)
call the zone of disruption. Bedding is nearly vertical on both sides of the fault.
Drag on the beds flanking the fault and very small drag folds on the fault indicate
the north side moved steeply up and west relative to the south side. Look for the
tight isoclinal fold about 50 feet south of mile marker 16 and for steeply-dipping
splay faults and beds sheared off by well-developed cleavage within the fault zone.
Slate present within the Needles Gray-Wolf lithic assemblage here, and at the previous
stop, is the result of shale and mudstone being subducted into the accretionary wedge,
subjecting it to increased temperatures and pressures. Tabor and Cady (1978b) found
a general increase in the metamorphic grade from west to east based on the presence
of various index minerals in samples they collected in the central and eastern Olympic
Mountains and other workers (Stewart, 1974, in the western part of the Olympic Structural
Complex and Hawkins, 1967, in the Mount Olympus area). Brandon and Calderwood (1990)
concur with Tabor and Cady’s metamorphic zonation. Based on fission-track dates
for sandstones from the eastern zone they place its temperatures between 100±100
and 200±500 C, the blocking temperatures for apatite (a mineral consisting of some
combination of fluorine, chlorine, hydroxyl or carbonate) and zircon. The slate
in the Needles Gray-Wolf rocks was, very likely, formed within this range of temperatures.
In the area of Mount Olympus, the topographically highest part of the mountains,
Brandon and Calderwood (1990) identified an adjacent zone with an assemblage of minerals
that indicate higher temperature (~190 0C) and pressure (~300 MPa or 3000 kg/cm2)
conditions. Assuming the rocks have an overall density of 2,700 kg/m3, they calculated
the rocks in this zone were subducted to a depth of 11 km (6.8 miles) before they
began their upward ascent to the surface.
Mile 10.7, Stop 6. Hurricane Ridge Road (Mile Marker 10.7). This roadcut in the
Crescent Formation is dominated by a volcanic breccia and pillow basalt. It provides
an excellent display of the faulting experienced by the basalt, juxtaposing different
rock types. Note the presence of both moderately dipping and steeply dipping faults
and what appears to be large conjugate shears filled with secondary minerals.
There are two basic models for the origin of the Coast Range basalts (Crescent and
Siletz formations), a seamount/plume model (a spreading ridge reorganization model
is a variation of this) and a marginal basin model. In the seamount/plume model
a seamount chain, that formed over a mantle plume, was accreted to the continent
(Simpson and Cox, 1977; Duncan, 1982). Its variation, the spreading ridge reorganization
model, involves reorganizations of spreading on the Kula-Farallon ridge between 61
and 48 Ma resulted in Coast Range basalts erupting as seamounts and volcanic ridges
along leaky transform faults and fractures during changes in spreading directions.
The marginal basin rift model involves the outpouring of oceanic basalt during rifting
of the continental margin as a result of highly oblique motion of the Kula and Farallon
plates relative to the North American plate (Wells et al., 1984; Babcock et al.,
1992; Snaveley and Wells, 1996). A study by Chan et al. (2012) of Pb isotopes in
the 42 – 37 Ma Grays River volcanics indicates these younger Coast Range basalts
at least partly shared a common mantle source with the older (ca. 56 – 45 Ma) Crescent
Formation basalts. J. H. Tepper (written communication, 2013) suggested the Crescent
basalts may reflect a combined seamount/plume and marginal basin model, like the
model proposed by Chan et al. (2012) for the Grays River volcanics. In their model
the Grays River volcanics (MORB) erupted in a marginal basin, formed in response
to oblique subduction of the Kula-Farallon spreading ridge, while oceanic island
basalts (OIB) from a mantle plume fed into it.
Schmandt and Humpreys (2011) place the accretion (of the “Siletzia microplate”) at
~55 Ma in the early Eocene while Brandon and Vance (1992) favor a younger age of
42-24 Ma for accretion. Brandon and Vance (1992) base the time of accretion primarily
on the movement history of the Leech River fault on Vancouver Island, the only place
where the continental suture boundary of the Coast Range terrane is exposed.