Olympic Mountains Field Trip

Guide Stops

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 similar fold.  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.