Beach 4: Turbidites - submarine mass flows of sediment on the edge of the continent
Stop 3: Ruby Beach: Melange - rubble on the boundary of a fault
Hurricane Ridge: Turbidites at high elevation - extreme structure
Hurricane Hill Trail: Needles-Grey Wolf greywacke (turbidite) sediments, standing
Hurricane Road: Mile post 16.0
Sediments contorted like playdough
Hard Rock Pillows: Round balls of lava spilled out on the ocean floor. Right is
a pillow which has been split by a fault.
Fundamental to the geology of the Olympic mountains is the basalt which makes up
so much of them. This is rock created at mid ocean ridges which sea floor spreading
brings to continental margins where subduction is occurring, part of the conveyor
belt of earth surface crust with formation in the ocean and consumption beneath continents.
Basalt is also made in oceanic islands where hot spots are burning through the ocean
floor to feed volcanoes like Hawaii. Subduction wedges basalt and deep ocean sediments
into the continental margin and builds out the land mass over time using slices of
the oceanic material as construction material.
Crescent Basalt: Flows of black pillow basalt striking approximately east-west and
dipping steeply (~850) north. Pillows show that the lava was extruded onto the seafloor
where it chilled rapidly into a solidified mass. This basalt dates from 53 to 45
million years ago when it rose at a sea floor spreading center. Spreading carried
it continentward, and subduction jammed it into the margin of N. America.
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.
Turbidites are sediment accumulations at the base of the continental submarine margin
where the seabed flattens out to the deep sea floor. As these spread out onto ocean
crust, they are like material on a conveyor belt, carried into the continental margin
by subduction, so they wind up piled into the growing continental edge (Olympic Mountains).
Dr. James Aldrich explains what’s on view: the sedimentary layers bent into folds
Squeezing of the sediments as they are rafted into the continental margin warps the
initially flat layers into folds which in the extreme bend back on themselves, making
some layers lie upside-down (think of a rug being shoved against a wall).
How can you tell if sediment layers (beds) are upside down? Since they are deposited
progressively, look for features that indicate direction of deposition. Erosion
surfaces are good because something has to be deposited first before it can be eroded.
An erosion surface below the deposit it is truncating means reversed bedding.
Faults: Rock layers under stress can break along planes and then slide along those
faults. This happened on large and small scales.
Unconformity: A boundary between rock units representing the passage of time. In
this case Pleistocene age sediments (up to a couple of million years old) sit on
an erosion surface with sediment layers which are around 20 million years old below.
Faults are surfaces along which masses of rock slide past one another. These aren’t
clean, flat, planes, but zones of breakage under great pressure. As you can imagine,
there is a lot of rubble generated as the rocks slip past one another. That rubble
Trace the layers on any of these pictures. The sedimentary beds have been deformed
like playdo squeezed between two blocks. The ‘rocks and a hard place’ are the ocean
crust seaward which is rafting into the continental margin, and the interior of the
continent. This material was scraped off the ocean floor and jammed into the continent,
building out the margin and the Olympic Mountains in the process.
Stop 6: what was seafloor underlying all the sediments, pillow basalts carved by
Field trip Extra: Beach Four Deformation of Pleistocene Gravels (recent tectonics)
Courtesy of Carol Serdar
These younger deposits date from Ice Age times and have been deformed by faults and
earthquakes. The picture below shows a cobble bed which has been folded upwards
and then wrapped towards the viewer, so the cobbles go from flat and horizontal to
vertical and turned towards the viewer.
This image shows drag folds imposed on the gravel beds giving an ‘s’ shape form to
the sediment layers.