By Dave Tucker
With thanks to Dr. Chris Suczek, WWU Geology Department, for reviewing the geology. Any errors are mine.
Beach 4, north of Kalaloch on the wild coast of Olympic National Park, rewards the geologically-minded with some outstanding geology. The well-exposed, tightly-folded rocks are very unusual for western Washington. Also, with a little effort and knowledge, you can easily determine that some of the rock layers are completely upside down. As a bonus, there is an excellent example of an angular unconformity. This trip is a sneak preview excerpt of a chapter in my upcoming book, Geology Underfoot in Western Washington, to be published by Mountain Press Publishers. Other titles in the series are here.
Beach 4 is on the wild Pacific shore of Olympic National Park, 2.7 miles (4.3 km) north of Kalaloch Campground. From the south, drive 72 miles (115 km) north from Hoquiam on US 101. From the north, drive west on US 101 from Port Angeles for 76 miles (122 km) to reach the Forks, then continue south another 30 miles (48 km) on 101. The well-signed turn into the paved Beach 4 parking lot is halfway between mile posts 161 and 160 on US 101. The parking lot has restrooms and interpretive signs.
From the parking lot, a trail descends an easy 100 feet along a small stream, reaching the beach and Stop 1 in 200 yards (meters). Unless the tide is high, walk another 200 yards (meters) or so up the beach to the north to reach Stop 2. It is possible to walk further along this gorgeous beach for miles to the south or north; popular Ruby Beach is about 4 miles (6.4 km) to the north. Be very careful you don’t get trapped against the coastal bluff by high tide and surf. Always consult a tide chart before setting out. Never try to go far if the tide is rising.
Beach 4 provides textbook examples of folded rocks and overturned sedimentary rock layers, two kinds of unconformities, and an uplifted wave-cut bench. It provides easy access to miles of wonderful beach strolling with crashing surf.
From the trail head at the parking lot, the main trail descends to the beach. First, walk out the right trail fork for 50 yards to a fine overlook above the beach bluffs, and a chance to check the tide level and surf situation. Flat-topped Destruction Island lies 3 ½ miles (5.6 km) to the northeast. Once part of the coast, it is now isolated by coastal subsidence and erosion. Take the main trail downhill 200 yards (meters) to the beach. Stop 1 is the outcrop just north of the foot of the Beach 4 trail. Here is a fine example of an angular unconformity. The steeply tilted sedimentary rock layers rising out of the sandy beach were originally deposited as close-set horizontally stratified layers of sediment on the sea floor. The grains in the sediment were compressed and cemented together, or lithified, by pressure within the crust and slowly became rock. Later, the layers were compressed, folded, and in many cases fractured to bits as they were subducted under North America. The rocks were then uplifted to sea level, where wave erosion could plane them off; this truncated the beds. The eroded rocks were later covered by flat-lying young sedimentary beds, which have yet to be lithified. It’s what we don’t see that is a major focus of this vignette, represented by that angular contact between the dipping rocks and the gravel. Herein lies a lot of geology!
An unconformity is a time gap in the geologic record- rocks of very different ages are in contact with each other. The contact between the vertical and the horizontal layers at Stop 1 is an angular unconformity. This means that two rock formations, each with layered beds dipping at different angles, are separated by an erosional surface truncating the lower rocks. For this to occur, compression or faulting must first fold or otherwise tilt the lower rocks. Then, sufficient time must pass to allow erosion to bevel off the rock layers and strip off any that may have once lain higher in the geologic sequence, before younger sediment is deposited on the erosion surface. An angular unconformity of necessity represents a great gap in time. Consequently, the single small rock outcrop at Beach 4 encapsulates a lot of geology, though much of it is necessarily missing due to erosion.
You may find references that call the older sedimentary rocks along the Olympic coast the ‘Hoh Formation’. Geologists use the term ‘formation’ for rocks that are contemporaneous, distinctive from others in the area, related in their depositional environments, and that are extensive enough to be mapped out on the surface or traceable in the subsurface by geophysical detection methods. The various sedimentary rock units along the Olympic Coast are a mishmash of different-looking rocks of widely varying ages and degrees of deformation. Some are highly contorted and fragmented; others, like the rocks at Beach 4, are folded but not much contorted. None of the ‘Hoh’ rocks are laterally continuous enough to be practically mapped as distinct units over any distance. Consequently, the steeply-dipping rocks at Beach 4, and other different looking sedimentary rocks up and down the Olympic National Park coastal strip, cannot be considered a formation any longer and have been lumped into a catchall group, ‘the Olympic Subduction Complex’ (OSC). This name recognizes a common historic thread- all are marine rocks deposited somewhere off the coast, and all were subducted beneath North America to some degree.
Turbidity currents and turbidites
At Beach 4, the tilted OSC rocks are Miocene-aged turbidites deposited between 16 and 24 million years ago. Turbidites are sedimentary rocks resulting from lithification of sediment which flowed across a sea or lake bottom; the submarine OSC turbidites are now sandstone and shale. Turbidites have a distinctive repetitive pattern of beds that result from repeated flows, or ‘turbidity currents ‘, of suspended mud, sand, and even gravel flowing on and above the seafloor. These currents, rich in suspended sediment, may be derived from rivers or debris flows entering the sea, or submarine landslides from the continental shelf moving into deeper water. When turbidity currents first begin to slow down, coarse grained sand and gravel particles settle onto the bottom quickly. Fine grained mud remains suspended in the water, settling much more slowly over the coarser sediment. Mixed with water and driven by gravity, the clouds of really fine-grained mud typically form a density current that can sweep onward over the seafloor at high velocities for great distances. Fine grained suspended sediment settles slowly. Sediment layers consisting of coarser particles at the base and finer particles at the top are said to be ‘graded’ in a ‘fining upward’ sequence. Repeated couplets of coarse and fine sedimentary layers are the hallmark of turbidites. At stop 1, these graded turbidite beds form couplets of buff sandstone and darker gray shale. Find a place where a buff-colored layer is sandwiched between two darker layers. Look very closely (use your ever-present hand lens) to see that it is made of sand-sized grains. If the contact, or boundary, between this sandstone and a darker shale layer is very sharp, then you are looking at shale from one turbidite couplet, and sandstone from an overlying one. A single graded turbidite bed will consist of sandstone grading to shale; observe carefully that the sandstone–shale contact is actually diffuse, reflecting the gradation during a single event. Be careful here: most of the beds along this beach are upside down, so that the deposit of an older turbidity current lies above of a younger one. The tipped OCS rocks at Beach 4 record scores of turbidity events, seen in the many repetitive layers of alternating fine and coarser grained beds. Their sheer numbers indicate that unstable slopes persisted for a long time off the continental coast where the sediment was deposited.
The sediment that becomes this type of rock is typically deposited seaward of the margin of continental shelves, in deep ocean basins. The fossils preserved in OSC rocks shows that sedimentation occurred in water depths exceeding 2000 meters (6600 feet) west of the margin of the North American plate as it overrides the oceanic Juan de Fuca plate about 80 miles (130 km) off the Olympic Coast. A blanket of sediment about 1 ½ miles (2.4 km) thick lies on top of the eastward-migrating basalt slab of the Juan de Fuca plate. In a process that continues today, this thick sedimentary blanket is crushed against the North America plate at the plate boundary and is scraped off to form an accretionary wedge along the outer edge of North America. Accretion of more seafloor sediment to the western margin of the wedge shoves the previously accreted sediment further to the east. Burial beneath the sediment blanket and pressure from the plate collision is the main way that the sediment is lithified to rock. The lateral pressure crumples the lithifying sediment and fractures the sedimentary rocks, as well, rumpling them like a throw rug your dog made a high-speed turn on. The chaotic jumble of the Miocene part of this wedge, now exposed along the Olympic coast, is the OCS. The tilted rocks at Beach 4 have migrated laterally 80 miles (130 km) or so from the subduction margin off the coast, but have neither been subducted nor uplifted very far. The turbidite layers were so deformed that they have been overturned beyond vertical in places- the youngest tilted rocks at Stop 1 are the ones nearest the sea.
The rocks at Beach 4 were gradually uplifted until they are now just above sea level due to a combination of downward-directed erosion removing the uppermost rocks and sediment on the North America plate, and lateral and slightly upward-directed migration of rocks from within the accretionary wedge. Once the rocks rose to sea level, they were subject to wave erosion at the shoreline. A calculation based on the area of the wedge and the steady, uniform movement of the two plates indicates that it took around 22 million years for rocks to migrate to the coast line, 80 miles east of the plate boundary. This estimate independently agrees with the age of these rocks.
Rock boring clams
The tilted sedimentary strata at Beach 4 are pitted with shallow holes a little above the high tide line. These unoccupied holes are rarely more than an inch or so deep, and 1- 2 inches (2.5- 5 cm) across. They are the remains of deeper burrows of a rock-boring clam, Penitella penita, also known as piddock clams. When these clams are young and small, they begin to burrow into soft rocks, rather than into mud on the seafloor, as most clams do. A clam attaches its fleshy foot to the rock, and rocks the hard edges of the two halves of its shell back and forth to grind away grain by grain. The resulting pit becomes deeper; as the clam grows, the cavity widens within the rock, and the clam becomes trapped in its own hole. The clam, which may grow to 3 inches (7.5 cm) in length, may end up at the bottom of a pit 6 inches (15 cm) deep. These clams either live in the lower part of the intertidal zone or entirely subtidally. Borings above sea level are further evidence for uplift and erosion of these rocks.
The Angular Unconformity:
The OSC turbidites were truncated by waves around 122,000 years ago (see the photograph at the top of this field trip). At Beach 4, this wave-cut bench is about 9 feet above today’s mean sea level. The bench can be traced for 50 miles along the coast. In places it dips below sea level or is uplifted as much as 160 feet (49 m) above sea level south of Kalaloch. The crudely stratified sediment above the truncated top of the tilted turbidites, and extending to the top of the bluff is stratified sand, gravel and peat of the Pleistocene Browns Point Formation. This sediment is outwash deposited by streams flowing from now-vanished large glaciers in the Olympic Mountains during Pleistocene ice advances. Radiocarbon ages collected from the Browns Point Formation range from over 47,000 years old at the bottom to about 16,700 years ago near the top. The contact between the OCS turbidites and the Browns Point Formation is an angular unconformity. The missing time represented by the unconformity’s surface is the difference between the minimum and maximum ages of the two sedimentary packages– around 75,000 years. This is but an eye blink compared to Siccar Point’s 80 million, or the Great Unconformity at Grand Canyon, which represents 250 million years or more of missing time.
Folded and overturned rocks
Walk north along the beach from the foot of the trail. Pay attention to the changing dip of the turbidite beds. In places they are nearly horizontal, but entirely upside down, overturned during folding. This is not immediately obvious except by very carefully examining the size of the grains in individual layers and also the contact between sandstone and adjacent mudstone beds. Since these beds result from deposition of pulses of sand along the seafloor, followed by the settling of a ‘cloud’ of finer grained sediment, each individual bed consists of a gradation of grain sizes, from coarser at the bottom to finer at the top. Also, each sudden pulse of sand across the seafloor has a sharp, and usually planar, contact with the slightly older mudstone it flowed over. By looking carefully at the features in these beds, the geologist will see which way was originally ‘up’ and younger in the sequence, even if the whole sequence is now upside down.
You’ll find a real treat at Stop 2, about 200 yards (meters) north of the trail. The base of the bluff, where it meets the top of the beach, has a wonderful sequence of several chevron-shaped folds. If you are navigating by GPS, this place is at about N 47°39.111, W124°23.377. When I was there, beach sand was piled at the base of the Miocene rock, so the exposed folds were only 3 feet (1 meter) high. Clean, visible folds like these are a rarity in western Washington, and are alone worth the pilgrimage.
A short distance before the rocky point at the north end of the beach (locally known as “Starfish Point’) you may see a different breed of unconformity. In places, the turbidite sandstone beds are horizontal, whether flipped upside down or not, and are overlain by the Browns Point sand and gravel, also horizontal. However, there is still a time gap, called a disconformity in this case. This term is applied when successive geologic layers are parallel, but still of different ages. A disconformity between two similar-looking rock formations might be overlooked by a geologist until detailed observations elsewhere revealed the fact that erosion had made a time gap between them. Here, the difference is obvious because of the very different looking OSC turbidite rocks and the obviously younger, unlithified Browns Point sand and gravel.
Your stroll along the beach reaches Starfish Point less than 400 yards (meters) from the trail and Stop 1. Beyond the point, the beach continues north 4 miles (6.4 km) to Ruby Beach, passing one or two other access trails along the way. If you choose to continue, check a tide table first to reduce your chance of getting caught by an incoming high tide.
The sea stack at Starfish Point consists of relatively erosion resistant turbidite beds. It is a remnant of the coastline when it was a few hundred yards further west than today. The implacable ocean is eroding the point to fragments; eventually the sea stack will disappear altogether. To the northwest, flat-topped Destruction Island, about 3 ½ mile offshore is a remnant of when the coast was even further west, during the great relative lowering of global sea level during the Pleistocene glaciations. The same wave cut unconformity seen at Stop 1 forms the island’s surface. Destruction is the first island of any size north of the Farallons (west of San Francisco) and the largest off the Pacific coast of Washington and Oregon. Seastacks become increasingly common further north along the wilderness coast of Olympic National Park; some of them are several hundred feet high. All will eventually be worn away, replaced by new ones as the hungry ocean eats into rock of the coast.