Washington Department of Natural Resources Geology and Earth Resources Division Bulletin No. 72 Washington Coastal Geology between the Hoh and Quillayute Rivers Weldon W. Rau

GENERAL STATEMENT

Rock formations exposed along the Washington coast tell a fascinating story of complex geologic history. The area between the Hoh River and La Push is no exception. Data recorded in these rocks reveal how and in what environments these strata were formed, millions of years ago, as well as the severe conditions they have since undergone. Because crustal forces of the earth have dealt harshly with some of the rock formations of this area, the geologic history is garbled and can be reconstructed only by piecing together data from individual outcrops. Where the rock record has been completely erased by erosion or has been jumbled by forces within the earth beyond comprehension, geologists can only theorize, basing their concepts on knowledge of the earth's crust in other parts of the world where geologic data is less complex.

ROCK FORMATIONS OF THE EARTH'S CRUST

According to the available data, the oldest rock formations exposed along this part of the Washington coast date back no more than 50 million years. This is relatively young when compared to an age of 4.5 billion years usually calculated for the older rocks of the earth's crust in other areas of the world (fig. 1). During the past 50 million years rock formations of the Washington coast have nevertheless undergone a surprising amount of deformation and alteration.

GEOLOGIC TIME CHART (fig. 1). (click on image for an enlargement in a new window)

Rock formations exposed along the Washington coast, particularly between the Hoh and Quillayute Rivers, are largely sedimentary in origin. The sediments (silt, sand, and gravel) that make up these rocks were first deposited on a sea floor millions of years ago in the same way modern sediments, transported by streams from continents to the oceans, accumulate today on the sea floor. Much has happened to these ancient sedimentary deposits since their accumulation, not only because they have become consolidated into rock but because they are found today above sea level and as a part of the continent. Generally, most of these rock formations can be grouped into three major categories—siltstones, sandstones, and conglomerates. These basic terms simply indicate the relative grain size of the individual sediments that make up lithified sedimentary rocks. Silt refers to very fine-grained, essentially mudlike sediment, and sand refers to sandy or sugary size particles similar to sand that accumulates on a beach. Conglomerates are composed of rounded pebbles of various sizes and can be thought of as consolidated gravel. These three types of sedimentary rocks are found associated with each other usually as laminated, thin-to-thick layers or sometimes as very massive individual beds. As sediments accumulate, their weight produces STATIC PRESSURE that compacts and solidifies the deeply buried layers. Eventually, with the aid of minerals precipitated from ground water, the deeply buried sediments are lithified into formations and become a part of the earth's crust. Sediments usually accumulate in nearly horizontal layers or beds on the ocean floor. However, as can be seen in many outcrops along the coast, the orientation of bedding in sedimentary rocks is usually far from horizontal. Thus, indications are that forces within the earth, over a period of millions of years, have changed the original orientation of these beds by the gradual processes of FOLDING (bending) and FAULTING (breaking). These rock strata are therefore steeply tilted or even in overturned positions (fig. 2).

STEEPLY DIPPING, OVERTURNED, well-stratified sedimentary rocks exposed at Strawberry Point (fig. 2).

CRUSTAL FORCES

To logically account for the forces responsible for major crustal disturbances and to understand the mechanism or method this force has applied to the rocks of the earth's crust, it is necessary to rely on a relatively new geologic concept known as plate tectonics. Before this idea was developed some 15 or 20 years ago, geologists found it difficult to adequately explain the complex structural relations that are seen in the rock formations of the Washington coast and the Olympic Mountains. The application of this concept to the Olympic structures has been discussed in some detail in both professional journals and semipopular reports (Stewart, 1971; Rau, 1973, 1979; Cady, 1975; Tabor, 1975; Tabor and Cady, 1978) and therefore only a brief summary is presented here.

The term PLATE TECTONICS refers to the concept that the earth's crust is made up of a series of large, relatively rigid segments or plates (fig. 3) that essentially float on the less rigid or plastic inner part of the earth known as the MANTLE (fig. 4). Each plate may maintain a generally constant direction of motion for long, periods of time. Although movement of each plate may only be a fraction of an inch each year, over a period of millions of years, many miles of relative motion can take place. Plate motion originates along a mountainous area on the sea floor known as an OCEANIC RIDGE. The ridge, in effect, is an elongated vent or crack in the earth's crust where, as the vent opens, magma rises from below to fill the crack, cools, solidifies, and is added to crustal plates on each side of the ridge. As the process is repeated periodically over millions of year, the ocean s volcanic crustal floor appears to move or spread in opposite directions away from the oceanic ridge or crack—thus a closely related term SEA FLOOR SPREADING is often applied to this part of the plate tectonic concept. The leading edge of each plate may be hundreds of miles from the oceanic ridge. In such areas the expanding plate is not only in contact with, but is continually moving toward, another plate. To accommodate the continual expansion of the first plate, it usually forces its way beneath the second plate. Eventually rocks of the first plate reach such depths within the earth that they are converted to molten magma by high pressures and temperatures. Areas where one plate moves beneath another are known as SUBDUCTION ZONES.

THE MAJOR PLATE SYSTEM of the world. The earth's crust is believe to be made up of large, rigid plates (fig. 3) (click on image for an enlargement in a new window)

BLOCK DIAGRAM showing the basics of the plate tectonic concept. Molten magma from the mantle rises at the top oceanic ridge, cools and solidifies, continually forming a crustal plate. Hundreds to thousands of miles from the ridge the plate moves downward into the mantle at the contact with another plate and melts. The continuous process resembling a large "conveyor belt" moves the crustal plate a few centimeters each year. (fig. 4).

The nature of the force that drives the plates away from the ridge in opposite directions is not entirely known but one theory of explanation is based on convection currents. A simple analogy can be made to a boiling kettle of water where the hottest or least dense water on the bottom rises to the top and forces the cooler, heavier water to the sides of the kettle. If convection currents are functioning within the somewhat plastic inner earth, they may be causing the relatively rigid crustal plates of volcanic rock to continually move away from the oceanic ridge where lighter molten magma rises. The entire system may then be compared to a continuous conveyor belt that slowly, but over a period of millions of years, continues to move in one general direction.

During all of this time, the seas continually receive sediments that have been transported from the continents by streams. Therefore, in millions of years of time, thousands of feet of sediments (sand, silt, and gravel) can accumulate on the volcanic "conveyor belt" and become consolidated to sedimentary rocks. By the time the oceanic crustal plate reaches the subduction zone, it not only consists of a thick belt of volcanic rock but is blanketed with a considerable thickness of sedimentary strata as well.

CRUSTAL FORCES AND THE OLYMPIC PENINSULA

Scientists have found that the plate tectonic model may well be applied to the origin of many of the rock formations of the Olympic Peninsula. A northerly trending ridge is known to lie a few hundred miles off the Washington coast. It is referred to as the JUAN DE FUCA RIDGE (fig. 5). This ridge has generated the Juan de Fuca plate to the east and the adjacent part of the Pacific plate to the west. According to the concept, the Juan de Fuca plate has been colliding with the North American plate during much of this time. Although normal subduction of the volcanic part of this oceanic plate has taken place over eons of time, much of the sedimentary rock sequence has not been subducted. Instead, it is believed to have been skimmed off, foreshortened by intense folding and faulting, and accreted to the continental plate. HOH ROCKS (deep marine sandstones, siltstones, and conglomerates) of the Washington coast and the somewhat altered sedimentary rocks of the Olympic Mountains are believed to be those accreted materials.

A DIAGRAMMATIC SECTION showing how the structurally complex rocks of the Olympic Mountains and of the west coastal area may have been formed. Sediments, now lithified to rock, such as the Hoh rocks, have been carried eastward, relative to the continent, on a thick oceanic crust of volcanic rock a few centimeters a year for millions of years. Where the heavier oceanic rocks of the Juan de Fuca plate met the lighter rocks of the North American plate, most of the oceanic rocks moved beneath those of the continent and were dragged into the depths of the earth and converted to magma. However, it is believed some of the materials were not thrust under the continent but were "skimmed off," foreshortened by crumpling and successive underthrusting, piled up, and accreted to the western edge of the North American plate. The present-day Olympic Mountains and complexly folded and faulted Hoh rocks along the coast are believed to represent the rocks of this "pile" (fig. 5). (click on image for an enlargement in a new window)