Rock Formations and Their Geologic History
Weaver (1937) originally applied the term Hoh Formation to many of the rocks found throughout much of the westernmost part of the Olympic Peninsula. Subsequent studies have found that these rocks not only are varied lithologically but are known to range greatly in age. Furthermore, they have undergone various amounts of tectonism from moderate folding and faulting to intense shearing and disarrangement of individual strata. Because these rocks are not a single mappable unit with a definable top and base, present-day workers no longer formally regard Weaver's rock assemblage as a formation.
Attempts have been made to subdivide these rocks into smaller mappable units. Glover (1940), for example, suggested that some of these rocks, largely the sandstone beds exposed in the vicinity of Browns Point and Destruction Island, be separated from Weaver's Hoh Formation and be referred to as the Browns Point Formation. However, recent studies of the extent of these rocks inland reveal that satisfactory boundaries are not easily mapped and, therefore, even these rocks do not constitute a well-defined formation.
Thus, in conformity with present-day usage of the term formation, the rocks that Weaver originally assigned to his Hoh Formation will be informally referred to in this report as the "Hoh rock assemblage."
Two major groups of rocks make up the Hoh rock assemblage between Point Grenville and the Hoh River. One group is a highly folded, steeply tilted sandstone and siltstone sequence. Good exposures of these rocks can be seen in the Browns Point-Starfish Point area (figs. 7 and 64). Although broken in places by faulting, generally they constitute a coherent sequence of sandstone and siltstone strata. The other group of rocks is called a "tectonic melange." These rocks are distinct from the first group in that they are a chaotic assemblage of siltstone, sandstone, conglomerate, and volcanic material. Such rocks are exposed in a number of places along the coast hut are well exemplified in the cliffs for a distance of about 212 miles immediately south of Raft River in the Hogsbacks area (fig. 4).
Although fossils are extremely rare in Hoh rocks, microfossils (Foraminifera) have been found in scattered places. Paleontologists have concluded that almost all collections represent an early to middle part of the Miocene Epoch and the containing rocks are therefore some 15 to 22 million years old (fig. 2). Rare isolated collections may be slightly oldera late part of the Oligocene Epoch, in which case they may be as old as 25 to 30 million years.
The major collections of megafossils from Hoh rocks have come from the north side of the Hoh River about 1-1/2 miles upstream from the mouth. Some of the more commonly occurring species from there were identified by Dr. Warren O. Addicott of the U.S. Geological Survey, and are listed below:
According to Dr. Addicott, these fossils also indicate an early or middle Miocene age. They further suggest a relatively shallow-water marine environment, possibly at water depths between 30 and 100 feet.
Many of the sandstone and siltstone strata of the Hoh rock assemblage, as typified by the rocks of the headlands and coves of the Browns Point-Starfish Point area, are known as turbidites, a type of sedimentary rock with a rather special and interesting depositional history. Although these beds are now steeply dipping and in many places are even overturned, originally they were deposited horizontally, or nearly so, on the sea floor as layers of sediments. They have since been lithified by pressure and mineralization. Furthermore, lateral forces applied over many millions of years on the earth's crust, probably have moved these rocks relatively eastward to their present place along our coast where they are seen today in a nearly vertical and, in places, overturned position.
As the term suggests, turbidites are sediments that originally settled out from turbid or muddy water; thus, a definite gradation of particle size usually occurs in each sequence of deposition. Each sequence probably was deposited in a relatively short period of time. The source of sediment for these deposits is believed to have been from the edge of an ocean basin, perhaps from the outer slope of some ancient continental shelf. Due possibly to earthquakes or the overweighting of a slope with sediments, large masses of unstable sediments were suddenly dislodged and moved downslope much like a landslide, except that many of these materials were placed in suspension in water to form a very large "cloud." Almost immediately redeposition or settling out of these sediments began to take place in a deeper part of the ocean basin. The coarsest materials were deposited first and nearest to their source. Therefore, these sediments formed the base of each sequence of deposition. The very coarsest materials probably slid along the sea floor somewhat lubricated by water, perhaps more than actually being completely suspended. Typical examples of such coarse grained beds form the main and prominent headland masses of both Browns Point and Starfish Point (fig. 65). These deposits of coarse-grained sand, grit, and, in places, even conglomerate are technically called "graywackes," because many of the individual grains or clasts are fragments of other rocks rather than individual mineral grains. In many of the graywacke sandstones seen along the coast, angular fragments of various sizes of dark-gray siltstone occur in a light-colored sand matrix. Most likely these siltstone fragments were ripped from siltstone beds on the sea floor as the turbidite "cloud" passed over. They then became incorporated in the coarse-grained part of the turbidite sequence.
Following the initial deposition of the coarse material of each turbidite sequence, fine-grained material gradually began to fall out of suspension. In many places, therefore, the strata display a grading of grain size from coarse-grained sandstone to fine-grained siltstone. The thickness of each sequence varies from that of a few inches to many feet. Frequently, a series of rhythmically bedded thin sandstone and siltstone strata were formed, some of which represent many small graded sandstone-to-siltstone sequences. Others may be the result of pulsating currents that intermittently supplied coarse-grained material (sand) for a short period of time to an area otherwise normally receiving finer materials (silt and clay particles). Such rhythmically bedded strata can be seen in a steeply dipping position on the beach in the vicinity and at the foot of Beach Trail 4 (figs. 7 and 8).
Aside from the graywacke sandstone and interbedded finer siltstone beds, substantial thicknesses of massive siltstone may also occur in association with turbidite deposition. These deposits most likely represent relatively quiet and uniform periods of deposition, during which time only fine-grained particles settled out of suspension. Such fine-grained deposits are usually widespread, but because they are less resistant to erosion, they are more likely to be eroded and covered by debris, such as soil, sand, gravel, and vegetation. The low-lying cliffs and sand-covered beaches of many of the coves along the coast are underlain by such siltstone beds.
Well-formed structures of sedimentation are excellently preserved in some turbidite sequences. "Flamelike" structures and "convolute" or contorted bedding are examples (fig. 9). These features were formed when unconsolidated sediments were either squeezed or wrinkled into overlying beds. Subsequent lateral movement of the entire bed, while still relatively soft, has bent these features mostly in the same direction. Other features are casts of "flutes" or small channels that were formed on the surface of a one-time muddy sea bottom. They are now preserved on the underside of what was an overlying sand bed. Such flute casts are common in turbidite sediments and are some of the features that not only indicate that the beds at Browns Point have been rotated beyond vertical to a slightly overturned position but also indicate the direction of currents during their formation. (Suggested technical reading on turbidite sedimentation: Bouma and Brouwer, 1964).
Certain outcrops of the Hoh rock assemblage consist of chaotically mixed resistant blocks of rock set in a fine-grained matrix of softer rock materials much like peanuts in a "chunky-style" peanut butter (fig. 4). Locally these deposits are known as smell muds, because an odor of petroleum frequently can be detected. Technically these mixtures of rock are referred to as "tectonic melange" (ma'lä nzh), implying that the jumbled condition is believed to be the result of major deformation of the earth's crust. Such materials (see cover photo) are well exposed in the cliffs for about 2-1/2 miles along the Hogsback area (fig. 45). Here the hard blocks of rock ranging greatly in size consist variously of conglomerates, sandstones, well-stratified sandstone and siltstone, and, in places, altered volcanics. The softer, finer grained materials of the matrix are mostly clays and badly broken siltstone. The slumped condition of melange deposits in the Hogsbacks area, as well as in other coastal areas attests to the extreme structural weakness of such deposits. This weakness is due largely to the expanding nature of many of the clay minerals after they have become moistened. Therefore, the periodic wetting of the base of the cliffs by ocean waves, together with much precipitation, makes coastal outcrops of the melange materials particularly vulnerable to slumping. Similar expanding materials have been encountered in many of the petroleum test wells drilled in the Ocean City area a few miles to the south, and in areas offshore. There, such deposits were referred to as "heaving shale." Because these materials tend to expand, the drill hole was often reduced in size or completely closed.
Some idea of the great amount of force involved in the formation of jumbled deposits of a tectonic melange can be realized by noting the very large size of some of the resistant blocks incorporated in these deposits. A group of these blocks, some larger than houses, now rest on the beach at Boulder Point (fig. 10 and cover photo), where they were left after the fine-grained materials of the melange were eroded away by the sea. Furthermore, the promontories of Little Hogsback (fig. 49) and Hogsback (fig. 5), as well as the offshore rocks of Split Rock (fig. 48) and Willoughby Rock (fig. 34), are all huge resistant blocks, largely of volcanic origin, that were once incorporated in the melange but now are erosional remnants along the coast resting on the Continental Shelf.
Microfossils from some of the siltstones of the tectonic melange are known to have lived during early to middle Miocene time, some 15 to 22 million years ago (fig. 2). These fossils further indicate a deep marine depositional environment where temperatures were cold. Therefore, melange siltstones in which these fossils are found today were originally deposited as marine silts in an ocean basin somewhere below depths of the outer edge of a continental shelf, such as exists today well off the coast of Washington.
The complicated structural patterns so apparent in many of the coastal outcrops of Hoh rocks no doubt have aroused the curiosity of many people. What conditions were these rocks subjected to in order to produce the chaotic mixtures of materials such as those in the cliffs of the Hogsbacks area? What forces were involved to tightly fold and place coherent sequences of sedimentary rocks in nearly vertical or overturned positions like those at Browns Point and on Destruction Island? Geologists' answers to these questions are not completely without reservations. However, geophysical and geologic studies of ocean floors in recent years have provided new fundamental concepts about the earth's crust. These theories, scientists now believe, are basic for logical explanations to many of the structural relations of the Hoh rock assemblage of the Washington coast, as well as the rocks of much of the Olympic Mountains.
Most early attempts to explain structural conditions of the Hoh rocks, particularly those of the jumbled melange rocks, were based simply on landsliding or slumping of structurally weak siltstone beds. This, however, did not adequately explain such problems as the large "exotic" boulders or blocks incorporated in the siltstone.
In recent years, Weissenborn and Snavely (1968) and Koch (1968), by using the same basic mechanism of gravity sliding, emphasized the magnitude of the force involved by suggesting that it took place on a tectonic or major crustal scale. They pointed out that these rocks may have been formed by gravity sliding of very large masses, possibly in combination with gravity thrusting or overriding of large blocks of rock in a submarine environment. Stewart (1971) suggested that melange deposits may represent major faulting and are materials brought together in shear zones created by differential movement between major segments of the earth's crust. Variations of the later concept are largely with respect to the specific type of faulting involved. Regardless of the mechanism of motion, it is now generally accepted that the chaotic condition of these rocks reflects major tectonism or movements of the earth's crust.
How and Why
Plate TectonicsSea Floor Spreading
Speculations on how, and to some extent why, these forces of tectonism were generated is also of interest to geologists. The new data about the ocean floor has aided greatly in the formulation of theories for possible answers to these questions. The relatively new concept known as "plate tectonics" is particularly significant. It generally maintains that the earth's outer crust consists of a series of large, somewhat mobile, plates (Matthews, 1973). This idea has evolved from a nearly forgotten theory proposed some sixty years ago and known as Continental Drift. Because the configuration of some of the continents appears as though they could be fitted together, for example, the east coast of South America and the west coast of Africa, some workers thought that the continents were together at one time but have since drifted apart. It was not until recent studies were made of the floor of the ocean basins that a mechanism was discovered explaining why continents may have moved apart. The available data of today strongly indicates that over millions of years sea floors have spread or expanded at various rates between one-half to over 2-1/4 inches a year in each direction from central ridges. The ridge in the Pacific extends in a northerly direction through much of the main part of the South Pacific and is known as the east Pacific Ridge (National Geographic Society, 1969). The axis of this and all ridges is a major fissure or crack in the earth's crust where volcanic materials are extruded and accreted periodically to the crust or sea floor, thus forcing the existing floor away both to the east and west from the ridge. Therefore, in a given plate, the farther away an individual volcanic unit is now from the ridge, the longer ago that unit was extruded from the ridge, and the older the unit should be.
Where two plates come in contact with each other from opposite directions, it is believed that the heaviest, usually the oceanic plate, underthrusts or slides beneath the other and the materials of the former are once again placed slowly back into the depths of the earth to become magma.
With respect to our Pacific Northwest, in the ocean basin some 250 miles off the Washington coast a ridge exists that, although offset considerably to the west, corresponds to a segment of the east Pacific Ridge. It is known as the Juan de Fuca Ridge (fig. 11). The present-day contact between the Juan de Fuca plate and the North American plate is believed to be immediately west of the Continental Shelf of the Pacific Northwest. However, some workers speculate that in the geologic past this contact was farther to the east relatively, perhaps somewhere between the present-day Olympic Peninsula and the Cascade range. During the millions of years that volcanic rock of the oceanic crust has moved away from the Juan de Fuca Ridge, probably at a rate of slightly more than 1 inch a year, marine sediments, such as the deep-water siltstones and sandstones of the Hoh rocks, were deposited and carried on the volcanic floor eastward toward the outer edge of the Juan de Fuca plate. Miocene rocks as young as 15 to 22 million years of age (Hoh rocks) are believed to have moved at least as far eastward as our present-day coast. Older rocks of the Eocene Epoch (36 to 55 million years of age) predominate even farther east in the eastern part of the Olympic Mountains. Therefore, according to this concept, the extensively folded rocks of the coastal area and the Olympic Mountains are believed to be sedimentary and volcanic rocks of the Juan de Fuca plate that were not thrust under the North American plate but instead were "skimmed off' the oceanic crust, foreshortened by crumpling and successive underthrusting, piled up, and accreted to the western edge of the North American plate. The present-day Olympic Mountains are believed to represent much of this "pile." Erosion by wave action has since beveled off the western part of this pile of complexly folded and sheared rock, leaving the Continental Shelf and the low-lying western coastal area of the Olympic Peninsula.
Listed below are selected references on plate tectonics and other subjects related to geologic structure along the Washington coast: Atwater, 1970; Dewey, 1972; Dewey and Bird, 1970; Dietz and Holden, 1970; McKee, 1972, p. 164-169; Phinney, 1968; Schiller, 1971; Silver, 1971; Stewart, 1970, 1971; Tabor, Cady, and Yeats, 1970.
The extremely distorted condition of some Hoh melange rocks may be the result of having been squeezed into, and in some places through, overlying strata of younger formations. Certain rocks when charged with liquids and gases under high pressure are known to have the ability to flow much like plastic. Therefore, where they are overloaded by another more rigid formation, the plastic material can be squeezed upward into areas of weakness in overlying, more rigid formations like putty into a crack. In so doing, the surrounding areas of more rigid materials become bent or dragged upward. In some cases, the plastic materials do not break completely through the overlying beds but simply dome them upward. Such structures are known to exist in a number of places in the world. For example, in the Gulf of Mexico coastal area, salt is the plastic material that is forced upward in the form of a "plug," thus producing salt domes.
The local occurrence of piercement structures is strongly supported by continuous seismic profiles recorded by various research groups in recent years over the Continental Shelf of Washington, Oregon and Vancouver Island. (Grim and Bennett, 1969; Snavely and MacLeod, 1971; Tiffin, Cameron, and Murray, 1972). Such records generally show, in cross section, geologic structures beneath traverse lines. In records from off the Washington coast such structures are well defined (fig. 12).
One of the better onshore examples of a possible piercement structure can be seen in the cliffs along the beach about three-quarters of a mile south of the mouth of Duck Creek and extending for one-quarter of a mile southward (fig. 29). There, dark-gray melange rocks are believed to have penetrated into the overlying lighter colored Quinault Formation (fig. 53).
Last Updated: 28-Mar-2006