Washington Department of Natural Resources Bulletin No. 62 Foraminifera, Stratigraphy, and Paleoecology of the Quinault Formation, Point Grenville-Raft River Coastal Area, Washington

ASSOCIATED ROCKS

OLDER ROCKS

The Quinault Formation rests unconformably on older rocks, commonly called the Hoh Formation. However in many places within the type area, faulting has also played an important part in bringing older rocks into juxtaposition with the Quinault Formation. Continuous seismic profile records from off the Washington coast (Snavely, P. D. Jr., U.S. Geological Survey, oral communication, 1970) substantiate such an extensive unconformity on the Continental Shelf. The actual contact can be seen in several places within the type area. It is probably best exposed in an outcrop along the coast in the NE1/4 sec. 9, T. 22 N., R. 13 W., at the end of a foot trail leading to the beach (Fig. 5, p. 9). At this point the Quinault Formation dips some 25° SE. and rests on a somewhat irregular surface that also dips gently southward. The contact is well defined by color contrast between the relatively light-colored sandy siltstone of the Quinault Formation and the very dark gray older rocks beneath. Although a certain amount of differential movement may have taken place on this surface, the undulating and low-angle nature of this contact suggests that in this area it may well be essentially unfaulted. Older rocks are exposed in the sea cliffs for a distance of about 2-1/2 miles from this point northward to within about 1/2 mile of the mouth of the Raft River. Koch (1968), in an unpublished research paper, described and discussed the rocks exposed in this area. Here, and in nearly all other places along the coast where the Quinault Formation is in association, the older rocks consist of a completely chaotic mass of nearly structureless melange. This disturbed rock unit has been referred to the Hoh Formation by Weaver (1916a). Locally, it has become known unofficially as the "Hoh Breccia" because of its completely chaotic nature. For the most part, this rock is an incompetent siltstone that appears to have undergone intensive squeezing. Almost all clasts of siltstone display slickensided surfaces. Within the general matrix of the squeezed siltstone, more resistant clasts of many different rock types occur, some boulders of which are several tens of feet in diameter. Rock types commonly included are altered andesitic volcanic rock, indurated sandstone or graywacke, indurated siltstone, conglomerate, greenstone, and other metamorphic rocks. The volcanic headlands known as "The Hogsback" and "Little Hogsback" are within this outcrop of melange and may well be large resistant blocks within a matrix which is generally of siltstone. Although the structure within these older rocks is largely chaotic, faint pseudobedding or perhaps shearing-planes are apparent locally. Within the major area of outcrop immediately south of the mouth of the Raft River this bedding (?) strikes dominantly northeast and dips southeast some 35° to 40°.

Foraminiferal assemblages from the Hoh Formation from 15 coastal localities between Cape Elizabeth and the Raft River indicate various ages ranging from late Oligocene (Zemorrian stage) to middle Miocene (Relizian stage), but most of them are early Miocene (Saucesian stage) in age.

Although Glover (1945) suggested that faulting probably had some effect on the nature of these rocks, he also implied that slumping might have been the main force that caused their chaotic condition. Weissenborn and Snavely (1968) referred to these rocks as a tectonic melange and suggested that they are similar to the so-called "Argille Scagliose" of the foothills of the northern Apennines in Italy (Sitter, 1956). As postulated for the "Argille Scagliose" Weissenborn and Snavely suggested that the chaotic condition of some of the rocks along the Washington coast may also be a result of a combination of gravity thrust and intraformational gravity sliding.

Current investigations by the writer and by Stewart (1970) in areas immediately east and north of the area of this report show that much of the lowlands and westernmost foothills of the Olympic Mountains are underlain by intensely folded and broken massive to thick-bedded graywackes, thin-bedded sandstones and siltstones, and massive siltstones that also range in age largely from late Oligocene to middle Miocene (Zemorrian stage to Relizian stage). These rocks are distinct both faunally and lithologically, as well as structurally, from rocks of the same general age, both to the north along the northern border of the Olympic Peninsula and to the south in the Grays Harbor basin and southward into Oregon, and therefore appear foreign to the general surrounding province. Melange similar to that of the Taholah-Raft River area is also present in local areas of the west-central part of the Olympic Peninsula.

Speculations on the tectonics of the Olympic Mountains are beginning to emerge, and popular among these ideas are those employing plate tectonics, for which the basic mechanism is relative eastward movement of the sea-floor materials (Stewart, 1970). Briefly, it is postulated that these materials, largely turbidite sedimentary rocks at least as young as middle Miocene age, have moved eastward against the North American continent and have been restricted possibly by the upturned buttress of the Crescent Volcanics, as well as by pre-Cenozoic rocks farther to the east. This restriction of eastward-moving oceanic materials has caused intense folding, together with successive underthrusting of these beds (Tabor and Cady, 1965; Tabor and others, 1970; Stewart, 1970), the most recent breaks and downward thrusting having taken place farthest to the west. Those disturbed rocks present along the coast in the Taholah-Raft River area may well be part of a zone, or zones, of thrusting. However, because of the apparent mobility of these rocks, it is further suggested that at least in some instances they may have been placed in juxtaposition with the Quinault Formation by piercement or diapiric methods, which in turn may have added even further to their chaotic condition. Diapiric structures are known to occur on the Continental Shelf off Washington and Vancouver Island, where they have been identified in continuous seismic profiles obtained by various petroleum companies and research groups such as the U.S. Geological Survey (P. D. Snavely Jr., written communication, 1970); University of Washington (Bennett and Grim, 1968); and by the Department of Energy, Mines, and Resources of Canada (D. L. Tiffin, oral communication, 1970). A possible onshore example can be seen along the coast between the mouth of Duck Creek and Cape Elizabeth, where a melange of older rocks is exposed for a distance of some 1,400 feet. In this area the Quinault Formation appears to be in fault contact with both the north and south ends of the outcrop of the melange. The older rocks are completely broken and structureless near both contacts, showing only faint shearing planes nearly parallel to the contacts. Large blocks of graywacke, volcanic rocks, and well-bedded sandstone and siltstone are present in a matrix of finely broken siltstone and gougelike clayey material. Structural relations and the nature and composition of the older rocks, therefore, suggest that in this area, at least, the Quinault Formation may well have been penetrated by the underlying, relatively mobile rocks of the Hoh Formation.

YOUNGER BEDS

The Quinault Formation is overlain by nearly horizontal Pleistocene(?) deposits of silt, sand, and gravel. These overlying deposits have been measured in several places and are as much as 100 feet thick. Particularly well exposed and thick beds of these deposits can be seen in the cliff a mile or so south of the town of Taholah. Inland from the coast, Pleistocene(?) deposits are extensive, and with the exception of a few outcrops of older rocks in streambeds and roadcuts, these deposits almost completely cover the Quinault Formation and older rocks. In places in secs. 10, 15, and 22, T. 22 N., R. 13 W., the Pleistocene(?) cover is relatively thin and a number of outcrops of the Quinault Formation can be seen.