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

REGIONAL CONSIDERATIONS, STRUCTURAL AND STRATIGRAPHIC

Arnold (1906) described the outcrop area of the Quinault Formation as a great syncline between Cape Elizabeth and Point Grenville. Weaver (1916a), in the same area, also mapped a syncline, the axis of which extends northeastward about one-half mile south of Taholah. In 1937 he stated that the Quinault Formation lies within a shallow syncline complicated by numerous small fractures and normal faults. The gross structural configuration implied by the attitudes within the coastal outcrops of the Quinault Formation is that of a syncline, the axis of which lies somewhere in the vicinity of Taholah. However, it would appear that this gross feature may be modified by faulting somewhat more than some of the earlier workers suggest.

The Duck Creek-Pratt Cliff section, a part of the north limb of this apparent synclinal feature, based on continuity and uniformity of direction of dip, most likely constitutes a reliable, essentially continuous section (Fig. 5). Furthermore, its base most likely represents one of the lowest known horizons in the outcrop area of the Quinault Formation, because it rests with apparent unconformity on older rocks (Hoh Formation). The only other possible place along the coast that may represent a basal horizon in the formation is immediately north of the mouth of the Quinault River. However, at this point there is no appreciable section above. The Duck Creek-Pratt Cliff section, therefore, most likely represents some 4,600 feet of essentially continuous section. Those beds exposed in the vicinity of both sides of the mouth of Duck Creek constitute the upper part of the section.

The Cape Elizabeth section (Fig. 4, p. 8), on the basis of continuity and uniformity of structure, also represents an essentially continuous section, which also is a part of the north limb of this synclinal structure. However, because this section is bounded at both the bottom and top by faults, its stratigraphic relation to the Duck Creek-Pratt Cliff section is uncertain. Both lithologically and faunally there appears to be no correlation between the two sections, in that the Duck Creek-Pratt Cliff section generally suggests an offshore marine environment, whereas the Cape Elizabeth section appears to represent a nearshore, possibly estuarine, and, in some places, even nonmarine environment. Most of the very few foraminiferal species found in the Cape Elizabeth section are either rare or absent in the Duck Creek-Pratt Cliff section. Aside from displacement that may have taken place on the faulting at the base of the Cape Elizabeth section regionally the apparent updip position of the Cape Elizabeth section with respect to the Duck Creek-Pratt Cliff section suggests that the Cape Elizabeth section is the younger of the two. This relation is substantiated lithologically, and to some extent faunally, by subsurface records from onshore and offshore wells drilled mainly to the south in the Ocean City area. Coarse clastic rocks, such as are present in the Cape Elizabeth section are encountered in these wells above finer grained sedimentary rocks that contain Foraminifera similar to those of the Duck Creek-Pratt Cliff section. The available evidence, therefore, although not conclusive, suggests that the Cape Elizabeth section constitutes an upper part of the Quinault Formation and is stratigraphically above the entire Duck Creek-Pratt Cliff section (Fig. 6).

FIGURE 6.—Suggested correlations of the four measured sections of the Quinault Formation.

The section south of Taholah dips generally north and northeast, therefore suggesting that it represents a part of the south limb of the supposed regional downwarp. Disregarding possible structural complications, it would therefore be expected that this section could be correlated with some part of the north limb, particularly the Cape Elizabeth section. However, there appears to be no correlation, either lithologically or faunally. The section south of Taholah is basically rather massive, fossiliferous siltstone. It contains Foraminifera that suggest substantial water depths during deposition, and therefore is totally unlike the conglomerate and sandstone and meager shallow-water fauna of the Cape Elizabeth section. The beds on the north side of the Quinault River immediately north of Taholah and the mouth of the river (Fig. 3, p. 7), although mostly slumped, are essentially identical both faunally and lithologically to the section south of Taholah. Therefore, it is suggested that the massive siltstone section south of Taholah may continue northward beneath alluvial cover to this point north of the river, where it rests on older rocks but is separated from the Cape Elizabeth section by a fault with major displacement a few hundred feet to the west (Fig. 4, p. 8). Parts of the Duck Creek-Pratt Cliff section are reasonably similar to the section south of Taholah. Generally the rocks of the latter section are more massive and finer grained, and some differences are present also in the faunas. However, a thickness estimated to be greater than the total section south of Taholah is inaccessible at Pratt Cliff for sampling and examination. It is possible, therefore, that the section south of Taholah represents a stratigraphic position similar to some part of the Duck Creek-Pratt Cliff section, but conclusive evidence is lacking at this time.

The section north of Point Grenville, as shown in Figure 2, p. 6, also dips generally northeast, and therefore would appear to lie below the section south of Taholah. However, for a distance of about three-quarters of a mile between the two sections, the Quinault Formation is either covered with Pleistocene(?) gravel or is badly slumped, and therefore the relation between the two sections cannot be definitely determined. As pointed out previously, both the north end (top) and the south end (base) of the Point Grenville section are probably terminated by eastward-trending faults, older rocks being in juxtaposition on the south end and some other part of the Quinault Formation on the north. From this evidence it appears that the Point Grenville section is isolated and probably is not continuous with and therefore not necessarily stratigraphically below the section south of Taholah.

Lithologically, the Point Grenville section is unique mainly because of the numerous large sedimentary structures. It does not particularly compare either faunally or lithologically with the Cape Elizabeth section, although both have elements of shallow-water deposition. Any probable correlation of these sections would most likely be confined to a lower sandstone part of the Cape Elizabeth section and all or part of the Point Grenville section (Fig. 6). None of the Duck Creek-Pratt Cliff section is particularly similar, either lithologically or faunally, to the Point Grenville section. Furthermore, the siltstone section south of Taholah is also lithologically unlike the sandstone of the Point Grenville section, but faunally they are similar.

Because the Point Grenville section is isolated and lithologically distinct from all other sections, it is not possible to make a definite correlation of these beds with the other sections. However, faunal similarities suggest that deposition of the section north of Point Grenville could have been penecontemporaneous, in part, and (or) possibly later than that of the section south of Taholah.