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Bulletin No. 62

Foraminifera, Stratigraphy, and Paleoecology of the Quinault Formation, Point Grenville-Raft River Coastal Area, Washington




One new species and fifty-six additional species of Foraminifera from the Quinault Formation are illustrated and their significance discussed. Assemblages were collected from 101 localities in 4 measured sections and in isolated outcrops exposed along the Washington coast between Point Grenville and the vicinity of the Raft River.

The total stratigraphic thickness of the heds included may be as much as 6,200 feet, although relations between sections are not known precisely. The Duck Creek-Pratt Cliff section alone, composed of siltstone and fine-grained sandstone, is some 4,600 feet thick. Sandstone and conglomerate of the Cape Elizabeth section are believed to lie stratigraphically above, and they constitute some 1,600 feet of thickness. The siltstone section south of Taholah and the sandstone section north of Point Grenville both may correlate with some part of the Duck Creek-Pratt Cliff section.

The Quinault Formation rests unconformably on highly contorted older rocks of the Hoh Formation that are largely of early to middle Miocene age (Saucesian and Relizian stages). In places where these formations are in fault contact, a tectonic melange of the older (Hoh) rocks appears to have been injected in a diapiric manner into the Quinault Formation.

Locally, the Quinault Formation represents an upper part of the upper Tertiary-lower Quaternary sequence, most of which is stratigraphically higher than the upper Miocene Montesano Formation of southwest Washington. The stratigraphic distribution of Foraminifera in the Quinault Formation compares broadly with that of the standard west coast Pliocene section of the Repetto Pico sequence (Repettian stage to possibly lower Hallian stage of Natland) of the Los Angeles basin and vicinity. Furthermore, in other basins along the coast, the Foxen Mudstone of the Santa Maria area, the Purisima and Merced Formations of the San Francisco area, the Rio Del Formation and Scotia Bluffs Sandstone of northern California, and the Port Orford and Elk River Formations exposed near Cape Blanco, Oregon, all either contain foraminiferal assemblages that, in a general way, compare with those of parts of the Quinault Formation, or the stratigraphic sequence of occurrence of the contained Foraminifera is similar to that of the Quinault Formation. Therefore, the Quinault Formation may represent an interval in the development or filling of its basin of deposition similar to the intervals represented by other formations in a number of young basins of deposition along the west coast, including that of the standard west coast Pliocene section in the Los Angeles area.

Environmental conditions of deposition in the four measured sections of the Quinault Formation are noticeably varied, as revealed by the foraminiferal assemblages. The section north of Point Grenville was deposited largely in neritic conditions, probably at no greater depth than 200 feet in temperate-to-cool water (13° to 8° C). Minor foraminiferal evidence also suggests that some of the sediments may have been redeposited at greater depths, but not necessarily deeper than 1,500 feet. In the section south of Taholah faunas indicate more than one condition of deposition. Final deposition was likely at upper bathyal depths (900 to 2,000 feet) in cool-to-cold water (9° to 4° C). However, much of the material was derived, possibly by turbidity currents, from sediments originally deposited in a neritic environment (0 to 600 feet) under temperate water conditions. In the Cape Elizabeth section, the very few Foraminifera known suggest shallow-water conditions of deposition and therefore support lithologic evidence for upper neritic conditions in the lower siltstone-sandstone part and for littoral deposition of carbonaceous coarse-grained sandstone and conglomerate in the upper part. The Duck Creek-Pratt Cliff section contains Foraminifera indicating that deposition of these beds took place largely on the outer edge of the Continental Shelf in lower neritic to uppermost bathyal depths (400 to 1,000 feet) in cool-to-cold water (12° to 7° C).

Quinault Foraminifera, therefore, reflect varied depositional environments. Stratigraphic distribution suggests a sequence of ecological events comparable to that in other young west coast basins of deposition, most of which are considered Pliocene in age.

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Last Updated: 01-Jun-2006