Year: 1976
Type: report
Abstract: The Clallam Formation is a predominantly marine sandstone with minor conglomerate and siltstone exposed along the Strait of Juan de Fuca in northwestern Washington.Marine mollusks occur throughout the 600-to 800-m-thick formation but are especially abundant in the lowest 240m.New molluscan data from the Clallam provide the paleontologic ba se for the Pillarian Stage , a provincial timestratigraphic unit of late early Miocene age.Until this study, the Clallam had been assigned to the middle Miocene.The new lower Miocene stage , characterized by the restricted stratigraphic range of Vertipecten fu canus (Dall) and the restricted and overlapping ranges of several other mollusks, can also be recognized in coastal Oregon and southwestern Washington.Formations coeval with the Clallam, and referable to the Pillarian Stage, include the Nye Mudstone, the lower part of the Astoria Formation in the Grays Harbor and Astoria-Grays River embayments, and at least part of the Hoh rock assemblage.On the basis of molluscan data, the Clallam Formation is also correlative with the upper part of the "Vaqueros" Stage of California and, presumably, t he uppermost part of the Poul Creek Formation in the Gulf of Alaska.These correlations are compatible with evidence from benthonic foraminiferal assemblages.The molluscan fauna of the Clallam consists of 76 taxa, about evenly divided between gastropods and pelecypods.Several species are undescribed but only five new species-Semicassis pyshtensis, Priscofusus goweri, P .slipensis, Ophiodermella olympicensis, and Solamen snavelyi-are represented by material adequate for formal description.All the Clallam molluscan assemblages represent inner sublittoral depths (less than about 100 m).Qualitative analysis of these assemblages indicates a gradual shallowing during deposition of the form ation.The highest strata in the Clallam contain coal beds and are presumably nonmarine.This trend is the concluding phase of an Eocene to Miocene depositional cycle in which maximum depths, as determined from foraminiferal and lithologic evidence, were attained during the Oligocene.
Source: USGS professional paper
DOI: 10.3133/pp976
Author: Warren O. Addicott
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W7085496