Molluscan faunas of the Lincoln Creek Formation, southwestern Washington, represent 6 provincial invertebrate Opellian zones. Continuous late Eocene to early Miocene sequences have been studied in 6 sections where both mollusks and benthonic foraminifers are abundant. The stratigraphic control on both faunas in these and 7 other partial sections allows for detailed biostratigraphic analysis and the synthesis of superpositionally controlled megainvertebrate zones. These zones, moreover, are correlatable in the same sections with the standard Californian benthonic foraminiferal zones and stages.

The molluscan sequence includes faunas previously assigned by Weaver et al. to the Keasey, Lincoln, and Blakeley "Stages." These faunal assignments were made from geographically isolated sections. Durham defined 7 Oligocene megafaunal zones for northwestern Washington on the basis of 7 partial sections and other fossiliferous localities. The lack of superpositional control on boundaries for both these studies suggested the need for further biostratigraphic refinement of the Oligocene of western Washington.

The foraminiferal sequence includes faunas previously assigned by Rau to the Narizian Stage of Mallory, the Refugian Stage of Schenck and Kleinpell, and the Zemorrian and lower Saucesian Stages of Kleinpell.

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