The upper part of the Montesano Formation near Aberdeen, Washington, contains early late Miocene marine diatoms of Schrader’s North Pacific Diatom Zones XIII and XIV and Subzone b of combined Zones XV–XVI as modified by Barron. Silicoflagellates are referable to Bukry’s Distephanus pseudofibula Zone and support the diatom correlations.

The type Empire Formation at Coos Bay, Oregon, and Diller’s Empire Formation near Cape Blanco, Oregon, contain a somewhat younger (late late Miocene) diatom assemblage.

Diatoms from the uppermost part of the Astoria(?) Formation of Rau, which underlies the Montesano Formation along the Middle Fork of the Wishkah River, are indicative of the middle Miocene North Pacific Diatom Zone XXI. The middle Miocene/upper Miocene boundary lies either in the diatom-poor lower Montesano or at the unconformity with the underlying Astoria(?) Formation.