The benthic foraminiferal biostratigraphic framework for marine Tertiary strata of the Pacific Northwest, as it has developed over the past 30 years, has been fundamental to geologic mapping by the U.S. Geological Survey and the state of Washington. However, the refinement of foraminiferal zonation in this area varies considerably because foraminiferal faunas sometimes do not differ greatly throughout thick sequences or the available data from certain areas have been limited. Furthermore, correlations are most reliable within local areas because differences in benthic foraminiferal faunas are largely the result of paleoecologic variations through time, as well as over geographic area, rather than evolutionary change.
Studies, beginning in southwest Washington in the late 1940s and continuing into the 1950s, resulted in the initial correlation of Washington’s Tertiary sequence with California foraminiferal stages and the establishment of provincial zones. This framework was later extended to the early and middle Tertiary rocks of northwest Washington. Refinements and additions were made in the 1960s in connection with Washington State and U.S. Geological Survey geologic mapping in the Grays Harbor basin. In western Oregon during much of the past 30 years, foraminiferal studies have also been an integral part of U.S. Geological Survey geologic mapping. California stages have been recognized and the local faunal distribution has been informally established. In the 1970s, foraminiferal studies and geologic mapping in several coastal areas of the Olympic Peninsula by both the U.S. Geological Survey and the state of Washington made it possible to present biostratigraphic and paleoecologic interpretations for the Tertiary rocks of that area.
As a result of these studies, eight foraminiferal zones, two informal subdivisions of a stage, and four distinctive assemblages from additional parts of the Pacific Northwest Tertiary sequence have been described. The characteristics of these early Eocene to Pliocene faunal divisions are summarized in this report.