AGE AND CORRELATION
A number of relatively young basins of deposition along the Oregon and particularly the California coast have been receiving sediments almost continuously since late Miocene time. Deposition in these basins, at least in part, probably took place penecontemporaneously with the deposition along the Washington coast of the upper Tertiary-lower Quaternary sequence, of which the Quinault Formation is a part. However, detailed correlations of the major outcrops of the Quinault Formation with certain segments of the depositional sequence in each basin are made with some reservations. The length of geologic time during which upper Tertiary-lower Quaternary strata were deposited is relatively short, and therefore evolutionary changes are minor among the foraminiferal species involved. However, ecological differences, such as water depth and temperature, had a pronounced effect on the composition of faunas in these young basins of deposition. Therefore, correlations from basin to basin based on evolutionary changes may be largely masked by the greater effects of variations or similarities of environments of deposition. Ecological conditions are considerably varied, not only throughout time, but obviously at any given moment in various parts of all basins, as well as within each basin. Even so, the overall ecological evolution or the sequence of ecological events in each of these basins is broadly comparable; that is, the similarity is essentially that of progressive basin filling. Relatively deep-water conditions are indicated in the lower part of each sequence, followed by shoaling conditions, succeeded by shallow-water to nonmarine conditions that dominate at the top. Therefore rather than to attempt conventional correlations based largely on evolution of species, it is perhaps more realistic to compare the dominating ecological conditions reflected by the Foraminifera in each of these basins with those of the Quinault Formation. To this extent the foraminiferal assemblages of the various sections of the Quinault Formation are compared with those of other late Tertiary-early Quaternary basins of the west coast.
Last Updated: 01-Jun-2006