FORAMINIFERA FROM THE ALDWELL FORMATION
Foraminifera are not abundant in the Aldwell formation, but 49 species from nine localities in different parts of the formation were identified (table 2).
TABLE 2.Foraminifera from the Aldwell
The foraminiferal fauna of the Aldwell formation can best be correlated with Laiming's A-2 zone of the Eocene of California (1940; see also fig. 1, this report). Approximately one-third of the species known from the Aldwell formation (table 2) were recorded by Laiming from his A-2 zone, and many others in the Aldwell formation are either comparable with or similar to species of his A-2 zone. Uvigerina churchi Cushman and Siegfus, U. garzaensis Cushman and Siegfus, Bulimina corrugata Cushman and Siegfus, and B. lirata Cushman and Parker, all of which are in the Aldwell, are regarded as characteristic of the A-2 zone. A low position in the A-2 zone is suggested for the Aldwell fauna because it contains such species from Laiming's C-zone as Asterigerina crassiformis Cushman and Siegfus, Bifarina nuttalli Cushman and Siegfus, Bolivinopsis directa (Cushman and Siegfus), and Tritaxilina colei Cushman and Siegfus. Laiming (1940) showed that these species, which are common in his C zone, also occur in the lower part of his A-2 zone. The fauna of the Aldwell formation is correlated with the A-2 rather than C-zone faunas because it contains only about one-half as many species that are known from the C zone as are known from the A-2 zone or higher. This correlation is supported also by stratigraphic evidence because B-zone assemblages which normally separates C-zone and A-zone assemblages have not been found in the relatively complete section above the Aldwell formation.
Of the faunas noted in Mallory's division of rocks of Eocene age in the California Coast Ranges (1959), the fauna of the Aldwell formation is best compared with that of his Bulimina corrugata zone of the lower part of his Narizian stage. The Aldwell formation contains Amphimorphina californica Cushman and McMasters, a form that Mallory indicated is restricted to the upper part of his subjacent Ulatisian stage in California. Therefore a part of the Ulatisian stage may be represented in the Aldwell formation. However, A. californica is found frequently in western Washington and Oregon in association with Foraminifera that compare best with those of Mallory's B. corrugata zone and therefore, locally at least, A. californica may not be necessarily indicative of a pre-Narizian age. Faunas resembling those in the Aldwell formation are also found in the Canoas siltstone member of the Kreyenhagen shale (Cushman and Siegfus, 1942) and in the Alhambra formation (Smith, 1957) of California; similar faunas have also been found by the writer in the upper part of the Yamhill formation and the lower parts of the Nestucca and Toledo formations in Oregon, and in the McIntosh formation of Washington.
The similarity of all known assemblages of the Aldwell formation suggests that they probably lived in similar ecological conditions, but conclusions regarding these conditions are necessarily based on considerable supposition. It is possible to be reasonably certain of only a very generalized concept of the environment in which the Foraminifera of the Aldwell formation lived.
Bandy's work (1953) with Recent Foraminifera from three widely spaced profiles normal to the California coast, off San Diego, Point Arguello, and San Francisco, is the only record of west coast assemblages that are at all similar to the fauna of the Aldwell formation. These assemblages are confined to the deepest and coldest water that was sampled, and are largely from the Point Arguello profile. Several of the species in these Recent assemblages are similar to species of the Aldwell formation and most of the genera are represented in the fauna of the Aldwell formation. The comparable forms from off the California coast are recorded chiefly from depths between 3,000 feet and 12,000 feet and at temperatures between 15°C and 0°C. The number of individuals with similarities to the Foraminifera of the Aldwell formation generally decreases in shallower and warmer water.
From these generalized comparisons with Bandy's work on Recent Foraminifera it can only be concluded that some of the Foraminifera in the Aldwell formation probably would have thrived in an environment of relatively deep and cold water.
Last Updated: 28-Mar-2006