FORAMINIFERA FROM THE CRESCENT FORMATION
Two distinctly different types of foraminiferal faunas were found in the Crescent formation. One of these consists almost entirely of planktonic Foraminifera and is known only from lenticular bodies of reddish argillaceous limestone interbedded with volcanic rocks. The other type of fauna consists chiefly of benthonic Foraminifera, and is known to occur only in basaltic sandstone and tuffaceous siltstone in the upper part of the Crescent formation.
Foraminifera were first reported from the lenticular bodies of reddish argillaceous limestone of the Crescent formation by Pardee (1921, p. 232) and were latermentioned by Park (1946, p. 310). About 50 thin sections of reddish argillaceous limestone from 12 localities were examined in connection with the present study. Five of these localities, f11711 to f11715 (pl. 1), are within the mapped area. Because the fossils could not be separated from the matrix, they were examined only in thin section and were not specifically identified. Most of the Foraminifera are planktonic and are either globigerinids or globorotalids, but none are globotrun canids (pl. 7, figs. 12, 13). The Foraminifera are abundant locally and are commonly concentrated in thin layers. Such concentrations of planktonic Foraminifera as are found in the calcareous rocks of the Crescent formation are rarely found in other Tertiary rocks of the Pacific Northwest.
The abundance of planktonic foraminiferal remains and the lack of benthonic foraminiferal remains in these red argillaceous limestone beds could be indirectly attributed to contemporaneous volcanic activity. The small lenticular bodies of red limestone probably represent periods during which volcanism was locally inactive. Nevertheless volcanism probably continued in nearby areas because volcanic debris is incorporated in the red limestone. It is unlikely that any type of Foraminifera can live in or near areas that are undergoing submarine volcanism. Planktonic Foraminifera can be brought into such an environment by ocean currents, but they probably would die in mass and their tests would be deposited in concentrations as part of the accumulating sediments. Benthonic or nonfloating forms are less likely to be transported in large numbers by ocean currents and therefore in general only planktonic foraminiferal tests would be deposited.
The planktonic assemblages of the Crescent formation are probably post-Cretaceous in age because they consist of globigerinids and globorotalids without any globotruncanids, which are indicative of a Cretaceous age.
In addition to the planktonic fauna, 25 species of benthonic Foraminifera were collected from six other localities, f11716 to f11721, in basaltic sandstone and tuffaceous siltstone of the upper part of the Crescent formation (table 1). Although locality f11721 was mapped as Twin River formation by Brown and others (1960), the foraminiferal fauna from there is typical of the Crescent formation; it is therefore regarded here as fauna of the Crescent formation. Locality f11720 at Observatory Point yielded the most complete assemblage. Berthiaume (1938) described several species of orbitoid Foraminifera from this locality. Samples collected during the present study also contain a few orbitoid Foraminifera, but Amphistegina californica Cushman and M. A. Hanna is the most common form in these collections.
TABLE 1.Benthonic Foraminifera from the
The association of orbitoid Foraminifera with many Amphistegina suggests a marine environment with warm, shallow water. Vaughan (1945, p. 69,70) stated:
Cushman (1950, p. 300, 301) stated:
On the basis of orbitoid Foraminifera, Berthiaume (1938) correlated the Crescent formation exposed at Observatory Point with the orbitoid-bearing Sierra Blanca limestone of Santa Barbara County, Calif., and referred them both to an early middle Eocene age. Mallory (1953) agreed with this correlation, but also suggested a correlation of the Crescent formation with the "Mabury Reef sandstone" and "basal Spiroglyphus sands" at Media Agua Creek in California. He assigned all these units to his Penutian stage, which he regarded as early Eocene (Capay) in age (Mallory, 1959).
The fauna here described from the Crescent formation substantiates a general age assignment of early to early middle Eocene. However, in terms of Mallory's stages the present fauna suggests a Ulatisian age rather than a Penutian age because of the common occurrence of Amphistegina californica. This species was recorded by Mallory only from his Ulatisian stage (1959, p. 84), which he regarded as middle Eocene and possibly early Eocene in part (Mallory, 1959, p. 77)
In addition to the previously suggested correlations the fauna of the Crescent formation may also be correlated with that described from rocks of Eocene age at Cape Blanco, Oreg. (Bandy, 1944). Practically all the species in the Crescent fauna are represented by closely related forms in the Cape Blanco fauna and five species are regarded as identical.
Last Updated: 28-Mar-2006