FORAMINIFERA FROM THE TWIN RIVER FORMATION
Foraminifera are more common in the Twin River formation than in any other formation in the mapped area. Assemblages were found in nearly 400 samples and, of these, approximately 160 samples contained sufficient foraminiferal material to be useful in this study. Foraminifera were collected from three measured sections, the Lyre River (pl. 2), Deep Creek (pl. 3), and on the coast between the mouths of the East Twin River and Murdock Creek (pl. 4). Many samples were also obtained from isolated outcrops and short sections throughout the mapped extent of the Twin River formation (table 3).
Most samples contain only a few species of Foraminifera and are only sparsely fossiliferous, but because many samples were studied, a sizeable number of species were identified from the formation. Because they are poorly preserved, many of the Foraminifera are tentatively or questionably identified.
Precise correlations and age determination of foraminiferal assemblages in the Twin River formation are hampered by the scarcity and poor preservation of Foraminifera. Furthermore, some species that are known to have different stratigraphic ranges in other areas are found together in parts of the Twin River formation. This apparently anomalous association of species may be due to varied local ecology or perhaps to a reworking of fossils from lower parts of the section; therefore, age determinations of assemblages from isolated outcrops and correlations of faunas from parts of measured sections are based on assemblages or groups of assemblages rather than on individual species.
Four faunal divisions are recognized and they are referred to the upper Eocene upper Narizian stage (Mallory, 1959), the Eocene and Oligocene Refugian stage (Schenck and Kleinpell, 1936), and a lower part and an upper part of the Zemorrian stage (Kleinpell, 1938) of Oligocene or Miocene age.
Foraminifera were collected from all three members of the formation in a measured section in the Lyre River area (pl. 2); this section is used as the basic reference for the study of the foraminiferal sequence in the Twin River formation.
LYRE RIVER SECTION
Foraminifera are scarce in the lower member of the Twin River formation in the Lyre River section. Those species which are present, however, suggest a late Eocene age and for the most part they are recorded from the upper part of the Narizian stage of Mallory (1959). Angulogerina hannai Beck, Cassidulina globosa Hantken, and Globigerina cf. G. yeguaensis Weinzierl and Applin are the most significant species because they are most commonly known from rocks of late Eocene age in other areas.
No Foraminifera were obtained from approximately 1,400 feet of sandy strata in the upper part of the lower member, and the age of the upper part of the lower member could not be determined. Foraminifera indicative of the Refugian stage make their lowest occurrence in the Lyre River section at the base of the middle member, immediately above the sandy strata of the upper part of the lower member. Because the highest assemblage of Narizian age and the lowest assemblage of Refugian age are separated by 1,400 feet of unfossiliferous section, the boundary between these two stages cannot be placed precisely, and therefore, in this report, is questionably placed in the middle of the unfossiliferous part of the section. The following, whose lowest occurrence is above the unfossiliferous sandstone, are the more significant species of the Ref ugian stage:
Quinqueloculina weaveri Rau
Refugian Foraminifera occur throughout all but approximately the upper 300 feet of the middle member of the Twin River formation in the Lyre River section. Such species as Cancris joaquinensis Smith and Cassidulina globosa Hantken occur in the underlying strata of the Narizian stage as well as in the Refugian stage, but none occur in rocks of the overlying Zemorrian stage; therefore a combination of Narizian and Refugian species in an assemblage is used as evidence to differentiate faunas of the Refugian stage from those of the overlying Zemorrian stage. The base of the Zemorrian stage is indicated by the lowest occurrence of Bulimina cf. B. alsatica Cushman and Parker, Cassidulina crassipunctata Cushman and Hobson, and Sphaeroidina variabilis Reuss. The known occurrence of these species is limited to rocks no older than those assigned to the Zemorrian stage. Additional species that characterize the Zemorrian stage in the Pacific Northwest and which are also present in the Lyre River section are Entosolenia sp., Robulus cf. R. calcar (Linne), and Nonion incisum (Cushman).
A lower fauna and an upper fauna of the Zemorrian stage are recognized in the Twin River formation, but these two faunas do not necessarily correspond to the lower and upper faunas of the Zemorrian stage of Kleinpell in California. In the Lyre River section the lowest occurrence of the upper fauna of the Zemorrian stage is best indicated by the lowest occurrence of Nonion incisum (Cushman), about 100 feet below the top of the measured section.
DEEP CREEK SECTION
Foraminiferal assemblages from the Deep Creek section suggest an age no older than that of the Zemorrian stage (pl. 3). Foraminifera were not found in the lowest part of the formation in the Deep Creek section, but the lowest assemblages obtained resemble those of the lower part of the Zemorrian stage in the Lyre River section. No Foraminifera characteristic of either the Refugian or the Narizian stage were found in the lower assemblages from the Deep Creek section. The occurrence of Cassidulina crassipunctata Cushman and Hobson in the lowest assemblages from the Deep Creek section is significant because this species not only is a common species in the Zemorrian stage throughout the Pacific Northwest, but its lowest occurrence is in the Lyre River section above the base of the Zemorrian stage. Bulimina cf. B. alsatica Cushman and Parker, although not a common form in the Twin River formation, is found also in the lower assemblages of the Deep Creek section. Its lowest occurrence in the Lyre River section is in the lowest beds assigned to the Zemorrian stage.
The lowest occurrence of Nonion incisum (Cushman) is a few hundred feet below the top of the measured section in Deep Creek and best marks the base of the upper part of the Zemorrian stage in this section. Buliminella subfusiformis Cushman, Bolivina marginata adelaidana Cushman and Kleinpell, and Uvigerina gallowayi Cushman also make their lowest occurrence in the uppermost part of the measured section in Deep Creek, and Eponides mansfieldi oregonensis Cushman and R. E. and K. C. Stewart was found in several samples in the upper half of the measured section. None of these species are abundant, but they are important because they rarely occur in rocks older than those assigned to the Zemorrian stage. Records of U. gallowayi Cushman are confined largely to rocks assigned to the Zemorrian stage.
Faunas of the upper part of the Zemorrian stage are well represented in the rocks exposed along the coast between the East Twin River and Murdock Creek, where a thick section of rocks is assigned to the upper part of this stage (pl. 4). Because in the area of this report the coastal section contains the most complete section of rocks assigned to the upper part of the Zemorrian stage, it is regarded as a reference section. Nonion incisum (Cushman) occurs at the base of the coastal section and is found throughout the lower half of this measured section. Elphidium cf. E. minutum (Reuss) is common in several samples from the upper part of the coastal section, and within the mapped area this species is known only from the upper part of the Zemorrian stage. Cassidulina crassipunctata Cushman and Hobson is present in a number of the samples from the coastal section and is regionally indicative of the Zemorrian stage. A high position in the Zemorrian stage is suggested by the presence of Bulimina alligata Cushman and Laiming, Uvigerina gallowayi Cushman, and Cassidulinoides sp. because of their common occurrence in the upper part of the Zemorrian stage in other regions of the Pacific Northwest.
Studies of foraminiferal assemblages from the Twin River formation are based chiefly on collections from the three measured sections, but faunas from these sections are augmented by many collections from scattered localities in other parts of the northern Olympic Peninsula. About 300 samples of Foraminifera were collected at random and 65 of these contain fossils that are useful in dating the containing rocks (table 3). The relative age of assemblages from these samples was determined by comparing them with faunas in the measured sections. The 65 samples from scattered localities contain assemblages that range in age from Narizian to late Zemorrian.
Assemblages referred to the Narizian stage are generally of two types. Some are similar to those of the Aldwell formation, but most are similar to assemblages that are believed to be younger than those of the Aldwell formation. However, the younger appearing assemblages do not always occur in the highest stratigraphic position. This anomalous condition may be due to reworking, or it may indicate that the fauna is more indicative of facies than of age.
The following species were not found in the measured sections but are present in assemblages from isolated outcrops. They are at least locally indicative of the Narizian stage and are useful in delimiting its upper extent.
Quinqueloculina goodspeedi Beck
Refugian, lower Zemorrian, and upper Zemorrian as semblages from isolated outcrops do not differ significantly from assemblages from the measured sections.
CHARACTERISTICS OP FAUNAL UNITS
Faunal divisions in the Twin River formation are based primarily on observations of the Foraminifera in the Lyre River section. Additional and substantiating information was obtained from other sections and from isolated outcrops. Particular use was made of the observed range of species, the recorded range in other areas of species present in the Twin River formation, the association of species, and the frequency of occurrence of species.
Many species do not characterize any one faunal unit because they are found throughout the formation. Other species occur so rarely that their significance is not known. Species displaying the greatest significance are listed on figure 2. This figure shows the observed ranges, combination of occurrence, and frequency of occurrence of selected species. The frequency of occurrence of a species is given as a ratio, in percent, of the number of samples in which a given species occurred with respect to the total number of samples obtained from a given stage or stage subdivision. Because many more samples were available in the younger stages, the evidence concerning the limits of these stages and ranges of the species in them is much more reliable than is that concerning the relatively unfossiliferous lower part of the section. Therefore, the calculations shown on figure 2 should be considered only as a broad guide for determining which species are probably the most useful stratigraphically in the Twin River formation.
Foraminiferal assemblages of the Twin River formation that are comparable to those referred to Mallory's Eocene Narizian stage are known in southwest Washington from an upper part of the McIntosh formation and parts of the Skookumchuck formation. There they are referred to either the Bulimina schencki-Plectofrondicularia cf. P. jenkinsi zone or the Uvigerina cf. U. yazooensis zone (Rau, 1958). In western Oregon similar assemblages are known from the Coaledo formation (Cushman, Stewart, and Stewart, 1947, p. 57-69) and from a lower part of the Toledo formation (Cushman, Stewart, and Stewart, 1949), and they have been observed by the writer from parts of the Nestucca formation. In California similar assemblages are known from the Alhambra formation (Smith, 1957) and the Kreyenhagen shale (Cushman and Siegfus, 1942). Although faunas similar to those in both the lower and upper part of the Kreyenhagen shale are present in the Twin River formation, they do not necessarily occur in the same stratigraphic order as they do in the Kreyenhagen shale. In the Twin River formation these two faunas may represent two different environments or one of them may have been reworked into the Twin River formation from older rocks.
All the above-mentioned west coast assemblages have been referred to Laiming's A-1 or his A-2 zone of California. In the standard west coast section (Weaver and others, 1944) these zones are considered late Eocene in age (fig. 1).
Twin River assemblages that are assigned to the Refugian stage are comparable with Refugian assemblages from the lower and middle part of the Lincoln formation of Weaver (1912) in southwest Washington, where they are referred to the Sigmomorphina schencki and Eponides kleinpelli zones (Rau, 1958). In western Oregon similar Refugian assemblages are known from the Keasey formation and Bastendorff shale (Cushman and Schenck, 1928; Detling, 1946) and they have been observed by the writer from a middle part of the Toledo formation near Waldport, Oreg. Comparable Refugian assemblages of California are known from the Tumey formation (Cushman and Simonson, 1944), the Gaviota formation (Wilson, 1954), and the Wagonwheel formation (Smith, 1956). In terms of the standard west coast section (Weaver and others, 1944), the Refugian stage is "Eo-Oligocene" and Oligocene in age.
Zemorrian assemblages similar to those of the Twin River formation are known from the upper part of Weaver's Lincoln formation in southwest Washington, where they are referred to the Pseudoglandulina aff. P. inflata zone (Rau, 1958), the upper part of the Toledo formation of western Oregon, and the upper part of the San Lorenzo formation of California (Cushman and Hobson, 1935). The Zemorrian stage is generally regarded as "Oligo-Miocene" in age.
Only generalized interpretations can be made of the environment in which the Foraminifera of the Twin River formation lived because many of the species are extinct. Conculsions are necessarily based largely on the assumption that fossil species required the same environment as similar living species. Furthermore, data on many living species are incomplete and in some cases controversial. The following generalized interpretations are based on depth and temperature records of living species either identical with or similar to species known from the Twin River formation. Data on the depths and temperatures under which certain Foraminifera are known to live come from several independent records and, in many cases, are too numerous to cite in the discussion. The following references are cited as the major sources of depth and temperature data used in this report: Cushman (1927), Norton (1930), Natland (1933), Kleinpell (1938, p. 11-19, fig. 5), Glaessner (1947, p. 183-194), Parker (1948), Phleger and Parker (1951), Crouch (1952), and Bandy (1953). Particular attention has been given to species that were found in substantial numbers or that occurred persistently throughout thick sections of the formation. Because the occurrence of many species is rare or sporadic, the number of species used in determining paleoecology is limited.
The Foraminifera of the Twin River formation generally suggest an environment of deep cold water in an open sea. Gyroidina orbicularis planata Cushman is the most common and persistently occurring species known in the formation. Records of this species and similar species show that they prefer cold water at bathyal (600 to 6,000 feet) to abyssal (6,000 feet or more) depths. A similar environment is indicated by the commonly occurring species Uvigerina cocoaensis Cushman, U. garzaensis Cushman and Siegfus, and Cassidulina crassipunctata Cushman and Hobson. Although it is never found in large numbers, the planktonic form Globigerina is found in places and suggests that the mapped area may have had reasonably good access to an open sea.
Although the evidence is not conclusive, Foraminifera from the lower part of the formation suggest a more shallow environment than do the Foraminifera from the middle and much of the upper part of the formation. Angulogerina and certain species of Cibicides that are similar to species generally recorded from upper bathyal and neritic depths (approximately 2,000 feet to tidal depth) were found in many of the samples from the lower part of the formation. Deepwater species, such as Gyroidina orbicularis planata, also are present. The depth at which the combination of species from the lower part of the Twin River formation lived is there fore estimated as moderate, uppermost bathyal to lower neritic (approximately 1,000 to 300 feet).
Several assemblages from isolated localities within the lower part of the formation are comparable to the cold-, deep-water fauna of the Aldwell formation and may have been reworked into the Twin River formation. However, if they lived during the deposition of the Twin River formation, a depth greater than 1,000 feet may have also existed in local areas during deposition of the lower part of the formation.
Costate Uvigerina, Plectofondicularia, and Nonion pomplioides suggest bathyal depths (6,000 to 600 feet), whereas Cancris and Alabamina are recorded from cold but shallower water. These Foraminifera occur together in the middle and in much of the upper part of the Twin River formation and suggest bathyal depths, between 6,000 and 1,000 feet.
During the deposition of the uppermost part of the upper member of the Twin River formation, depths probably were decreased but water temperatures remained at least cool. Gyroidina, together with several other forms characteristic of bathyal to lower neritic depths, persist throughout the middle and upper members of the formation. However, Elphidium cf. E. minutum (Reuss) and Nonion incisum (Cushman) become common in the fauna of the uppermost part of the upper member of the formation. Foraminifera of this type suggest shallowwater conditions. N. incisum suggests cool temperatures in a somewhat sheltered environment. Cool conditions are further suggested by the presence of abundant Cassidulina of the type that is found in cool water of Recent seas. Although the uppermost part of the formation does contain some deep-water Foraminifera the greater abundance of shallow forms here than in any other part of the formation suggests that the water became more shallow but that the temperature remained cool. Probably in the final stages of deposition of the Twin River formation upper neritic depths became dominant, a prelude to distinct neritic conditions that prevailed during the deposition of the overlying Clallam formation.
Last Updated: 28-Mar-2006