The middle Eocene lower Coaledo Formation was interpreted as ten shoaling upward delta-margin cycles based on sediments and macrofauna. The strata, however, contains deep-water foraminifers. Explanations to resolve this anomaly included reworking, bathymetric range extension, or upward migration of water masses. Paleoecology analysis of foraminifers indicates that the few shelf species are poorly preserved whereas the well-preserved lower bathyal species dominate, and planktic organisms are present. Evidence for reworking, bathymetric range extension, or upward migration of water masses was not found in any of the cycles. The paleoecologic utility of hummocky cross-bedded sandstones is questioned as these features are controversial. In addition, there is no evidence of sea-level changes or tectonic activity to accommodate the bathymetric changes needed. Deposition of the lower Coaledo Formation on a submarine fan at lower bathyal depths eliminates the need to explain bathymetric anomalies or lack of tectonic movement.

The Coaledo Formation is considered a middle to late Eocene deltaic deposit (Chan & Dott, 1983, 1986), found along the southern Oregon coast in the Coos Bay basin (Fig. 1). Studies suggest that the Tyee and Coaledo Formations were deposited in a subsiding forearc basin with an active magmatic arc to the southeast and east (Dott, 1966; Rooth, 1974; Ryberg, 1978; Dott & Bird, 1979; Chan & Dott, 1983, 1986). The Tyee Formation was believed to contain fluvial, marsh-swamp, and delta-distributary channel facies, as well as basinal facies (Chan & Dott, 1983), whereas the Coaledo Formation contained marsh-swamp, delta-margin, delta-distributary channel, delta-front, and prodelta-shelf facies (Chan & Dott, 1986).

Foraminiferal analyses of the Tyee and Coaledo Formations have in most cases supported these interpretations (Bird, 1967; Rooth, 1974; McKeel & Lipps, 1975; McKeel, 1979, 1980). However, Bird (1967) had difficulty interpreting the depth at which deposition occurred because the microfossils suggested bathyal (>150 m) or deeper environments, but macrofossils and sediments suggested deposition occurred at inner shelf depths (<50 m). Although Bird (1967) considered reworking and broader bathymetric ranges for the microfossils, he discounted the reworking as there was no source for the specimens, and he could not find evidence to support a broad enough bathymetric range that would allow deposition of lower bathyal species at inner shelf depths. So, the anomaly remained. Rooth (1974) also suggested that the “lower member of the Coaledo Formation was deposited in a predominantly shallow marine environment, 120 to 240 feet (36.6 to 73.2 m) based on mollusks, and that there was evidence of strong current action in the lower and middle parts of the lower member, whereas deposition of the upper part of the lower Coaledo was in a quieter environment.” Rooth (1974) also noted that the bathymetric interpretation of at least two foraminiferal species (Gyroidina girardana planata of Rooth, 1974, and Cyclammina pacifica) was not consistent with sedimentary or macrofossil evidence; therefore, he suggested that perhaps the bathymetric ranges may not be as restricted in the Eocene as today. McKeel (1979, p. 4) noted: “Abundant specimens of the bathyal genus Gyroidina occur with rare inner neritic Elphidium californicum” and considered this occurrence to be incompatible with a deep water interpretation so suggested that “since the benthonic fauna in this interval represents cold bottom water (very low species diversity), an outer neritic environment is suggested here; the deep-water species of Gyroidina could have migrated onto the outer shelf, finding similar temperatures to its normal bathyal habitat.” Based on this interpretation McKeel (1979, 1980) indicated deposition occurred at neritic depths.

Although suggestions were made to resolve the discrepancy between the foraminiferal and mollusk/sediment bathymetric interpretations, none have been accepted. To resolve this issue, closely spaced samples were taken from cycle 2 of Chan & Dott (1986) and examined. Cycle 2 is near the top of the lower member of the Coaledo Formation and one of the shoaling upward delta-margin sequences of Chan & Dott (1986). This study documents the foraminiferal assemblages and 1) provides a detailed analysis of the paleoecology of cycle 2 based on the foraminiferal assemblages and ecological implications of the species, as well as 2) examines and compares microfossil assemblages in the other lower Coaledo cycles to those of cycle 2.

The Coaledo Formation outcrops in southwestern Oregon around Coos Bay in what is considered the Coos Bay basin (Niem et al., 1992; Ryu, 1995; Armentrout, 2021; Fig. 1). The Coos Bay basin includes 3400 m (11000 ft) of middle Eocene to Late Miocene deltaic to deep marine sediments (Armentrout, 2021; Ryu, 1995; Chan & Dott, 1986; Fig. 2). The adjacent Tyee Forearc Basin contains 7600 m (25000 ft) of early to middle Eocene sediments and overlies the older Umpqua Group (Baldwin, 1974; Ryu, 1995; Ryu et al., 1996). Late middle Eocene to middle Oligocene strata most likely extended across the forearc basin to the foothills of the Cascade arc but were removed by late Oligocene and younger uplift and erosion (Armentrout, 2021).

Deposition in the Coos Bay basin began with the early Eocene sandstone of Fivemile Point of Wiley et al. (2015), which was deposited at middle to lower bathyal depths (Fig. 2; Snavely et al, 1982). This unit is unconformably overlain by the Tyee Mountain Member of the Tyee Formation, which is interpreted as a deep-water turbidite unit (Wiley et al., 2015). The contact between the siltstones and associated channel and turbidite sandstones of the Sacchi Beach and Tyee Mountain Members of the Tyee Formation has not been observed (Wiley et al., 2015). The Sacchi Beach Member is interpreted as lower slope deposits (Rooth, 1974; Bird, 1967; Ragan et al., 2023) and is overlain by the Coaledo Formation. The base of the lower Coaledo Formation is described as a sandstone with minor conglomerate and pebbly sandstones (Baldwin & Beaulieu, 1973). Thin, local coal beds (1–4 inches thick; Bird, 1967) and coalified wood (Armentrout, 2021) are recognized in some outcrops. Baldwin & Beaulieu (1973) suggested an unconformity between the Sacchi Beach Member of the Tyee Formation and the overlying lower Coaledo Formation. However, Dott (1966), Bird (1967), and Rooth (1974) considered the contact between the two units to be gradational. Rooth (1974) believed the upper beds of the Sacchi Beach Member shallowed to neritic depths. This interpretation was also supported by Chan & Dott (1986), as they correlated the base of the Coaledo Formation with a sea level fall in the late early Eocene [zones NP12/13 of the Vail et al. (1977) sea level curve].

The overlying Coaledo Formation is divided into three members that comprise approximately 2000 m (Baldwin, 1974; Chan & Dott, 1986). The lower member of the Coaledo Formation is composed of ten shoaling progradational successions and is believed to represent delta progradation and lobe shifting as well as relative changes in sea level (Dott, 1966; Frazier, 1967; Baldwin, 1974; Rooth, 1974; Coleman & Wright, 1975; Ryberg, 1978; Dott & Bird, 1979; Chan & Dott, 1986; Armentrout, 2021). Deposition of this member is thought to have occurred at shallow depths of ≤50 m (Chan & Dott, 1986). Contact between the lower and middle members of the Coaledo is gradational. The middle member of the Coaledo Formation is interpreted as reflecting a period of sudden deepening of hundreds of meters (Rooth, 1974; Armentrout, 2021), which resulted in shelf mudstones being deposited over the delta-front sandstones. The upper member of the Coaledo Formation is interpreted as a prograding deltaic sandstone and conglomerate, with additional delta-front and delta-distributary channel facies (Baldwin, 1974; Chan & Dott, 1986). Conformably overlying the upper Coaledo Formation is the late Eocene Bastendorff Shales, suggesting that deposition occurred near the shelf edge to upper bathyal biofacies (Baldwin, 1974; Tipton, 1975).

Cycle 2 of the lower Coaledo Formation is exposed in Sunset Bay, Oregon. At this location, cycles 1 (youngest) through 4 (oldest) are exposed, as is a portion of the middle Coaledo Formation (Fig. 3). Cycle 2 represents the delta-front facies of Chan & Dott (1986) that was previously interpreted as being deposited at depths of 50 m and shoaling upward. The base of cycle 2 is composed of a massive mudstone (USGS Micropaleontology Laboratory numbers Mf14402–Mf14411) that becomes a silty mudstone (Mf14412) and a laminated silty mudstone (Mf14413–Mf14425). The first hummocky cross-bedded sandstone occurs between samples Mf14425 and Mf14426. Above this, the middle part of the cycle is described as a laminated muddy siltstone with thin hummocky sandstones or silty sandstones (Mf14427–Mf14453; Ragan et al., 2023). The upper part of cycle 2 is a hummocky sandstone with interbedded sandy siltstones and occasional muddy siltstones (Fault block B; Mf14454–Mf14469).

Microfossil samples from cycle 2 from the lower Coaledo Formation were collected on three separate occasions (Ragan et al., 2023): 1) widely spaced samples (seven randomly spaced samples throughout the cycle) collected in 2017 and 2019; 2) closely spaced samples (∼ 1-m stratigraphic intervals), collected in 2019, to provide detailed analysis of the paleoenvironmental changes; and 3) samples collected in 2021 from the sandier upper parts of cycle 3, underlying cycle 2, and upper part of cycle 2, top 20 m of fault block B. Microfossil assemblages were also examined from cycle 10 or older sediments at Ocean View or Collapse Cave Point; from cycles 9 through 6 in and near Simpson Cove and from cycle 4 along the Qochyax Island Tombolo (Fig. 4; Ragan et al., 2023). Samples were assigned laboratory numbers (MfXXXXX), processed, and examined in the U.S. Geological Survey Micropaleontology Laboratory in Flagstaff, Arizona (Appendix A). The foraminiferal samples were processed with solvent (kerosene) and Quaternary-O, washed through a 63-μm mesh screen, and dried at low temperatures (<40°C). The entire ≥150-μm fraction or a known split obtained using a microsplitter was picked for foraminifers, and the presence of fragments, planktic foraminifers, and other organic remains (radiolarians, diatoms, ostracodes, and fish debris) were noted (Tables 1, 2, 3). Benthic foraminifer species identifications were reviewed for consistency with modern taxonomy and ecological considerations (Ragan et al., 2023). Foraminiferal slides and residues are on file at the U.S. Geological Survey Micropaleontology Laboratory in Flagstaff, Arizona.

Overall preservation of the benthic foraminiferal assemblages was assessed based on the absolute preservational scale (APS) of Nguyen et al. (2009). The condition of the tests (intact or broken chambers and number), etching, holes, and pyrite fillings were noted (Table 3). Assemblages that contained <300 specimens or were barren are believed to have been subjected to post-depositional factors. Criteria used to interpret the paleoecological setting of these assemblages includes bathymetry, species trends, agglutinated foraminiferal morphogroups, epi/infaunal ratios, and dissolved oxygen in addition to quantitative measures, such as foraminiferal abundance, species richness, planktic/benthic ratio, and associated microfossils.

Abundance is given as a percent of the total number of benthic foraminifers counted in Tables 13. Diversity is given as species richness (number of benthic foraminiferal species in each sample), Shannon H(S), and Evenness indices. The Shannon H(S) and Evenness (e^H/S) indices were calculated for the benthic foraminiferal assemblages using the PAST-software program (Hammer et al., 2001).

Paleobathymetric interpretations are based on foraminiferal bathymetric biofacies of Ingle (1980), which are specific to the tectonically active East Pacific Margin and reflect the distribution of foraminiferal species based on water depth, water mass, and physicochemical parameters. The approach taken here builds on the biofacies developed by Ingle (1980) and allows coeval foraminiferal assemblages along the East Pacific Margin to be compared. Assignment of species to a biofacies is based on overviews of Pacific benthic foraminifers, calcareous and agglutinated species by Ingle (1980), Ingle & Keller (1980), and studies of cosmopolitan benthic foraminifers by Douglas (1981), Tjalsma & Lohmann (1983), Woodruff (1985), van Morkhoven et al. (1986), Kaminski & Gradstein (2005), and Hayward et al. (2012). Since foraminiferal assemblages can contain indigenous and transported species, the abundance of each biofacies was determined by the percent of benthic foraminiferal specimens with upper depth limits (UDL) in each of the bathymetrically defined biofacies. Depths associated with the bathymetric biofacies follows Ingle (1980): inner neritic (0–50 m), outer neritic (50–150 m), upper bathyal (>150–500 m), upper middle bathyal (>500–1500 m), lower middle bathyal (>1500–2000 m), lower bathyal (>2000–4000 m), and abyssal (>4000 m). Species UDLs and other ecological and environmental information are given in the Taxonomic Notes (Fig. 5; Appendix B; Ragan et al., 2023). Further analysis of these assemblages considered: epi/infauna abundances, dominant species, environmental associations, habitat, food preferences, and oxygen constraints (Corliss & Chen, 1988; Kaiho, 1991; Jorissen et al., 1995; Kaminski & Gradstein, 2005; Jorissen et al., 2007; Thomas, 2007; Alegret et al., 2021).

The lower Coaledo Formation is Eocene in age: late Eocene (Bird, 1967; Baldwin, 1974; Armentrout, 1981) and middle Eocene, late Ulatisian to early Narizian Stages (Rooth, 1974; Chan & Dott, 1986). Recent work on detrital zircons suggests that the lower Coaledo Formation is middle Eocene, ranging from 43.4 to 42.0 Ma; samples from the top of the lower member (cycle 1) were reported as 40.9±1.1 Ma (Fig. 6; Darin et al., 2022). Paleomagnetic analysis of the Sacchi Beach Member of the Tyee Formation and the Coaledo Formation (Blackwell et al., 2021) suggest that the lower Coaledo Formation ranges from the latter part of Chron C20r through C19n, approximately 44.5–42.1 Ma.

Foraminifers in cycle 2 of the lower Coaledo Formation contain several age diagnostic species that suggest a middle Eocene age, ranging from the very latest Ulatisian to early Narizian Stages. This interpretation is based on the first appearances of Caucasina schencki (coeval with planktic zones E9–E16), Cibicides natlandi (E10–E15), Eggerella elongata (E10–E16), Plectofrondicularia packardi (E10–E15), and Pullenia salisburyi (E11 and younger), and the last appearance of Spiroplectammina richardi (P4–E8 and questionably from E9–E14). The planktic foraminifer Pseudohastigerina micra, also present, ranges from zone E7a through O1, and thus overlaps the range based on benthic foraminifers. Based on the global ranges of the benthic foraminifers, cycle 2 is correlative with planktic zones E10 through E14 and ranges from 43.1 to 36.3 Ma. However, restrictions imposed by the zircon dates (Darin et al., 2022) and paleomagnetic data (Blackwell et al., 2021) indicate the age of cycle 2 is approximately 42.0 Ma and coeval with planktic zones late E10 through early E11 (Fig. 6).

Abundance and Diversity

Foraminifera in cycle 2 are common to abundant in most samples: 44 samples contained ≥300 specimens, 16 samples contained <300 specimens, and 9 samples were barren (Appendix C; Fig. 7). Barren samples and low abundance samples occurred at the top of cycle 2. Species richness averages 18 species per sample in the lower part (samples Mf14402–Mf14425), declining to ten per sample in the middle part (samples Mf14426–Mf14453) and to two species per sample in the upper part (Fault Block B, samples Mf14454–Mf14469). The Shannon index declines slightly upsection, whereas the Evenness index increases (Fig. 8). The decline in the foraminiferal number and various diversity indices corresponds to the change in lithology and increase in sand content: massive mudstones in samples Mf14402–Mf14411, laminated silty mudstones in samples Mf14412–Mf14421, muddy siltstones with thin hummocky cross-bedded sandstones or sandy siltstones from Mf14422–14453, and hummocky sandstones with interbedded sandy siltstones and occasional muddy siltstones in samples Mf14454–Mf14469. The first hummocky cross-bedded sandstone (HCS) occurs between sample Mf14425 and Mf14426.

Planktic foraminiferal abundance and the ratio of planktic to benthic foraminifers (P/B) in cycle 2 of the Coaledo Formation are low (Fig. 7). Few planktic foraminifers occur in the lower part of the cycle but are very rare above sample Mf14425. Diatoms and radiolarians are common to abundant in the lower part of the cycle but decline above sample Mf14425 (Fig. 7).


Foraminiferal assemblages containing 300 or more specimens in lower Coaledo cycle 2 are moderately well preserved, but the condition of specimens within the assemblage is highly variable. Neritic and upper bathyal species are poorly to moderately preserved, whereas lower bathyal species are well preserved. Rare inner neritic specimens are characterized by well-rounded and worn tests (E. californicum; Fig. 9.1) with most of the surface ornamentation obscured by broken and pyrite-filled tests with little original shell material (Quinqueloculina, Figs. 9.59.6). Outer neritic specimens (Eponides, Lenticulina, and Plectofrondicularia) show similar signs of wear and are broken or crushed and are sometimes filled with pyrite (Figs. 9.79.12). Upper bathyal specimens (e.g., Caucasina; Figs. 9.169.17) are moderately well preserved with little to no breakage. Lower bathyal Gyroidina specimens are well preserved, generally intact, and show little sign of wear or breakage, although some tests may contain some pyrite (Figs. 9.219.23). Based on the Absolute Preservation Scale (APS) of Nguyen et al. (2009), inner neritic species have an APS of 2 or 3; outer neritic species have a 3 or 4, upper and middle bathyal species have a 5 or 6, and lower bathyal species have a 7 or 8 (Table 3). This preservation pattern occurs in all cycle 2 assemblages.

Bathymetry and Epi/Infaunal Characteristics

Biofacies analysis of the foraminiferal assemblages in cycle 2 of the lower Coaledo Formation indicates that species with UDLs in the inner neritic occur sporadically (≤7%); outer neritic species are common throughout (≤42%); upper and middle bathyal species are few (≤10% and ≤2%, respectively); and lower bathyal species dominate (∼54%; Fig. 10). The dominance of lower bathyal species remains consistent throughout the entire cycle, decreasing only in the poorly preserved samples at the top where barren and low abundance samples occur. Outer neritic species decline up-section as upper and middle bathyal species increase. Although specimens are rare in the upper part of cycle 2 (Mf14454–Mf14469), the outer neritic, upper bathyal, and lower bathyal biofacies are present and in abundances similar to that seen in the lower assemblages.

Epifaunal species are not common and occur primarily in the lower part of cycle 2, whereas infaunal species dominate the foraminiferal assemblages. Specimens from the inner neritic biofacies are primarily infaunal (Nonion applini and rare Elphidium californicum) with rare epifaunal (Quiqueloculina goodspeedi and Q. imperialis) specimens. Common outer neritic epifaunal species are found in the lower part of the cycle, and several specimens are found in association with the hummocky cross-bedded sandstones at the top. Few outer neritic infaunal species occur. Bathyal assemblages are dominated by infaunal species. Caucasina schencki and Eggerella elongata are particularly abundant in the upper part of the section. Lower bathyal infaunal species Haplophragmoides porrectus and Gyroidina soldanii dominate. Tests of Haplophragmoides walteri and the deeper water H. porrectus are fine-grained, smoothly finished and use a dissolution-resistant cement, which is characteristic of the lower part of the species depth range and below or in proximity to the calcium compensation depth (CCD). Although Gyroidina soldanii is typically considered an epifaunal species common to well-oxygenated bottom waters, this species has been found as shallow infauna in lower oxygen environments, explained by its dependence on a supply of high-quality food particles (Jorisson et al., 2007).

Agglutinated Benthic Foraminifers

Agglutinated foraminifers are not common in cycle 2 and average only 25% of the benthic foraminiferal assemblage (Fig. 11). Exceptions to this low abundance are spikes in the abundance of H. porrectus and H. walteri (in samples Mf14407, Mf14412, Mf14454, and Mf14457) that indicate a physical disturbance, such as current activity. In both samples Mf14407 and Mf14412, an increase in upper bathyal infaunal species suggests that an upper slope failure may have occurred. In samples Mf14453, Mf14454, and Mf14457 from the top of the cycle, the abundance spike occurs in association with an increase in sand and a probable increase in current activity plus poor preservation possibly related to weathering, as foraminifers are rare in these samples.

Morphogroup B3 (Murray et al., 2011; morphogroup 4 of Kaminiski & Gradstein, 2005) dominates cycle 2 and includes species of Haplophragmoides, Cyclammina, and Cribrostomoides (Fig. 11). Peak abundances of H. walteri alternate with H. porrectus. Few specimens of Cyclammina placenta are present in cycle 2. Although Ingle (1980) considered C. pacifica an upper middle bathyal species along the east Pacific margin, subsequent studies have placed this species in synonymy with Cyclammina placenta and recognize the UDL, as in the outer neritic biofacies (Lagoe, 1988; Kaminiski & Gradstein, 2005; Murray et al., 2011). Morphogroup C1 is only moderately represented in cycle 2 and includes few to common Reophax, Spiroplectammina, Gaudryina, and Eggerella. Although Eggerella elongata is characteristic of similar environmental conditions, it is more common in deeper water and occurs in association with Haplophragmoides langsdalensis and H. walteri. Morphogroup A of Murray et al. (2011) occurs rarely in cycle 2. Few specimens of Bathysiphon spp. occur randomly throughout.

Calcareous Benthic Foraminifers

Calcareous benthic foraminiferal specimens dominate the cycle 2 assemblages; few species are inner neritic and upper or middle bathyal, but outer neritic and lower bathyal species are common (Fig. 10). Dominant calcareous species (i.e., species with abundances greater than 10% in multiple samples) contained Nonion applini (average = 5%) from the inner neritic biofacies; Cibicidoides natlandi (2.5%), Epondies mexicanus (∼4%), Lenticulina inornata (13%), and Plectofrondicularia packardi (1.3%) from the outer neritic biofacies; Caucasina schencki (4.6%) from the upper bathyal biofacies; and Gyroidina soldanii (28%) from the lower bathyal biofacies (Fig. 12). The abundance of these species drops considerably in the hummocky cross-bedded sands in the top of the cycle where Caucasina schencki (13%), Gyroidina soldanii (7%), and Lenticulina inornata (5.6%) dominate and the other species decline [Nonion applini (2.5%), Eponides mexicanus (0.34%)], or disappear from the assemblages (Cibicidoides natlandi and Plectofrondicularia packardi). The distribution of Globocassidulina globosa is also noted, as this is an important upwelling indicator species (Gooday, 1994; Mackensen et al., 1995; Suhr et al., 2003; Ortiz & Thomas, 2015), but only rare specimens were observed in cycle 2, occurring in the lower part of the cycle (average < 0.14%; Fig. 12).

Oxygen Conditions

The dominance of infaunal species in cycle 2 (Fig. 10) suggests that lower oxygen conditions prevailed. Although epifaunal species are present in the lower part of the cycle, these specimens are transported from the shelf and do not represent the conditions at which deposition was occurring. The benthic foraminiferal oxygen index (BFOI) of Kaiho (1994, 1999) and abundances of the species associated with various oxygen levels (Kaiho, 1999; Cannariato & Kennett, 1999) indicates that the benthic foraminifers in cycle 2 were deposited under suboxic conditions (0.3–1.2 mL O2/L; Fig. 13).

Genera characteristic of oxic conditions (>1.5 mL O2/L) include various Cibicides (C. elmaensis, C. lobatulus, C. natlandi), Eponides, Globocassidulina, and Quinqueloculina. This group occurs mainly in the lower part of cycle 2 and represents neritic biofacies identified as transported, due to preservation. Suboxic (0.3–1.5 mL O2/L) genera include Lenticulina, Marginulina, Dentalina, Bulimina, Bolivina, rare Anomalina, Caucasina, Gyroidina, and Nonion. These dominate throughout cycle 2 and are represented by both epifaunal and infaunal species. Only rare dsyoxic (0.1–0.3 mL O2/L) species are present in cycle 2. The abundance in Mf14424 is due to a poorly preserved assemblage and low foraminiferal number.

Ten cycles were identified by Chan & Dott (1986; Fig. 4). Each cycle, except cycle 5, was examined for microfossils, but sampling was not as detailed as in cycle 2. No foraminifers were found in samples from cycle 10 or older sediments at Ocean View or Collapse Cave Point (Fig. 4). Only samples from cycles 9 through 6 in and near Simpson Cove and cycle 4 along the Qochyax Island Tombolo contained foraminiferal assemblages that could be compared with cycle 2.

Samples from cycles 9 through 6 of Chan & Dott (1986) were collected from several outcrops in North and South Simpson Coves and Bathers Cove (Figs. 1417). Microfossil samples are from the silty mudstones between the hummocky cross-bedded sandstones (Table 1; Ragan et al., 2023). Most samples in cycles 9 through lower cycle 7 at all three locations are poorly preserved due to weathering and contain rare foraminifers (<300 specimens/sample), but preservation and foraminiferal abundance are better in the middle to upper part of the section, cycles 7 and 6 (Figs. 14, 16). Preservation of the benthic foraminifers is similar to that seen in cycle 2: neritic species are poorly preserved and have an APS score of 3 or less, upper and middle bathyal species have a 5 or 6, and lower bathyal species have a score of 7 or 8. Planktic foraminifers are rare, but diatoms and radiolarians are common (Table 1). Planktic organisms are preserved as siliceous molds with little original shell material or pyrite molds with no original shell material.

Bathymetric biofacies analysis of the foraminiferal assemblages at Simpson Cove indicates that the neritic biofacies are not well represented, averaging about 20%, while the lower part of the Simpson Cove samples contains abundant upper bathyal species with the lower bathyal species dominating cycles 9 through 6 (Fig. 14). At Bathers Cove, the neritic biofacies is better represented: 33.3%, 0%, 18.9%, 30.2%, and 23.5%, respectively for cycles C10 through C6 (Fig. 16). However, samples Mf14348 through Mf14351 are large samples specifically designed to sample lenses rich in macrofossils. These samples are skewed towards the neritic biofacies as the macrofossils and unidentified shell fragments are primarily transported and from the shelf edge (C. Hickman, University of California, written communication, 2022). When these samples are removed from the analysis, the neritic biofacies, cycle 7, declines to an average of 26.8%.

Distribution patterns of the diagnostic foraminiferal species observed in cycle 2 are not evident in these cycles, probably due to lack of detailed samples, low foraminiferal numbers, and poor preservation (Fig. 14, 16). However, Gyroidina soldanii is the dominant species throughout all cycles. Poorly preserved intervals, intervals with <300 specimens/sample, are dominated by agglutinated genera Haplophragmoides and Eggerella with dissolution resistant tests that are common to abundant throughout cycles 9 through 7, and Gaudryina (Fig. 9.14) is common in the middle part of cycle 7 at all three locations. Poorly preserved specimens of Elphidium californicum are common in samples associated with shell fragments in the foraminiferal residue or visible in the outcrop.

Cycle 4 was sampled at low tide along the Qochyax Island Tombolo (Fig. 4). Although detailed sampling was not done for this cycle and many samples have few to no foraminifers (<300 specimens/sample), a number of samples do contain common to abundant foraminifers (Fig. 18; Table 2). Planktic foraminifers were not observed in this section, but diatoms and radiolarians are common to abundant in the lower part of cycle 4. Preservation of the foraminiferal assemblages is similar to that seen in cycle 2: neritic species are poorly preserved and have an APS score of 3 or less, upper and middle bathyal species have a 5 or 6, and lower bathyal species have a 7 or 8. Bathymetric biofacies analysis indicates that inner neritic species are rare (averages ∼3%), outer neritic species are common (average 33.4%), with few upper bathyal species (16%), rare middle bathyal species (<1%), and common lower bathyal species (30.6%). The distribution of key species is like that observed in cycle 2, with outer neritic epifaunal species with heavy tests (Lenticulina spp., E. mexicanus, C. natlandi, and P. packardi) most common in the lower part of the section and poorly preserved (APS of 2–3). Caucasina schencki and species of Bolivina and Bulimina appear in the middle to upper part of the section, along with rare Globocassidulina globosa (Fig. 19). Lower bathyal species, Gyroidina soldanii and Haplophragmoides spp., dominate the section (Fig. 19). The dissolution-resistant tests of Haplophragmoides walteri and H. porrectus allow these species to dominate the poorly preserved samples. Foraminifers decline in abundance as the sand component increases at the top of the section.

Deposition of cycle 2 of the lower Coaledo Formation occurred in the middle Eocene, early Narizian Stage of Mallory (1959; as modified by McDougall, 2007). An age of 41.8–42.0 Ma is indicated for Cycle 2 based on the overlapping age ranges of foraminiferal biostratigraphy, detrital zircons, and paleomagnetic analysis. This age suggests that cycle 2, which is near the top of the lower Coaledo Formation, was deposited when sea level was approximately 50 m higher than present (Westerhold & Rohl, 2013; Intxauspe-Zubiaurre et al., 2018; Westerhold et al., 2020) and the CCD was 4 to 4.5 km below sea level (Palike et al., 2012). This interval coincides with the onset of a calcium carbonate accumulation event (CAE) that is marked by a high-carbonate burial, a relatively deep CCD, and evidence of cool global conditions (CAE3; Lyle et al., 2005, 2008; Moore et al., 2008).

Foraminifers are common to abundant in cycle 2 except near the top of the cycle where hummocky cross-bedded sandstones dominate. Abundance declines gradually as sand becomes more common. Foraminifers are scarce or absent at the top of the section, as is most organic matter, and occur primarily in the interbedded silty mudstones. Overall preservation of the assemblages in cycle 2 is moderate; however, preservation within an assemblage is highly variable. Neritic specimens are poorly preserved, often broken, crushed, or dissolved, but bathyal specimens are well preserved. The low abundance and poor preservation of the neritic species indicates transport from the shelf, whereas the well-preserved middle to lower bathyal species suggest in situ deposition. Displaced or transported specimens are common along an active margin and can comprise more than 50% of the assemblage at lower bathyal depths (Ingle, 1980). The low numbers of planktic foraminifers suggest proximity to the coast and deposition on an active margin but may also indicate exclusion due to the influx of freshwater. The abundance of diatoms and radiolarians in the lower part of cycle 2 suggests deposition is occurring near the base of the slope.

Biofacies analysis of foraminiferal assemblages indicates that cycle 2 was deposited at lower bathyal depths (2000–4000 m) in suboxic conditions and with abundant organic material. The massive and laminated silty mudstones at the base of the cycle (Mf14402-Mf14425) are dominated by lower bathyal species in association with planktic organisms (diatoms, radiolarians, and rare to few planktic foraminifers) and transported benthic species. Gyroidina soldanii and several species of Haplophragmoides are the dominant lower bathyal species in this interval. Gyroidina soldanii is a shallow infaunal species found in lower oxygen environments where there is a supply of high-quality food particles and increased terrigenous detrital material (Heinze & Wefer, 1992; Jorisson et al., 2007). Haplophragmoides walteri and H. porrectus, members of morphogroup B3, are tolerant of oxygen-poor environments and significant organic matter (Waśkowska, 2021) and have smooth, dissolution-resistant tests, which implies deposition at the deeper portion of their depth range (bathyal to abyssal; Kaminiski & Gradstein, 2005). These species also indicate pulsed or seasonal food sources, lower oxygen conditions, and vigorous bottom currents and sandy sediments (Gooday, 1994; Schmiedl et al., 1997; Ortiz & Thomas, 2015). Morphogroup C1, particularily Gaudryina laevigata, are common in this interval and indicate low dissolved oxygen, high productivity, and increased organic matter flux (Kaminiski & Gradstein, 2005; Murray et al., 2011). The transported benthic foraminifers (Lenticulina, Cibicides, Eponides, and Plectofrondicularia) are primarily from the outer neritic biofacies, epifaunal, tend to inhabit areas with current activity, and have sturdy tests that can withstand transport (Beck, 1943; Lagoe, 1988). Transported inner neritic species are few to rare, but the most consistent species, Nonion applini, is believed to be similar to the modern Nonionella stella, which is widely distributed, opportunistic, and common in oligotrophic environments with high amounts of organic matter (Gooday & Hughes, 2002; Alve, 2010; Goineau, et al., 2012; Rasmussen & Thomsen, 2017). Nonionella stella is commonly found in reduced oxygen conditions of the California borderland basins after transport from well-oxygenated shelf regions (Uchio, 1960; Douglas & Heitman, 1979; Stott et al., 1996; Bernhard et al., 1997). Transported Nonion applini may also represent deeper waters in the Eocene. Rare specimens of Globocassidulina globosa, an important upwelling indicator species, were observed in the lower part of the cycle 2. The distribution of this species is related to water masses, pulsed food inputs, and upwelling (Gooday, 1994; Mackensen et al., 1995; Suhr et al., 2003; Ortiz & Thomas, 2015), and in the modern eastern South Atlantic, it has been associated with vigorous bottom currents and sandy sediments (Schmiedl et al., 1997).

The muddy siltstones with thin HCS above the first hummocky cross-bedded sandstone in the middle of the cycle 2 (Mf14426-Mff14453) are dominated by lower bathyal species with reduced numbers of transported species (Lenticulina and Eponides) and rare to no planktic organisms. In the upper part of this interval, lower bathyal species continue to dominate, but bathyal infaunal species (Caucasina schencki and rare Bolivina, Praeglobobulimina, Alabamina, Anomalinoides, and Dentalina) appear and indicate that transported material is from the upper slope. Caucasina schencki may be synonymous with the middle Eocene (Lutetian and Bartonian) species Bulimina elongata, which has an upper depth limit in the upper bathyal biofacies. Buliminid species in general are common in areas of high organic matter fluxes and low bottom or pore-water oxygenation (Jorissen et al., 2007; Thomas, 2007; Ortiz & Thomas, 2015). The distribution of Caucasina schencki suggests that the influx of organic matter was high and oxygen conditions were low. The agglutinated specimens (Haplophragmoides) with dissolution-resistant tests are abundant in this interval, suggesting proximity to the CCD.

Microfossil samples from the hummocky cross-bedded sandstones with interbeds of silty sandstones at the top of cycle 2 (Mf14454–Mf14469) contain primarily quartz with little to no mica, organic material, or pyrite. Fossiliferous samples are rare but still dominated by lower bathyal species (Gyroidina and Haplophragmoides), although transported neritic and upper bathyal species are also present. Planktic organisms are not present.

Foraminiferal patterns recognized in cycle 2 include: the dominance of lower bathyal species, primarily Gyroidina soldanii and several species of Haplophragmoides; transported neritic species; abundant planktic organisms at the base of the cycle; transported upper and middle bathyal species near the middle of the cycle; and reduced numbers of specimens at the top of the cycle. The assemblages suggest lower oxygen conditions and abundant organic material. The low abundances of agglutinated foraminifers and dominance of dissolution-resistant tests noted in the lower Coaledo Formation is thought to represent deposition between the lysocline and the CCD, as is seen in the Eocene off Brazil (De Mello et al., 2017). These patterns are also recognized in the other lower Coaledo cycles, despite the limited number of samples.

Each cycle is dominated by lower bathyal species and contains transported neritic specimens. Neritic species are dominated by epifaunal forms with heavy tests that are poorly preserved. Restriction of planktic organisms to part of a cycle is not evident in the older cycles, although planktic organisms are not common in the older cycles. Upper and middle bathyal species appear in the middle to upper part of the cycle and are usually few in number. Transported species in the older cycles are worn specimens of Elphidium californicum in association with common to abundant macrofaunal shell fragments and outer neritic species with sturdy tests (Lenticulina, Eponides, and Cibicides). Macrofossil lenses occur in the older cycles (9–6) and indicate transport, primarily from the shelf edge (C. Hickman, University of California, written communication, 2022).

Evidence of shallowing within a cycle or within the lower Coaledo Formation has not been observed. Lower bathyal conditions persist despite changes in lithology. Reworked fossil material is not likely since preservation of the specimens reflects transport from the shelf rather than reworking from an older or coeval formation. Preservation of shelf species is always poor, whereas bathyal, particularly lower bathyal, species are moderately well preserved.

Although the lower Coaledo Formation was interpreted as a delta (Chan & Dott, 1986), large discrepancies exist between the mollusk/sediment and foraminiferal bathymetric interpretations. Based on mollusks and sediments, the lower Coaledo cycles were interpreted as shoaling upward sequences within a wave-dominated delta and capped by hummocky cross-bedded sandstones (Chan & Dott, 1986). In this interpretation, the presence of bathyal foraminifers was explained as due to reworking, extension of species bathymetric ranges, upward migration of water masses, and upwelling of a cold nutrient-rich bottom water mass.

Reworking was discounted by Bird (1967) and continues to be discounted because no formations in the region contain a foraminiferal assemblage composed of greater than 50% lower bathyal species, and the lower bathyal species in the lower Coaledo Formation are well preserved. Extension of some species bathymetric ranges has been documented (e.g., Plectofrondicularia packardi), but the upper depth limit of the dominant species, Gyroidina soldanii, remains at ≥2000 m in the North Pacific, therefore confirming deposition of the lower Coaledo at lower bathyal depths.

Microfossils diagnostic of upwelling, such as abundant planktic organisms and low oxygen tolerant benthic foraminifers (Bolivina, Bulimina, and Praeglobobulimina), are rare in the lower Coaledo cycles. Also, global sea level remained at approximately 50 m above modern sea level during the middle Eocene (Kominz et al., 2009), and homogeneous marine conditions in the North Pacific Ocean (Borrelli & Katz, 2015) led to reduced stratification of the water column and limited bottom water production (Pak & Miller, 1995; Lyle et al., 2008; Borrelli & Katz, 2015). The pre-Middle Eocene Climatic Optimum (MECO) is associated with the deepening of the CCD and CAE3 (Palike et al., 2012; Sluijs et al., 2013; Arimoto et al., 2020). Lowering the sea level would cause the CCD to be depressed, and foraminiferal assemblages to move to deeper water to maintain required physiochemical conditions (Kuhnt & Collins, 1996; Kaminiski & Gradstein, 2005; De Mello et al., 2017). Together, these conditions make it unlikely that deep bottom waters were present or that assemblages migrated up to the shelf.

Biofacies studies of foraminifers from modern deltas link foraminifers or assemblages with bathymetry, sediments, and environments (Vilela, 2003; Fillon, 2009; Adegoke et al., 2017; De Mello et al., 2017). Lithobiofacies described by Fillon (2009) for the Mississippi River delta indicate that the lower Coaledo microfossil assemblages contain elements of the unconfined slope and rise hemipelagites (bathyal foraminiferal taxa), deltaic-prodeltaic debrite (transported foraminifera characteristic of deltaic and prodeltaic environments), heterolithic turbidites and thermohaline current deposits (flysch-type faunas), and massive well-sorted sand bodies (sands carried into deep water by submarine canyons); these together suggest deposition on a submarine fan complex. Biofacies described by Adegoke et al. (2017) for the Niger River delta indicate lower Coaledo microfossil assemblages incorporate components of the lower slope (lower bathyal), rare inner and middle neritic, and few to common outer neritic and upper slope environmental biofacies, thus indicating deposition occurred in the bathyal environment and not directly associated with the delta. Foraminiferal studies of the Amazon delta and fan (Vilela, 1995; Vilela & Maslin, 1997; Vilela, 2003) indicate that the lower Coaledo assemblages contain only rare components of the Quinqueloculina bicostata relict assemblage (outer shelf) and some components of the upper and middle slope assemblages. Based on biofacies developed for the Brazilian margin (De Mello et al., 2017), the lower Coaledo microfossil assemblages are most similar to Biofacies C, which is a middle to upper Eocene assemblage that indicates lower bathyal depths.

Along the active east Pacific Margin, continental shelves are narrow and typically associated with submarine canyon systems (Somme et al., 2009). However, microfossil studies of the Gulf of California and Colorado River delta (Bandy, 1961) suggest the lower Coaledo assemblages are most like the lower bathyal faunas (2438–2743 m) but also contain rare to few components of the shallower faunas. Faunal studies of the Monterey Fan indicate that the lower Coaledo assemblages are characteristic of the lower bathyal biofacies, and the distribution of transported specimens in the lower Coaledo Formation resembles the transported specimens observed in the Monterey Fan at lower bathyal depths (McGann, 2014). In addition to faunal similarities, the sequence of sediments in the lower Coaledo cycles is similar to that seen in most fan lobes (Shanmugam, 2016, 2020), i.e., basin plain deposits with planktic organisms, overlain by turbidites that contain transported shelf material and an increase in sand, and sands at the top of a lobe.

In the lower Coaledo Formation, sands that occur at various levels and at the top of the cycles were described as hummocky cross-bedded sands and used by Dott & Bourgeois (1982, 1983) and Chan & Dott (1986) to restrict the deposition of the lower Coaledo Formation to depths of ≤50 m, since these structures were believed to result from surface storm activity (Harms et al., 1975; Pomar, 2020). Although these features are present in the geologic record, no direct observations of their formation on continental shelves or in the laboratory exist, and there is controversy about the paleoenvironmental significance of this feature (Cheel & Leckie, 1993; Mulder et al., 2009; Tinterri, 2011; Quin, 2011; Morsilli & Pomar, 2012; Pomar, 2020). Tinterri (2011) listed a variety of environments in which these features are recognized from fluvial to turbidite deposits (table 2 in Tinterri, 2011). Examples of HCS in deep-water environments are discussed by Prave & Duke (1990), Mulder et al. (2009), and Morsilli & Pomar (2012). The Mulder et al. (2009) example was questioned by Higgs (2011) who discounted the evidence for deep water (ichnofauna, foraminifera, basin evolution, and lithology) in order to assert formation of HCS in shallow water (Mulder et al., 2011).

In the lower Coaledo Formation, lower bathyal foraminiferal assemblages dominate the entire unit, and there is no indication of shallowing, upward migration of water masses, reworking, or ecological changes in diagnostic species. Although sea-level changes and tectonic activity were suggested to explain the water depth change between the Tyee, lower Coaledo, and middle Coaledo formations: 1) the lower Coaledo microfossil assemblages vary little from those in the underlying Sacchi Beach Member of the Tyee Formation or the overlying middle Coaledo Formation (Ragan et al., 2023); 2) no unconformity or hiatus is observed at either of these boundaries; 3) global sea-level curves indicate sea level was approximately 50 m above modern during this time interval; 4) local sea-level change is not observed in the Coaledo or coeval formations at this time; and 5) there is no evidence of tectonic activity resulting in 2000 m of elevation change in this unit or in coeval formations in southwestern Oregon (e.g., Elkton and Yamhill formations; Bird, 1967; Gaston, 1974; Wells et al., 2014). Paleogeographic reconstructions of southwestern Oregon indicate that paleo-rivers entered the Tyee Forearc basin from the paleo-east starting at about 50 Ma (Dumitru et al., 2013; Santra et al., 2013). The shelf edge prograded across the basin and deltas, slope channels, deep-water fans, and basinal areas with water depths >500 m developed (Santra et al., 2013). The Coos Bay basin and the Coaledo Formation are considered basinal, although only limited data from the area were considered in the development of the paleogeographic map. Deposition of the lower Coaledo Formation at lower bathyal depths as a submarine fan resolves the anomalies and eliminates the need for unidentified or undocumented rapid tectonic movements or faunal changes.

Benthic foraminiferal analyses show that the lower Coaledo Formation was deposited at lower bathyal depths (2000–4000 m). This interpretation is based on: 1) dominance of foraminiferal species with UDLs in the lower bathyal biofacies; 2) poor preservation of the shelf and upper slope fauna, which indicates downslope transport rather than reworking of the well-preserved deep-water specimens; 3) agglutinated foraminifers with dissolution-resistant tests, which indicate deposition at lower bathyal depths and proximity to the CCD; 4) benthic foraminiferal species that suggest suboxic conditions and abundant organic matter; and 5) planktic organisms, which indicate deposition occurred at bathyal or abyssal depths.

Interpretations to explain the presence of lower bathyal foraminifers in a deltaic environment are discounted by the lack of evidence for reworking, bathymetric range extension of species, upwelling or the upward migration of water masses. Restriction of hummocky cross-bedded sandstones to the tidal zone is questioned as these beds are associated with lower bathyal microfossil assemblages, and there are no associated sea-level changes or tectonic activity to accommodate the more than 2000 m of movement necessary to uplift the lower Coaledo Formation into the inner neritic zone. The striking similarities of the microfossil patterns and biofacies to submarine fans, as well as lithology, suggest this is a plausible alternative.

This research used samples and data of the U.S. Geological Survey (USGS) and John M. Armentrout, University of Oregon emeriti. No funding was received from any specific grant funding agencies in the public, commercial, or not-for-profit sectors. I thank Paige Latendresse and Brandon Ragan for assistance with laboratory and database work. In addition, I thank members of the SUDs group and the Pacific Northwest Project for thoughtful comments and discussions. I also thank Marci Robinson, Mary McGann, the assistant editor of the Journal of Foraminiferal Research, and journal reviewers for helpful comments. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Appendices A, B and C can be found linked to the online version of this article.

Appendix Captions

Appendix A. Location and description of samples.

Appendix B. Taxonomic notes.

Appendix C. Distribution and abundance of foraminifers and associated microfossils in cycle 2 of the lower Coaledo Formation, Sunset Bay, Coos Bay, Oregon. Samples are arranged stratigraphically from oldest to youngest. Abundances indicate the percentage of the total benthic foraminiferal fauna or absent (-). Species richness is the number of species identified in the sample. Samples barren of microfossils are indicated by light grey shading. Associated microfossils noted are the number of planktic foraminifers and ostracods observed in the sample and the relative abundance of other microfossils. Relative abundances are indicated by R (rare) = 1 specimens; F (few) = 2–10 specimens; C (common) = 11–50 specimens; and A (abundant) ≥ 50 specimens. Data available from Ragan et al. (2023).

Supplementary data