Geology and Vertebrate Paleontology of Western ... - Donald Prothero

GEOLOGY AND VERTEBRATE PALEONTOLOGY OF WESTERN AND SOUTHERN NORTH AMERICA

Geology and Vertebrate Paleontology of Western and Southern North America Edited By Xiaoming Wang and Lawrence G. Barnes

Contributions in Honor of David P. Whistler WANG | BARNES

900 Exposition Boulevard Los Angeles, California 90007

Natural History Museum of Los Angeles County Science Series 41

May 28, 2008

Paleomagnetism of the Oligocene and Miocene Lincoln Creek and Astoria formations, Knappton, Washington Donald R. Prothero,1,2 Jonathan M. Hoffman,3 and James L. Goedert2,4

ABSTRACT. The upper Oligocene Lincoln Creek Formation and the lower Miocene Astoria Formation, exposed on the north bank of the Columbia River near Knappton, Washington, have long been important localities for marine fossils. Oriented magnetic samples were taken at low tide from 12 sites spanning the exposed parts of both formations and analyzed by both thermal and alternating field demagnetization. The characteristic remanence was held primarily in magnetite, with minor amounts of goethite overprinting. Both reversed and normal polarities were obtained, and they passed a reversal test for stability. The rocks show a mean direction of D 5 67.2, I 5 62.5, k 5 5.8, a95 5 11.3, which suggests almost 77 6 11 degrees of clockwise tectonic rotation, consistent with other results reported for the region, but they show no significant latitudinal translation. The lower part of the Lincoln Creek Formation is reversed in polarity, followed by a short normal magnetozone. The contact between the two formations is covered, but the available exposures of the Astoria Formation are entirely reversed in polarity. On the basis of the presence of late Oligocene molluscan faunas (Liracassis apta Zone) and correlation with the magnetic stratigraphy of the Lincoln Creek Formation at Canyon River on the south flank of the Olympic Mountains, we correlate the Knappton section of the Lincoln Creek Formation with either Chrons C7Ar–C6Cn2r (25.7–23.3 Ma) or C7r–C6Cn2r (25.4–23.3 Ma). The age of the Astoria reversed magnetozone is poorly constrained, and it could be as young as 20 Ma (million years before present). The locations of key fossil horizons within this stratigraphic sequence are discussed.

INTRODUCTION Fossiliferous rocks of the Lincoln Creek and Astoria formations near the now-defunct cement plant and sawmill at the abandoned town site of Knappton, Washington, are exposed at low tide on the north bank of the Columbia River, across from Astoria, Oregon (Fig. 1). They were first mapped by Weaver (1937), later mapped in more detail by Wells (1979, 1989), and described further by Moore (1984a). According to Moore (1984a), several different fossiliferous levels are within the Lincoln Creek sequence. The lowest, Unit I (LACMIP 5844; Natural History Museum

1 Department of Geology, Occidental College, Los Angeles, California 90041. Email: [email protected] 2 Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007 3 Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 4 Burke Museum of Natural History and Culture, University of Washington, Seattle, Washington 98195

Science Series, Number 41, pp. 63–72 Natural History Museum of Los Angeles County, 2008

of Los Angeles County, Department of Invertebrate Paleontology), contains abundant gooseneck barnacles, which were described by Zullo (1982). Unit II (LACMIP 5843) contains many decapod crustaceans (not yet described). Unit III (LACMIP 5852) contains glass sponges described by Rigby and Jenkins (1983). Unit IV (LACMIP 5842) is known as the Aturia bed because of the abundance of the nautiloid Aturia (Bronn, 1838). Fossils from the Lincoln Creek Formation near Knappton have also been illustrated, described, or both in a number of other reports (e.g., Rathbun, 1926; Miller, 1947; Moore, 1984b, 1988; Squires, 1989; Goedert and Squires, 1993; Squires and Goedert, 1994), as well as in a brief summary of the fish fossils by Welton (in Moore, 1984a:7). In the overlying Astoria Formation are several LACM localities, including the type locality of the desmatophocine otarioid pinniped Desmatophoca brachycephala Barnes, 1987. Fossil crabs have also been reported from the Astoria Formation near Knappton (Berglund and Goedert, 1992; Schweitzer and Salva, 2000). Not only are the Knappton localities the source of many

64 & Science Series 41

Prothero et al.: Paleomagnetism of Knappton, Washington

assigned the sequence to the upper part of the Liracassis apta Zone (Moore, 1984b, 1988) and suggested that it is earliest Miocene in age. She indicated that most of the mollusks belong to the Juanian Molluscan Stage (of Addicott, 1976a, 1976b), although the assemblage has strong resemblances to the overlying early Miocene Pillarian Stage. Squires and Goedert (1994) assigned the sequence a tentative age range of latest early Oligocene to earliest late Oligocene on the basis of the presence of specimens of Liracassis Moore, 1984b, that appear to be transitional between Liracassis rex (Tegland, 1931) and L. apta (Tegland, 1931). In the last two decades, considerable progress has been made in dating the marine rocks of the Pacific Northwest. Through the use of planktonic microfossils and magnetic stratigraphy, most of the key sections have now been dated and correlated to the global timescale of Berggren et al. (1995). The longest, most complete section through the Lincoln Creek and Astoria formations, at Canyon River on the southern Olympic Peninsula of Washington, was studied by Prothero and Armentrout (1985). The type section of the Juanian Molluscan Stage in the Pysht Formation (Durham, 1944) was studied by Prothero et al. (2001c), and the type section of the Pillarian Stage in the Clallam Formation was studied by Prothero and Burns (2001). Correlative sections in the Yaquina Formation and Nye Mudstone in Oregon were studied by Prothero et al. (2001b), and in the overlying Astoria Formation by Prothero et al. (2001a). With the use of these sections as standards, it should now be possible to further refine the age of the Knappton localities with magnetic stratigraphy. METHODS AND MATERIALS

Figure 1 Index map showing location of Knappton and other localities that are mentioned in text in western Oregon and Washington.

important specimens in the LACM (Natural History Museum of Los Angeles County) collections, but they could also be of historical importance as well. Some of the first fossils from the West Coast of North America, collected by J.D. Dana with the Wilkes Exploring Expedition, might have been found in Knappton outcrops (Weaver, 1942:76). However, the ages of these strata and fossils are still not well constrained. Zullo (1982) assigned the Lincoln Creek sequence to the upper Eocene and upper Oligocene. Rigby and Jenkins (1983) suggested the sequence ranges in age from late Eocene to middle Miocene. Moore (1984a)

The stratigraphic section follows the measurements of George Moore (personal communication) and the descriptions and photographs in Moore (1984a). A map of the localities is shown in Figure 2. Samples were collected during low tide in the summer of 2001 with the use of simple hand tools to recover oriented blocks of rock (three samples per site to obtain site statistics). Oriented cores were then drilled from the blocks with a drill press or prepared from smaller pieces of rock with Zircar aluminum oxide ceramic. The samples were measured on a 2G cryogenic magnetometer at the California Institute of Technology with the use of an automatic sample changer. After measurement of natural remanent magnetization (NRM), each sample was demagnetized in alternating fields (AFs) of 25, 50, and 100 gauss to determine the coercivity behavior (and magnetite content) of the samples and demagnetize any multidomain grains before their remanence was baked in. After AF demagnetiza-

Science Series 41

Prothero et al.: Paleomagnetism of Knappton, Washington & 65

Figure 2 Detail of the topographic map of the Columbia River northern shoreline, showing the location of Moore’s (1984a) LACMIP fossil localities and of our paleomagnetic sites. Map based on the United States Geological Survey Knappton, Oregon–Washington 7.59 quadrangle, with geologic features after Wells (1979).

tion, each sample was then thermally demagnetized at 50uC steps from 200u to 600uC to remove any chemical remanence held by iron hydroxides such as goethite and to determine whether any remanence held by hematite remained after the Curie point of magnetite (580uC) had been exceeded. About 0.1 g of powdered rock from several samples was subjected to increasing isothermal remanent magnetization (IRM) to determine their IRM acquisition behavior, and thus the relative abundance of magnetite or hematite They were also AF demagnetized twice: once after having acquired an IRM produced in a 100-mT (millitesla) peak field and once after having acquired an anhysteretic remanent magnetization (ARM) in a 100-mT oscillating field. Such data are useful in conducting a modified Lowrie–Fuller test (Pluhar et al., 1991), which indicates whether single or multidomain grains are present. Fossils of taxa mentioned in the text are deposited in the collections of the LACM, the UWBM (Burke Museum of Natural History and Culture), or both. ABBREVIATIONS AF IRM

Alternating fields Isothermal remanent magnetization

LACM

Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, Los Angeles, California USA LACMIP Natural History Museum of Los Angeles County, Department of Invertebrate Paleontology locality NRM Natural remanent magnetization UWBM Burke Museum of Natural History and Culture, University of Washington, Seattle, Washington USA RESULTS Orthogonal demagnetization (‘‘Zijderveld’’) plots of representative samples are shown in Figure 3. In most samples, there is a dramatic response to AF demagnetization, suggesting that the remanence is held largely in a low-coercivity mineral such as magnetite, although the slight decrease in magnetization in the first AF steps suggests a minor overprinting by an iron hydroxide such as goethite. The abrupt decrease in intensity at 200uC (above the dehydration temperature of goethite) suggests the removal of a high-coercivity remanence held in iron hydroxides. In a few samples (e.g., Fig. 3C), thermal demagnetization at 200uC removed a slight overprint to reveal a stable reversed direction. In nearly every sample,



Figure 3 Orthogonal demagnetization plots of representative samples from the Lincoln Creek and Astoria formations (tilt-corrected). Solid squares indicate horizontal component; open squares indicate vertical component. First four AF steps are NRM, 25, 50, and 100 gauss, followed by thermal steps of 200, 300, 350, 400, 450, 500, 550, and 600uC. Each division 5 1026 emu.

Science Series 41


Figure 4 IRM acquisition (ascending curve on right) and Lowrie–Fuller test (two descending curves on left) of representative powdered samples from the Lincoln Creek and Astoria formations. Open circles 5 IRM; solid circles 5 ARM.

the remanence was completely gone by 500uC, which is more consistent with a mineral such as magnetite, which has a Curie temperature of 580uC. Thus, it seems likely that the remanence is held mainly in magnetite, with minor chemical overprinting by goethite or other iron hydroxides. Representative IRM acquisition analyses are shown in Figure 4. Most samples showed near saturation at 300 mT, suggesting that the remanence is held largely in magnetite, although the continuing increase in IRM suggests some hematite was present as well. In most samples, the ARM was more resistant to AF demagnetization than the IRM, suggesting that the remanence is held in single-domain or pseudo–single-domain grains. Most of the samples showed a direction pointed southwest and up at NRM and showed only a single component of magnetization, which decayed steadily to the origin (Fig. 3A and 3B). Because these data are dip-corrected, this is clearly a reversed direction that has undergone clockwise rotation. A few samples (Fig. 3C) had a slight overprint but showed the characteristic reversed and rotated direction by 300uC. Two sites (Fig. 3D) pointed east and down from NRM to 600uC, which is a normal direction with clockwise rotation. Thus, it is apparent that the direction is a primary or characteristic remanence and not an overprint or later magnetic component. This is confirmed by the reversal test (Fig. 5). The mean for the five normal samples was D 5

Figure 5 Stereoplot of mean poles and circles of confidence for the data discussed in this study. Solid circles indicate lower hemisphere projections; dashed lines and open circles indicate upper hemisphere projections. Solid square indicates mean of reversed samples projected through the origin to the lower hemisphere.



Table 1 Site statistics. ABBREVIATIONS: n 5 number of samples; D 5 declination (u); I 5 inclination (u); k 5 precision parameter; a95 5 ellipse of 95% confidence interval. Site

n

D

I

k

a95

1 2 3 4 5 6 7 8 9 10 11 12 Normal samples Reversed samples Formational mean

2 3 3 3 3 3 2 3 3 3 3 3 5 30 35

182.0 249.4 238.5 203.2 213.6 208.5 101.0 115.2 222.6 295.4 230.5 243.7 108.4 233.7 67.2

235.2 274.9 256.8 259.0 247.4 268.1 24.8 48.5 271.0 270.3 261.5 244.4 39.2 264.4 62.5

13.9 111.4 10.4 2.3 4.8 7.9 208.0 42.4 6.1 24.3 13.4 3.0 22.7 6.1 5.8

73.0 11.7 40.3 113.0 64.0 47.2 17.4 19.2 54.8 25.6 35.1 90.8 16.4 11.8 11.3

108.4, I 5 39.2, k 5 22.7, a95 5 16.4; the mean for the 30 reversed samples was D 5 233.7, I 5 264.4, k 5 6.1, a95 5 11.8. These directions, although not directly opposite one another, show overlapping circles of confidence, so they are antipodal within confidence limits (Fig. 5). Clearly, these directions are primary or characteristic directions, and not modern normal overprints, in that they are largely reversed and show a rotation consistent with that seen elsewhere in this area in rocks of this age. Inverting the reversed directions through the origin, the mean for the formation was D 5 67.2, I 5 62.5, k 5 5.8, a95 5 11.3. Once the overprinting had been removed and a stable component was isolated, each direction was summarized by the least squares method of Kirschvink (1980) and averaged with Fisher (1953) statistics (Table 1). Each site was then ranked according to the scheme of Opdyke et al. (1977). Of the 12 sites, seven were statistically separated from a random distribution at the 95% confidence level (Class I sites of Opdyke et al., 1977). Two sites had only two usable samples, so no site statistics could be calculated (Class II sites of Opdyke et al., 1977). The remaining three sites showed a clear polarity preference, but the third sample was divergent (Class III sites of Opdyke et al., 1977). The magnetic polarity stratigraphy is shown in Figure 6. The lower 100 m of the sampled section of the Lincoln Creek Formation is entirely reversed in polarity, including sites containing the gooseneck barnacle bed (Unit I of Moore, 1984a; 5 LACMIP 5844) and the decapod crustacean bed (Unit II of Moore, 1984a; 5 LACMIP 5843). The next 70 m of section are normal in polarity. The interval from 250 m to approximately 500 m in the section was covered by landslides, so it could not be sampled for paleomagnetic analysis. It includes the Aturia locality (Unit IV; 5 LACMIP 5842). From 500 m

to more than 600 m in the section, the exposed outcrop of Astoria Formation is entirely reversed in polarity, including localities LACMIP 5863 and LACMIP 5864 and the type locality of the desmatophocine otarioid pinniped Desmatophoca brachycephala Barnes, 1987. DISCUSSION A tentative correlation of this stratigraphic section with the global timescale of Berggren et al. (1995) and the Pacific Coast timescale of Prothero (2001) is shown in Figure 7. The presence of Liracassis apta and other Juanian Stage mollusks places the Lincoln Creek part of the section in the late Oligocene, and Moore (1984a) indicated that some of the other mollusks suggest affinities with those of the early Miocene Pillarian Stage, so she argued that the Knappton Lincoln Creek molluscan fauna is late Juanian in age. In the Canyon River section of the Lincoln Creek and Astoria formations (Prothero and Armentrout, 1985), the L. apta (Moore, 1984b) Zone (and Juanian Stage) spans magnetic Chrons C10n–C6Cr; this is also true of the type Juanian section in the Pysht Formation (Prothero et al., 2001c). Thus, the best correlation of the long reversed interval followed by the short normal magnetozone of the Knappton Lincoln Creek section is either Chrons C7r–C7n, or C7Ar–C7An (Fig. 7). The long talus-covered interval stratigraphically higher than site 8 (Fig. 6) makes the remaining correlations less certain. It could potentially obscure several magnetic polarity changes, so when the reversed section of the Astoria Formation is encountered, it is difficult to decide which of the many reversed early Miocene intervals best correlate with it. If the covered interval is a long one, or if the section has hidden unconformities, this reversed interval of Astoria Formation could correlate with

Science Series 41


Figure 6 Lithostratigraphy and magnetic stratigraphy of the Lincoln Creek and Astoria formations (stratigraphy after G.W. Moore [personal communication] and Moore [1984a]). LACMIP fossil localities of Moore (1984a) indicated. Solid circles indicate Class I sites (following Opdyke et al., 1977), which are statistically distinct from a random distribution at the 95% confidence level; circles with diagonal slash pattern are Class II sites, in which one sample was missing, so site statistics could not be calculated; open circles are Class III sites, in which one site was divergent, but the other two gave a clear indication of polarity.

Astoria beds as young as C5Br (15–16 Ma), as shown by magnetostratigraphic analysis of the Astoria Formation in the Newport, Oregon, area (Prothero et al., 2001a). However, several speci-

mens of Aturia angustata Conrad, 1848, have been found at LACMIP 5863 (encompassing our sites 9 and 10), low in the Astoria Formation. According to Moore and Moore (2002) this nautiloid became



Figure 7 Two possible correlations of the Lincoln Creek and Astoria formations at Knappton to the global and local timescale (following Prothero [2001] and Berggren et al. [1995]), and to the magnetostratigraphy of the Lincoln Creek and Astoria formations at Canyon River (after Prothero and Armentrout, 1985).

Science Series 41


extinct approximately 17 Ma, so we know that sites 9 and 10 are at least that old. Furthermore, fragments of the pectinid bivalve Vertipecten fucanus (Dall, 1909), a zonal mollusk for the Pillarian Stage (Addicott 1976a, 1981), have also been found at LACMIP 5863. Farther east, and higher in the Astoria Formation, LACMIP 5864 (encompassing our sites 11 and 12) marks the stratigraphically lowest occurrence, locally, of the gastropod Fulgoraria (Miopleiona) indurata (Conrad, 1849). According to Addicott (1976a) this would also mark the lower boundary of the Pillarian Stage, although it also occurs in the middle Miocene Newportian Stage. On the basis of correlations with the type Pillarian section in the Clallam Formation (Prothero and Burns, 2001), we suggest that the Astoria Formation east of Knappton best corresponds to one of the reversed intervals in Chron C6C: C6C1r, C6C2r, or later reversed intervals. We suggest that the Knappton section of the uppermost Lincoln Creek and Astoria formations is temporally equivalent to at least the lower part of the original (and now concealed) type section of the Astoria Formation, approximately 10 km to the south, in the city of Astoria, Oregon. It is significant that Dall and Harris (1892) divided the exposures of the Astoria Formation at Astoria, Oregon, into three stratigraphically successive members (Astoria shale, Aturia bed, and Astoria sandstone), and this would apply equally well to the section that is exposed at Knappton, Washington. Additionally, Addicott (1976a, 1976c) and Moore and Addicott (1987) referred the lower part of the original type section of the Astoria Formation to the Pillarian Stage. On the basis of correlations with the type section of the Pillarian Stage in the Clallam Formation (Prothero and Burns, 2001), we suggest that the Lincoln Creek and Astoria formations section at Knappton best corresponds to one of the reversed intervals in Chron CBC: CBC1r, CBC2r, or later reversed intervals, We cannot precisely date the very top of the section, but we know that it must have an age between 15 and 23 Ma. The mean direction D 5 67.2, I 5 62.5, k 5 5.8, a95 511.3 suggests about 77 6 11 degrees of clockwise tectonic rotation compared with the Oligocene cratonic pole of Diehl et al. (1983) and corrected according to Demarest (1983). The nearest basement rock, the Eocene Crescent Volcanics Domain 7 (of Wells and Coe, 1985; Table 1), has a clockwise rotation of 65 6 17u, and is thus statistically indistinguishable from our results. South of Knappton, in northern Oregon, rotations of 74 6 5u were found in the upper Eocene–lower Oligocene Keasey Formation (Prothero and Hankins, 2000) and of 72 6 11u in the lower Oligocene Pittsburg Bluff Formation (Prothero and Hankins, 2001). Thus, our rotational results are consistent with those of similar age reported in the nearest studies to the north and south.

The mean inclination of 62.5 6 11u suggests a paleolatitude of 46u, which is virtually identical to its present latitude of 46u179N. Thus, there is no indication in these rocks of any significant northward translation since the early Miocene (as expected for such young rocks after the emplacement of most of the Cascades and their basement terranes). ACKNOWLEDGMENTS We thank George Moore and Ellen Moore for the use of their unpublished stratigraphic section and for comments on the manuscript. Joseph Kirschvink provided access to his paleomagnetics laboratory. We acknowledge Lawrence G. Barnes, Ellen Moore, and two anonymous reviewers for helpful comments on the paper. This research was supported by a grant to D.R.P. from the Donors of the Petroleum Research Fund of the American Chemical Society, and by NSF grant EAR00-00174.

LITERATURE CITED Addicott, W.O. 1976a. Neogene molluscan stages of Oregon and Washington. In The Neogene Symposium, ed. A.E. Fritsche, H. TerBest, Jr., and W.W. Wornardt, 95–115. Pacific Section, SEPM. ———. 1976b. New molluscan assemblages from the upper member of the Twin River Formation, western Washington: Significance in Neogene biostratigraphy. United States Geological Survey Journal of Research 4:437–447. ———. 1976c. On the significance of the bivalve Acila gettysburgensis (Reagan) in the Middle Tertiary chronostratigraphy of the Pacific Coast. The Veliger 19:121–124. ———. 1981. Significance of pectinids in Tertiary biochronology of the Pacific Northwest. Geological Society of America Special Paper 184:17–37. Barnes, L.G. 1987. An early Miocene pinniped of the genus Desmatophoca (Mammalia: Otariidae) from Washington. Contributions in Science, Natural History Museum of Los Angeles County 382:1–20. Berggren, W.A., D.V. Kent, C.C. Swisher, III, and M.-P. Aubry. 1995. A revised Cenozoic geochronology and chronostratigraphy. SEPM, Special Publication, 54:129–212. Berglund, R.E., and J.L. Goedert. 1992. A new species of Cancer (Decapoda: Brachyura) from the Miocene Astoria Formation in Washington. Burke Museum Contributions in Anthropology and Natural History 9:1–11. Dall, W.H., and G.D. Harris. 1892. Neocene of North America. United States Geological Survey Bulletin 84:15–349. Demarest, H.H., Jr. 1983. Error analysis for the determination of tectonic rotation from paleomagnetic data. Journal of Geophysical Research 88:4321–4328. Diehl, J.F., M.E. Beck, Jr., S. Beske-Diehl, D. Jacobson, and B.C. Hearn, Jr. 1983. Paleomagnetism of the Late Cretaceous-early Tertiary north-central Montana alkalic province. Journal of Geophysical Research 88:10,593–10,609. Durham, J.W. 1944. Megafaunal zones of the Oligocene of northwestern Washington. University of California Publications in Geological Sciences 27:101–211.

72 & Science Series 41 Fisher, R.A. 1953. Dispersion on a sphere. Proceedings of the Royal Society of London, Series A, 217:295–305. Goedert, J.L., and R.L. Squires. 1993. First Oligocene records of Calyptogena (Bivalvia: Vesicomyidae). The Veliger 36:72–77. Kirschvink, J.L. 1980. The least-squares line and plane and analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society 62:699–718. Miller, A.K. 1947. Tertiary nautiloids of the Americas. Geological Society of America Memoir 23:1–234. Moore, E.J. 1984a. Molluscan paleontology and biostratigraphy of the lower Miocene upper part of the Lincoln Creek Formation in southwestern Washington. Contributions in Science, Natural History Museum of Los Angeles County 351:1–42. ———. 1984b. Middle Tertiary molluscan zones of the Pacific Northwest. Journal of Paleontology 58:718–737. ———. 1988. Diagenetic history of sequential calcite, barite, and quartz within the chambers of the Tertiary nautiloid Aturia, southwestern Washington. Saito Ho-on Kai Special Publication (Professor Tamio Kotaka Commemorative Volume), 193–201. Moore, E.J., and W.O. Addicott. 1987. The Miocene Pillarian and Newportian (molluscan) stages of Washington and Oregon and their usefulness in correlations from Alaska to California. United States Geological Survey Bulletin 1664:A1–A12. Moore, E.J., and G.W. Moore. 2002. Miocene molluscan fossils and stratigraphy, Newport, Oregon. Field Guide to Geologic Processes in Cascadia. Oregon Department of Geology and Mineral Industries Special Paper 36:187–200. Opdyke, N.D., E.H. Lindsay, N.M. Johnson, and T. Downs. 1977. The paleomagnetism and magnetic polarity stratigraphy of the mammal-bearing section of Anza-Borrego State Park, California. Quaternary Research 7:316–329. Pluhar, C.J., J.L. Kirschvink, and R.W. Adams. 1991. Magnetostratigraphy and clockwise rotation of the Plio-Pleistocene Mojave River Formation, central Mojave Desert, California. In Abstracts of Proceedings, 5th Annual Mojave Desert Quaternary Research Symposium, ed. J. Reynolds. San Bernardino County Museum Association Quarterly 38(2):31–42. Prothero, D.R. 2001. Chronostratigraphic calibration of the Pacific Coast Cenozoic: A summary. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:377–394. Prothero, D.R., and J.M. Armentrout. 1985. Magnetostratigraphic correlation of the Lincoln Creek Formation, Washington: Implications for the age of the Eocene/Oligocene boundary. Geology 13:208–211. Prothero, D.R., C.Z. Bitboul, G.W. Moore, and E.J. Moore. 2001a. Magnetic stratigraphy of the lower and middle Miocene Astoria Formation, Lincoln County, Oregon. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:272–283. Prothero, D.R., C.Z. Bitboul, G.W. Moore, and A.R. Niem. 2001b. Magnetic stratigraphy and tectonic rotation of the Oligocene Alsea, Yaquina, and Nye formations, Lincoln County, Oregon. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:184–194.

Prothero et al.: Paleomagnetism of Knappton, Washington Prothero, D.R., and C. Burns. 2001. Magnetic stratigraphy and tectonic rotation of the upper Oligocene– ?lower Miocene (type Pillarian stage) Clallam Formation, Clallam County, Washington. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:234–241. Prothero, D.R., and K.G. Hankins. 2000. Magnetic stratigraphy and tectonic rotation of the upper Eocene–lower Oligocene Keasey Formation, northwest Oregon. Journal of Geophysical Research 105(B7):16473–16480. ———. 2001. Magnetic stratigraphy and tectonic rotation of the lower Oligocene Pittsburg Bluff Formation, Columbia County, Oregon. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:201–209. Prothero, D.R., A. Streig, and C. Burns. 2001c. Magnetic stratigraphy and tectonic rotation of the upper Oligocene Pysht Formation, Clallam County, Washington. In Magnetic stratigraphy of the Pacific Coast Cenozoic, ed. D.R. Prothero. Pacific Section, SEPM, 91:224–233. Rathbun, M.J. 1926. The fossil stalk-eyed Crustacea of the Pacific Slope of North America. Smithsonian Institution Bulletin 138:1–153. Rigby, J.K., and D.E. Jenkins. 1983. The Tertiary sponges Aphrocallistes and Eurete from western Washington and Oregon. Contributions in Science 344:1–13. Schweitzer, C.E., and E.W. Salva. 2000. First recognition of the Cheiragonidae (Decapoda) in the fossil record and comparison of the family with the Atelecyclidae. Journal of Crustacean Biology 20:285–298. Squires, R.L. 1989. Pteropods (Mollusca: Gastropoda) from Tertiary formations of Washington and Oregon. Journal of Paleontology 63:443–448. Squires, R.L., and J.L. Goedert. 1994. A new species of the volutid gastropod Fulgoraria (Musashia) from the Oligocene of Washington. The Veliger 37:400–409. Weaver, C.E. 1937. Tertiary stratigraphy of western Washington and northwestern Oregon. University of Washington Publications in Geology 4:1–266. ———. 1942 (1943). Paleontology of the marine Tertiary formations of Oregon and Washington. University of Washington Publications in Geology 5:1–789. Wells, R.E. 1979. Geologic map of the Cape Disappointment–Naselle River area, Pacific County, Washington. United States Geological Survey, Open-File Report. 79-389, scale 1:48,000. ———. 1989. Geologic map of the Cape Disappointment– Naselle River area, Pacific and Wahkiakum counties, Washington. United States Geological Survey Miscellaneous Investigations Series, Map I-1832. Wells, R.E., and R.S. Coe. 1985. Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington: Implications for mechanisms of tectonic rotation. Journal of Geophysical Research 90:1925–1947. Zullo, V.A. 1982. Arcoscapellum Hoek and Solidobalanus Hoek (Cirripedia, Thoracica) from the Paleogene of Pacific County, Washington, with a description of a new species of Arcoscalpellum. Contributions in Science 336:1–19. Received 3 April 2003; accepted 21 May 2007.