Lithostratigraphy, Tephrochronology, and Rare Earth Element Geochemistry of Fossils at the Classical Pleistocene Fossil Lake Area, South Central Oregon James E. Martin, Doreena Patrick,1 Allen J. Kihm,2 Franklin F. Foit Jr.,3 and D. E. Grandstaff4 Museum of Geology, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, South Dakota 57701, U.S.A. (e-mail: [email protected]) ABSTRACT One of the most famous fossiliferous Pleistocene sites in the Pacific Northwest is Fossil Lake, Oregon. Until recently, fossil collections from the area were not stratigraphically controlled, owing to the lack of a detailed stratigraphic and chronologic framework. Our field studies reveal at least nine exposed thin rhythmic fining-upward depositional packages, most separated by disconformities. Analysis of interbedded tephras reveals that the Rye Patch Dam (∼646 ka), Dibekulewe (∼610 ka), Tulelake T64 (∼95 ka), Marble Bluff (47 ka), and Trego Hot Springs (23.2 ka) tephra layers are present in the section, indicating deposition from more than ∼646 ka to less than 23 ka, which includes both the late Irvingtonian and Rancholabrean North American land mammal ages, a much longer time span than previously believed. Bones analyzed from eight of the defined units have distinctly different rare earth element (REE) signatures. Fossils obtain REE during early diagenesis, and signatures are probably closely related to lake water compositions. REE signatures in fossils from lower packages suggest uptake from neutral pH waters. In contrast, REE signatures become increasingly heavy REE–enriched up-section, with positive Ce anomalies in the upper units. REE signatures in fossils from the upper units are very similar to waters from modern alkaline lakes, such as Lake Abert, Oregon, suggesting diagenetic uptake in increasingly alkaline and saline waters. These REE changes suggest increasing aridity up-section, a contention reinforced by the habitat preferences of the terrestrial vertebrates preserved. Introduction The Fossil Lake area of Lake County, Oregon, may brates. Although the region is well known, rela- represent the most important Quaternary verte- tively little dating of the deposits has been under- brate paleontological region in the Pacific North- taken. Most age speculations have been made on west. Fossil vertebrates and invertebrates have the basis of specimens collected, with the excep- been known from Fossil Lake since 1876, and since tion of a radiometric date of 29,000 yr B.P. (Allison 1989, very large collections have been made from 1966, 1979) from snail shells near the base of the these Pleistocene deposits, culminating in tens of stratigraphic section. This date and many fossils thousands of stratigraphically collected verte- suggest the Late Pleistocene (Rancholabrean North American land mammal age; NALMA), which be- Manuscript received May 4, 2004; accepted November 4, gins at 150 ka (Bell et al. 2004). However, no evi- 2004. dence of Bison, the primary index fossil for the Ran- 1 Author for correspondence: Department of Earth and En- cholabrean NALMA, had been published, and only vironmental Science, University of Pennsylvania, 254-B Hayden Hall, 240 South 33rd Street, Philadelphia, Pennsylvania 19104- two possible specimens have been identified sub- 6316, U.S.A.; e-mail: [email protected]. sequently. The lack of diagnostic Bison remains 2 Department of Geosciences, Minot State University, Minot, made the Rancholabrean designation somewhat North Dakota 58707, U.S.A.; e-mail: [email protected]. problematic. To refine the temporal relationships 3 Department of Geology, Washington State University, Pull- man, Washington 99164, U.S.A.; e-mail: [email protected]. and environmental changes during the Late Pleis- 4 Department of Geology, Temple University, Philadelphia, tocene, our investigations have concentrated on Pennsylvania 19122, U.S.A.; e-mail: [email protected]. careful stratigraphic collection of paleontological [The Journal of Geology, 2005, volume 113, p. 139–155] ᭧ 2005 by The University of Chicago. All rights reserved. 0022-1376/2005/11302-0002$15.00 139 140 J. E. MARTIN ET AL. Table 1. Glass Chemistry of Composite Tephra Samples from Fossil Lake, Lake County, Oregon Rye Patch Dibekulewe Sample T64 Fossil Lake Dam S28- Fossil Lake ash bed Fossil Lake Tulelake Fossil Lake Marble Bluff Fossil Lake Trego Hot Oxide tephra A 505.8 tephra B LD-19 tephra C (17.01 m) tephra D MSH set C tephra E Springs SiO2 71.75 (.36) 72.20 77.11 (.11) 76.7 70.19 (.26) 70.6 76.74 (.10) 76.5 75.93 (.31) 75.50 (.20) Al2O3 14.58 (.10) 14.55 12.83 (.03) 13.2 15.09 (.06) 14.9 13.76 (.07) 14.0 13.44 (.06) 13.60 (.10) Fe2O3 3.44 (.12) 3.40 1.32 (.03) 1.30 3.16 (.03) 3.15 .95 (.02) .99 1.64 (.10) 1.58 (.04) TiO2 .47 (.04) .44 .07 (.01) .07 .63 (.01) .66 .10 (.01) .10 .23 (.01) .23 (.01) Na2O 4.24 (.18) 3.70 3.58 (.02) 4.06 4.58 (.24) 4.8 4.10 (.01) 4.1 4.02 (.20) 4.50 (.10) K2O 3.52 (.13) 3.68 4.28 (.09) 4.06 2.82 (.01) 2.92 2.49 (.02) 2.4 3.34 (.04) 3.32 (.06) MgO .42 (.05) .35 .06 (.00) .05 .81 (.01) .70 .25 (.01) .24 .24 (.01) .21 (.01) CaO 1.44 (.11) 1.46 .59 (.01) .61 2.55 (.01) 2.29 1.55 (.00) 1.50 1.05 (.03) 1.02 (.03) Cl .11 (.02) .13 .12 (.01) n.d. .14 (.01) n.d. .05 (.00) .07 .10 (.01) n.d. Totala 100 100 100 100 100 100 100 100 100 100 No. tephra samples 14324 No. shards analyzed 17 67 49 32 79 Similarity coefficient .97 .96 .96 .98 .97 Note. Also shown are compositions of reference tephras, similarity coefficients, numbers of samples and shards analyzed, and the analytical standard deviation (SD); 1 SD of the analyses given in parentheses.MSH p Mount St. Helens; n.d. p not determined. Source. Rye Patch Dam, Williams (1994); Dibekulewe ash bed, Davis (1978); Sample T64 Tulelake and Trego Hot Springs, Rieck et al. (1992); Marble Bluff MSH set C, Davis (1985). a Analyses normalized to 100% on a volatile-free basis. samples from discrete lithologies. We identified ticle presents the first major study of REE in fossils tephra layers interbedded within these lithological from a lake-dominated environment. units that aid in the temporal resolution. Herein, we describe the detailed lithostratigraphy, tephro- chronology, and rare earth element (REE) signatures Analytical Methods of the fossils and their implications for the history Electron Microprobe Analysis of Tephra. Mounts and evolution of Fossil Lake. for analysis of tephra samples consisted of four 8- Since its discovery in 1876, Fossil Lake has pro- mm-diameter glass rings (cells) fixed to a standard duced thousands of invertebrate and vertebrate fos- petrographic slide using epoxy, allowing four sam- sils; many were described in the late nineteenth ples to be prepared simultaneously. A few hundred and early twentieth centuries. Fish (Cope 1883), milligrams of tephra followed by a few drops of birds (Shufeldt 1913; Howard 1946), and mammals epoxy were added to each cell, thoroughly mixed, (Elftman 1931; Martin 1996) are particularly abun- and allowed to cure. The cells were trimmed using dant. However, none of the early collections were a microdiamond saw, ground to thin section thick- stratigraphically controlled. Therefore, the area has ness, polished, and carbon coated. been recollected over the last 15 yr, resulting in The composition of the glass in the tephra was thousands of specimens tied to thin lithostrati- determined using the Cameca Camebax electron graphic units. A future goal is to analyze the new microprobe in the GeoAnalytical Laboratory lo- collections in a temporal framework; therefore, a cated in the Department of Geology at Washington detailed lithostratigraphic framework combined State University. The analytical conditions are the with tephrochronology was required. The first por- same as those described in Hallett et al. (2001), and tion of this contribution represents the results of the tephra glass compositions are presented in table these undertakings. Based on the framework de- 1. The compositions of the glasses were compared scribed herein, the second portion of the contri- to one another and to the standard glasses in the bution represents an analysis of REE signatures in GeoAnalytical Laboratory’s database of more than fossils, which was to provide an independent as- 1500 Pacific Northwest tephra samples using the sessment of paleoenvironments that could be com- similarity coefficient (SC) of Borchardt et al. (1972) pared with results derived from the fossil succes- as a discriminator. The SC is the average of eight sions. REE signatures in fossils reflect the REE oxide concentration ratios between the glasses in content of the original pore waters during the fos- the tephra sample and tephra standard. It was cal- silization process (Henderson et al. 1983; Trueman culated using a weighting of 1.0 for the Si, Al, Ca, 1999) and may be used to infer original paleoen- Fe, and K oxide concentration ratios, 0.50 for the vironmental or early diagenetic conditions (Elder- Na oxide ratio, and 0.25 for the Mg and Ti oxide field and Pagett 1986; Patrick et al. 2004). This ar- ratios. Na was a given lower weighting because of Journal of Geology FOSSIL LAKE TEPHROCHRONOLOGY 141 well-recognized impediments to accurate measure- Ultrex-grade HNO3 and diluted to 100 mL. When ment and susceptibility to post-depositional envi- only smaller masses of bone were available, the ronmental modification. Mg and Ti were given initial solution was diluted to smaller volumes to lower weighting because of their low concentra- preserve the mass/volume ratio.