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PaleoBios 26(1):27–32,26(1):27–32, MayMay 115,5, 2006 © 2006 University of California Museum of Paleontology Magnetic stratigraphy of the lower portion of the middle Miocene Mascall Formation, central Oregon DONALD R. PROTHERO1, ELIZABETH DRAUS2, and SCOTT FOSS3 1Department of Geology, Occidental College, Los Angeles, CA 90041. 2Department of Geology, University of Nebraska, Lincoln, NE 68588. 3Bureau of Land Management, Utah State Offi ce, P.O. Box 45155, Salt Lake City, UT 84145 The Mascall Formation in central Oregon consists of about 350 m of volcaniclastic fl oodplain siltstones and sandstones exposed in numerous fault blocks in the John Day region of central Oregon. It yields a famous early Barstovian mam- malian fauna (part of the Wood Committee’s 1941 original concept of the Barstovian) that includes at least 33 species of mammals, as well as birds, turtles, fi sh, and freshwater gastropods. The most complete section in the type area was sampled using oriented block sampling. The samples were then subjected to both alternating fi eld demagnetization at 25, 50, and 100 Gauss, followed by thermal demagnetization at 50°C steps from 200 to 630°C. Most samples yielded a stable single component of remanence that passed a reversal test, and was held largely in magnetite with minor goethite overprints. The lower half of the section is of reversed polarity, followed by shorter magnetozones of normal, reversed, and normal polarity to the top of the section. Based on dates of 16.2±1.4 Ma at the base of the section, and 15.77± 0.07 Ma in the lower part of the section, we correlate the Mascall Formation with Chron C5Br to Chron C5Bn1n (14.8–16.0 Ma). This correlation is consistent with the early Barstovian age of the fauna, and it matches the pattern seen in other Barstovian magnetostratigraphic sections, such as those at Barstow, California; Virgin Valley and Massacre Lake, Nevada, and Pawnee Creek, Colorado. INTRODUCTION The Mascall Formation in the John Day region of central Oregon (Fig. 1) has been an important locality for fossil vertebrates since it was fi rst collected in the 1870s. Merriam John Day Fossil Beds (1901) was the fi rst to describe the Mascall Formation and National Monument A. its fauna, and Merriam and Sinclair (1907) gave the fi rst John Day comprehensive review of the fauna. Many other authors fol- lowed with discussions about Mascall fossils or geology (sum- marized in Downs, 1956, Fremd et al., 1994, and Bestland, 1998). Originally, some specimens of the Mascall fauna were mixed with those of the underlying John Day Formation Hwy. 19 N (Arikareean-Hemingfordian) and the overlying Rattlesnake J P o S h Formation (Hemphillian), but studies by Downs (1956) and n B D o k a u subsequent authors have largely clarifi ed this confusion. The e y n re R d C i a B. v r Wood Committee (1941) regarded the Mascall Fauna as one k y c e o r of the typical assemblages on which they based their concept R of the Barstovian, and subsequent authors (Downs, 1956; Tedford et al., 1987, 1994) have consistently regarded its age as early Barstovian. The fauna contains at least 33 species of “The Bowl” mammals (Fremd et al., 1994, p. 32), including at least six “First k ree Red Hill” taxa of horses, the rhinoceroses Teleoceras medicornutum and e C ak sn Aphelops megalodus (Prothero,(Prothero, 2005), rabbits, twotwo generagenera ttle “Second Red Hill” Ra ek each from the families Canidae, Amphicyonidae, and Mus- re H C w d y o . telidae, fi ve families of rodents, two genera of proboscideans o 2 nw 6 tto (contra TedfordTedford et al., 11987,987, p. 1161),61), as wwellell as ororeodonts,eodonts, Co camels, dromomerycids and blastomerycids, antilocaprids, 1 mile and tayassuids. Small collections from the Mascall Formation were fi rst made for Cope and Marsh in the early 1870s, but Fig. 1. Map of localities discussed in this paper. A. Location map most of the stratigraphically useful collections were made by of the Mascall Formation within Oregon (area 1) (after Downs, 1956). B. Detail of location of “The Bowl” and “First Red Hill” the UCMP, starting with Merriam’s expeditions in 1899. and “Second Red Hill” sections (after Bestland, 1998, Fig. 4). There is an important fl oral locality in the Mascall Formation 28 PALEOBIOS, VOL. 26, NUMBER 1, MAY 2006 that was described by Chaney (1925). Results were plotted on orthogonal demagnetization The classic stratigraphic study of the Mascall Formation (“Zijderveld’) plots, and average directions of each sample was published by Downs (1956). Kuiper (1988) conducted were determined by the least-squares method of Kirschvink a more detailed stratigraphic study that remains unpublished, (1980). Mean directions for each sample were then analyzed and Bestland (1998) and Bestland et al. (this volume) pub- using Fisher (1953) statistics, and classifi ed according to the lished a summary of the paleosols in the Mascall Formation. scheme of Opdyke et al. (1977). Swisher (1992) reported a 40Ar/39Ar datedate of 15.77±15.77± 0.070.07 About 0.1 g of powdered rock from several samples was Ma on plagioclase from a tuff in unit 2 of Downs (1956), subjected to increasing isothermal remanent magnetization near the base of the formation and 25 feet below the level (IRM) to determine their IRM acquisition behavior, and thus of Downs’ (1956) mammal-bearing unit 5. A prominent tuff the relative abundance of magnetite or hematite. They were bed at the base of the Mascall Formation was dated using also AF demagnetized twice, once after having acquired an K-Ar methods at 16.2 ± 1.4 Ma (Fiebelkorn et al., 1983). The IRM produced in a 100 mT (millitesla) peak fi eld and once Dayville Basalt of the Columbia River basalt group below after having acquired an anhysteretic remanent magnetization the Mascall Formation has been dated using K-Ar methods (ARM) in a 100 mT oscillating fi eld. Such data are useful at 16.3–16.5 Ma (Long and Duncan, 1982; Hooper and in conducting a modifi ed Lowrie-Fuller test (Pluhar et al., Swanson, 1990), and diatomaceous sediments interbedded 1991), which indicates whether single or multi-domain grains with the basalt fl ows at Picture Gorge have been dated using are present. 40Ar/39Ar methodsmethods at 116.06.0 Ma (Swisher(Swisher,, 11992,992, in BesBestland,tland, 1998). Thus, the lower age of the Mascall Formation is well RESULTS constrained, but there are no dates on the upper part of the Orthogonal demagnetization (“Zijderveld”) plots of section as yet. representative samples are shown in Figure 2. In nearly every sample, there was a single component of remanence METHODS that was pointed either north and up at NRM, indicating In the summer of 2001, sampling was conducted on the that the sample is of normal polarity (Fig. 2A), or south thickest, most continuous, and most fossiliferous section of and down at NRM, indicating that the sample is reversed the Mascall Formation (Fig. 1), known as the “type section” in polarity (Fig. 2B-C). There was a slight high-coercivity of Downs (1956), Fremd et al. (1994), and Bestland (1998). component (shown by the minimal drop of intensity in the The section was measured by Foss and Matt Kohn, following the sections published by Bestland (1998). The lower part of the section followed Bestland’s (1998, Figs. 4, 6) section in “The Bowl”, and the upper part of the section was taken in Bestland’s (1998, Figs. 4, 7, 8) section through the “First Red Hill” and “Second Red Hill”, in the SE sec. 19 T12S R26E, Picture Gorge 7.5-minute quadrangle, Grant County, Oregon. Thirteen paleomagnetic sites (three samples per site) were taken from regularly spaced intervals spanning 110 m of section, thus covering nearly all the 125 m of Mascall Forma- tion exposed in the area (Bestland, 1998, Fig. 5). Samples were taken as oriented blocks of rock with simple hand tools, and then wrapped and carried back to the laboratory. There they were subsampled into cores using a drill press, or if the sample was too crumbly, casts into disks of Zircar aluminum ceramic. The samples were then analyzed on a 2G Enterprises cryogenic magnetometer with an automatic sample changer at the California Institute of Technology. After measure- ment of natural remanent magnetization (NRM), they were demagnetized in alternating fi elds (AF) of 25, 50, and 100 Gauss to prevent the remanence of multi-domain grains from being baked in, and to examine the coercivity behavior of A. B. C. D. each specimen. AF demagnetization was followed by thermal Fig. 2. Orthogonal demagnetization (“Zijderveld”) plots of rep- demagnetization of every sample at steps from 300 to 600°C resentative samples. Solid squares indicate declination (horizontal to get rid of high-coercivity chemical overprints due to iron component); open squares indicate inclination (vertical compo- hydroxides such as goethite, and to determine how much nent). First step is NRM, followed by AF steps of 25, 50, and remanence was left after the Curie temperature of magnetite 100 Gauss, then thermal steps from 300 to 600°C. Each division (580°C) was exceeded. equals 10-6 emu. PROTHERO ET AL.—MAGNETIC STRATIGRAPHY OF THE MASCALL FORMATION 29 fi rst three demagnetization steps in Fig. 2), probably due to (1998, Fig. 6) “The Bowl” section is of reversed polarity as some chemical remanence due to iron hydroxides, such as well. The lower part of this section contains the dated tuffs goethite. Apparently the overprint was not signifi cant, be- of Swisher (1992) and Fiebelkorn et al.

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