Rattlesnake Formation, Central Oregon

PaleoBios 26(1):21–26,26(1):21–26, MayMay 115,5, 2006 © 2006 University of California Museum of Paleontology Magnetic stratigraphy of the Upper Miocene (Hemphillian) Rattlesnake Formation, central Oregon DONALD R. PROTHERO1, JONATHAN M. HOFFMAN2, and SCOTT E. FOSS3 1Department of Geology, Occidental College, Los Angeles, CA 90041 2Department of Geology, University of Florida, Gainesville, FL 32611 3Bureau of Land Management, Utah State Offi ce, P.O. Box 45155, Salt Lake City, UT 84145 The Rattlesnake Formation near Picture Gorge in the John Day region of central Oregon consists of about 120 m of siltstones and conglomerates punctuated by several tuff beds. This formation is well known for its early Hemphillian mammals, and it was originally part of the Wood Committee’s (1941) concept of the Hemphillian. Paleomagnetic samples were collected from the type section of the Rattlesnake Formation between Rattlesnake and Cottonwood Creeks 2 km south of Picture Gorge. Samples were demagnetized with both alternating fi eld and thermal demagnetization, and yielded a stable remanence held mainly in magnetite. After cleaning, the normal and reversed directions passed a reversal test, so the remanence is interpreted to be primary. Almost the entire section is reversed in polarity except for the basal 10 m and a single site near the top of the section. Based on 40Ar/39Ar datesdates of eithereither 77.2.2 Ma or 7.05 ± 0.01 Ma on the Rattlesnake Ash Flow Tuff near the top of the section, we correlate the section with magnetic Chrons C3Bn to C3Br2n (6.9–7.3 Ma), or late early Hemphillian in age (Hh2 of Tedford et al., 2004). INTRODUCTION the term “Rattlesnake Formation” to refer to the entire unit Merriam (1901) fi rst described the Rattlesnake Forma- in the original sense of Merriam (1901). tion, which is the youngest Miocene unit in the John Day The early Hemphillian age of the Rattlesnake Fauna was region. Merriam and Sinclair (1907) and Merriam et al. apparent to Wood et al. (1941), even though at that time (1925) documented its fossils and stratigraphy, and several they thought the Hemphillian was middle Pliocene. Revisions paleontologists have examined and expanded the collections of the Miocene-Pliocene boundary caused Tedford et al. since then (reviewed in Martin, 1983; see also Martin and (1987) to correlate the Hemphillian with the late Miocene. Fremd, 2001). The Wood Committee (1941) designated the Fremd et al. (1994, Fig. 12) reported 23 genera of mammals, Rattlesnake Fauna as one of their principal reference faunas along with turtle and frog remains, from the fossiliferous for the early Hemphillian. conglomerates of the Rattlesnake Formation; that faunal list Merriam (1901), Merriam and Sinclair (1907), and Mer- will be refi ned soon (Martin, pers. commun.). riam et al. (1925) used the name “Rattlesnake Formation” Numerous radiometric dates have been reported for the for the entire fossiliferous mammal-bearing unit that uncon- RAFT. Parker and Armstrong (1972) obtained K-Ar dates of formably overlies the Mascall Formation. This terminology 6.6 ± 0.1 Ma and 6.8 ± 0.2 Ma on sanidines from the RAFT. was followed by Enlows (1973, 1976), who described the Walker (1979) reported K-Ar dates ranging from 5.95 ± 0.18 lithology of the “Rattlesnake Formation” in detail. However, Ma to 6.7 ± 0.4 Ma. Streck and Grunder (1995) reported a 40 39 most of the conglomerates and siltstones of the unit are very Ar/ Ar datedate of 7.057.05 ± 0.010.01 Ma, based on 1515 single-crystalsingle-crystal unresistant and localized, whereas the thick resistant Rattle- analyses of alkali feldspars. Swisher (in Martin and Fremd, 40 39 snake Ash Flow Tuff (RAFT) forms a prominent bench. This 2001) reported a Ar/Ar/ ArAr ddateate ooff 77.2.2 MMaa ((nono eerrorrror eestimatestimate bench occurs not only in the John Day region, but the RAFT or details given). Thus, a variety of ages for the RAFT exist was once widespread over central Oregon, covering an area in the literature, although they are beginning to converge in of at least 9000 square kilometers, and possibly as great as their age estimates and the error estimates are decreasing. 40,000 square kilometers with a tuff sheet at least 30 m thick, and up to 70 m thick in places (Streck and Grunder, 1995). METHODS Because of the prominence of this tuff unit, Walker (1979, Sampling of the lower part of the section was conducted 1990) restricted the name “Rattlesnake Formation” to just in the summer of 2001 along the south-facing exposures the RAFT. This leaves the fossiliferous sediments above and (Fig. 1) beneath the RAFT in the type section (NW SE below the RAFT without a name, and ignores the histori- NE SW section 19 T11S R26E, Picture Gorge 7.5-minute cal precedent of Merriam, Enlows and others. J.E. Martin quadrangle, Grant County, Oregon; GPS coordinates UTM (pers. commun.) is currently revising the lithostratigraphy NAD27 Zone 11 E0289782m N4931979m). Retallack et and biostratigraphy of this unit and tentatively recommends al. (2002) described this section in detail. Nine sites (three that the RAFT be designated a formation-rank unit within samples per site) were collected from the 77 m interval below the Rattlesnake Group (Martin and Fremd, 2001). Until the RAFT. The uppermost tuffaceous silty beds above the this change is formally published, we will continue to use RAFT (fi ve sites from a 30 m interval) were sampled to the 22 PALEOBIOS, VOL. 26, NUMBER 1, MAY 2006 A. John Day Fossil Beds National Monument John Day Hwy. 19 N B. J P o S h n B D o k a u e y n re R d C i a v r k y c e o r R “Lower Rattlesnake Section” k ree e C nak tles Rat ek re H C w d y o . o 2 “Upper Rattlesnake nw 6 tto Section” Co 1 mile Fig. 1. A. Index map showing location of the Rattlesnake For- mation in central Oregon. B. Detail of the Picture Gorge 7.5- minute quadrangle, showing the location of the two stratigraphic Fig. 2. Orthogonal demagnetization (“Zijderveld”) plots of rep- sections in the Picture Gorge-Cottonwood Creek area. resentative samples. Solid squares indicate declination (horizontal component); open squares indicate inclination (vertical component). First step is NRM, followed by AF steps of 25, 50, and south of the type section (GPS coordinates UTM NAD27 100 Gauss, then thermal steps from 300 to 600°C. Each division Zone 11 E0290540m N4292591m). The RAFT itself was not equals 10-5 emu. sampled because its paleomagnetism has already been studied; it is reversed in polarity (Stimac and Weldon, 1996). Each oriented sample was taken with simple hand tools by (“Zijderveld”) plots, and average directions of each sample scraping a horizontal surface at the top of the block of rock. were determined by the least-squares method of Kirschvink Samples that were too crumbly in the fi eld were hardened (1980). Mean directions for each sample were then analyzed with sodium silicate. In the laboratory, the block samples using Fisher (1953) statistics, and classifi ed according to the were cored with a drill press; those too small or crumbly to scheme of Opdyke et al. (1977). withstand the drilling were fi rst molded into disks of Zircar About 0.1 g of powdered rock from several samples was aluminum ceramic . The core samples were then measured subjected to increasing isothermal remanent magnetization on a 2G cryogenic magnetometer with an automatic sample (IRM) to determine their IRM acquisition behavior, and thus changer at Caltech. After measurement of NRM (natural re- the relative abundance of magnetite or hematite. They were also manent magnetization), they were demagnetized in alternating AF demagnetized twice, once after having acquired an IRM fi elds (AF) of 25, 50, and 100 Gauss to prevent the remanence produced in a 100 mT (millitesla) peak fi eld and once after of multi-domain grains from being baked in and to examine having acquired an anhysteretic remanent magnetization (ARM) the coercivity behavior of each specimen. AF demagnetization in a 100 mT oscillating fi eld. Such data are useful in conduct- was followed by thermal demagnetization of every sample at ing a modifi ed Lowrie-Fuller test (Pluhar et al., 1991), which steps from 300 to 600°C to remove high-coercivity chemi- indicates whether single or multi-domain grains are present. cal overprints due to iron hydroxides such as goethite, and to determine how much remanence was left after the Curie RESULTS temperature of magnetite (580°C) was exceeded. Representative orthogonal demagnetization plots are Results were plotted on orthogonal demagnetization shown in Fig. 2. In Fig. 2A, the sample exhibits normal PROTHERO ET AL.—MAGNETIC STRATIGRAPHY OF THE RATTLESNAKE FORMATION 23 GRAY TUFFACEOUS SILTSTONE suggesting that the remanence is held largely in magnetite, 100 although the continuing slight increase in IRM suggests some hematite was present as well. In most samples, the ARM 90 was more resistant to AF demagnetization than the IRM, 80 suggesting that the remanence is held in single-domain or 70 pseudo-single-domain grains. M R Mean directions for all sites are given in Table 1. All but I 60 f o three of the 14 sites were reversed in polarity, and all but two t 50 n of the sites were statistically signifi cant, i.e., separated from e c r 40 a random distribution at the 95% confi dence level (Class I e P 30 sites of Opdyke et al., 1977). One site was missing a sample that crumbled, so it was considered a Class II site (Opdyke 20 et al., 1977).

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