PaleoBios 26(1):27–32,26(1):27–32, MaMayy 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 TTedfordedford 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 1 15.77±5.77± 0.0 0.077 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 metmethodshods 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. (1983). Most of the cause it was quickly removed by thermal demagnetization at mammal fossils come from the upper part of this lower half 200°C (above the temperature at which iron hydroxides are of the section (unit 5 of Downs, 1956, p. 205), or from the dehydrated to hematite). The rest of the remanence appears section around the Mascall Tuff. to be held in magnetite, since it declined in intensity rapidly The Mascall Tuff (site 7), however, was of normal polar- through thermal demagnetization, and vanished above the ity, as was the site immediately above it (site 8, located 2 m Curie temperature of magnetite and no remanence was left above the Mascall Tuff on the “First Red Hill” section of at 600°C (Fig. 2). A few samples (Fig. 2D) had a slight Bestland, 1998, Fig. 7). The rest of the “First Red Hill” overprint, which was removed by 300°C. section of Bestland (1998, Fig. 7) is of reversed polarity. The A representative IRM acquisition analysis is shown in entire “Second Red Hill” section of Bestland (1998, Fig. 8) Figure 3. Most samples showed near saturation at 300 mT, is of normal polarity (sites 11–13). suggesting that the remanence is held largely in magnetite, although the continuing slight increase in IRM suggests some DISCUSSION hematite was present as well. In most samples, the ARM The magnetostratigraphic correlation of the type section was more resistant to AF demagnetization than the IRM, of the Mascall Formation is shown in Fig. 6. The dates suggesting that the remanence is held in single-domain or of 15.77±0.07 Ma and 16.2±1.4 Ma near the base of the pseudo-single-domain grains. section constrain the correlation of the long basal reversed Since both normal and reversed polarities were obtained, a magnetozone, so that it must correlate with magnetic Chron reversal test for stability was conducted (Fig. 4). The normal C5Br (15.2–16.0 Ma). Although there are no radiometric mean (D=353.8, I=46.3, k=13.1, α95=11.0, n=15) and the age constraints on the top of the section, the presence of reversed mean (D=200.3, I=-53.0, k=14.1, α95=8.2, n=24) early Barstovian mammals in this part of the section suggests are antipodal within error estimates, so the remanence is a that the normal-reversed-normal pattern of the upper half primary or characteristic remanence, and overprinting has been removed. Eight of the 13 sites were normal in polarity; the rest N were reversed in polarity (Fig. 5). Their site statistics are given in Table 1. All sites were statistically signifi cant, i.e., separated from a random distribution at the 95% confi dence level (Class I sites of Opdyke et al., 1977). The entire basal 40 m (sites 1–6) of the type section (up to the Mascall Tuff) was reversed in polarity, so the lower 40 m in Bestland’s

GRAY SILTSTONE 100

90

80

70 M R

I 60 f o

t 50 n e c

r 40 e

P 30

20

10 Fig. 4. Stereonet of means of normal and reversed sites. Solid 0 circle and solid line indicates lower hemisphere projection of the 1 3 5 10 30 50 100 300 1000 mean of all normal samples, and its ellipse of confi dence. Open Magnetic Field (mT) circle and dashed line indicate mean of reversed samples (upper Fig. 3. IRM acquisition (ascending curve on right) and Low- hemisphere projection). Solid square shows projection of reversed rie-Fuller test (two descending curves on left) of a represen- mean into lower hemisphere. Means are antipodal within error tative powdered sample from the Mascall Formation. Open estimates, so the directions are primary and overprinting has been squares =IRM; solid squares =ARM. removed. 30 PALEOBIOS, VOL. 26, NUMBER 1, MAY 2006

LITHO- MAGN. DECLINATION INCLINATION LOGY SITES 90 180 270 0 90 -90 +90 Ft M 13

300 12 100 11

10

9 200 8 50 Mascall Tuff 7

100 6

5 4 Tuffaceous ss. with black 3 glass 2 0 0 1

Fig. 5. Lithostratigraphy and magnetic stratigraphy of the type section of the Mascall Formation. Stratigraphy after Bestland (1998). Declination and inclination of magnetic sites are shown. Solid circles are sites that are statistically removed from a random distribution at the 95% confi dence level (Class I sites of Opdyke et al., 1977). Y

Table 1. Paleomagnetic data from the Mascall Formation. T I N H A R O C A M L R L O H A P O

SITE DEC INC K α95 Ma E P C N 14 C5ACr 2

1 172.7 -59.5 17.1 30.7 n a i Proboscidean Datum v o 2 197.2 -40.4 35.5 21.0 C5ADn t s r a B 3 202.1 -48.0 15.2 32.7 e MASCALL n

e C5ADr FORMATION c

4 217.2 -66.2 54.6 16.8 o i C5Bn1n 15 M C5Bn1r e 1 Early 5 225.4 -68.1 21.9 27.0 l C5Bn2n d n Barstovian Mascall Tuff d a i i mammals v

6 183.7 -46.3 160.0 9.8 M

o Ar/Ar 15.8 Ma t s r

7 330.2 41.7 6.9 50.9 a Ar/Ar 16.2 ± 1.4 Ma

C5Br B 8 3.0 51.9 51.4 17.4 9 230.0 -50.6 10.2 40.8 16 10 191.5 -33.7 16.4 31.4 Fig. 6. Correlation of the Mascall Formation paleomagnetic 11 15.7 53.4 270.3 7.5 section, based on the dates and age constraints discussed in the text. Radiometric dates after Swisher (1992) and Fiebelkorn et al. 12 5.5 37.0 7.0 50.6 (1983). Time scale after Berggren et al. (1995). “Proboscidean Datum” after Tedford et al. (1987). 13 358.3 41.1 16.8 31.1

the interval from 14.8–16.0 Ma, and most of the Mascall of the section probably correlates best with Chrons C5Bn2n fauna occurs at the top of Chron C5Br or in Chron C5Bn2n to C5Bn1n (14.8–15.2 Ma). Thus, the total section spans (15.1–15.3 Ma). This is consistent with the correlations sug- PROTHERO ET AL.—MAGNETIC STRATIGRAPHY OF THE MASCALL FORMATION 31 Y T I N A R O A M L R L A H O

Ma N C P

C5Ar CALIFORNIA 13 n a i

C5AA . C5ABn v o m n t o a i s i t r C5AB v a

a COLORADO NEVADA OREGON o t B m . r s

C5AC + e r t e o m t a a C5AC F a F

14 C5ADn L B L k PROB. w e 14.93 Ma o t e s r

C5ADn 14.3 Ma r

PROB. n C a n a i B e a . v i C5ADr 14.5 Ma e C5Bn v o m n t o y F l s t

w PROB.

C5Bn r r l 15 s l a r a a a P a E B 15.18 Ma c 15.8 Ma B Virgin s MEXICO PROB. a y PROB. 16.2 Ma l FLORIDA Valley

r PROB. C5Br M a PROB.

16.28 Ma n E a i d

16 r

16.2 Ma o Massacre f n g a n i PROB. C5Cn Lake i d r m o e f H g n i e 17 t m C5Cr a e L

H C5Dn e

t C5Dn a L

18 C5Dr n a i d r C5En o f g n 19 i C5Er m e H y l

r C6n a E 20 Fig. 7. Comparison of the Mascall Formation paleomagnetic correlation with those of other Barstovian localities. Correlation of Bar- stow Formation section after MacFadden et al. (1990), and other Barstovian magnetic sections Prothero and Davis (in prep.), and Prothero et al. (in prep.). “PROB.” indicates fi rst appearance of proboscideans in these localities. gested by Tedford et al. (1987, 2004). It also suggests that and Skull Springs in eastern Oregon, and now the Mascall the Mascall fauna is late early Barstovian in age, or slightly Formation in central Oregon. Even the Barstow Formation younger than earliest Barstovian faunas reported from the itself now yields early Barstovian proboscidean trackways. If Barstow Formation (MacFadden et al., 1990), the Virgin the age of the Massacre Lake local fauna in northwestern Valley Formation (Prothero and Davis, in prep.), and other Nevada is correct, there may also be a late Hemingford- localities (Fig. 7). ian occurrence of proboscideans in North America. Only The occurrence of proboscideans (both the mammutid the High Plains and the Texas Gulf Coastal Plain show no Zygolophodon and tthehe gomgomphotherephothere Gomphotherium) in the evidence of early Barstovian proboscideans, despite the large Mascall Formation has important implications for the “Pro- collections from the Olcott Formation of Nebraska and the boscidean Datum” (Fig. 7). Originally, Tedford et al. (1987) Fleming Formation of Texas. The question of the Probos- used the fi rst appearance of proboscideans as the indicator cidean Datum will be discussed in greater detail elsewhere of the late Barstovian, since they were known to appear only (Prothero et al., in prep.). in the late Barstovian in the High Plains and in the Barstow Formation. Since that work, however, numerous early Bar- CONCLUSION stovian appearances of proboscideans have been documented Magnetostratigraphic analysis of the lower portion of the (Tedford et al., 2004), including High Rock and Virgin Valley type section of the Mascall Formation shows that it yields in Nevada, Deep River in Montana, localities in Florida and a stable magnetic signal, held largely in magnetite, which Mexico, the Coalinga local fauna in California, Sucker Creek 32 PALEOBIOS, VOL. 26, NUMBER 1, MAY 2006 passes a reversal test. The lower 40 m of the type section analysis of paleomagnetic data: examples from Siberia and Mo- was reversed in polarity, but the Mascall Tuff is normal in rocco. Geophysical Journal of the Royal Astronomical Society 62: polarity. The upper part of the section has alternating zones 699–718. of normal and reversed polarity. Based on the 40Ar/39Ar datdatee Kuiper, J.L. 1988. The stratigraphy and sedimentary petrology of of 15.77±0.07 Ma on the lowest part of the section, and the Mascall Formation, eastern Oregon. M.S. thesis. Oregon 16.2±1.4 Ma on the base of the section, and constrained by State University, Corvallis. the early Barstovian mammals in the upper part of the section, Long, P.E., and R.A. Duncan. 1982. 40Ar/39Ar agageses of Columbia we correlate the type section of the Mascall Formation with River Basalt from deep boreholes in south-central Washington. Chrons C5Bn1n-C5Br (14.8–16.0 Ma). The Mascall fauna Rockwell Hanford Operations Report RHO-BW-SA-233p. 11 itself is about 15.1–15.3 Ma in age, or late early Barstovian, pp. slightly younger than other earliest Barstovian fossil mam- MacFadden, B.J., C.C. Swisher III, N.D. Opdyke, and M.O. mal localities. This also documents the fi rst appearance of Woodburne. 1990. Paleomagnetism, geochronology, and pos- Proboscidea in the early Barstovian, before the late Barstovian sible tectonic rotation of the middle Miocene Barstow Forma- “Proboscidean Datum” of Tedford et al. (1987). tion, Mojave Desert, southern California. Geological Society of America Bulletin 102:478–493. ACKNOWLEDGEMENTS Merriam, J.C. 1901. A contribution to the geology of the John Day We thank Jonathan Hoffman for help with sampling, and Basin. University of California Bulletin, Department of Geology Matt Kohn for help in measuring the section. We thank Dr. 2 (9):269–314. Joseph Kirschvink for access to the Caltech paleomagnetics Merriam, J.C., and W.J. Sinclair. 1907. Tertiary faunas of the John lab. We thank Ted Fremd for permits to work on National Day region. University of California Publications in Geological Park Service land. We thank two anonymous reviewers for Sciences 5:171–205.5:171–205. helpful comments on this paper. This research was supported Opdyke, N.D., E.H. Lindsay, N.M. Johnson, and T. Downs. 1977. by grants to Prothero by the Donors of the Petroleum Re- The paleomagnetism and magnetic polarity stratigraphy of the search Fund of the American Chemical Society, and by NSF mammal-bearing section of Anza-Borrego State Park, California. grant 00-00174. Quaternary Research 7:316–329.7:316–329. Pluhar, C., J.L. Kirschvink, and R.W. Adams. 1991. Magnetostratig- LITERATURE CITED raphy and clockwise rotation of the Plio-Pleistocene Mojave River Formation, central Mojave Desert, California. San Bernardino Berggren, W.A., D.V. Kent, C.C. Swisher III, and M.-P. Aubry. County Museum Association Quarterly 38(2):31–42. 1995. A revised Cenozoic geochronology and chronostratigra- Prothero, D.R. 2004. The Evolution of North American Rhinoc- phy. SEPM Special Publication 54:129–212.54:129–212. eroses. Cambridge University Press, Cambridge. 218 pp. Bestland, E.A.1998. Stratigraphy of the mid-Miocene Mascall For- Swisher, C.C., III. 1992. 40Ar/Ar/39ArAr datingdating andand itsits applicationapplication toto thethe mation (lower part) in its type area. Publication of the Bureau of calibration of North American land-mammal ages. Ph.D. diss. Land Management, Prineville District, and John Day Fossil Beds University of California, Berkeley. National Monument. 3434 pp. Tedford, R.H., L.B. Albright III, A.D. Barnosky, I. Ferrusquia- Chaney, R.W. 1925. A comparative study of the Bridge Creek fl ora Villafranca, R.M. Hunt Jr., J.E. Storer, C.C. Swisher III, M.R. and the modern redwood forest. Contributions in Paleontology, Voorhies, S.D. Webb, and D.P. Whistler. 2004. Mammalian Carnegie Institution Publications 349:1–22.349:1–22. biochronology of the Arikareean through Hemphillian interval Downs, T. 1956.The Mascall fauna from the Miocene of Oregon. (late Oligocene through early Pliocene epochs). Pp. 169–231 in University of California Publications in Geological Sciences 31:31: M.O. Woodburne (ed.). Late Cretaceous and Cenozoic Mam- 199–354. mals of North America: Biostratigraphy and Geochronology. Fiebelkorn, R.B., G.W. Walker, N.S. MacLeod, E.H. McKee, and Columbia University Press, New York. J.G. Smith. 1983. Index to K-Ar determinations for the state of Tedford, R.H., T. Galusha, M.F. Skinner, B.E. Taylor, R.W. Fields, Oregon. Isochron/West 37:1–60.37:1–60. J.R. Macdonald, J.M. Rensberger, S.D. Webb, and D.P.Whistler. Fisher, R.A. 1953. Dispersion on a sphere. Proceedings of the Royal 1987. Faunal succes sion and biochronology of the Arikareean Society A217:295–305.A217:295–305. through Hemphillian interval (late Oligocene through earliest Fremd, T., E.A. Bestland, and G.J. Retallack. 1994. John Day Basin Pliocene Epochs) in North America. Pp. 153–210 in M M.O..O. paleontology fi eld trip guide and road log.Society of Vertebrate Woodburne (ed.). Cenozoic Mammals of North America, Paleontology Field Trip Guide. 56 pp. Geochronology and Bios tratigraphy. Columbia University Hooper, P.R., and D.A. Swanson.1990. The Columbia River Basalt Press, New York. Group and associated volcanic rocks of the Blue Mountains Wood, H.E., III, R.W. Chaney, J. Clark, E.H. Colbert, G.L. Jepsen, Province. Pp. 69–99 in GG.W..W. WWalker.alker. ((ed.).ed.). GGeologyeology ooff tthehe BBluelue J.B. Reeside Jr., and C. Stock. 1941. Nomenclature and cor- Mountains Region of Oregon, Idaho, and Washington. Cenozoic relation of the North American continental Tertiary. Geological Geology of the Blue Mountains Region. Society of America Bulletin 52:1–48. Kirschvink, J.L. 1980. The least-squares line and plane and the