Geological Society of America Bulletin, Published Online on 24 February 2012 As Doi:10.1130/B30545.1

Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Geological Society of America Bulletin

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins

Kaori Tsukui and William C. Clyde

Geological Society of America Bulletin published online 24 February 2012; doi: 10.1130/B30545.1

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Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins

Kaori Tsukui† and William C. Clyde Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA

ABSTRACT INTRODUCTION Ogg and Smith (2004), radioisotopic ages of the tie points were obtained with different fl uence Age calibration of the early to middle Eo- The geomagnetic polarity time scale plays monitor standards, and thus the accuracy of the cene geomagnetic polarity time scale remains an integral role in interpreting geologic records resultant time scale was compromised. Further- highly uncertain due to confl icting magneto- ranging from biologic evolution and climate more, there is as much as a 1 m.y. difference in stratigraphic, radioisotopic, and astro chrono- change to seafl oor spreading, making the preci- the age of the C20r-C21n chron boundary and logic results. In this study, new paleomagnetic sion of its calibration a matter of fundamental ~1.2 m.y. difference in the duration of the early polarity determinations of 29 ash-fall tuffs importance to our understanding of Earth his- Eocene between the two time scales. Given that preserved in strata of fi ve Laramide foreland tory (e.g., DeMets et al., 1994). However, for the the geomagnetic polarity time scale serves as a basins were used in conjunction with previ- pre-Neogene geomagnetic polarity time scale, global reference to which radioisotopic, mag- ously published 40Ar/39Ar ages from the same where theoretical uncertainties in the orbital netostratigraphic, and biostratigraphic data are tuffs to evaluate eight different calibration calculations do not permit reliable astronomi- correlated, these types of uncertainties in the models for the early to middle Eocene part of cal calibration of magnetic polarity reversals, geomagnetic polarity time scale calibration can the geomagnetic polarity time scale. Reliable numerical calibration has been achieved mainly quickly propagate and affect studies that rely paleomagnetic information was recovered via interpolation among a limited number of on it for chronostratigraphic purposes (Machlus from 23 tuffs, of which 17 showed normal po- radioisotopic age determinations that are cor- et al., 2004). larity and six showed reversed polarity. After related to marine magnetic anomalies, and Some of the most complete sedimentary rec- comparison of the models with the paleomag- an assumption of smoothly varying seafl oor ords from the early and middle Eocene are pre- netic and radioisotopic data from the tuffs spreading (Cande and Kent, 1992; Ogg and served in the Laramide basins of southwestern and an array of independent chronostrati- Smith, 2004; Laskar et al., 2004). However, cal- Wyoming, northeastern Utah, and northwestern graphic observations, the new Willwood ibration of intervening magnetic chrons using Colorado (Bradley, 1964; Roehler, 1992a; Mur- model is herein selected as the best alternative this interpolation method is highly sensitive to phey, 2001). Within the last decade, a suite of to the current geomagnetic polarity time scale the number and accuracy of the tie points used, ash-fall deposits of volcanic origin from these calibration for the early to middle Eocene. making the interpolated segments poorly de- basins has been dated using high-precision Three important implications are apparent in fi ned in absolute time if insuffi cient numbers of 40Ar/39Ar geochronology, resulting in several our proposed model. First, the early Eocene is tie points are used. Furthermore, the assumption different efforts to calibrate this interval of the shortened by 0.6 m.y., and the middle Eocene about the rates of seafl oor spreading, on which geomagnetic polarity time scale (Murphey et al., is lengthened by 0.8 m.y. compared with the the accuracy of the geomagnetic polarity time 1999; Wing et al., 2000; Machlus et al., 2004; 2004 geomagnetic polarity time scale. Also, scale calibration relies, is not routinely tested Murphey and Evanoff, 2007; Smith et al., 2003, the early Eocene climatic optimum is esti- by empirical data. A more precise and reliable 2006, 2008a). Additionally, Westerhold and mated to have lasted from 52.9 to 50.7 Ma, time scale is needed to correlate geologic and Röhl (2009) developed the fi rst astronomically ~1 m.y. longer than previously suggested, and paleoclimatologic records that are being recov- calibrated age model for the early to middle Eo- overlapping in time with the inferred age of ered at the temporal scale of astronomical forc- cene (base of C21r to base of C24n) based on the Wasatchian-Bridgerian faunal transition. ing (Lourens et al., 2005; Zachos et al., 2005; marine sedimentary records from the western Our new model agrees with a previous astro- Sexton et al., 2011). Atlantic (Demerara Rise, Ocean Drilling Pro- nomical model when it is tied to the oldest The early Eocene is a relatively poorly gram [ODP] Leg 207, Site 1258). Although the proposed age for the Paleocene-Eocene Ther- calibrated interval of the geomagnetic polar- astronomical age model is not calibrated in ab- mal Maximum at 56.33 Ma. ity time scale. In the most recent time scale solute time because it is beyond 42 Ma, the limit (Ogg and Smith, 2004, hereafter GOS2004), of precise astronomical solutions, it provides an †Present address: Division of Geochemistry, the ~22-m.y.-long Eocene segment of the geo- independent basis for determining the durations Lamont-Doherty Earth Observatory of Columbia magnetic polarity time scale is calibrated by of chrons and forces reevaluation of the current University, Palisades, New York 10964, USA, and applying a cubic spline function through only calibration scheme for the Eocene part of the Richard Gilder Graduate School at the American Museum of Natural History, Central Park West at fi ve tie points that are spaced on average every geomagnetic polarity time scale. 79th Street, New York, New York 10024, USA; ~4–5 m.y. In the previous two time scales by One way to assess the accuracy of the pro- e-mail: [email protected] Cande and Kent (1995, hereafter CK95) and posed calibration models is to determine the

GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1–16; doi: 10.1130/B30545.1; 9 ﬁ gures; 2 tables; Data Repository item 2012102.

Tsukui and Clyde

100 km N

Wind River Uplift Wind River Basin

WL Granite Mtns TB Uplift CP

Greater Green River Basin 42°N B R Great Divide Basin Sa K A Bridger Basin Rock A'A’ G 6 Springs L S CB M Uplift A Fossil Basin B C F SB B’ LC SCM Washakie Basin Figure 1. Geologic map of the HF BE WY study area showing sampling i CO locations of tuffs and lines of Sand Wash cross sections. See Table 1 for Uinta Uplift Basin abbreviations of tuffs. The base map is after Burchﬁ el (1993).

St C O Y Fa 40°N W Cu Bl C’C'

Uinta Basin Piceance Creek Basin UT CO

ID WY Early Paleogene Sample locality Lines of cross sections Cretaceous X X' UT CO Cambrian-Jurassic clastic Precambrian cored uplifts 38°N 112°W 110°W 108°W paleomagnetic polarity of radioisotopically sins (Fig. 1; see Table DR1 for detailed de- every ~0.36 m.y., and thus they allow us to test dated tuffs and compare the observed polarity scription of the sampled tuffs1). According to the competing calibration models at a tempo- to that predicted by each age model. McIntosh the 40Ar/39Ar geochronology of Smith et al. ral resolution much fi ner than that at which the et al. (1992) used a similar approach on a se- (2008a), these tuffs were sampled on average Eocene geomagnetic polarity time scale is cur- quence of ignimbrites from the southwestern rently calibrated. United States to provide four possible calibra- 1GSA Data Repository item 2012102, three addi- An improved calibration of the early to tional fi gures and fi ve additional tables that contain tions for the late Eocene to Oligocene. In this sample descriptions, measurements, and details of middle Eocene time scale has important impli- study, we determined the magnetic polarity of our method of age model evaluation, is available at cations for studies that rely on temporal syn- 29 tuffs from the Greater Green River, Wind http://www.geosociety.org/pubs/ft2012.htm or by re- chroneity of geologic data for understanding River, Uinta, Fossil, and Piceance Creek Ba- quest to [email protected]. causal mechanisms. For instance, a coupling

2 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

Figure 2. Representative fence Rock diagrams for Fossil Basin (A-A′), Fossil Basin Bridger Springs Washakie Indian Gate Basin Uplift Basin Canyon Canyon Greater Green River Basin Sa BE SCM (B-B′), and Uinta Basin (C-C′) 1200 Tgsl Tu along lines of cross sections 150 HF TB Twk St shown in Figure 1. Approxi- Tfbh O mate stratigraphic positions of 300 Tb sampled tuffs are shown rela- 120 LC CP tive to major lithologic bound- CB 900 Tgs aries (solid line) and marker Fa beds (broken line, b—Buff Tfs 90 Tgu marker bed, m—Mahogany 200 Tgl Bl h marker bed, h—Horse Bench b 600 SB sandstone). Dotted lines show A Twb m W 60 6 general interbasinal correlation. L Cu M Twc For global positioning system Tga Tgw Stratigraphic thickness (m) Tgtr (GPS) coordinates and abbre- 100 K G C 300 viations of the tuffs, see Table 1. 30 F B Tgf R Lithologic units are colored ac- Tgt S Tg cording to lake type assign ment of Smith et al. (2008a). Abbre- Twm TwmTwm Twm 0 0 0 viations of lithologic units are as ABA′ B′ CC′ follows: Tfbh—Bulldog Hollow Member of the Fowkes Forma- Volcaniclastic Fluvial-lacustrine Interbedded evaporites tion; Tfs—Sillem Member of Alluvial Fluctuating profundal Evaporative the Fowkes Formation; Twb— Bullpen Member of the Wasatch Formation; Tga—Angelo Member of the Green River Formation; Tgf—Fossil Butte Member of the Green River Formation; Twm—main body of the Wasatch Formation; Twk—Washakie Formation; Tb—Bridger Formation; Tgl—Laney Member of the Green River Formation; Tgw—Wilkins Peak Member of the Green River Formation; Tgt—Tipton Member of the Green River Forma- tion; Twc—Cathedral Bluffs Tongue of the Wasatch Formation; Tu—Uinta Formation; Tgsl—sandstone and limestone facies of the Green River Formation; Tgs—saline facies of the Green River Formation; Tgu—upper member of the Green River Formation; Tgtr—transitional interval of the Green River Formation; Tg—main body of the Green River Formation. Lithostratigraphic data are adapted from Buchheim (1994); Oriel and Tracey (1970; profi le A); Culbertson (1961); Roehler (1992a, 1992b); Murphey and Evanoff (2007; profi le B); Remy (1992); Dane (1954); Bryant et al. (1989); Prothero (1996; profi le C). between the early Eocene climatic optimum, Geologic Setting more than 30 cm thick and vary in color from which represents the long-term Cenozoic white to light gray to orange in outcrop. Grain peak in warmth (Zachos et al., 2001), and the The ash-fall tuffs collected for this study were size of the tuffs varies from very fi ne sand to Wasatchian-Bridgerian North American Land deposited in fl uvial and lacustrine environments clay, and textural differences correlate with Mammal Age (NALMA; Wood et al., 1941) after being transported from the Absaroka and different depositional settings (see GSA Data faunal turnover has been suggested based on Challis volcanic fi elds of northwestern Wyo- Repository Table DR1 [see footnote 1]). For their co-occurrences in chron C23r (Clyde ming and Idaho via winds or rivers (Surdam and instance, the lacustrine tuffs (e.g., K-spar and et al., 2001; Woodburne et al., 2009). How- Stanley, 1980; Chetel et al., 2011). In general, Sixth tuffs) are relatively fi ne grained and are ever, other studies dispute the result (Smith the tuff-bearing formations record geomorphic more distinctive in outcrop, having sharp upper et al., 2003, 2004; Clyde et al., 2004), and the and hydrologic changes resulting from the inter- and lower contacts with the surrounding mud- underlying mechanism for this coupling re- play between regional tectonics and climate stone or limestone units. They also typically mains elusive. A resolution to these questions (Fig. 2; Pietras et al., 2003). The Wasatch and lack the evidence of postdepositional rework- will require an improved chronostratigraphy Wind River Formations contain mostly fl uvial ing that is otherwise common in the fl uvial tuffs for the Greater Green River Basin and a more facies (e.g., red paleosol mudstones and channel (e.g., Basal Bridger E tuff), which are relatively reliable geomagnetic polarity time scale to sandstones), whereas the Green River Forma- coarser grained, have more diffuse contacts with make precise correlations between deep-sea tion is dominated by lacustrine facies (e.g., lami- the facies above and below, and often contain climatic proxies and terrestrial biotic records. nated shales interbedded with thin sandstones pumice clasts or fragments. Signs of postdepo- Finally, a refi ned geomagnetic polarity time and limestones). The Bridger Formation and sitional zeolitization caused by saline and alka- scale can help reconstruct the history of sea- its lateral correlatives, the Fowkes Formation line lake water are common in the matrix of the fl oor spreading rates and thus test the assump- in Fossil Basin and the Washakie Formation in lacustrine tuffs (Ratterman and Surdam, 1981). tion of smoothly and continuously varying Washakie Basin, are composed of fi ne-grained A grading-upward pattern of the biotite crystals, seafl oor spreading on which the precision of volcaniclastic deposits with laterally extensive which were abundant in many tuffs, indicates the geomagnetic polarity time scale has tradi- limestone and sandstone marker beds. On fresh that the tuffs were deposited in single events, tionally relied. surfaces, these water-laid tuffs are usually no thus ensuring that the paleomagnetic data from

Geological Society of America Bulletin, Month/Month 2012 3 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Tsukui and Clyde these tuffs record an instantaneous polarity at nent magnetization (NRM) was measured on over three orders of magnitude between 0.092 the time of deposition. Previous studies (e.g., an HSM 2 SQUID-based spinner magnetom- mA/m and 21.912 mA/m before demagneti- Reynolds, 1979; Hayashida et al., 1996; Iwaki eter inside a three-dimensional, direct current zation (Table DR2 [see footnote 1]). Neither and Hayashida, 2003) have shown that water- (DC) coil, low-fi eld cage at the Paleomagnetism intensity nor grain size appears to control de- laid ash-fall tuffs typically yield early acquired Laboratory at the University of New Hamp- magnetization behavior or the quality of the detrital remanent magnetizations, probably shire. Characteristic components of NRM were isolation of a ChRM. Tuffs deposited in fl uvial of detrital origin, that record paleomagnetic calculated by least squares analysis through the environments (Wind River, Bridger, and Fowkes fi eld directions acquired in a relatively short high-coercivity or high-unblocking-temperature Formations) generally have higher NRM inten- time period. demagnetization paths that trended toward sities than those deposited in lacustrine settings the origin (Kirschvink, 1980). The maximum (Green River Formation). The fl uvial tuffs also METHODS angular deviation (MAD) of the ChRM was have higher magnetic susceptibility compared calculated for each sample, and at least three with lacustrine tuffs, likely due to higher con- Paleomagnetic Sampling samples with MADs less than 20° were used to centrations of volcanigenic ferro/ferrimagnetic determine a statistically robust site mean polar- minerals (Dunlop and Özdemir, 1997; Table Paleomagnetic samples were collected ity. Those samples with MADs above 20° were DR3 [see footnote 1]). After initial pilot studies from 29 previously dated ash-fall tuffs in fi ve rejected from further analyses. Sites that passed were carried out to determine the most effective Laramide Basins (Figs. 1 and 2; Smith et al., Watson’s test for randomness at the 95% signifi - demagnetization protocol for each tuff, 73 out of 2003, 2006, 2008a, 2010; Murphey et al., 1999; cance level at N = 4 with directional precision 128 samples were demagnetized using thermal Machlus et al., 2004; Murphey and Evanoff, parameter (k) of >10 (<10) were classifi ed as al- demagnetization, and the remaining 55 samples α 2007). In the fi eld, tuffs were located with the pha (beta) sites (Watson, 1956). Alpha 95 ( 95) were demagnetized with AF demagnetization. help of colleagues (see Acknowledgments) or expresses within-site scatter as the 95% cone of In some cases, both methods were combined to by global positioning system (GPS) coordinates confi dence around the estimated mean direc- resolve a ChRM. and rock descriptions provided in the literature tion. The relatively liberal data quality criteria Demagnetization revealed either one or two (Table DR1 [see footnote 1]). Effort was made on MAD and precision parameter were chosen NRM components in every sample. Most re- to sample from the exact same locations that because the objective of the study is to deter- versed polarity samples exhibited overprint had been sampled for 40Ar/39Ar analysis in the mine polarity for specifi c tuff horizons, rather components, which were unblocked by 400 °C previous studies (Smith et al., 2003, 2008a). At than a precise paleopole. For the samples col- or randomized by 40 mT (Fig. 3). The overprint each site, weathered materials were removed lected from beds with measurable dips, tectonic components show highly variable directions, to expose a fresh surface for sampling. Five to correction was performed to the measured dec- making it diffi cult to infer the cause or timing ten separately oriented samples were collected linations and inclinations. Virtual geomagnetic of the secondary magnetization acquisition. as 2.5-cm-diameter cylinders with a portable poles (VGP) were calculated for the alpha and After removal of the low-temperature or low- gas-powered core drill (85 samples total) or as beta sites, and VGP latitude was used to infer coercivity component, ChRM was progressively oriented hand samples (43 samples total), which magnetic polarity of sites, assuming that the demagnetized up to 690 °C or 100 mT (Table were later cut into 8 cm3 samples with a saw. ChRM is primary in origin. DR2 [see footnote 1]). Five samples that had Whenever possible, the fi nest-grained parts of To investigate the magnetic mineralogy, we best-fi t lines to the ChRM with MADs above a tuff were sampled for paleomagnetic analy- conducted isothermal remanent magnetiza- 20° were rejected from further analyses. The av- sis because they may contain single-domain tion (IRM) acquisition experiments on a suite erage MAD of the ChRM for the remaining 123 grains, which are more likely to preserve reli- of samples cut into 1 cm3 cubes. Using a ASC samples was 8.16°, and the ChRM from those able characteristic remanent magnetizations IM10 impulse magnet, samples were sub- samples was used to estimate site mean direc- (ChRM; Butler, 1992). However, Smith et al. jected to a magnetic fi eld of different intensities tions. Of the 23 sites that passed Watson’s test (2003, 2008a) sampled the coarsest fraction of (0.12 T, 0.4 T, and 1.1 T) along three orthogo- for randomness at the 95% signifi cance level, each deposit (i.e., base) in order to collect the nal axes, isolating low-, medium-, and high- six sites were characterized by reversed polar- largest possible crystals for radioisotopic dat- coercivity fractions in the x, y, and z directions, ity with ChRM showing S-SE declinations and ing. In one case (Boar tuff), the tuff bed was too respectively (Lowrie, 1990). Subsequently, the moderate to steep negative inclinations (mean of friable for paleomagnetic sampling, so samples acquired IRM was thermally demagnetized at Dec = 159°, Inc = −62° [Dec —declina- tec tec tec were collected from a siliciclastic layer immedi- steps of 25, 50, 75, 100, 125, 150, 200, 250, tion in tectonic coordinates, Inctec—inclination α ately (~20 cm) above. Also, where stratigraphic 300, 400, 500, 540, and 580 °C. The three-axis in tectonic coordinates], 95 = 25.3°). The re- integrity of the tuffs was unclear (e.g., due to IRM experiment shows the ferromagnetic min- maining 17 sites showed normal polarity with suspected slumping as in the case of the White eral content of a sample using the characteristic N-NW directions and moderate to steep posi-

Lignitic and Wavy tuffs), samples were col- coercivities and temperature-dependent proper- tive inclinations (mean of Dectec = 1.6°, Inctec = α lected from multiple locations within the same ties of different magnetic minerals. Bulk sus- 56°, 95 = 7.8). Eighteen sites with k > 10 are bed to average out the possible effect of post- ceptibility was measured on selected samples to referred to as alpha sites; ﬁ ve sites with k < 10 depo si tional disturbance. further facilitate interpretation of the magnetic are referred to as beta sites. The beta sites are mineralogy of the tuffs. characterized by anomalous site mean declina- Laboratory and Data Selection Procedures tions between 7° and 59° for normal polar- PALEOMAGNETIC RESULTS ity sites and 56.9° for the one reversed polarity The paleomagnetic samples were demagne- site. Inclinations of their ChRM, however, are tized by alternating ﬁ eld (AF) and/or thermal Statistics for all paleomagnetic sites exam- largely consistent between samples within a methods between 2.5 mT and 100 mT and/or ined in this study are summarized in Table 1. site and therefore are included in the calculation 25 °C and 690 °C, respectively. Natural rema- NRM intensities are highly variable, ranging of site VGPs, but we caution that those polarity

4 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale §§ vgp (°) Long §§ vgp (°) Lat †† 95 (°) α ** k # (°) R Inclination 7.3 53.3 3.5 2.69 58.2 81.5 23.5 (°) 38.9 35.7 3.5 5.43 43.5 50.5 2.3 Declination (°) 37 Inclination (°) Declination In geographic coordinates In tectonic coordinates § N † (°) Dip † 0 0 5 132.1 66.1 132.1 66.1 1.6 1.17 180 7.9 279.5 0 0 4 330.7 53 330.7 53 3.9 24.07 19.1 65.8 154.1 0 0 4 160 −60.7 160 −60.7 4 76.7 10.6 −75.1 345.7 0 0 3 179.9 −24.2 179.9 −24.2 2.4 3.5 79.5 −60.2 250.8 0 0 4 351.3 54.2 351.3 54.2 3.9 53.68 12.7 80.2 115.9 0 0 4 35.4 47.3 35.4 47.3 2.9 2.81 67.9 58.8 350.1 0 0 4 1750 −45.9 0 4 175 353.5 −45.9 58.4 3.8 15.79 353.5 23.9 −75.6 268 58.4 4 128.300 8.1 0 84.5 0 5 6 136.4 7.3 59.4 53.3 70.8 59.4 70.8 5.5 9.3 23.2 48.7 297.6 0 0 4 13.2 63.2 13.2 63.2 3.9 32.9 16.3 80 320.3 0 0 4 160.7 −65.1 160.7 −65.1 3.9 47.03 13.5 −75.5 7.4 0 0 4 346.1 58.8 346.1 58.8 3.9 45.9 13.7 79.3 156.2 0 0 4 344.5 55.5 344.5 55.5 4 68.48 11.2 76.6 139.6 0 0 4 179.1 −35.5 179.1 −35.5 4 159.6 7.3 −67.2 252.8 0 0 4 227.4 58.5 227.4 58.5 3 2.98 64.9 2.6 216.9 96 18 4 23 66.7 62.8 82.1 3.8 18.4 22 45.5 271.7 12 7 4 168.2 −68.4 186.3 −70.2 3.8 16.13 23.6 −76.3 85.5 (°) 211 6 3 315.7 −42.2 317.3 −48 1.6 1.41 180 9.2 107.6 337 10 4 53.5 74.1 58.4 64.2 1 0.99 180 48.4 314.5 315 35 4 96.9 43.2 82 17.6 2.3 1.74 107 12 340.5 232 7 4227 44 6 4 3.7 48.7 359.2 44.4 4 198.7 6.5 74.5 73.2 339 6 4 342.6 54.3 350.7 53.5 3.7 11.62 28.2 79.8 118.9 227 6 4 1.4 58.1 355.5 53.6 4 88.1 9.8 81.8298 97.6 12 5 4.3 35.7 7 24.6 3.8 3.44 48.5 62 54.8 227 6 5 17.3 61.1 8.8 57.7 5 82.62 8.5 82.5 3 122 3 4 353.5 39.8 351.8 42.1 4 73.52 10.8 71.8 94.5 270 9 3 49.8 −41.9 56.9 −47.3 2.7 6.76 51.7 2.6 23.1 189 6 4 4 61.4 353.4 60.3 4 404.7 4.6 85 161.5 Strike ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ TABLE 1. SUMMARY SITE STATISTICS 1. SUMMARY TABLE 30.23 12.31 35.21 24.50 11.20 8.70 53.59 55.89 34.63 17.79 53.01 57.31 0.59 9.37 7.40 3.10 11.80 52.22 17.60 6.69 42.10 5.60 41.45 59.82 11.38 ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ 53.20 27.59 4.29 5.34 15.81 ′ ′ ′ ′ ′ (°) Long designation did not satisfy the Watson’s test (Watson, 1956) and were excluded from polarity determination. test (Watson, β designation did not satisfy the Watson’s or α W110°15 W109°19 W108°40 W109°15 W108°11 W109°22 W108°6 W108°18 W108°40 W110°11 W110°9 W109°28 W110°8 W110°31 W110°33 W109°28 W109°17 W109°28 W110°8 W110°12 W110°8 W110°57 W109°22 W110°15 W109°24 W110°37 W109°22 W108°43 W110°43 W109°15 ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ 2.32 33.80 9.89 2.94 47.20 36.46 3.67 59.02 43.13 35.12 1.84 42.43 47.07 49.53 6.02 54.30 41.50 5.40 44.11 34.72 37.59 25.00 32.75 12.67 47.92 ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ 1.90 56.80 54.00 34.41 52.47 (°) ′ ′ ′ ′ ′ Lat N42°42 N42°26 N41°20 N42°51 N41°20 N41°21 N39°50 N39°50 N41°72 N41°28 N41°32 N41°32 N41°39 N41°14 N41°46 N41°57 6 N41°32 B N41°57 R G O N40°2 M Fa N39°58 TB TB N42°26 CP N42°16 SCM N41°7 Peak Butte Butte Pumice dence about estimated mean direction. refer to latitude and longitude of the site virtual geomagnetic pole (VGP) where positive is north east. vgp Bridger Tabernacle Bridger Tabernacle Bridger Henrys Fork HF Bridger Church Butte CB BridgerBridger Basal Bridger EBridger Leavitt Creek BE Continental LC N41°7 Bridger Sage Creek Mtn Fowkes Sage Sa Wind River Halfway Draw HD Wagon BedWagon White Lignitic WL Green River Curly Cu N39°50 Green River Wavy W Green River Analcite A Green River Scheggs S N41°31 Green River Yellow Y N40°1 Green River Grey Green River Main Green RiverGreen River Sixth LayeredGreen River L C Bed C N41°21 Green RiverGreen River Boar K-sparGreen RiverGreen River KGreen River Fat Strawberry Oily N41°46 St N40°9 Green River Firehole F Green River Rife Green River Sand Butte SB Green River Blind Canyon Bl refer to alpha and beta sites, respectively (see text). Sites without β and Long α β α β α α α α α α α α α α α α β β α α α α β vgp is cone of 95% conﬁ 95 is precision parameter. is number of samples analyzed for paleomagnetic polarity determination. and k α Lat α Strike and dip are measured using right-hand rule (dip to the right of strike). N R is resultant vector. * ** † § # †† §§ BR0510 KT0703 KT0715 KT0702 KT0714 BR0502 KT0701 KT0713 BR0504 BR0505 BR0509 KT0707 KT0704 BR0503 CP0646 KT0709 KT0708 KT0716 BR0506 KT0711 KT0712 BR0507 KT0710 BR0501 BR0508 KT0705 Site* Formation Name Abbreviation BR0513 BR0511 BR0512 KT0706

Geological Society of America Bulletin, Month/Month 2012 5 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Tsukui and Clyde

A B Our paleomagnetic results show that ash-fall N, Up W, Up deposits in general can reliably record the an- BR0506F KT0703D cient geomagnetic ﬁ eld via the acquisition of a detrital remanent magnetization and can typically provide paleomagnetic polarity data even 50 mT when the surrounding carbonate and siliciclas- 10 mT tic rocks are not well suited for paleomagnetic analysis (Strangway and McMahon, 1973; W 560 °C Reynolds, 1979; Sheriff and Shive, 1982; 300 °C Hayashida et al., 1996; Iwaki and Hayashida , 660 °C 100 mT 2003). The sites for which results could not be statistically distinguished from random may 50 mT 130 °C have been handicapped by possible postmag- S netization slumping (e.g., Wavy and White 560 °C Lignitic tuffs) or severe postdepositional 10 mT chemical alteration (likely in the Analcite , MAD = 1.6° MAD = 1.9° Curly, and Wavy tuffs). Step-wise thermal demagnetization of three-component IRM indicates a dominance of magnetite in some tuffs (e.g., Henrys Fork and Grey tuffs) and a domi- C nance of hematite in others (e.g., Basal Bridger N, Up E and Leavitt Creek tuffs; Fig. 5). These in- KT0706D terpretations of the IRM results are consistent with the laboratory unblocking temperatures of 400 °C the ChRM of the respective tuffs (Table DR2 525 °C [see footnote 1]).

EVALUATION OF AGE MODELS W Evaluation of the Input Data 200 °C 400 °C Paleomagnetic Data of the Tuffs The reliability of our paleomagnetic results is demonstrated by the passage of a reversal test based on 23 sites, the correspondence between the mean direction for all 18 alpha sites and the MAD = 7.7° expected direction for the early Eocene, and the lack of complications in magnetic mineralogy, Figure 3. Vector end-point diagrams (Zijderveld, 1967) for three representative samples as indicated by IRM experiments. However, with different textures and from different depositional environments. Open (fi lled) squares postdepositional diagenesis, which has been show vector end points in the vertical (horizontal) plane. (A) BR0506F (Layered tuff) is shown to have affected carbonate and siliciclas- a laminated tuff deposited in a lacustrine setting. Alternating fi eld (AF) demagnetization tic deposits of the lacustrine Green River For- isolated a single component for which characteristic remanent magnetization (ChRM) was mation, could potentially have obscured the demagnetized by 100 mT with a maximum angular deviation (MAD) of 1.6°. It is of normal primary ChRM in some of the tuffs (Strangway polarity. (B) KT0703D (Tabernacle Butte tuff) was deposited along with pumice clasts in a and McMahon, 1973; Sheriff and Shive, 1982). fl uvial setting. Thermal demagnetization isolated a single component for which ChRM was In addition, some of the beta sites with simi- demagnetized by 660 °C with a MAD of 1.9°. It is of reversed polarity. (C) KT0706D (K-spar lar inclinations but variable declinations may tuff) is a homogeneous crystalline tuff deposited in a lacustrine setting. Thermal demagneti- record short-term fi eld behavior. Although the zation revealed two components. An overprint component was removed by 400 °C, leaving possibility of unrecognized overprints cannot be the characteristic component that was demagnetized by 525 °C with a MAD of 7.7°. The refuted due to a lack of opportunity to perform a ChRM is of reversed polarity. Final demagnetization step and two intermediate steps are fold test in the subhorizontal strata that charac- shown. Tics represent intensity increments of 0.5 mA/M. terize the study area, the criteria described here attest to the reliability of the polarity determination of the tuffs. determinations are of lesser quality than those 1998). The mean direction (dec/inc) when all α 40 39 for the alpha sites. These sites could record tran- reversed sites are inverted is 353.6°/57° ( 95 = Ar/ Ar Data of the Tuffs from sitional polarities or may have been affected by 5.5, k = 40.23, N = 18), which is statistically in- Previous Studies vertical-axis rotations. The 18 alpha sites are of distinguishable from the expected early Eocene The 40Ar/39Ar geochronologic data reported both normal and reversed polarity and pass a re- direction for southwestern Wyoming (349°/61°; by Smith et al. (2008a, 2010), together with versal test at the 95% confi dence level (Tauxe, Fig. 4; Diehl et al., 1983). our paleomagnetic results, comprise the input

6 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

North North data used to evaluate competing age models for the geomagnetic polarity time scale. Smith et al. (2008a) determined 40Ar/39Ar ages of 29 tuff beds (22 ash beds and 3 volcaniclastic sand Dec = 173°, Inc = –56° beds) based on 2234 analyses of sanidine and k = 28, α = 15° 95 biotite crystals using laser incremental heating experiments and/or laser fusion experiments on single- and multiple-crystal aliquots. The types of 40Ar/39Ar analyses used include (in order of accu racy): (1) sanidine single-crystal Dec = 354°, Inc = 57° laser fusion , (2) biotite single-crystal step heat- k = 44, α = 6° 95 ing, (3) sanidine multiple-crystal laser fusion , (4) biotite single-crystal laser fusion, and (5) biotite multiple-crystal analyses. These age determinations are consistent with the strati- Figure 4. Equal-area projections of mean characteristic remanent graphic order of the tuff beds and thus are magnetization defi ned by progressive demagnetization directions of deemed reliable to the fi rst degree. 18 alpha sites in tectonic coordinates. Circles show a 95% cone Those ages with no distinct outliers due to of confi dence around the estimated mean direction. Filled (open) contamination by xenocrysts or without indi- symbols lie on the lower (upper) hemisphere of the projection. Sta- cations of 40Ar* loss are most preferred. Con- α tistical parameters (directional precision parameter [k] and 95) are tamination by xenocrysts is readily identifi able also shown. When the reversed sites are inverted, the mean direction using single-crystal fusion analyses because (dec/inc) for all the alpha sites is 353.6°/57°, which is very close to of their signifi cantly older ages. In the case of the expected direction for the early Eocene based on the Eocene ref- the Sixth tuff, in which <10% contamination by erence pole for North America (349°/61°; Diehl et al., 1983). xenocrystic or detrital grains was found, those analyses derived via only concordant incremental heating of individual biotite crystals A C

180 800

160 BR0501A 700 BR0512B

140 600

120 500 100 Figure 5. Step-wise thermal 400 80 demagnetization of acquired 300 isothermal remanent magneti- 60 zation for selected samples. The

Magnetization (mA/m) 200 40 samples were demagnetized in

20 100 three orthogonal axes after ﬁ elds of 0.12 T, 0.4 T, and 1.1 T were 0 0 0100200 300 400 500 600 0 100 200 300 400 500 600 700 applied to the x (diamonds), D y (squares), and z (triangles) B axes, respectively. Observed 300 400 unblocking temperatures indicate the presence of magne- BR0502A 350 BR0513I 250 tite in (A) BR0501A (Henrys 300 Fork tuff) and (B) BR0502A 200 (Grey tuffs) and hematite in 250 (C) BR0512B (Basal Bridger E

150 200 tuff) and (D) BR0513I (Leavitt Creek tuff). 150 100

Magnetization (mA/m) 100 50 50

0 0 0100200 300 400 500 600 0 100 200 300 400 500 600 700 Temperature (°C) Temperature (°C)

Geological Society of America Bulletin, Month/Month 2012 7 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Tsukui and Clyde were used to infer the depositional age (Min not only refl ect growing interest in Eocene time interval between chrons C15 and C29 based et al., 2001; Smith et al., 2006, 2008a). Incre- (e.g., Jaramillo et al., 2010; Sexton et al., 2011) on different calibration points, but in all cases, mental heating experiments on multiple-crystal but also underscore uncertainties in this part of they used a natural cubic spline fi t. In model B, sanidine aliquots yielded internally concordant the time scale. Although one of the goals of the the two older tie points of CK95 were replaced plateau ages that were consistent with fusion geomagnetic polarity time scale is to provide with an unpublished age (45.6 Ma) of Swisher ages, thus demonstrating the absence of 40Ar* an age estimate for magnetic chron bound- and Montanari (in Berggren et al., 1995) and loss in these samples. With the exception of the aries, the radioisotopic age determinations of an interpolated age for the Paleocene-Eocene Yellow and Strawberry tuffs, only 3.7% of all the tie points and width estimates of the ma- boundary (55.3 Ma; Wing et al., 2000). In sanidine analyses were excluded from all age rine magnetic anomalies, which are two major model C, the two older tie points of CK95 were estimates. pillars in the construction of the geomagnetic replaced with a single-crystal biotite 40Ar/39Ar In the absence of sanidine, biotite was ana- polarity time scale, have inherent analytical age for the Sixth tuff (48.8 Ma; Machlus et al., lyzed instead. Laser incremental heating ex- and measure ment uncertainties. The resultant 2004) and the aforementioned Paleocene- periments and electron probe microanalyses calibration of the chrons thus has built-in un- Eocene boundary age. In model D, the two were performed on euhedral biotite crystals certainties. Such uncertainties are not always older tie points of CK95 were replaced by the to determine the presence of alteration-related quantifi ed or acknowledged, but it is crucial to age for the Sixth tuff only. In these models, the 40 39 Ar* loss and ArK recoil induced during irra- recognize them and try to further improve the Cretaceous-Paleogene and Eocene-Oligocene diation. Discordant age spectra were correlated accuracy and precision of the time scale using boundaries were left unchanged as in CK95, with the presence of K-depleted alteration additional empirical data from deposits that resulting in a time scale where the early Eocene phases and resulted in considerable age scat- yield both paleomagnetic polarity and radioiso- was lengthened at the expense of the Paleocene ter (Smith et al., 2008b). For high-precision topic data. The age models we evaluate in this and middle Eocene. age determination, age spectra that were con- study include the two most recent geomagnetic cordant and reproducible were used with polarity time scales (CK95, GOS2004; Cande Model by Smith et al. (2008a, 2010) the weighted mean as the best estimate of the and Kent, 1995; Ogg and Smith, 2004), models Smith et al. (2008a, 2010) recalibrated the eruptive age. B, C, and D of Machlus et al. (2004), models by interval, C24 through C20 based on 40Ar/39Ar All 40Ar/39Ar ages were calculated relative Smith et al. (2008a, 2010), an extended version ages of ash beds, existing magnetostratigraphic to the Taylor Creek rhyolite (28.34 ± 0.28 Ma; by Wing et al. (2000), and the astronomical data, NALMA biostratigraphy from the Big- Renne et al., 1998) in Smith et al. (2008a) but models of Westerhold and Röhl (2009) (Table horn Basin, Wind River Basin, Greater Green were recalculated to the astronomically cali- DR5; Fig. DR1 [see footnote 1]). River Basin, Uinta Basin, Devil’s Graveyard brated age of 28.201 Ma for the Fish Canyon Formation in Texas, and Absaroka volcanic sanidine standard (FCsK08; Kuiper et al., 2008) Geomagnetic Polarity Time Scales province, as well as marine biostratigraphy in Smith et al. (2010). By intercalibrating all CK95 is a revised version of the time scale from San Diego region (see table 4 in Smith 40 39 of the Ar/ Ar ages to FCsK08, it is possible to by Cande and Kent (1992), and it incorporates et al., 2008a). However, uncertainties remain make precise comparison with astrochronol- changes that arose as a result of a new age esti- in the correlation of some of the tuffs to local ogy and ages derived by other methods such as mate for the Cretaceous-Paleogene (K-Pg) magnetostratigraphic records (e.g., Layered

U-Pb chronometer, because the FCsK08 reduces boundary (65 Ma rather than 66 Ma) and an tuff, Sixth tuff, and Continental Peak tuff). This the absolute uncertainty from ~2.5% to less astronomical age estimate for the base of chron model implies the presence of several short- than 0.25% and it eliminates the ~1% discrep- C3n.4n (5.23 Ma). Its Eocene part is cali- duration polarity chrons that are not shown in ancy between radioisotopic and astronomical brated by fi tting a cubic spline through three tie the original marine magnetic anomaly records dating. This is an important point because the points: 33.7 Ma at the Eocene-Oligocene (E-O) of Cande and Kent (1992). Smith et al. (2008a) age models under consideration in this study are boundary, 46.8 Ma in the middle Eocene, and attributed those to tiny wiggles described in based on astrochronology as well as calibration 55.0 Ma at the Paleocene-Eocene (P-E) bound- Cande and Kent (1992). points that were obtained under different cali- ary. However, as pointed out by Machlus et al. brations. In the following discussion, to allow (2004), there are important uncertainties on the Model by Wing et al. (2000) intercomparison of ages obtained by 40Ar/39Ar age estimates of the two older tie points. Con- In Wing et al. (2000), a 40Ar/39Ar sani- dating using different fl uence monitor standards fl icting K-Ar ages (45.8 ± 0.5 Ma and 46.8 ± dine age from the bentonitic tuff at the base and astronomically determined ages, we used 0.5 Ma) have been proposed for the middle Eo- of chron C24n.1n (also referred to as “Will- 40 39 the FCsK08-calibrated Ar/ Ar ages as reported cene tie point (Bryan and Duncan, 1983), and wood Ash”) was used in place of the calibra- in Smith et al. (2010; see their Supplement the age estimate for the early Eocene tie point tion point at the Paleocene-Eocene boundary table DR2) with a 2σ fully propagated uncer- is questionable because of an unconformity of of CK95 (Wing et al., 1991; Tauxe et al., tainty in all cases. A summary of all of the input unknown duration that separates the dated tuff 1994). Chrons between C29 and C22 were data used to evaluate the age models is available and the tie point (Aubry et al., 1996). GOS2004 calibrated by linear interpolation between in Table DR4 (see footnote 1). uses the same method of interpolation as CK95 the bentonitic tuff and two calibration points but is based on fi ve calibration points, three of of CK95 at the Cretaceous-Paleogene bound- Previous Age Models which are new additions since CK95 (however, ary and in the middle Eocene. We have ex- note that a new 40Ar/39Ar age was used for the tie trapolated the original calibration to chron Since the early compilation of the seafl oor point at C21n.33; sensu Ogg and Smith, 2004). C20n to accommodate some of the younger magnetic anomaly pattern by Cande and Kent tuffs collected from the Uinta Basin in this (1992), eight calibration models have been pro- Models by Machlus et al. (2004) study. This new version, which includes posed for the early to middle Eocene part of the Machlus et al. (2004) proposed three alter- the extrapolated segment, is referred to as the geomagnetic polarity time scale. These efforts native models (models B, C, and D) for the Willwood model in the following discussion.

8 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

Models by Westerhold and Röhl (2009) summarizes all of the calibration models con- as category I according to one age model but as Westerhold and Röhl (2009) used a funda- sidered in this study, showing different ways in category II according to another. The tuffs with mentally different approach than those previ- which chrons are scaled according to different relatively large uncertainties in 40Ar/39Ar age ously discussed, which relied on radioisotopic tie points and interpolation methods used (see determinations are weighted less in this method ages of discrete tuff horizons. Instead, the footnote 1). of evaluation because they are more likely to be Westerhold and Röhl (2009) models are based classifi ed as category II (e.g., Yellow and Straw- on orbital tuning and cycle counting of Fe in- Method of Age Model Evaluation berry tuffs). tensity data from ODP Leg 207, Site 1258, and An index of agreement (IA = number of cat- they provide estimates of chron durations for The paleomagnetic polarity results from egory I tuffs with concordant measured and pre- the interval between chrons C20 and C24. In the the 23 alpha and beta sites were considered in dicted polarity divided by the total number of tuned model, the cycles are tuned to the stable conjunction with the most recently published category I tuffs within a particular age model) 405-k.y.-long eccentricity cycle, whereas the 40Ar/39Ar ages of the tuffs (Smith et al., 2008a, was calculated for each model and used to defi ne cycle-counted model is based on cycle counting 2010) to evaluate geomagnetic polarity time the model with maximum congruence between assuming 21 k.y. and 95 k.y. for the precession scale age models for the interval between ca. 53 expected and measured polarity. Because the and short eccentricity cycles, respectively. The and 44 Ma. For the purpose of evaluating these number of category I tuffs (i.e., denominator) accuracy of an astronomical time scale based on age models, the 40Ar/39Ar and paleomagnetic varies among the models, the index of agreement orbital cyclicities depends on the accuracy of the polarity data are assumed reliable (see previous provides a semiquantitative means to measure orbital solution used (although it is presently not section on “Evaluation of the Input Data” for the effective explanatory power of each model available for the Paleogene), the stratigraphic uncertainties of the polarity and 40Ar/39Ar data). with an expectation that a perfectly congruent completeness of climatic proxy records, proper Because every age model predicts a polarity for calibration model would have an IA of 1. correlation between the proxy records and the the 40Ar/39Ar age range of each tuff defi ned by orbital solution, the accuracy of magnetostrati- a 2σ fully propagated uncertainty, we deter- Results of Age Model Evaluation graphic data for the section in which the proxy mined the number of tuffs that have concor- records are identifi ed and correlated, and fi nally, dant predicted and measured polarities within Calculated IAs range from 0.29 to 0.60, with the assumed value of sediment accumulation a framework of a particular age model. The the highest value recorded for the Willwood rates between two astronomically tuned cali- tuffs for which 40Ar/39Ar age ranges fall within model, followed by model C and T-3 (IA = bration points. Uncertainties in the Westerhold a single chron in a particular age model are re- 0.56; Fig. 6; Table DR5 [see footnote 1]). No and Röhl (2009) geochemical data set include: ferred to as category I tuffs and are included age model has an IA close to 1, and we interpret potential ambiguities in magnetostratigraphy of in the calculation of the “index of agreement” this result to indicate that further modifi cations ODP 1258 due to near-equatorial paleolatitudes (see following). However, 40Ar/39Ar age ranges to the models are necessary to reconcile the and weak magnetization; potential unrecog- of some tuffs in any given age model are likely paleomagnetic polarity data with the radioiso- nized faults in Hole 1258A; and ambiguities in to span chron boundaries by chance, preventing topic data. Because the use of IA was not sat- construction of the composite section (Wester- an unequivocal assessment of the congruence isfactory in defi ning one best model, the three hold and Röhl, 2009). between the predicted and observed polarity. models with the highest IA values (Willwood, In contrast to the models based on 40Ar/39Ar These tuffs are classifi ed as category II and are model C, and T-3) are further discussed in the age determinations, testing the Westerhold and excluded from the evaluation of the particular following. It is worth noting that the same test Röhl (2009) models requires us to fi rst anchor age model because they are equivocal. Note was performed using the 40Ar/39Ar age estimates them to a reference point, as they are fl oat- that the category assignment is age model spe- considering only the 2σ analytical uncertainty ing age models due to the uncertain age of the cifi c, and thus any given tuff may be classifi ed (as opposed to fully propagated uncertainty), Paleocene-Eocene Thermal Maximum and uncertainties in orbital solutions for this interval (Laskar et al., 2004; Westerhold et al., 2007, 0.70

2008). To calibrate their models in absolute 0.60 time, we used three astronomically proposed 0.50 0.60 0.56 ages for the top of chron C24r as anchor points 0.56 0.50 0.50 0.50 (53.53 Ma, 53.93 Ma, and 54.33 Ma according 0.50 0.40 0.44 to option 1, option 2, and option 3, respectively; 0.44 0.40

Westerhold et al., 2007). Because Westerhold 0.30 0.38

and Röhl (2009) used two methods to calibrate 0.31 0.29 the observed cycles (tuning and cycle counting), Index of agreement 0.20 and each of them is anchored to three proposed 0.10 age estimates for the top of chron C24r, in total, six variations are considered for evaluation in 0.00

this study (these models will be referred to as T-1 T-1 T-2 T-3 CC-1 CC-2 CC-3 CK95

to T-3 and CC-1 to CC-3, where T and CC stand model (2010) Model B Model C Model D Willwood for tuning and cycle counting, respectively, and GOS2004 (Machlus et al., 2004) (Westerhold and Röhl, 2009) the number following the hyphen corresponds Smith et al. to either option 1, 2, or 3). Tuffs that are younger Figure 6. Index of agreement (IA) for all the calibration models than the younger limit of a particular age model evaluated in this study. See text for derivation of IA and abbrevia- are not considered in such cases. Figure DR1 tions of the model names.

Geological Society of America Bulletin, Month/Month 2012 9 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Tsukui and Clyde but the IA values did not change signifi cantly, polarity time scale calibration in absolute time. (5) The Continental Peak tuff is exposed in implying that the match between the model and Comparison of the three models with these in- the South Pass section in a stratigraphic interval the input data is not controlled by the size of dependent chronostratigraphic data favors the that is interpreted to be Bridger B (or uppermost uncertainty associated with each 40Ar/39Ar age. new Willwood model over the new model C Bridger A; Zeller and Stephens, 1969; Gunnell, Further adjustments were made to the three and the new T-3 model, although the new T-3 2006, personal commun.). The new Willwood models by shifting the chron ages in order to model is very similar to the older half of the and new T-3 models place the tuff in chron C21r; force at least a minimum overlap between ex- new Willwood model. The chronostratigraphic this is consistent with the magneto stratigraphy pected and measured polarity for all category I data used to support the new Willwood model of Clyde et al. (2001) from the same South Pass tuffs. Chron boundaries nearest to discordant are summarized below and in Table 2: section, which correlates the lower part of the category I tuffs (those with unmatched expected (1) Walsh (1996) and Prothero (1996) showed Bridger Formation with chron C22n. This con- and measured polarities) were shifted to either that the Bridgerian-Uintan NALMA bound- trasts with the new model C because it correlates the top or bottom of the 40Ar/39Ar age range of ary should lie in chron C21n. The Sage Creek the Continental Peak tuff with chron C23n.1r, the tuffs so that predicted and measured polari- Mountain pumice from the Bridger E lithol- which has been associated with the older ties matched at least minimally within the age ogy approximates the position of the boundary Wasatch Formation in the magneto stratigraphy range determined for the tuffs. The ages of either because the Bridger E faunal assemblage that of Clyde et al. (2001). the top or base of the discordant category I tuffs contains a mix of Bridgerian and Uintan fauna (6) The Willwood Ash is a bed of particular toward which the chron bound aries were shifted is found ~7 m above the base of the Bridger E interest for calibration of the geomagnetic polar- were then tentatively used as best approximate lithology (Evanoff et al., 1994; Robinson et al., ity time scale because it is located at the base of age estimates for the polarity boundaries. For 2004; Murphey and Walsh, 2007). The new chron C24n.1n and is used as one of the fi ve cali- example, four chron boundaries (C20r-C20n, Willwood model places the Sage Creek Moun- bration points in GOS2004 and in the original C21n-C20r, C22n-C21r, and C22r-C22n) in tain pumice in chron C21n. age model by Wing et al. (2000) (Tauxe et al., the Willwood model were incrementally shifted (2) The Tabernacle Butte locality yields a 1994; Ogg and Smith, 2004). It was dated at to the top of the Strawberry tuff age range Br-3 fauna (the youngest subage of the Bridg- ca. 52.8 Ma by Wing et al. (1991) but has been at 43.34 Ma, base of the Oily tuff age range at erian NALMA) according to West (1973). The recently redated and recalibrated to 52.9 ± 45.57 Ma, base of the Church Butte tuff age new Willwood and new T-3 models place the 0.18 Ma (2σ fully propagated uncertainty) rela- range at 49.21 Ma, and top of the Main tuff Tabernacle Butte tuff in chron C21r. This place- tive to the FCsK08 by Smith et al. (2004, 2010). age range at 50.11 Ma, respectively (Fig. 7). ment is consistent with the bio- and magneto- The new Willwood model is expected to meet Hereafter, the three adjusted models will be re- stratigraphic records of Clyde et al. (2001), these magnetostratigraphic and radioisotopic ferred to as new Willwood, new model C, and which place Br-1b in C22n, and records from criteria, since these data were used in building new T-3 to differentiate them from the original Prothero (1996), which place the Bridgerian- the calibration model by Wing et al. (2000) from versions (Figs. DR2 and DR3; Table DR5, see Uintan boundary in chron C21n. which the Willwood model was derived. How- the last three columns [see footnote 1]). (3) The Church Butte tuff is exposed at the ever, the new model C and new T-3 model place fossiliferous Grizzly Buttes locality, which the base of chron C24n.1n outside the error of DISCUSSION has been associated with a Br-2 fauna (upper the Willwood Ash. Blacksforkian subage; Alexander and Burger, (7) The tuff just below the Alamo Creek basalt Comparison with 2001). The Br-2 fauna was found in chron C21r in the Lower Member of the Devil’s Graveyard Chronostratigraphic Data in the Delmar Formation in California (Rob- Formation lies just below rocks of reversed po- inson et al., 2004). The Delmar Formation has larity and above the Junction faunas, which con- The three modifi ed models (new Willwood, been correlated directly to marine biostratigra- tain a mixture of the Bridgerian-Uintan faunas new model C, and new T-3) were evaluated in phy and thus has a reliable chron assignment. (Prothero and Emry, 1996; Walton, 1992). Smith a regional chronostratigraphic context to de- The new Willwood and new T-3 models place et al. (2010) recalibrated the age of this tuff to termine which one integrates existing chrono- the Church Butte tuff in chron C22n. This cor- 46.85 ± 0.14 Ma using the FCsK08. Based on its stratigraphic constraints in the most coherent relation is also consistent with that of Clyde 40Ar/39Ar age, the new Willwood model places way. To do so, we used bio- and magnetostrati- et al. (2001), who placed the underlying Br-1b the tuff in chron C21n, i.e., the same chron the graphic data from sections in the Bighorn Ba- in chron C22n. Bridgerian-Uintan boundary is predicted to lie sin (Clyde et al., 1994), Greater Green River (4) Prothero (1996) proposed a polarity stra- by Prothero (1996). In contrary, the new model C Basin (Clyde et al., 1997, 2001), and Uinta tigraphy from Indian Canyon of the Uinta Ba- places the tuff in chron C22n, which is correlated Basin (Prothero, 1996). These bio- and magneto- sin that spans from the Horse Bench Sandstone to Br-1b by Clyde et al. (2001). stratigraphic records were placed within each marker bed to the limestone and sandstone (8) The Mission Valley Ash bed has been of the three calibration frameworks in exactly facies of the Green River Formation (Fig. 2). correlated to Ui-3 and chron C20n (Prothero the same manner as they were correlated to the The Fat and Oily tuffs can be correlated to his and Emry, 1996; Walsh et al., 1996; Robinson geomagnetic polarity time scale in the origi- section based on their meter levels with respect et al., 2004). Smith et al. (2010) recalibrated nal studies (Fig. 7; Figs. DR2 and DR3 [see to the Horse Bench Sandstone and the saline the age of this tuff to 43.35 ± 0.49 Ma using the footnote 1]). These magnetostratigraphic sec- facies–limestone-sandstone facies transition. FCsK08. The new Willwood model places the tuff tions were chosen because they had been cor- The new Willwood model places the Fat tuff in chron C20n based on its 40Ar/39Ar age, con- related to the NALMA biostratigraphy, which within chron C21n, and this correlation is in sistent with the bio- and magnetostratigraphic is important for evaluating the veracity of the agreement with Prothero (1996). His magneto- records of Prothero (1996). The new model C age models because the biostratigraphy is es- stratigraphic correlation places the Oily tuff in places the tuff in chron C21n instead. tablished with respect to magnetostratigraphy chron C19r, which confl icts with the polarity (9) The Blue Point Ash (Hiza, 1999) lies near for a given area, independent of geomagnetic determination of this study. the base of chron C21r (Sundell et al., 1984;

10 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

Tuffs (alpha and beta sites only) Bio-magnetostratigraphy from previous studies St O Fa Bl SCMSa TB HF LC CP CB 6 L M G B Y F R K

42 Ui-3 C20n 43 Ui-2

44 C20r Ui-1 45

C21n 47 Br-3

48 Prothero (1996) Age (Ma) C21r 49

C22n 50 Br-1b

C22r 51 Br-1a C23n.2n Br-0 52 C23r Wa-7 C24n.1n ~Wa-7 Clyde et al. 53 Wa-6 (1997,2001) C24n.3n Wa-5 Willwood Ash Clyde et C24r Wa-4 al. (1994) 54

Figure 7. Paleomagnetic polarity and 40Ar/39Ar ages of alpha and beta sites shown within the calibration framework of the proposed new Willwood model. Bio- and magnetostratigraphic data from the relevant sections are shown to the right. Filled (open) boxes indicate sites of normal (reversed) polarity, whereas height of each box indicates 2σ fully propagated uncertainty of a 40Ar/39Ar age as determined by Smith et al. (2010). Ovals show the younger or older end of discordant category I tuffs toward which some chron boundaries were shifted. The position of the Willwood Ash is also shown with its associated 2σ fully propagated uncertainty. Wa—Wasatchian North American Land Mammal Age (NALMA), Br—Bridgerian NALMA, Ui—Uintan NALMA. Biostratigraphic boundaries are shown with broken lines because their stratigraphic positions are not precisely known. See Table 1 for abbreviations of the tuffs.

Geological Society of America Bulletin, Month/Month 2012 11 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Tsukui and Clyde )

√ Eaton, 1985). Its age has been recalibrated by

) Smith et al. (2010) to 48.41 ± 0.17 Ma using ) ) √ √ √ (n22CniBC (r1 (r12CniBT the FCsK08. The new Willwood and new T-3 † † † * .D.N .D.N

2CniPC models place the ash in chron C21r, which falls N.D. N.D. between Br-1b and Br-3 according to bio- and predicts magnetostratigraphy of Clyde et al. (2001) and Prothero (1996). In contrary, the new model C What new T-3 model T-3 What new

Blue Point ash in C21r ( places it in chron C22r through C23n.1n, which were correlated to the early Bridgerian NALMA by Clyde et al. (2001). Sundell et al. (1984) as- signed the late Bridgerian age for the part of their measured section in which the Blue Point )

√ Ash lies, so the new Willwood and new T-3 ) ) √ ) ) )

) models are both consistent with their observa- √ √ (X) indicates agreement (disagreement) (n12CniMCS √ ) √ √ √ (n22CniBC (n12Cni ( (r12CniBT

√ tions regarding biostratigraphy. r 12 C

niPC Implications of New Calibration in C21n ( a F

C20n and C20r ( The new Willwood model provides a calibra-

Blue Point ash in C21r ( tion framework in which the regional bio- and The Mission Valley Ash bed in The Mission Valley ) but correspondence is expected C24n.1n at 53.42 Ma (X) √

e explained in Table 1. Table e explained in magnetostratigraphic data can be integrated ( Tuff just above the Alamo Creek basalt just above the Tuff most coherently (Table 2; Fig. 7), and thus it is tentatively considered to be the best alternative for calibrating the early to middle Eocene geomagnetic polarity time scale (Fig. 8). In addition, the similarity between the new Willwood and new T-3 models provides independent support for the oldest age for the Paleocene-Eocene ) )X(n22CniMCS )X(n32Cn X )X(n22C )X(r22CniBT ( Thermal Maximum (56.33 Ma) as proposed by r 1. Westerhold et al. (2008). In light of new infor- n 32C mation regarding the cyclostratigraphically de- n iBC i − n termined position of the ash 17 in Site 550, iPC in C22n (X) a

F Westerhold et al. (2009) also concluded that the option 3 age estimate is most consistent with the C24n.1n at 51.85 Ma (X) published 40Ar/39Ar sanidine age of ash −17 if

Mission Valley Ash bed in C21n (X) Mission Valley it is calculated relative to FCs (Storey et al.,

Tuff just above the Alamo Creek basalt just above the Tuff K08 Blue Point ash in C22r and C23n.1n (X) 2007). Using the astronomically determined durations for the Paleocene (Westerhold et al., 2008), the option 3 age for the Paleocene-Eocene Thermal Maximum implies an age of 66.05 Ma for the Cretaceous-Paleogene boundary, which is very close to the estimate by Kuiper et al. TABLE 2. CHRONOSTRATIGRAPHIC CRITERIA USED TO ASSESS AGE MODELS ASSESS TO USED CRITERIA 2. CHRONOSTRATIGRAPHIC TABLE (2008) (Westerhold et al., 2009). The agreement between the new Willwood model derived by

n12CniMCS paleomagnetic polarity and radioisotopic age n n12CniaF 1

2CniaF determinations of terrestrial tuffs with the new T-3 marine astronomical age model represents

Ui-3 fauna an important step toward integrating astronomi- 52.9±0.18 Ma cal, radioisotopic, and paleomagnetic data from TB in C21n or C21r Blue Point ash in C21r Blue Point ash in C21r CP in C22n or younger CP both marine and terrestrial records to construct CB in C21n, C21r, or C22n CB in C21n, C21r, CP from Bridger B lithology CP CB associated with Br-2 fauna Mission Valley Ash bed in C20n Mission Valley TB locality produces Br-3 fauna TB a more robust calibration for the geomagnetic polarity time scale for this time interval. Addi- the Bridgerian/Uintan NALMA boundary the Bridgerian/Uintan NALMA tional progress in this regard could come from

Willwood Ash at the base of C24n.1n and dated Willwood a well-resolved and reproducible land-based cyclostratigraphy in the Green River Formation )sbO( )pxE( )pxE(

data and expected (Exp) outcome according to age models What new model C predicts What new Willwood model predicts (Machlus et al., 2008; Meyers, 2008). Observed data (Obs) from previous bio- and magnetostratigraphic (Exp) (Exp) C24n.1n (base) lies between 53.08 and 52.72 Ma (Exp) (Exp)Alamo Creek basalt in C21n or above just below the Tuff (Exp) (Exp) (Exp) (Obs) (Obs)Ash bed is correlated to C20n and Valley Mission (Obs) (Obs)Alamo Creek basalt lies above just below the Tuff (Obs) (Obs) (Obs) boundary in C21n Bridgerian/Uintan NALMA (Obs) For the most part, our proposed new Will-

More detailed discussion and references can be found under the corresponding criteria number in text. Abbreviations of tuffs ar Abbreviations of tuffs More detailed discussion and references can be found under the corresponding criteria number in text. wood model makes the study interval older than previously calibrated in GOS2004 (chrons N.D.—no data and indicates that the tuffs of interest fall outside the age range particular model. N.D.—no data and indicates that the tuffs † Note: *Criteria number corresponds to the in text “Comparison with Chronostratigraphic Data.” C20n through C24n), with appreciable length- 7 Criteria 3 8 4 5 6 2 1 9 between the observed data and prediction. number*

12 Geological Society of America Bulletin, Month/Month 2012 Geological Society of America Bulletin, published online on 24 February 2012 as doi:10.1130/B30545.1

Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

1986), and in agreement with calculated rates Chron Distance (km) Age calibration (Ma) % difference for the Eocene by Mosar et al. (2002). It is pos- from South Atlantic GOS2004 New Willwood sible that a very slow spreading ridge like the

GOS2004 New Willwood model spreading center model Atlantic may be characterized by sporadic mag- 40 C18r matic supply, resulting in nonuniform rates in 947.96 40.439 40.21 0.57 C19r C19n seafl oor spreading over these relatively short 954.12 40.671 40.53 0.35 42 C19r time scales (e.g., Allerton et al., 2000). C20n 977.65 41.590 41.79 -0.48 C20n An important but commonly overlooked addi- 1006.06 42.774 43.34 -1.32 C20r tional source of error in calibrating the geomag- 1060.24 45.346 45.57 -0.49 44 C20r C21n netic polarity time scale and calculating rates of 1094.71 47.235 48.03 -1.68 C21r seafl oor spreading is the uncertainty of the esti- 1117.55 48.599 49.21 -1.26 C22n mated positions of seafl oor magnetic anomaly 46 1130.78 49.427 50.11 -1.38 C21n C22r boundaries with respect to the spreading ridge 1150.83 50.730 51.03 -0.59 C23n.1n (see table 4 in Cande and Kent, 1992; Wester- 1153.90 50.932 51.19 -0.51 C21r C23n.1r hold et al., 2008). For the South Atlantic, the

Age (Ma) 48 1155.75 51.057 51.29 -0.46 C23n.2n uncertainties in chron spacings between chrons C22n 1168.20 51.901 51.96 -0.10 C23r C19 and C24 amount to 27.082 km, with chron 1178.96 52.648 52.53 0.23 50 C22r C24n.1n C23 having the largest uncertainty (17.3% of 1184.03 53.004 52.80 0.38 C24n.1r the reported width; Fig. 9). These uncertainties C23n 1185.61 53.116 52.91 0.39 C24n.2n are expressed in distance but can be converted 52 1186.34 53.167 52.96 0.40 C23r C24n.2r 1188.05 53.286 53.07 0.40 to time using an age model. Thus, temporal un- C24n.3n C24n 1195.35 53.808 53.57 0.45 certainties inherent in the geomagnetic polarity 54 time scale calibration are derived not only from the commonly acknowledged problems asso- Figure 8. Comparison of GOS2004 (Ogg and Smith, 2004) and proposed (new Willwood ciated with the quality of radioisotopic age model) calibration of the early to middle Eocene geomagnetic polarity time scale shown estimates of the tie points and the assumption schematically and in table format. The broken line indicates the Ypresian-Lutetian stage of smoothly varying seafl oor spreading rates, boundary. Distance data are from Cande and Kent (1992). Underlined ages are linearly ex- but also from uncertain positions of magnetic trapolated ages from Wing et al. (2000), whereas bold ages are new in this study derived via anomalies, making it diffi cult to identify domi- incremental adjustment of chron boundaries near discordant category I tuffs (see text for nant sources of uncertainty in the nonastronomi- discussion). Ages in italics are the same as originally proposed in Wing et al. (2000). cally calibrated part of the time scale. Because of the large uncertainties in the underpinnings of the geomagnetic polarity time scale, this study suggests that calibration of the pre-Neogene ening of four contiguous normal chrons (C19n, standing climate dynamics at the height of geomagnetic polarity time scale is best achieved C20n, C21n, and C22n) at the expense of their the Cenozoic greenhouse climate and its effect through directly integrating radioisotopic dat- corresponding reversed chrons (except for on mammalian evolution on land. The new esti- ing with magnetostratigraphy (e.g., McIntosh chron C19r, which was lengthened; Fig. 8). mates on the timing of the early Eocene climatic et al., 1992), although the seafl oor magnetic The chrons in the older part of the study in- optimum and Wasatchian-Bridgerian faunal anomaly record will remain the only continuous terval (between chrons C22r and C24n) are all transition overlap signifi cantly and are thus con- blueprint of magnetic reversals. The departure shortened as a result. The proposed calibra- sistent with hypotheses linking biotic change to from the traditional paradigm of relying heavily tion model also changes the age calibration of climatic forcing at this time (Clyde et al., 2001; on the seafl oor magnetic anomaly profi le as a major biological and stage boundaries. Within Woodburne et al., 2009). standard for scaling magnetic polarity chrons in the new calibration framework, the Wasatchian- the development of a time scale is demonstrated Bridgerian NALMA boundary in chron C23r Implications for Seafl oor Spreading Rates by the recent efforts to calibrate the entire Ceno- is bracketed between 51.96 and 52.53 Ma, and zoic geomagnetic polarity time scale using the Bridgerian-Uintan NALMA in chron C21n Our proposed model implies more frequent astronomical tuning (e.g., Pälike et al., 2001; is bracketed between 48.03 and 45.57 Ma. variations in South Atlantic seafl oor spread- Lourens et al., 2004; Westerhold et al., 2008). The new Willwood model places the Ypresian- ing rates, thus challenging the assumption of Lutetian stage boundary at ca. 49.21 Ma, out- smoothly varying seafl oor spreading employed CONCLUSIONS side the 2σ error quoted for the stage boundary in the development of the geomagnetic polar- in GOS2004, making the Ypresian stage (early ity time scale (Fig. 9; Cande and Kent, 1992; This study combines new paleomagnetic po- Eocene) ~0.61 m.y. shorter in duration, whereas Ogg and Smith, 2004). However, it is important larity data from volcanic tuffs with previously the Lutetian is lengthened by ~0.84 m.y. in dura- to note that our estimates of seafl oor spreading published radioisotopic ages to systematically tion compared to GOS2004 (Fig. 8). Finally, the rates are highly dependent on the estimates of assess eight proposed calibration models for the new calibration implies that the early Eocene the anomaly widths, which themselves are un- early to middle Eocene part of the geomagnetic climatic optimum is ~2.2 m.y. in duration from certain (see following). Our estimates of ~15–25 polarity time scale. The ash-fall deposits were 52.9 to 50.7 Ma. This is ~1 m.y. longer than has mm/yr for the absolute rates are within the range sampled on average every ~0.36 m.y. and thus been previously proposed (Zachos et al., 2001) found for the last 16 m.y. in the northern South allow for precise calibration of the interpolated and could have important implications for under- Atlantic, although relatively low (Brozena, segments of the time scale. The new Willwood

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Spreading rate Spreading rate Brozena, J.M., 1986, Temporal and spatial variability of 10 seafl oor spreading processes in the northern South At- 940 990 1040 1090 1140 1190 lantic: Journal of Geophysical Research, v. 91, no. B1, p. 497–510, doi:10.1029/JB091iB01p00497. Bryan, W.B., and Duncan, R.A., 1983, Age and provenance Distance (km) from South Atlantic spreading center of clastic horizons from Hole 516F, in Whalen, E., ed., Initial Reports of the Deep Sea Drilling Project, Vol- Figure 9. Age-distance chart for the study interval based on GOS2004 (solid line) and new ume 72: Washington, D.C., U.S. Government Printing Willwood model (broken line). Locations of magnetic chrons are shown by black vertical Offi ce, p. 475–477. Bryant, B., Naeser, C.W., Marvin, R.F., and Mehnert, H.H., lines, the positions of which are relative to the mid-ocean-ridge crest in the South Atlantic 1989, Upper Cretaceous and Paleogene Sedimentary (Cande and Kent, 1992). 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Chetel, L.M., Janecke, S.U., Carroll, A.R., Beard, B.L., John- resulting in a shortening of the early Eocene by Thomas Westerhold generously provided unpublished son, C.M., and Singer, B.S., 2011, Paleogeographic 0.61 m.y. and lengthening of the middle Eo- data. Julie Bryce and Joel Johnson provided useful reconstruction of the Eocene Idaho River, North Amer- comments on an earlier draft. We also thank our re- ican Cordillera: Geological Society of America Bulle- cene by 0.84 m.y. The new model supports the viewers, Brad Singer, John Geissman, Mike Smith, tin, v. 123, no. 1–2, p. 71–88, doi:10.1130/B30213.1. temporal overlap of the Wasatchian-Bridgerian and an anonymous reviewer, for their extensive re- Clyde, W.C., Stamatakos, J., and Gingerich, P.D., 1994, faunal turnover in North America with the early views and comments, which improved the manuscript Chronology of the Wasatchian land-mammal age: Mag- signifi cantly. This project was funded by the National netostratigraphic results from the McCullough Peaks Eocene climatic optimum, suggesting a possible section, northern Bighorn Basin, Wyoming: The Journal Science Foundation grant EAR-0642291 to W. C. relationship between climatic and biotic change of Geology, v. 102, p. 367–377, doi:10.1086/629680. Clyde and a Geological Society of America Research Clyde, W.C., Zonneveld, J.-P., Stamatakos, J., Gunnell, G.F., at this time. Our proposed new Willwood model Grant and an American Association of Petroleum and Bartels, W.S., 1997, Magnetostratigraphy across agrees with the orbitally tuned model of Wester- Geol ogists Grant-in-Aid to K. Tsukui. the Wasatchian/Bridgerian NALMA boundary (early

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Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

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