Revised chronostratigraphy and biostratigraphy of the ...

U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho

Revised chronostratigraphy and biostratigraphy of the early–middle Miocene Railroad Canyon section of central-eastern Idaho, USA Elisha B. Harris1,2,†, Caroline A.E. Strömberg1,2, Nathan D. Sheldon3, Selena Y. Smith3,4, and Mauricio Ibañez-Mejia5,6 Department of Biology, University of Washington, Box 351800, 24 Kincaid Hall, Seattle, Washington 98195, USA Burke Museum of Natural History and Culture, 4331 Memorial Way Northeast, Seattle, Washington 98195, USA 3 Department of Earth and Environmental Sciences, University of Michigan, 2534 CC Little Building, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA 4 Museum of Paleontology, University of Michigan, 1109 Geddes Avenue, Ann Arbor, Michigan 48109, USA 5 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 54-918, Cambridge, Massachusetts 02139, USA 6 Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, P.O. Box 270221, Rochester, New York 14627, USA 1 2

ABSTRACT The early–middle Miocene was an important transitional period in the evolution of Earth’s biota and climate that has been poorly understood in North America due to a paucity of continuous, fossil-bearing rock records in this interval for which the ages have been robustly constrained. In the northern Rocky Mountains, United States, one site in particular, known as the Railroad Canyon section, has provided biostratigraphic, magnetostratigraphic, and lithostratigraphic evidence suggesting a late early–middle Miocene age; however, radiometrically calibrated age models have been notoriously lacking. To better constrain the age of the Railroad Canyon section and the abundant fossils preserved therein, we employed moderate- and highprecision U-Pb dating of single zircon crystals from four ash horizons throughout the section. The resulting dates span from 22.65 ± 0.37 Ma to 15.76 ± 0.22 Ma. Using these dates, we developed a radiometrically calibrated age model for the Railroad Canyon section that constrains the age of the section to ca. 22.9–15.2 Ma, ~5 m.y. older than previous estimates. These results firmly establish that the Railroad Canyon section was deposited during buildup to peak warming of the mid-Miocene climatic optimum. Additionally, these dates provide definitive age

[email protected]

†

estimations for the initiation and cessation of the early Miocene unconformity, a regional unconformity exposed in many intermontane basins across the northern Rocky Mountains, as ca. 21.5 and 21.4 Ma, respectively, in the Railroad Canyon section. This new chronostratigraphic analysis provides an impetus for reassessment of the biochronology of the region, in turn suggesting earlier first appearances of many biostratigraphically important taxa found in the northern Rocky Mountains, Great Plains, and American Northwest. INTRODUCTION The early–middle Miocene was a critical transitional period in Earth’s geologic history. Long-term global climatic cooling was temporarily reversed, culminating in the mid-Miocene climatic optimum (ca. 17–14.75 Ma; Zachos et al., 2001), and modern ecosystems were established around the world (e.g., Graham, 1999, 2011; Pound et al., 2012). Grass-dominated biomes, which cover up to 40% of Earth’s land surface today (Gibson, 2009), spread across North America, Eurasia, Australia, and Africa (Jacobs et al., 1999; Strömberg, 2011). These changes were accompanied by replacement of the archaic browser-dominated fauna(s) by grazing herbivores through evolution and migration (e.g., North America—MacFadden, 2000; Barnosky and Carrasco, 2002; Janis et al., 2004; Eurasia—Fortelius et al., 2006; van Dam, 2006; Africa—Bobe, 2006; South America—Flynn et al., 2003; Pascual, 2006). In addition, many

parts of the South and North American Cordilleras experienced either accelerated or renewed uplift during the early–middle Miocene (e.g., Gregory-Wodzicki, 2000; Horton et al., 2004), with profound impacts on faunal biogeography and diversification (Kohn and Fremd, 2008; Finarelli and Badgley, 2010). In North America, the timing of many of these events, particularly those associated with the mid-Miocene climatic optimum, remains ambiguous due to the few, continuous, fossilbearing rock records for which the age can be robustly constrained to the early–middle Miocene, and a general paucity of absolute age determinations using modern methods. For example, Fritz et al. (2007) compiled existing K-Ar ages for Montana and Idaho and reported just five between 27 and 10 Ma, with only a single age in strata of mid-Miocene climatic optimum age. As a result, much of the regional age control (e.g., Chamberlain et al., 2012) in the northern Rocky Mountains and western Great Plains of North America (e.g., Sjostrom et al., 2006) relies on relative and semiquantitative ages based on mammalian biostratigraphy (e.g., Tedford et al., 2004). However, the relatively coarse temporal resolution based on mammalian faunas has led to questionable conclusions about the timing of biologic, tectonic, and climatic events (e.g., Tedford et al., 2004; Kent-Corson et al., 2006, 2013; Barnosky et al., 2007). In particular, we point out that both the timing and rates of vegetation change, faunal turnover and evolution, and tectonic deformation and/or uplift in the area can still be significantly improved

GSA Bulletin; September/October 2017; v. 129; no. 9/10; p. 1241–1251; doi: 10.1130/B31655.1; 5 figures; 1 table; Data Repository item 2017184; published online 23 June 2017.

For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 129, no. 9/10 1241 © 2017 Geological Society of America

Harris et al. nosky et al. (2007). The primary purpose of this paper is to provide the first absolute age model for the Railroad Canyon section, using U-Pb dating of zircons extracted from three volcanic ash layers found above and below an erosional unconformity that separates the Renova and Six Mile Creek Formations. Using the improved age model, we then discuss the potential implications of this important locality for Miocene biostratigraphy of the northwestern United States and its chronostratigraphic significance for studying tectonic uplift history in the northern Rocky Mountains and the mid-Miocene climatic optimum in North America more broadly.

Missoula

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Railroad Canyon Lithostratigraphic, Biostratigraphic, and Magnetostratigraphic Context The lowermost ~70 m of the composite Railroad Canyon section belong to the Renova Formation (Fig. 2), which locally consists of gray to white mudstone and siltstone deposits with occasional gypsum and halite deposits poten-

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tially indicative of an arid, closed basin with intermittent saline lakes (Fields et al., 1985; Barnosky et al., 2007). The Renova Formation is overlain by the Six Mile Creek Formation (Fig. 2), which is locally distinguished by pinkish to tan siltstone and sandstone beds with occasional conglomeratic lenses indicative of a sediment-choked fluvial system (Fields et al., 1985; Rasmussen, 2003; Barnosky et al., 2007). These two formations are separated by an erosional contact that has been described in many other intermontane basins (including the Ruby River, Beaverhead, Jefferson River, and Horse Prairie basins of southwestern Montana) as the mid-Tertiary unconformity (Fields et al., 1985; Hanneman and Wideman, 1991, 2006; Rasmussen, 2003; Barnosky et al., 2007). Herein, we discard the use of the name mid-Tertiary unconformity and instead resume use of the name “early Miocene unconformity,” proposed by Fields et al. (1985), because this is a more accurate and precise name for this unconformity given what we know about its timing and stratigraphic occurrence. In addition, “Tertiary” is

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through the development of a more robust regional absolute chronology framework. The Railroad Canyon section (Fig. 1) of centraleastern Idaho contains the most complete geologic record for the early–middle Miocene in the northern Rocky Mountains. The Railroad Canyon section is located in Bannock Pass, ~19 km northeast of Leadore, Idaho, and exposes nearly 360 m of sedimentary rock section from the upper Renova and lower Six Mile Creek Formations (Fig. 1; Barnosky et al., 2007). Prior to this study, the age of the Railroad Canyon section was poorly constrained. Interpretation of biostratigraphic and magnetostratigraphic data by Barnosky et al. (2007) suggested the Railroad Canyon section was deposited ca. 17.3–13 Ma (see also Zheng, 1996). This contrasts with a revised age model by Retallack (2009), who proposed that Railroad Canyon section sediments were deposited between 16.4 and 10.7 Ma, based on an amended geologic time scale (Ogg and Smith [2004] vs. Cande and Kent [1995]) and by constraining plausible alternative age estimates using faunal biostratigraphic data from Bar-

Figure 1. Railroad Canyon section (RCS) locality information. (A) Map showing the location of the Railroad Canyon section in central-eastern Idaho. The locations of four additional fossil sites in southwestern Montana are also included, namely, Trace Fossil Fun Time (TFFT; Cotton et al., 2012), Timber Hills A (TH; Cotton et al., 2012), Madison Buffalo Jump (MBJ; Chen et al., 2015), and Beaverhead Basin Flora (BHL). (B) Location of vertebrate fossil sites within the Railroad Canyon section that were also sampled for magnetostratigraphic analysis (Zheng, 1996; Barnosky et al., 2007), modified from Barnosky et al. (2007). ID—Idaho; MT—Montana; WY—Wyoming. Site name abbreviations: WH4—Whiskey Springs 4; WRC—West Railroad Cut; SFe—Snowfence east; SFw—Snowfence west; ST1—Snowfence Turtle 1; ST2—Snowfence Turtle 2; ST3—Snowfence Turtle 3; DS3—Dead Squirrel 3; DS4—Dead Squirrel 4; HDS3— High Dead Squirrel 3; HDS2—High Dead Squirrel 2.

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U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho an archaic term that has been replaced by both the International Commission on Stratigraphy (www.stratigraphy.org) and North American Stratigraphic Code (www.nacstrat.org) with Paleogene and Neogene at the period level, so this revised designation both represents a return to the original nomenclature and an update to modern stratigraphic terminology. The Railroad Canyon section fauna has been known for quite some time and was traditionally believed to belong to the early Barstovian North America Land Mammal Age (NALMA; Fields

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tilocapridae), Bouromeryx (Palaeomerycidae), Rakomeryx (Palaeomerycidae), Aepycamelus (Camelidae), Brachycrus laticeps (Merycoidodontidae), and Tylocephalonyx skinneri (Chalicotheriidae) (Fig. 2), Barnosky et al. (2007) proposed that meters 150–250 in the composite Railroad Canyon section are no older than the late Hemingfordian (He2). Additionally, they suggested that this interval is no younger than late Barstovian (Ba2) based on the cooccurrence of Oreolagus (Ochotonidae), Plesiosminthus (Zapodidae), Peridiomys (Heteromyidae),

et al., 1985; Tedford et al., 1987; Barnosky, 2001). Barnosky et al. (2007) published a comprehensive biostratigraphic and magnetostratigraphic analysis of the Railroad Canyon section and suggested that the Railroad Canyon section fauna assemblage was characteristic of the late Hemingfordian to late Barstovian NALMAs (He2-Ba2). Based on the presence of key taxa such as Pliocyon (Amphicyonidae), Hypolagus (Leporidae), Harrymys irvini (Heteromyidae), Alphagaulus vetus (Mylagaulidae), Paracosoryx wilsoni (Antilocapridae), Merycodus (An-

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Figure 2. Railroad Canyon composite stratigraphic section and stratigraphic ranges of biostratigraphically significant taxa collected throughout the section (modified from Barnosky et al., 2007). Age model is based on U-Pb dating of zircons from three dated ash horizons (RCS1 = 22.65 ± 0.37 Ma; RCS2 = no good age estimate; RCS3 = 21.24 ± 0.27 Ma; RCS4 = 15.76 ± 0.22 Ma). Solid black bars indicate where taxa were found in the Railroad Canyon section. Black wavy lines at the top or bottom of a range bar indicate that the taxon-range boundaries are within the vertical line, but the exact placement of the taxon range is not possible due to specimens being collected as float. Dashed gray bars indicate previously published first appearances and taxonomic ranges of specific taxa (Albright et al., 2008; Tedford et al., 2004; source data in Table DR1 [see text footnote 1]). Single asterisks denote taxa that have an earlier first appearance in the Railroad Canyon section (RCS) based on our new age model. Double asterisks denote taxa that might have an earlier first appearance in the Railroad Canyon section if stratigraphic placement of the fossils could be better constrained. NALMA—North American Land Mammal Age; EMU—early Miocene unconformity; Ar4—late late Arikareean; He1—early Hemingfordian; He2—late Hemingfordian; Ba1—early Barstovian. Site name abbreviations are as in Figure 1. See Figure 3 for lithological key to stratigraphic section.

clay silt sand

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aracosoryx wilsoni (Antilocapridae), and P Hypohippus cf. H. osborni (Equidae). This biostratigraphic assessment was then used to correlate the Railroad Canyon section polarity sequence to the geomagnetic polarity time scale (GPTS) of Cande and Kent (1995). Given that the Railroad Canyon section polarity sequence was tied to the GPTS using local biostratigraphic age constraints only, the temporal window was limited. For this reason, Barnosky et al. (2007) presented two different interpretations of the Railroad Canyon section magnetostratigraphy, “basic unrevised” and “least squares 2 match.” Both interpretations placed the top of the Railroad Canyon section at ca. 13 Ma and the bottom at ca. 17.3 Ma, although the authors admitted that this latter estimate could be older, depending on the amount of unrepresented time due to the mid-Tertiary unconformity (herein early Miocene unconformity). Zheng (1996) provided additional interpretations of the Railroad Canyon section polarity sequence including a “least squares 1 match” that estimated the section to be ~22–15 m.y. old and a “least squares 3 match” that estimated an age of ~15–11.5 m.y. old. Both of these alternative hypotheses were rejected because the dates (1) did not agree with the existing constraints from faunal data, and (2) would have conflicted with the accepted age estimate of the mid-Tertiary unconformity (early Miocene unconformity) at that time (ca. 17 Ma; Barnosky et al., 2007).

agU/ 20 es 6P b GP TS

Harris et al.

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Figure 3. Correlation of the Railroad Canyon section (RCS) magnetostratigraphy (Zheng, 1996) with the global geomagnetic polarity time scale (GPTS; Gradstein et al., 2012). Isotopic age determinations are from U-Pb dating of three ash horizons (RCS1, RCS3, RCS4) that can be directly tied from the Railroad Canyon section into the GPTS (solid black lines). Gray dashed lines show possible chron correlation of rock units. North American Land Mammal Age (NALMA) durations and age boundaries are from Woodburne (2004) and Albright et al. (2008). Star denotes location of reworked ash, RCS2. Ar4—late late Arikareean; He1—early Hemingfordian; He2—late Hemingfordian; Ba1—early Barstovian; EMU—early Miocene unconformity.

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In addition to the work outlined herein, other efforts have been made to produce independent

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Other Attempts at Constraining the Age of the Railroad Canyon Section

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24 Claystone

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Conglomerate

Covered


U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho age constraints for the Railroad Canyon section. The first attempt was to establish a tephrachronology by comparing the elemental composition of Railroad Canyon section tephras to the University of Utah’s tephra database (Barnosky et al., 2007). This work resulted in parts of the section being consistent with a late Hemingfordian (He2) or early Barstovian (Ba1) age. However, the sampled ash horizons that were used to establish this tephrachronology were collected from sites that were not directly tied to the measured composite section due to separation by large covered intervals (e.g., Whiskey Springs 3 site located ~250 m west of Whiskey Springs 4 site and Deadman Pass 2 site located ~1.9 km northeast of Dead Squirrel 4 site). Therefore, the tephrachronology cannot be used as a reliable independent measure of age for the Railroad Canyon section. Ar-Ar dating of feldspars from numerous ash samples constituted a second attempt to produce an independent age constraint for the Railroad Canyon section (Barnosky et al., 2007). However, as the authors noted, they were unable to obtain reliable Ar-Ar dates for any of the Railroad Canyon section ashes because they were dominated by detrital feldspars that had been reworked after deposition. Herein, we propose a new geochronology for the Railroad Canyon section based on radioisotopic dating of zircons from well-defined ashes and compare these results with previously published magnetostratigraphic and biostratigraphic data (Zheng, 1996; Barnosky et al., 2007). We seek to: (1) provide a reappraisal and refinement of the age of the Railroad Canyon section; (2) further constrain the age of the early Miocene unconformity that separates the Renova and Six Mile Creek Formations regionally and provide a discussion of the implications for regional tectonic evolution; and (3) suggest refinements to the timing of the first appearance of biostratigraphically important taxa in the northern Rocky Mountains. MATERIALS AND METHODS Within the measured composite stratigraphic section, series of volcanic ash layers were identified and sampled for zircon U-Pb geochronology (Fig. 3). Four ash layers were dated from within the composite section, three collected from Whiskey Springs 4 (RCS1–RCS3) and one from High Dead Squirrel 2 (RCS4). Previous chronostratigraphic and biostratigraphic work in the Railroad Canyon section has included data from nearby sites (e.g., from the Lemhi Valley sequence) in their analysis, including from Cruikshank Creek, Peterson Creek, Mollie Gulch, and South Portal. For the purposes of this paper, we excluded from our analyses all data

from these four sites because they have not been accurately correlated to the composite Railroad Canyon section, and thus including them would introduce unnecessary additional uncertainty. Zircon Sampling and U-Pb Geochronologic Analyses

Pb/238U values, and the quoted uncertainty represents the final propagation of within-run analytical uncertainties, reproducibility of the primary reference material, and sources of systematic error (i.e., standard calibration and U decay constant uncertainties).

206

CA-TIMS U-Pb Geochronologic Analyses Zircon sampling and dating analyses were conducted by Ibañez-Mejia. Zircon crystals concentrated from whole-rock ash samples were picked under a binocular microscope, mounted in epoxy resin, and polished to expose the interior of the grain prior to cathodoluminescence imaging. U-Pb geochronologic analyses were first conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICPMS) at the Arizona LaserChron Center, using a Photon Machines Analyte G2 laser coupled to a Nu Plasma multicollector ICP-MS. Instrumental bias, drift, and interelement fractionation corrections were performed by the standardsample bracketing approach, using an in-house Sri Lanka zircon crystal with a known age of 563.5 ± 3.2 Ma (Gehrels et al., 2008) as primary reference material. After LA-ICP-MS analyses, one of the samples from the Whiskey Springs 4 interval (RCS3) was selected for further highprecision dating by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), in order to validate the accuracy of the new absolute-age model proposed herein. Select zircons were plucked out of the epoxy mount and subjected to a modified version of the CA-TIMS method of Mattinson (2005) at the Massachusetts Institute of Technology Isotope Geochemistry Laboratories (MIT-IG). Analyses conducted at the Arizona LaserChron laboratory were performed using a laserbeam diameter of 20 µm, firing at a repetition rate of 7 Hz for ~14 s and using a constant energy fluence of ~7.0 J cm–2 on the sample surface. All Pb masses (i.e., 208, 207, 206, and 204) were simultaneously monitored using discrete-dynode ion-multipliers, while 232Th and 238U were measured using Faraday detectors equipped with 3 × 1011 Ω resistors. Data processing and uncertainty calculations followed the approach described in Ibañez-Mejia et al. (2014). To assess age accuracy, zircon crystals with a well-established CA-TIMS age of 48.13 ± 0.02 Ma (M.P. Eddy and M. Ibañez-Mejia, Personal commun.) were frequently analyzed and treated as unknowns during the analytical session; a calculated age of 48.59 ± 0.50 Ma (2σ, n = 58, mean square of weighted deviates [MSWD] = 1.3) using the LA-ICP-MS data indicates that the results are accurate within the quoted uncertainties of ~1%–1.5%. Eruption ages discussed in the text are weighted mean

Selected crystals previously dated by LAICP-MS and plucked out of the epoxy resin were subjected to a modified version of the chemical abrasion method of Mattinson (2005) at the MIT-IG. Zircons were annealed in quartz crucibles in a muffle furnace at 900 °C for 60 h, chemically abraded using a single abrasion step in concentrated HF at 205 °C for 12 h, and processed for ID-TIMS using the EARTHTIME 205Pb-233U-235U mixed tracer (Condon et al., 2015). Pb and U were loaded on a single outgassed Re filament in 2 µL of a silica-gel/ phosphoric acid mixture (Gerstenberger and Haase, 1997), and U and Pb isotopic measurements were made on a IsotopX PhoeniX-62 TIMS at MIT. Pb isotopes were measured by peak-hopping using a Daly photomultiplier and corrected for instrumental mass fractionation (0.18 ± 0.02%/amu) based on repeat analyses of the NBS 981 Pb isotopic standard. Uranium was measured as UO2+ ions using Faraday cups with 1011 Ω resistors in multicollector mode and corrected for instrumental mass fractionation using the known ratio of 233U/235U in the ET535 tracer (Condon et al., 2015), assuming a sample 238U/235U ratio of 137.818 (Hiess et al., 2012) and a 18O/16O value of 0.00205 (Condon et al., 2015). U-Pb dates and uncertainties were calculated from the TIMS data using U-Pb Redux (Bowring et al., 2011), following the algorithms of McLean et al. (2011) and the U decay constants of Jaffey et al. (1971). The 206Pb/238U ratios and dates were corrected for initial 230Th disequilibrium using a Th/U[magma] of 2.8 ± 1.0. All common Pb in the analyses was attributed to laboratory blank and subtracted based on the measured laboratory Pb isotopic composition at MIT and associated uncertainty, determined from total procedural blank measurements (see Data Repository1). Quoted errors for individual analyses are presented in the form ± x(y)[z] following the scheme of Schoene et al. (2006), where x is solely analytical uncertainty, y is

1 GSA Data Repository item 2017184, which includes biostratigraphic source data and results from zircon LA-ICP-MS and CA-TIMS dating, is available at http://www.geosociety.org/datarepository/2017 or by request to [email protected].


Harris et al. the combined analytical and tracer uncertainty (i.e., ±50% increase in sedimentation rate. DISCUSSION Revised Age Estimate for the Railroad Canyon Section The new U-Pb dates from zircons preserved in ashes indicate that the Railroad Canyon section ranges in age from ca. 22.9 to 15.2 Ma, suggesting that the Railroad Canyon section is ~5 m.y. older than previously suggested (ca. 17.3–13 Ma from Zheng, 1996; Barnosky et al., 2007; 16.4–10.7 Ma from Retallack, 2009). These new age constraints indicate that the Railroad Canyon section captures pre–mid-Miocene climatic optimum warming as well as buildup to peak warming during the global mid-Miocene climatic optimum, rather than documenting peak warming and subsequent cooling (e.g., Retallack, 2009). However, an alternative possibility is that the substantially older dates for the Railroad Canyon section could be a result of inheritance, producing inaccurate eruption ages. Several observations help to reject this interpretation: (1) The reported weighted means for the three dated intervals result in ages that progressively decrease


U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho

A

D

data-point error ellipses are 2σ

EBH-RCS14-Ash06 RCS4


110

EBH-RCS14-Ash04 RCS2

LA-ICP-MS

LA-ICP-MS

0.016

17 0.0026

90 0.012

0.0025

16

70

0.0024

50

0.008

Reworked Cretaceous zircons

15

15.76 ± 0.22 Ma 95% conf., n= 21, MSWD= 1.0

206

14

0.01

B

207

0.02

Pb/238U

Weighted mean 206Pb/ 238U age

206

Pb/238U

0.0023

Pb/235U

0.04

0.06

0.08

Pb/235U


CA-TIMS

0.00334

0.0036

207 0.02

RCS3

24

LA-ICP-MS

Youngest zircons ~ 22 Ma

10

E


RCS3 EBH-RCS14-Ash05

30

(Th-corrected)

21.50

23 21. .40 40 21.40 0.00332

22

0.0034

2 1.30 30 21.30

21

0.00330

0.0032

2 1.20 21.20 0.00328


19

21.24 ± 0.27 Ma 95% conf., n= 26, MSWD= 1.1

206

Pb/238U

20

C

0.018

0.020

0.022

0.024

Pb/ U

207

235


RCS1 EBH-RCS14-Ash01 LA-ICP-MS

0.0039

0.00326

21.10

21. 21 00 21.00 0.020

0.022

Weighted mean

0.024

Pb/

206

238

Pb/235U

207

U age

21.217 ± 0.027/0.031/0.039 Ma n= 7, MSWD= 0.67

25 24

0.0037

23 0.0035

22 0.0033

Pb/238U

21

206

20 19 0.01

0.02


22.65 ± 0.37 Ma 95% conf., n= 23, MSWD= 2.0 0.03

0.04

207

Pb/235U

Figure 5. U-Pb concordia diagrams for zircon analyses from four dated tuffs within the Railroad Canyon section. (A–C) Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) results (open ellipses) for three ash horizons (A) RCS4, (B) RCS3, and (C) RCS1. (D) LA-ICP-MS results from reworked ash RCS2. (E) Chemical abrasion– isotope dilution–thermal ionization mass spectrometry results (gray filled ellipses) for RCS3. All sample error ellipses are plotted at 2σ. MSWD—mean square of weighted deviates.


Harris et al. up section, thus not violating stratigraphic first principles; (2) the three horizons from which weighted mean ages are reported contain a proportion between 85% and 95% of zircons with apparent 206Pb/238U dates that are statistically undistinguishable, thus validating the hypothesis that they represent a single population within the assigned uncertainties; and (3) although one ash layer collected from the Renova Formation (sample RCS2 at 68.5 m in WH4) yielded a zircon age distribution clearly dominated by inheritance (Fig. 5D; Table DR2 [see footnote 1]), indicating an important component of Late Cretaceous detritus being delivered to the Miocene basins of the Railroad Canyon section, the other three dated samples have virtually no zircons with pre-Cenozoic ages, thus suggesting that an inherited zircon component is not prevalent in these horizons. In addition, high-precision CATIMS analyses obtained for seven zircons from sample RCS3 (Table DR3 [see footnote 1]) also define a statistically single population and are in excellent agreement with the age obtained by LA-ICP-MS. Altogether, these lines of evidence support the notion that the reported dates accurately approximate the age of eruption of these ash horizons and are not detrital ages influenced by reworking. Revised Age of the Early Miocene Unconformity in the Railroad Canyon Section At ~70 m in the composite section (in WH4), the erosional early Miocene unconformity separates the white beds of the Renova Formation from predominately buff and pink beds of the Six Mile Creek Formation. The early Miocene unconformity has been widely discussed in paleontological and geological studies in the northern Rocky Mountains (previously referred to as the mid-Tertiary unconformity; e.g., Fields

et al., 1985; Hanneman and Wideman, 1991; Barnosky, 2001; Rasmussen, 2003; Barnosky et al., 2007) and is thought to have been timetransgressive between ca. 20 Ma to ca. 17 Ma in a region spanning from southwestern Montana and to northwestern Wyoming (Table 1; Fields et al., 1985; Barnosky and Labar, 1989; Burbank and Barnosky, 1990; Hanneman and Wideman, 2006). Based on these previous studies, as well as biostratigraphy and magnetostratigraphy, Barnosky et al. (2007) suggested that the early Miocene unconformity in the Railroad Canyon section occurred between ca. 17.3 Ma and 16.73 Ma. However, given the new age model for the Railroad Canyon section, the age of the early Miocene unconformity in central-eastern Idaho is now bracketed between ca. 21.5 and 21.4 Ma. This new age estimate shows that the early Miocene unconformity is indeed time transgressive in the region and lends support to the idea of diachronous regional uplift in western North America leading into the early and middle Miocene (Barnosky et al., 2007; Chamberlain et al., 2012). Tectonic and Paleoelevation Implications Several researchers have used d18O values from pedogenic carbonates to reconstruct the uplift history of the northern Rocky Mountains (e.g., Kent-Corson et al., 2006; Chamberlain et al., 2012). This work has shown that the pattern of elevation changes in this region included significant spatial and temporal heterogeneity (e.g., Horton et al., 2004; Chamberlain et al., 2012; Mix and Chamberlain, 2014; Mulch et al., 2015), and large elevation gradients (Sjostrom et al., 2006). However, ambiguities remain for given subregions because of uncertainties associated with the ages of the stratigraphic sections under study. For example, in KentCorson et al. (2006), the composite isotopic re-

cord for Idaho has a stratigraphic gap between ca. 36 Ma and 19 Ma, making it difficult to assess local tectonic changes. Our new dates for the Railroad Canyon section narrow that gap by >3 m.y. and, in addition, make it possible to compare the absolute d18O values from the Railroad Canyon section with the Sage Creek Basin of southwestern Montana (Kent-Corson et al., 2006) to look for evidence of regional heterogeneity. Paleosol d18O values from both of these areas range between +14‰ and +16‰ (relative to Vienna standard mean ocean water [VSMOW]) without any temporal trend, pointing to (1) regional stability during the deposition of early–middle Miocene strata and (2) similar absolute elevations in the region, at the scale of tens of kilometers. This reanalysis is consistent with the SWEEP (southwest encroachment of an Eocene plateau) hypothesis of Chamberlain et al. (2012), which holds that the western North American Cordillera underwent major uplift starting in the Paleogene, developing into a high, rugged mountain range by the late Eocene (ca. 40 Ma), and that collapse of these highlands, resulting in development of the modern Basin and Range, would have occurred only after 15 Ma. Parsons et al. (1994) furthermore proposed that the impingement of the Yellowstone hotspot may have influenced topographic relief up to 2000 km away from the plume head, beginning ca. 17–16 Ma. However, the regional stability and topographic uniformity now inferred from the d18O data from the Sage Creek Basin and Railroad Canyon section suggest that there were no significant far-field topographic effects from the Yellowstone hotspot in the region until after ca. 15 Ma. Potential Biostratigraphic Implications The age model proposed herein differs considerably from previous age-model analyses

TABLE 1. EXAMPLES OF AGE ESTIMATES FOR THE EARLY MIOCENE UNCONFORMITY (EMU; PREVIOUSLY REFERRED TO AS THE MID-TERTIARY UNCONFORMITY) IN SOME INTERMONTANE BASINS IN NORTH AMERICA Basin location

South Killdeer Mountains, southwestern North Dakota

Arikaree

Approximate age of unconformity (Ma) ca. 20

Monroe Canyon, Nebraska

Harrison

ca. 20

Fission-track age of 19.2 + 0.5 Ma overlying the unconformity; radiometric age of 21.9 Ma of Agate Ash below unconformity

Hunt (1990); MacFadden and Hunt (1998)

Yellowstone Valley, southwestern Montana

Hepburn’s Mesa

ca. 16.8 (cessation of EMU)

Magnetostratigraphy;biostratigraphy; lithostratigraphy

Barnosky (1984, 1986); Barnosky and Labar (1989); Burbank and Barnosky (1990)

Jackson Hole, Wyoming

Colter

ca. 17–18 (onset of EMU)

Biostratigraphy

Barnosky (1984, 1986); Barnosky and Labar (1989); Burbank and Barnosky (1990)

Beaverhead Mountains, Idaho

Sixmile Creek and Renova

ca. 21.5–21.4

U-Pb radiometric dating, magnetostratigraphy

This study

1248

Formation

Age constraint(s)

References

Fission-track age of 25.1 + 2.2 Ma at base of burrowed marker unit; co-occurrence of Merychyus and Merycochoerus 27 m above fission-track age

Delimata (1975); Murphy et al. (1993); Hoganson et al. (1998)


U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho based on faunal occurrence data from the Railroad Canyon section. The majority of faunal data for the Railroad Canyon section comes from two sites, Snowfence east and Snowfence west, that occur between 125 and 177 m in the composite section. Based on our new age model, these faunas date between ca. 19.8 and 18.8 Ma (Ar4). In contrast, previous workers proposed that the Railroad Canyon section fauna was late Hemingfordian–late Barstovian (He2–Ba2; ca. 17.5–13 Ma) in age, based on how it and nearby faunas (e.g., Mollie Gulch beds)—which were presumed to be coeval (Barnosky et al., 2007)—compared with regional faunas from the northern Rocky Mountains, the northern Great Plains, and the Northwest (e.g., Barnosky, 2001). Due to this discrepancy in ages, we suggest that the early–middle Miocene biostratigraphy of northern Rocky Mountains strata needs to be reassessed. Although a full biostratigraphic reevaluation is beyond the scope of this paper, we are able to highlight a few important changes in taxonomic occurrence data that have stratigraphic and potentially evolutionary implications. Thus, the new age model revises the NALMA assignments for Railroad Canyon section strata (Fig. 2) and thereby the biochronologic ranges of certain taxa deemed biostratigraphically important (“index fossils”) by Barnosky et al. (2007; specimens are curated at either the University of California Museum of Paleontology or University of Montana Paleontology Center; additional details can be found in the Carrasco et al., 2005). Specifically, it pushes back the biostratigraphic ranges of these index taxa. A regional comparison of first appearances of these taxa now suggests that some of these taxa may have appeared earlier in the northern Rocky Mountains compared to the U.S. West Coast or Great Plains (Tedford et al., 2004). Based on our new age model, as well as published taxonomic range limits (Tedford et al., 2004; Albright et al., 2008), and assuming that the Railroad Canyon section taxa have been accurately classified, we propose that the following taxa had an earlier first appearance in the late Arikareean (Ar4; ca. 22.8–18.5 Ma) than previously reported regionally (Fig. 2): Alphagaulus vetus (Mylagaulidae), Mesogaulus (Mylagaulidae), Cupidinimus (Heteromyidae), Harrymys irvini (Heteromyidae), Merychyus elegans (Merycoidodontidae), Oreolagus (Ochotonidae), Pliocyon (Amphicyonidae), and Tylocephalonyx cf. T. skinneri (Chalicotheriidae). Additionally, the following taxa appear to have had an earlier first appearance either in the late Arikareean (Ar4) or early Hemingfordian (He1; 18.5 to ca. 17.5 Ma): Aepycamelus (Camelidae), Paramiolabis (Camelidae), Brachycrus laticeps (Merycoidodontidae), Hypolagus (Leporidae),

Merycodus (Antilocapridae), and Rakomeryx (Palaeomerycidae) (Fig. 2). Because many fossils collected in the Railroad Canyon section were recovered as float after weathering out, it is impossible with our data set to estimate the exact range of some Railroad Canyon section taxa. Therefore, the following taxa may have had an earlier first appearance in the Railroad Canyon section than they did elsewhere in the region, but because of the method of fossil recovery, we cannot be certain at this point: Bouromeryx (Palaeomerycidae), Hypohippus cf. H. osborni (Equidae), Merychippus cf. M. insignis (Equidae), and Ticholeptus zygomaticus (Merycoidodontidae). Overall, we suggest these biostratigraphic modifications as a first step toward reconciling our understanding of regional biostratigraphy with new radiometric dates from the Railroad Canyon section. We also note that these earlier appearance dates in the northern Rocky Mountains for many taxa compared to those in western America and the Great Plains imply marked diachrony (up to ~2 m.y.; Fig. 2) in Cenozoic faunas across North America during the early–middle Miocene, well above the 0.75 m.y. discrepancy that can be expected solely because of sampling error (undersampling of faunas) according to Alroy’s analysis (Alroy, 1998). This appearance pattern across western North America may indicate that these taxa evolved in the Cordilleran region and later spread to the west and east, which is consistent with the broad notion of this topographically complex region as an engine of biodiversity (Kohn and Fremd, 2008; Finarelli and Badgley, 2010; Badgley et al., 2017), and it also adds to pre–mid-Miocene climatic optimum taxon richness in the northern Rocky Mountains (e.g., Kohn and Fremd, 2008). However, to fully evaluate how our new ages for Railroad Canyon section faunas influence faunal patterns, a more thorough taxonomic review coupled with precise dating of early–middle Miocene faunas is likely necessary. Our study therefore stresses the importance of precise dating of more than just a handful of Miocene faunas for a full understanding of biogeographic leads and lags in faunal occurrences and Cenozoic diversity patterns across western North America. CONCLUSION We provide a revised and radiometrically calibrated age model for the Railroad Canyon section, an important sequence of Mioceneaged rocks in northwestern North America, constraining its age to ca. 22.9–15.2 Ma, i.e., ~5 m.y. older than previous age models. The radiometric dates reported here provide ample evidence that the Railroad Canyon section

was deposited during buildup to peak warming during the global mid-Miocene climatic optimum, highlighting the importance of this site in discussions of local and global effects of the mid-Miocene climatic optimum. Furthermore, this new age model suggests the early Miocene unconformity (previously also known as the Mid-Tertiary unconformity) occurred between ca. 21.5 and 21.4 Ma in the Railroad Canyon section and supports the idea that this erosional episode was time transgressive across the intermontane basins of the northern Rocky Mountains. Our new age constraint on the early Miocene unconformity is particularly important because it is the first time that both the top and bottom of this regional unconformity have been bounded by radiometric dates from a single basin. A comparison between this new chronostratigraphic framework and previously reported biostratigraphy from the Railroad Canyon section potentially pushes back the dates of first appearance for many biostratigraphically important taxa found throughout the region. Many of these taxa were previously known only from early–late Hemingfordian (He1–He2) strata, but our results suggest they may have had earlier first appearances in the northern Rocky Mountains. This work also highlights the importance of radiometric dating for calibration of Miocene biostratigraphic range limits and NALMA assignments in western North America, which are notorious for being poorly temporally constrained. ACKNOWLEDGMENTS

We would like to thank C. Trinh-Le, A. Padgett, C. Bitting, E. Fredrickson, K. Smith, A. Jijina, T.-Y. Le, J. Benca, M. Dennis, and E. Hyland for field assistance during this project. Additionally, we thank R.E. Dunn for laboratory support and assistance with paleomagnetic correlation, A.D. Barnosky for assistance acquiring field notes and general discussion about the Railroad Canyon section, and the Arizona LaserChron and MIT-IG laboratories for making their analytical facilities available to Ibañez-Mejia for zircon U-Pb analysis. Finally, we would like to thank Brad Singer, A.E. Troy Rasbury, and two anonymous reviewers for their comprehensive and thorough reviews of the manuscript. Funding for this project was provided by National Science Foundation grants EAR-1024681 to C.A.E. Strömberg, and EAR-1024535 to N.D. Sheldon and S.Y. Smith, an Evolving Earth Foundation grant to Harris, and a University of Washington Biology Iuvo Award to Harris, as well as funding from the Burke Museum of Natural History and Culture. REFERENCES CITED Albright, L.B., Woodburne, M.O., Fremd, T.J., Swisher, C.C., III, MacFadden, B.J., and Scott, G.R., 2008, Revised chronostratigraphy and biostratigraphy of the John Day Formation (Turtle Cove and Kimberly Members), Oregon, with implications for updated calibration of the Arikareean North American Land Mammal Age: The Journal of Geology, v. 116, p. 211–237, doi:10.1086/587650.


Harris et al. Alroy, J., 1998, Diachrony of mammalian appearance events: Implications for biochronology: Geology, v. 26, p. 23– 26, doi:10.1130/0091-7613(1998)0262.3.CO;2. Badgley, C., Smiley, T.M., Terry, R., Davis, E.B., DeSantis, L.R.G., Fox, D.L., Hopkins, S.S.B., Jezkova, T., Matocq, M.D., Matzke, N., McGuire, J.L., Mulch, A., Riddle, B.R., Roth, L., et al., 2017, Biodiversity and topographic complexity: Modern and geohistorical perspectives: Trends in Ecology & Evolution, v. 32, no. 3, p. 211–226, doi:10.1016/j.tree.2016.12.010. Barnosky, A.D., 1984, The Coulter Formation: Evidence for Miocene volcanism in Jackson Hole, Teton County, Wyoming: Wyoming Geological Association Earth Science Bulletin, v. 16, p. 50–101. Barnosky, A.D., 1986, Arikareean, Hemingfordian, and Barstovian mammals from the Miocene Colter Formation, Jackson Hole, Teton, County, Wyoming: Carnegie Museum of Natural History Bulletin, p. 69. Barnosky, A.D., 2001, Distinguishing the effects of the Red Queen and Court Jester on Miocene mammal evolution in the northern Rocky Mountains: Journal of Vertebrate Paleontology, v. 21, p. 172–185, doi:10.1671/0272-4634(2001)021[0172:DTEOTR]2 .0.CO;2. Barnosky, A.D., and Carrasco, M.A., 2002, Effects of OligoMiocene global climate changes on mammalian species richness in the northwestern quarter of the USA: Evolutionary Ecology Research, v. 4, p. 811–841. Barnosky, A.D., and Labar, W.J., 1989, Mid-Miocene (Barstovian) environmental and tectonic setting near Yellowstone Park, Wyoming and Montana: Geological Society of America Bulletin, v. 101, p. 1448–1456, doi:10.1130/0016-7606(1989)1012.3.CO;2. Barnosky, A.D., Bibi, F., Hopkins, S.S.B., and Nichols, R., 2007, Biostratigraphy and magnetostratigraphy of the mid-Miocene Railroad Canyon sequence, Montana and Idaho, and age of the Mid-Tertiary unconformity west of the Continental Divide: Journal of Vertebrate Paleontology, v. 27, p. 204–224, doi:10.1671/0272-4634(2007)27[204:BAMOTM]2.0 .CO;2. Bobe, R., 2006, The evolution of arid ecosystems in eastern Africa: Journal of Arid Environments, v. 66, p. 564– 584, doi:10.1016/j.jaridenv.2006.01.010. Bowring, J.F., McLean, N.M., and Bowring, S.A., 2011, Engineering cyber infrastructure for U-Pb geochronology: Tripoli and U-Pb-Redux: Geochemistry Geophysics Geosystems, v. 12, Q0AA19, doi:10.1029/2010GC003479. Burbank, D.W., and Barnosky, A.D., 1990, The magnetochronology of Barstovian mammals in southwestern Montana and implications for the initiation of Neogene crustal extension in the northern Rocky Mountains: Geological Society of America Bulletin, v. 102, p. 1093–1104, doi:10.1130/0016-7606(1990)102