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 1Department of Biology, University of Washington, Box 351800, 24 Kincaid Hall, Seattle, Washington 98195, USA 2Burke Museum of Natural History and Culture, 4331 Memorial Way Northeast, Seattle, Washington 98195, USA 3Department of Earth and Environmental Sciences, University of Michigan, 2534 CC Little Building, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA 4Museum of Paleontology, University of Michigan, 1109 Geddes Avenue, Ann Arbor, Michigan 48109, USA 5Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 54-918, Cambridge, Massachusetts 02139, USA 6Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, P.O. Box 270221, Rochester, New York 14627, USA
ABSTRACT estimations for the initiation and cessation of parts of the South and North American Cordil- the early Miocene unconformity, a regional leras experienced either accelerated or renewed The early–middle Miocene was an im- unconformity exposed in many intermontane uplift during the early–middle Miocene (e.g., portant transitional period in the evolution basins across the northern Rocky Mountains, Gregory-Wodzicki, 2000; Horton et al., 2004), of Earth’s biota and climate that has been as ca. 21.5 and 21.4 Ma, respectively, in the with profound impacts on faunal biogeography poorly understood in North America due to a Railroad Canyon section. This new chro- and diversification (Kohn and Fremd, 2008; Fi- paucity of continuous, fossil-bearing rock re- nostratigraphic analysis provides an impetus narelli and Badgley, 2010). cords in this interval for which the ages have for reassessment of the biochronology of the In North America, the timing of many of been robustly constrained. In the northern region, in turn suggesting earlier first appear- these events, particularly those associated with Rocky Mountains, United States, one site in ances of many biostratigraphically important the mid-Miocene climatic optimum, remains particular, known as the Railroad Canyon taxa found in the northern Rocky Mountains, ambiguous due to the few, continuous, fossil- section, has provided biostratigraphic, mag- Great Plains, and American Northwest. bearing rock records for which the age can be netostratigraphic, and lithostratigraphic evi- robustly constrained to the early–middle Mio- dence suggesting a late early–middle Miocene INTRODUCTION cene, and a general paucity of absolute age de- age; however, radiometrically calibrated age terminations using modern methods. For exam- models have been notoriously lacking. To bet- The early–middle Miocene was a critical ple, Fritz et al. (2007) compiled existing K-Ar ter constrain the age of the Railroad Canyon transitional period in Earth’s geologic history. ages for Montana and Idaho and reported just section and the abundant fossils preserved Long-term global climatic cooling was tempo- five between 27 and 10 Ma, with only a single therein, we employed moderate- and high- rarily reversed, culminating in the mid-Miocene age in strata of mid-Miocene climatic optimum precision U-Pb dating of single zircon crystals climatic optimum (ca. 17–14.75 Ma; Zachos et age. As a result, much of the regional age control from four ash horizons throughout the sec- al., 2001), and modern ecosystems were estab- (e.g., Chamberlain et al., 2012) in the northern tion. The resulting dates span from 22.65 ± lished around the world (e.g., Graham, 1999, Rocky Mountains and western Great Plains of 0.37 Ma to 15.76 ± 0.22 Ma. Using these 2011; Pound et al., 2012). Grass-dominated bi- North America (e.g., Sjostrom et al., 2006) re- dates, we developed a radiometrically cali- omes, which cover up to 40% of Earth’s land lies on relative and semiquantitative ages based brated age model for the Railroad Canyon surface today (Gibson, 2009), spread across on mammalian biostratigraphy (e.g., Tedford et section that constrains the age of the section North America, Eurasia, Australia, and Africa al., 2004). However, the relatively coarse tem- to ca. 22.9–15.2 Ma, ~5 m.y. older than pre- (Jacobs et al., 1999; Strömberg, 2011). These poral resolution based on mammalian faunas vious estimates. These results firmly estab- changes were accompanied by replacement of has led to questionable conclusions about the lish that the Railroad Canyon section was the archaic browser-dominated fauna(s) by graz- timing of biologic, tectonic, and climatic events deposited during buildup to peak warming ing herbivores through evolution and migration (e.g., Tedford et al., 2004; Kent-Corson et al., of the mid-Miocene climatic optimum. Ad- (e.g., North America—MacFadden, 2000; Bar- 2006, 2013; Barnosky et al., 2007). In particu- ditionally, these dates provide definitive age nosky and Carrasco, 2002; Janis et al., 2004; lar, we point out that both the timing and rates Eurasia—Fortelius et al., 2006; van Dam, 2006; of vegetation change, faunal turnover and evo- Africa—Bobe, 2006; South America—Flynn lution, and tectonic deformation and/or uplift †[email protected] et al., 2003; Pascual, 2006). In addition, many in the area can still be significantly improved
GSA Bulletin; Month/Month 2017; v. 129; no. X/X; p. 000–000; doi: 10.1130/B31655.1; 5 figures; 1 table; Data Repository item 2017184.
GeologicalFor Society permission of to America copy, contact Bulletin [email protected], v. 1XX, no. XX/XX 1 © 2017 Geological Society of America Harris et al. through the development of a more robust re- nosky et al. (2007). The primary purpose of this tially indicative of an arid, closed basin with gional absolute chronology framework. paper is to provide the first absolute age model intermittent saline lakes (Fields et al., 1985; The Railroad Canyon section (Fig. 1) of central- for the Railroad Canyon section, using U-Pb Barnosky et al., 2007). The Renova Forma- eastern Idaho contains the most complete geo- dating of zircons extracted from three volcanic tion is overlain by the Six Mile Creek Forma- logic record for the early–middle Miocene in the ash layers found above and below an erosional tion (Fig. 2), which is locally distinguished by northern Rocky Mountains. The Railroad Can- unconformity that separates the Renova and Six pinkish to tan siltstone and sandstone beds with yon section is located in Bannock Pass, ~19 km Mile Creek Formations. Using the improved age occasional conglomeratic lenses indicative of a northeast of Leadore, Idaho, and exposes nearly model, we then discuss the potential implica- sediment-choked fluvial system (Fields et al., 360 m of sedimentary rock section from the up- tions of this important locality for Miocene bio- 1985; Rasmussen, 2003; Barnosky et al., 2007). per Renova and lower Six Mile Creek Forma- stratigraphy of the northwestern United States These two formations are separated by an ero- tions (Fig. 1; Barnosky et al., 2007). Prior to this and its chronostratigraphic significance for sional contact that has been described in many study, the age of the Railroad Canyon section studying tectonic uplift history in the northern other intermontane basins (including the Ruby was poorly constrained. Interpretation of bio- Rocky Mountains and the mid-Miocene climatic River, Beaverhead, Jefferson River, and Horse stratigraphic and magnetostratigraphic data by optimum in North America more broadly. Prairie basins of southwestern Montana) as the Barnosky et al. (2007) suggested the Railroad mid-Tertiary unconformity (Fields et al., 1985; Canyon section was deposited ca. 17.3–13 Ma Railroad Canyon Lithostratigraphic, Hanneman and Wideman, 1991, 2006; Rasmus- (see also Zheng, 1996). This contrasts with a re- Biostratigraphic, and sen, 2003; Barnosky et al., 2007). Herein, we vised age model by Retallack (2009), who pro- Magnetostratigraphic Context discard the use of the name mid-Tertiary un- posed that Railroad Canyon section sediments conformity and instead resume use of the name were deposited between 16.4 and 10.7 Ma, based The lowermost ~70 m of the composite Rail- “early Miocene unconformity,” proposed by on an amended geologic time scale (Ogg and road Canyon section belong to the Renova For- Fields et al. (1985), because this is a more ac- Smith [2004] vs. Cande and Kent [1995]) and by mation (Fig. 2), which locally consists of gray curate and precise name for this unconformity constraining plausible alternative age estimates to white mudstone and siltstone deposits with given what we know about its timing and strati- using faunal biostratigraphic data from Bar- occasional gypsum and halite deposits poten- graphic occurrence. In addition, “Tertiary” is
Missoula 113°W 11113°3° 15 ′ W Helena A B 90
O
ANA T 15
IDAH
MON 93 Butte 46°N 90 MONTANA MBJ IDAHO ST1 BHL ST2 SFw 15 Salmon SFe ST3 WRC TH TFFT
RCS HDS 2/3 DS3 Leadore DS4 15 44°N 93 28 75 N N
MT ″ Idaho Falls N ″ WH4 15 15 N ′ 20 mi ID WY 29 1 km
40 Km 44° 46
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 Buf- falo 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.
2 Geological Society of America Bulletin, v. 1XX, no. XX/XX U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho an archaic term that has been replaced by both et al., 1985; Tedford et al., 1987; Barnosky, tilocapridae), Bouromeryx (Palaeomerycidae), the International Commission on Stratigraphy 2001). Barnosky et al. (2007) published a com- Rakomeryx (Palaeomerycidae), Aepycamelus (www.stratigraphy.org) and North American prehensive biostratigraphic and magnetostrati- (Camelidae), Brachycrus laticeps (Merycoid- Stratigraphic Code (www.nacstrat.org) with graphic analysis of the Railroad Canyon section odontidae), and Tylocephalonyx skinneri (Chal- Paleogene and Neogene at the period level, so and suggested that the Railroad Canyon section icotheriidae) (Fig. 2), Barnosky et al. (2007) this revised designation both represents a return fauna assemblage was characteristic of the late proposed that meters 150–250 in the composite to the original nomenclature and an update to Hemingfordian to late Barstovian NALMAs Railroad Canyon section are no older than the modern stratigraphic terminology. (He2-Ba2). Based on the presence of key taxa late Hemingfordian (He2). Additionally, they The Railroad Canyon section fauna has been such as Pliocyon (Amphicyonidae), Hypolagus suggested that this interval is no younger than known for quite some time and was traditionally (Leporidae), Harrymys irvini (Heteromyidae), late Barstovian (Ba2) based on the cooccur- believed to belong to the early Barstovian North Alphagaulus vetus (Mylagaulidae), Paracoso- rence of Oreolagus (Ochotonidae), Plesiosmin- America Land Mammal Age (NALMA; Fields ryx wilsoni (Antilocapridae), Merycodus (An- thus (Zapodidae), Peridiomys ( Heteromyidae),
NALMA Ma 15.5
RCS4
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y HDS2 Figure 2. Railroad Canyon compos- 16.0 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Ba1 * ite stratigraphic section and strati- graphic ranges of biostratigraphically significant taxa collected throughout the section (modified from Barnosky et al., 2007). Age model is based on 16.5 U-Pb dating of zircons from three HDS3 dated ash horizons (RCS1 = 22.65 ± 0.37 Ma; RCS2 = no good age estimate;
He2 RCS3 = 21.24 ± 0.27 Ma; RCS4 = 17.0 15.76 ± 0.22 Ma). Solid black bars in- dicate where taxa were found in the Railroad Canyon section. Black wavy 17.5 lines at the top or bottom of a range DS 3/4 bar indicate that the taxon-range mation r boundaries are within the vertical line, Fo ST3 18.0 but the exact placement of the taxon He1 range is not possible due to specimens ST2 being collected as float. Dashed gray 18.5 bars indicate previously published
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y ST1
Six Mile Creek first appearances and taxonomic
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y ranges of specific taxa (Albright et
w 19.0 al., 2008; Tedford et al., 2004; source
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y data in Table DR1 [see text footnote
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
SF e & 1]). Single asterisks denote taxa that 19.5 have an earlier first appearance in i i n i
x the Railroad Canyon section (RCS) us yx rn yo ry cr etus
ving based on our new age model. Double hy me
20.0 ycodus
osbo asterisks denote taxa that might have ko skinner ys ir * Plioc ycamelus H. “Cynorca” T. ramiolabis WRC
. an earlier first appearance in the Rail- . ym * Hypolagus * Mer * Brac * Ra Pa * Mesogaulus road Canyon section if stratigraphic ** Bouromer *
20.5 x cf * Aep Plesiosminthus Ar4
ny placement of the fossils could be better * Harr
* Alphagaulus v constrained. NALMA—North Ameri- U 21.0
RCS3 s
EM can Land Mammal Age; EMU—early Ticholeptus zygomaticus ** locephalo Miocene unconformity; Ar4—late 21.5* ** Hypohippus cf x wilsoni Ty
* late Arikareean; He1—early Heming-
*RCS2 ry yus elegans * Oreolagus penultimus
22.0 ch fordian; He2—late Hemingfordian; ation
WH4 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y * Cupidinimu ry Ba1—early Barstovian. Site name ychippus insignis rm racoso
Fo abbreviations are as in Figure 1. See * Me Pa
22.5 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Mer . Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y va Figure 3 for lithological key to strati- Archaeohippus ultimus
RCS1 . ** cf and A. * cf graphic section. Reno m
10 22.7 clay silt sand
Geological Society of America Bulletin, v. 1XX, no. XX/XX 3 Harris et al.
Paracosoryx wilsoni (Antilocapridae), and Hypohippus cf. H. osborni (Equidae). This biostratigraphic assessment was then used to 206 Composite correlate the Railroad Canyon section polarity Ma sequence to the geomagnetic polarity time scale U/ Pb RCS 238 GPTS (GPTS) of Cande and Kent (1995). 14 ages
Given that the Railroad Canyon section po-
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y larity sequence was tied to the GPTS using lo- Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 5AD
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y cal biostratigraphic age constraints only, the Ba1 temporal window was limited. For this reason, Barnosky et al. (2007) presented two different 15 interpretations of the Railroad Canyon section
magnetostratigraphy, “basic unrevised” and 5B “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, al- 16 He2 though the authors admitted that this latter esti- mate could be older, depending on the amount RCS4
of unrepresented time due to the mid-Tertiary 5C unconformity (herein early Miocene unconfor- mity). Zheng (1996) provided additional inter- 17 pretations of the Railroad Canyon section polar- ity sequence including a “least squares 1 match” D that estimated the section to be ~22–15 m.y. old and a “least squares 3 match” that estimated an 18 He1 age of ~15–11.5 m.y. old. Both of these alter- native hypotheses were rejected because the E5 dates (1) did not agree with the existing con- straints from faunal data, and (2) would have
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
conflicted with the accepted age estimate of the Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 19 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y mid-Tertiary unconformity (early Miocene un- conformity) at that time (ca. 17 Ma; Barnosky 65
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y YY YY YY
et al., 2007). Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Other Attempts at Constraining the Age of 20 the Railroad Canyon Section
In addition to the work outlined herein, other efforts have been made to produce independent 6A 21 RCS3 Ar4 Figure 3. Correlation of the Railroad Can- 6AA yon section (RCS) magnetostratigraphy
(Zheng, 1996) with the global geomagnetic 22 U 6B
polarity time scale (GPTS; Gradstein et al., RCS2 EM 2012). Isotopic age determinations are from
U-Pb dating of three ash horizons (RCS1, Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y RCS3, RCS4) that can be directly tied from
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y the Railroad Canyon section into the GPTS 23 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y (solid black lines). Gray dashed lines show Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y possible chron correlation of rock units. 6C RCS1 North American Land Mammal Age (NA- 10 LMA) durations and age boundaries are m from Woodburne (2004) and Albright et al. 24 (2008). Star denotes location of reworked Sandy Claystone Sandstone ash, RCS2. Ar4—late late Arikareean; Siltstone Ash He1—early Hemingfordian; He2—late Silty Conglomerate Hemingfordian; Ba1—early Barstovian; Siltstone Sandstone Covered EMU—early Miocene unconformity.
4 Geological Society of America Bulletin, v. 1XX, no. XX/XX U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho age constraints for the Railroad Canyon section. from these four sites because they have not been 206Pb/238U values, and the quoted uncertainty The first attempt was to establish a tephrachro- accurately correlated to the composite Railroad represents the final propagation of within-run nology by comparing the elemental composi- Canyon section, and thus including them would analytical uncertainties, reproducibility of the tion of Railroad Canyon section tephras to the introduce unnecessary additional uncertainty. primary reference material, and sources of sys- University of Utah’s tephra database (Barnosky tematic error (i.e., standard calibration and U et al., 2007). This work resulted in parts of the Zircon Sampling and U-Pb decay constant uncertainties). section being consistent with a late Hemingford- Geochronologic Analyses ian (He2) or early Barstovian (Ba1) age. How- CA-TIMS U-Pb Geochronologic Analyses ever, the sampled ash horizons that were used to Zircon sampling and dating analyses were establish this tephrachronology were collected conducted by Ibañez-Mejia. Zircon crystals Selected crystals previously dated by LA- from sites that were not directly tied to the mea- concentrated from whole-rock ash samples were ICP-MS and plucked out of the epoxy resin sured composite section due to separation by picked under a binocular microscope, mounted were subjected to a modified version of the large covered intervals (e.g., Whiskey Springs 3 in epoxy resin, and polished to expose the inte- chemical abrasion method of Mattinson (2005) site located ~250 m west of Whiskey Springs 4 rior of the grain prior to cathodoluminescence at the MIT-IG. Zircons were annealed in quartz site and Deadman Pass 2 site located ~1.9 km imaging. U-Pb geochronologic analyses were crucibles in a muffle furnace at 900 °C for northeast of Dead Squirrel 4 site). Therefore, the first conducted by laser ablation–inductively 60 h, chemically abraded using a single abra- tephrachronology cannot be used as a reliable coupled plasma–mass spectrometry (LA-ICP- sion step in concentrated HF at 205 °C for 12 h, independent measure of age for the Railroad MS) at the Arizona LaserChron Center, using and processed for ID-TIMS using the EARTH- Canyon section. Ar-Ar dating of feldspars from a Photon Machines Analyte G2 laser coupled TIME 205Pb-233U-235U mixed tracer (Condon et numerous ash samples constituted a second at- to a Nu Plasma multicollector ICP-MS. Instru- al., 2015). Pb and U were loaded on a single tempt to produce an independent age constraint mental bias, drift, and interelement fractionation outgassed Re filament in 2 µL of a silica-gel/ for the Railroad Canyon section (Barnosky et corrections were performed by the standard- phosphoric acid mixture (Gerstenberger and al., 2007). However, as the authors noted, they sample bracketing approach, using an in-house Haase, 1997), and U and Pb isotopic measure- were unable to obtain reliable Ar-Ar dates for Sri Lanka zircon crystal with a known age of ments were made on a IsotopX PhoeniX-62 any of the Railroad Canyon section ashes be- 563.5 ± 3.2 Ma (Gehrels et al., 2008) as primary TIMS at MIT. Pb isotopes were measured by cause they were dominated by detrital feldspars reference material. After LA-ICP-MS analyses, peak-hopping using a Daly photomultiplier and that had been reworked after deposition. one of the samples from the Whiskey Springs 4 corrected for instrumental mass fractionation Herein, we propose a new geochronology for interval (RCS3) was selected for further high- (0.18 ± 0.02%/amu) based on repeat analyses the Railroad Canyon section based on radioiso- precision dating by chemical abrasion–isotope of the NBS 981 Pb isotopic standard. Uranium topic dating of zircons from well-defined ashes dilution–thermal ionization mass spectrometry was measured as UO2+ ions using Faraday cups and compare these results with previously pub- (CA-ID-TIMS), in order to validate the accuracy with 1011 Ω resistors in multicollector mode lished magnetostratigraphic and biostratigraphic of the new absolute-age model proposed herein. and corrected for instrumental mass fraction- data (Zheng, 1996; Barnosky et al., 2007). We Select zircons were plucked out of the epoxy ation using the known ratio of 233U/235U in the seek to: (1) provide a reappraisal and refine- mount and subjected to a modified version of ET535 tracer (Condon et al., 2015), assuming a ment of the age of the Railroad Canyon section; the CA-TIMS method of Mattinson (2005) at sample 238U/235U ratio of 137.818 (Hiess et al., (2) further constrain the age of the early Mio- the Massachusetts Institute of Technology Iso- 2012) and a 18O/16O value of 0.00205 (Condon cene unconformity that separates the Renova tope Geochemistry Laboratories (MIT-IG). et al., 2015). and Six Mile Creek Formations regionally and Analyses conducted at the Arizona Laser- U-Pb dates and uncertainties were calculated provide a discussion of the implications for Chron laboratory were performed using a laser- from the TIMS data using U-Pb Redux (Bow- regional tectonic evolution; and (3) suggest re- beam diameter of 20 µm, firing at a repetition ring et al., 2011), following the algorithms of finements to the timing of the first appearance of rate of 7 Hz for ~14 s and using a constant en- McLean et al. (2011) and the U decay constants biostratigraphically important taxa in the north- ergy fluence of ~7.0 J cm–2 on the sample sur- of Jaffey et al. (1971). The 206Pb/238U ratios and ern Rocky Mountains. face. All Pb masses (i.e., 208, 207, 206, and dates were corrected for initial 230Th disequi- 204) were simultaneously monitored using librium using a Th/U[magma] of 2.8 ± 1.0. All MATERIALS AND METHODS discrete-dynode ion-multipliers, while 232Th common Pb in the analyses was attributed to and 238U were measured using Faraday detectors laboratory blank and subtracted based on the Within the measured composite stratigraphic equipped with 3 × 1011 Ω resistors. Data pro- measured laboratory Pb isotopic composition section, series of volcanic ash layers were iden- cessing and uncertainty calculations followed at MIT and associated uncertainty, determined tified and sampled for zircon U-Pb geochronol- the approach described in Ibañez-Mejia et al. from total procedural blank measurements (see ogy (Fig. 3). Four ash layers were dated from (2014). To assess age accuracy, zircon crystals Data Repository1). Quoted errors for individual within the composite section, three collected with a well-established CA-TIMS age of 48.13 ± analyses are presented in the form ± x(y)[z] fol- from Whiskey Springs 4 (RCS1–RCS3) and one 0.02 Ma (M.P. Eddy and M. Ibañez-Mejia, lowing the scheme of Schoene et al. (2006), from High Dead Squirrel 2 (RCS4). Previous Personal commun.) were frequently analyzed where x is solely analytical uncertainty, y is chronostratigraphic and biostratigraphic work and treated as unknowns during the analytical in the Railroad Canyon section has included session; a calculated age of 48.59 ± 0.50 Ma data from nearby sites (e.g., from the Lemhi (2σ, n = 58, mean square of weighted deviates Valley sequence) in their analysis, including [MSWD] = 1.3) using the LA-ICP-MS data in- 1GSA Data Repository item 2017184, which in- cludes biostratigraphic source data and results from from Cruik shank Creek, Peterson Creek, Mollie dicates that the results are accurate within the zircon LA-ICP-MS and CA-TIMS dating, is available Gulch, and South Portal. For the purposes of this quoted uncertainties of ~1%–1.5%. Eruption at http://www.geosociety.org/datarepository/2017 or paper, we excluded from our analyses all data ages discussed in the text are weighted mean by request to [email protected].
Geological Society of America Bulletin, v. 1XX, no. XX/XX 5 Harris et al. the combined analytical and tracer uncertainty polynomial model to avoid the assumption that tribution dominated by older inherited zircons (i.e., ±<0.03%; McLean et al., 2015), and z is sedimentation rates were constant throughout and was therefore not included in the age model the combined analytical, tracer, and 238U decay deposition of the Railroad Canyon section strata for the Railroad Canyon section. Further refine- constant uncertainty (i.e., ±0.108%; Jaffey et al., (Fig. 4). The resulting age model was used to ment of the age obtained for sample RCS3 by 1971). The weighted mean age error includes calculate the age range of the Railroad Canyon the CA-TIMS method resulted in a 206Pb/238U analytical uncertainties based on counting sta- section strata, as well as the age of specific lev- weighted mean crystallization age of 21.217 ± tistics, mass fractionation correction, spike and els within the composite section. 0.027/0.031/0.039 Ma (2σ, n = 7, MSWD = blank subtraction, and 230Th disequilibrium 0.67), which is in excellent agreement with the correction, and these values are appropriate to RESULTS initial LA-ICP-MS result and thus provides use when comparing with other 206Pb/238U ages further support to the accuracy of the proposed obtained with spikes cross-calibrated with the Isotopic results and corresponding concordia age model. EARTHTIME gravimetric standards. diagrams from zircon U-Pb analyses using the The ages of the top and bottom of the Rail- LA-ICP-MS and CA-TIMS methods are pre- road Canyon section, as well as ages of specific Age Model Calculations sented in Tables DR2 and DR3 (see footnote levels within the composite section, were deter- 1) and in Figure 5. The two ash layers collected mined using the following equation: Three new radiometric dates were used to within the WH4 interval, near the top of the correlate the Railroad Canyon section magne- Renova Formation (RCS1 at 31.2 m in WH4) T = [(42.58 × 10−3) × M] – [(16.16 × 10−4) × tostratigraphic record (Zheng, 1996; Barnosky and the base of the Six Mile Creek Formation M2] + [(15.99 × 10−6) × M3] – et al., 2007) with the global GPTS (Gradstein (RCS3 at 75 m in WH4), yielded zircon crys- [(76.44 × 10−9) × M4] + [(1.76 × 10−10) × M5] – et al., 2012; Fig. 3) to create an updated and re- tallization ages of 22.65 ± 0.37 Ma (2σ, n = [(15.59 × 10−14) × M6] + 22.48, (1) vised age model for the Railroad Canyon sec- 23, MSWD = 2.0) and 21.24 ± 0.27 Ma (2σ, tion. This made it possible to assign ages to spe- n = 26, MSWD = 1.1), respectively. The third where T is the age in millions of years (Ma), and cific levels within the Railroad Canyon section dated horizon, corresponding to a tuff in the M is the height (in meters) of the sample of in- that were associated with transitions between upper portion of the Six Mile Creek Formation terest in the composite Railroad Canyon section. normal and reversed magnetic events. Based (RCS4 at 339.9 m in HDS2), yielded a zircon Based on this model, the sedimentation rate be- on this scheme, chron ages (Ma) were graphed crystallization age of 15.76 ± 0.22 Ma (2σ, n = tween RCS1 and RCS3 was ~31.8 m m.y.–1, and against meter level height within the Railroad 21, MSWD = 1.0). A fourth ash layer collected the sedimentation rate between RCS3 and RCS4 Canyon section, and the data were fitted with a from WH4 (RCS2 at 68.5 m) yielded an age dis- was ~48.1 m m.y.–1, showing that sedimentation rates were not constant throughout deposition of the Railroad Canyon section (Fig. 4). These data suggest that deposition of the Six Mile Creek • Formation was associated with a >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 sec- • tion 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 Rail- ••• road 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., Re- tallack, 2009). However, an alternative possibility is that the substantially older dates for the Railroad Can- Figure 4. Revised age model for the Railroad Canyon section based on new yon section could be a result of inheritance, pro- U-Pb dates. Stars indicate locations of radioisotope ages reported in this study. ducing inaccurate eruption ages. Several obser- Variability in sedimentation rates is apparent in the changes in slope of age vations help to reject this interpretation: (1) The model (dotted line). Arrow points to location of the early Miocene unconfor- reported weighted means for the three dated in- mity (EMU). See Methods section for age model equation. tervals result in ages that progressively decrease
6 Geological Society of America Bulletin, v. 1XX, no. XX/XX U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho
A data-point error ellipses are 2σ D data-point error ellipses are 2σ EBH-RCS14-Ash06RCS4 RCS2EBH-RCS14-Ash04 110 LA-ICP-MS LA-ICP-MS 0.016 17 0.0026 90
0.012 0.0025 16 70
0.0024 0.008 50 Reworked Cretaceous 15 zircons 0.0023
U 30 U 8
206 238 238
23 Weighted mean Pb/ U age 14 15.76 ± 0.22 Ma
Pb/ Youngest zircons ~ 22 Ma Pb/ 95% conf., n= 21, MSWD= 1.0 10 206 206 207 235 207 Pb/235 U 0.01 0.02 Pb/ U 0.02 0.04 0.06 0.08 BEdata-point error ellipses are 2σ data-point error ellipses are 2σ RCS3EBH-RCS14-Ash05 24 RCS3 LA-ICP-MS CA-TIMS (Th-corrected) 0.00334 21.521.50 0.0036 23
2121.4.4400 22 0.00332 0.0034 22121.3.300 21 0.00330 0.0032 22121.21..200 20
U 0.00328
8 221.11.10 206 238
23 Weighted mean Pb/ U age 19 21.24 ± 0.27 Ma
Pb/ 95% conf., n= 26, MSWD= 1.1 2121.0.000 0.00326 206 207 235 207 235 0.018 0.020 0.022 0.024 Pb/ U 0.020 0.022 0.024 Pb/ U Weighted mean 206Pb/ 238U age C data-point error ellipses are 2σ 21.217 ± 0.027/0.031/0.039 Ma EBH-RCS14-Ash01RCS1 n= 7, MSWD= 0.67 LA-ICP-MS
0.0039 25
24 0.0037
23 0.0035 22
0.0033 21 U 8 206 238
23 Weighted mean Pb/ U age 20 22.65 ± 0.37 Ma
Pb/ 95% conf., n= 23, MSWD= 2.0
206 19 207 235 0.01 0.02 0.03 0.04 Pb/ U
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.
Geological Society of America Bulletin, v. 1XX, no. XX/XX 7 Harris et al. up section, thus not violating stratigraphic first et al., 1985; Hanneman and Wideman, 1991; cord for Idaho has a stratigraphic gap between principles; (2) the three horizons from which Barnosky, 2001; Rasmussen, 2003; Barnosky ca. 36 Ma and 19 Ma, making it difficult to as- weighted mean ages are reported contain a pro- et al., 2007) and is thought to have been time- sess local tectonic changes. Our new dates for portion between 85% and 95% of zircons with transgressive between ca. 20 Ma to ca. 17 Ma in the Railroad Canyon section narrow that gap apparent 206Pb/238U dates that are statistically un- a region spanning from southwestern Montana by >3 m.y. and, in addition, make it possible distinguishable, thus validating the hypothesis and to northwestern Wyoming (Table 1; Fields to compare the absolute δ18O values from the that they represent a single population within et al., 1985; Barnosky and Labar, 1989; Burbank Railroad Canyon section with the Sage Creek the assigned uncertainties; and (3) although one and Barnosky, 1990; Hanneman and Wideman, Basin of southwestern Montana (Kent-Corson ash layer collected from the Renova Formation 2006). Based on these previous studies, as well et al., 2006) to look for evidence of regional (sample RCS2 at 68.5 m in WH4) yielded a as biostratigraphy and magnetostratigraphy, heterogeneity. Paleosol δ18O values from both zircon age distribution clearly dominated by in- Barnosky et al. (2007) suggested that the early of these areas range between +14‰ and +16‰ heritance (Fig. 5D; Table DR2 [see footnote 1]), Miocene unconformity in the Railroad Can- (relative to Vienna standard mean ocean water indicating an important component of Late Cre- yon section occurred between ca. 17.3 Ma and [VSMOW]) without any temporal trend, point- taceous detritus being delivered to the Miocene 16.73 Ma. However, given the new age model ing to (1) regional stability during the deposition basins of the Railroad Canyon section, the other for the Railroad Canyon section, the age of the of early–middle Miocene strata and (2) similar three dated samples have virtually no zircons early Miocene unconformity in central-eastern absolute elevations in the region, at the scale of with pre-Cenozoic ages, thus suggesting that an Idaho is now bracketed between ca. 21.5 and tens of kilometers. This reanalysis is consistent inherited zircon component is not prevalent in 21.4 Ma. This new age estimate shows that the with the SWEEP (southwest encroachment of these horizons. In addition, high-precision CA- early Miocene unconformity is indeed time an Eocene plateau) hypothesis of Chamber- TIMS analyses obtained for seven zircons from transgressive in the region and lends support to lain et al. (2012), which holds that the western sample RCS3 (Table DR3 [see footnote 1]) also the idea of diachronous regional uplift in west- North American Cordillera underwent major define a statistically single population and are ern North America leading into the early and uplift starting in the Paleogene, developing in excellent agreement with the age obtained by middle Miocene (Barnosky et al., 2007; Cham- into a high, rugged mountain range by the late LA-ICP-MS. Altogether, these lines of evidence berlain et al., 2012). Eocene (ca. 40 Ma), and that collapse of these support the notion that the reported dates accu- highlands, resulting in development of the mod- rately approximate the age of eruption of these Tectonic and Paleoelevation Implications ern Basin and Range, would have occurred only ash horizons and are not detrital ages influenced after 15 Ma. Parsons et al. (1994) furthermore by reworking. Several researchers have used δ18O values proposed that the impingement of the Yellow- from pedogenic carbonates to reconstruct the stone hotspot may have influenced topographic Revised Age of the Early Miocene uplift history of the northern Rocky Mountains relief up to 2000 km away from the plume head, Unconformity in the Railroad (e.g., Kent-Corson et al., 2006; Chamberlain et beginning ca. 17–16 Ma. However, the regional Canyon Section al., 2012). This work has shown that the pat- stability and topographic uniformity now in- tern of elevation changes in this region included ferred from the δ18O data from the Sage Creek At ~70 m in the composite section (in WH4), significant spatial and temporal heterogeneity Basin and Railroad Canyon section suggest that the erosional early Miocene unconformity sepa- (e.g., Horton et al., 2004; Chamberlain et al., there were no significant far-field topographic rates the white beds of the Renova Formation 2012; Mix and Chamberlain, 2014; Mulch et effects from the Yellowstone hotspot in the re- from predominately buff and pink beds of the al., 2015), and large elevation gradients (Sjos- gion until after ca. 15 Ma. Six Mile Creek Formation. The early Miocene trom et al., 2006). However, ambiguities remain unconformity has been widely discussed in for given subregions because of uncertainties Potential Biostratigraphic Implications paleontological and geological studies in the associated with the ages of the stratigraphic northern Rocky Mountains (previously referred sections under study. For example, in Kent- The age model proposed herein differs con- to as the mid-Tertiary unconformity; e.g., Fields Corson et al. (2006), the composite isotopic re- siderably 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 Formation Approximate age of Age constraint(s) References unconformity (Ma) South Killdeer Mountains, Arikaree ca. 20 Fission-track age of 25.1 + 2.2 Ma at base of Delimata (1975); southwestern North Dakota burrowed marker unit; co-occurrence of Merychyus Murphy et al. (1993); and Merycochoerus 27 m above fi ssion-track age Hoganson et al. (1998) Monroe Canyon, Nebraska Harrison ca. 20 Fission-track age of 19.2 + 0.5 Ma overlying the Hunt (1990); unconformity; radiometric age of 21.9 Ma of Agate MacFadden and Hunt (1998) Ash below unconformity Yellowstone Valley, Hepburn’s Mesa ca. 16.8 Magnetostratigraphy; biostratigraphy; lithostratigraphy Barnosky (1984, 1986); southwestern Montana (cessation of EMU) Barnosky and Labar (1989); Burbank and Barnosky (1990) Jackson Hole, Wyoming Colter ca. 17–18 Biostratigraphy Barnosky (1984, 1986); (onset of EMU) Barnosky and Labar (1989); Burbank and Barnosky (1990) Beaverhead Mountains, Sixmile Creek ca. 21.5–21.4 U-Pb radiometric dating, magnetostratigraphyThis study Idaho and Renova
8 Geological Society of America Bulletin, v. 1XX, no. XX/XX U-Pb chronology and revised biostratigraphy for Railroad Canyon section, Idaho based on faunal occurrence data from the Rail- Merycodus (Antilocapridae), and Rakomeryx was deposited during buildup to peak warm- road Canyon section. The majority of faunal (Palaeomerycidae) (Fig. 2). Because many fos- ing during the global mid-Miocene climatic data for the Railroad Canyon section comes sils collected in the Railroad Canyon section optimum, highlighting the importance of this from two sites, Snowfence east and Snow- were recovered as float after weathering out, it site in discussions of local and global effects of fence west, that occur between 125 and 177 m is impossible with our data set to estimate the the mid-Miocene climatic optimum. Further- in the composite section. Based on our new exact range of some Railroad Canyon section more, this new age model suggests the early age model, these faunas date between ca. 19.8 taxa. Therefore, the following taxa may have Miocene unconformity (previously also known and 18.8 Ma (Ar4). In contrast, previous work- had an earlier first appearance in the Railroad as the Mid-Tertiary unconformity) occurred ers proposed that the Railroad Canyon section Canyon section than they did elsewhere in the between ca. 21.5 and 21.4 Ma in the Railroad fauna was late Hemingfordian–late Barstovian region, but because of the method of fossil re- Canyon section and supports the idea that this (He2–Ba2; ca. 17.5–13 Ma) in age, based on covery, we cannot be certain at this point: Bou- erosional episode was time transgressive across how it and nearby faunas (e.g., Mollie Gulch romeryx (Palaeomerycidae), Hypohippus cf. H. the intermontane basins of the northern Rocky beds)—which were presumed to be coeval (Bar- osborni (Equidae), Merychippus cf. M. insignis Mountains. Our new age constraint on the early nosky et al., 2007)—compared with regional (Equidae), and Ticholeptus zygomaticus (Mery- Miocene unconformity is particularly impor- faunas from the northern Rocky Mountains, coidodontidae). Overall, we suggest these bio- tant because it is the first time that both the the northern Great Plains, and the Northwest stratigraphic modifications as a first step toward top and bottom of this regional unconformity (e.g., Barnosky, 2001). Due to this discrepancy reconciling our understanding of regional bio- have been bounded by radiometric dates from in ages, we suggest that the early–middle Mio- stratigraphy with new radiometric dates from a single basin. A comparison between this new cene biostratigraphy of northern Rocky Moun- the Railroad Canyon section. chronostratigraphic framework and previously tains strata needs to be reassessed. Although a We also note that these earlier appearance reported biostratigraphy from the Railroad full biostratigraphic reevaluation is beyond the dates in the northern Rocky Mountains for many Canyon section potentially pushes back the scope of this paper, we are able to highlight a taxa compared to those in western America and dates of first appearance for many biostrati- few important changes in taxonomic occurrence the Great Plains imply marked diachrony (up to graphically important taxa found throughout data that have stratigraphic and potentially evo- ~2 m.y.; Fig. 2) in Cenozoic faunas across North the region. Many of these taxa were previously lutionary implications. Thus, the new age model America during the early–middle Miocene, known only from early–late Hemingfordian revises the NALMA assignments for Railroad well above the 0.75 m.y. discrepancy that can (He1–He2) strata, but our results suggest they Canyon section strata (Fig. 2) and thereby the be expected solely because of sampling error may have had earlier first appearances in the biochronologic ranges of certain taxa deemed (undersampling of faunas) according to Alroy’s northern Rocky Mountains. This work also biostratigraphically important (“index fossils”) analysis (Alroy, 1998). This appearance pattern highlights the importance of radiometric dat- by Barnosky et al. (2007; specimens are curated across western North America may indicate that ing for calibration of Miocene biostratigraphic at either the University of California Museum of these taxa evolved in the Cordilleran region range limits and NALMA assignments in west- Paleontology or University of Montana Paleon- and later spread to the west and east, which is ern North America, which are notorious for be- tology Center; additional details can be found in consistent with the broad notion of this topo- ing poorly temporally constrained. the Carrasco et al., 2005). Specifically, it pushes graphically complex region as an engine of bio- back the biostratigraphic ranges of these index diversity (Kohn and Fremd, 2008; Finarelli and ACKNOWLEDGMENTS taxa. A regional comparison of first appearances Badgley, 2010; Badgley et al., 2017), and it also We would like to thank C. Trinh-Le, A. Padgett, C. of these taxa now suggests that some of these adds to pre–mid-Miocene climatic optimum Bitting, E. Fredrickson, K. Smith, A. Jijina, T.-Y. Le, taxa may have appeared earlier in the northern taxon richness in the northern Rocky Mountains J. Benca, M. Dennis, and E. Hyland for field assis- Rocky Mountains compared to the U.S. West (e.g., Kohn and Fremd, 2008). However, to fully tance during this project. Additionally, we thank R.E. Dunn for laboratory support and assistance with pa- Coast or Great Plains (Tedford et al., 2004). evaluate how our new ages for Railroad Can- leomagnetic correlation, A.D. Barnosky for assistance Based on our new age model, as well as pub- yon section faunas influence faunal patterns, a acquiring field notes and general discussion about the lished taxonomic range limits (Tedford et al., more thorough taxonomic review coupled with Railroad Canyon section, and the Arizona LaserChron 2004; Albright et al., 2008), and assuming that precise dating of early–middle Miocene faunas and MIT-IG laboratories for making their analytical facilities available to Ibañez-Mejia for zircon U-Pb the Railroad Canyon section taxa have been ac- is likely necessary. Our study therefore stresses analysis. Finally, we would like to thank Brad Singer, curately classified, we propose that the follow- the importance of precise dating of more than A.E. Troy Rasbury, and two anonymous reviewers ing taxa had an earlier first appearance in the late just a handful of Miocene faunas for a full un- for their comprehensive and thorough reviews of the Arikareean (Ar4; ca. 22.8–18.5 Ma) than previ- derstanding of biogeographic leads and lags in manuscript. Funding for this project was provided by ously reported regionally (Fig. 2): Alphagaulus faunal occurrences and Cenozoic diversity pat- National Science Foundation grants EAR-1024681 to C.A.E. Strömberg, and EAR-1024535 to N.D. Shel- vetus (Mylagaulidae), Mesogaulus (Mylagauli- terns across western North America. don and S.Y. Smith, an Evolving Earth Foundation dae), Cupidinimus (Heteromyidae), Harrymys grant to Harris, and a University of Washington Biol- irvini (Heteromyidae), Merychyus elegans CONCLUSION ogy Iuvo Award to Harris, as well as funding from the (Merycoidodontidae), Oreolagus (Ochotoni- Burke Museum of Natural History and Culture. dae), Pliocyon (Amphicyonidae), and Tylo- We provide a revised and radiometrically REFERENCES CITED cephalonyx cf. T. skinneri (Chalicotheriidae). calibrated age model for the Railroad Canyon Additionally, the following taxa appear to have section, an important sequence of Miocene- Albright, L.B., Woodburne, M.O., Fremd, T.J., Swisher, C.C., III, MacFadden, B.J., and Scott, G.R., 2008, had an earlier first appearance either in the late aged rocks in northwestern North America, Revised chronostratigraphy and biostratigraphy of Arikareean (Ar4) or early Hemingfordian (He1; constraining its age to ca. 22.9–15.2 Ma, i.e., the John Day Formation (Turtle Cove and Kimberly 18.5 to ca. 17.5 Ma): Aepycamelus (Camelidae), ~5 m.y. older than previous age models. 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