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. For permission to copy, contact [email protected] Geological Society of America Bulletin, 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.