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HighprecisionU–PbzircongeochronologyforCenomanianDakotaFormationflorasinUtah

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Cretaceous Research 52 (2015) 213e237

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Cretaceous Research

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High precision UePb zircon geochronology for Cenomanian DakotaFormation floras in Utah

Richard S. Barclay a, *, Matthew Rioux b, d, Laura B. Meyer b, Samuel A. Bowring b,Kirk R. Johnson c, Ian M. Miller c

a Smithsonian Institution, National Museum of Natural History, Department of Paleobiology, NHB121, Washington, D.C. 20013-7012, USAb Massachusetts Institute of Technology, Department of Earth, Atmosphere, and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, MA 02139, USAc Denver Museum of Nature & Science, Department of Earth Sciences, 2001 Colorado Boulevard, Denver, CO 80205, USAd Earth Research Institute, University of California, Santa Barbara, CA 93106, USA

a r t i c l e i n f o

Article history:Received 11 March 2014Accepted in revised form 19 August 2014Available online

Keywords:Zircon UePb geochronologyPaleobotanyOcean Anoxic Event 2mid-CretaceousDakota FormationPaleoclimate

* Corresponding author.E-mail addresses: barclay.rich@gmail.com, barclay

http://dx.doi.org/10.1016/j.cretres.2014.08.0060195-6671/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Cretaceous Dakota Formation on and near the Colorado Plateau represents the time-transgressiveshoreline deposits of the Western Interior Seaway and is a rich repository of fossil plants and animals.However, given the basin architecture and depositional setting, correlations between localities aredifficult without geochronology. Here we present new high precision ID-TIMS UePb zircon dates fromfive air-fall ash deposits from Westwater (n ¼ 3) and Henrieville (n ¼ 2) that obey stratigraphic super-position and precisely constrain the ages of these two floras. Three air-fall ashes from the Westwaterlocality record weighted mean Th-corrected 206Pb/238U dates of 97.949 ± 0.037/0.12 Ma, 97.943 ± 0.023/0.12 Ma, and 97.601 ± 0.049/0.13 Ma, providing the first UePb dates from this locality. Two ash beds fromthe Henrieville locality record weighted mean Th-corrected 206Pb/238U dates of 95.070 ± 0.036/0.12 Maand 94.879 ± 0.032/0.11 Ma. The new dates indicate that the two floras are separated by ca 2.5 Ma andare both within the Cenomanian. The assemblage of fossil plants collected from the mudstone and ash-bed localities at Westwater show no compositional overlap, despite the presence of 28 distinct mor-photypes. This lack of compositional overlap is repeated between the mudstone facies from the West-water and Henrieville localities, where two different morphotypes of “Liriodendron” are the taxa incommon. The species incongruence at Westwater is likely due to rapid colonization of early successionalplants post deposition of the volcanic ash. For the mudstone floras at Westwater and Henrieville, thedifferent floral composition may be due to rapid species evolution during the rise of angiosperms, theinfluence of climate in the mid-Cretaceous, the expression of high regional diversity of localities sepa-rated by 300 km, or a highly partitioned floodplain vegetation. Estimated sediment accumulation ratesfrom the new radioisotopic dates, combined with existing proxy records for pCO2, suggest that the rise inpCO2 preceding Ocean Anoxic Event 2 (OAE2) began 513 ka (range from 384 to 641 ka) prior to thepositive d13C excursion that defines the event. This estimate from terrestrial rocks is within error ofestimates for the timing of changes in d34Ssulfate (570 ka; range from 420 to 814 ka) in marine sections atthe GSSP for the CenomanianeTuronian boundary. The overlap indicates synchronous perturbation ofmarine and terrestrial environments related to an increase in pCO2 prior to the onset of OAE2, providingfurther support for the volcanic initiation hypothesis.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Dakota Formation consists of non-marine and marginal-marine sedimentary rocks exposed in the central United States,deposited during Albian to Cenomanian time. The deposits of theDakota formed along the shorelines of theWestern Interior Seaway

rs@si.edu (R.S. Barclay).

(Kauffman and Caldwell,1993), an epicontinental sea that stretchedfrom the Gulf of Mexico to the Arctic Ocean, splitting the continentinto two halves during the mid-Cretaceous (Roberts andKirschbaum, 1995). The historic type area of the Dakota Forma-tion lies in the Missouri and Sioux River Valleys of Nebraska andIowa (Witzke and Ludvigson, 1994), and together with correlativeexposures in Kansas, South Dakota, North Dakota, and Minnesotarecord deposition along the eastern margin of the seaway (Witzkeand Ludvigson, 1994; Brenner et al., 2000; Ludvigson et al., 2010).

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237214

To the west, the Dakota Formation or Dakota Group designationsare applied to sedimentary rocks in Utah, Arizona, Colorado,Oklahoma, and NewMexico (Young,1960; Peterson and Kirk, 1977).These were deposited along the western margin of the WesternInterior Seaway in foreland basins related to the Sevier Orogenicbelt (Currie, 2002; DeCelles, 2004).

The Dakota flora, collected on both sides of the Western InteriorSeaway, provides an important record of the early evolution anddiversification of angiosperms in North America (Dilcher et al.,1976; Hickey and Doyle, 1977; Dilcher, 1979; Basinger and Dilcher,1984). Fossils from the eastern shoreline of the Western InteriorSeaway preserve an extensive record of Early Cretaceous plants,one that has been studied for more than a century (Lesquereux,1891). The angiosperms have been particularly well studied fromthe eastern shore of the seaway (Lesquereux, 1891; Dilcher andCrane, 1984; Wolfe and Upchurch, 1987; Upchurch and Dilcher,1990; Wang and Dilcher, 2006a,b; Hu et al., 2008; Dilcher andWang, 2009; Wang and Dilcher, 2009; Wang et al., 2011; Wanget al., 2013). Initial analysis of the level of angiosperm speciesrichness suggested that there were 437 distinct species(Lesquereux, 1891), which is an extremely high number and skewsdiversity curves for this time period (Lidgard and Crane, 1990).Subsequent researchers argued that the species were over-split inthe earlier studies, because individual quarries typically preserveonly 20 species, a common level for the middle Cretaceous (Wang,2002; Wang and Dilcher, 2006b).

The flora from the Dakota Formation along the western marginof the Western Interior Seaway received some attention during theoriginal surveys of the western states in the late 1800's and early1900's (Newberry, 1861, 1868, 1898; Cockerell, 1916; Rushforth,1971; McClammer and Crabtree, 1989), and then again in themid-20th century (Brown, 1950; Rushforth and Tidwell, 1968;Rushforth, 1971; Tidwell et al., 1976), with the most detailedstudy by Rushforth (1971). No other work has been published onthe macrofossil plants in the Dakota Formation from the westernmargin of the seaway for over four decades, although specimenspublished here were collected beginning in the late 1980s.

The non-marine faunal record of the Dakota in Colorado, NewMexico, Nebraska, Kansas, and Oklahoma preserves an extensiverecord of dinosaur trackways (Lockley et al., 1992; Lockley andHunt, 1994; Lockley and Hunt, 1995; Joeckel et al., 2004).Mammal fossils have been well characterized in Utah, usually fromdisarticulated remains recovered by sieving sediment (Cifelli andEaton, 1987; Eaton and Cifelli, 1988; Eaton, 1993; Cifelli et al.,1997; Eaton et al., 1999). In addition to vertebrates, the DakotaFormation near Henrieville has yielded flying insects, includingsome of the oldest-known bee burrows (Elliott and Nations, 1998),and the only known Cenomanian body fossils of dragon-fly larvaein the Western Interior (Titus et al., 2010).

Due to eustatic sea-level fluctuations in the mid-Cretaceous(Haq et al., 1987), the shoreline deposits that comprise theDakota Formation produced a time-transgressive record of depo-sition, often of unknown temporal extent (Averitt, 1962; Shanleyand McCabe, 1991; Shanley and McCabe, 1993; Shanley andMcCabe, 1995). Depending upon the geographic location, theDakota Formation (or Dakota Group) on the western margin of theWestern Interior Seaway can range in age from Albian to the latestCenomanian (Witzke and Ludvigson, 1994; Currie et al., 2008;Ludvigson et al., 2010; Sprinkel et al., 2012). The age of theDakota Formation has typically been determined based uponbiostratigraphic methods, a practice which has served to highlightthe large temporal differences between the disparate Dakota For-mation sections, rather than allow for meaningful correlation.Frequent sea level fluctuations of the Western Interior Seaway arepreserved in some of the sedimentary rocks of the Dakota

Formation (Leckie et al., 1991; Shanley and McCabe, 1991; Shanleyand McCabe, 1993; Shanley and McCabe, 1995), providing preciseammonite and inoceramid biostratigraphy that helps determinethe age of the marine strata and also creates bounding ages for theadjacent terrestrial units (Cobban and Scott, 1972; Kauffman andCaldwell, 1993; Elder et al., 1994; Cobban et al., 2000). For theexclusively non-marine portions of the Dakota Formation in Utah,of particular interest to this study, the relative age continues to bedetermined using palynology (May and Traverse, 1973; Nichols andSweet, 1993; Oboh-Ikuenobe et al., 2007; Currie et al., 2008;Sprinkel et al., 2012). Despite the many tools researchers have usedto determine the age of the Dakota Fm., most of the fossil localitieswithin the Dakota Formation can only be constrained to the Stagelevel using biostratigraphy (McClammer and Crabtree,1989), whichis problematic because mid-Cretaceous Stages are 4e12 Ma long.Almost all fossil plant and animal sites lack precise radioisotopicdates.

Here we present new high precision isotope dilution-thermalionization mass spectrometry (ID-TIMS) UePb zircon dates andfossil plant descriptions from two Dakota Formation exposures onthe Colorado Plateau in Utah. The UePb dates come from five air-fall ash beds associated with fossil plant localities held in the col-lections at the Denver Museum of Nature & Science (DMNS). Thenew data and descriptions update and greatly expand the originalfloral study conducted by Rushforth (1971), allowing thesegeographically isolated paleobotanical localities to be placed intocontext with other contemporaneous paleobotanical records fromthe Cretaceous Western Interior Seaway.

2. Geological setting

2.1. Sampling localities

The samples dated in this study come from outcrops of theDakota Formation near the town of Henrieville and near theWestwater Canyon of the Colorado River in Utah, close to the Utah/Colorado state line (Fig. 1; Table 1). In southern Utah, the DakotaFormation typically rests on an angular unconformity with Jurassicstrata, and to a lesser amount overlies older Cretaceous deposits.The sedimentary sequences represent fluvial channel and overbankdeposits, preserved within contemporaneously carved paleo-valleys (Currie et al., 2008).

At the Westwater locality (Fig. 2) in eastern Utah, the DakotaFormation generally rests unconformably on the Jurassic MorrisonFormation, but locally overlies the Lower Cretaceous Cedar Moun-tain/Burrow Canyon Formation (Currie et al., 2008). The DakotaFormation near Westwater is divided into two unconformity-bounded sequences, based upon outcrop measurements of thestratigraphic architecture of incised river channel sandstones (Ryeret al., 1987; Currie, 2002). Each sequence consists of a basal coarsesandstone or conglomerate overlain by mudstones, siltstones, finegrained sandstones, and coals (Currie et al., 2008). The lowersequence consists of fluvial sandstone, conglomerate, and alluvialmudstones filling valleys cut into the underlying rocks (Currie,2002). The upper sequence cuts into the lower-Dakota sequenceby as much as 15 m, filling with mudstone, sandstone, andconglomerate that was deposited in fluvial, alluvial, and tidal facies.TheDakota Formation in theWestwater region is always overlain bymarine strata of the Mancos Shale (Currie, 2002), representingflooding during sea level rise in the Western Interior Seaway.

Rushforth (1971) conducted themost extensive site-based studyin the Dakota Formation when he described the flora and theenvironment of deposition at the Westwater locality. The siteprovided an excellent floral example of “an admixture of an olderJurassic-Wealden floristic typewith amodern angiospermous floral

Fig. 1. Map of Utah showing the extent of Cretaceous to lower Tertiary rocks (Simplified from Hintze et al., 2000). Location of samples at Westwater and Henrieville indicated bystars.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237 215

type” (Rushforth,1971). The florawas dominated bywell-preservedfern fronds and a few poorly preserved and incomplete angio-sperms. He described three new species, from a flora that included19 species in 14 genera. Rushforth (1971) appropriately ascribed theWestwater Flora to the Cretaceous, following the original diagnosisof Meek and Hayden (Meek and Hayden, 1856, 1858), and to theCenomanian Stage based upon floristic similarities with theWoodbine flora of Texas, a flora considered to be Cenomanian usingboth plants (MacNeal, 1958) and invertebrate faunas (Stephenson,1952).

The sections near Henrieville, Utah (Fig. 3) are located along thewestern margin of the Kaiparowits Plateau (Kirschbaum andMcCabe, 1992; Uli�cný, 1999), which exposes a thick section of

Table 1Geographic coordinates for bentonite and paleobotanical samples.

General location Sample type DMNH locality name orsample number

Westwater Fossil plant Coal DrawWestwater Fossil plant Westwater AngiospermWestwater Fossil plant Slide AshWestwater Fossil plant Westwater IIWestwater Bentonite KJ08105Westwater Bentonite KJ08107Westwater Bentonite KJ08108Henrieville Fossil plant HenrievilleHenrieville Bentonite KJ08142Henrieville Bentonite KJ08143

a Coordinates are relative to the WGS84 datum, using the Simple Cylindrical projectio

Upper Cretaceous (Cenomanian to Campanian) sedimentary rocksunconformably deposited on the tilted Jurassic Entrada orMorrisonFormations (Eaton et al., 1999; Uli�cný, 1999). Peterson (1969)divided the Dakota Formation on the Kaiparowits Plateau intothree informal members, separated by disconformities. The LowerMember erodes into the underlying Jurassic deposits and is char-acterized by thick sequences of conglomerate (typically one meterthick, but exceeding 10 m locally) and sandstone, deposited inbraided river channels flowing from the Sevier Orogeny (Gustason,1989; Kirschbaum and McCabe, 1992). The Middle Member iscomposed of largely non-marine sandstone, mudstone, carbona-ceous mudstone and thin coal horizons, with occasional marinestrata representing short-lived marine incursions. Kirschbaum and

DMNH localitynumber

Latitudea

(Degrees)Longitudea

(Degrees)

947 39.108752 �109.1466991191 39.105940 �109.1520371192 39.105782 �109.1522771193 39.109907 �109.145729e 39.106037 �109.152258e 39.105782 �109.152277e 39.105782 �109.152277404 37.588211 �111.979714

e 37.588221 �111.979681e 37.588247 �111.979808

n.

Fig. 2. Stratigraphic column of the Westwater site showing the position of bentonitesand paleobotanical samples. Reported ages are weighted mean 206Pb/238U dates. Alluncertainties are ± 2s, reported as ± analytical uncertainties/analytical þ tracer þdecay constant.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237216

McCabe (1992) named one of these marine deposits as the ‘TropicShale Tongue’, but Uli�cný (1999) treated these as sub-units of theDakota Formation (Fig. 3). Marginal-marine sandstone, mudstone,and shale characterize the Upper Member which is overlain by theTropic shale. The progression from fluvial to marine depositionalenvironments is interpreted as a transgressive sequence associatedwith the Greenhorn cycle (Elder et al., 1994; Laurin and Sageman,2001; Laurin, 2003; Sageman et al., 2006; Laurin and Sageman,2007).

2.2. Existing geochronology

Palynomorphs from the Westwater exposures indicate that theDakota Formation was deposited in the late Albian to early Cen-omanian and the overlying Mancos Shale was deposited in themiddle Cenomanian to Turonian (Currie et al., 2008). Existingconstraints on the age of the Henrieville locality include paleon-tology and 40Ar/39Ar dates. Palynology and ammonite biostratig-raphy indicate that the formation was deposited between the earlyto late Cenomanian, although the lowest conglomerate may be lateAlbian (May and Traverse, 1973; am Ende, 1991; Uli�cný, 1999;Cobban et al., 2000; Dyman et al., 2002a). Ammonites from theSciponoceras gracile biozone recovered from the base of the over-lying Tropic Shale are consistent with late Cenomanian depositionof this unit (am Ende,1991). Several unpublished 40Ar/39Ar sanidinedates from bentonites within the Dakota Formation (or boundingunits) near Henrieville also support a Cenomanian depositional age

(Table 2). Sanidine 40Ar/39Ar dates from three air-fall ashes withinthe Middle Member of the Dakota Formation and base of the TropicShale near Henrieville ranged from 92.9 ± 0.2 Ma, 91.5 ± 0.1 Ma(Dakota Formation) and 90.5 ± 0.1 Ma (Tropic shale; Bohor, 1991;±2 sigma), assuming an age of 513.9 Ma for the MMhb standard(Table 2). A subsequent re-analysis of the two younger samplesyielded older dates of 94.34 ± 0.60 Ma and 93.66 ± 0.44 Ma(Obradovich, pers. comm., 1992), assuming an age of 520.4 Ma forthe MMhb standard. A final unpublished sanidine 40Ar/39Ar anal-ysis from the Middle Member of the Dakota Formation near Tropic,Utah, yielded a date of 96.06 ± 0.3 Ma (Dyman et al., 2002a; Dymanet al., 2002b), again assuming an age of 520.4 Ma for the MMhbstandard. Re-calculated dates for these analyses using modernstandard and decay constant values are given in Table 2 and will bediscussed below.

3. Materials and methods

3.1. UePb zircon geochronology

Ongoing advances in UePb geochronology have led toimproved precision and accuracy. These advances include loweranalytical blanks, new data acquisition and reduction protocols(Schmitz and Schoene, 2007; Bowring et al., 2011; McLean et al.,2011), new high-precision measurements of the isotopic compo-sition of uranium in standards and zircon (Condon et al., 2010;Hiess et al., 2012), and improved techniques for the eliminationof Pb-loss (Mattinson, 2005). The improved precision and accu-racy often highlight complexities that could not be resolved withlower precision methodologies. To extract the best estimate of asingle magmatic population requires a relatively large number ofanalyses and identification of outliers, including grains that areboth distinctly older and younger than the interpreted eruptionage.

Ash samples were collected as “bricks” cut from cleanedbedding planes to avoid contamination with soil and surroundingsedimentary rocks. In the lab, samples were soaked in water anddisaggregated using an ultrasonic device with zircons and otherminerals effectively separated from clays (Hoke et al., 2014).Further isolation followed standard procedures using high-densityliquids and magnetic separation.

Single grainUePbdatingwas conducted in the radiogenic isotopelab at the Massachusetts Institute of Technology. Individual grainswere annealedat 900 �C for60handdissolved in two steps followingthe chemical abrasionmethod (Mattinson, 2005),modified for singlegrain analyses (e.g. Rioux et al., 2012). Initial leach stepswere held at210 �C for 12e14 h. Samples were spiked with the EARTHTIME205Pbe233Ue235U tracer (ET535; www.earth-time.org). All commonPbwas assumed to be laboratory blank and corrected using isotopiccompositions determined by repeat analyses of spiked total proce-dural blanks. Isotopic analyses were carried out on a VG Sector 54mass spectrometer, following theproceduresdescribed inRiouxet al.(2010). The UePb Redux software package (Bowring et al., 2011;McLean et al., 2011) was employed for all data reduction and errorpropagation. All dates reported in the text are corrected for initialexclusion of 230Th from the 238U decay chain using the Th/U of thezircon, calculated from the measured 208Pb/206Pb, and an assumedmagma Th/U ¼ 2.8 ± 1 (2s). Uncertainties are reported as ± 2sanalytical/± 2s analytical þ tracer þ decay constant.

3.2. Paleobotany

Fossil plant specimens in this study were collected from theDakota Formation during multiple collecting trips in Utah startingin the late 1980's and early 1990's. All samples are housed in the

Fig. 3. Stratigraphic column of the Henrieville site comparing previously published sections for the local area at the same scale. The position of bentonites with UePb dates (thisstudy) and paleobotanical samples (DMNH loc. 404) are plotted against a section measured for this study. Recalibrated 40Ar/39Ar dates taken from unpublished sources are plottedagainst the master section of Kirschbaum and McCabe (1992). The two stratigraphic sections to the left are modified from Uli�cný (1999). All uncertainties are ± 2s, reportedas ± analytical uncertainties/analytical þ tracer þ decay constant.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237 217

Table 2Recalculation of historical 40Ar/39Ar sanidine dates from the Dakota Formation and overlying Tropic Shale in the region, with comparative newly measured UePb ages fromzircons. Uncertainties are 2s internal/2s internal þ standard þ decay constant.

Sample information Unpublished dates (Ma) Recalibrated dates (Ma, 2e6) Similar U/Pbsamples(this study) (7)

Unit Strat. Position Generalgeographiclocation

Samplenumber

Bohor,1991 (2)

Obrad, pers.comm.,1992 (3)

Dyman et al.,2002a,b (3)

Bohor, 1991(4e6)

Obrad, pers.comm.,1992 (4e6)

Dyman et al.,2002a,b (4e6)

TropicShale

Base S.gracile(1)

Near Henrieville,Utah

DMT89-22

90.5 ± 0.1 93.66 ± 0.44 92.9 ± 0.1/3.6 94.83 ± 0.4/3.7

DakotaFm.

17 m abovebasal sample

Near Henrieville,Utah

DMT89-25

91.5 ± 0.1 94.34 ± 0.60 94.0 ± 0.1/3.7 95.52 ± 0.6/3.8 94.879 ± 0.032/0.11 (KJ08142)

DakotaFm.

Base Near Henrieville,Utah

e 92.9 ± 0.2 95.4 ± 0.2/3.7 95.070 ± 0.036/0.12 (KJ08143)

DakotaFm.

Middle (?) 2 miles south oftown of Tropic

e 96.06 ± 0.3 97.26 ± 0.3/3.8

(1) S. gracile e Sciponoceras gracile ammonite biozone (Obradovich, pers. comm., 1992); (2) Assumed age of 513.9 Ma for MMhb standard (Lanphere et al., 1990); (3) Assumedage of 520.4 Ma for MMhb standard, sanidine etched for 5 min in 12% hydrofluoric acid prior to analysis (Obradovich, pers. comm., 1992); (4) Conversion applied betweenMMhb and Fish Canyon standards (Renne et al., 1998); (5) K decay constant of Min et al. (2000); (6) Age of Fish Canyon Tuff from Kuiper et al. (2008). (7) Samples analyzed forUePb dates are different samples than those used for Ar/Ar analysis, but are presented here for comparison for the samples that are relatively close stratigraphically. Theprecise position of the samples (with the recalibrated Ar/Ar dates) is indicated on the stratigraphic column for Henrieville in Fig. 3.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237218

Paleobotany Collections at the DenverMuseum of Nature& Science(DMNS) in Colorado, which retains the DMNH abbreviation forlocality numbers and specimen numbers, despite having changedthe name of the public side of the museum. The specimens fromHenrieville (DMNH loc. 404) were discovered and collected by KirkJohnson in 1991. At the Westwater locality, Howard and DarleneEmry collected the majority (~75%) of the ash-bed flora specimensin the late 1980's (DMNH loc. 947) and thesewere supplemented byadditional specimens collected by Johnson in 1994 (DMNH locs.1192 and 1193). Johnson also collected the floras in the mudstoneunit at Westwater (DMNH loc. 1191) in 1994.

Macrofossil plant specimens were removed from thin layers ofrock from individual mudstone and ash beds. Fossils were dug fromthe outcrop using hand tools, typically from small quarries of lessthan one meter square. No attempt was made to quantitativelysample the floras in the field. As a result, the collections at theDMNS consist of a selectively collected assemblage, one that con-tains the best preserved examples of common plant leaves andother plant parts, along with rare taxa that often are not as wellpreserved.

Leaf fossils were separated into morphotype categories basedupon venation characters (Hickey, 1971, 1973; Ellis et al., 2009) andeach category of plant morphotype was assigned a hol-omorphotype specimen. The floras in the mudstone- and ash-bedsat Westwater were stratigraphically separated by a few meters, sowere morphotyped using the same two letter prefix of WW(Westwater), followed by sequential numbers (e.g. WW001,WW002, etc.). The geochronology results were already knownwhen morphotype efforts began and given the time differencebetween the Westwater and Henrieville localities, a separate two-letter morphotype prefix was used for the fossils at Henrieville(HD for Henrieville Dakota), followed by sequential numbers. Allmorphotypes were compared to the appropriate literature to assignLinnaean nameswhere possible, and specimens from the ash bed atWestwater were compared directly to Rushforth (1971).

4. Geochronology results

4.1. UePb zircon geochronology

To determine the eruptive/depositional age of the samples, wepreferentially selected and analyzed elongate zircons, often withmelt inclusions parallel to the c-axis, which in our experience is acommon morphology for volcanic zircon. Individual samples yiel-ded a range of single grain 206Pb/238U dates, some as much as5.1 Ma older than the interpreted eruption age (Table 3 and

Figs. 4e5). The data for each sample are characterized by a cluster ofconcordant or near concordant analyses that define statisticallysignificant 206Pb/238U weighted mean dates. Three samples fromthe Westwater locality yielded weighted mean dates of97.949 ± 0.037/0.12 Ma (KJ08107), 97.943 ± 0.023/0.12 Ma(KJ08108), and 97.601 ± 0.049/0.13 Ma (KJ08105; Fig. 2 and Fig. 5).Two samples from the Henrieville locality yielded weighted meandates of 95.070 ± 0.036/0.12 Ma (KJ08143) and 94.879 ± 0.032/0.11 Ma (KJ08142; Fig. 3 and Fig. 5). We interpret these weightedmean dates as the best estimate of the eruption/depositional age ofthe tuffs. The dates are consistent with stratigraphic superposition,within analytical uncertainties.

The older zircons in each sample likely reflect either slightlyolder volcanic ejecta incorporated into the eruption column, or thepresence of small inherited cores related to formation of theparental magma. Three of the five samples also contain one ormoreanomalously young grain(s) which we interpret as reflecting re-sidual post-eruption Pb-loss, which was not removed by thechemical abrasion process.

5. Paleobotany results

Representative morphotypes from the Westwater flora are dis-played in Fig. 6 (ash-bed) and Fig. 7 (mudstone), and typical mor-photypes from the flora at Henrieville are presented in Fig. 8(mudstone). The complete set of morphotypes for all of the fossillocalities are documented in Figs. S1eS8 (online supplementarymaterial). The complete list ofmorphotypes and associate taxonomicnames are in Table 4 (Westwater) and Table 5 (Henrieville).

5.1. Westwater Flora

The mudstone and ash-bed floras from the Westwater localitywere morphotyped using the same suite of comparative hol-omorphotype specimens (WW001, WW002, etc.). Taken togetherthere were a total of 556 fossil plant specimens investigated, with488 of those identifiable to morphotype (88% identifiable, Table 6).A total of 28 different morphotypes were identified in the flora,composed of six ferns, one sphenopsid, one lycopsid, one conifer,and nineteen dicot angiosperm morphotypes. As an aggregate, theWestwater flora was dominated in abundance by ferns (87% of thespecimens), but dicot angiosperms had a higher species richness(19 dicot morphotypes vs. 6 fern morphotypes).

The ash-bed floras are greatly dominated by ferns (93.2% of leafmorphotypes), with eleven specimens of a sphenopsid (Equisetumsp., WW006), one conifer morphotype (Geinitzia sp., WW038), a

Table 3Single grain206Pb/238U zircon data.

Composition Isotopic ratios

Pb*/a Pb*b Pbcb Ub Th/c 206Pb/d 208Pb/e 206Pb*/e 2s 207Pb*/e 2s 207Pb*/e 2s corr.f

Pbc (pg) (pg) (pg) U 204Pb 206Pb 238U (%err) 235U (%err) 206Pb* (% err) coef.

KJ08105z3 7.0 4.55 0.65 236.89 1.293 366 0.41222 0.015266 0.235 0.10225 2.844 0.048599 2.787 0.282z4 13.0 9.53 0.74 597.45 0.531 791 0.16924 0.015235 0.147 0.09951 1.492 0.047395 1.445 0.358z5 14.8 22.10 1.49 1420.20 0.424 925 0.13521 0.015285 0.101 0.10134 1.078 0.048107 1.055 0.270z6 8.3 3.51 0.42 185.99 1.227 435 0.39136 0.015229 0.214 0.10174 2.860 0.048472 2.773 0.437z7 22.4 12.88 0.57 795.66 0.574 1340 0.18301 0.015284 0.097 0.10144 0.847 0.048155 0.821 0.313z9 8.2 5.49 0.67 294.27 1.175 434 0.37473 0.015229 0.231 0.10136 2.528 0.048290 2.459 0.338z10 7.5 7.83 1.05 492.27 0.552 461 0.17595 0.015110 0.181 0.10033 2.275 0.048179 2.233 0.269z16 10.3 3.39 0.33 214.18 0.506 637 0.16146 0.015233 0.142 0.10022 1.753 0.047740 1.711 0.331z17 6.0 3.49 0.58 216.95 0.557 373 0.17756 0.015236 0.230 0.10133 2.872 0.048259 2.815 0.287z18 37.1 11.38 0.31 720.83 0.487 2256 0.15531 0.015247 0.067 0.10065 0.503 0.047896 0.484 0.320z19 3.2 1.17 0.37 73.72 0.532 209 0.16966 0.015210 0.449 0.09916 5.660 0.047304 5.544 0.294z20 9.1 3.24 0.36 201.43 0.575 557 0.18341 0.015202 0.179 0.09935 2.063 0.047418 2.015 0.310z21 59.1 17.65 0.30 1118.02 0.481 3590 0.15332 0.015274 0.059 0.10090 0.363 0.047932 0.344 0.368KJ08107z11 18.2 8.32 0.46 524.65 0.464 1125 0.14810 0.015415 0.086 0.10217 0.942 0.048092 0.923 0.249z12 21.1 9.70 0.46 613.40 0.483 1289 0.15391 0.015288 0.093 0.10097 0.985 0.047921 0.953 0.375z13 18.2 7.64 0.42 496.14 0.385 1147 0.12289 0.015288 0.093 0.10124 0.957 0.048052 0.929 0.340z14 22.5 9.57 0.43 606.53 0.467 1381 0.14907 0.015324 0.079 0.10165 0.774 0.048131 0.752 0.326z15 11.3 6.12 0.54 390.91 0.445 710 0.14205 0.015298 0.138 0.10095 1.483 0.047880 1.445 0.312z16 9.2 5.32 0.58 332.19 0.535 569 0.17078 0.015282 0.151 0.10052 1.856 0.047729 1.815 0.306z17 24.0 7.13 0.30 447.27 0.508 1456 0.16194 0.015308 0.087 0.10091 0.861 0.047833 0.828 0.416z18 23.5 7.46 0.32 472.11 0.479 1440 0.15277 0.015297 0.085 0.10085 0.809 0.047839 0.786 0.318z19 12.7 4.12 0.32 262.41 0.454 789 0.14475 0.015291 0.136 0.10074 1.419 0.047803 1.369 0.413z20 10.9 4.56 0.42 266.97 0.779 629 0.24837 0.015320 0.147 0.10142 1.724 0.048037 1.680 0.334z21 22.9 5.83 0.26 378.63 0.377 1438 0.12039 0.015327 0.087 0.10176 0.847 0.048175 0.820 0.349KJ08108z1 12.9 22.93 1.77 1405.96 0.608 774 0.19409 0.015254 0.119 0.10211 1.274 0.048571 1.248 0.256z2 11.5 9.19 0.80 595.59 0.327 745 0.10417 0.015556 0.117 0.10544 1.331 0.049180 1.303 0.278z3 22.0 11.51 0.52 734.50 0.446 1359 0.14224 0.015304 0.080 0.10125 0.792 0.048006 0.772 0.287z7 12.2 4.61 0.38 246.21 1.138 647 0.36298 0.015391 0.148 0.10294 1.799 0.048529 1.747 0.386z8 19.4 17.56 0.91 1141.31 0.380 1222 0.12112 0.015303 0.096 0.10113 0.864 0.047950 0.837 0.328z10 28.2 14.82 0.52 920.84 0.519 1706 0.16550 0.015405 0.076 0.10308 0.677 0.048553 0.656 0.328z11 54.7 25.14 0.46 1571.88 0.525 3281 0.16761 0.015292 0.060 0.10116 0.347 0.047998 0.327 0.385z17 35.1 24.59 0.70 1525.86 0.552 2100 0.17620 0.015298 0.069 0.10135 0.495 0.048070 0.470 0.398z18 40.3 19.38 0.48 1214.93 0.516 2429 0.16446 0.015293 0.064 0.10095 0.463 0.047895 0.447 0.290z19 25.6 11.95 0.47 759.73 0.459 1571 0.14640 0.015303 0.075 0.10147 0.678 0.048113 0.658 0.299z20 18.0 9.20 0.51 595.78 0.392 1130 0.12518 0.015293 0.083 0.10183 0.906 0.048316 0.888 0.252z21 17.8 8.30 0.47 516.21 0.546 1075 0.17431 0.015289 0.094 0.10110 1.004 0.047981 0.975 0.346z22 60.6 26.71 0.44 1706.68 0.446 3711 0.14222 0.015291 0.058 0.10058 0.324 0.047728 0.308 0.331KJ08142z1 32.8 22.94 0.70 1346.23 0.877 1811 0.27965 0.014903 0.064 0.09960 0.560 0.048495 0.546 0.269z3 18.1 9.59 0.53 553.29 0.972 982 0.30995 0.014821 0.115 0.09804 1.118 0.047996 1.090 0.288z4 18.3 24.25 1.33 1459.03 0.797 1035 0.25434 0.014821 0.089 0.09869 0.972 0.048318 0.955 0.222z5 21.0 26.34 1.26 1548.37 0.889 1161 0.28341 0.014839 0.083 0.09838 0.865 0.048103 0.845 0.275z6 17.4 12.17 0.70 691.29 1.036 933 0.33053 0.014823 0.094 0.09861 1.115 0.048269 1.092 0.273z7 21.3 12.03 0.57 700.47 0.931 1165 0.29695 0.014824 0.083 0.09822 0.869 0.048078 0.853 0.230z8 18.8 15.04 0.80 896.54 0.841 1057 0.26841 0.014802 0.095 0.09785 1.016 0.047965 0.996 0.252z9 36.3 14.24 0.39 882.38 0.675 2101 0.21537 0.014847 0.086 0.09801 0.657 0.047900 0.643 0.212z10 8.9 6.10 0.68 351.39 0.985 495 0.31434 0.014795 0.179 0.09745 2.331 0.047790 2.287 0.278z16 20.8 6.24 0.30 356.59 1.015 1119 0.32361 0.014811 0.100 0.09758 0.993 0.047805 0.962 0.351z18 53.9 14.27 0.27 865.46 0.770 3038 0.24548 0.014812 0.071 0.09776 0.449 0.047888 0.438 0.208z19 86.5 28.33 0.33 1747.46 0.499 5218 0.15907 0.015611 0.055 0.10336 0.235 0.048042 0.215 0.427z20 9.4 5.51 0.59 317.72 0.971 521 0.30980 0.014832 0.180 0.09842 2.096 0.048148 2.044 0.328z21 77.0 30.94 0.40 1828.21 0.868 4231 0.27692 0.014836 0.086 0.09791 0.379 0.047885 0.357 0.348KJ08143z1 32.4 15.58 0.48 959.26 0.525 1952 0.16740 0.015525 0.067 0.10365 0.540 0.048443 0.523 0.293z2 16.8 7.97 0.47 529.98 0.410 1056 0.13085 0.014842 0.098 0.09741 1.037 0.047621 1.012 0.294z3 16.2 22.14 1.37 1478.60 0.341 1035 0.10887 0.015046 0.092 0.10017 0.961 0.048307 0.939 0.275z4 42.4 22.69 0.53 1495.12 0.421 2621 0.13443 0.014913 0.082 0.09972 0.455 0.048518 0.435 0.315z6 2.6 2.12 0.81 137.55 0.478 176 0.15264 0.014907 0.562 0.09832 6.773 0.047854 6.613 0.323z7 13.0 5.70 0.44 377.26 0.442 814 0.14107 0.014789 0.131 0.09544 1.470 0.046828 1.422 0.405z8 9.1 6.36 0.70 416.92 0.460 573 0.14687 0.014849 0.143 0.09753 1.802 0.047657 1.768 0.267z9 6.1 2.40 0.39 154.57 0.515 382 0.16441 0.014863 0.226 0.09764 3.034 0.047665 2.962 0.350z19 16.5 8.02 0.49 522.61 0.483 1015 0.15411 0.014847 0.109 0.09821 1.165 0.047995 1.132 0.342z20 32.0 9.01 0.28 566.74 0.616 1885 0.19641 0.014840 0.065 0.09836 0.585 0.048092 0.570 0.266z21 7.7 2.52 0.33 160.90 0.567 476 0.18103 0.014836 0.173 0.09770 2.204 0.047780 2.162 0.282z22 3.2 1.88 0.58 118.77 0.607 207 0.19357 0.014830 0.392 0.09825 5.202 0.048071 5.114 0.260z24 16.0 5.05 0.32 330.48 0.476 984 0.15191 0.014805 0.100 0.09779 1.142 0.047923 1.110 0.360z25 13.2 6.92 0.53 461.49 0.397 832 0.12675 0.014839 0.113 0.09881 1.282 0.048318 1.249 0.324z27 9.3 3.79 0.41 246.82 0.516 575 0.16458 0.014736 0.150 0.09610 1.980 0.047317 1.940 0.297z28 16.1 5.92 0.37 396.13 0.383 1020 0.12208 0.014847 0.113 0.09719 1.064 0.047496 1.039 0.270

(continued on next page)

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237 219

Table 3 (continued)

Th corrected isotopic ratios Dates (Ma) Th corrected dates (Ma)

206Pb*/g 2s 207Pb*/g 2s corr.f 206Pb/h 2s 207Pb/h 2s 207Pb/h 2s 206Pb/h 2s 207Pb/h 2s

238U (% err) 206Pb* (% err) coef. 238U (abs) 235U (abs) 206Pb (abs) 238U (abs) 206Pb (abs) % disci

KJ08105z3 0.015275 0.235 0.048570 2.787 0.280 97.669 0.227 98.85 2.68 127.5 65.6 97.727 0.228 126.1 65.6 23.4z4 0.015248 0.147 0.047352 1.446 0.357 97.468 0.143 96.32 1.37 68.1 34.4 97.555 0.143 65.9 34.4 �43.2z5 0.015299 0.101 0.048062 1.055 0.269 97.785 0.098 98.01 1.01 103.5 24.9 97.877 0.098 101.3 25.0 5.5z6 0.015239 0.215 0.048442 2.773 0.435 97.435 0.207 98.38 2.68 121.3 65.3 97.495 0.208 119.9 65.4 19.7z7 0.015298 0.097 0.048113 0.821 0.311 97.782 0.094 98.10 0.79 105.8 19.4 97.867 0.094 103.8 19.4 7.6z9 0.015239 0.232 0.048259 2.460 0.336 97.435 0.224 98.03 2.36 112.4 58.0 97.497 0.224 110.9 58.1 13.3z10 0.015124 0.181 0.048136 2.233 0.268 96.679 0.174 97.08 2.11 107.0 52.7 96.765 0.174 104.9 52.8 9.6z16 0.015247 0.142 0.047696 1.712 0.330 97.456 0.137 96.98 1.62 85.3 40.6 97.544 0.137 83.2 40.6 �14.2z17 0.015249 0.230 0.048216 2.815 0.286 97.476 0.223 98.01 2.68 110.9 66.5 97.562 0.223 108.8 66.5 12.1z18 0.015261 0.067 0.047852 0.484 0.318 97.547 0.065 97.37 0.47 93.1 11.5 97.636 0.065 90.9 11.5 �4.8z19 0.015224 0.448 0.047261 5.545 0.293 97.312 0.434 96.00 5.18 63.5 132.0 97.399 0.433 61.4 132.1 �53.2z20 0.015215 0.179 0.047376 2.015 0.309 97.261 0.173 96.17 1.89 69.3 47.9 97.346 0.173 67.2 48.0 �40.4z21 0.015288 0.060 0.047888 0.344 0.365 97.717 0.058 97.60 0.34 94.8 8.2 97.806 0.058 92.7 8.2 �3.0KJ08107z11 0.015429 0.086 0.048048 0.924 0.248 98.616 0.084 98.78 0.89 102.7 21.8 98.705 0.084 100.5 21.9 4.0z12 0.015302 0.093 0.047877 0.953 0.373 97.808 0.090 97.67 0.92 94.3 22.6 97.897 0.090 92.1 22.6 �3.7z13 0.015302 0.093 0.048006 0.929 0.339 97.805 0.090 97.92 0.89 100.8 22.0 97.898 0.091 98.5 22.0 2.9z14 0.015338 0.079 0.048087 0.752 0.324 98.038 0.077 98.30 0.73 104.7 17.8 98.128 0.077 102.5 17.8 6.3z15 0.015313 0.138 0.047835 1.446 0.311 97.873 0.134 97.65 1.38 92.3 34.2 97.963 0.134 90.1 34.3 �6.1z16 0.015296 0.151 0.047686 1.815 0.305 97.770 0.146 97.26 1.72 84.8 43.1 97.857 0.146 82.7 43.1 �15.3z17 0.015322 0.087 0.047789 0.828 0.414 97.936 0.084 97.62 0.80 89.9 19.6 98.024 0.085 87.8 19.6 �8.9z18 0.015311 0.086 0.047795 0.786 0.316 97.863 0.083 97.56 0.75 90.3 18.6 97.952 0.083 88.1 18.6 �8.4z19 0.015306 0.136 0.047758 1.369 0.412 97.828 0.132 97.46 1.32 88.4 32.5 97.918 0.132 86.2 32.5 �10.6z20 0.015332 0.147 0.047998 1.680 0.332 98.010 0.143 98.09 1.61 100.0 39.8 98.088 0.143 98.1 39.8 2.0z21 0.015342 0.087 0.048129 0.820 0.348 98.057 0.085 98.40 0.79 106.8 19.4 98.150 0.085 104.5 19.4 8.2KJ08108z1 0.015267 0.119 0.048528 1.248 0.254 97.589 0.115 98.72 1.20 126.1 29.4 97.673 0.115 124.1 29.4 22.6z2 0.015571 0.117 0.049132 1.303 0.277 99.508 0.116 101.78 1.29 155.3 30.5 99.603 0.116 153.1 30.5 35.9z3 0.015318 0.080 0.047962 0.772 0.286 97.907 0.077 97.93 0.74 98.5 18.3 97.997 0.077 96.3 18.3 0.6z7 0.015401 0.148 0.048497 1.748 0.383 98.458 0.144 99.48 1.71 124.1 41.2 98.522 0.145 122.5 41.2 20.7z8 0.015318 0.096 0.047904 0.837 0.327 97.902 0.093 97.82 0.81 95.7 19.8 97.995 0.093 93.5 19.8 �2.3z10 0.015418 0.076 0.048509 0.656 0.326 98.547 0.074 99.61 0.64 125.2 15.5 98.635 0.074 123.1 15.5 21.3z11 0.015306 0.060 0.047955 0.327 0.382 97.834 0.058 97.85 0.32 98.1 7.8 97.921 0.058 96.0 7.8 0.3z17 0.015312 0.070 0.048028 0.470 0.395 97.872 0.067 98.02 0.46 101.7 11.1 97.959 0.068 99.6 11.2 3.7z18 0.015307 0.064 0.047851 0.447 0.288 97.840 0.062 97.65 0.43 93.0 10.6 97.928 0.063 90.9 10.6 �5.2z19 0.015317 0.075 0.048068 0.658 0.297 97.903 0.073 98.13 0.63 103.8 15.6 97.993 0.073 101.6 15.6 5.6z20 0.015307 0.083 0.048271 0.888 0.250 97.837 0.081 98.47 0.85 113.7 21.0 97.929 0.081 111.5 21.0 14.0z21 0.015302 0.095 0.047938 0.975 0.344 97.811 0.092 97.79 0.94 97.2 23.1 97.897 0.092 95.1 23.1 �0.6z22 0.015305 0.058 0.047683 0.309 0.328 97.823 0.056 97.31 0.30 84.7 7.4 97.913 0.057 82.5 7.4 �15.5KJ08142z1 0.014914 0.065 0.048457 0.546 0.263 95.360 0.061 96.41 0.52 122.4 12.9 95.434 0.062 120.6 12.9 22.1z3 0.014832 0.116 0.047961 1.090 0.285 94.840 0.109 94.96 1.01 98.0 25.8 94.911 0.109 96.3 25.8 3.2z4 0.014833 0.090 0.048279 0.955 0.219 94.839 0.084 95.57 0.89 113.8 22.6 94.916 0.084 111.9 22.6 16.7z5 0.014851 0.084 0.048066 0.845 0.272 94.958 0.078 95.28 0.79 103.3 20.0 95.031 0.079 101.4 20.0 8.1z6 0.014834 0.095 0.048234 1.092 0.269 94.855 0.088 95.49 1.02 111.4 25.8 94.922 0.089 109.7 25.8 14.9z7 0.014835 0.084 0.048042 0.853 0.227 94.858 0.078 95.13 0.79 102.1 20.2 94.930 0.079 100.3 20.2 7.1z8 0.014814 0.095 0.047927 0.996 0.249 94.723 0.089 94.79 0.92 96.5 23.6 94.798 0.090 94.6 23.6 1.8z9 0.014860 0.087 0.047858 0.643 0.210 95.009 0.081 94.94 0.60 93.3 15.3 95.090 0.082 91.2 15.3 �1.9z10 0.014806 0.179 0.047754 2.287 0.277 94.678 0.168 94.42 2.10 87.8 54.2 94.748 0.168 86.1 54.3 �7.8z16 0.014822 0.101 0.047770 0.962 0.347 94.778 0.094 94.54 0.90 88.6 22.8 94.847 0.095 86.8 22.8 �7.0z18 0.014825 0.071 0.047848 0.438 0.205 94.786 0.066 94.71 0.41 92.7 10.4 94.864 0.067 90.7 10.4 �2.3z19 0.015625 0.055 0.047999 0.215 0.424 99.860 0.054 99.88 0.22 100.3 5.1 99.948 0.054 98.1 5.2 0.4z20 0.014843 0.181 0.048112 2.044 0.327 94.909 0.170 95.31 1.91 105.5 48.3 94.979 0.170 103.7 48.3 10.0z21 0.014848 0.087 0.047847 0.357 0.344 94.936 0.081 94.84 0.34 92.5 8.5 95.010 0.082 90.6 8.5 �2.6KJ08143z1 0.015539 0.067 0.048400 0.523 0.291 99.313 0.066 100.14 0.51 119.9 12.3 99.400 0.066 117.8 12.4 17.2z2 0.014857 0.098 0.047575 1.012 0.293 94.977 0.092 94.38 0.93 79.4 24.0 95.069 0.093 77.1 24.0 �19.6z3 0.015061 0.092 0.048260 0.939 0.274 96.270 0.088 96.93 0.89 113.3 22.2 96.364 0.088 111.0 22.2 15.0z4 0.014927 0.083 0.048471 0.435 0.314 95.426 0.078 96.52 0.42 123.5 10.3 95.517 0.078 121.2 10.3 22.7z6 0.014921 0.561 0.047809 6.614 0.322 95.389 0.532 95.22 6.16 91.0 156.7 95.479 0.532 88.8 156.8 �4.8z7 0.014803 0.131 0.046783 1.422 0.404 94.635 0.123 92.56 1.30 39.4 34.0 94.726 0.123 37.1 34.0 �140.1z8 0.014863 0.143 0.047612 1.769 0.266 95.018 0.135 94.49 1.63 81.2 42.0 95.108 0.135 78.9 42.0 �17.0z9 0.014877 0.226 0.047621 2.963 0.349 95.109 0.214 94.59 2.74 81.6 70.3 95.197 0.214 79.4 70.4 �16.6z19 0.014861 0.109 0.047950 1.132 0.341 95.006 0.102 95.12 1.06 98.0 26.8 95.095 0.103 95.7 26.8 3.0z20 0.014853 0.065 0.048050 0.570 0.263 94.962 0.061 95.26 0.53 102.8 13.5 95.046 0.062 100.6 13.5 7.6z21 0.014850 0.173 0.047737 2.162 0.280 94.936 0.163 94.65 1.99 87.3 51.3 95.022 0.163 85.2 51.3 �8.7z22 0.014844 0.391 0.048028 5.115 0.259 94.901 0.369 95.16 4.73 101.7 120.9 94.985 0.369 99.6 121.0 6.7z24 0.014819 0.100 0.047878 1.110 0.358 94.742 0.094 94.73 1.03 94.4 26.3 94.831 0.094 92.2 26.3 �0.3z25 0.014853 0.113 0.048271 1.249 0.323 94.954 0.106 95.68 1.17 113.8 29.5 95.047 0.106 111.5 29.5 16.6z27 0.014750 0.150 0.047272 1.941 0.296 94.304 0.140 93.17 1.76 64.2 46.2 94.391 0.140 61.9 46.2 �47.0z28 0.014862 0.113 0.047449 1.039 0.269 95.009 0.106 94.18 0.96 73.2 24.7 95.102 0.106 70.8 24.7 �29.8

a Ratio of radiogenic to common Pb.b Total radiogenic Pb, common Pb and U (picograms).c Th/U ratio calculated from 208Pb/206Pb and the206Pb/238U date of the sample.d Fractionation and spike corrected isotopic ratios.e Fractionation, spike, and common Pb corrected radiogenic isotope ratios.f Correlation coefficient of radiogenic207Pb*/235U and206Pb*/238U.g Fractionation, spike, common Pb, and Th corrected radiogenic isotope ratios: Th corrections were calculated using model Th/Uzircon calculated from 208Pb/206Pb andTh/Umagma ¼ 2.8 ± 1 (2s).

h Dates (Ma) calculated using238U/235U ¼ 137.88, and decay constants of 238U ¼ 1.5513 � 10�10 yr�1 and235U ¼ 9.8485 � 10�10 yr�1. Uncertainties are 2s absolute errors.i % discordance ¼ 100 � (100 � (206Pb/238U date)/(207Pb/206Pb date)).

Fig. 4. Concordia diagrams. Plotted ellipses and reported dates are Th-corrected, as described in the text. Ages on concordia are in Ma. Gray bands represent 2s uncertainties onconcordia based on decay constant uncertainties of 0.107% (238U) and 0.136% (235U) (Jaffey et al., 1971). Reported dates are weighted mean 206Pb/238U dates and data included in theweighted mean dates are shaded gray. All uncertainties are ± 2s and are reported as ± analytical uncertainties/analytical þ tracer þ decay constant. Plots, weighted mean dates andmean square of the weighted deviates (MSWD) of the weighted mean were generated using the UePb Redux software package (Bowring et al., 2011; McLean et al., 2011). Threeanalyses with dates >99 Ma from KJ08108, KJ08142 and KJ08143 and three analyses with uncertainties > ± 0.3 Ma from KJ08105 and KJ08143 were excluded from the plots andweighted mean calculations.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237 221

single specimen of a lycopsid (Selaginella sp., WW010) and 28specimens of dicot angiosperms (6 morphotypes, Table 6).Although the collection at Westwater in the ash-bed was notconducted using an unbiased census method, ferns clearly

dominate the assemblage. The top 4 morphotypes (all ferns) areessentially the entire flora, representing 93% of the assemblage ofleaf taxa (Table 6). The fern counts certainly overestimate theirabundance, partly because fern fronds are larger than angiosperm

Fig. 5. Th-corrected 206Pb/238U dates in millions of years (Ma). Reported dates are weighted mean 206Pb/238U dates and data included in the weighted mean calculations are shownin black. The horizontal black line and gray bar for each sample represent the weighted mean date and ± 2s analytical uncertainties. The weighted mean dates are consistent withthe stratigraphic order of the bentonites. Uncertainties and excluded analyses are the same as Fig. 4.

R.S. Barclay et al. / Cretaceous Research 52 (2015) 213e237222

and conifer leaves, but also because they are predominantly brokenfragments, both of which skew the fern counts to higher numbers.

The dominance structure is the reverse for themudstone flora atWestwater, which is almost entirely composed of dicot angio-sperms (one specimen of Sectilopteris sp., WW014), and thereforelacks the taxonomic breadth found in the ash-bed flora. There wereconsiderably fewer specimens collected from the mudstone flora(51 total, 35 identified), but the diversity is slightly higher than forthe ash-beds (15morphotypes in themudstone locality vs. 13 in theash-bed at Westwater), despite less collection intensity.

5.2. Henrieville flora

From the Henrieville flora (DMNH loc. 404), 266 fossils wereexamined, but only 134 of those specimens were identifiable tomorphotype (51% identifiable; Table 7). The low percentage isattributable to the moderate preservation of venation detail on thefossils, although for the better preserved specimens a high level ofdetail was retained. The Henrieville plant assemblage was domi-nated by angiosperms, both in abundance (97%) and species rich-ness (75%). The flora consisted of 6 dicot leaf morphotypes (119spec.), 2 fern leaf morphotypes, (4 spec.), plus 6 reproductivemorphotypes (11 spec.). Dicot morphotype HD001 was the domi-nant leaf type (47 of 123 leaf specimens, 38%), with the top fivedicot leaf morphotypes comprising 93.5% of the whole leaf flora(Table 7).

6. Systematic Paleobotany

A complete taxonomic treatment of the Dakota Formation florafrom Utah provides a challenge because ferns and angiosperms areboth dominant components. The taxonomic identity of mid-Cretaceous ferns has been much better constrained (Miller and

Hickey, 2008). For the ferns (and gymnosperms) presented here,we place them in their proper phylogenetic context, using theapproach outlined in Miller and Hickey (2008). This includescomplete morphological descriptions, a full list of synonymies, adiscussion of the diagnostic features, and previous usage of thespecies in the relevant literature. Ferns and gymnosperms are or-dered based upon their phylogenetic relationships.

Accurate taxonomic assignment of most mid-Cretaceous angio-sperms remains highly speculative, and this is the case for the an-giosperms from the Westwater and Henrieville localities. It wasbeyond the scope of this paper to determine theproper phylogeneticposition of the angiosperm morphotypes. In addition, we did notattempt to compare thefloras fromWestwater andHenrieville to thebetter studied, but still incompletely described floras from theeastern side of the Western Interior Seaway. We feel that the mostappropriate course of action is to completely illustrate the angio-sperm specimens and to precisely describe their features, butwheretheir taxonomic identity is questionable, to assign them to a “mor-photype” using the system of Johnson et al. (1989), and to presentthem as a morphotype catalog (Peppe et al., 2008; pp. 183 & 187).Categories are: 1) specimens assigned to a morphotype only, desig-nated either with a WW### (Westwater), or HD### (Henrieville);2a) Genus is incorrect or invalid, but the species name is valid (e.g.“Ilex” serrata Rushforth, 1971); 2b) Specimen associated with apreviously published, but uncertain or incorrect identification (e. g.“Platanus newberryana” Heer); 3) Attributable to a valid and legiti-mate genus and species (e.g. Landonia calliiWang and Dilcher).

Complete morphological descriptions are included to aid withsubsequent taxonomic revision, using the characters defined in theManual of Leaf Architecture (Ellis et al., 2009). In this sectionwe listthe angiosperms by organ type, with leaves organized first by theirvenation (pinnate or palmate) and then by their margin (toothed orentire). The complete flora is illustrated in Figs. S1eS8 (online

Fig. 6. Representative leaf macrofossils from the ash-bed flora at Westwater. A, Matonidium brownii (WW001 HM; DMNH 12564). B, Gleicheniaceaephyllum comptoniaefolia(WW002 HM; DMNH 13343). C, “Ilex” serrata (WW004 HM; DMNH 12679). D, Gleicheniaceaephyllum sp. aff. G. delicatula (WW005 HM; DMNH 12501). E, Astralopteris coloradica(WW003 HM; DMNH 12550). F, Selaginella sp. (WW010 HM; DMNH 12837). Scale bars 1 cm.

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Fig. 7. Representative leaf macrofossils from the mudstone flora at Westwater. A, WW021 HM (DMNH 12911). B, WW037 HM (DMNH 35837). C, WW011 HM (DMNH 12920). D, aff.Liriodendron sp. A (WW012 HM; DMNH 12915). E, WW013 HM (DMNH 12918). F, WW016 HM (DMNH 12944). Scale bars 1 cm.

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supplementary material), or highlighted in Figs. 6e8 of the textbecause they are dominant.

A few species/morphotypes are described in detail below. Theseare included in the main text because we can confidently provide ataxonomic placement for the specimens, and they have alsonot been described before from the applicable literature for thesites. The entire flora is figured and described in the online sup-plementary material.

6.1. Lycophytes

Class: Lycopodiopsida Bartling, 1830.Order: Selaginellales Prantl, 1874.Family: Selaginellaceae Willkomm, 1854.Genus: Selaginella Palisot de Beauvois, 1805.Species: Selaginella sp.

Morphotype: WW010; Fig. 6F, Fig. S2B

Fig. 8. Representative leaf macrofossils from the mudstone flora at Henrieville. A, HD010 HM (DMNH 13408). B, aff. Liriodendron sp. B (HD001 HM; DMNH 12948). C, HD009 HM(DMNH 13391). D, HD007HM. E, HD014 HM (DMNH 13451). F, HD006 HM (DMNH 5924). G, HD002 HM (DMNH 13395). H, HD013 HM (DMNH 13407). Scale bars 1 cm.

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Table 4Listing of species and morphotypes present in the Westwater flora (DMNH locs. 947, 1191, 1192, & 1193) of the Dakota Formation. Abbreviations: i.s. ¼ incertae sedis;cp ¼ counterpart.

Morphotype Organ DMNH number Order/Group Family Genus and species Epithet Figure Name in Rushforth (1971)

WW001 Leaf 12564 Polypodiales Matoniaceae Matonidium brownii 6A, S1A Matonidium browniiWW002 Leaf 12679; cp13343 Gleicheniales Gleincheniaceae Gleicheniaceaephyllum comptoniaefolia 6B, S1B Gleichenia comptaniaefoliaWW003 Leaf 12550 Polypodiales Polypodiaceae Astralopteris coloradica 6E, S1D Astralopteris coloradicaWW004 Leaf 12679; cp13343 Dicot i.s. “Ilex” serrata 6C, S3A Ilex serratusWW005 Leaf 12501 Gleicheniales Gleincheniaceae Gleicheniaceaephyllum sp. aff. G. delicatula 6D, S1E Gleichenia delicatulaWW006 Leaf 12643; cp12646 Equisetales Equisetaceae Equisetum sp. S2A Equisetum lyelliWW007 Leaf 12655; cp12654 Dicot i.s. i.s. S4AWW008 Leaf 12652; cp12676 Dicot i.s. i.s. S4BWW009 Leaf 12832; cp12825 Fern i.s. Sectilopteris dicksonianum S1E Asplenium dakotensisWW010 Leaf 12837 Selaginellales Selaginellaceae Selaginella sp. 6F, S2BWW011 Leaf 12920; cp12928 Dicot i.s. i.s. 7C, S6AWW012 Leaf 12915 Dicot i.s. aff. Liriodendron sp. A 7D, S7AWW013 Leaf 12918; cp12919 Dicot i.s. i.s. 7E, S7BWW014 Leaf 12930; cp12940 Fern i.s. Sectilopteris sp. S1FWW015 Leaf 12926; cp12923 Dicot i.s. i.s. S7CWW016 Leaf 12944 Dicot i.s. i.s. 7F, S4CWW017 Leaf 12933; cp12929 Dicot i.s. i.s. S6BWW018 Leaf 12942 Dicot i.s. i.s. S4DWW020 Leaf 12903; cp12924 Dicot i.s. i.s. S7DWW021 Leaf 12911 Dicot i.s. i.s. 7A, S4EWW033 Leaf 35836 Dicot i.s. i.s. S5AWW034 Leaf 12686 Dicot i.s. i.s. S5BWW035 Leaf 12846 Dicot i.s. i.s. S6CWW036 Leaf 12912 Dicot i.s. i.s. S5CWW037 Leaf 35837 Dicot i.s. i.s. 7B, S3BWW038 Leaf 35835 Pinales i.s. Geinitzia sp. S2CWW040 Leaf 12934 Dicot i.s. i.s. S7EWW041 Leaf 12937 Dicot i.s. i.s. S7F

TabList

M

HHHHHHHHHHHHHH

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Morphotype exemplar: DMNH 12837, DMNH loc. 1192

Description: Dichotomizing plant stem, branches spacedapproximately 1 cm apart, stem stout and coalified. Leavesdimorphic, appearing anisophyllous, overlapping, appearingspirally arranged, arranged in at least 2 ranks, potentially asmany as 3 or 4 ranks, high rank leaves spreading, lower rankleaves appressed; leaf length up to 2 mm, width up to 1.5 mm,L:W ratio 1:0.75, leaf shape round or ovate or oblong; leafvenation largely obscured, where partially visible appearing as ifone or two thin veins enter the leaf. Fertile material unknown.

Discussion: Based on the stem branching, which dichotomizesregularly and repeatedly, leaf size, shape, consistency of leaf sizeand shape, and weak leaf venation, we attribute this fossil to thelycopod clade. The pronounced leaf dimorphism and potentialanisophylly exhibited by the specimen (Fig. 6F), demonstratesthat it belongs to the Selaginellaceae as opposed to the Lyco-podiaceae, which shows monomorphic leaves. We assign thespecimen to Selaginella because it shows no vegetative

le 5ing of species and morphotypes present in the Henrieville flora (DMNH loc. 404) of t

orphotype Organ DMNH# Order

D001 Dicot leaf 12948 MagnolialesD002 Dicot leaf 13395 i.s.D003 Dicot reproductive 13352 i.s.D004 Dicot reproductive 13466 i.s.D005 Reproductive 13450 i.s.D006 Dicot leaf 05924 i.s.D007 Dicot reproductive 24612 i.s.D008 Fern leaf 13460 aff. PolypodiD009 Dicot reproductive 13391 i.s.D010 Dicot reproductive 13408 i.s.D011 Reproductive 13465 i.s.D012 Reproductive 13467 i.s.D013 Dicot reproductive 13407 i.s.D014 Fern leaf 13451 Salviniales

characters that may be used to distinguish it from this extantgenus. Precedent exists for the presence of this genus in themid-Cretaceous; numerous examples have been found ofMesozoic and Paleozoic age on multiple continents (Watson,1969; Ash, 1972; Thomas, 1992; Banks, 2009). Though manyfossil species have been named in the genus, we do not attemptfurther identification, since only one sterile stem has thus farbeen recovered from the Westwater locality.

6.2. Pteridosperms

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998.Subdivision: Pteridophytina Engl., 1886.Class: Polypodiopsida Cronquist, Takhtajan, and Zimmerman,1966.Order: Gleicheniales Link, 1833.Family: Gleicheniaceae C. Presl, 1825.Genus: Gleicheniaceaephyllum Crabtree emend. Nagalingum andCantrill, 2006.

he Dakota Formation. Abbreviations: i.s. ¼ incertae sedis.

Family Genus and species Figure

Magnoliaceae aff. Liriodendron sp. B 8B, S7Gi.s. i.s. 8G, S5Di.s. i.s. S8Ai.s. i.s. S8Bi.s. i.s. S8Ci.s. i.s. 8F, S6Di.s. i.s. 8D, S8D

ales i.s. i.s. S1Gi.s. i.s. 8C, S5Ei.s. i.s. 8A, S3Ci.s. i.s. S8Ei.s. i.s. S8Fi.s. i.s. 8H, S5Fi.s. i.s. 8E, S1H

Table 6Numerical counts of Westwater macrofloral specimens contained in the collections at the Denver Museum of Nature& Science. Data is comprehensive, but has been separatedinto the ash-bed and mudstone floras. Non-leaf morphotypes removed for % leaf flora calculation. Abbreviations: i.s. ¼ incertae sedis.

Morphotype number Genus and species (if known) Plant group Number of specimens % Leaf Flora

Ash-bed flora (DMNH locs. 947, 1192, 1193)WW002 Gleicheniaceaephyllum comptoniaefolia Fern 204 46.15WW001 Matonidium brownii Fern 129 29.19WW003 Astralopteris coloradica Fern 53 11.76WW005 Gleicheniaceaephyllum sp. aff. G. delicatula Fern 25 5.66WW006 Equisetum sp. Equisetum 11WW007 i.s. Dicot 8 1.81WW004 “Ilex” serrata Dicot 7 1.58WW033 i.s. Dicot 7 1.58WW037 i.s. Dicot 5 1.13WW009 Sectilopteris dicksonianum Fern 1 0.23WW010 Selaginella sp. Lycopsid 1 0.23WW034 i.s. Dicot 1 0.23WW038 Geinitzia sp. Conifer 1 0.2313 Taxa 453 Total SpecimensMudstone flora (DMNH loc. 1191)WW021 i.s. Dicot 9 25.71WW011 i.s. Dicot 4 11.43WW015 i.s. Dicot 4 11.43WW013 i.s. Dicot 3 8.57WW016 i.s. Dicot 3 8.57WW012 aff. Liriodendron sp. A Dicot 2 5.71WW017 i.s. Dicot 2 5.71WW008 i.s. Dicot 1 2.86WW014 Sectilopteris sp. Fern 1 2.86WW018 i.s. Dicot 1 2.86WW020 i.s. Dicot 1 2.86WW035 i.s. Dicot 1 2.86WW036 i.s. Dicot 1 2.86WW040 i.s. Dicot 1 2.86WW041 i.s. Dicot 1 2.8615 Taxa 35 Total Specimens

TabNumheldi.s.

H

Mn

HHHHHHHHHHHHHH

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Species: Gleicheniaceaephyllum comptoniaefolia (Debey andEttingshausen) Barclay et al. comb. nov.

Morphotype: WW002; Fig. 6B, Fig. S1BMorphotype exemplar: DMNH 13343, DMNH loc. 947Referred Material: DMNH 12812 (loc. 1192), DMNH 12813(loc. 1192). Resting bud examples - DMNH 12452 (loc. 1192),DMNH 35841 (loc. 1192).Synonymy: ≡ Didymosorus comptoniaefolia Debey andEttingshausen, 1859, Pl. 1, figs. 1-5. ¼ Gleichenia comp-toniaefolia (Debey and Ettingshausen) Rushforth, 1971, pg.20; figs. 8-6, 10-1, 10-6, 12-1, 12-3, 13-1, 13-3, 14-3, 20a.

le 7erical counts of macrofossil plant specimens from Henrieville (DMNH loc. 404)in the collections at the Denver Museum of Nature & Science. Abbreviations:

¼ incertae sedis.

enrieville flora (DMNH loc. 404)

orphotypeumber

Genus and species(if known)

Plantgroup

Number ofspecimens

% LeafFlora

D001 aff. Liriodendron sp. B Dicot 47 38.21D002 i.s. Dicot 4 3.25D003 i.s. Seed 1 e

D004 i.s. Seed 1 e

D005 i.s. Seed 1 e

D006 i.s. Dicot 21 17.07D007 i.s. Seed 2 e

D008 i.s. Fern 1 0.81D009 i.s. Dicot 14 11.38D010 i.s. Dicot 22 17.89D011 i.s. Seed 1 e

D012 i.s. Seed 5 e

D013 i.s. Dicot 11 8.94D014 i.s. Fern 3 2.44

134 Total Specimens 123 spec.

Description: Complete fronds unknown. Frond appearingbipinnate or once pinnateepinnatifid to pinnatisect, appearingimparipinnate, length in excess of 10 cm, width in excess of10 cm; rachis stout, width up to 1 mm, bifurcates at approxi-mately 90�, with a resting bud located in the axil of thebifurcation; costa stout, width up to 2.5 mm, appearing sulcateon costa in which the coalified stem is removed, linear tocurvilinear. Pinnae pinnate to pinnatifid or pinnatisect, lengthup to 12 cm, but commonly 6e8 cm or shorter, maximumwidth ~1 cm, shape linearelanceolate, rachial attachmentalternate to subalternate, forming approximately 90� anglethat decreases slightly toward the apex, dissection typicallycatadromous though often appearing equal. Pinnules oppositeto subopposite, length up to 5 mm, width up to 3 mm, sizedecreasing toward apex, l/w ratio 1:0.6, oblong and typicallylinear, occasionally slightly falcate, base truncate, broadly andevenly attached to costa, apex rounded or pointed, sinusesangular or occasionally rounded, margins entire and some-what undulating, attachment angle approximately 60e80�.Midvein present, two orders of venation, midvein slightlydeflected at the point where second order veins originate;second order venation branched once or rarely twice, occa-sionally arising from the costa, veins open, terminating atmargin, dissection appearing catadromous. Fertile morphologyunknown.

Discussion:WefollowRushforth (1971) andattribute this species toDidymosorus comptoniaefolia Debey and Ettingshausen (1859)based on overall frond organization, pinnule shape, size andvenation, and size, position and distribution of sori, though thepresent collection does not show fertile structures. Both theRushforth (1971) material and the present collection contain ex-amples of fronds with rachi that dichotomize. In the axil of these

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dichotomies are resting buds, which are diagnostic of the Glei-cheniaceae (Nagalingum and Cantrill, 2006). As a result, werecombine this species to Gleicheniaceaephyllum comptoniaefolia(Debey and Ettingshausen) Barclay et al. comb. nov. following theemended diagnosis of Gleicheniaceaephyllum, which is used forfossil ferns attributable to the Gleicheniaceae (Nagalingum andCantrill, 2006).

Sterile and fertile fossil foliage of gleicheniaceaous affinity havebeen overspeciated. For example, Heer (1868, 1874, 1882) nameddozens of species of gleicheniaceaous ferns for what are likely onlya handful of true biological species. An up-to-date comprehensivesynonymy of G. comptoniaefolia is beyond the scope of the currentstudy, so we direct the reader to Rushforth (1971) for any futureattempt at such a synonymy. G. comptoniaefolia is the most abun-dant taxon in the present collection at the Westwater locality. Itoccurs exclusively in the ash bed deposits often overlying or un-derlying Matonidium brownii. As noted by Rushforth (1971), thisspecies forms an important part of the successional flora colonizingthe ash fall deposits.

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998Subdivision: Pteridophytina Engl., 1886 Class: PolypodiopsidaCronquist, Takhtajan, and Zimmerman, 1966.Order and Family: incertae sedisGenus: Sectilopteris Miller and Hickey, 2008Species: Sectilopteris dicksonianum (Heer) Barclay et al. comb.nov.

Morphotype: WW009; Fig. S1EMorphotype exemplar: DMNH 12832, DMNH loc. 1192Synonymy:≡ Asplenium dicksonianumHeer, 1874, pg. 31, Pl. 1, figs. 1 (excl.b, c), 1aa, 2-4, 5 (excl. a, b).¼ Asplenium johnstrupi Heer, 1874, pg. 32, Pl. 1, figs. 6, 6b, 7,7b.¼ Asplenium nordenskiӧldi Heer, 1874, pg. 33, Pl. 2, fig. 17a, b.¼ Asplenium sp. Brown, 1950, pg. 48, Pl. 10, fig. 4.¼ Asplenium dicksonianum (Heer) Rushforth,1971, pg.16, figs.8-4, 11-3, 12-2, 15-4.¼AspleniumdakotensisRushforth,1971, pg.16,figs.11-5,12-6.

Description: Single specimen. Complete fronds unknown.Frond organization pinnate to pinnatifid/pinnatisect tobipinnate, appearing imparipinnate; length in excess of100 mm; width in excess of 80 mm; rachis stout, widthapproximately 1.5 mm, sulcate, slightly winged particularlyimmediately below pinnae; costa gracile, costal width<0.5 mm, more or less dichotomously branched into pinnules.Pinnae pinnate to pinnatifid or pinnatisect; length up to ~3;width up to 10 mm; shape oblong; attachment alternate,forming a super-adjacent angle of approximately 30e45� withthe rachis; dissection typically catadromous. Pinnules oppo-site to subopposite; length up to 10 mm; width up to 3 mm,L:W ratio 1:0.33; shape ovate to elliptical or occasionallyobovate; base decurrent, attachment broad; apex pointed orless frequently rounded, generally straight; sinuses angular;margins entire. Ultimate laminar divisions without midveins;one order of open dichotomous venation, appearing to have~3 forks, terminating at the margin. Fertile morphologyunknown.

Discussion: This species belongs to the population of specimensattributed to Asplenium dicksonianum Heer by Rushforth (1971)and Asplenium dakotensis Rushforth (1971) based on frond or-ganization, and pinnule shape, size and venation. All of thespecimens, including the synonymized material from other

authors, lack fertile remains. As a result, this group of fossilscannot be confidently assigned to the extant genus Asplenium.We elect to recombine the species in the morphogenus Secti-lopteris Miller and Hickey (2008). This genus was established tocollect the fossil sterile fern foliage that fell within thefollowing diagnosis: “Sterile ferns of uncertain relationship.Fronds bipinnate to tripinnate, margins lobed, toothed orentire, ultimate laminar divisions dichotomous, narrow andwithout mid-veins, veins dichotomous and open, veinstapering uniformly to their terminations” (Miller and Hickey,2008; pg. 171).

We have chosen to include A. dakotensis within S. dicksonianumbecause Rushforth (1971) based the species on distinguishing fea-tures that we consider to be within the range of S. dicksonianum.Moreover, specimens of A. dakotensis are rare and poorly preserved.Sectilopteris dicksonianum occurs exclusively in the ash bed de-posits at Westwater. It is represented by only three specimens inthe current collection.

6.3. Aquatic Pteridosperms

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998Subdivision: Pteridophytina Engl., 1886Class: Polypodiopsida Cronquist, Takhtajan, and Zimmerman,1966Order: cf. SalvinialesFamily, and Genus: incertae sedisSpecies: incertae sedis

Morphotype: HD014; Fig. 8E, Fig. S1HMorphotype exemplar: DMNH 13451, DMNH loc. 404.

Description: Known only from a single, incomplete laminarsegment that appears to represent a distal section of a pinnule.Pinnule appears palmate; length and width of preserved ma-terial is 11 mm and 15 mm, respectively; attachment, orienta-tion, and base and sinus shape are unknown; margins toothedwith large, serrate, irregular and non-glandular teeth; appearingwithout a midvein; one order of anastomosing venation, ter-minating at the margin. Fertile morphology unknown.

Discussion: This species is represented by a single, incompletelaminar segment that appears to be the distal section of apinnule. The pinnule appears palmate and has one-order ofanastomosing venation that terminates at the margin. The teethon the margin of the specimen are relatively large compared tothe pinnule size and are serrate, irregular and non-glandular(Fig. 8D). Based on the characteristics of the venation andoverall pinnule shape, size, and margin, this specimen may bean aquatic fern in the Salviniales. However, without additionalmaterial, we leave assignment to the class level. At present, wedo not have a form genus like Sectilopteris or Furcillopteris thatwe can use to classify the specimen. As a result, it is identifiedonly by its morphotype number (HD014). The specimen ofHD014 represents only the second fern specimen in the collec-tion from Henrieville.

6.4. Conifers

Subdivision: Pinophytina Cronquist, Takhtajan, and Zimmer-man, 1966Superclass: Pinopsae Miller and Hickey, 2010Class: Pinopsida Burnett, 1835Order: Pinales Gorozhankin, 1904Family: Incertae sedis

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Genus: GeinitziaSpecies: Geinitzia sp.

Morphotype: WW038; Fig. S2CMorphotype exemplar: DMNH 35835, DMNH loc. 1192.

Description: Partial branch, width of main stem up to 2 mm,bearing alternate shoots, branch appearing furrowed. Leaf-bearing shoots compound or simple, when compound organi-zation subopposite, length up to 25 mm, width up to 4 mm,rachis appearing gracile. Shoots appear fertile and probablyovulate, consisting of sterile bracts subtending tightly packedprobably spirally arranged swollen structures that are inter-preted as ovules. Bracts appearing approximately as long as theyare wide, length and width ~1.3 mm, apex acute or apiculate;base unknown, attachment unknown; bracts spreading. Vena-tion unknown. Ovules appearing obscured by bracts, estimatedheight 0.75 mm, width 0.75 mm, shape more or less spherical.

Discussion: This fossil represents a small section of a coniferbranch bearing what appear to be fertile shoots with spirallyarranged bracts subtending swollen structures that we interpretas ovules. The arrangement, shape and size of ovulate structures,plus the overall arrangement of the branch and shoots suggestthat this specimen is a gymnosperm in the order Pinales.However, given that the specimen is highly compressed, doesnot preserve venation of the sterile foliage, the bracts obscurethe ovules, and the limited amount of material recovered, we areunable to definitively assign it to family. Since the bracts arespreading and are about as long as they are wide, we provi-sionally assign the specimen to the morphotaxon Geinitzia.

Rushforth (1970,1971) did not report any conifer fossils from theWestwater section. As a result, this single specimen in the currentcollection expands the range of plants recovered from the area. Thefossil occurs within the ash beds amongst the herbaceous, earlysuccessional plants that make up the majority of fossils from thesite. Despite it being a single, partial specimen, we interpret it as alikely member of the woody late successional plants that colonizedthe stable environment that existed at Westwater prior to theash fall.

6.5. Angiosperms

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998Subdivision: MagnoliophytinaClass: MagnoliopsidaOrder: MagnolialesFamily: MagnoliaceaeGenus: aff. LiriodendronSpecies: aff. Liriodendron sp. A.

Morphotype: WW012; Fig. 7D, Fig. S7AMorphotype exemplar: DMNH 12915 (part and counterpart);DMNH loc. 1191.

Distinguishing features: Three prominent midveins that areclearly primary in gage. The lateral two gently arch towards theapex, forming brochidodromous loops. No true secondaries onmidvein present until above the midpoint of the lamina. Sec-ondaries emerging from lateral primaries form distinct brochi-dodromous loops. These latter features help to distinguish fromWW036, which has much finer gage of eucamptodromousveins, and are thus classified as secondaries.

Description: Lamina incomplete; mesophyll; shape obovate.Margin pinnately lobed and untoothed. Apex not well pre-served, but apical sinus is present andmidvein terminates at leafapex. Base not preserved. Primary venation actinodromous.

Major secondaries simple brochidodromous; attachmentexcurrent. Marginal secondaries present. Intercostal tertiaryveins reticulate to occasionally sinuous opposite percurrent.Quaternary vein fabric irregular reticulate. Quinternary veinfabric irregular reticulate. Good areolation development. Mar-ginal ultimate venation looped.

Discussion: The fossils show the emarginate apex and lobing thatare the hallmark features of both species of modern Liriodendron[Liriodendron chinense (Hemsl.) Sarg. and Liriodendron tulipi-fera], but the fossils also have many characteristics which aresignificantly different from the modern genus, especially at thehigher orders of venation. Based upon the depth of lobing in thefossils, they are more similar to L. chinense than to L. tulipifera.Liriodendron chinense is distinct due to the parting of an inter-secondary vein that courses directly to the sinus, separatinginside of the margin and feeding the intercostal area to eitherside of the sinus of each lobe; the fossils have secondaries thatfeed an intermarginal vein at the sinus, but not at the base of thesinus. Also L. chinense has decurrent secondary vein attachmentto the primary vein, which is the case forWW012, and to a lesserdegree in HD001, but more commonly in the basal half of theleaf. The strongly developed reticulum of 4th order (þ5th or-der?) in both fossil morphotypes is not as well developed in themodern L. chinense, but the comparison is somewhat hamperedbecause the 3rd order veins are more dominant (alternate per-current), but there is at least a tendency for L. chinense to havepolygonal ultimate veins in the aereoles, before extending freelyending veinlets. These differences preclude assigning the fossilsto the modern genus of Liriodendron, despite the generallysimilar aspect of the leaf morphology. We have not attempted acomplete survey of fossil Liriodendron-type leaves, of whichthere are many genera described from the Upper Cretaceous ofNorth America, so only describe the features of the two fossilmorphotypes and suggest that their affinity to Liriodendron.

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998Subdivision: MagnoliophytinaClass: MagnoliopsidaOrder: MagnolialesFamily: MagnoliaceaeGenus: aff. LiriodendronSpecies: aff. Liriodendron sp. B.

Morphotype: HD001; Fig. 8B, Fig. S7GMorphotype exemplar: DMNH 12948; DMNH loc. 404.Referred Material: DMNH 13471 (loc. 404)

Distinguishing features: Actinodromous venation and a trun-cated apex characterize this morphotype as well as WW012.HD001 has a larger number of secondaries and intersecondariesalong the primary vein, better development of agrophic veins onthe lateral primaries, and a greater degree of lobing than onWW012. Both have a reticulum of venation at the tertiary andhigher vein levels.

Description: Leaf attachment petiolate. Blade attachmentmarginate. Lamina 130 mm long, ~140 mm wide; mesophyll;L:W ratio ~1:1; laminar shape probably obovate; mediallysymmetric; base symmetrical. Margin palmately lobed anduntoothed. Apex angle reflex; shape emarginate lobed. Baseangle obtuse; shape concavo-convex. Primary venation supra-basal actinodromous; naked basal veins absent; agrophic veinssimple. Major secondaries eucamptodromous becoming bro-chidodromous; spacing irregular; variation of secondary angleuniform; attachment excurrent. Interior secondaries absent;minor secondary course simple brochidodromous; proximal

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course parallel to major secondaries. Intersecondaries greaterthan 50% of subjacent secondary; distal course parallel to majorsecondary; greater than one per intercostal area. Intercostaltertiary veins opposite percurrent; convex to midvein; veinangle inconsistent. Epimedial tertiaries not well preserved.Exterior tertiary course looped. Quaternary vein fabric irregularreticulate. Quinternary vein fabric irregular reticulate. Gooddevelopment of areolation. Marginal ultimate venation looped.

Discussion: See comparison of the two fossil morphotypes of aff.Liriodendron to the modern species, Liriodendron chinense andLiriodendron tulipifera, in the discussion of WW012 above.

Division: Tracheophyta Sinnott, 1935 ex Cavalier-Smith, 1998Subdivision: MagnoliophytinaClass: MagnoliopsidaOrder: ProtealesFamily and Genus: incertae sedisSpecies: incertae sedis

Morphotype: HD007; Fig 8D, Fig. S8DMorphotype exemplar: DMNH 24612, DMNH loc. 404.

Description: Two fragmentary catkins showing several complete,

attached, staminate or pistillate, globose inflorescences bornalternately on a central stalk. Stalks thick and coalified, butotherwise nondescript; up to 4 mm in width; length unknown;stalk apex and base unknown. Inflorescence attachment appearsalternate and appressed to the stalk; spacing appearing uniformat ~7 mm. Inflorescences globose or spherical; diameter ~8 mm;containing an aggregate of minute flowers; arrangement un-known; flowers indeterminate staminate or pistillate.

Discussion: Catkin stalks with alternately and regularly arrangedglobose inflorescences similar to extant Platanus catkins arecommon in the mid-Cretaceous (e.g. Hickey and Wolfe, 1975;Doyle and Hickey, 1976; Hickey and Doyle, 1977; Crane et al.,1986; Friis et al., 1988; Crane et al., 1993; Pederson et al.,1994). These catkins have been assigned to at least four generaincluding Platanocarpus, Platananthus, Aquia, and Hamatiadistinguished by sex, number of flowers in the globose in-florescences, and whether the inflorescences are sessile orstalked (Crane et al., 1993; Pederson et al., 1994). The catkinsfrom the Henrieville flora clearly belong to one of these generabased on their macromorphology; however, given the limitedmaterial, which precludes determining whether the flowers arepistillate or staminate, we have elected to leave classificationonly to the ordinal level.

Several studies have argued that mid-Cretaceous catkins withthis morphology belong to the same plant as Sapindopsis based onassociation and similar cuticle structure (Hickey and Wolfe, 1975;Hickey and Doyle, 1977; Crane et al., 1986; Friis et al., 1988; Craneet al., 1993; Pederson et al., 1994). Several Sapindopsis-like leafmorphotypes occur in the Henrieville flora, but we did not findthat the leaves belonged to that genus. Without a direct associa-tion with the foliage, we are not confident in placing this mor-photype within Sapindopsis. Moreover, the Henrieville flora isyounger than other occurrences of Sapindopsis, which are typicallyAlbian in age.

7. Discussion

7.1. Age of the Dakota Formation in Utah

Weighted mean 206Pb/238U dates from the two localities rangefrom97.949±0.037/0.12Ma to97.601±0.049/0.13Ma atWestwater

and 95.070 ± 0.036/0.12 to 94.879 ± 0.032/0.11 Ma at Henrieville.Uncertainties are ± 2s analytical/± 2s analytical þ tracer þ decayconstant. The Cenomanian Stage spans more than six million years,beginning at 100.5 Ma, ending at 93.9 Ma (Gradstein et al., 2012;Meyers et al., 2012). Thus, the new UePb dates are consistent withCenomanian deposition for the Dakota Formation at both West-water and Henrieville, as originally suggested by palynology for theHenrieville section (May and Traverse, 1973), but the UePb datesprovide much greater precision. The new UePb dates indicate a2.53 ± 0.061 Ma age difference between the Henrieville and West-water localities.

To compare the new UePb dates to the unpublished 40Ar/39Ardates from the Henrieville locality, it was necessary to recalculatethe 40Ar/39Ar dates using modern standard and decay constantvalues (Table 2). To recalculate the dates, we used the proposed Kdecay constant of Min et al. (2000) and the recalibrated date for theFish Canyon Tuff from Kuiper et al. (2008). We assumed originalMcClure Mountain hornblende (MMhb; Alexander et al., 1978;Samson and Alexander, 1987) standard values of 513.9 Ma forBohor's dates (Lanphere et al., 1990; Bohor, 1991) and 520.4 Ma forthe Obradovich (pers. comm.,1992) and Dyman et al. (2002b) dates,and converted between theMMhb and Fish Canyon standards usingRenneet al. (1998). Uncertainties for the recalculated 40Ar/39Ardatesare reported as ± 2s internal/± 2s internal þ standard þ decayconstant.

Bohor's samples (1991; Table 2) produce recalibrated dates of95.4 ± 0.2/3.7 Ma (basal Dakota), 94.0 ± 0.1/3.7 Ma (17 m intoDakota) and 92.9 ± 0.1/3.6 Ma (Tropic shale). The two samplesreanalyzed by Obradovich produce recalibrated dates of95.52 ± 0.6/3.8 Ma and 94.83 ± 0.4/3.7 Ma (Obradovich, pers.comm., 1992). The date reported by Dyman et al. (2002a, 2002b)from the Dakota Formation south of Tropic, Utah, yielded a recali-brated date of 97.26 ± 0.3/3.8 Ma.

The two bentonites from the Henrieville location dated in thisstudy come from a similar stratigraphic position to the lowermostsamples dated by Bohor (1991) and Obradovich (pers. comm.,1992;Fig. 3). The UePb and 40Ar/39Ar dates overlap within external un-certainties, however, the large external uncertainties for therecalculated 40Ar/39Ar dates, which result from large uncertaintiesin the K decay constant, preclude detailed comparison. The offsetbetween dates in each system can be compared more precisely, andwill be discussed in terms of sediment accumulation rates below.

The UePb dates presented here provide the first precise con-straints for the depositional age of the ash-beds and mudstonefloras at Westwater. The only existing temporal constraints on theDakota Formation at Westwater come from palynological samplesinterpreted to span from the late Albian (first occurrences of mul-tiple taxa e see Currie et al., 2008) to above the Cen-omanianeTuronian boundary. At present it is not known how theinterval section that contains the mudstone- and ash-bed floras(Fig. 2) fits into the stratigraphic framework created by Currie et al.(2008), but future work will facilitate placing the interval section ofFig. 2 into the surrounding geologic context.

7.2. Sediment accumulation rates

The geochronological control allows us to estimate sedimentaccumulation rates for the Middle Member of the Dakota Forma-tion based on rates for the lower portion of the section.We considerthese to be reasonable estimates for the depositional duration ofthe undivided Dakota Formation at Westwater, and the MiddleMember of the Dakota Formation at Henrieville. We calculatedrates using the oldest and youngest samples to integrate thepotentially highly variable rates of deposition for individual

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lithologies, as well as different amounts of post-depositionalcompaction.

At Westwater, the base of the stratigraphically-lowest datedbentonite (KJ08107) is indistinguishable from the one directlyabove it (KJ08108) and 0.348 ± 0.061 Ma older than the top of thestratigraphically-highest dated bentonite (KJ08105), representedby 7.58 m of section (Fig. 2). This yields a range of sediment accu-mulation rates from 1.9 to 2.6 cm/ka for the Dakota Formation atWestwater. If we extrapolate this range of accumulation rates overthe 47.5 m thick section presented in Figure 3 of Currie et al. (2008),it results in a duration of 1.80e2.57 Ma for the (undivided) DakotaFormation at Westwater.

The same calculation of sediment accumulation rate was madefor Henrieville. KJ08143 is 0.191 ± 0.048 Ma older than KJ08142,represented by 9.28 m of rock (Fig. 3). This provides a range inaverage sediment accumulation rate from 3.9 to 6.5 cm/ka, nearlydouble the rate calculated atWestwater. The measured thickness ofthe Middle Member of the Dakota Formation near Henrievillevaries depending upon location; Kirschbaum and McCabe's (1992)master section (52.5 m) was intermediate in thickness betweenUli�cný’s (1999) Henrieville West section (55 m) and the HenrievilleEast section (46.5 m). Using the range of the average accumulationrates calculated above, the minimum duration represented by theMiddle Member of the Dakota Formation is then 0.72 Ma (Eastsection), with a maximum duration of 1.42 Ma (West section). Themaximum duration of 1.42 Ma from the Henrieville West sectionmay be a more reliable estimate because Unit 1 (Lower Dakota) ofUli�cný (1999) is not represented in the Henrieville East section,suggesting it is partially incomplete, and the longer duration iscloser to the average value of Kirschbaum and McCabe (1992).

The recalibrated 40Ar/39Ar dates from Henrieville predict muchslower sediment accumulation rates than the new UePb results.The two bentonites dated by Bohor (1991) are separated by 17 m ofsection and the recalculated dates record a duration of 1.4 ± 0.22million years. The 40Ar/39Ar data require a sediment accumulationrate of 1.05e1.44 cm/ka, significantly slower than the 3.9e6.5 cm/ka predicted by the new UePb dates. Sediment accumulation ratesusing the 40Ar/39Ar dates require that the deposition of the entiresequence at Henrieville West would have taken more than 4.5 Ma.This is odds with existing correlation schemes (Uli�cný, 1999; Laurinand Sageman, 2007) and is thus not considered further.

7.3. Ecologic and biologic significance of the Westwater andHenrieville floras

Ferns are overwhelmingly the most abundant fossils found inthe ash-bed flora at Westwater. The collection we studied at theDenver Museum of Nature & Science (DMNS) produced the samenumber of ferns species in the ash-bed flora as found by Rushforth(1971) when he originally described the flora. We update theirtaxonomy (See x6. Systematic Paleobotany, and supplementaryonline material), but all of the species described by Rushforth areaccounted for in the collections at DMNS (Table 4). The mostcommon fossil species in the ash-bed flora at Westwater is Glei-cheniaceaephyllum comptoniaefolia (46%, WW002), followed byMatonidium brownii (29%, WW001), Astralopteris coloradica (12%,WW003), and then Gleicheniaceaephyllum sp. aff. G. delicatula (6%,WW005). These four species of ferns make up almost 93% of allspecimens collected from the ash, and 86% of all specimenscollected from both the ash-bed and mudstone localities at West-water. A fifth fern, Sectilopteris dicksonianum (WW009), is repre-sented by only one specimen in the current collection. These fivefern species occur exclusively in the ash-bed deposits, often over-lapping each other in mats. Eleven Equisetum (WW006) rhizomeswere recovered from the Westwater section from DMNH localities

947 and 1192, though no aerial stemswere recovered. The rhizomesoccur in the ash beds (presumably) in growth position, suggestingthat Equisetum may have been an early colonizer of the ash beddeposits. We conclude that the ferns and Equisetum formedimportant components of the early successional flora colonizingthe ash fall deposits, as also noted by Rushforth (1971).

Several specimens from the DMNS collections increase thetaxonomic breadth of the ash-bed flora from when it was initiallydescribed by Rushforth (1970, 1971). The current collection con-tains a single specimen of conifer in the genus Geinitzia (WW038),previously not known from Westwater. The conifer fossil occurs inthe ash amongst the herbaceous, early successional plants thatmake up the majority of fossils from the site. However, it is a single,partial specimen that could be interpreted in two ways. It mostlikely was a member of the woody late successional plants thatcolonized the stable environment that existed at Westwater priorto the ash fall, but could also represent a plant growing in moreupland communities transported by fluvial processes. Lycophyteswere also present; a single specimen of Selaginella sp. (WW010)was collected at DMNH locality 1192 (Slide Ash). Five dicotyle-donous angiosperm morphotypes were recovered from the ashbeds (WW004, WW007, WW033, WW034, WW037), but are arelatively rare portion of the flora (28 specimens collected), slightlymore than 6% of the whole ash-bed flora.

By contrast, themudstone flora atWestwater is overwhelminglydominated by dicotyledonous angiosperms (97%). Only a singlespecimen of the fern morphogenus Sectilopteris sp. (WW014) wasrecovered. There is no compositional overlap between themudstone flora (DMNH loc. 1191) and the underlying ash-bed floraat Westwater (DMNH locs. 947, 1192, and 1193) even though theyare only separated by a few meters of section (Fig. 2). Despite thesmall collection of leaves (51 total specimens) and the relativelylow percentage of identifiable specimens (68% attributed to amorphotype), the species/morphotype richness of leaves in themudstone flora is actually slightly higher than the ash-bed flora (15leaf morphotypes vs. 13 leaf morphotypes, respectively). Moreintensive collecting is certain to uncover more rare taxa, which islikely to show that the mudstone locality is even more species-richthan the ash-bed flora at Westwater.

The lack of species overlap between the ash-bed and mudstonefloras at Westwater, as well as a complete switch to angiospermdominance, suggests not just facies dependence but also stages ofsuccessional vegetation. For the two flora types the most likelyscenario is that the ash-bed flora is unique in that it preserved theearly phase of ecological succession by rapid colonizers, unlike themudstone locality that better represents the woody vegetation.Ferns, lycopsids, and sphenopsids reproduce by spores, which al-lows them to more rapidly colonize landscapes removed of theirvegetation. This was the case for the re-colonization of Krakatauafter the massive volcanic eruption denuded the island of plant andanimal life; studies of the progression of vegetation colonizationshowed that the first plants to return were ferns (Treub, 1888;Whittaker et al., 1984). The re-colonization of Krakatau providedthe modern example used to explain the spike in fern-sporeabundance that occurs in the first few centimeters of the earlyPaleogene (Nichols et al., 1986; Nichols and Fleming, 2002; Barclayet al., 2003; Nichols and Johnson, 2008).

There is a surprising lack of compositional overlap between thetwo mudstone floras at Henrieville and Westwater despite angio-sperm dominance in both; there are only two similar morphotypes(WW012 and HD001) of dicot angiosperm identified at the twolocalities. These two morphotypes have strong affinities to themodern genus Liriodendron, based upon the truncated apex, deeplobes, smooth margin, and reticulated tertiary venation patternspresent in both fossil morphotypes and the modern genus

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Liriodendron. We have chosen to keep them separated as mor-photypes until more complete and better preserved specimens arecollected, but also because of notable differences. The most sig-nificant difference is the number and degree of lobing, but also theway that the intersecondary veins interact with the sinus of eachlobe (see description in x6, Systematic Paleobotany). The remainingdicotyledonous angiosperms collected from the mudstone localityat Henrieville bear little resemblance to the morphotypes fromeither the ash-bed or the mudstone flora at Westwater.

The new geochronology indicates that the Henrieville andWestwater flora were deposited at different times, 2.5 Ma apart,and they are presently separated by 300 km, a distance whichprobably hasn't changed significantly since the mid-Cretaceous.Four potential explanations for the compositional differences be-tween the two sites include: 1) rapid species evolution during acritical phase of angiosperm radiation; 2) climate change in themid-Cretaceous or associated with Ocean Anoxic Event 2 (OAE2)that influenced rates of species evolution and/or led to a geographicshift in species ranges; 3) high regional diversity as a result ofmicro-climates or geographic barriers; or 4) a high degree of par-titioning of plant assemblages on Cretaceous floodplains. Wediscuss each hypothesis in detail in the following paragraphs.

The lack of species-level overlap between themudstone floras atWestwater and Henrieville may document an episode of rapidspecies evolution within the 2.5 Ma timespan between the floras.The second half of the Cenomanian represents a time of increas-ingly rapid angiosperm diversification, indicated by the upwardinflection of within-flora floristic diversity (Lupia et al., 1999).Therefore, the lack of species overlap could indicate an almostcomplete species turnover in the Cretaceous forests of Utah withinan evolutionarily rapid time frame. To our knowledge no estimatesfor the average duration of species is available for plants, but formarine invertebrates, average species durations are close to 5million years (Buzas and Culver, 1984; Sepkoski, 1984; Buzas andCulver, 2004), so the complete replacement of the flora by speciesevolution would represent abnormally high rates of evolution, butcannot be discounted.

Rates of speciation may also have been influenced by a globalchange in climate, which in turnmay have led to regional migrationof populations of plant species allowing for subsequent allopatricspeciation in the study area. Changes in temperature due toincreased pCO2 concentrations associated with the lead-up toOcean Anoxic Event 2 (Turgeon and Creaser, 2008; Adams et al.,2010; Barclay et al., 2010) could have been the cause for themigration of species, as environments would have shifted latitu-dinally. A full understanding of how global climate may haveaffected the regional distribution of mid-Cretaceous floras in NorthAmerica is limited at this time. Further work is necessary to betteridentify and understand potential climate effects for this timeperiod.

We favor the hypothesis that the difference in speciescomposition between the two mudstone floras represents highregional diversity as a result of micro-climates or geographicbarriers. The two mudstone floras were likely preserved in similarenvironments, namely shallow lacustrine settings with highsediment accumulation rates. Therefore, the lack of overlapwould require high regional heterogeneity in species composi-tion. Given that the two localities were separated by 300 km, asmall topographic barrier may have separated the two regions,creating two isolated populations (DiMichele et al., 2004). TheDakota flora from the Eastern side of the Western Interior Seawayprovides a good comparison, because it also appears to contain ahigh degree of compositional heterogeneity. Wang (2002)described 87 species from six localities in Kansas and Minne-sota, and the overlap in species composition between sites was

less than 25%. The 87 species described by Wang (2002) was farless than the 437 species described by Lesquereux (1891),potentially a function of over-splitting of species by Lesquereux(Lidgard and Crane, 1990). However, if the true number of speciesin the Dakota Formation is closer to the number described byLesquereux (1891) because he studied a greater number of lo-calities, then a large degree of compositional heterogeneityexisted on the eastern shoreline of the Western Interior Seaway.Indeed, the high angiosperm diversity in the Dakota Formationmay have been a function of the many different environmentsrepresented along the large geographic extent of the shoreline onboth margins of the Western Interior Seaway (Ludvigson et al.,2010). For the mudstone floras at Westwater and Henrieville onthe western margin of the seaway, small microclimates may havealso existed as a result of differences in their geographic location,creating enough difference in rainfall or topography to host theentirely different floras.

The most conservative explanation is that the mudstone de-posits preserved slightly different depositional settings on a highlypartitioned floodplain landscape. Despite the similarity in the grainsize and aspect of the two deposits, it is possible that the deposi-tional settings were considerably different and therefore preserveda different local vegetation. Previous research found evidence forpartitioning of the distribution of species within the floodplainduring the diversification of angiosperms (Hickey and Doyle, 1977;Feild et al., 2003; Feild et al., 2004), so the lack of overlap could havemerely been a function of preserving a different portion of similarfloodplains, although we consider it unlikely that this produced thetotal lack of species overlap between Westwater and Henrieville.

7.4. Implications for Ocean Anoxic Event 2 at the Cenomanian-Turonian boundary

The new dates from the Henrieville section, combined withpCO2 proxies and isotopic records from marine and terrestrialsections, provide new insight into the conditions leading up tothe CenomanianeTuronian mass extinction. This boundary isstraddled by Oceanic Anoxic Event 2 (OAE2), a second-ordermass extinction event (Elder, 1985, 1989), identified globallyby an initial þ2‰ and then a þ4‰ excursion in d13Corg. The d13Cexcursion coincides with the onset of widespread marineanoxia, widely considered the kill mechanism for the Cen-omanianeTuronian mass extinction (Elder, 1989). The d13Cexcursion is pre-dated by excursions in multiple isotopes sys-tems (lead, osmium, strontium, and sulfur), and volcanism haslong been implicated as the driver of marine anoxia duringOAE2 (Kerr, 1998; Turgeon and Creaser, 2008; Adams et al.,2010; Barclay et al., 2010). The Caribbean large igneous prov-ince has been invoked as the most likely volcanic source, andvolcanism associated with its emplacement probably occurredin multiple phases (Sinton and Duncan, 1997; Kerr, 1998; Snowet al., 2005).

A detailed timescale for these chemical proxies is crucial forevaluating a link with volcanism, but has not been developed dueto a lack of radioisotopic dates in rocks belowOAE2. However, manygeochemical proxies shift in advance of the d13C excursion thatsignals the onset of anoxia, so estimates for the timing of changeshave been attempted, but are largely based on sediment accumu-lation rates determined within OAE2 and extrapolated down sec-tion, as is the case for the following examples. Lead isotope ratios(208Pb/204Pbinitial) shift towards values more similar to largeigneous province signatures, starting 1.5 m below OAE2, an esti-mated 480 ka prior to OAE2 based upon sediment accumulationrates calculated from within the Bonarelli interval in Italy(Ohkouchi et al., 1999; Kuroda et al., 2007). Strontium isotopes shift

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towards less radiogenic values at least 600 ka prior to OAE2, againestimated by extrapolation of sediment accumulation rates fromwithin OAE2 (Frijia and Parente, 2008; Ando et al., 2009). Osmiumisotopes make an initial shift towards less radiogenic values start-ing ~400 ka prior to OAE2. A more rapid shift occurs in isotopiccomposition, accompanied by an increase in concentration 23 kabefore the d13C excursion (Turgeon and Creaser, 2008). Finally,there is evidence from d34Ssulfate for an increase in marine sulfatefrom submarine volcanism, initiated 430 ka prior to OAE2 (Adamset al., 2010). Together, the shifts in these marine proxies areconsistent with a major episode of volcanism, interpreted to havemodified the chemistry of the oceans and triggered widespreadmarine anoxia during OAE2 (Jarvis et al., 2011; Monteiro et al.,2012; Du Vivier et al., 2014). There is relatively good agreementfor the timing of changes in this set of geochemical proxies (400-600ka prior to OAE2), but extrapolating sediment accumulationrates calculated fromwithin OAE2 to rock intervals outside of OAE2(where marine conditions were clearly different) is tenuous.Radioisotopic control is required to unravel the timing of eachgeochemical response relative to ocean anoxia and marineextinction.

The lack of temporal control from precise radioisotopic datesbelow OAE2 has also limited correlations between marine andterrestrial sections, making it impossible to determine if the in-crease in pCO2 from terrestrial proxies prior to OAE2 (Barclay et al.,2010) was temporally correlative to the shifts observed in marineproxies. However, a recent 40Ar/39Ar sanidine date from an ashwithin the Dunveganoceras pondi biozone (Lincoln LimestoneMember of the Greenhorn Formation; Ma et al., 2014) and an ashdated from within the Euomphaloceras septemseriatum biozone atthe base of OAE2 (Meyers et al., 2012), provide two control pointsthat constrain the age and duration of isotope shifts below OAE2 inthis section. The Hartland Shale Member overlies the LincolnLimestoneMember (Sageman et al., 2006), and estimated sedimentaccumulation rates between the d13C excursion for OAE2 and theunderlying Lincoln Limestone are 1.69e3.28 cm/ka (Ma et al., 2014)based upon the thickness of the section between the two 40Ar/39Ardates. Ma et al. (2014) applied an evolutive average spectral misfitapproach (Meyers et al., 2001; Meyers et al., 2012) to produce anoptimal fit of 2.41 cm/ka for the sediment accumulation in theHartland Shale. Using these values, our best estimate is thatchanges observed in d34Ssulfate from the GSSP at Pueblo wereinitiated 570 ka (optimal fit) prior to the onset of d13C excursionthat defines OAE2, with a range between 420 and 814 ka.

Extensive field and laboratory work traced the onset of the d13Cexcursion for OAE2 into the terrestrial sedimentary rocks on thewestern margin of the Western Interior Seaway, allowing forcomparison of the timing of events in the terrestrial realm withthose in the marine realm (Elder et al., 1994; Laurin and Sageman,2007). The excursion in d13C is located just below the formationalcontact of the Middle Dakota with the overlying marine TropicShale, a contact that is readily identifiable throughout southernUtah and represents a flooding surface associated with a major sea-level transgression that reached maximum high stand in the earlyTuronian (Haq et al., 1987; Roberts and Kirschbaum, 1995). Thistransgression provided the initial framework for correlating theDakota Formation and Tropic Shale on the Colorado Plateau withthe Greenhorn Cycle (Averitt, 1962; Peterson, 1969; Peterson andKirk, 1977). The correlation from western Utah to the Cen-omanianeTuronian GSSP in Pueblo, Colorado was improved usingbentonite (Kowallis et al., 1989) and limestone marker beds (Elderet al., 1994), coupled with associated ammonite and bivalve indexfossils (Cobban and Scott, 1972; Kauffman, 1977; Cobban, 1984;Kauffman and Caldwell, 1993). Further constraint from studies ofcombined outcrop and well log data (Laurin and Sageman, 2001;

Laurin, 2003; Laurin and Sageman, 2007) significantly increasedthe resolution of the CenomanianeTuronian correlation in south-western Utah.

The characteristic positive d13C excursion that defines OAE2 wasidentified in the Kanarra Mountains of southwestern Utah (Barclayet al., 2010). The stratigraphic section is tightly correlated to thesections measured at Henrieville (Laurin and Sageman, 2001;Laurin and Sageman, 2007) because the d13C excursion for OAE2coincides with the base of the Vascoceras diartianum biozone(Sageman et al., 2006), which, in this region of southwestern Utahis the readily identifiable formation contact between the MiddleDakota and overlying marine Tropic Shale (Uli�cný, 1999; Laurin andSageman, 2001; Laurin, 2003; Laurin and Sageman, 2007). TheUePb dates presented in this report from Henrieville thereforeprovide the first precise temporal control on the terrestrial sedi-mentary record deposited prior to OAE2.

Stomatal-index proxy data collected from Wahweap Creek,Utah (Barclay et al., 2010) suggested a steady increase in pCO225 m before the d13C excursion that defines OAE2. WahweapCreek is also tightly correlated to the Henrieville section (Uli�cný,1999; Laurin and Sageman, 2007), so the precise numerical con-trol on the deposits in the Middle Member of the Dakota For-mation provides a unique opportunity to compare the increase inpCO2 prior to OAE2 (Barclay et al., 2010) with contemporaneousmarine proxy data. Sediment accumulation rates in the Henrie-ville section (3.9e6.5 cm/ka) are about twice the rate of those forthe Hartland Shale in Pueblo, CO, so there is correspondinglymore section (25 m) between the initial increase in pCO2 and theonset of the d13C excursion (Barclay et al., 2010). Using the sedi-ment accumulation rates from Henrieville, we estimate that theinitial rise in pCO2 started 513 ka (range from 384 to 641 ka) priorto the onset of OAE2. The estimates for the timing of changes toisotope systems prior to OAE2 in the marine realm (570 ka) and arise in pCO2 documented in terrestrial plant material from thestomatal index proxy (513 ka) are equivalent, within error esti-mates. These estimates represent the first temporal correlationbetween the timing of events in the marine and terrestrial realmsfor the multitude of events leading to widespread anoxia duringOAE2. The contemporaneous dates in both terrestrial and marinesequences lends further support for volcanism as the masterdriver causing widespread anoxia and mass extinction duringOAE2.

8. Conclusion

The time transgressive Dakota Formation deposited on theeastern and western margins of the Western Interior Seaway con-tains abundant plant fossils that preserve an important record ofearly angiosperm evolution and diversification. We present newhigh precision ID-TIMS UePb zircons dates from five air-fall ashesthat precisely constrain three fossil localities, from two locations insouthern Utah separated by 300 km. Three air-fall ash beds brackettwo fossiliferous layers near the Westwater bend of the ColoradoRiver, and record weighted mean Th-corrected 206Pb/238U dates of97.949 ± 0.037/0.12 Ma, 97.943 ± 0.023/0.12 Ma, and97.601 ± 0.049/0.13 Ma. Two ash beds that bracket fossiliferousrocks northeast of the town of Henrieville, Utah record weightedmean Th-corrected 206Pb/238U dates of 95.070 ± 0.036/0.12 Ma and94.879 ± 0.032/0.11 Ma.

At Westwater, an ash bed and mudstone layer yielded a total of556 fossils; 488 fossil leaves were assigned to 28 different mor-photypes. The ash bed flora is dominated by ferns, with a limitedabundance and diversity of dicot angiosperms, plus single speci-mens of a conifer, a sphenopsid, and a lycopsid. By contrast, themudstone flora is entirely composed of dicot angiosperms, with the

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exception of one fern specimen. We conclude that the lack oftaxonomic overlap is due to the ash-bed flora representing an earlysuccessional vegetation that colonized the area directly after theash-fall event, while the mudstone flora records the more typicalwoody floodplain flora.

At the Henrieville locality, a mudstone layer yielded 266 fossils,137 were identified and assigned to 14 morphotypes (8 leaf mor-photypes; 6 reproductive morphotypes). The leaf flora is dominatedby ferns and dicot angiosperms. There is a surprising lack of overlapbetween the Westwater and Henrieville floras; only two morpho-types, probably within the same genus of “Liriodendron”, are incommon between the two localities, despite 29 taxa between them.Thedisparatefloral compositionmay reflect a combinationof factors,including rapid rates of species evolutionduring the2.5Ma timespanthat separates the Henrieville andWestwater floras, the influence ofmid-Cretaceous climate change, high regional diversity as a result ofgeographic barriers and micro-climates, or a high degree of parti-tioning of plant assemblages on mid-Cretaceous floodplains.

The new UePb dates from the Henrieville locality, throughcorrelation to other sedimentary sections in Utah, place temporalconstraints on increases in pCO2 inferred in terrestrial sections ofthe Dakota Formation prior to OAE2. We find that the increase inpCO2 documented from stomatal index on terrestrial plants in Utahis contemporaneous (within error) with changes to d34Ssulfate in theHartland Shale Member of the Greenhorn Formation at the GSSP inPueblo, Colorado. Our best estimate of the duration of thesechanges is between 570 ka (Pueblo) and 513 ka (Utah) indicating forthe first time, near synchronous perturbation of marine andterrestrial environments related to an increase in pCO2. The cor-relation is consistent with existing models that link OAE2 withvolcanism associated with the emplacement of the Caribbean LIP,and which also require simultaneous perturbation of the ocean andatmosphere.

Acknowledgments

We would like to thank Mark Kirschbaum (USGS) for assistancein placing the unpublished 40Ar/39Ar dates from Henrieville, Utahinto their proper stratigraphic position. Many individuals at DMNSshould be thanked for help with collecting, preparing, and mor-photyping the flora. Howard and Darlene Emry were the principalcollectors of the specimens from Westwater. The “Leaf Whackers”prepared the leaf fossils at DMNS, allowing the fossils to be fullyexposed and more easily identified. Michele Reynolds and NicoleBoyle of DMNS made considerable progress (a decade ago) towardsmorphotyping thefloras, funded inpart bya grant fromtheColoradoDepartment of Transportation. Doug Kline and SarahMaccracken ofDMNS are thanked for their assistance in the final stages of mor-photyping the flora. David Uli�cný provided locality information forhis East and West sections at Henrieville, allowing for precise cor-relation of the fossil and ash material into his stratigraphic frame-work.Wewould also like to thank reviewers Daniel Peppe and GregLudvigson for their very helpful comments. This work was sup-ported by NSF grants EAR-0642838 and EAR-0643158, awarded toJohnson and Bowring, respectively. Sample collection was part of afield excursion for MIT first year students (DEAPS) led by Johnson,Miller, and Bowring and supported by the MIT Department of EAPS.RSB acknowledges current stipend funding from the Peter BuckFellowship Endowment at the National Museum of Natural History,Smithsonian Institution, Washington, D.C.

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Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.cretres.2014.08.006