Canadian Journal of Earth Sciences

High-precision U-Pb CA-ID-TIMS dating and chronostratigraphy of the dinosaur-rich Horseshoe Canyon Formation (Upper Cretaceous, Campanian–Maastrichtian), Red Deer River valley, Alberta, Canada

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2019-0019.R1

Manuscript Type: Article

Date Submitted by the 30-Jul-2019 Author:

Complete List of Authors: Eberth, David; Royal Tyrrell Museum of Palaeontology, Kamo, Sandra;Draft University of Toronto, Earth Sciences Horseshoe Canyon Formation, radiometric dating, dinosaurs, Alberta, Keyword: Maastrichtian

Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :

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1 High-precision U-Pb CA-ID-TIMS dating and chronostratigraphy of the 2 dinosaur-rich Horseshoe Canyon Formation (Upper Cretaceous, Campanian– 3 Maastrichtian), Red Deer River valley, Alberta, Canada 4 5 6 David A. Eberth* and Sandra L. Kamo** 7 8 9 10 *Royal Tyrrell Museum of Palaeontology 11 Box 7500 12 Drumheller, Alberta T0J0Y0 13 [email protected] 14 15 **Jack Satterly Geochronology Laboratory 16 Department of Earth Sciences 17 University of Toronto 18 22 Russell St. 19 Toronto, ON, M5S 3B1 20 [email protected] 21 Draft

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23 Abstract

24 The non-marine Horseshoe Canyon Formation (HCFm, southern Alberta) yields

25 taxonomically diverse, late Campanian to middle Maastrichtian dinosaur assemblages that play a

26 central role in documenting dinosaur evolution, paleoecology, and paleobiogeography leading up

27 to the end-Cretaceous extinction. Here, we present high-precision U-Pb CA-ID-TIMS ages and

28 the first calibrated chronostratigraphy for the HCFm using zircon grains from (1) four HCFm

29 bentonites distributed through 129 m of section, (2) one bentonite from the underlying Bearpaw

30 Formation, and (3) a bentonite from the overlying Battle Formation that we dated previously. In

31 its type area, the HCFm ranges in age from 73.1–68.0 Ma. Significant paleoenvironmental and

32 climatic changes are recorded in the formation, including (1) a transition from a warm-and-wet

33 deltaic setting to a cooler, seasonally wet-dryDraft coastal plain at 71.5 Ma, (2) maximum

34 transgression of the Drumheller Marine Tongue at 70.896 ± 0.048 Ma, and (3) transition to a

35 warm-wet alluvial plain at 69.6 Ma. The HCFm’s three mega-herbivore dinosaur assemblage

36 zones track these changes and are calibrated as follows: Edmontosaurus regalis-

37 Pachyrhinosaurus canadensis zone, 73.1–71.5 Ma; Hypacrosaurus altispinus-Saurolophus

38 osborni zone, 71.5–69.6 Ma; and Eotriceratops xerinsularis zone, 69.6–68.2 Ma. The

39 Albertosaurus bonebed—a monodominant assemblage of tyrannosaurids in the Tolman

40 Member—is assessed an age of 70.1 Ma. The unusual triceratopsin, Eotriceratops xerinsularis,

41 from the Carbon Member, is assessed an age of 68.8 Ma. This chronostratigraphy is useful for

42 refining correlations with dinosaur-bearing upper Campanian-middle Maastrichtian units in

43 Alberta, and elsewhere in North America.

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45 Keywords

46 Horseshoe Canyon Formation, radiometric dating, dinosaurs, Alberta, Maastrichtian

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48 Introduction

49 The Horseshoe Canyon Formation (HCFm) of south-central Alberta (Figs. 1–3) is unique

50 among non-marine formations in North America in yielding a nearly continuous record of late

51 Campanian to middle Maastrichtian dinosaurs. In turn, these provide insight into evolutionary,

52 paleoenvironmental, and paleobiogeographic patterns and events that were unfolding in North

53 America 7 to 2 million years prior to the end-Cretaceous extinction event (e.g., Eberth and 54 Braman 2012; Eberth et al. 2013; EberthDraft and Bell 2014). Until now, chronostratigraphic 55 calibrations of the formation and its dinosaur assemblages have yielded inconsistent results. This

56 is because previous calibrations have relied on local magnetostratigraphic data that were

57 constrained by (1) a few, poorly documented radiometric dates and ages (see discussions in

58 Eberth and Braman 2012; Eberth et al. 2013; and Eberth and Kamo 2019), and (2) comparisons

59 with calibrated magnetostratigraphies from other regions (i.e., International Stratigraphic

60 Commission Geologic Time Scales presented in 2004 and 2012; Gradstein et al. 2004, 2012;

61 Eberth et al. 2013; Fowler 2017; Eberth and Kamo 2019).

62 Here, we present four high-precision U-Pb CA-ID-TIMS ages for the HCFm derived

63 from altered volcanic airfall deposits (bentonites) that are exposed in HCFm strata in the Red

64 Deer River valley, north of the town of Drumheller, Alberta (Figs. 1–2). We also present a U-Pb

65 CA-ID-TIMS age for the underlying Bearpaw Formation and include previously determined

66 high-precision ages from the overlying Battle bentonite (Eberth and Kamo 2019) and the K-Pg

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67 boundary (Renne et al. 2013), which enable us to interpolate lower and upper age-limits for the

68 HCFm in our field area.

69 The four HCFm bentonites are semi-evenly distributed through 129 m of stratigraphic

70 section (Figs. 2–3) and provide an opportunity to interpolate the ages of significant horizons and

71 fossil sites in the HCFm. In turn, the combined U-Pb and interpolated ages allow us to present a

72 robust, internally consistent chronostratigraphy for the HCFm that forms the basis for more

73 precise correlations with other upper Campanian–middle Maastrichtian dinosaur-rich units in

74 Alberta and elsewhere in North America (e.g., Fowler 2017).

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76 Geologic context and background

77 The Horseshoe Canyon FormationDraft represents the lower three-quarters of the Edmonton

78 Group (Fig. 3; Irish 1970; Gibson 1977), which was deposited during late-stage accretion of the

79 Insular Superterrane with the Intermontane Superterrane (Cant and Stockmal 1989). The ongoing

80 collision resulted in thrust-belt uplift and sedimentation of Edmonton Group clastics into the

81 adjacent subsiding foreland basin (Western Canada Sedimentary Basin; Hamblin 2004; Eberth

82 and Braman 2012). The Red Deer River valley and its drainages at Drumheller (Fig. 1) are the

83 type area for the HCFm, and a composite measured section of the formation in that area is 240–

84 250 m thick (Figs. 1–3; Eberth and Braman 2012). The formation thickens and becomes

85 significantly older to the west (Fig. 3). Near Calgary, the formation (subsurface data only) is

86 more than 500 m thick (Eberth and Braman 2012). The HCFm was deposited as an overall

87 prograding, non-marine clastic wedge with distinctive south-eastward thinning tongues that

88 interfinger with marine shales of the Bearpaw Formation (Fig. 3; Eberth and Braman 2012).

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89 Throughout south-central Alberta, the HCFm is a coal-bearing, paralic-to-non-marine

90 unit with members that were variously deposited in deltaic, coastal plain, and alluvial plain

91 settings, and under a variety of warm-temperate monsoonal to seasonally wet-dry climatic

92 conditions (Hamblin 2004; Eberth and Braman 2012; Eberth and Bell 2014). HCFm stratigraphy,

93 depositional history, coals, and vertebrate and trace fossils are well studied (see references in

94 Hamblin 2004; Eberth and Braman 2012). More recent information on the formation is available

95 in Eberth et al. (2013), Eberth and Bell (2014), and Ainsworth et al. (2015).

96 Eberth and Braman (2012) interpreted late Cretaceous south-central Alberta as a distal

97 foredeep setting where deposition was influenced by upstream and downstream controls,

98 including tectonism, volcanism, and climate and sea-level changes. By documenting the relative

99 influences of these controls on sedimentationDraft in different stratigraphic intervals of the HCFm,

100 they established the following seven formal lithomembers (in ascending order): Strathmore,

101 Drumheller, Horsethief, Morrin, Tolman, Carbon, and Whitemud (Figs. 2–3). Whereas the

102 Strathmore Member comprises paralic to non-marine sediments west of Strathmore, it is replaced

103 by marine sediments of the Bearpaw Formation in the Drumheller type area (and farther east).

104 Accordingly, in our field area, the base of the Horseshoe Canyon Formation is placed at the base

105 of the Drumheller Member (Figs. 2–3).

106 Eberth et al. (2013) assessed dinosaur composition and biostratigraphic patterns in most

107 of the members, identifying three dinosaur assemblage zones that track paleoenvironmental

108 changes in the formation. Eberth and Braman (2012) and Eberth et al. (2013) both presented

109 provisional age assessments for the formation and its dinosaur assemblage zones that were based

110 on poorly constrained radiometric dating and the magnetochron calibrations of Ogg et al. (2004).

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112 Bentonites and sampling methods

113 Table 1 includes information from six bentonites from which zircon grains were collected

114 and analyzed during this study and our previous study on the Battle bentonite (Eberth and Kamo

115 2019). In ascending stratigraphic order these are: 1) the Bearpaw bentonite (BpB;

116 TMP2018.100.0001) preserved near Iddesleigh, along the eastern boundary of Dinosaur

117 Provincial Park; 2) the overflow-parking bentonite at the Royal Tyrrell Museum (OPB;

118 TMP2018.100.0002); 3) the Trentham road bentonite (TRB; TMP2018.100.0003) southwest of

119 Morrin Bridge along Highway 27; 4) the Morrin Bridge bentonite (MBB; TMP2018.100.0004)

120 northeast of the Morrin Bridge; and 5) the Albertosaurus Bonebed bentonite (ABB;

121 TMP2018.100.0005) north of Dry Island Provincial Park. Bulk samples of each of the six

122 bentonites in this study maintained in theDraft collections of the Royal Tyrrell Museum of

123 Palaeontology for comparative analyses, and each is assigned a collection number (TMP prefix;

124 Table 1).

125 Table 1 and Figure 1 include location data for the six bentonites that were obtained from

126 hand-held GPS confirmed by Google Earth calibrated satellite imagery (April 30, 2007; May 04,

127 2007; August 23, 2011; May 10, 2012; and August, 19, 2012). Stratigraphic occurrences (in

128 meters) are calculated relative to the base of the Drumheller Member (of the HCFm) at the

129 Hoodoos Recreational Area using a composite measured section and subsurface thickness data

130 from Eberth and Braman (2012)(Figs. 2–3, 6).

131 In this study, the currently accepted age for the K-Pg boundary claystone as preserved in

132 eastern Montana is included (Renne et al. 2013; Sprain et al. 2014; Figures 6–7). However, no

133 field examinations, lithologic descriptions, or radiometric analyses of this material were

134 conducted by us.

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135 The six bentonites were excavated until fresh (unweathered) portions were exposed (Fig.

136 1D; Eberth and Kamo 2019). Freshly exposed bentonites appear compact and waxy, and exhibit

137 a variety of tan-to-yellow-to-green colors (Thomas et al. 1990). In contrast, weathered portions

138 are consistently grey-to-white in color and exhibit deeply fractured, popcorn-like textures that

139 result from repeated episodes of expansion and contraction (Fig. 1D). Within hours after

140 exposure, each freshly exposed bentonite began to dry, shrink, and lighten in color, typically

141 stabilizing in a color range of 5Y 7/2 to 5GY 7/2 (GSA 1991).

142 The excavated bentonites typically exhibited sharp contacts on underlying sediments, and

143 sharp-to-diffuse upper contacts. Because the sampled beds were thin (5–20 cm thick) and large

144 sample volumes were required (2–5 liters), we exposed approximately 0.5–1.0 m2 of fresh

145 bentonite in plan-view during each excavation.Draft Extensive overburden removal prevented

146 contamination from non-bentonitic layers during sampling.

147 All bentonites were examined in the field with a hand lens, and attempts were made to

148 identify and sample sub-horizons where fresh biotite crystals were notably abundant and large,

149 typically near the base of each deposit (e.g. Thomas et al. 1990; Eberth and Kamo 2019). After a

150 bentonite (or a potentially crystal-rich sub-horizon) was exposed, samples were collected using a

151 putty-knife to lift layers of fresh, crystal-rich bentonite from above the lower bounding surface.

152

153 Analytical methods for U-Pb geochronology

154 Bentonite samples were disaggregated in a blender or disk mill and a heavy mineral

155 concentrate was produced by re-processing heavy mineral splits on the Wilfley table. This was

156 followed by standard mineral separation procedures using magnetic (Isodynamic Frantz) and

157 heavy liquid (methylene iodide) methods.

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158 U-Pb analysis was by isotope dilution-thermal ionization mass spectrometry methods on

159 chemically abraded zircon grains (CA-ID-TIMS) at the University of Toronto. Prior to

160 dissolution and analysis, zircon crystals were thermally annealed at 900º C for 48 hours to repair

161 radiation damage in the crystal lattice. The grains were then partially dissolved in ~0.1 ml ~50%

162 hydrofluoric acid and ~0.020 ml of HNO3 at 200º C for 12 hours (Mattinson, 2005). Zircon

163 grains were rinsed with 6N HCl followed by 8N HNO3 at room temperature prior to dissolution.

164 A 205Pb-233-235U spike from the EARTHTIME Project was added to the Teflon dissolution

165 capsules during sample loading. Zircon was dissolved using ~0.10 ml of concentrated HF acid

166 and ~0.020 ml of 7N HNO3 at 200° C for 5 days, then dried to a precipitate and re-dissolved in

167 ~0.15 ml of 3N HCl overnight (Krogh 1973). U and Pb were isolated from the zircon using 50 μl

168 anion exchange columns using HCl, driedDraft down in 0.05N H3PO4, deposited onto outgassed

169 rhenium filaments with silica gel (Gerstenberger and Haase 1997), and analyzed with a VG354

170 mass spectrometer using a single Daly detector in pulse counting mode for Pb, and 3 Faraday

171 cups in static analysis mode for U. Corrections to the 206Pb-238U ages for initial 230Th

172 disequilibrium in the zircon have been made assuming a Th/U ratio in the magma of 4.2. All

173 common Pb in each analysis was assigned the isotopic composition of procedural Pb blank. Dead

174 time of the measuring system for Pb was 16 nanoseconds. The mass discrimination correction for

175 the Daly detector is constant at 0.05% per atomic mass unit. The thermal mass fractionation

176 correction for Pb was 0.10% per atomic mass unit (± 0.076%, 2σ); and the U thermal mass

177 fractionation correction was measured and corrected within each measurement cycle. Amplifier

178 gains and Daly characteristics were monitored using the SRM 982 Pb standard. Decay constants

179 are those of Jaffey et al. (1971). Age errors quoted in the text and table, and error ellipses in the

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180 concordia diagram and weighted mean age plot are given at the 95% confidence interval.

181 Plotting of U-Pb data employed Isoplot 3.31 (Ludwig 2003).

182

183 U-Pb results

184 U-Pb dates reported below for single zircon grains are from the 238U – 206Pb decay

185 scheme. This is the most robust system for geologically young rocks due to the much greater

186 abundance of 206Pb, which make it less sensitive to common Pb corrections, possible minor

187 effects from initial deficiencies of 230Th that lead to deficits in 206Pb, or measurement bias issues,

188 compared to the 235U – 207Pb decay scheme. Correction for initial 230Th disequilibrium has been

189 made with an assumed Th/U of the magma of 4.2. However, for zircon grains with low Th/U, a

190 lower value of 2.5 may be more appropriateDraft (Wotzlaw et al., 2014). In our study, zircon grains

191 have Th/U ranging from 0.4 – 0.6 and an assumed Th/U of 2.5 will have the effect of reducing

192 the final age by about ~10 ka. To facilitate age correlations in future studies, dates presented

193 herein are shown with uncertainties following Schoene et al. (2006), i.e. age ±x/y/z, where x

194 includes internal uncertainties only (for comparison of U-Pb ages produced in the same

195 laboratory), y includes x in addition to spike calibration error (for comparison of dates from

196 different U-Pb laboratories), z includes both x, y and the 238U decay constant uncertainty to

197 compare to dates acquired by other isotopic dating methods. A summary of the U-Pb zircon

198 isotopic data is presented in Table 2. Concordia diagrams and plots of 206Pb/238U dates for each

199 sample are presented in figures 4 and 5, respectively.

200 Each sample contains abundant, unaltered, euhedral zircon grains that are mainly short

201 prismatic, multi-facetted crystals (2/1 aspect ratio) or needle-like, long prismatic grains (5/1 to

202 >7/1 aspect ratio) with occasional elongate melt inclusions and apparent mineral (apatite?)

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203 inclusions. After zircon grains were chemically abraded, those with mineral/melt inclusions

204 and/or fractures, or visible surface abrasion were excluded. Translucent crystals with no apparent

205 defects, limited to minor inclusions, were selected for U-Pb analysis. Each bentonite contained

206 older inherited grains, some of which are plotted in figures 4 and 5. Data for those not plotted are

207 presented in Table 2. Such grains may have been incorporated during post-depositional

208 reworking of the deposited volcanic horizon, or earlier from the magma source chamber, or

209 during eruption and passage through older crust.

210

211 Albertosaurus Bonebed bentonite (ABB)

212 The youngest three dates from a suite of seven long-prismatic grains (typically ~200

213 microns) overlap and have a weighted meanDraft 206Pb/238U age of 70.675 ± 0.047/0.091/0.201 Ma

214 (MSWD=0.083), which is interpreted as the best estimate for the time of deposition of the

215 original volcanic ash. Four older grains, two with concordant data at ca. 78 Ma, and two with

216 discordant results from probable inherited cores, are also reported.

217

218 Morrin Bridge bentonite (MBB)

219 Elongate zircon crystals (up to 400–500 microns long) and short prismatic to equant

220 grains are gem-quality and fresh. These gave five overlapping results with a weighted mean

221 206Pb/238U age of 70.896 ± 0.048/0.092/0.202 Ma (MSWD=1.2), which is considered the best age

222 for formation of the volcanics. This interpretation may be slightly biased towards a marginally

223 older age than the true time of ash deposition if the five grains crystallized over a prolonged time

224 period (for example, ~100 kyrs, which reflects the approximate range between the oldest and

225 youngest results). For comparison, the mean of the youngest three results is 70.853 ± 0.061 Ma

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226 (MSWD=0.09), which is ~0.043 Ma younger than our interpretation based on five grains, but

227 still resolvably older than the age of the overlying ABB unit.

228

229 Trentham road bentonite (TRB)

230 The four youngest results for short prismatic multifaceted crystals overlap and provide a

231 weighted mean 206Pb/238U age of 71.832 ± 0.044/0.086/0.196 Ma (MSWD=1.07), which is

232 interpreted as the best estimate for the time of deposition of the original volcanic ash. Two older

233 grains, one concordant at ca. 74 Ma, and one discordant with data plotting to the upper right of

234 the cluster, indicate a discrete inherited grain and one with a significantly older core component,

235 respectively.

236 Draft

237 Overflow parking bentonite (OPB)

238 The five youngest results from three elongate and two short prismatic multifaceted zircon

239 crystals minimally overlap within their 2σ uncertainties to give a weighted mean age of 72.353 ±

240 0.037/0.075/0.185 Ma (MSWD=2.1), which provides a conservative estimate of the time of

241 deposition of the original volcanic ash. A slightly older grain is concordant at ca. 72.5 Ma and

242 overlaps only the oldest in the age cluster. The age for a 7th grain indicates that it contains a

243 much older core component, as it plots to the upper right of the cluster. Age interpretations, in

244 general, can include suggestions of potential Pb loss in the youngest analyses, and the presence

245 of possible antecrysts at the older end of the age spectrum. Although there is slight scatter in the

246 five results that produce the age cluster, they overlap within their 2σ uncertainties and, therefore,

247 72.353 ± 0.037/0.075/0.185 Ma remains the preferred age interpretation for this bentonite.

248

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249 Bearpaw bentonite (BpB)

250 The seven youngest grains give overlapping 206Pb/238U dates with a weighted mean age

251 of 74.308 ± 0.031/0.050/0.130 Ma (MSWD=0.97). The data were obtained from fresh, euhedral,

252 long-prismatic zircon grains. One xenocrystic grain is ca. 74.7 Ma and two others contain an

253 inherited core component (z1 and z3). We interpret the age of 74.308 ± 0.031/0.050/0.130 Ma as

254 the best estimate for the time of deposition of the original volcanic ash.

255

256 Horseshoe Canyon Formation chronostratigraphy

257 We combined the five new U-Pb CA-ID-TIMS ages reported here with (1) the U-Pb age

258 of 66.936 ± 0.047 Ma for the Battle Formation (Eberth and Kamo 2019), and (2) the U-Pb age of

259 66.043 ± 0.043 Ma for the K-Pg boundaryDraft (Renne et al. 2013; Sprain et al. 2014), in order to

260 construct a high-precision chronostratigraphy for the HCFm and its fossil assemblages (Figs. 6–

261 8, Table 3).

262 We used an age-stratigraphy-line (ASL) intercept approach (Fig. 6) to interpolate the

263 ages of non-dated stratigraphic-positions-of-interest in the HCFm (e.g., fossil occurrences, and

264 litho-, magneto- and biostratigraphic boundaries). The ASL was established simply by plotting

265 the intercepts of the stratigraphic positions (y-axis) of the dated bentonites and their U-Pb ages

266 and 2σ error envelopes (x-axis), and then drawing lines between the intercept points. The width

267 of the 2σ error envelope for the ASL based on our radioisotopic dating is indicated by light grey

268 shading surrounding the ASL in Figure 6. It ranges from a minimum of 0.062 Ma for BpB (2σ

269 error of ± 0.031 Ma) to a maximum of 0.096 Ma for MBB (2σ error of ± 0.048 Ma), and thus

270 spans less than 0.1 Ma throughout its extent. Accordingly, the 2σ error envelope is very difficult

271 to see along most of the ASL in Figure 6 because its width closely matches the graphic width of

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272 the ASL itself. It is thus a graphic illustration of the emerging fine resolution resulting from the

273 use of high-precision U-Pb CA-ID-TIMS dating techniques.

274 As a matter of protocol, for each interpolated age in the HCFm we set 2σ error at ± 0.1

275 Ma (Fig. 6–7, Table 3). In Figure 6, the ± 0.1 Ma error envelope for interpolated ages of the

276 HCFm is illustrated by the finely dotted lines extending along the length of the ASL. Although

277 the error envelopes for interpolated ages (± 0.1 Ma) encompasses twice the error that we

278 calculated for our dated bentonites (see above) this protocol still allows us to easily compare

279 interpolated ages in the HCFm with interpolated ages for many of the same magnetostratigraphic

280 boundaries and biostratigraphic events and ranges that have been reported from co-eval strata in

281 other regions (dashed blue line in Figure 6; Ogg and Hinnov 2012) and that typically have

282 greater associated error (Ogg et al. 2004;Draft Ogg and Hinnov 2012).

283 Using the intercept method and the ± 0.1-Ma-error protocol described above for

284 interpolated ages, we interpolate an age range of 73.1–68.0 Ma, a duration of 5.1 Ma, and an

285 average rate of sedimentary rock accumulation of 4.7 cm/1000 years (240 m/5.1 Ma) for the

286 HCFm in the Drumheller area (Fig. 7, Table 3). In the following sections, we assess other

287 interpolated ages within the HCFm and discuss their importance.

288

289 Dorothy Bentonite

290 The Dorothy Bentonite, an approximately 9 m thick deposit of bentonite-rich mudstone

291 in the upper Bearpaw Formation southeast of Drumheller. It is interpreted as a secondarily

292 thickened volcanic ashfall deposit (Lerbekmo 2002), and has been further discussed by

293 Lerbekmo and Braman (2002), Pyle (2003), Eberth and Braman (2012), and Eberth et al. (2013).

294 Lerbekmo (2002) reported a Rb-Sr age of 73.5 ± 0.4 Ma for the Dorothy Bentonite based on a

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295 personal communication from H. Baadsgaard. The age was derived from an analysis of biotites,

296 but no analytical data were included in the report. Furthermore, the age was incorrectly reported

297 as 73.2 Ma by Lerbekmo and Braman (2002; D.R. Braman personal communication, September,

298 2018). These age reports were included in Eberth and Braman (2012) and Eberth et al. (2013).

299 Our interpolated age for the Dorothy Bentonite is 73.7 ± 0.1 Ma, which closely matches

300 the age of 73.5 ± 0.4 Ma reported by Lerbekmo (2002; Fig. 6) and falls within its ± 0.4 Ma 2σ

301 error-range (Fig. 6).

302 Because fragmentary specimens of Baculites cuneatus have been retrieved from above

303 the Dorothy Bentonite (Tsujita 1995; near the base of the 32r magnetochron, Lerbekmo and

304 Braman 2002), and because the small reversed interval near the top of the 33n magnetochron

305 yields Baculites compressus (LerbekmoDraft and Braman 2002), we regard the Dorothy Bentonite as

306 occurring at, or straddling the boundary between these two ammonite biozones. Braman (2018)

307 approximated the boundary between the Pseudoaquilapollenites parallelus-Parviprojectus

308 leucocephalus palynostratigraphic biozone and the overlying Wodehouseia gracile-Mancicorpus

309 glaber biozone as at the Dorothy Bentonite. Accordingly, we regard the Dorothy Bentonite as

310 marking an important biostratigraphic transition in the region, just prior to the onset of HCFm

311 deposition.

312

313 Drumheller Member and the Drumheller bentonite zone

314 We interpolate the base of the HCFm in the type area as having an age of 73.1 ± 0.1 Ma

315 (Figs. 6–7, Table 3), which is approximately 0.5 Ma older than suggested by Eberth and Braman

316 (2012) and Eberth et al. (2013). In the Drumheller area, the base of the Horseshoe Canyon

317 Formation coincides with the base of the Drumheller Member, and occurs near the top of the 32r

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318 magnetochron and in the lower portion of the B. reesidei ammonite biozone (Lerbekmo and

319 Braman 2002).

320 Braman (2018) recognized the base of the Kurtzipites andersonii palynostratigraphic

321 biozone in the middle of the Drumheller Member, at or slightly above the #5 coal (palynological

322 data on file at RTMP; D.R. Braman, personal communication, September, 2018). We interpolate

323 its basal age as 72.4 ± 0.1 Ma (Figs. 6–7, Table 3).

324 Eberth and Braman (2012) identified the 6-7 coal zone at the top of the Drumheller

325 Member, and recognized the Drumheller bentonite zone (DBZ) as a widespread bentonite-rich

326 zone that occurs within the 6-7 coal zone, and extends slightly up-section (locally) into the

327 bottom of the overlying Horsethief Member. The OPB occurs in the lowest portion of the DBZ

328 (on coal 6; Fig. 1D), and was dated in ourDraft study at 72.342 ± 0.069 Ma (Figs. 6–8, Table 3). We

329 interpolate an age of 72.2 ± 0.1 Ma for the Drumheller-Horsethief boundary, and propose that the

330 bentonites that comprise the DBZ accumulated over a time span of ~0.1–0.2 Ma.

331 Eberth and Braman (2012) noted that DBZ volcanism and the Drumheller-Horsethief

332 member transition coincide broadly with an increased rate of withdrawal for the Bearpaw Sea,

333 and documented an up-section shift from stacked, lower-coastal-plain deposits dominated by

334 distributary channels in the Drumheller Member, to off-lapping upper-coastal-plain deposits

335 dominated by meandering channel deposits in the Horsethief Member (Eberth and Braman

336 2012). Although vertebrate fossil occurrences and quality of preservation are quantifiably better

337 in the Horsethief Member (Eberth et al. 2013), there are no discernable taxonomic changes

338 across the Drumheller-Horsethief member boundary.

339

340 Horsethief and Morrin members

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341 Eberth and Braman (2012) placed the boundary between the Horsethief and Morrin

342 members at the top of the 8-9 coal zone, and noted that this boundary approximates an up-section

343 transition from warm-and-wet to cool-and-dry climatic conditions. They also noted a decrease in

344 overall grain size and paleochannel dimensions, compatible with changing climatic (and possibly

345 tectonic) influences on weathering, erosion, and sediment supply in source area. Eberth et al.

346 (2013) noted a significant faunal transition in mega-herbivore dinosaurs at this boundary, with

347 the Edmontosaurus regalis-Pachyrhinosaurus canadensis fossil assemblage being replaced by

348 the Hypacrosaurus altispinus-Saurolophus osborni assemblage. They also noted that

349 centrosaurine dinosaurs disappear from the region at this horizon. Lastly, Braman (2018)

350 approximated the placement of the boundary between the Kurtzipites andersonii and

351 Mancicorpus rostratus-Mancicorpus vancampoiDraft palynostratigraphic biozones at this position.

352 The TRB, with an age of 71.832 ± 0.044 Ma, occurs within the #8 coal swarm in the

353 upper one-half of the Horsethief Member. Our interpolated age for the overlying Horsethief-

354 Morrin member boundary, the onset of regional climatic cooling-and-drying, and the associated

355 changes in dinosaur and palynostratigraphic assemblages described above, is 71.5 ± 0.1 Ma.

356

357 Morrin and Tolman members

358 Along the Red Deer River valley, from the villages of Morrin to Rumsey, the boundary

359 between the Morrin and Tolman members occurs at the base of a widespread bentonite

360 succession that includes the MBB. Laterally continuous outcrop and subsurface expressions of

361 this bentonite succession were interpreted by Eberth and Braman (2012) as reflecting the

362 maximum transgression of the Drumheller Marine Tongue (DMT) in the region. Although an

363 40Ar/39Ar age of 70.44 ± 0.17 Ma was reported for the MBB by Eberth and Deino (2005), no

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364 analytical data were presented by them. Thus, our U-Pb age of 70.896 ± 0.048 Ma for the MBB

365 supersedes the results of that previous study, and stands as the best age estimate for the Morrin-

366 Tolman member boundary and the maximum transgression.

367 Eberth and Currie (2010) recognized the Albertosaurus bonebed as occurring near the top

368 of the Tolman Member (their “Unit 4”), ~8 m below the Tolman-Carbon member contact, and 9

369 m above the Albertosaurus Bonebed bentonite (ABB). We interpolate an age of 70.1 ± 0.1 Ma

370 for the bonebed.

371

372 31r-31n magnetochron boundary and the Campanian-Maastrichtian boundary

373 Lerbekmo and Coulter (1985), Lerbekmo and Braman (2002), Lerbekmo and Braman

374 (2005), and Eberth and Braman (2012, appendixDraft AS1-9), recognize the 31r-32n magnetochron

375 boundary 9.5 m below the MBB in the Morrin Member, and we interpolate its age as 71.1 ± 0.1

376 Ma. This is 0.3 Ma younger than an age of 71.4 Ma interpolated by Ogg and Hinnov (2012) that

377 was based on 40Ar/39Ar dates, biostratigraphic data, and spline-fit calculations.

378 Although Lerbekmo and Braman (2002) and Eberth and Braman (2012) placed the

379 Campanian-Maastrichtian stage boundary at the 31r-32n magnetochron boundary, we agree with

380 Ogg and Hinnov (2012, and references therein) that the C-M stage boundary likely occurs lower

381 in magnetochron 32n. However, it remains unclear exactly where to place the Campanian-

382 Maastrichtian (C-M) stage boundary in the Red Deer River valley stratigraphic section due to

383 uncertainties in interpreting the magnetostratigraphic data of Ogg and Hinnov (2012).

384 Although Ogg and Hinnov (2012) proposed that the C-M boundary be placed in

385 magnetochron 32n.2n, Lerbekmo and co-workers have consistently documented that the 32n

386 magnetochron contains at least 6 distinct normal-reverse pairs in western Canada, many of which

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387 are differentially preserved across the region (cf. Lerbekmo and Braman 2002, figs. 6, 14).

388 Because Ogg and Hinnov (2012, and references therein) indicate the presence of only one brief

389 reversal (32n.1r) in the upper one-half of 32n, it is unclear which of Lerbekmo’s three sub-chron

390 normals in the upper one-half of 32n actually represent 32.2n of Ogg and Hinnov (2012).

391 Because independent biostratigraphic evidence for the C-M boundary is derived mostly from

392 marine deposits (Ogg and Hinnov 2012), there are no independent biostratigraphic data (i.e., the

393 ammonoid B. baculus) in the Red Deer River valley section with which to resolve this

394 uncertainty. Accordingly, we propose that the C-M boundary occurs somewhere in the upper

395 one-half of the Horsethief Member where portions of at least three normal sub-chrons (32n.2n,

396 32n.3n, and 32n.4n; Lerbekmo and Braman 2002) may equate to Ogg and Hinnov’s 32n.2n

397 (Figs. 4–5). We consider it equally likelyDraft that any of these sub-chrons (or combinations of them)

398 may correlate with 32n.2n of Ogg et al. (2012), and interpolate the age of the C-M boundary in

399 the Red Deer River valley section as occurring somewhere in the range of 71.8–71.4 Ma.

400

401 Carbon-Whitemud members

402 Eberth and Braman (2012) noted a variety of lithofacies features that signaled a return to

403 climatically warm-wet conditions near the top of the HCFm, and used them to establish the

404 Carbon Member. However, Eberth and Braman (2012) also noted that the up-section transition

405 from the Tolman to the Carbon Member was gradual, and difficult to identify consistently from

406 section to section. Here, we recognize placement of that boundary at 212.5 m in our composite

407 measured section (Fig. 2), coincident with the 31r-31n magnetochron boundary, the base of the

408 Scollardia trapaformis-Mancicorpus gibbus biozone, and the base of the Eotriceratops

409 xerinsularis dinosaur assemblage zone (Figs. 6–8; also see Koppelhus and Braman 2010; and

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410 Braman 2018). Our interpolated age for this significant, combined lithostratigraphic and

411 biostratigraphic boundary is 69.6 ± 0.1 Ma.

412 Eotriceratops xerinsularis was described by Wu et al. (2007) as a new taxon of

413 triceratops-size chasmosaurine dinosaur. The only known specimen (TMP2002.057.0007; type)

414 was collected 13.5 m above the base of the Carbon Member (between the #11 and #12 coals).

415 Our interpolated age for the Eotriceratops xerinsularis quarry is 68.8 ± 0.1 Ma. This age

416 supersedes the much younger age range of 67.6–68.0 Ma estimated by Wu et al. (2007) using

417 calibrated magnetochron data from the 2004 Geologic Time Scale (Ogg et al. 2004).

418 Eotriceratops plays an important role in clarifying the timing and manner in which

419 Tyrannosaurus-Triceratops-grade dinosaur assemblages became established in North America

420 (e.g., Wu et al. 2007; Eberth et al. 2013;Draft Scannela et al. 2014), and Eberth et al. (2013)

421 tentatively proposed a distinct, Eotriceratops xerinsularis dinosaur assemblage zone based on the

422 presence of that taxon. They speculated that the Eotriceratops assemblage may have persisted

423 through the time represented by the combined Carbon and Whitemud members, interpolated as

424 ranging in age from 69.6–68.0 Ma in this study.

425 Binda (1992), Binda and Nambudiri (2000), Hamblin (2004), Lerbekmo (2009), Eberth

426 and Braman (2012), and Eberth and Kamo (2019) described the Whitemud Member of the

427 HCFm and the Battle Formation as kaolinized stratigraphic intervals reflecting times of (1) very

428 low accommodation, (2) low sediment-supply dominated by volcanic ashfalls, and (3)

429 widespread substrate modification due to exposure/weathering, soil formation, and dinosaur

430 trampling. Eberth and Braman (2012) emphasized climatic and tectonic influences that increased

431 subsidence in the proximal foredeep and sediment starvation in the distal foredeep. Lerbekmo

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432 (2009) proposed that the kaolinized Whitemud Member (and Colgate Member in Montana)

433 reflect a time of glacioeustatic sea level fall during a time range of 200 ka.

434 The base of the Whitemud is coincident with the bases of the 30n magnetochron and the

435 Pseudoaquilapollenites bertillonites palynostratigraphic biozone (Lerbekmo and Braman 2002;

436 Braman 2018), and can be correlated into Saskatchewan, Montana, and South Dakota, where

437 vertebrate assemblages are present (Eberth and Kamo 2019). We interpolate the age of this

438 boundary as 68.2 ± 0.1 Ma, as previously proposed by Ogg and Hinnov (2012) and Eberth and

439 Kamo (2019). We interpolate the age of the top of the Whitemud and, thus, the top of the

440 Horseshoe Canyon Formation as 68.0 ± 0.1 Ma. Our estimated age range of 200 ka for the

441 Whitemud Member matches that of Lerbekmo (2009) and supports his interpretation of a

442 glacioeustatic sea-level fall in western CanadaDraft at that time.

443

444 Comparison with interpolated ages in GTS 2012

445 Figure 6 includes the “GTS 2012 ASL” (dashed blue line) that we constructed by

446 combining late Cretaceous magnetostratigraphic and lithostratigraphic data from Alberta

447 (Lerbekmo and Braman 2002; Eberth and Braman 2012) with magnetochron boundary-ages for

448 magnetochrons 29–33 as interpolated by Ogg and Hinnov (2012). Ogg and Hinnov’s

449 interpolations are part of the International Commission on Stratigraphy’s (ICS) Global Time

450 Scale (GTS) 2012 (Gradstein et al. 2012), currently the most widely used geologic time scale in

451 the global geologic community. We calibrated the GTS 2012 ASL in Figure 6 using six

452 magnetochron boundary-ages as presented by Ogg and Hinnov (2012, figure 27.6). This study

453 also includes the currently accepted age for the K-Pg boundary (see above) and recent

454 refinements to magnetochron 29 boundary-ages (Sprain et al. 2014).

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455 The shapes of the two ASLs constructed from our radioisotopic data (solid line) and those

456 data of Ogg and Hinnov (dashed line) are similar, indicating a close correspondence in rates of

457 sediment accumulation (slopes) in all portions of the Red Deer River valley section, regardless of

458 which age calibration (this study or that of Ogg and Hinnov) is used. Thus, for example,

459 although there are no dated bentonites in the Red Deer River valley section between ABB and

460 BB — an approximately 3.75 Ma gap in our understanding of the age of these upper HCFm

461 strata — the similar shapes of both ASLs indicate that our age interpolations in this interval are

462 at least consistent from horizon to horizon when compared with those of Ogg and Hinnov

463 (2012).

464 More significantly, comparisons of the two ASLs in Figure 6 reveal consistent age

465 differences between them, and thus, differencesDraft in the age-calibration methods used for both sets

466 of data. In our ASL (derived from Red Deer River valley data only), the interpolated boundary-

467 ages of magnetochrons 32n and 32r are 0.3–0.9 Ma younger than those presented by Ogg and

468 Hinnov (2012), whereas the interpolated boundary-ages for magnetochrons 31r, 31n, 30r, and

469 30n are 0.0–0.6 Ma older in this study than those presented by Ogg and Hinnov (2012).

470 Differences in calibrated data sets may result from a variety of factors. For example,

471 subchrons (and sometimes chrons), in magnetostratigraphic data sets may be incorrectly

472 identified (see discussion in Lerbekmo and Braman 2002). Alternatively, magnetochrons may be

473 differentially preserved from place to place — a common problem for calibrated

474 magnetostratigraphic data sets compiled from non-marine alluvial units (such as the HCFm). In

475 such cases, effective comparisons of magnetostratigraphic data often require the use of

476 biostratigraphic data to constrain which portions of the magnetostratigraphic record are

477 present/absent in a given section (e.g., Lerbekmo and Lehtola 2011; Ogg and Hinnov 2012).

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478 Problems may also arise where calibrations employ different radiometric dating methods (e.g.,

479 U-Pb, K-Ar, Ar/Ar), or a single method where improvements in analytical procedures are

480 ongoing (e.g., Kuiper et al. 2008; Renne et al. 2010; Fowler 2017). In such cases, differences of

481 up to 1% may occur (e.g., compare the ICP’s Cretaceous time scales in GTS 2004 and 2012, and

482 see discussion in Gradstein et al. 2012 and Fowler 2017).

483 Although uncertainty exists as to why our age-calibrations differ from those of Ogg and

484 Hinnov (2012), the similar shapes of the two ASLs indicate that the age differences are likely

485 due to a systemic problem through the two portions of the section identified above, and thus are

486 not likely the result of a uniquely local pattern of magnetochron completeness in the Red Deer

487 River valley section. If so, one would expect the overall shapes of the two ASLs to differ even

488 more between stratigraphic points-of-interestDraft or magnetochrons. For now, we regard the

489 differences as reflecting our use of a small data set from, and applied to, the Red Deer River

490 valley section, versus Ogg and Hinnov’s use of large data-compilations (from a variety of places)

491 that have been calibrated and “smoothed” using a variety of statistical methods. Although we

492 remain biased in favor of the validity of our data sets that were gathered from the local Red Deer

493 River valley area, clearly, additional study of both data sets is required to determine more

494 precisely where systemic improvements and corrections are necessary.

495

496 Rates of sedimentary rock accumulation

497 Different slopes in segments of our ASL (Fig. 6) reflect differences in rates of

498 sedimentary rock accumulation (RSA) through the HCFm. Overall, the RSA in the lower ¾ of

499 the HCFm remains stable at ~8.1 cm/ka (196 m/2.425 Ma; Fig. 6). Because this rate is consistent

500 throughout much of the HCFm, as well as down through the Bearpaw Formation to the top of the

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501 Dinosaur Park Formation, it suggests that sediment supply and subsidence were likely balanced

502 in this region during the Bearpaw transgression-regression cycle. In turn, this interpretation

503 supports previous conclusions that the Bearpaw cycle was eustatic in origin (Catuneanu et al.

504 2000; Ogg and Hinnov 2012; Eberth and Braman 2012). However, a rate of 8.1 cm/ka is

505 relatively high compared to rates from other upper Cretaceous non-marine sections in southern

506 Alberta (cf. 3.5–4.8 cm/ka reported by Eberth 2005 and Lerbekmo 2005). Thus, the development

507 of the Drumheller delta in this region (lower portion of the HCFm stratigraphic section across

508 south-central Alberta, including the Drumheller area; Eberth and Braman 2012) suggests that the

509 distal foredeep experienced high rates of sediment supply relative to subsidence.

510 A significant decrease in the RSA occurred sometime after deposition of the

511 Albertosaurus Bonebed bentonite (TolmanDraft Member, Fig. 6). We estimate that the RSA

512 decreased to ~1.6 cm/ka (44 m/2.724 Ma), reflecting a significantly reduced sediment supply, a

513 reduction in accommodation that resulted in sediment bypassing, or both. Vertebrate fossil

514 preservation is quantifiably poorer in the Tolman and Carbon members (Eberth et al. 2013)

515 compared to the Drumheller and Horsethief members, and provides potential insight into how

516 rates of sediment accumulation declined. Specifically, ubiquitous occurrences of highly

517 fragmented, weathered, and isolated fossil bones in the Tolman Member fit well with an

518 interpretation of reduced sediment supply that would have resulted in greater exposure times and

519 greater degradation of vertebrate skeletal remains. In contrast, the rarity of vertebrate fossils of

520 all types in the Carbon Member (Eberth et al. 2013), combined with geological evidence for an

521 increased sediment supply due to a shift to a wetter climate (Eberth and Braman 2012) suggest

522 that sediment by-passing may have increased during Carbon Member deposition.

523

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524 Alberta’s late Campanian–middle Maastrichtian dinosaur biostratigraphy

525 The HCFm chronostratigraphy presented here (Figs. 6–7, Table 3) provides a baseline for

526 assessing and comparing the ages of upper Campanian–middle Maastrichtian dinosaur bearing

527 strata in the Red Deer River valley region, and elsewhere in Alberta and North America.

528

529 Revised ages for the HCFm’s dinosaur assemblage zones

530 Here, we revise the age ranges of the HCFm’s dinosaur assemblage zones as follows

531 (Figs. 7–8, Table 3): 1) Edmontosaurus regalis-Pachyrhinosaurus canadensis, 73.1–71.5 Ma

532 (duration 1.5 Ma); 2) Hypacrosaurus altispinus-Saurolophus osborni, 71.5–69.6 (duration 1.9

533 Ma); 3) Eotriceratops xerinsularis, 69.6–68.2 Ma (duration 1.4 Ma). These age ranges are ~0.5–

534 1.0 Ma older than those provisionally assessedDraft by Eberth et al. (2013).

535

536 Revised age of the Danek Bonebed, Horseshoe Canyon Formation, Edmonton

537 Davies et al. (2014) reported a U-Pb ID-TIMS date of 71.923 ± 0.068 from a detrital

538 zircon collected from a bentonitic sandstone lying 30 cm below the Edmontosaurus-dominated

539 Danek Bonebed in the HCFm, southwest of Edmonton. They were unable to calculate a weighted

540 mean age due to the absence of a population of high-precision U-Pb ID-TIMS dates in the

541 youngest portion of the age range. Nonetheless, they were able to confidently interpret this date

542 as a maximum age for deposition of the Danek Bonebed based on the distribution of ID-TIMS

543 U-Pb dates from all zircon grains in the sample, and overlap of this ID-TIMS date with an age of

544 71.32 ± 0.78 Ma, calculated from the youngest population of statistically equivalent LA-ICP-MS

545 U-Pb dates.

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546 Using sub-surface data, Eberth and Bell (2014) correlated the Danek Bonebed with strata

547 in the Red Deer River valley that occur in the middle of the 8-9 coal zone. They proposed an age

548 of 71.0–71.3 Ma for the bonebed using the tentative HCFm chronostratigraphy of Eberth et al.

549 (2013), which was based on Ogg et al. (2004).

550 We revise these previous conclusions as follows. The age of the stratigraphic interval

551 between the #8 and #9 coals is well defined in the Drumheller area, and is bracketed in age

552 below by TRB (71.832 ± 0.044 Ma; middle of the #8 coal swarm) and above by the Horsethief-

553 Morrin member contact (interpolated at 71.5 ± 0.1 Ma). Thus, our modified estimate for the age-

554 range of the Danek Bonebed is 71.8–71.5. This age range overlaps both the LA-ICP-MS age

555 range and single zircon ID-TIMS date of Davies et al. (2014), and confirms the validity of their

556 maximum-age assessment. A revised age-rangeDraft of 71.8–71.5 Ma for the Danek Bonebed places

557 its fossil assemblage near the top of the Edmontosaurus-Pachyrhinosaurus dinosaur assemblage

558 zone, as originally proposed by Eberth and Bell (2014).

559

560 Age of fossiliferous Unit 4, Wapiti Formation, Grande Prairie region

561 Unit 4 of the Wapiti Formation, near Grande Prairie, is an interval that is rich in

562 Pachyrhinosaurus bonebeds and Edmontosaurus skeletal material (Fanti et al. 2015). Its lower

563 boundary is thought to mark the maximum transgression of the Bearpaw Sea in that region (Fanti

564 and Catuneanu 2009), and it yields 40Ar/39Ar ages from two horizons: 71.89 ± 0.14 Ma (Fanti et

565 al. 2015) and 73.73 ± 0.25 Ma (Fowler’s 2017 recalibration of Fanti and Catuneanu 2009). An

566 additional 40Ar/39Ar age of 72.58 ± 0.09 Ma was reported by Bell et al. (2014) from 2 m above

567 an Edmontosaurus locality along the Red Willow River in the Wapiti Formation, 75 km west of

568 Grande Prairie, but no stratigraphic or analytical data were included with that report. When

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569 correlated chronostratigraphically to the Red Deer River valley section, this temporal interval

570 ranges from the middle of the Bearpaw Formation to near the top of the Horsethief Member (Fig.

571 7). The upper two-thirds of the interval have produced all known specimens of

572 Pachyrhinosaurus and Edmontosaurus in the Drumheller area (Fig. 8). If this correlation can be

573 confirmed in the future with magneto- and palynostratigraphic data, the base of the

574 Edmontosaurus regalis-Pachyrhinosaurus canadensis dinosaur assemblage zone may have to be

575 adjusted older than is currently recognized in the Red Deer River valley section.

576

577 Age of the lower St. Mary River Formation, Scabby Butte

578 The lowest 30 m of the St. Mary River Formation at the geographically restricted Scabby

579 Butte locality, 25 km northwest of Lethbridge,Draft has produced a diverse and locally important

580 vertebrate assemblage in a paralic succession that marks the transition from the marine (Bearpaw

581 Formation) upward into the non-marine (St. Mary River Formation; Langston 1975, 1976). Most

582 notably, the site has produced cranial material and bonebeds of the centrosaurine,

583 Pachyrhinosaurus canadensis (Langston 1975, 1976), as well as more than 30 taxa of

584 vertebrates, including sharks, fishes, amphibians, mosasaurs, meso-reptiles, dinosaurs, and

585 mammals. The strata are similar in general appearance and lithology to those of the Drumheller

586 Member of the HCFm, and have been referred to as “the Edmonton facies of the St. Mary River

587 Formation” by Tozer (1956) and others. Eberth and Braman (2012, fig. 6) used well logs to

588 correlate HCFm strata from the Red Deer River valley south for ~90 km to T16 R26W4,

589 approximately 50 km north of Scabby Butte. They showed that, across that distance, the lowest

590 ~40–50 m of the HCFm is replaced by the marine Bearpaw Formation. Although no radiometric

591 dates or magnetostratigraphic data are known from the Scabby Butte site, Langston (1976), on

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592 the basis of palynostratigraphy, proposed that it correlates best with the middle and upper

593 portions of the “Edmonton A” stratigraphic zone of Ower (1960). Hamblin (2004) and Eberth

594 and Braman (2012) revised Edmonton Group stratigraphy, equating the upper one-half of Ower’s

595 “Edmonton A” zone with the Drumheller Member of the HCFm. Based on these observations,

596 we suggest that the Scabby Butte site fossil assemblage likely correlates with the upper one-half

597 of the Drumheller Member in the Red Deer River valley. This suggests an age range of 72.2–

598 72.6 Ma for the Scabby Butte fossil assemblage, and placement within the combined 32n.4n–5n

599 sub-chron range (cf., Lerbekmo and Braman 2002). These correlations and age assessments

600 place the Scabby Butte fossil assemblage in the E. regalis-P. canadensis dinosaur assemblage

601 zone, and either the upper Wodehouseia gracilis-Mancicorpus glaber or lower Kurtzipites

602 andersonii palynostratigraphic biozone Draftof Braman (2018).

603

604 Potential for correlations beyond Alberta

605 Fowler (2017) presented temporally calibrated correlations of Santonian to end-

606 Cretaceous non-marine strata in the Western Interior of North America, focusing on formations

607 that yield dinosaurs. His adjusted ages for the HCFm match the data in this study for some

608 stratigraphic positions (e.g, ages of the Morrin-Tolman and Horsethief-Morrin member

609 boundaries), but also reflect uncertainties inherited from earlier studies (e.g., age for the base of

610 the Drumheller Member). Regardless of the varying accuracy of the results, Fowler (2017)

611 reflects recent improvements in precision, consistency, and accuracy in U-Pb and 40Ar/39Ar

612 dating, and demonstrates that high-precision radioisotopic data may now be combined from

613 multiple high-precision studies without introducing significant error. Such combinations offer

614 the promise of precise correlations and more meaningful comparisons of ecologically and

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615 taxonomically disparate dinosaur assemblages. In turn, such data will likely improve our

616 understanding of late Cretaceous dinosaur evolution, paleobiogeography, and paleoecology.

617 For example, future comparisons of high-precision radiometric dates and ages from the

618 Cretaceous of Alberta and the Prince Creek Formation of northern Alaska may help refine the

619 ages of Pachyrhinosaurus occurrences across the northern paleolatitudes of North America,

620 eventually providing a test for recent proposals that pachyrhinosaurs, inhabiting refugia in

621 northern Alaska, were the last centrosaurines to survive, persisting into the early Maastrichtian

622 (Fiorillo and Tykoski 2012; Eberth and Bell 2014).

623

624 Conclusions

625 A suite of U-Pb CA-ID-TIMS agesDraft for bentonites from the HCFm and associated

626 formations in the Red Deer River valley of southern Alberta allows us to calibrate litho-,

627 magneto-, and biostratigraphic data sets in the HCFm, and more precisely correlate the formation

628 with other upper Campanian–middle Maastrichtian deposits in Alberta. Our data show that the

629 HCFm, in the type area surrounding Drumheller, ranges in age from 73.1–68.0 Ma (a duration of

630 5.1 Ma) and has an average rate of sediment accumulation of 4.7 cm/1000 years.

631 The Drumheller and Horsethief members represent deposition in a warm-wet deltaic

632 setting that persisted in the area from 73.1–71.5 Ma. The Morrin and Tolman members represent

633 deposition in cooler, seasonally wet-dry coastal plain and alluvial settings from 71.5–69.6 Ma,

634 and the Carbon and Whitemud members represent deposition in warm-wet alluvial to paludal

635 settings from 69.6–68.0 Ma. Each of these three climatic/paleoenvironmental intervals persisted

636 for 1–2 Ma in the region, and is characterized by a distinct dinosaur fossil assemblage zone. In

637 ascending order, these are: the Edmontosaurus regalis-Pachyrhinosaurus canadensis zone

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638 (73.1–71.5 Ma); the Hypacrosaurus altispinus-Saurolophus osborni zone (71.5–69.6 Ma); and

639 the Eotriceratops xerinsularis zone (69.6–68.2 Ma). In addition to the fossil assemblage zones,

640 two unique dinosaur occurrences are present in the HCFm: the Albertosaurus bonebed in the

641 upper Tolman Member (70.1 ± 0.1 Ma), and the triceratopsin, Eotriceratops xerinsularis in the

642 middle Carbon Member (68.8 ± 0.1 Ma).

643 Stratigraphic placement of the Campanian-Maastrichtian (C-M) boundary (and thus

644 assessment of its age) remains uncertain in the Red Deer River valley. This is due to the fact that

645 the upper one-half of the 32n magnetochron in the Red Deer River valley section (and other

646 western Canada sections) contains three subchron reversals, but that only one subchron reversal

647 is recognized in the upper portion of magnetochron 32n in Europe where the C-M boundary is

648 defined. In the absence of independent marineDraft biostratigraphic data in this part of the Red Deer

649 River valley section, accurate placement of the boundary is not yet possible. For now, we

650 propose that the C-M boundary lies in the upper Horsethief Member, and conservatively assign it

651 an interpolated age range of 71.8–71.4 Ma.

652 We compared our U-Pb chronostratigraphy with an alternative chronostratigraphy for the

653 HCFm that was inferred from calibrated magnetostratigraphic data presented in the ICS’s

654 Geological Time Scale 2012. Whereas the shapes of the two age-stratigraphy-lines are similar,

655 the two chronostratigraphies exhibit systemic age differences of up to 1%. Although our locally

656 derived chronostratigraphy for the HCFm is likely to be more accurate than that inferred using

657 GTS 2012 data, more study is required to identify and resolve the cause(s) of the systemic age

658 offsets.

659 The chronostratigraphy presented in this study provides a baseline for assessing and

660 adjusting the ages of upper Campanian–middle Maastrichtian dinosaur bearing strata elsewhere

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661 in Alberta. This allows us to adjust the age of the Edmontosaurus-dominated Danek Bonebed in

662 Edmonton to 71.8–71.5 Ma and places the bonebed’s fossil assemblage near the top of the

663 Edmontosaurus regalis-Pachyrhinosaurus canadensis dinosaur assemblage zone. Radiometric

664 data from Unit 4 in the Wapiti Formation near Grande Prairie correlate with an interval in the

665 Red Deer River valley that extends from the middle of the Bearpaw Formation to near the top of

666 the Horsethief Member. This suggests that the base of the E. regalis-P. canadensis dinosaur

667 assemblage zone in Alberta may have to be adjusted older than is currently recognized in the Red

668 Deer River valley section. The lowest 30 m of the St. Mary River Formation at Scabby Butte,

669 near Lethbridge, produces a diverse vertebrate assemblage, including the centrosaurine,

670 Pachyrhinosaurus canadensis. This interval correlates with the upper one-half of the Drumheller

671 Member in the Red Deer River valley, suggestingDraft an age of 72.2–72.6 Ma for the Scabby Butte

672 fossil assemblage, and placement of the assemblage within the E. regalis-P. canadensis dinosaur

673 assemblage zone. Comparisons of radiometric ages from the HCFm in west-central Alberta and

674 the Prince Creek Formation of Alaska may help refine the age range of Pachyrhinosaurus

675 occurrences across the northern paleolatitudes of North America, eventually providing a means

676 of assessing whether Alaska’s pachyrhinosaurs were the world’s last centrosaurines to survive.

677 Our data should encourage vertebrate biostratigraphers to revise and refine age comparisons of

678 Campanian-Maastrichtian age dinosaur assemblages throughout North America, especially

679 where high resolution age data and chronostratigraphic frameworks are beginning to emerge.

680

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681 Acknowledgements

682 DAE and SLK thank Jahandar Ramezani of MIT for providing samples of the Bearpaw

683 bentonite for dating. SLK thanks the staff of the Jack Satterly Geochronology Laboratory for

684 technical support. DAE thanks Dennis Braman, David Evans, Caleb Brown, James Gardner, and

685 Eric Roberts for advice, insightful discussions, and pre-submission assessments.

686 Allison Vitkus of the Royal Tyrrell Museum of Palaeontology kindly provided assistance

687 with Figure 1. The Government of Alberta provided financial support for this component of the

688 ‘End-of-Dinosaurs’ Chronostratigraphy Research Project. The Tyrrell Museum Cooperating

689 Society provided travel and logistics support, and we thank Patty Ralrick of that organization.

690 We thank the journal editors and two anonymous reviewers for their patience, and helpful

691 comments and criticisms. Any and all errorsDraft remain the responsibility of the authors.

692

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849 Sprain, C.J., Renne, P.R., Wilson, G.P., and Clemens, W.A. 2014. High resolution

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858 western Alberta. Geological Survey of Canada Memoir 280, 125 p.

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860 Bearpaw Formation in southern Alberta. PhD. Thesis, McMaster University, Hamilton,

861 Ontario, 357 p.

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865 Figure Captions

866

867 Figure 1. Study area in Alberta. A, Map of Alberta showing extent of Horseshoe Canyon

868 Formation (yellow shading), Bearpaw Formation (green shading), and Battle Formation

869 (red line). B, Location (white dots) of all bentonites dated here. C, Location of HCFm

870 bentonites dated here. D, typical example of weathered and freshly exposed bentonite

871 (overflow parking bentonite, OPB) near the Royal Tyrrell Museum. Knife marks sharp

872 base of bentonite. Scale bar is 10 cm. Abbreviations: ABB, Albertosaurus Bonebed

873 bentonite; BB, Battle bentonite; BFm, Battle Formation; BpB, Bearpaw bentonite; BpFm,

874 Bearpaw Formation; BRG, Belly River Group; HCFm, Horseshoe Canyon Formation;

875 km, kilometers; MBB, Morrin BridgeDraft bentonite; N, north; OPB, overflow parking

876 bentonite; TRB, Trentham road bentonite.

877 Figure 2. Composite measured section and dated bentonites in the Horseshoe Canyon and Battle

878 formations in the Red Deer River valley near Drumheller. Modified from Eberth and

879 Braman (2012). Darker tones indicate increased organic content. Abbreviations: ABB,

880 Albertosaurus Bonebed bentonite; BB, Battle bentonite; c, coarse sandstone; cg,

881 conglomerate; cl, claystone; DBZ, Drumheller bentonite zone; f, fine sandstone; Fm,

882 Formation; m, medium sandstone; m, meters; MBB, Morrin Bridge bentonite; Mbr,

883 Member; OPB, overflow parking bentonite; slt, siltstone; TRB, Trentham road bentonite;

884 Wmd, Whitemud.

885 Figure 3. Lithostratigraphy of the Edmonton Group in southern Alberta. Modified from Eberth

886 and Braman (2012). Caps indicate stratigraphic occurrences of Battle bentonite and K-Pg

887 boundary bentonite. Asterisks indicate stratigraphic occurrences of the four Horseshoe

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888 Canyon Formation bentonites dated and described by us. Abbreviations: BFm, Battle

889 Formation; cz, coal zone; DB, Dorothy Bentonite; DBZ, Drumheller bentonite zone;

890 DMT, Drumheller Marine Tongue; HCFm, Horseshoe Canyon Formation; K-Sc,

891 Cretaceous portion of the Scollard Formation; P-mag, magnetostratigraphy; Pal,

892 Paleocene; Pg-Sc, Paleogene portion of the Scollard Formation.

893 Figure 4. Five Concordia diagrams showing U-Pb zircon CA-ID-TIMS results for each of the

894 five bentonites that were dated in the Horseshoe Canyon and Bearpaw formations

895 exposed in the Red Deer River valley of southern Alberta (see text). Each diagram is

896 labelled with the name of the bentonite that it represents.

897 Figure 5. Chronostratigraphic plot of 206Pb-238U dates for each of the five bentonites in the

898 Horseshoe Canyon and BearpawDraft formations in the Red Deer River valley of southern

899 Alberta (see text). Y-axis indicates calibrated ages in mega-annums. Shaded/colored bars

900 and outlines indicate those dates included in weighted mean age; white bars indicate

901 dates that were excluded from age interpretations. Weighted mean ages indicated by

902 dotted lines. 2σ error indicated by vertical thickness of the grey shading.

903 Figure 6. Litho-, magneto-, bio-, and chronostratigraphy of Upper Cretaceous strata in the Red

904 Deer River valley. Age-stratigraphy lines (ASL) are based on (1) the U-Pb CA-ID-TIMS

905 ages presented here (large dots, solid line with error envelopes [see figure legend]), and

906 (2) Ogg and Hinnov’s (2012) calibrated magnetostratigraphy (small dots and dashed

907 line). Isolated dots indicate previously presented radiometric ages from Alberta

908 bentonites that did not include analytical data; those ages are superseded by the results

909 presented here. Asterisks in the lithostratigraphy column indicate stratigraphic positions

910 of dated bentonites. Note the multiple sub-chrons in magnetochron 32n. Reference key as

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911 follows: a, Lerbekmo and Braman (2002); b, Eberth and Braman (2012); c, Braman

912 (2018); d, Tsujita (1995); e, Lerbekmo and Lehtola (2011); f, Eberth et al. (2013); g,

913 Renne et al. (2013); h, Sprain et al. 2014; i, Eberth and Kamo (2019); j, Eberth and Deino

914 (2005); k, Lerbekmo (2002). Other abbreviations: ABB, Albertosaurus Bonebed

915 bentonite; BB, Battle bentonite; BpB, Bearpaw bentonite; MBB, Morrin Bridge

916 bentonite; OPB, overflow parking bentonite; TRB, Trentham road bentonite; BFm, Battle

917 Formation; C, Carbon Member; D, Drumheller Member; DB, Dorothy Bentonite; DBZ,

918 Drumheller bentonite zone; DPFm, Dinosaur Park Formation; H, Horsethief Member;

919 Fm, Formation; K-Pg, Cretaceous-Paleogene; ka, kilo-annum; Leth, Lethbridge; M,

920 Morrin Member; Ma, mega-annum; PAL, Paleocene; T, Tolman Member; W, Whitemud

921 Member. Draft

922 Figure 7. Chronostratigraphically calibrated litho-, magneto-, and biostratigraphies for Upper

923 Cretaceous strata in the Red Deer River valley. Asterisks indicate stratigraphic positions

924 of dated bentonites. Reference key as follows: a, Sprain et al. 2014; b, Eberth and Kamo

925 (2019). Abbreviations as in Figure 6, except Mbr (Member).

926 Figure 8. Chronostratigraphically calibrated dinosaur biostratigraphy for the Edmonton Group in

927 the Red Deer River valley in the Drumheller area. Modified from Eberth et al. (2013).

928 Vertical bars indicate well established age ranges for ornithischian (white) and theropod

929 (black) taxa based on stratigraphic occurrences. Numbers in bars indicate totals of

930 identifiable specimens known as of 2017. Dashed vertical lines indicate inferred age

931 ranges. Dinosaur assemblage zones indicated by horizontal shading. Abbreviations and

932 symbols as in figures 3, 6–7, except BB (bonebed).

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933 Table 1. Bentonites used to compile the current chronostratigraphy for the Horseshoe Canyon

934 Formation in the Red Deer River valley near Drumheller.

935 Table 2. U-Pb CA-ID-TIMS data for zircons from five bentonites in the Horseshoe Canyon and

936 Bearpaw formations, Red Deer River Valley, southern Alberta, Canada.

937 Table 3. Summary of dated and age-interpolated features and intervals in the Bearpaw,

938 Horseshoe Canyon, Battle, and Scollard formations in the Red Deer River valley near

939 Drumheller.

Draft

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Table 1. Bentonites used to compile our chronostratigraphy for the Horseshoe Canyon Formation in the Red Deer River valley near Drumheller. Bentonite Specimen Number Location Meters above Formation/Member U-Pb CA-ID-TIMS Age Notes local base of HCFm

Battle TMP2017.033.0001 N51.900978 244 m Battle Formation 66.936 ± 0.047/0.060/0.140 Ma Collected in 2017 from Knudsen's Farm locality. Uppermost bentonite (BB) W113.012015 (upper) portion of the Battle Formation, 75 cm below the contact with the Scollard Formation. Stratigraphic details presented in Eberth and Kamo (in press). Albertosaurus TMP2018.100.0005 N51.948364 196 m HCFm; Tolman Mbr 70.675 ± 0.047/0.064/0.141 Ma Collected in 2000 during measurement of the Albertosaurus Bonebed W112.943512 (upper) Bonebed section. Part of a small channel-fill, thus hydraulic reworking likely. Occurs stratigraphically above the stacked ss bentonite interval that forms the lower portion of the Tolman Mbr. (ABB) Located at 12 m in the measured section published by Eberth and Currie (2010) and Eberth and Braman (2012).

Morrin Bridge TMP2018.100.0004 N51.654671 169 m HCFm; 70.896 ± 0.048/0.065/0.142 Ma Collected in 2009. Occurs 3 m above the #10 coal, marks the bentonite W112.892479 Tolman/Morrin Mbr boundary between the Morrin and Tolman mbrs, and the max flooding surface of the Drumheller Marine Tongue (Eberth and (MBB) Draftcontact Braman 2012). This bentonite horizon is widespread. Locality is East of bridge along Highway 27 north-side roadcut, 1/2 way up hill. Excellent section exposed here. Measured section included in Eberth and Braman (2012). Trentham road TMP2018.100.0003 N51.645598 113 m HCFm; Horsethief 71.832 ± 0.044/0.061/0.140 Ma Collected in 2009. Occurs in the bentonite succession overlying bentonite W112.910114 Mbr (middle) the lowest exposed coal in the Morrin Bridge area (#8 coal swarm, SW of bridge). Collected just west of the gravel road to (TRB) Trentham's farm. Measured section included in Eberth and Braman (2012).

Overflow TMP2018.100.0002 N51.477004 67 m HCFm; Drumheller 72.353 ± 0.037/0.054/0.133 Ma Collected in 2009 from east side of the road to the overflow parking W112.789779 Mbr (upper) parking behind and below the Royal Tyrrell Museum and at the top of the #6 coal swarm. Excellent exposures and one of the bentonite better bentonites in the section. No associated measured section. (OPB) Bearpaw TMP2018.100.0001 N50.755531 -110 m Bearpaw Formation 74.308 ± 0.031/0.050/0.130 Ma Collected in 2017 from northeast Dinosaur Provincial Park bentonite W111.381729 (lower) (Iddesleigh area). Occurs ~5.5 m above base of the Bearpaw Formation. Placement in measured section shown in Eberth (BpB) (2005). Age of the Bearpaw bentonite indicated as 74.8 Ma in that publication. Revised to 74.26 ± 0.03 Ma (Eberth et al. 2016) and 74.308 ± 0.031 Ma here.

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TABLE 2. U-Pb CA-ID-TIMS data for zircons from five bentonites in the Horseshoe Canyon and Bearpaw formaitons, Red Deer River valley, southern Alberta, Canada.

a) b) c) d) e) e) f) Age (Ma)

206 204 207 235 206 238 207 206 207 206 207 235 206 238 No. Weight (μg) PbC (pg) PbT/PbC U (ppm) Th/U Pb/ Pb Pb/ U 2s Pb/* U 2s Err Corr Pb/* Pb 2s Pb/* Pb 2s Pb/ U 2s Pb/* U 2s Albertosaurus Bonebed bentonite (ABB) z1 2.1 0.3 16.7 125 0.47 1058 0.14164 0.00193 0.020841 0.000029 0.624 0.0493 0.0006 162 30 134.50 1.72 132.97 0.18 z2 10.4 1.2 6.5 57 0.51 422 0.08028 0.00121 0.012179 0.000035 0.449 0.0478 0.0007 90 33 78.41 1.14 78.03 0.22 z3 2.3 0.4 13.8 223 0.29 922 0.07981 0.00061 0.012124 0.000018 0.466 0.0477 0.0003 87 17 77.97 0.57 77.68 0.11 z4 1.7 0.5 7.9 222 0.20 552 0.07592 0.00154 0.011117 0.000020 0.511 0.0495 0.0010 173 46 74.30 1.46 71.27 0.13 z5 2.0 0.6 19.7 560 0.39 1277 0.07278 0.00044 0.011029 0.000013 0.491 0.0479 0.0003 92 13 71.34 0.42 70.71 0.08 z6 2.9 1.3 66.5 2697 0.35 4306 0.07217 0.00016 0.011024 0.000011 0.471 0.0475 0.0001 73 5 70.75 0.15 70.68 0.07 z7 2.9 2.2 5.1 359 0.25 359 0.07178 0.00144 0.011015 0.000016 0.760 0.0473 0.0009 63 45 70.39 1.37 70.62 0.10

Morrin Bridge bentonite (MBB) z1 2.6 0.4 14.0 194 0.46 892 0.07976 0.00066 0.011472 0.000014 0.490 0.0504 0.0004 214 18 77.91 0.62 73.534 0.086 z2 4.3 1.1 7.6 182 0.39 506 0.07265 0.00045 0.011126 0.000040 0.610 0.0474 0.0002 67 12 71.21 0.42 71.325 0.254 z3 3.0 0.3 17.0 167 0.49 1074 0.07298 0.00070 0.011119 0.000010 0.584 0.0476 0.0004 80 21 71.52 0.66 71.282 0.067 z4 4.4 0.5 18.5 172 0.71 1103 0.07302 0.00200 0.011070 0.000019 0.864 0.0478 0.0012 91 61 71.56 1.89 70.967 0.123 z5 4.2 0.6 16.4 198 0.43 1056 0.07266 0.00082 0.011067 0.000014 0.381 0.0476 0.0005 80 26 71.22 0.77 70.949 0.091 z6 2.9 0.3 22.9 187 0.43 1469 0.07274 0.00080 0.011053 0.000015 0.459 0.0477 0.0005 86 25 71.29 0.76 70.859 0.099 z7 1.8 0.3 10.0 137 0.57 628 0.07208 0.00095 0.011052 0.000014 0.565 0.0473 0.0006 64 30 70.67 0.90 70.856 0.090 z8 2.1 1.1 6.3 307 0.42 418 0.07222 0.00100 0.011046 0.000029 0.393 0.0474 0.0006 71 31 70.81 0.95 70.815 0.187

Trentham road bentonite (TRB) z1 10.2 1.2 12.2 128 0.39 798 0.07706 0.00036 0.011292 0.000014 0.746 0.0495 0.0002 171 9 75.38 0.34 72.385 0.091 z2 9.0 0.8 16.5 126 0.34 1089 0.07523 0.00034 0.011505 0.000012 0.762 0.0474 0.0002 71 9 73.65 0.32 73.739 0.078 z3 6.0 0.5 37.9 263 0.39 2437 0.07317 0.00029 0.011214 0.000019 0.692 0.0473 0.0001 66 7 71.70 0.28 71.885 0.122 z4 8.9 1.3 17.3 221 0.37 1129 0.07373 0.00049 0.011212 0.000019 0.583 0.0477 0.0003 84 14 72.24 0.46 71.872 0.123 z5 5.6 0.7 27.5 299 0.31 1809 0.07412 0.00042 0.011205 0.000009 0.447 0.0480 0.0003 98 13 72.60 0.40 71.831 0.059 z6 6.9 0.3 45.2 167 0.37 2921 0.07355 0.00030 0.011192 0.000018 Draft0.641 0.0477 0.0002 83 8 72.07 0.28 71.749 0.117 Overflow parking bentonite (OPB) z1 6.9 0.4 41.2 217 0.38 2658 0.07561 0.00033 0.011336 0.000010 0.466 0.0484 0.0002 118 9 74.01 0.31 72.667 0.061 z2 7.2 0.4 17.8 89 0.32 1178 0.07417 0.00030 0.011308 0.000008 0.563 0.0476 0.0002 78 9 72.65 0.29 72.489 0.050 z3 1.0 1.4 13.3 1641 0.28 894 0.07382 0.00085 0.011295 0.000010 0.477 0.0474 0.0005 70 26 72.32 0.80 72.404 0.066 z4 7.0 1.5 18.2 349 0.30 1210 0.07347 0.00015 0.011289 0.000004 0.356 0.0472 0.0001 59 5 71.99 0.14 72.364 0.026 z5 8.1 1.2 19.0 246 0.31 1262 0.07373 0.00023 0.011287 0.000007 0.405 0.0474 0.0001 68 7 72.23 0.22 72.351 0.045 z6 8.2 1.2 17.4 236 0.30 1156 0.07389 0.00082 0.011282 0.000008 0.918 0.0475 0.0005 75 25 72.39 0.78 72.321 0.049 z7 1.0 1.4 33.5 4296 0.30 2210 0.07383 0.00015 0.011278 0.000010 0.523 0.0475 0.0001 74 4 72.33 0.14 72.293 0.064

Bearpaw Formation Bentonite (BpB) z1 7.1 0.4 25.8 129 0.47 1626 0.07831 0.00029 0.011702 0.000013 0.295 0.04853 0.00017 125 8 76.55 0.27 74.998 0.084 z2 11.2 0.7 18.2 97 0.56 1132 0.07631 0.00043 0.011658 0.000009 0.288 0.04747 0.00026 73 13 74.67 0.40 74.720 0.057 z3 9.4 0.6 18.0 90 0.53 1122 0.07662 0.00033 0.011621 0.000013 0.231 0.04782 0.00020 90 10 74.96 0.31 74.483 0.083 z4 14.9 0.6 20.9 66 0.62 1272 0.07607 0.00037 0.011603 0.000018 0.363 0.04755 0.00021 77 11 74.44 0.35 74.368 0.112 z5 20 0.6 30.2 79 0.49 1894 0.07617 0.00029 0.011601 0.000014 0.354 0.04762 0.00017 80 8 74.54 0.27 74.355 0.091 z6 14.2 0.6 32.9 111 0.58 2019 0.07610 0.00024 0.011600 0.000012 0.359 0.04758 0.00014 78 7 74.47 0.23 74.348 0.079 z7 14.5 0.6 23.3 84 0.54 1447 0.07573 0.00029 0.011597 0.000020 0.333 0.04736 0.00017 68 9 74.12 0.27 74.328 0.128 z8 18.9 0.4 22.0 40 0.54 1370 0.07552 0.00033 0.011591 0.000012 0.272 0.04725 0.00020 62 10 73.92 0.31 74.293 0.077 z9 11 0.4 12.8 40 0.56 798 0.07574 0.00130 0.011591 0.000017 0.567 0.04739 0.00078 69 39 74.13 1.23 74.289 0.111 z10 11.1 0.5 30.8 107 0.52 1917 0.07602 0.00024 0.011587 0.000008 0.318 0.04758 0.00014 79 7 74.40 0.22 74.268 0.054

Notes: Zircon grains were chemically abraded ('CA', Mattinson, 2005). Errors are 2s absolute. a) total common Pb (in picograms); assumed isotopic composition of laboratory blank (206Pb/204Pb=18.49±0.4%; 207Pb/204Pb=15.59±0.4%;208Pb/204Pb=39.36±0.4%). b) ratio of total Pb in the analysis (radiogenic and common) to total common Pb c) Th/U calculated from radiogenic 208Pb/206Pb ratio and 207Pb/206Pb age assuming concordance. d) 206Pb/204Pb corrected for fractionation and common Pb in the spike. e) Pb/U ratios corrected for fractionation, common Pb in the spike, and blank. f) Error Corr is correlation coefficients of X-Y errors on the concordia plot. *Correction for 230Th disequilibrium in 206Pb/238U and 207Pb/206Pb assuming Th/U of 4.2 in the magma. Decay constants are those of Jaffey et al. (1971): 238U and 235U are 1.55125 x 10-10/yr and 9.8484 x 10-10/yr. 238U/235U ratio of 137.88 used for 207Pb/206Pb model age calculation.

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Table 3. Summary of dated and age-interpolated features and intervals in the Bearpaw, Horseshoe Canyon, Battle, and Scollard formations in the Red Deer River valley near Drumheller. Stratigraphic position or interval GTS 2012 Ages in this Source (Ma) report (Ma) K-Pg boundary 66.0 ± 0.1 66.043 ± 0.043 Renne et al. (2013); Sprain et al. (2014) Base of Scollard Fm; base of R. collaris-P. reductus biozone 66.88 Interpolated by Eberth and Kamo (2019)

BB (TMP2017.033.0001); 75 cm below base of Scollard Fm 66.936 ± 0.047 Eberth and Kamo (2019) Base of Battle Formation; approximate base of S. pyriformis-S. 67.20 Interpolated by Eberth and Kamo (2019) radiatus biozone; highest position for base of the Tyrannosaurus-Triceratops assemblage zone Battle unconformity 67.95-67.20 Interpolated by Eberth and Kamo (2019) 30n-30r boundary; base of Whitemud Mbr; base of P. 68.2 68.2 ± 0.1 Interpolated bertillonites biozone; lowest position for base of the Tyrannosaurus-Triceratops assemblage zone Eotriceratops xerinsularis (TMP2002.057.0007; type) 68.8 ± 0.1 Interpolated 30r-31n boundary 68.3 68.9 ± 0.1 Interpolated 31n-31r boundary; base of Carbon Mbr; base of S. trapaformis- 69.2 69.6 ± 0.1 Interpolated M. gibbus biozone; base of E. xerinsularis assemblage zone Draft Albertosaurus Bonebed; 9 m above ABB 70.1 ± 0.1 Interpolated ABB (TMP2018.100.0005); 27 m above base of Tolman Mbr; 70.675 ± 0.047 dated here 196 m above base of HCFm MBB (TMP2018.100.0004); base of Tolman Mbr; maximum 70.896 ± 0.048 dated here flooding surface of DMT 31r-32n boundary 71.4 71.1 ± 0.1 Interpolated Base of Morrin Mbr; base of M. rostratus-M. vancampoi 71.5 ± 0.1 Interpolated biozone; base of H. altispinus-S. osbornii zone; highest position for base of Maastrichtian Stage TRB (TMP2018.100.0003); top of #8 coal; lowest position for 71.832 ± 0.044 dated here base of Maastrichtian Stage Base of Horsethief Mbr 72.2 ± 0.1 Interpolated OPB (TMP2018.100.0002); top of coal #6 72.353 ± 0.037 dated here Base of K. andersonii biozone 72.4 ± 0.1 Interpolated 32n-32r boundary 73.4 73.0 ± 0.1 Interpolated Base of HCFm; base of E. regalis-P. canadensis assemblage zone 73.1 ± 0.1 Interpolated

32r-33n boundary 74.3 73.5 ± 0.1 Interpolated Dorothy Bentonite; base of W. gracile-M. glaber biozone; base 73.7 ± 0.1 Interpolated of B. cuneatus biozone BpB (TMP2018.100.0001); 5.5 m above base of BPFm 74.308 ± 0.031 dated here Base of Bearpaw Fm 74.4 ± 0.1 Interpolated

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BFm Post-HCFm N 200 km ABB 30 km HCFm N AB BB

USA BpFm TRB MBB OPB Drumheller

Grande Prairie Dinosaur Provincial Park Edmonton

BRG BpB HCFm B disturbed belt BFm

Deer 10 km Red ABB N Drumheller BB top of Calgary sect DraftRiver Post- HCFm Medicine Hat BFm A HCFm

MBB 27 TRB

9

OPB

Drumheller base of sect D C

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Scollard 250 meters Fm 120 245 BB (244 m) Battle Fm * 115 240 TRB (113 m) Mbr 110 *#8 coal Wmd 235

105 230 #12 coal zone

100 Fe 225

95 220 Horsethief Mbr Carbon Mbr

90 215 #11 coal zone

85 210

80 205 #7 coal 75 200

70 OPB (67 m) 195 (196 m) B ABB

DBZ * 65 #6 coal zone * Draft 190

60

#5 coal Mbr Tolman 185

55 180 stacked ss 50 #4 coal 175 #3 coal 45 #2 coal 170

Horseshoe Canyon Fm 40 MBB (169 m) #10 coal 165 * Drumheller Marine Tongue Drumheller Marine 35

Horseshoe Canyon Fm 160 Drumheller Mbr 30 155 #1 coal 25 150

20

#0 coal Morrin Mbr 145 15

140 10

135 5

130 #9 coal zone

0 Fe Fe

Bearpaw 125 Mbr Fm -5 Horsethief 120 cl slt f m c cg cl slt f m c cg

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Members/Sub-Units Stage Group FormationP-mag west east meters Dated Pal. 29n Pg-Sc 290 ^ 29r K-Sc Scollard Formation 250 30n Battle Formation BFm Whitemud Member ^ 30r 11-12 Carbon Member 31n cz

200 Maastrichtian 31r Tolman Member *

v v v v v v v DMT Draft 10 cz * v v v v v v 150 Morrin Member

8-9 cz Horsethief * Member 100

32n v v v v v v v v v v v v 6-7

cz DBZ

Edmonton Group * Drumheller 50 Member 2-5 (Drumheller) cz

Horseshoe Canyon Formation 0-1 cz base of HCFm (Drumheller) 0 Campanian 32r Dorothy tongue upper Drumheller Member (Calgary) tongue v v v DB v v v -50 33n Strathmore Member lower Bearpaw Formation

Bearpaw Formation tongue -80

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Albertosaurus71.6 Bonebed 74 0.01116 Morrin Bridge Bentonite (ABB) 0.0115 Bentonite (MBB) 71.4 z1 z4 0.01112 73 71.2 U 238 0.0113 U 0.01108 238 Pb/ 71 70.675 Ma 70.896 Ma

206 72 Pb/ (±0.048/0.065/0.142 Ma) al (± 0.047/0.064/0.141 Ma) (MSWD=0.83; N=3) 206 (MSWD=1.2; N=5) 0.01104 70.8 z2-3

0.0111 70.6 z1-3 not plotted 71 0.01100 z5-7 z4-8

70.4 207 235 207Pb/235U Pb/ U 70 0.01096 0.0109 0.068 0.070 0.072 0.074 0.076 0.078 0.069 0.071 0.073 0.075 0.077 0.079 0.081 0.083

0.0116 0.01137 74.2 72.8 73.8 0.01135 0.0115 z1 72.7 73.4 z1 0.01133 72.6 U 0.0114 U 73 71.832 Ma 238 238 (±0.044/0.061/0.140 Ma) z2 0.01131

Pb/ 72.5 Pb/ 72.6 (MSWD=1.07; N=4) 206 206 z3 0.0113 z2 72.4 72.2 0.01129 z4-7 72.353 Ma 72.3 (±0.037/0.054/0.133 Ma) 0.0112 71.8 Draft (MSWD=2.1; N=5) z3-6 0.01127 207 235 72.2 71.4 Pb/ U 207Pb/235U

0.0111 0.01125 0.071 0.073 0.075 0.077 0.079 0.072 0.073 0.074 0.075 0.076 0.077

75.2 0.01174 Bearpaw Fm Bentonite (BpB)

0.01170 z1 74.8 U 238 0.01166

Pb/ z2 74.308 Ma

206 (±0.031/0.05/0.13 Ma) 74.4 (MSWD=0.97; N=7) 0.01162 z3 z4-8, z10 z9

0.01158 74.0 207Pb/235U

0.01154 0.0735 0.0745 0.0755 0.0765 0.0775 0.0785 0.0795

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Albertosaurus Bonebed Bentonite (ABB) 70.5

70.675 ± 0.047/0.064/0.141 Ma

70.896 ± 0.048/0.065/0.142 Ma 71.0 Morrin Bridge Bentonite (MBB)

71.5 Draft 71.832 ± 0.044/0.061/0.140 Ma

72.0

Pb/(Ma) age U 206 238 72.353 ± 0.037/0.054/0.133 Ma 72.5

74.0 Bearpaw Formation Bentonite (BpB)

74.308 ± 0.031/0.050/0.130 Ma

74.5

75.0

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Litho- and a, b magnetostratigraphy

c d,e meters f Dinosaur 300 K-Pg Ammonite 29n Palynomorph datum assemblagezones g PAL. biozones biozones 66.043 ± 0.043 Triprojectus 275 29r quadricretaeus- Bratzevaea amurensis Tyrannosaurus

Fm rex- Reticorpus collaris- duration (partial) Triceratops Scollard Parviprojectus reductus 750 ka 250 h BFm 30n S. pyriformis-St. radiatus unconf 66.2 W* P. bertillonites ? 68.2 BB 12 30r 225 C Scollardia trapaformis- Eotriceratops 66.936 Mancicorpus gibbus xerinsularis 68.3 I 11 31n ± 0.047 69.2 200 ABB 70.675 ± 0.047 T*

MAASTRICHTIAN 31r Hypacrosaurus j 175 Mancicorpus rostratus- altispinus- 70.4 Mancicorpus vancampoi B. grandis Saurolophus MBB 70.896 ± 0.048 10 osborni * 71.4

M 32n.1n 150 ?

32n.2n 9 125 ? 32n.3n 8 TRB 71.832 ± 0.044 *32n.4n 100 H Draft Kurtzipites andersonii ?

7 Edmontosaurus 75 32n regalis- 6 DBZ Pachyrhinosaurus Horseshoe Canyon Formation OPB 72.353 ± 0.037 * ? canadensis 5 4 50 3 2 D ? ± 0.1 Ma error envelope (2σ error) for interpolated ages

25 1 32n.5n Alberta U-Pb CA-ID TIMS ages, ASL 0 2σ error envelope for dated bentonites (± 0.031–0.048 Ma) B. reesidei 32n.6n 73.4 Wodehouseia gracile- Alberta ages reported with no data 0 Mancicorpus glaber ? GTS 2012 (magnetochron & K-Pg ages, ASL) 32r -25 CAMPANIAN B. cuneatus ? 74.3 k 73.5 ± 0.4 Ma -50 DB ? ?

-75

Bearpaw Formation Baculites 33n compressus Pseudoaquilapollenites -100 j parallelus- Kirtlandian 74.8 Parviprojectus dinosaur * leucocephalus taxa BpB 74.308 ± 0.031 -125 Leth Coal ?

DPFm -150 ? ? 75 74 73 72 71 70 69 68 67 66 Ma

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Magneto- and lithostratigraphy, Palyno- Ammonite Dinosaur stages, new and interpolated ages stratigraphic Biozones Assemblage Ma Biozones Zones 65.5 29n Scollard Fm Wodehouseia fimbriata (Pg portion, a

Pal 65.832 partial) Wodehouseia spinata- 66.0 29r K-Pg datum 66.043 ± 0.043 Parviprojectus reticulatus T. quadricetaeus-B. amurensis 66.195a Scollard Fm (Cretaceous Reticorpus collaris- Tyrannosaurus portion) Parviprojectus reductus rex- b 66.88 Triceratops 67.0 BB Striaticorpus pyriformis- Battle Fm * 66.936 ± 0.047 b Striatellipollis radiatus 67.20 30n

Pseudoaquilapollenites ? b bertillonites 68.0 67.95 Whitemd Mbr 68.2 ± 0.1 ?

30r Eotriceratops (type) Scollardia Carbon (68.8 ± 0.1) Eotriceratops Mbr trapaformis- xerinsularis 69.0 Maastrichtian Mancicorpus gibbus 31n

69.6 ± 0.1 Draft ?

70.0 Albertosaurus bonebed Tolman (70.1 ± 0.1) 31r Mbr Mancicorpus Hypacrosaurus rostratus- altispinus- Mancicorpus Saurolophus ABB * 70.675 ± 0.047 vancampoi osborni MBB 70.896 ± 0.048 Baculites grandis 71.0 * Morrin

Horseshoe Canyon Fm Mbr

71.5 ± 0.1 ? ? Horse- thief Mbr 71.832 ± 0.044 TRB Kurtzipites ? 72.0 * andersonii 32n Edmontosaurus 72.2 ± 0.1 regalis- OPB Pachyrhinosaurus DBZ 72.353 ± 0.037 * ? canadensis Drum- heller Mbr B. reesidei 73.0 Wodehouseia gracile- 73.1 ± 0.1 Mancicorpus glaber ? 32r B. cuneatus ? Campanian Bearpaw Formation DB 73.7 ± 0.1 ? ? Pseudoaquilapollenites 33n 74.0 parallelus- Parviprojectus B. compressus “Kirtlandian” leucocephalus taxa BpB * 74.308 ± 0.031 74.5 Dinosaur Park 74.4 ± 0.1 ? ? ? Formation

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Stratigraphy Ornithischia Theropoda pachycephalosaurs ornithomimids Ma Lithostrat Coals/ Dinosaur small theropods Stage Paleomag bents zones ankylosaurs ceratopsians hadrosaurs thescelosaurids tyrannosaurids

66 29r 1 ScFm * 2 9 7 1 1 4 1 T. rex- T.

67 B Triceratops 30n * Tyrannosaurus ? Triceratops 68 Ankylosaurus W Leptoceratops Thescelosaurus 12 30r 1 69 C * 31n Maastrichtian 11 Draft E. xerinsularis

1 70 Eotriceratops BB 4 T 8 1 31r 33 BB 57 3 29 10 14

10 S. osborni 71 * 5 H. altispinus - 7 11 M * Albertonykus 11 25 9 Montanoceratops ? 2 3 2

Horseshoe Canyon Formation H 8 28 3 3 3 Saurolophus 72 Parksosaurus 1 32n Sphaerotholus 1 indet.

* indet. 4 7 indet. 6 indet. D 5 * E. regalis- Hypacrosaurus P. canadensis P. Edmontonia 73 0 indet. Apatoraptor indet. 32r Albertavenator Epichirostenotes Anodontosaurus Campanian

BPFm Arrhinoceratops Anchiceratops

(marine) Ornithomimus Struthiomimus 74 Atrociraptor 33n Dromaeosaurus Pachyrhinosaurus Albertosaurus Paronychodon troodontid indet. Edmontosaurus cf. 74.5 Richardoestesia DPFm Leth.*

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