Canadian Journal of Earth Sciences

Revised stratigraphy of the middle Simcoe Group (, upper Sandbian-Katian) in its type area: an integrated approach

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2018-0023.R2

Manuscript Type: Article

Date Submitted by the 28-Dec-2018 Author:

Complete List of Authors: Paton, Timothy; University of Tennessee, Knoxville, Earth and Planetary Sciences; University of Cincinnati, Geology Brett, Carlton;Draft University of Cincinnati, Geology Upper Ordovician, Formation, Formation, sequence Keyword: stratigraphy, chemostratigraphy

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

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1 Revised stratigraphy of the middle Simcoe Group (Ordovician, upper 2 Sandbian-Katian) in its type area: an integrated approach 3

4 Timothy R. Paton1 and Carlton E. Brett2

5

6 [email protected]

7 Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA

8 Currently [email protected]

9 Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 10 37996-1410

11

12 [email protected] 13 Department of Geology, University of Cincinnati,Draft Cincinnati, OH 45221, USA 14

15 Timothy Robert Paton

16 1621 Cumberland Avenue, 602 Strong Hall, Knoxville TN 37996-1526

17 Phone: 901-292-7976

18 Fax: 865-974-2368

19 Email: [email protected]

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20 Abstract

21 The Upper Ordovician Bobcaygeon Formation of southern is a widespread unit

22 that spans the Sandbian-Katian stage boundary and contains exceptionally preserved invertebrate

23 fossil assemblages, including the famed ‘Kirkfield echinoderm fauna.’ However, the precise

24 correlation of this interval remains poorly understood. This paper presents new data on high-

25 resolution sequence and carbon isotope chemostratigraphy of the Bobcaygeon interval based on

26 new quarry exposures and introduces refined definitions of unit boundaries based on

27 allostratigraphic criteria. Sequence, chemo-, and biostratigraphic evidence indicate that the

28 Bobcaygeon Formation represents a composite unit as it encompasses a major erosional

th 29 unconformity. The and KirkfieldDraft formations, described in the early 20 century, were 30 merged into a single unit, the Bobcaygeon, for concern that the original lithostratigraphic

31 divisions would be conflated with biostratigraphic zones of the same names. However, these

32 biostratigraphic zones are no longer favoured, and the lower member of the Bobcaygeon is here

33 elevated again to the status of formation (Coboconk Formation) and represents the uppermost

34 portion of the Sandbian M4 sequence. The middle and upper members of the Bobcaygeon, herein

35 reassigned to the Kirkfield Formation, represent the upper Sandbian to lower Katian M5A and

36 M5B sequences recognized widely in the eastern and central United States. The term

37 Bobcaygeon is retained and elevated to the rank of subgroup. The Kirkfield Formation is divided

38 into three members and contacts are refined, placing a 1–2 m transgressive grainstone at the base

39 of each sequence. These units are correlated with equivalent strata of New York and the

40 Cincinnati Arch.

41

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42 Keywords: Upper Ordovician, Kirkfield Formation, Bobcaygeon Formation, sequence

43 stratigraphy, chemostratigraphy

Draft

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44 Introduction

45 Upper Ordovician sedimentary rocks in the Lake Simcoe area of southern Ontario,

46 (Fig. 1), dominated by shallow-water subtropical carbonates, provide key insights into

47 paleoenvironments, sea level fluctuations, and basin dynamics. These rocks are well exposed in

48 several quarries in the City of and neighboring townships, as well as a few road

49 cuts, but contacts between certain stratigraphic units are rarely exposed and few formations are

50 continuously exposed in any given locality. The Simcoe Group of southern Ontario (equivalent

51 to the Black River–Trenton groups of New York) was studied and documented from the mid-19th

52 century to the early-21st century by many researchers, including Logan et al. (1863), Johnston 53 (1911), Kay (1943), Sinclair (1954), LibertyDraft (1969), Brookfield and Brett (1988), Melchin et al. 54 (1994), Armstrong (1999; 2000), Brunton et al. (2009), and Armstrong and Carter (2010).

55 However, some of these units have been overly generalized and advances in sequence

56 stratigraphy and chemostratigraphy (Holland and Patzkowsky 1996, Holland and Patzkowsky

57 1998; Bergström et al., 2010) reveal a need for revision. The Bobcaygeon Formation, as defined

58 by Liberty (1969), has been disfavoured as a lithological unit by many geologists involved in

59 active study and mapping of the geology in south-central Ontario, who instead retain Johnston’s

60 (1911) Coboconk and Kirkfield formations (Winder and Sanford, 1972; Johnson et al., 1992;

61 Sanford, 1993; Melchin et al., 1994; Armstrong, 1999; Armstrong, 2000; Brunton et al., 2009;

62 Armstrong and Carter, 2010; Haeri-Ardakani et al., 2012; Haeri-Ardakani et al., 2013; Pancost et

63 al., 2013).

64 The objectives of the present study include: a) documentation of high-resolution

65 stratigraphy of the Bobcaygeon interval in new reference sections and comparisons with data

66 from the Kirkfield Quarry; b) proposal of an objectively defined lithostratigraphy; c) production

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67 of a high resolution carbonate carbon isotope curve tied to detailed litho- and biostratigraphy; d)

68 identification of key surfaces and lithologic trends to develop a sequence stratigraphic

69 framework; and e) correlation of that framework with the established Mohawkian sequences of

70 the Cincinnati Arch region and New York (Holland and Patzkowsky, 1996; Brett et al., 2004).

71 Geologic Setting

72 The lower Paleozoic strata of south-central Ontario are exposed at the surface in east-

73 west trending outcrop belts that extend for several hundred kilometres, controlled by a slight

74 (<0.5°) dip to the south (Fig. 1). These undeformed strata are bounded to the north by the

75 metamorphic and granitic shield rocks of the Proterozoic Grenville Province. The Upper 76 Ordovician strata (the Basal, Simcoe, andDraft Nottawasaga groups of Liberty, 1969) onlap exposed 77 crystalline Grenville-age rocks, which would have supplied coarse siliciclastic sediments to the

78 basal transgressive Paleozoic strata, and locally to younger carbonate strata, from

79 paleotopographic basement highs which would have formed islands. carbonate rocks

80 form the Niagara Escarpment to the south and west. The Ordovician strata create a relatively

81 continuous outcrop belt to the northeast of the escarpment, connected in the subsurface with

82 equivalent rocks of the Michigan Basin and Appalachian Basin, and are slightly offset from the

83 Ottawa Embayment by a thin belt of exposed crystalline basement.

84 The Upper Ordovician strata preserved in southern Ontario record shallow, possibly cool-

85 water environments of the Trenton carbonate platform (Brookfield, 1988). This platform, on the

86 southeastern margin of Laurentia, was situated adjacent to the cratonic margin of the Taconic

87 peripheral foreland basin and was subjected to pulses of muddy sediments from the Taconic

88 Highlands and periodic volcanic ash accumulation as the Taconic volcanic island arc collided

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89 with Laurentia during the Late Ordovician. The Trenton carbonate platform lay in the southern

90 subtropics at ~20° south latitude (Mac Niocaill et al., 1997) and hosted a rich invertebrate fauna.

91 Subdivisions of the Simcoe Group, defined by Liberty (1969), include the ,

92 Bobcaygeon, Verulam, and Lindsay formations. Underlying the Simcoe Group is the Basal

93 Group (Jacobsville and Mount Simon formations, which only occur in the subsurface) and

94 overlying is the Nottawasaga Group, which includes the Blue Mountain (previously the upper

95 Whitby Formation; Russell and Telford, 1983), Georgian Bay, and Queenston formations.

96 Although originally included by Liberty (1969) in the Basal Group, the Shadow Lake Formation

97 is now assigned as the basal member of the Simcoe Group (Noor, 1989; Melchin et al., 1994; and 98 Armstrong and Dodge, 2007). The GullDraft River Formation and lower member of the Bobcaygeon 99 Formation consist of clean-water micrites and grainstones, respectively, of the ‘pre-Taconic

100 foreland’ (Holland and Patzkowsky, 1996), which preserve little evidence of tectonic influence,

101 although the Bobcaygeon Formation does host several bentonitic clays indicative of volcanic

102 activity associated with the approaching Taconic magmatic arc (Armstrong, 2000). The middle

103 and upper members of the Bobcaygeon Formation, Verulam Formation, and Lindsay Formation

104 contain a higher fraction of siliciclastic mudstones and shales derived from the prograding clastic

105 wedge and exhibit a strong back-stepping pattern associated with a major marine transgression

106 enhanced by tectonic loading and subsidence of the Laurentian margin by Taconic allochthons.

107 Several facies are preserved in the Simcoe Group; these facies and their inferred environments

108 include: a) supratidal and intertidal mudstones, b) shallow water shoal grainstones, c) protected

109 shelf and lagoonal micrites, d) subtidal muddy grainstones, and e) deeper-water siliciclastic

110 mudstones (Brookfield and Brett, 1988).

111 Methods

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112 Stratigraphic measurements were recorded in four quarries in the eastern Lake Simcoe

113 region of south-central Ontario (Fig. 2). From east to west, these include the now water-filled

114 Kirkfield Quarry in Kirkfield (44° 35' 8.76" N, 78° 58' 8.85" W), the Tomlinson Quarry in

115 Brechin (44° 35' 28.26" N, 79° 5'38.76"W), the Carden Quarry (Dufferin Aggregates) in Brechin

116 (44° 34' 04.5" N, 79° 06' 00.5" W), and the Gamebridge Quarry (James Dick Aggregates) in

117 Gamebridge (44° 30' 1.25" N, 79° 10' 16.97" W). Only the uppermost Kirkfield Formation and

118 lowest Verulam are currently exposed in the Kirkfield Quarry due to filling of the quarry with

119 water, so detailed measurements from Liberty (1969) were used as well. In the Tomlinson

120 Quarry and Gamebridge Quarry sections, whole samples of were collected for stable

121 carbon isotope analyses at regular intervals of 20 m with the exception of the lower portion of

122 the Tomlinson Quarry where 50 cm intervalsDraft were selected due to time constraints. Samples

123 were powdered using an electric drill then sent to the University of New Mexico stable isotope

124 lab for analysis using isotope-ratio mass spectrometry (IRMS). δ13C profiles were then paired

125 with measured stratigraphic sections.

126 Results

127 A) Lithostratigraphy

128 The lower (Coboconk), middle and upper (Kirkfield) members of the Bobcaygeon and

129 the basal Verulam formations are described below in terms of lithology, facies, and bounding

130 surfaces. In each case, we title sections on stratigraphic units with proposed revised names as

131 well as the terminology of Liberty’s (1969) Bobcaygeon units to reflect the dichotomy of

132 terminology that exists; however, the text primarily uses the revised terminology (Fig. 3). Of

133 particular importance is the recognition of sharply defined surfaces. The Coboconk Formation

134 (formerly the lower member of the Bobcaygeon) contains one member. The Kirkfield Formation

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135 as here defined is divided into three members: the lower member (formerly the middle member

136 of the Bobcaygeon) and the middle and upper members (formerly the upper member of the

137 Bobcaygeon). Because the original stratotype of the Kirkfield Formation, the old quarry NW of

138 Kirkfield, Ontario (44°35'8.76"N, 78°58'8.85"W) is now largely water filled, we herein designate

139 a new reference section for the Kirkfield Formation. Because a complete section is now well

140 exposed in the Carden Quarry of Dufferin Aggregates Co. Ltd, Regional Road 47, near the

141 village of Brechin, City of Kawartha Lakes (formerly Victoria County), Ontario (44°34'04.5"N

142 79°06'00.5"W), we designate this section as the new reference section for the redefined

143 Coboconk and Kirkfield formations (Fig. 4). Subsidiary reference sections are found in the

144 Tomlinson Quarry and Brechin Quarry. Draft 145 A1. Coboconk Formation / Lower Member of Bobcaygeon

146 The Coboconk Formation was defined by Johnston (1911) to include 3–6 m of nodular

147 and cherty limestones that overlie the Gull River, though the Coboconk was later combined with

148 the Kirkfield Formation by Liberty (1969) to form the Bobcaygeon Formation for concern that

149 the Kirkfield Formation would be conflated with a biostratigraphic zone designation of the same

150 name (Sanford, 1993). The lower member of the Bobcaygeon Formation (formerly the Coboconk

151 beds or member) defined by Liberty, is herein elevated again to formation status. The Coboconk

152 Formation, as recommended previously by Melchin et al. (1994), is composed of 6–8 m of grey-

153 brown, thin- to thick-bedded (8–100 cm-thick beds) micritic limestones, packstones, and fine-

154 grained (0.06–0.25 mm allochems) grainstones with a persistent zone of chert nodules near the

155 base. The Coboconk is fossiliferous, typically containing the corals Tetradium, Foerstephyllum,

156 Lambeophyllum, and Lichenaria as well as stromatoporoids and several brachiopods and

157 mollusks typical of the Chaumont fauna of the Black River Group in New York State (Liberty,

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158 1969). As exposed in the Carden Quarry, the base of the Coboconk sharply overlies the pale-

159 grey-coloured micritic limestones of the Gull River Formation (Fig. 5a). The lowest metre of the

160 Coboconk consists of 12–25 cm beds of brown-coloured, fenestral, micritic limestones and

161 wackestones, with a 12 cm bipartite bed of fine-grained grainstone 40 cm above the base. A 12

162 cm grey bentonitic clay approximately 1 m above the base of the Coboconk forms a reentrant

163 that is sharply overlain by 1.5 m of grey to grey-brown, thick-bedded brachiopod wackestones.

164 This bentonite was apparently not noted by Liberty (1969). The remaining 3.5 m of the

165 Coboconk consists of heavily bioturbated grey-brown, medium- to thick-bedded (12–100 cm

166 beds) micritic limestones and brachiopod-mollusk wackestones. Near the top, the matrix of the

167 wackestones coarsens to silt size (0.03–0.06 mm grains). In this upper 3.5 m in the Carden

168 Quarry, sparse tabulate corals and stromatoporoidsDraft are present. The uppermost 1–2 m, previously

169 assigned to the Coboconk (C2 submember of lower Bobcaygeon in Liberty, 1969), appears

170 distinctive from the lower beds and is sharply separated from them by a probable unconformity,

171 whereas it appears to have an abrupt, but conformable upper contact. It is herein assigned to the

172 base of the Kirkfield Formation and described below.

173

174 A2. Kirkfield Formation / Middle and Upper Members of the Bobcaygeon Formation

175 Kay (1937) redefined the Hull of Raymond (1914) to include both the upper

176 beds famed for their rich echinoderm faunas and a 3–4 m lower interval exceptionally rich in

177 dalmanellid brachiopods, Johnston's (1911) ‘Dalmanella beds.’ Subsequently, Kay (1947)

178 renamed this composited interval ‘Kirkfield’ for the then-excellent exposures at Kirkfield

179 Quarry. Because of its possible time connotation, Liberty (1969) reassigned the Kirkfield interval

180 to his enlarged Bobcaygeon Formation (along with the Coboconk Formation) and eliminated the

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181 term Kirkfield from formal nomenclature (Sanford, 1993); we argue for a return to the original

182 terminology with some minor but significantly more objective definitions of unit boundaries.

183 The Kirkfield Formation, as defined herein, comprises the middle and upper members of the

184 Bobcaygeon Formation of Liberty (1969) and is comparably subdivided into lower, middle, and

185 upper members, of which the latter two are equivalent to Liberty's upper Bobcaygeon (Fig. 3).

186 Lower Member of Kirkfield / Middle Member of Bobcaygeon:

187 Liberty (1969) proposed the term ‘middle Bobcaygeon’ (his D submember) to refer to a

188 6–7 m, medium-bedded but fine-grained, argillaceous interval previously termed lower member

189 of the Kirkfield Formation, a term used by more recent workers in Ontario (Winder and Sanford, 190 1972; Johnson, 1992; Melchin et al., 1994;Draft Armstrong, 1999; Armstrong, 2000; Brunton et al., 191 2009; Armstrong and Carter, 2010; Haeri-Ardakani et al., 2012; Haeri-Ardakani et al., 2013),

192 and which we propose to re-introduce with slight modification. The lower member of the

193 Kirkfield Formation (Melchin et al., 1994; Armstrong, 2000), consists of a 2 m basal coarse-

194 grained grainstone (unit a, formerly the C2 submember of Liberty’s lower member of the

195 Bobcaygeon), and ~5 m of pale-grey-weathering, medium-bedded, argillaceous calcisiltites and

196 fine-grained grainstones (sublithographic limestones of Liberty, 1969), thin shales, and grey-

197 brown wackestones (unit b, Fig. 4).

198 As described by Liberty (1969), the uppermost 2 m (the C2 submember) of the lower

199 member of the Bobcaygeon Formation (Coboconk Formation of Johnston, 1911, and herein)

200 consist of medium-bedded (15–40 cm beds), coarse-grained (0.5–2 mm allochems) grainstones

201 with intraclasts of micritic lithology as well as chert and fossil fragments replaced by beekite. In

202 the Carden Quarry, this package possesses a sharp basal surface and contains abundant clasts, up

203 to 6 cm in length, of white micrite throughout the lower metre (Fig. 5d). Liberty (1969)

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204 recognized these clasts as possessing a lithology identical to the Gull River micrites. In the

205 Carden Quarry, this package consists entirely of 30–40 cm-thick coarse-grained grainstone beds

206 with minor shale partings. However, in the Tomlinson Quarry, the lower 20 cm of this package

207 locally contains lenses of brown-coloured micrite. White micritic clasts are abundant in this bed,

208 occasionally lying directly on the contact with the underlying silty (carbonate silt) wackestones

209 (Fig. 5c), although none are present below. This succession of grainstones and local brown

210 micrites are here reassigned to the base of the overlying Kirkfield Formation and designated ‘unit

211 a.’

212 Although Liberty (1969) referred to the coarse-grained 1–2 m grainstone package as 213 submember C2 of the lower member of Draftthe Bobcaygeon Formation, in earlier reports he mapped 214 the interval as the basal bed of the lower member of the Kirkfield Formation (Liberty, 1955) as is

215 done here. He noted that submember C2 locally pinched out to the west of the town of Coboconk

216 such that fine-grained argillaceous beds of his middle member rested directly upon the lower

217 submember (C1) of the lower Bobcaygeon. However, he also noted that the C2 submember

218 persisted as an important marker horizon in the subsurface of southwestern Ontario.

219 The remainder of the lower member of the Kirkfield Formation, unit b, is a 4–5 m

220 interval of thin- to medium-bedded (5–25 cm-thick beds) brownish- to bluish-grey, sparsely

221 fossiliferous, argillaceous calcisiltite with abundant dark shale partings (Fig. 5b).

222 Petrographically, the limestones are very fine grained (0.06–0.125 mm allochems), peloidal

223 grainstones (very fine calcarenites and calcisiltites). The lower half of the interval is more thinly

224 bedded and shaly and shows numerous bedding planes with storm lags of dalmanellid

225 brachiopods. Fossils include an unusual mix of typically offshore dalmanellids (Resserella

226 resupinata) and Skenidioides, as well as receptaculitids, gastropods, and trilobites. This fauna is

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227 suggestive of that in the Napanee Formation, with which the lower Kirkfield has been correlated

228 (Liberty, 1969; Melchin et al., 1994). A thin clay layer near the base of unit b of the lower

229 Kirkfield has been termed the MR metabentonite (Liberty, 1969) or CR seam (Forman and Lake,

230 1954). This bentonite was tentatively correlated by Kolata et al. (1996) with the Millbrig K-

231 bentonite. Given the biostratigraphic and isotopic constraints (see below) this is unlikely.

232 However, the bentonite could possibly correlate with the Capitol shaly metabentonite in the

233 upper middle part of the Curdsville Member in the Lexington-Frankfort area of Kentucky.

234 In the Carden Quarry, the lower member of the Kirkfield Formation, above basal unit a,

235 contains approximately 1 m of fine-grained, medium-bedded (18–40 cm beds) grainstones and 236 shales, occasionally with hummocky crossDraft stratification. Above this lower metre is a 2.6 m 237 succession of medium-bedded (5–30 cm beds) packstones and minor shale partings with several

238 fine-grained grainstone beds near the top. The uppermost 0.8–1.4 m of the lower member consist

239 of tabular, thin- to medium-bedded (2–8 cm beds), argillaceous calcisiltites and packstones with

240 dark grey, barren shales. In the Tomlinson Quarry, above unit a, the lower member contains 2 m

241 of medium-bedded (8–23 cm beds), sparsely fossiliferous micritic limestones with minor shale

242 partings overlain by 2.5 m of medium-bedded (5–15 cm beds) fine-grained grainstones. The

243 uppermost 80 cm of the lower member is notably more shaly in both quarries.

244 Middle Member of Kirkfield / Lower Unit of Upper Member of Bobcaygeon:

245 As noted above, when Kay (1947) renamed Raymond's (1914) Hull beds to the Kirkfield

246 Formation, he included Johnston's (1911) ‘Dalmanella beds.’ This 3 m shale and grainstone

247 interval, included as the base of the upper member of the Bobcaygeon Formation by Liberty

248 (1969; see below), was assigned to the middle member of the Bobcaygeon by Melchin et al.

249 (1994). Because of this ambiguity, and its distinctive lithologic characters, we set this interval off

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250 as an informal ‘middle member’ of the Kirkfield Formation. Included within this interval as well

251 is the underlying 1–1.2 m of coarse-grained grainstones (unit c).

252 The middle member of the Kirkfield Formation contains 5–6 m of fossiliferous fine- to

253 medium-grained, dark grey-weathering packstones, grainstones, and shales (Fig. 6a). The lower

254 contact of the middle member is marked by the sharp base of the 1–1.2 m coarse-grained

255 grainstone package (unit c) previously assigned to the top of the middle member of the

256 Bobcaygeon Formation by Liberty (1969). This grainstone lies in sharp, erosional contact with

257 the underlying fine-grained grainstones of the lower member of the Kirkfield Formation. Unit c

258 is medium-bedded (10–25 cm beds) and contains abundant reworked clasts of micritic limestone. 259 In the Carden Quarry, the base of this packageDraft locally displays small channels (up to 3 m in 260 width and up to 0.8 m deep) filled with oolitic grainstone (Fig. 6b). These channel fills are cross-

261 bedded and contain abundant reworked clasts. The bases of these channels incise and truncate

262 the underlying argillaceous grainstones of the lower member of the Kirkfield Formation. The

263 thickness of these channel fills ranges from 0–80 cm. This channelized oolitic grainstone is not

264 observed in Tomlinson Quarry, although it likely correlates with an oncolite bed at the same

265 stratigraphic position reported by Melchin et al. (1994) from the nearby Brechin Quarry.

266 Above the basal grainstone (unit c) is 2.5 m of fossiliferous bluish grey-weathering,

267 argillaceous, fine-grained grainstones (unit d) and 2–3 m of grey, medium-bedded (7–40 cm

268 beds), fine- to medium-grained grainstones (unit e). Units c and d were termed the ‘Dalmanella

269 beds’ by Johnston (1911) owing to the high abundance of dalmanellid brachiopods. The upper

270 portion of unit d in the Tomlinson Quarry is marked by a thick limestone with a hardground on

271 its upper surface, overlain by a rusty weathering clay, probably a K-bentonite, 1.5 m below the

272 contact with the upper member. As yet these beds have not been traced beyond the Tomlinson

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273 Quarry. The uppermost 1.5 m of the middle member consists of 20–40 cm beds of pale grey,

274 medium-grained grainstone with abundant echinoderm and bryozoan remains (unit e). The top of

275 the middle member is drawn at the sharp contact with a 1.5 m cross-bedded grainstone (unit f) of

276 the upper member with abundant lithoclasts at its base (Fig. 6c).

277 Upper Member of Kirkfield / Upper Unit of Upper Member of Bobcaygeon:

278 At its type locality in the Kirkfield Quarry, the upper member of the

279 Kirkfield/Bobcaygeon (submember E of Liberty, 1969), as originally defined, was 10.4 m thick,

280 and a similar thickness was later measured at the Brechin Quarry (Melchin et al., 1994).

281 However, the thickness of the informal upper member of the Kirkfield Formation, as now 282 redefined, is herein reduced to 5–6 m becauseDraft of two alterations: first, the lower ~ 5 m of the 283 upper member of the Bobcaygeon Formation, comprising the ‘Dalmanella beds’ (unit d) and the

284 overlying 2–3 m of grainstones (unit e), have been reassigned to the middle member of the

285 Kirkfield Formation; and second, the uppermost 2 m-thick grainstone previously assigned

286 tentatively to the Kirkfield is herein assigned to the basal Verulam Formation (see discussion).

287 Ironically, this brings the definition of the upper member essentially back to Raymond's (1914)

288 definition of the Hull Limestone and it might be useful to resurrect this term as a formal

289 replacement for ‘upper member.’ At this time, however, pending re-examination of the type Hull

290 near Ottawa, we favour the informal terminology.

291 A bentonite from the upper member of the Bobcaygeon Formation, reported from the

292 Carden Quarry by Armstrong (2000), yielded an age of 452.6 ± 0.8 Ma making it close to, but

293 about a million years younger than the age reported for the Millbrig (453.7 ± 1.8 Ma) by Tucker

294 et al., 1990. This bentonite was not re-located during this study and has not been correlated

295 elsewhere here.

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296 As redefined, the upper member of the Kirkfield (uppermost Bobcaygeon) comprises 5–6

297 m of richly fossiliferous, medium-grained grainstones and calcisiltites with minor shale. These

298 beds weather uniformly grey, but fresh exposures show a sharp lithologic and colour contrast, 4

299 m below the upper contact with the Verulam Formation, from coarse-bedded pink grainstones

300 below to dark grey, fine-bedded shaly limestones above (Fig. 7a). This contact marks a strong

301 shift to finer grained and more shale-rich facies in the higher portion of the upper member. The

302 upper member of the Kirkfield Formation is divisible into two units or submembers: unit f, a

303 lower 1–1.5 m interval of pinkish-grey grainstone bounded at its base by a 50 cm-thick

304 grainstone bed with abundant micritic intraclasts (Fig. 6d); and unit g, an upper medium- to dark-

305 grey, somewhat shaly packstone and calcisiltite succession including two distinct hardgrounds

306 near the middle. Draft

307 The basal, 50 cm-thick, coarse-grained grainstone of the upper member (lower portion of

308 unit f) exhibits trough cross-bedding and at its base and contains abundant reworked micritic

309 clasts. A distinctive marker bed ranging from 12–41 cm of light, pinkish grey, cross-bedded

310 grainstone with abundant ‘lithoclasts’ was reported at 4.3 m above the base of the ‘Dalmanella

311 beds’ at Kirkfield Quarry (Liberty, 1969). This bed appears to correlate with unit f at the

312 Tomlinson Quarry. The remainder of unit f contains 1 m of medium-bedded (20–30 cm beds),

313 pinkish-grey, crinoidal grainstones.

314 Fresh exposures show a sharp colour contrast, ~1.5 m above the limestone clast marker

315 bed, from the pink grainstones (unit f) below to dark grey, thin-bedded shaly limestones (unit g)

316 above (Fig. 7a). A distinctive shaly zone ~30 cm-thick at the base of the dark grey interval was

317 correlated between the Tomlinson, Kirkfield, and Carden quarries. Liberty (1969) reported a

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318 well-preserved crinoid assemblage from this interval, 5.9–6.2 m above the base of the quarry at

319 Kirkfield (the 1st crinoid bed).

320 Two hardgrounds occur 2–2.6 m above the unit f/unit g contact (or 3.5–4.1 m above the

321 base of the upper Kirkfield member at Tomlinson Quarry). These beds, 0.6 m apart, which

322 preserve a rich echinoderm-bryozoan Lagerstätte (Paton and Brett, in prep). The lower

323 hardground exhibits considerable topographic relief in the form of large (up to 3 m long and 60

324 cm high) hardground mounds (Fig. 7b). This bed is probably equivalent in stratigraphic position

325 to the second crinoid bed reported by Liberty (1969) at 7.8–7.9 m above the base of the Kirkfield

326 Quarry. This zone and the aforementioned bed at 5.9–6.2 m at Kirkfield were named the ‘Crinoid 327 beds’ by W. A. Johnston (1911) and P. E.Draft Raymond (1914) due to the exceptional preservation 328 and abundance of these echinoderms. Between the two hardgrounds are two 15–20 cm-thick

329 quartz-rich calcarenites. This succession of beds was located in the Tomlinson, Carden, and

330 Gamebridge quarries and was used as the datum for correlating these sections (Fig. 4).

331 The remainder of unit g, the uppermost Kirkfield Formation, is missing by post-

332 Ordovician erosion at the Tomlinson Quarry but at the Kirkfield Quarry this interval consists of

333 about 1.8 m of thinly-bedded, argillaceous limestone that weathers shaly and rubbly (Liberty,

334 1969); a similarly shale-rich interval was observed in the uppermost Kirkfield Formation at the

335 Gamebridge and Carden quarries. This interval forms a recess below the more resistant overlying

336 grainstones herein assigned to the Verulam Formation.

337 A 1–2 m grainstone package, previously assigned to the top of the upper member of the

338 Bobcaygeon Formation, consists of hard, typically massive, blue-grey, rusty-weathering,

339 medium-grained grainstones (Fig. 7c). The contact between the Bobcaygeon and Verulam

340 formations was drawn by Liberty (1969) at the upper contact of these medium-grained

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341 grainstones with overlying argillaceous, thinly-bedded limestones containing very abundant

342 Prasopora simultarix (Prasopora beds). However, at Gamebridge the 1 metre-thick grainstone

343 also contains abundant Prasopora and appears to grade upward into shaly beds of the typical

344 lower Verulam Formation. This grainstone contains trough cross beds in the Carden and

345 Gamebridge quarries, has a sharp base, locally truncates the uppermost beds of the Kirkfield

346 Formation, and contains abundant reworked clasts of micritic limestone (Fig. 7c). This

347 succession is herein assigned to the base of the Verulam Formation based on allostratigraphic

348 criteria (Fig. 7d, see discussion).

349 Summary of Lithostratigraphy: Proposed Revision of Stratigraphic Terminology 350 As noted above, there is already Drafta dichotomy in usage of stratigraphic terminology on the 351 upper Turinian and lower Chatfieldian strata of the Lake Simcoe area, with some authors

352 following Liberty (1969) in using Bobcaygeon (Ludvigsen, 1978; Von Bitter and Eley, 1984;

353 Williams and Telford, 1987; Hessin, 1989) and others opting to retain the older terminology of

354 Coboconk and Kirkfield (Beards, 1967; Winder and Sanford, 1972; Johnson et al., 1992;

355 Sanford, 1993; Melchin et al., 1994; Armstrong, 1999; Grimwood et al., 1999; Armstrong, 2000;

356 Brunton et al., 2009; Armstrong and Carter, 2010; Haeri-Ardakani et al., 2012; Haeri-Ardakani

357 et al., 2013; Pancost et al., 2013). Recent detailed field studies corroborate earlier views that a

358 possibly large unconformity and several minor discontinuities exist within the Bobcaygeon

359 Formation as presently defined. As noted by the North American Stratigraphic Code,

360 “establishment of formal units that straddle known, identifiable, regional disconformities is to be

361 avoided, if at all possible” (NASCN, 2005; p. 1566). Therefore, we support subdivision of the

362 Bobcaygeon into two formation scale units, each bounded by unambiguous sharp surfaces that

363 are interpreted as unconformities. Furthermore, the two intervals are distinct lithologically and

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364 each is thick enough to be represented on maps, so fitting the definition of a formation. We

365 favour retention of older terminology rather than promoting new names and accordingly suggest

366 division of the Bobcaygeon into the Coboconk and Kirkfield formations but with slightly

367 modified, non-arbitrary boundaries and using the measured section at Carden Quarry as a

368 primary reference section, supplemented by sections at Tomlinson, Brechin (Miller Aggregate,

369 44° 36' 42.6" N, 79° 06' 4.88"W), Gamebridge, old Kirkfield, and new Kirkfield quarries (Halton

370 Crushed Stone Ltd, 44° 35' 25.3" N, 78° 55' 38"W). In addition, we have subdivided the

371 Kirkfield Formation and its members into a series of seven distinctive intervals (at the rank of

372 submembers), herein simply designated with letters a–g. 373 The base of the Coboconk, Liberty’sDraft C1 submember of the lower member of the 374 Bobcaygeon, is placed at the sharp (probably disconformable) contact with the pale grey micrites

375 of the Gull River Formation and its top is placed at the sharp base of a 1–2 m-thick intraclastic

376 grainstone (unit a) formerly assigned to the C2 submember. This latter discontinuity also defines

377 the base of the Kirkfield Formation as modified herein, which is approximately equivalent to the

378 middle and upper members of the Bobcaygeon of Liberty’s (1969) terminology. The top of the

379 Kirkfield Formation is defined at the sharp and erosional contact at the base of a distinctive

380 intraclastic grainstone bed, approximately coinciding with the base of the Prasopora simultarix

381 epibole and assigned to the base of the Verulam Formation.

382 The Kirkfield Formation is further subdivided into three informal members, rather than

383 the previous two, each of them bounded by sharp surfaces with evidence for erosion and at least

384 minor disconformity. The lower member of the Kirkfield is approximately equivalent to the

385 former middle member of Bobcaygeon and extends from the previously noted disconformity at

386 the base of the ‘unit a’ grainstone upward through calcisiltites and fine-grained grainstones and

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387 shales (unit b) to a second sharp and locally channeled base of a thick intraclastic, crinoidal to

388 oolitic grainstone, here identified as unit c. The middle member of the Kirkfield Formation,

389 which corresponds to the lower part of the former upper member of Bobcaygeon, extends

390 upward from the base of bed c to encompass the shaly ‘Dalmanella beds’ to the base of a

391 massive, cross-bedded, intraclastic grainstone (unit f) recognized by Liberty (1969) at the

392 Kirkfield quarry and subsequently identified at the Carden and Tomlinson quarries. Finally, the

393 upper member of Kirkfield extends from the base of unit f to the upper contact of the Kirkfield

394 Formation with the overlying Verulam.

395 Although we are tempted to propose formal names for the members and submembers of 396 the Kirkfield Formation, to prevent ambiguitiesDraft between the system of middle and upper 397 Bobcaygeon versus lower, middle and upper Kirkfield, at present we refrain from adding further

398 terminology. To provide some measure of continuity given its common use we recommend that

399 the term Bobcaygeon be retained with nearly its present boundaries, though here redefined, at the

400 rank of subgroup (i.e. the Bobcaygeon subgroup of the Simcoe Group). The division of the

401 Bobcaygeon is also corroborated by the division of equivalent strata of the Trenton Platform,

402 Jessamine Dome, and Nashville Dome into two group-level divisions at a disconformity

403 (designated the M4/M5 sequence boundary; Holland and Patzkowsky, 1996) correlated with the

404 unconformable boundary between the Coboconk and Kirkfield formations (see discussion), i.e.

405 the Black River-Trenton group boundary (Kay, 1937; Cameron and Mangion, 1977; Melchin et

406 al., 1994), the High Bridge Group-Lexington Limestone boundary (Cressman and Noger, 1976;

407 Pope and Read,1997; Brett et al., 2004), and the Stones River-Nashville group boundary

408 (Bassler, 1932; Wilson, 1935, 1949; Holland and Patzkowsky, 1998) respectively.

409 B) Biostratigraphic Evidence

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410 Conodont data from the Bobcaygeon of the Lake Simcoe area are sparse and only permit

411 broad subdivision. Studies by Schopf (1966) indicate correlation of the underlying upper Gull

412 River with the Chaumont or Watertown (upper Black River Group) in New York. The age of the

413 Coboconk (or lower member of Bobcaygeon) has been controversial for many years (see

414 summaries in Liberty, 1969 and Melchin et al., 1994). Benthic macrofossils in the lower 2 metres

415 suggest affinities with the upper Black-Riveran strata of New York, whereas brachiopod and

416 coral taxa typical of the Trenton Group (Chatfieldian) occur above. However, conodonts from

417 the C1 submember, including ‘neurodonts,’ indicate an upper Turinian (Black-Riveran) age

418 (Winder et al., 1975). In addition, the chitinozoan fauna of the C1 submember is composed

419 completely of species common to the Turinian Gull River Formation (Assemblage zone ChA-1

420 of Melchin et al., 1994). In contrast, theDraft grainstones of the C2 submember (herein assigned as the

421 basal bed of the Kirkfield Formation, unit a) yield a distinctive and more diverse suite of

422 chitinozoans including Belonechitina and Kalochitina with simple and complex spines, the latter

423 of which persist into the overlying middle Bobcaygeon or lower Kirkfield (assemblage ChA-2 of

424 Melchin et al., 1994). Hart (1986) also found the same chitinozoan assemblage in the

425 ‘Rocklandian’ Napanee Formation of Ontario and New York State. The lower member of the

426 Kirkfield Formation, including the basal unit a, coincides with the Phragmodus undatus

427 conodont zone (Sweet, 1982; Melchin et al., 1994).

428 The upper member of the Kirkfield Formation (uppermost Bobcaygeon) and lowermost

429 Verulam Formation have yielded a chitinozoan assemblage (ChA-3) dominated by Belonechitina

430 with simple spines, Desmochitina, and Cyathochitina latipatagium, an assemblage shared with

431 the lower Kings Falls Formation in New York, which may be considered as typical of the

432 formerly used Kirkfieldian Substage of Kay (Melchin and Legault, 1985; Melchin et al., 1994).

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433 The presence of the conodont Polyplacognathus is also typical of the higher Trenton Group

434 (Kirkfieldian-Shermanian of Kay's 1943 terminology). The remainder of the Verulam Formation,

435 above the basal few metres but below the upper member, contains a different chitinozoan

436 assemblage, ChA-4, with many of the same taxa as ChA-3 but dominated by Belonechitina with

437 complex spines and Hercochitina (Melchin et al., 1994). This assemblage zone appears to

438 coincide in part with a widespread epibole of the gumdrop-shaped bryozoan Prasopora

439 simulatrix.

440 C) Carbon Isotope Chemostratigraphy

441 Bulk samples of unweathered carbonates were collected at the Tomlinson and 442 Gamebridge Quarries at regular stratigraphicDraft intervals for the purpose of constructing a carbon 13 443 isotope curve. A composite stratigraphic section and carbon isotope curve (δ Ccarb) were created

444 from these two sections to encompass the lower, middle, and upper members of the Kirkfield

445 Formation as well as the lower Verulam Formation (Fig. 8). The carbon isotopes record two

446 positive excursions. The lower and more prominent (up to 3 ‰) excursion is present primarily in

447 the 6 m wackestone interval herein termed lower member of Kirkfield Formation (middle

448 Bobcaygeon of Liberty, 1969), but also extends through the 1 m grainstone into the overlying

449 dalmanellid-rich shale in the middle member of Kirkfield. Following a ~10 m interval of slightly

450 descending values (1.9–1.0 ‰) corresponding to the upper member of the Kirkfield Formation, a

451 narrow spike of elevated values (2.5 ‰) is documented in the lowest shaly Prasopora beds of

452 the Verulam in the composite section.

453 Discussion

454 Biostratigraphic and Chemostratigraphic Correlation

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455 As noted above, the Coboconk Formation yields megafauna, conodonts, and chitinozoans

456 typical of the upper Sandbian or Turinian (Blackriveran) Stage (Melchin et al., 1994), whereas

457 the lower Kirkfield, including unit a, bears a chitinozoan assemblage zone ChA-2, which is

458 aligned with the Phragmodus undatus conodont zone (Sweet, 1982; Melchin et al., 1994). The

459 base of the Kirkfield thus may lie close to the Turinian-Chatfieldian stage boundary, presently

460 defined as occurring at the Millbrig K-bentonite (Leslie, 2000). Although that K-bentonite has

461 not definitely been identified in the Lake Simcoe area (but see Cornell, 2008 for possible

462 identification), work by Mitchell et al. (2004) has seemingly located the Millbrig high in the

463 Selby Formation in northwestern New York. Based upon the conodonts and chitinozoan

464 assemblages, most of the Selby probably correlates with the Coboconk below unit a/submember

465 C2 (Melchin et al., 1994), but some elementsDraft of the ChA-2 assemblage zone (Desmochitina) have

466 been found in the uppermost interval of the Selby Formation (Hart, 1986). Winder et al. (1975)

467 also recorded the abrupt influx of typical ‘Trentonian’ conodonts from this same bed. This

468 evidence supports correlation of the unit a/submember C2 interval with the uppermost Selby and

469 the location of the North American Turinian-Chatfieldian boundary slightly below this level. In

470 this regard, the discovery of a K-bentonite low in the Coboconk Formation at Carden Quarry

471 warrants further investigation, as it could prove to be the Millbrig. Both macrofossil and

472 microfossil evidence strongly support correlation of the lower Kirkfield (middle Bobcaygeon)

473 with the Napanee Formation in New York and the type ‘Rocklandian’ sections in Ontario. This

474 correlation is further corroborated by the occurrence of a strong positive isotopic excursion in

475 both the lower Kirkfield and in the Napanee Formation (Barta et al., 2007). The middle Kirkfield

476 of the present report probably correlates with the lower part of Kings Falls Formation and its

477 sharp basal contact may be confluent with the sharp base of the latter; this is corroborated by the

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478 presence of the descending limb of a major carbon isotope excursion, probably the Logana

479 excursion in both units (see below).

480 In the Cincinnati Arch reference section at Frankfort, KY, the lowest beds assigned to the

481 Chatfieldian, the Curdsville Member of the Lexington Limestone, overlie a widespread sequence

482 boundary identified by Holland and Patzkowsky (1996) as the M4-M5 (M for Mohawkian)

483 sequence boundary. The Curdsville Formation has yielded conodonts of the Phragmodus

484 undatus Zone (Mitchell et al., 2004), placing it in approximately the same position as the

485 Coboconk to lowermost Kirkfield Formation. Carbon isotope values of the Curdsville are low

486 relative to the underlying Tyrone Formation and overlying strata, ranging from about 0.5 to 1.5 487 ‰ (Bergström et al., 2010; Coates et al.,Draft 2010). A strong lithological shift to argillaceous 488 calcisiltites of the Logana Member is a strong shift in carbon isotope values to above 3 ‰. This

489 shift and the entire interval of relatively higher values of the next ~10 m that was originally

490 referred to as the Guttenburg isotopic excursion, or GICE (Bergström et al., 2010), but has

491 recently been subdivided into two separate excursions (Coates et al., 2010; Young et al., 2015).

492 The lower of the two excursions, the ‘Logana excursion,’ coincides approximately with the

493 Logana Member of the Lexington Limestone of the Jessamine Dome, Kentucky, and yields

13 494  Ccarb values that exceed 2 ‰ with a strongest peak near its base and values that descend

495 gradually upwards to around 1–1.5 ‰. The end of this excursion occurs slightly below the sharp

496 contact of the Grier Member of the Lexington. This level also coincides approximately with the

497 incoming of abundant Plectodina tenuis (Richardson and Bergström, 2003), which has been used

498 to identify the approximate position of the international Sandbian-Katian boundary. The second

499 excursion occurs 12–15 m higher in the Macedonia bed of the Grier Member, and is accordingly

500 termed the Macedonia Excursion and has a much narrower double peak of about 2.5 ‰, typically

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501 contained within 3–4 m of stratigraphic thickness where observed (Young et al., 2015). This

502 second excursion also coincides with a Prasopora bryozoan epibole on a regional scale.

503 The Logana excursion occurs in ‘Rocklandian age’ strata and has been identified in the

504 Napanee and lower Kings Falls formations of the Trenton Group in New York State (Barta et al.,

505 2007) and the Hermitage Formation of the Nashville Dome (Bergström et al., 2010). The major

506 positive shift in the fine-grained argillaceous beds of the lower Kirkfield to the middle Kirkfield

507 members appears to correlate with the Logana excursion, thus correlating these units to the

508 Logana Member, Hermitage and upper Napanee-lower Kings Falls formations.

13 509 A higher spike of δ Ccarbvalues occurring near the base of the Verulam Formation in the 510 James B. Dick Quarry at Gamebridge, OntarioDraft may represent the Macedonia excursion, which 511 occurs in ‘Kirkfieldian to Shermanian age’ strata (Barta, 2004; Brett et al., 2004; Bergström et

512 al., 2010; Coates et al., 2010). It is notable that this sharp peak falls within the first appearance of

513 abundant Prasopora simulatrix, (although this bryozoan is also known from scattered specimens

514 in the upper Kirkfield). However, this correlation is somewhat more tenuous as it is constrained

515 by just two rising values and no double peak was observed in contrast to that seen in the type

516 Macedonia area.

517 These biostratigraphic occurrences, as well as the character of the two isotope curves and

518 the position of the Prasopora epibole within the Verulam, thus indicate that the lower of the two

519 carbon isotope excursions reported here in the lower and middle members of the Kirkfield

520 Formation (or middle Bobcaygeon) is the Logana excursion and the higher excursion, in the

521 basal Verulam Formation, is possibly the Macedonia Excursion. As discussed below, these

522 correlations are corroborated by placing the Coboconk, Kirkfield, and Verulam Formations

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523 within the sequence stratigraphic framework of Holland and Patzkowsky (1996; 1998) and Brett

524 et al. (2004).

525 Sequence Stratigraphy

526 The Kirkfield Formation is divisible into three depositional sequences and the Coboconk

527 Formation represents at least the upper portion of a fourth sequence. Each sequence shows a

528 relatively condensed grainstone interval with a general deepening-upwards pattern in the lower

529 third corresponding to a transgressive systems tract (TST), and the upper two thirds of each

530 sequence comprise thinner-bedded, argillaceous limestones and shales that exhibit a slight

531 shallowing which corresponds to a highstand systems tract (HST) or early falling stage systems 532 tract (FSST). The sequences are here definedDraft as packages bounded by disconformities. 533 Disconformities are recognized by: 1) local truncation of the underlying units; 2) direct evidence

534 for erosion such as lags of concentrated erosional clasts with an exotic lithology (not derived

535 from immediately subjacent beds); and 3) sharp facies dislocations that place shallow-water

536 facies, such as coarse, trough-cross-bedded shoal grainstones, directly above deeper-water facies

537 such as barren dark shales and argillaceous micritic limestones. Above the basal transgressive

538 grainstones are several metres of backstepping facies, typically with a fining-upwards pattern.

539 Flooding surfaces at the tops of the transgressive grainstone packages are also recognized by a

540 sharp discontinuity with overlying sediments that juxtapose deeper water facies (dark-grey shales

541 or thin-bedded argillaceous packstones) above the coarse intraclastic grainstones. These surfaces

542 may be developed as hardgrounds, recognized by borings, encrusting sclerobionts, or

543 mineralized (ferruginous or phosphatic) coatings. Hardgrounds form during periods of low net

544 sedimentation, typically induced by rapid sea level rise that traps clastic sediments in nearshore

545 settings and reduces carbonate production by increasing water depths (Brett and Liddell, 1978;

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546 Brett and Brookfield, 1984; Wilson and Palmer, 1992; Wright and Cherns, 2016; Paton, 2017).

547 The remaining portion of each sequence is characterized by aggradation to slight progradation,

548 typical of HST and early FSST. This definition of depositional sequences is consistent with that

549 used by other authors for mixed carbonate-clastic platform successions in the Ordovician of

550 eastern North America (Holland and Patzkowsky, 1996, 1998; Brett et al., 2004; Bergström et

551 al., 2010). Based upon limited biostratigraphic data, carbon isotopes, faunal epiboles, and

552 stacking patterns, the sequences are tentatively correlated with 3rd order depositional sequences

553 of the Cincinnati Arch region originally recognized by Holland and Patzkowsky (1996) and 4th

554 and 5th order subdivisions recognized and tentatively correlated from this region by Brett et al.

555 (2004). Draft 556 The base of the Kirkfield Formation, as defined herein, is a sequence boundary. As noted,

557 the presence of lithoclasts in the basal ‘unit a’ grainstone, including clasts of Gull River micrites,

558 indicates a substantial period of erosion that cut down locally through the entire thickness of the

559 Coboconk Formation. This erosion may have been focused in nearshore settings. Based upon the

560 constraints of biostratigraphy and the carbon isotope patterns discussed above, this erosion

561 surface comprises the M4-M5 sequence boundary of Holland and Patzkowsky (1996). This

562 boundary had been argued to occur within the Bobcaygeon Formation by Swisher et al. (2015)

563 based on conodont biostratigraphy but has not previously been formally placed.

564 The lower and middle members of the Kirkfield Formation (formerly the C2 grainstone,

565 the middle Bobcaygeon, and the lower portion of the upper Bobcaygeon) comprise the TST and

566 HST of the Katian M5A sequence, as defined by Brett et al. (2004) to include the ‘Rocklandian’

567 Curdsville and Logana member of the Lexington Formation in the Cincinnati Arch region, and

568 tentatively correlated with the Napanee Formation of New York (Fig. 9). Although the

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569 Curdsville was originally argued to be Kirkfieldian (see Cressman, 1973; Holland and

570 Patzkowsky, 1996; Barta et al., 2007) based on a similar echinoderm assemblage to the upper

571 member of the Kirkfield Formation, these two occurrences are diachronous and the Curdsville is

572 now accepted to be Rocklandian (Brett et al., 2004; Barta et al., 2007). The assignment of the

573 lower and middle members of the Kirkfield Formation to sequence M5A, the Curdsville and

574 Logana members of the Lexington Limestone, is also supported by carbon isotope data. The

13 575 occurrence of relatively low  Ccarb values in the basal thick grainstone (unit a of Kirkfield; C2

576 of Liberty, 1969) supports its correlation with the grainstones of the Curdsville Member, the

13 577 transgressive systems tract of sequence M5A. Similarly, the presence of the strong  Ccarb 578 carbon isotope excursion (apparently theDraft Logana excursion) within the lower and middle 579 members corroborates biostratigraphic data that support correlation with the Logana Member,

580 the highstand of sequence M5A, as well as the tentatively correlated Napanee Formation of

581 Ontario and New York, which possesses similar lithological characteristics (see Barta et al.,

582 2007). The 5.5 m middle member of the Kirkfield Formation contains medium-grey to bluish

583 grey-coloured calcisiltites (wackestone), medium-grained calcarenites, and shales and is

584 diagnosed by an abundance of Dalmanella (Resserella), a zone termed the ‘Dalmanella beds’ by

585 Johnston (1911). This is similar to the upper Logana and lower Grier members of the Lexington

586 Formation, which contain abundant dalmanellid brachiopods. The strongly descending limb of

587 the Logana carbon isotope excursion also occurs within the upper part of the Logana Member in

588 Kentucky (Bergström et al., 2010).

589 However, the 1 metre-thick grainstone noted above, as the base of Johnston's (1911)

590 ‘Dalmanella beds,’ and herein designated as the base of middle Kirkfield, may actually represent

591 the mid Logana limestone bed, the TST of a 5th order sequence which is persistent in the

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592 Cincinnati Arch region, and the upper shaly ‘Dalmanella beds’ and overlying thicker beds

593 (middle member of Kirkfield) would then correlate with the upper submember of the Logana, a

594 5th order HST (Brett et al., 2004). The base of the 1 m ‘unit c’ grainstone would represent a

595 small-scale sequence boundary (ssb) within M5A. This boundary has been argued to be a

596 significant sequence-stratigraphic surface by Coates et al. (2010). Our observations in the

597 Danville area of Kentucky, where both the Logana facies and much of the Logana carbon

598 isotopic excursion are lacking suggest that in rare cases this erosion surface may truncate the

599 lower Logana, rarely removing it entirely. However, the more important boundary within the

600 Kirkfield may be the limestone clast bed at the base of the second 1.5 metre-thick pink

601 grainstone package (unit f) above the middle member. The abrupt shift to medium- to coarse-

602 grained grainstones at this level as well Draftas the limestone lithoclasts may signify a more major

603 disconformity. As thus defined, the upper member of the Kirkfield Formation likely represents

604 the Katian M5B sequence of Brett et al. (2004); i.e. it is correlative with the lower Grier Member

605 of the Lexington Formation in the Cincinnati Arch region, as well as the Kings Falls Formation

606 of New York (Brett et al., 2004).

607 As noted, we the include 1 m package of grainstones, formerly placed in the top of the

608 Bobcaygeon, within the base of the Verulam Formation. The sharp base of these grainstones is

609 interpreted as the sequence boundary of 4th order sequence M5C, whereas the overlying shaly

610 lower Prasopora-rich beds may constitute a small-scale highstand. These shaly lower Verulam

611 Prasopora beds may correlate with the ‘Macedonia Bed’ of the Cincinnati Arch as evidenced by

612 the shared faunal epibole and presence of the brief positive isotopic excursion in each (Fig. 8).

613 Conclusions

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614 The ‘Bobcaygeon Formation’ should be split into its original components described by

615 Johnston (1911) and Kay (1943), the Coboconk and Kirkfield formations, on the basis that there

616 exists a significant erosional unconformity between these two units, the two formations possess

617 disparate micro- and macrofauna, and both preserve distinct facies. The term Bobcaygeon is

618 retained as a subgroup of the Simcoe Group that includes the Coboconk and Kirkfield

619 formations. These two units exhibit contrasting lithology and contain distinct faunas and should

620 be treated as separate formations. As defined by Johnston (1911), these units contain a coarse

621 grainstone at their tops. However, we revise the Kirkfield and Verulam formations to contain

622 grainstones at their bases because these coarse skeletal units appear to overlie regional erosion

623 surfaces. These units are interpreted to represent the basal transgressive grainstones of 4th and

624 5th order sequences. The 2 m grainstoneDraft (unit a) with lithoclasts of Gull River lithology

625 previously included in the top of the Coboconk is placed at the base of the Kirkfield Formation.

626 The 1 m grainstone (unit c) at the base of the ‘Dalmanella beds’ is designated as the base of the

627 middle member of the Kirkfield Formation. The cross-bedded grainstone with reworked

628 limestone clasts (the base of unit f), previously in the middle of the upper member of the

629 Bobcaygeon Formation, is placed at the base of the upper member of the Kirkfield Formation.

630 The ~1 m grainstone previously placed at the top of the Bobcaygeon Formation is here redefined

631 as the base of the Verulam Formation.

632 The Coboconk Formation, except for the uppermost beds previously included (unit a),

633 comprises the upper portion of the Sandbian M4 sequence. The lower and middle members of

634 the Kirkfield Formation, including the 2 m grainstone (unit a), previously assigned to the top of

635 the lower member of Bobcaygeon Formation (Coboconk), and overlying wackestones,

636 calcisiltites, and shale partings (previously middle member and part of the upper member of

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637 Bobcaygeon), comprise the TST and HST, respectively, of the Katian M5A fourth order

638 sequence of Brett et al. (2004). In addition, the upper ~1 m grainstone (unit c) and overlying

639 medium-bedded, dark-grey shaly packstones of the middle Kirkfield, rich in dalmanellid

640 brachiopods, probably comprises a highest small scale (5th order) cycle within M5A. These

641 shaly offshore facies correspond to a widespread eustatic highstand near the Sandbian-Katian

642 boundary. The upper member of the Kirkfield Formation, herein defined as having a lower

643 contact at the base of pinkish medium grainstones (unit f), comprises the Katian M5B sequence.

644 The upper Kirkfield Formation is bounded by two positive isotopic carbon excursions,

645 here correlated with the Logana excursion of M5A and, tentatively, the higher Macedonia 646 excursion of sequence M5C. One or bothDraft of these probably also correlate with the Guttenburg 647 Isotopic Carbon Excursion (GICE). The lower portion of this excursion (i.e. Logana) is observed

648 in the lower and middle Kirkfield Formation, and the upper portion (i.e. Macedonia) is observed

649 in the lower Verulam Formation, coincident with a Prasopora epibole.

650 Acknowledgements

651 We thank Dr. Brian Pratt, Derek Armstrong, and Dave Rudkin for their insight and

652 careful reviews. We acknowledge George Kampouris of Ottawa, Ontario for his assistance in

653 this project as well as discovering exceptional exposures of the Kirkfield Formation and rich

654 hardground faunas in the Tomlinson Quarry. We would also like to thank Dr. Forest Gahn,

655 Cameron Schwalbach, Dr. Michael Brookfield, Dr. Patrick McLaughlin, and Frank Brunton for

656 their contributions to this research. Funding for this project was provided by grants from the

657 National Geographic Society and the Cincinnati Dry Dredgers.

658 References

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757 Liberty, B.A. 1969. Paleozoic geology of the Lake Simcoe area, Ontario. Geological Survey of

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759 Logan, W. E., Murray, A., Hunt, T. S., and Billings, E. 1863. Geology of Canada; Geological

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767 sedimentology and biostratigraphy of the Ordovician rocks of the Lake Simcoe area,

768 south-central Ontario. Geological Association of Canada B Mineralogical Association of

769 Canada, Joint Annual Meeting, Ottawa 1994, Field Trip A4 Guidebook. University of

770 Waterloo, Waterloo, Ont., pp. 101.

771 Melchin, M. and Legault, J.A. 1985. Evolutionary lineages in some Ordovician

772 Chitinozoa. Palynology, 9(1): 199–210.

773 Mitchell, C.E., Adhya, S., Bergström, S.M.,Draft Joy, M.P. and Delano, J.W. 2004. Discovery of the 774 Ordovician Millbrig K-bentonite Bed in the Trenton Group of New York State:

775 implications for regional correlation and sequence stratigraphy in eastern North

776 America. Palaeogeography, Palaeoclimatology, Palaeoecology, 210(2-4): 331–346.

777 NACSN (North American Commission on Stratigraphic Nomenclature). 1983. North American

778 Stratigraphic Code, 1983. American Association of Petroleum Geologists, Bulletin. 67:

779 841–875. 2005. North American Stratigraphic Code, 2005. American Association of

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781 Noor, I. 1989. Lithostratigraphy, environmental interpretation, and paleogeography of the

782 Middle Ordovician Shadow Lake, Gull River, and Bobcaygeon formations in parts of

783 southern Ontario. Doctoral dissertation. Department of Earth Sciences, University of

784 Toronto, Toronto, ON.

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785 Pancost, R. D., Freeman, K. H., Herrmann, A. D., Patzkowsky, M. E., Ainsaar, L., and Martma,

786 T. 2013. Reconstructing Late Ordovician carbon cycle variations. Geochimica et

787 Cosmochimica Acta, 105: 433–454.

788 Paton, T.R. 2017. Paleo Pompeii; Genesis and Preservation of an Upper Ordovician Mounded

789 Hardground with a Diverse Encrusting Community. M.Sc. thesis, Department of

790 Geology, University of Cincinnati, Cincinnati, Ohio.

791 Pope, M.C. and Read, J.F. 1997. High-resolution stratigraphy of the Lexington Limestone (Late

792 Middle Ordovician), Kentucky, USA: A cool-water carbonate-clastic ramp in a

793 tectonically active foreland basin.

794 Raymond, P. E. 1914. The Trenton groupDraft in Ontario and Quebec. Geol. Survey Canada, Summ. 795 Rept. for 1912, pp. 342–350.

796 Richardson, J.G. and Bergström, S.M. 2003. Regional stratigraphic relations of the Trenton

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799 Russell, D.J. and Telford, P.G. 1983. Revisions to the stratigraphy of the Upper Ordovician

800 Collingwood Beds of Ontario - A potential oil shale; Canadian Journal of Earth Sciences,

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802 Sanford, B. 1993. Stratigraphic and structural framework of Upper Middle Ordovician rocks in

803 the Head Lake-Burleigh Falls area of south-central Ontario. Géographie physique et

804 Quaternaire, 47(3): 253 –268.

805 Schopf, T.J. 1966. Conodonts of the Trenton Group (Ordovician) in New York, southern

806 Ontario, and Quebec.

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807 Sinclair, G. W. 1954. The age of the Kirkfield formation in Ontario. Ohio Journal of Science, 54:

808 31–41.

809 Sweet, W.C. 1982. Conodonts from the Winnipeg Formation (Middle Ordovician) of the

810 northern Black Hills, South Dakota. Journal of Paleontology, pp. 1029-1049.

811 Swisher, R.E., Westrop, S.R. and Amati, L. 2015. The Upper Ordovician trilobite Raymondites

812 Sinclair, 1944 in North America. Journal of Paleontology, 89(1): 110–134.

813 Tucker, R.D., Krogh, T.E., Ross Jr., R.J., Williams, S.H. 1990. Time-scale calibration by high-

814 precision U–Pb zircon dating of interstratified volcanic ashes in the Ordovician and

815 Lower Silurian stratotypes of Britain. Earth and Planetary Science Letters, 100: 51–58. 816 Von Bitter, P.H. and Eley, B. 1984. BalsamDraft Lake chert from the upper member of the Middle 817 Ordovician Bobcaygeon formation of southern Ontario. Canadian Journal of

818 Archaeology/Journal Canadien d'Archéologie, pp. 135–148.

819 Williams, D.A. and Telford, P.G. 1987. Structure and Ordovician stratigraphy of the Ottawa

820 area, southern Ontario. Geological Society of America, Centennial Field Guide-

821 Northeastern Section, pp. 349–352.

822 Wilson Jr, C.W. 1935. The pre-Chattanooga development of the Nashville dome. The Journal of

823 Geology, 43(5): 449–481.

824 Wilson, C.W. 1949. Pre-Chattanooga stratigraphy in central Tennessee. State of Tennessee. 56.

825 Wilson, M.A., Palmer, T.J. 1992. Hardgrounds and hardground faunas. University of Wales,

826 Aberystwyth, Institute of Earth Studies Publications, 9: 1–131.

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827 Winder, C.G. and Sanford, B.V. 1972. Stratigraphy and paleontology of the Paleozoic rocks of

828 southern Ontario. 24th International Geological Congress, Montreal, Quebec, Excursion

829 A45-C45, pp. 73.

830 Winder, C.G., Barnes, C.R., Telford, P.G., Uyeno, T.T. and Telford, D.G. 1975. Fieldtrip Nos. 4

831 and 5. Ordovician to stratigraphy and conodont biostratigraphy of southern

832 Ontario. Field Excursions Guidebook Part B: Phanerozoic Ge-ology. Geological

833 Association of Canada, Mineralogical Association of Canada, Geological Society of

834 America, North-Central Section, pp. 119–160.

835 Wright, V.P. and Cherns, L. 2016. How far did feedback between biodiversity and early 836 diagenesis affect the nature of EarlyDraft Palaeozoic sea floors?. Palaeontology, 59(6): 753– 837 765.

838 Young, A. L., Brett, C. E., and McLaughlin, P. I. 2015. Upper Ordovician (Sandbian-Katian)

839 sub-surface stratigraphy of the Cincinnati Region (Ohio, USA): transition into the Sebree

840 Trough. Stratigraphy, 12(3-4): 297–305.

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841 Figure captions

842

843 Figure 1. Simplified geologic map of Southern Ontario.

844

845 Figure 2. Map of the eastern Lake Simcoe region showing sample localities: a) Kirkfield Quarry,

846 presently Kirkfield Lake, in Kirkfield, ON; b) Tomlinson Quarry near Brechin, ON; c) Carden

847 Quarry near Brechin, ON; and d) Gamebridge Quarry (James Dick Aggregates) in Gamebridge,

848 ON. 849 Draft 850 Figure 3. Historical nomenclature of the middle Simcoe Group.

851

852 Figure 4. Correlations of the Kirkfield Formation in the Simcoe District of southern Ontario.

853 Sections measured at the Kirkfield Quarry, Kirkfield ON (with measurements from Liberty,

854 1969), the Tomlinson Quarry near Brechin, the Carden Quarry near Brechin, and the

855 Gamebridge Quarry in Gamebridge. The datum is the base of a mounded hardground with a

856 distinct echinoderm-bryozoan fauna and considerable topographic relief, overlain by two 15–20

857 cm-thick calcarenites and a second hardground bed.

858

859 Figure 5. Field photographs of the Coboconk Formation and lower member of the Kirkfield

860 Formation: a) the contact between the light-coloured micritic limestones of the Gull River

861 Formation and the overlying Coboconk Formation in Carden Quarry; b) the contact between the

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862 Coboconk and the Kirkfield formations in Tomlinson Quarry indicating informal submembers a,

863 b, and c. Scale = 1 m; c) reworked erosional clast of Gull River lithology (yellow arrow) in the

864 unit a grainstone of the lower member of the Kirkfield Formation in Tomlinson Quarry. Scale =

865 5 cm; d) the contact between the fossiliferous wackestones of the Coboconk and the overlying

866 coarse-grained grainstones (unit a) of the basal lower member of the Kirkfield Formation in

867 Carden Quarry. Yellow arrow indicates erosional clast.

868

869 Figure 6. Field photographs of the Kirkfield Formation: a and b) the contact between the lower

870 and middle members of the Kirkfield Formation in Carden Quarry. The white arrow indicates an 871 oolite-filled channel at the base of the unitDraft c grainstone (basal middle member) that locally 872 incises the uppermost fine-grained grainstones and calcisiltites of the lower member (unit b); c)

873 the contact between the middle and upper members at the base of the unit f grainstone. Scale = 1

874 m; d) the base of the unit f cross-bedded grainstone (basal upper member) indicating reworked

875 erosional clasts (yellow arrow).

876

877 Figure 7. Field Photographs of the upper Kirkfield Formation and lower Verulam Formation: a)

878 the contact between the middle and upper members of the Kirkfield Formation. Red arrow

879 indicates the approximate position of the lower mounded hardground. Note the strong

880 backstepping pattern between units f and g with a flooding surface separating the pink-coloured

881 coarse-grained grainstones below from the overlying dark-coloured argillaceous, fine-grained

882 grainstones and packstones above; b) a surface view of the mounded hardground. Note the large

883 encrusting crinoid holdfasts and hummocky surface topography; c) the basal grainstone of the

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884 Verulam Formation, formerly designated the uppermost package of the Bobcaygeon Formation.

885 Yellow arrows indicate micritic erosional clasts. Scale = 10 cm; d) The contact between the

886 Kirkfield and Verulam Formations. Scale = 50 cm.

887

13 888 Figure 8. Composite stratigraphic section and carbon isotope profile (δ Ccarb) for the Kirkfield

889 Formation. Shown are the unit divisions within the Kirkfield and comparisons with the previous

890 divisions within the Bobcaygeon after Liberty (1969) and the fourth order sequence designations

891 after Brett et al. (2004). Disconformities are marked as sb (sequence boundaries) and ssb (small-

892 scale sequence boundaries). The carbon isotope profile displays two prominent positive 893 excursions: the larger, lower Logana excursionDraft and tentatively the upper Macedonia excursion, 894 coinciding with the M5A and M5C sequences, respectively. sb = sequence boundary, ssb =

895 subsequence boundary or small-scale sequence boundary.

896

897 Figure 9. Regional correlation of the Lower Simcoe Group with equivalent allostratigraphic units

898 of New York, the Jessamine Dome (Kentucky, Ohio, and Indiana), and the Nashville Dome

899 (Tennessee).

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Draft

900

901 Figure 1

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902

903 Figure 2 Draft

904

905 Figure 3

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Draft

906

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907 Figure 4

Draft

908

909 Figure 5

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Draft

910

911 Figure 6

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Draft

912

913 Figure 7

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Draft

914

915 Figure 8

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916

917 Figure 9 Draft

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