Canadian Journal of Earth Sciences
Revised stratigraphy of the middle Simcoe Group (Ordovician, 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, Kirkfield Formation, Bobcaygeon 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
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 Ontario 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 Coboconk 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 Canada (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 Kawartha Lakes 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. Silurian 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 Gull River,
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 limestones 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 Limestone 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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766 Melchin, M.J., Brookfield, M.E., Armstrong, D.K., Coniglio, M. 1994. Stratigraphy,
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
780 Petroleum Geologists, Bulletin, 89: 1547–1591.
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
797 Limestone (Chatfieldian) in the eastern North American Midcontinent. Northeastern
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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
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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 Devonian 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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914
915 Figure 8
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916
917 Figure 9 Draft
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