Canadian Journal of Earth Sciences

Sequence stratigraphic model for repeated “butter shale” Lagerstätten in the (Katian) of the Cincinnati region, USA

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2015-0219.R1

Manuscript Type: Article

Date Submitted by the Author: 10-Feb-2016

Complete List of Authors: Aucoin, Christopher; University of Cincinnati, Geology Brett, Carlton;Draft University of Cincinnati, Geology Dattilo, Benjamin F.; Department of Geosciences Thomka, James; University of Akron, Geosciences

Keyword: Claystone, Mixed siliciclastic, , , Lithofacies

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1 Sequence stratigraphic model for repeated “butter shale” Lagerstätten in the Ordovician

2 (Katian) of the Cincinnati region, USA

3 Christopher D. Aucoin 1* , Carlton E. Brett 1, Benjamin F. Dattilo 2 , James R. Thomka 3

4 1Department of Geology, University of Cincinnati, Cincinnati, Ohio, 45221

5 [email protected], [email protected]

6

7 2Geoscience Department, Indiana University Purdue University Fort Wayne, [email protected]

8 3Department of Geosciences, University of Akron, Akron, Ohio 44325, USA;

9 [email protected] Draft

10 *Corresponding author (C.D. Aucoin)

11 500 Geology Physics Building

12 University of Cincinnati

13 Cincinnati OH 45221-0013

14 Email: [email protected]

15

16

17

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18 Sequence stratigraphic model for repeated “butter” shale Lagerstätten in the Ordovician

19 (Katian) of the Cincinnati region, USA

20 Christopher D. Aucoin 1* , Carlton E. Brett 1, Benjamin F. Dattilo 2 , James R. Thomka 3

21

22 Abstract:

23 The “butter shale” Lagerstätten of the Cincinnati Arch have produced an abundance of

24 articulated , along with assorted bivalves and cephalopods. These bluish-gray shales are

25 rich in clay, poorly calcified, and show vague internal bedding in outcrop. “Butter shales” form a 26 repetitive motif with similar lithological and paleontologicalDraft characteristics suggesting 27 conditions existed that can be explained by the interference between different orders of sequence

28 stratigraphic cyclicity. The characteristics that define "butter shales" include: rarity of coarser

29 interbeds, homogenous, fine grain-size, and abundance of burial horizons. The overriding control

30 is siliciclastic sediment supply. During 3rd order transgressions sediment supply to the basin is

31 too low to produce thick shale-prone intervals. Conversely, during third-order falling stages

32 sediment supply is generally too high to favor "butter shale" deposition. “Butter shales” formed

33 preferentially during 3 rd order HST and two subtly different variants resulted from the

34 superimposed effects of higher order cycles. Highstands moderated by small-scale transgressions

35 are characterized by lower background sedimentation and fewer/thinner mud deposition events.

36 Superposition of small-scale sea level fall on highstands produced increased background

37 sedimentation, higher silt, and patchy occurrences. Juxtaposition of various scaled HSTs

38 provided the optimal “butter shale” conditions, characterized by elevated mud influx and

39 frequent episodic burial events, leading to abundant, articulated trilobites and associated fauna.

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40 In these scenarios, episodic events provide sufficient mud to smother local faunas and create a

41 soft, fine-grained substrate that prohibited recolonization by taxa adapted to firm substrates. Each

42 scenario differs from the others with respect to sedimentology and faunal composition.

43 Keywords: Claystone, Mixed siliciclastic, Trilobite, Waynesville Formation, Lithofacies 44

Draft

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45 46 Introduction

47 Konservat-Lagerstätten, deposits containing exceptionally well-preserved , occur

48 repeatedly in the fossil record (Seilacher et al. 1985; Nudds and Selden 2008). Many of these

49 deposits, commonly genetically related to obrution (rapid burial) events, are famous for their

50 preservation of articulated multi-element skeletons (i.e., intact, delicate echinoderm and

51 remains) and, fittingly, considerable work has been done on the taphonomy of these

52 assemblages (e.g., Brett et al. 1997; Brett and Seilacher 1991). These deposits are unusually

53 valuable for the reconstruction of whole skeletal anatomy, permitting recognition of life and

54 mortality postures, and enabling interpretation of original community density and structure. To

55 preserve such readily disarticulated organisms in an articulatedDraft state and/or in life position

56 requires that the organism be buried rapidly enough to keep the skeleton intact and sufficiently

57 deep to prevent exhumation or scavenging. Hence, the depositional processes associated with

58 these Lagerstätten are restricted to certain paleoenvironments, and therefore may recur

59 predictably within stratigraphic sequences (Brett and Baird 1986).

60 The Upper Ordovician (Katian; Edenian-Richmondian) of the Cincinnati Arch region of

61 North America contains numerous obrution Lagerstätten referred to informally as “butter shales”

62 or ‘trilobite shales’ (Fig. 1). These shales derive this name from their soft homogenous, fine

63 grained nature and are sought by collectors for articulated trilobites, as well as extraordinarily

64 preserved echinoderms, bivalves, nautiloids and other fossils (Frey 1987a, 1987b; Schumacher

65 and Shrake 1997; Hunda et al. 2006; Aucoin et al. 2015). The shales are typically one to three

66 meters thick and are characterized by a bluish-green coloration, soft, sticky claystone

67 consistency, and a scarcity of interbedded limestones (Brandt Velbel 1984; Frey 1987a, 1987b;

68 Hunda et al. 2006, Aucoin et al. 2015) (Fig. 2). Thus, they form lithological motif as well as a

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69 suite of distinctive taphofacies (sensu Brett and Baird 1986). In this paper we explore the

70 possibility of a sedimentological control, linked to eustatic fluctuations, over the distribution of

71 “butter shales” and the composition and preservation of their faunas. In particular, we explore

72 the possible variations in siliciclastic sediment input that control obrution occurrences and may

73 result from the constructive and destructive interference between cyclic climatic/sea-level

74 oscillations of different scales, thereby allowing the distribution of “butter shales” to be modeled

75 and predicted based on sequence stratigraphy. This research may lead to a better understanding

76 not only of “butter shale” type obrution deposits but of a variety of other taphofacies (cf. Brett

77 1995).

78 Geologic setting Draft 79 During the Late Ordovician the present-day Ohio, Kentucky, and Indiana tri-state region,

80 east-central USA, was covered by a shallow epicontinental sea. The region had a ramp geometry

81 with shallow "lagoonal" to peritidal environments in south-central Kentucky deepening gradually

82 north-northwestwardly into southern Ohio and Indiana (Brett and Algeo 2001; Meyer and Davis

83 2008; Brett et al. 2015). At this time, the Cincinnati Arch region was located approximately 20 oS

84 of the equator and Laurentia was rotated clockwise 45 o relative to present orientation (Holland

85 1993; Brett and Algeo 2001; Holland and Patzkowsky 2007). Upper Ordovician depositional

86 sequences in the Cincinnati region exhibit mixed carbonate-siliciclastic facies with transgressive

87 systems tracts being dominated by carbonates, and highstand and falling stages predominantly

88 siliciclastic muds and silts sourced from the Taconic Mountains to the east.

89 Characteristics of “butter shales” and a predictive model for their occurrence

90

91 “Butter shale” taphofacies and biofacies

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92 As noted, “butter shales” are thicker than average intervals of soft, poorly calcareous

93 claystone, mainly illitic with low total organic carbon and a typically bluish-gray. They contain

94 abundant pyrite and may show very slender pyritic burrows. Small carbonate concretions (~5

95 cm in diameter) may occur at particular horizons, as do very thin shell hash beds and minor

96 calcareous siltstones, but overall these intervals may be nearly pure clay with few fossils except

97 in certain levels (Aucoin et al. 2015).

98 In terms of taphonomy, typical features of "butter shale" taphofacies include: a) abundant

99 articulated, closed or butterflied bivalves, typically as robust composite molds and with black

100 periostracal films preserved; b) three-dimensionally preserved nautiloid cephalopods and 101 gastropods, commonly with calcitic chamber fills; c)Draft abundant and articulated trilobites, both as 102 prone, and typically inverted, carcasses and as enrolled specimens (Hunda et al. 2006). Other

103 fossils, including intact and disarticulated crinoids, brachiopods and disarticulated bivalves,

104 "hash" of trilobite exuviae, and shell debris occur on individual bedding planes (e.g.,

105 Schumacher and Shrake 1997).

106 The faunas and paleoecology of three distinct "butter shale" intervals have been well

107 documented (Frey 1987a, 1987b; Schumacher and Shrake 1997; Hunda et al. 2006; Aucoin et al.

108 2015). All of these shales in the Cincinnatian share common features with respect to faunas.

109 These include: a) lower abundance of otherwise typical shelly, suspension-feeing epibenthos

110 (e.g., articulate brachiopods, bryozoans, crinoids) compared to other intervals of the type

111 Cincinnatian (see Holland and Patzkowsky 2007), and b) a relatively high abundances of vagrant

112 to slightly mobile organisms such as endobyssate and burrowing bivalves, gastropods, nautiloids,

113 and trilobites. In addition, cryptic bioturbation may be pervasive in the mudstones and common

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114 scolecodonts in some intervals points to an originally abundant soft-bodied infauna including

115 polychaete worms (Eriksson and Bergman 2003).

116 “Butter shales”: basic conditions and assumptions

117 Any model to explain the recurrent "butter shales" must consider both taphonomic and

118 paleoecological aspects of these deposits. Three basic factors are considered essential for the

119 development of any “butter shale” deposit. The first is background sediment influx, which

120 controls both substrate consistency (i.e., inhibiting shell bed growth; preventing hardground

121 development via mobilizing redox boundaries in response to migration of the sediment-water

122 interface) and governing faunal composition (i.e., precluding turbidity-intolerant epifaunal taxa;

123 increasing abundance of mobile, turbidity-tolerant, and/orDraft infaunal taxa). Many sedentary

124 suspension feeders, such as crinoids, require firm substrates on which to attach, at least initially

125 (Brett et al. 2008). Strong pulses of mud from the distal outfall of storms and other events create

126 soft substrates prohibiting most crinoids and many brachiopods from colonizing. In areas where

127 the mud has been winnowed or pauses in episodic events have occurred, harder substrates are

128 expected to return and crinoids and brachiopods may be more prevalent.

129 Related to the substrate issue is evidence for a relatively high rates of mud sedimentation.

130 The associated high turbidity and soft, fluid substratum would inhibit colonization by certain

131 groups of organisms. Although mobile epifauna, such as gastropods, nautiloids, and trilobites

132 were better equipped to handle higher background sedimentation and softer substrates, it is more

133 likely that the rate of sedimentation would have to have been more moderate for filter feeding

134 semi-infaunal bivalves to be successful. It appears that these organisms may have been more

135 turbidity tolerant than a majority of brachiopods. High sedimentation rates do not explain the

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136 occasional limestone hash beds with brachiopods and crinoids that occur within the shale and

137 thus substrate controlled by episodic events appears more probable. In the context of this model,

138 the skeletal debris beds would be expected to form in areas of increased winnowing, exposing

139 harder substrates for colonization. Subsequent storm events would continue to winnow in these

140 areas of reduced mud leading to an accumulation of shell hash over multiple generations (Dattilo

141 et al. 2008).

142 The next factor is the fine grain size of these deposits. Cincinnatian “butter shales” are

143 composed primarily of clays largely undiluted by carbonate material, which gives them their

144 soft, butter-like consistency. In addition the lithological contrast with indurated limestone beds 145 makes these intervals stand out sharply, both lithologicallyDraft from the shelly carbonates that 146 comprise much of the Cincinnatian Series (Brett and Algeo 2001; Brett et al. 2008).

147 In addition, the accumulation of soft, clay-rich substrates had ecological effects. Fine-

148 grained sediment is comparatively low in permeability, potentially promoting subsurface anoxia

149 close to the sediment-water interface. Low oxygen within the sediments may have inhibited deep

150 infaunal burrowing and reduced rates of decay. This is supported by large quantities of pyrite

151 framboids, pyritic fossils and pyritic burrows retrieved during disaggregation of the butter shales

152 for microfossil extraction. On the other hand, fine particulate organic detritus accumulates

153 preferentially in muds, making them a rich food source for deposit feeders that tolerate low

154 oxygen conditions (Rhoades and Morse, 1971).

155 A third factor required for development of certain “butter shale” taphofacies is episodic

156 rapid burial by fine-grained siliciclastic sediment. Although background sedimentation can be

157 sufficient to preserve organisms as fossils, the majority of the fossils preserved during low-

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158 sedimentation, quiescent intervals represent individuals that died, became disarticulated, and

159 remained exposed on the sea floor during a significant residence time in the taphonomically

160 active zone (Speyer and Brett 1991). To preserve the abundant articulated multi-element

161 skeletons, commonly observed in “butter shales”, episodic events, which dramatically increase

162 sedimentation beyond normal background conditions, are required to bury organisms to the point

163 where they are smothered and preserved intact. For sessile organisms, such as byssate bivalves,

164 the increase in sedimentation rate associated with burial does not need to be as high as for mobile

165 fauna because they cannot disinter themselves as mobile fauna can and they are more commonly

166 preserved intact. Further, the resultant softer substrate would prohibit a resurgence of these 167 sessile fauna (i.e., inhibitory taphonomic feedback; KidwellDraft and Jablonski 1983; Freeman et al. 168 2013). As noted, a majority of the organisms preserved in "butter shales" were at least mobile to

169 some extent and thus higher rates of event sedimentation and/or other sources of mortality would

170 be required to entomb their intact remains. In fact, many enrolled or partially enrolled trilobites

171 in "butter shale" intervals occur at random orientations with respect to bedding, suggesting that

172 many of the best-preserved horizons may reflect entrainment of organisms in bottom flows (type

173 2 obrution deposits of Brett et al. 2012). This implies episodic input of viscous mudflows as

174 distal tempestites or mud turbidites.

175 These three sedimentation characteristics (rate of background sedimentation, grain size of

176 sediment, and thickness of event deposits) may ultimately be modulated by sea-level

177 fluctuations. Sequence stratigraphy can provide a predictive model for the occurrence of various

178 facies including “butter shales” that qualify as trilobite Lagerstätten (Brett 1995). To better

179 understand the occurrence of "butter shale" taphofacies within the context of sequence

180 stratigraphy it is important to recognize that each of the 3rd order sequences generally accepted

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181 for the greater Cincinnati Arch region (Holland and Patzkowsky 1996, and subsequent papers) is

182 composed of a hierarchy of smaller cyclic sedimentary units that display properties mirroring

183 those of larger sequences. These small-scale cycles comprise condensed limestone-rich

184 transgressive intervals overlain by shalier highstand intervals) and range from meter- to

185 decameter-scale (Dattilo et al. 2012). For the present purposes we have simplified this

186 discussion by using two orders of cyclicity: that are termed 3rd and 4 th order sequences.

187 Interactions of cycles at these two scales may have the net effect of amplifying or dampening

188 particular sedimentary processes related to development of “butter shales”. For the present

189 model we are most concerned with factors that may enhance or suppress offshore mud 190 sedimentation. The precise manner in which nested cyclesDraft may interact is complex but is most 191 readily considered from the perspective of small scale oscillations at shorter time scales (e.g. a

192 few 100 kyr) occurring during times when base-level as a whole was rising or falling. For

193 example, a higher-order transgression occurring during a time of lower-order (larger-scale) high

194 and rising sea level might have the effect of particularly strong mud sequestration in estuarine

195 areas-leading to intensified mud starvation offshore (i.e., amplification of transgression).

196 Conversely, a short term regression superimposed on overall low sea level might lead to stronger

197 than average progradation and elevated deposition of mud and/or silt in offshore areas.

198 For the purposes of a simplified conceptual model we consider six combinations of two

199 "states" of lower order cycles (transgression and highstand/regression) superimposed on three

200 different phases of higher order cycle: transgression, highstand, and falling stage (terminology of

201 Catuneaunu 2006) as might occur during a third order sequence. The lowstand systems tract

202 (LST) was not factored into our analyses because during this systems tract, sea-level is at its

203 lowest causing much of the epeiric basin under study to experience erosion rather than

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204 deposition. For this reason, the LST is mostly absent in the Cincinnatian, with sequence

205 boundaries generally representing co-planar sequence boundaries and transgressive surfaces

206 (Holland 1993; Schramm 2011).

207 Predictive models for “butter shales”

208 A conceptual model linking sequence stratigraphy and taphofacies can be summarized by

209 looking at two nested sea-level curves (Fig. 3). On the rising limb of the diagram, enhanced sea

210 level rise can be caused by constructive inference of a smaller scale sea level rise. It is during

211 this interval that beds enriched in carbonate and phosphatized skeletal debris should accumulate.

212 Near the top of the curve, extending from the very end of the higher order TST to the start of the

213 higher order FSST and encompassing the entire HST,Draft is the interval of maximum potential for

214 "butter shale" development. This interval is characterized by maximum shale/mudstone

215 development and significantly less carbonate and it may also be modulated by the phases of

216 smaller scale cycles. Finally, on the falling limb of the curve, the period of maximum sea level

217 fall is characterized by low shell content, and high silt and mud deposition, but again, this may

218 be modified by effects at smaller scale.

219 Figure 4 shows a comparison of larger 3rd order and smaller (4th or 5 th order) systems

220 tract combinations with respect to the three environmental parameters discussed above. For

221 purposes of this model we utilize 3 rd order to indicate the larger depositional sequences of

222 approximately 0.5-2 million year durations, modified from the C1 to C5 sequences originally

223 recognized by Holland and Patzkowsky (1996); we use 4 th order to imply major subdivisions of

224 these intervals with durations of about 100-400 Kyr. In the following sections, we explore the

225 various nested combinations of cycle phases of the two nested scales of sequences as depicted in

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226 Figures 3, 4 and 5. Through this discussion we will attempt to demonstrate how a “butter shale”

227 style deposit may be expressed, or not, in different sequence pairings. "Butter shales" can

228 actually form in multiple sequence pairings, which will alter the way in which the mudstones are

229 expressed faunally and sedimentologically, although there is an optimal set of conditions for

230 thicker shale formation.

231 “Butter shale” scenarios

232 3rd Order HST - 4th Order TST

233 This case represents superimposition of the lower order TST on the higher order HST. 234 During this pairing the rapid sea level rise of the 4th orderDraft TST will cause a slightly higher than 235 normal rate of sea level rise overall for the HST (Fig. 4) along with a slightly higher maximum

236 landward progression of water. Despite the higher sea level rise and rate, siliciclastic

237 sedimentation would still be predominant and fine-grained siliciclastics would be deposited in

238 the basins at a low rate. Episodic events would create a patchwork of soft and hard substrates. In

239 this scenario, although mixed mobile and sessile faunas would be expected, there would likely be

240 a dominance of sessile suspension feeding organisms. Trace fossils may be present in small

241 quantities, but low sedimentation rates may lead to depletion of the detrital organic matter in the

242 sediment, inhibiting deposit feeders. This pairing may produce shales with occasional limestone

243 or siltstone interbeds (Fig. 5). The thickness of the shale will vary depending on the level of

244 sequestration produced by the TST.

245 As the lower 4th TST passes into the 4th order HST, i.e., there is an additive slowing of

246 the rate of rise, the system approaches the true maximum flooding surface causing a relatively

247 brief period of starvation to occur. During this transition, a buildup of carbonate material with

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248 intercalated shale is expected. This is similar to the conditions of the lower order TST-HST

249 transition.

250 An example of this type of interval is the " Reteocrinus bed", an interval of compact

251 mudstones and ledge-forming limestones that overlies the Upper G. insculpta submember of the

252 Liberty Formation. The interval is very consistent over Ohio, Indiana, and northern Kentucky

253 with a thickness of about 11’ (3.35m) from Waynesville to Madison. Greenish-gray shales in the

254 lower 6’ (1.8m) yielded the most diverse crinoid fauna of the Cincinnatian (including of

255 Cincinnaticrinus, Paradendrocrinus, Reteocrinus, Glyptocrinus, Compsocrinus and

256 Canistrocrinus ) in creeks in Warren and Clinton County, Ohio (Austin 1927; Morris and Felton 257 1993). Articulated bivalves and nautiloids are also typicalDraft of this interval.

258 3rd Order HST - 4th Order HST

259 The HST-HST pairing is considered to represent the ideal situation for deposition of

260 thicker "butter shale" intervals as it favors conditions that fulfill all three requirements for

261 deposition of muds with relatively little skeletal debris. At the beginning of the HST, sea level

262 has reached its farthest landwards extent and much of the terrigenous sediment is still being

263 sequestered in nearshore and coastal plain settings; however, as defined by Catuneaunu (2009)

264 the highstand is also characterized by sedimentation rates, which exceed those of base level rise,

265 thus allowing sediment to be deposited offshore in an aggradational to progradational pattern.

266 Continued nearshore sequestration in filling estuaries still traps the majority of the coarser

267 material but allows abundant fine-grained sediment to move offshore. Episodic storm turbulence

268 would resuspend this material allowing for burial and smothering of the existing fauna in a

269 “butter shale” style lagerstätten. This scenario would be expressed similarly to that of the 3rd

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270 order FSST 4 th order HST with small modifications. The HST-HST should have a larger quantity

271 of clay-sized sediment and less silt than would be anticipated in the FSST-HST. The

272 homogeneity of sediments would tend to obscure trace fossils, because of a lack of contrast

273 between burrow fillings and matrix. Offshore environments should contain a mixture of sessile

274 and mobile fauna with a greater number of mobile organisms than present in the 3rd order HST

275 4th order TST.

276 Carlucci and Westrop (2014) provide empirical data from the Bromide Formation in

277 Oklahoma, which indicate that deposits yielding quality trilobite preservation were typically

278 those of the HST. Even when similar assemblages were found in the TST deposits, the 279 preservation of multi-element skeletons was of lesserDraft quality, although overall diversity was 280 higher in the TST. This has been attributed to both winnowing and a lack of burial during

281 formation of the time-averaged skeletal accumulations of the TSTs.

282 This end-member is exemplified by the best studied “butter shale” from the Cincinnatian, Comment [??1]: I do not really like the addition of this new abbreviation system. If we do that it should be carefully explained; 283 the Harpers Run submember (Aucoin and Brett 2016), formerly termed Treptoceras duseri or otherwise it is just a confusing form of new jargon. 284 Trilobite shale (Frey 1987a, 1987b). The shale is situated within the Fort Ancient Member of the

285 Waynesville Formation and has been correlated well over 100km. Contained within this unit are

286 the trilobites Flexicalymene and Isotelus¸ the nautiloids Treptoceras duseri, Manitoulinoceras ,

287 molluscan bivalves Ambonychia , Cuneamya miamiensis, Caritodens, Modiolopsis concentrica,

288 Orthodesma curvatum , Lyrodesma the bryozoans Cyphotrypa clarksvillensis, Spatiopora , the

289 corals Tetradium, Labechia , the stromatoporoid Stromatocerium , lingulid brachiopod, crinoids,

290 graptolites, conodonts, gastropods Clathrospira and Sinuites and the ichnofossils Chondrites

291 (Foerste 1908; Austin 1927; Wolford 1930; Frey 1987a, 1987b, etc). This shale tends to be about

292 2m thick with very thin lenses of limestone throughout. The shale also contains discontinuous

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293 lenses of skeletal pavements. The Harpers Run submember is good example of an HST-HST

294 scenario where there environment is extremely muddy and dominated by mollusks and trilobites,

295 with only occasional brachiopods, trepostome bryozoans, and crinoids. However, diastems are

296 recorded by horizons of corroded and bored stromatoporoids and Tetradium corals (Frey 1987a,

297 1987b). These suggest deposition in relatively shallow water settings wherein pauses in

298 sedimentation permitted temporary colonization of the seafloor. Additional examples of HST-

299 HST butter shales include the Oldenburg submember of the Waynesville (Aucoin et al. 2015)

300 and the Mt Orab shale of the (Hunda et al. 2006).

301 3rd Order FSST - 4th Order TST and HST

302 A 4 th order transgression superimposed on a 3 Draftrd order falling would setup conditions in

303 which moderate sedimentation would occur as the general drop in sea level that would be

304 expected from an FSST would be temporarily slowed (Fig. 4). The majority of the sediment

305 deposited would be fine-grained siliciclastics. However, coarser silt- or even sand-sized

306 sediment could be mixed with carbonates forming calcareous, shelly siltstones or silty

307 packstones. As in the 3rd order FSST 4th order TST scenario, “butter shale” formation would be

308 possible, but thicker and siltier shales would be expected during the 3rd order FSST 4th order

309 HST pairing. Although silty beds are not unexpected in the other “butter shale” setups, this tract

310 pairing would produce greater thickness of siltstone beds (Fig. 5). This "butter shale" would still

311 likely contain a mixture of mobile and non-mobile fauna; however, the higher rate of

312 sedimentation and softer substrate would create a preference for mobile fauna. This higher

313 propensity for mobile fauna combined with the chance of slightly coarser material, would likely

314 create a deposit wherein trace fossils would be abundant and relatively well defined.

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315 For example, in the Liberty Formation (Fig. 6), there is a "butter shale", informally

316 known as the Minuens shale (named for the small species of Flexicalymene, F. minuens which is

317 found in abundance). The main body of the shale is the typical HST-HST 2m thick clay shale.

318 Just above the shale, is a series of stacked siltstone beds with interbedded clay shale. Another

319 excellent example of a 4th order TST and 3 rd order FSST combination is seen in the extraordinary

320 Glyptocrinus bed of the Maysville area where pockets of perfectly preserved crinoids occur

321 overlying scoured siltstone beds and buried in silty mudstones (Brett et al. 2008; Milam 2013).

322 Non-“butter shale” scenarios

323 Although the following scenarios fail to produce “butter shale” deposits, brief discussions of

324 their conditions are provided to contrast the “butter shale”Draft examples.

325 3rd Order TST - 4th Order TST

326 The 4 th order TST superimposed on the 3 rd order TST, presumably by amplified warming

327 during a longer warm interval, causes an accelerated rate of sea level rise by constructive

328 interference and thus accommodation would have strongly outpaced the rate of sedimentation.

329 During this time, siliciclastic sediments are sequestered in the estuaries and rivers (Fig. 4). This

330 excludes the deposition of fine-grained siliciclastic material in far offshore areas, although minor

331 carbonate mud may be produced locally. Sediment starvation in the downramp environments

332 would mean there would be little sediment for storm disturbances to resuspend and redeposit.

333 Instead, storms would winnow what sediment was present and break up and rework the shell

334 material (Brett et al. 2008). It is during this interval that the maximum carbonate buildup (Fig. 5),

335 consisting of reworked shelly material, often broken and variably biased, along with

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336 accumulation of phosphatic steinkerns, would build up in the absence of dilution (Brett et al.

337 2008; Dattilo et al. 2008, 2016).

338 3rd Order TST - 4th Order HST

339 A shorter-term cooling during a time of rising sea level may result in a slowing of the

340 general rate of rise, i.e., the superimposing of a 4th order HST on a 3rd order TST, permitting

341 some increased offshore sediment progradation. Such a situation sets up conditions, similar to

342 the TST-TST where sea level is higher than usual (Fig. 4). However, the sedimentation rate is

343 also increased due to the influence of the slowing rate of rise, which allows possible movement

344 of fine-grained sediment into the basin. This creates a transitional lithology of thinly interbedded

345 limestones and shales. The fine-grained sediment in theDraft basin would be available for

346 resuspension during storm events although the amount of terrigenous mud would still be minor.

347 These deposits are likely to be relatively thin (Fig. 5).There are occasional obrution deposits

348 within these intervals and the low net rate of sedimentation may allow a stacking of obrution

349 layers, as the only siliciclastic sediments to accumulate are those of extremely large storm

350 disturbances, which export fine-grained sediments offshore. However, these beds will not

351 resemble "butter shales". Rather, they will consist of thin layers of mudstone containing more

352 shelly material and may overlie encrusted hardgrounds. These mud layers will tend to

353 incorporate remains of organisms, such as bryozoans, edrioasteroids or crinoids, that thrive

354 during times of lowered sedimentation and turbidity. Examples include well described

355 edrioasteroid beds of the (Meyer 1990; Shroat-Lewis et al. 2011).

356 3rd Order FSST - 4th Order FSST

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357 Lastly, a nested FSST and FSST package would have constructive interference, which

358 would act to increase the rate of forced regression and the already rapid sedimentation rate

359 expected for a FSST, excluding most organisms from living in such an environment. When

360 present, this pairing would create thick, commonly deformed deposits of silty to sandy beds. One

361 might expect to find extensive discrete trace fossils on some bedding planes in this package. The

362 FSST-FSST and TST-TST pairings represent extreme end members, which largely exclude the

363 possibility of “butter shale” formation.

364 Discussion 365 A Waynesville Formation succession Draft 366 The Waynesville Formation (Fig. 6) of the Richmond Group of the Cincinnati Arch

367 provide the prime example of the expression of systems tract pairs (Fig. 7). At the base of the

368 Waynesville Formation is a bed, about 50cm thick, known as the South Gate Hill submember

369 (SGH), a pack-grainstone made up of Cincinnetina brachiopods (Jin 2012), bivalves and other

370 shells (Aucoin and Brett 2016). This bed represents the TST-TST condition, i.e., accelerated rate

371 of rise produced by constructive interference. Above the SGH is a 6m bed of clay rich barren

372 shale with occasional interbeds of limestone and siltstone. This package, called the Lower Fort

373 Ancient shale or "Barren shale", represents the 3rd order TST 4th order HST. and basal 3rd order

374 HST 4th order TST. The Bon Well Hill submember (BWH), a series of brachiopod rich pack-

375 grainstones separated by brachiopod rich shales represents the 3 rd order HST 4th order TST.

376 Lastly we have the HST-HST represented by the Harpers Run submember (Aucoin and Brett

377 2016). This shale is a 1-3m package of clay rich material containing interbeds of calcisiltite and

378 abundant trilobites, bivalves and cephalopods. Above this level the pattern is repeated, with the

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379 Stony Hollow Creek submember (Aucoin and Brett 2016) representing the TST-TST of the next

380 high order package. Above that, the TST-HST and the HST-TST is represented by the Middle

381 Clarksville submember composed of shale with thin limestone interbeds.

382 Exploring the HST

383 In all three highstand related combinations (HST-TST, HST-HST, FSST-HST), “butter

384 shale” deposits can form and are even likely to form. The 3rd order HST 4th order TST and 3rd

385 order FSST 4th order HST both act as end members for the “sweet spot” zone but what do those

386 end members really mean? The TST-HST scenario results in a slightly coarser substrate allowing

387 for benthic fauna such as crinoids, brachiopods, bryozoans and even corals to thrive. Trilobites

388 and bivalves are still present in this setting, although Draftcompetition from crinoids, brachiopods and

389 bryozoans makes them relatively less abundant. The HST-HST scenario marks the optimal

390 combination for the “butter shale” zone. With a greater influx of clay-sized sediment, the softer

391 substrate causes suspension feeders to decline. Trilobites, bivalves, lingulids and other organisms

392 that are more adapted for muddy substrates proliferate. Thus, these organisms increase relative to

393 sessile suspension feeders. In the FSST-HST scenario mud is replaced by silt as the primary

394 sediment and any non-mobile fauna, such as attached brachiopods, exist in small numbers.

395 Bivalves, trilobites, cephalopods and even lingulids, as well as soft-bodied burrowers, dominate

396 this silty environment.

397 Beyond the Waynesville

398 The primary examples of butter shales presented in this paper include those from the

399 Arnheim, Waynesville and Liberty Formations of the early Richmondian. However the question

400 could be posed, are there butter shales elsewhere in the Cincinnatian, and are there examples of

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401 butter shales elsewhere temporally and geographically? There are a number of other Cincinnatian

402 butter shale examples not discussed in this paper including the “granulosa” shale from the Kope

403 Formation (Gaines et al. 1999; Hughes and Cooper 1999), and the little studied Moranburg, and

404 Western Hill Trilobite shales from the Corryville. There are also number of Isotelus rich beds

405 known from Bromley Member of the as well as the Elkhorn Formation.

406 Although poorly studied, this beds may also be considered potential butter shales.

407 The Cincinnati Arch’s location relative to the Taconic orogen and consequent mixed

408 siliciclastic-carbonate system is a major contributing factor for the prevalence of butter shale

409 style Lagerstätten in the Cincinnatian. The sequestering of the majority of the coarse siliciclastics 410 near the orogen is important for butter shale development.Draft

411 Examples of butter shale style Lagerstätten persist beyond the Cincinnatian. The

412 (Wenlock) Waldron Shale from Indiana and the from New York provide

413 excellent examples. Although more calcareous than the Cincinnatian butter shales, the Rochester

414 and Waldron formations both show analogues of "butter shales". These are intervals of soft,

415 rather sparsely fossiliferous mudstone and they may contain obrution beds with abundant

416 articulated trilobites and small crinoids that record HST-HST deposits, whereas TSTs are

417 typified by dense bryozoan and brachiopod packstones (Taylor and Brett 1988, 1999; Peters and

418 Bork 1998; Brett 2015). Limited obrution beds with diverse echinoderm faunas and some

419 trilobites occur in thinner mudstone intervals interpreted as HST-TST or TST-HST pairs (Brett

420 2015). Inferred falling stage deposits consist of alternating silty dolomitic mudstones and

421 calcisiltites, which are generally sparsely fossiliferous, but contain rare well-preserved crinoid-

422 trilobite beds, probably associated with brief transgressions superimposed upon the general

423 forced regression (FSST- TST or HST combinations).

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424 Further examples include the Hamilton Group of New York, and Ontario

425 (Speyer and Brett 1986; Brett et al. 1986; Miller et al. 1988; Brett 1999; Tsujita et al. 2006).

426 Excellent analogs of Cincinnatian "butter shales" occur in the equivalent Silica Shale of Ohio. In

427 particular, unit 9 consists of 1-2 m of pure claystone which yields abundant, commonly pyritized

428 fossils including enrolled Eldredgeops trilobites (Kesling and Chilman 1975). These beds are

429 quite reasonably interpreted as HST-HST. A very good example of an TST-HST pair is unit 13

430 which shows obrution beds of complete crinoids in mudstones overlying skeletal debris

431 packstones. These examples suggest that the preliminary model presented here may be readily

432 generalized to numerous other small scale sequences in mixed siliciclastic-carbonate successions 433 (also see Brett et al., 2011). Draft 434 Conclusions

435 • Three primary environmental parameters are required in order for the generation of

436 "butter shale" type deposits: background sedimentation rates must be moderate to low,

437 deposition of predominantly fine-grained siliciclastic sediment, and episodic, rapid burial

438 the local system by fine-grained siliciclastic material.

439 • Sequence stratigraphy can be used to generate predictive models for the optimal

440 generation of "butter shale" as well as other facies.

441 • Although there is a spectrum of possible nested systems that can produce butter shale

442 style deposits, a combination of nested lower and higher order highstands represents the

443 optimal conditions for "butter shale" generation.

444 • “Butter shales” are found throughout the Cincinnatian as well during the Silurian and

445 Devonian. It is very likely that additional examples can be found elsewhere. Previous

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446 case studies largely conform to the model developed herein, with the analogs of

447 Cincinnatian "butter shales" occurring at in highstand combinations.

448

449

450

451 Acknowledgements

452 This project was funded by the 2013 Dry Dredgers Paleontological Research Grant (to CDA),

453 the 2014 Association of Applied Paleontological Sciences Grant (CDA), the American

454 Association of Petroleum Geologists Grant (CDA), the Clay Mineralogical Society Student 455 Grant (CDA) and an American Chemical Society PetrolDrafteum Research Fund Grant 528 # 55225- 456 UR8 (BFD). The authors would like to thank Dan Cooper who has repeatedly granted us access

457 to his trilobite quarries. Also we would like to thank Steve Westrop, Jinsu Jin, and the

458 anonymous reviewer that helped greatly improve this paper. This is a contribution to the

459 International Geoscience Programme (IGCP) Project No. 591 – The Early to Middle Paleozoic

460 Revolution .

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589 Geology, University of Cincinnati, Cincinnati, Ohio.

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593 Ecological, and Evolutionary Implications. Edited by Brett, C.E., and Baird, G.C.

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599 Paleoecologic assessment of an edrioasteroid (Echinodermata) encrusted hardground

600 from the Upper Ordovician (Maysvillian) Bellevue Member, Maysville, Kentucky.

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604 Speyer, S.E., and Brett, C.E. 1991. Taphofacies controls: Background and episodic processes in

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607 from the Silurian (Wenlockian) Rochester Shale. PALAIOS , 11 : 118-140.

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612 disturbance in the Middle Devonian Arkona Shale, southwestern Ontario. Journal of 613 Taphonomy, 4: 49-68 Draft 614 Wolford, J.J. 1930. The Stratigraphy of the Oregonia-Ft. Ancient Region, Southwestern Ohio.

615 The Ohio Journal of Science, 30 : 301-308.

616

617

618

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619 Fig. 1. Diagram showing the Cincinnatian strata of the tri-state region with some of the “butter

620 shale” horizons recorded. A =Moranburg Shale; B = Western Hill Trilobite Shale; C = Dent

621 Trilobite Shale; D = Mt Orab Shale; E = Harpers Run submember Shale; F = Oldenburg

622 submember; G = Roaring Brook submember; H = Minuens Shale. Sequence stratigraphy is

623 modified from Holland and Patzkowsky (1996) as presented in Aucoin and Brett (2016).

624 Fig. 2. Images of various Waynesville Formation “butter shales”. (A) Polished slab of Oldenburg

625 submember “butter shale” from Oldenburg, Indiana. Visible are two lenses limestone interbeds

626 and highly bioturbated claystone. Scale bar is 1 cm. Image modified from Aucoin et. al., 2015.

627 (B) Close up view of the Harpers Run submember “butter shale” from St Leon, Indiana. Image 628 clearly shows the lack of distinct bedding within the Draftclaystone. (C) In situ Flexicalymene 629 trilobite from the Harpers Run submember.

630 Fig. 3. Composited sea level curve showing the result of higher and lower order sequence

631 nesting.

632 Fig. 4. This diagram shows expected sea level changes and lithologic expression of specific 4 th /

633 5th and 3 rd order nested systems tracts.

634 Fig. 5. Schematic stratigraphic column representing theoretical 4 th / 5 th and 3 rd order nested

635 systems tracts. CL = Clay, Z = Silt, CS = Calcisiltite, SB = Shelly Bed

636 Fig. 6. Diagram showing some of the submember subdivisions in the Waynesville and Liberty

637 Formations along with revised 3 rd order cycles as per Aucoin and Brett (2016). Colors indicate

638 4th and 5 th order systems tracts. Orange is TST, blue is HST and green is FSST.

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639 Fig. 7. Photographs showing actual Waynesville Formation units. (A) Cincinnetina meeki

640 grainstone of South Gate Hill submember from St Leon, Indiana. (B) South Gate Hill

641 submember at the base, overlain by the Lower Fort Ancient Shale and capped but the

642 Cincinnetina meeki grainstone of the Bon Well Hill submember at the top. Succession from St

643 Leon, Indiana. (C) Cincinnetina meeki grainstone of the Bon Well Hill submember from

644 Brookville, Indiana. (D) Harpers Run submember from St Leon, Indiana. All terminology from

645 Aucoin and Brett (2016)

646 Draft

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Draft

Diagram showing the Cincinnatian strata of the tri-state region with some of the “butter shale” horizons recorded. A =Moranburg Shale; B = Western Hill Trilobite Shale; C = Dent Trilobite Shale; D = Mt Orab Shale; E = Harpers Run submember Shale; F = Oldenburg submember; G = Roaring Brook submember; H = Minuens Shale. Sequence stratigraphy is modified from Holland and Patzkowsky (1996) as presented in Aucoin and Brett (2016). 89x96mm (300 x 300 DPI)

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Draft

Images of various Waynesville Formation “butter shales”. (A) Polished slab of Oldenburg submember “butter shale” from Oldenburg, Indiana. Visible are two lenses limestone interbeds and highly bioturbated claystone. Scale bar is 1 cm. Image modified from Aucoin et. al., 2015. (B) Close up view of the Harpers Run submember “butter shale” from St Leon, Indiana. Image clearly shows the lack of distinct bedding within the claystone. (C) In situ Flexicalymene trilobite from the Harpers Run submember.

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Draft

Composited sea level curve showing the result of higher and lower order sequence nesting. 209x382mm (300 x 300 DPI)

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Draft

This diagram shows expected sea level changes and lithologic expression of specific 4th / 5th and 3rd order nested systems tracts. 85x106mm (300 x 300 DPI)

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Draft

Schematic stratigraphic column representing theoretical 4th / 5th and 3rd order nested systems tracts. CL = Clay, Z = Silt, CS = Calcisiltite, SB = Shelly Bed

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Diagram showing some of the submember subdivisions in the Waynesville and Liberty Formations along with revised 3rd order cycles as per Aucoin and Brett (2016). Colors indicate 4th and 5th order systems tracts. Orange is TST, blue is HST and green is FSST. 179x90mmDraft (300 x 300 DPI)

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Draft

Photographs showing actual Waynesville Formation units. (A) Cincinnetina meeki grainstone of South Gate Hill submember from St Leon, Indiana. (B) South Gate Hill submember at the base, overlain by the Lower Fort Ancient Shale a nd capped but the Cincinnetina meeki grainstone of the Bon Well Hill submember at the top. Succession from St Leon, Indiana. (C) Cincinnetina meeki grainstone of the Bon Well Hill submember from Brookville, Indiana. (D) Harpers Run submember from St Leon, Indiana. All terminology from Aucoin and Brett (2016)

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