Draft - University of Toronto T-Space · Manuscript ID cjes-2015-0219.R1 Manuscript Type: Article Date Submitted by the Author: 10-Feb-2016 Complete List of Authors: Aucoin, Christopher;

Draft

Sequence stratigraphic model for repeated “butter shale”

Lagerstätten in the Ordovician (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; University of Cincinnati, Geology Dattilo, Benjamin F.; Department of Geosciences Thomka, James; University of Akron, Geosciences

Keyword: Claystone, Mixed siliciclastic, Trilobite, Waynesville Formation, Lithofacies

https://mc06.manuscriptcentral.com/cjes-pubs

Canadian Journal of Earth Sciences

Draft

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

(Katian) of the Cincinnati region, USA 2

Christopher D. Aucoin1*, Carlton E. Brett1, Benjamin F. Dattilo2 , James R. Thomka3 3

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

[email protected], [email protected] 5

6

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

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

[email protected] 9

*Corresponding author (C.D. Aucoin) 10

500 Geology Physics Building 11

University of Cincinnati 12

Cincinnati OH 45221-0013 13

Email: [email protected] 14

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16

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

(Katian) of the Cincinnati region, USA 19

Christopher D. Aucoin1*, Carlton E. Brett1, Benjamin F. Dattilo2 , James R. Thomka3 20

21

Abstract: 22

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

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

rich in clay, poorly calcified, and show vague internal bedding in outcrop. “Butter shales” form a 25

repetitive motif with similar lithological and paleontological characteristics suggesting 26

conditions existed that can be explained by the interference between different orders of sequence 27

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Introduction 46

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

1995). 77

Geologic setting 78

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

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

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

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

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

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

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

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

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

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

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

90

“Butter shale” taphofacies and biofacies 91

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

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

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

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

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

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

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

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

periostracal films preserved; b) three-dimensionally preserved nautiloid cephalopods and 100

gastropods, commonly with calcitic chamber fills; c) abundant and articulated trilobites, both as 101

prone, and typically inverted, carcasses and as enrolled specimens (Hunda et al. 2006). Other 102

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

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

Schumacher and Shrake 1997). 105

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

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

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

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

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

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

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

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

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

polychaete worms (Eriksson and Bergman 2003). 115

“Butter shales”: basic conditions and assumptions 116

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

et al. 2008). 141

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

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

soft, butter-like consistency. In addition the lithological contrast with indurated limestone beds 144

makes these intervals stand out sharply, both lithologically from the shelly carbonates that 145

comprise much of the Cincinnatian Series (Brett and Algeo 2001; Brett et al. 2008). 146

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

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

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

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

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

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

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

oxygen conditions (Rhoades and Morse, 1971). 154

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

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

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

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

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

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

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

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

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

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

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

preserved intact. Further, the resultant softer substrate would prohibit a resurgence of these 166

sessile fauna (i.e., inhibitory taphonomic feedback; Kidwell and Jablonski 1983; Freeman et al. 167

2013). As noted, a majority of the organisms preserved in "butter shales" were at least mobile to 168

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

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

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

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

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

distal tempestites or mud turbidites. 174

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

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

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

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

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

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

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

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

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

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

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

discussion by using two orders of cyclicity: that are termed 3rd and 4

th order sequences. 186

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

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

model we are most concerned with factors that may enhance or suppress offshore mud 189

sedimentation. The precise manner in which nested cycles may interact is complex but is most 190

readily considered from the perspective of small scale oscillations at shorter time scales (e.g. a 191

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

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

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

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

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

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

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

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

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

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

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

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

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

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

(Holland 1993; Schramm 2011). 206

Predictive models for “butter shales” 207

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

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

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

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

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

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

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

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

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

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

be modified by effects at smaller scale. 218

Figure 4 shows a comparison of larger 3rd order and smaller (4

th or 5

th order) systems 219

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

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

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

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

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

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

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

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

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

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

thicker shale formation. 230

“Butter shale” scenarios 231

3rd

Order HST - 4th

Order TST 232

This case represents superimposition of the lower order TST on the higher order HST. 233

During this pairing the rapid sea level rise of the 4th order TST will cause a slightly higher than 234

normal rate of sea level rise overall for the HST (Fig. 4) along with a slightly higher maximum 235

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

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

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

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

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

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

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

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

sequestration produced by the TST. 244

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

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

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

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

transition. 249

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

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

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

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

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

Cincinnaticrinus, Paradendrocrinus, Reteocrinus, Glyptocrinus, Compsocrinus and 255

Canistrocrinus) in creeks in Warren and Clinton County, Ohio (Austin 1927; Morris and Felton 256

1993). Articulated bivalves and nautiloids are also typical of this interval. 257

3rd Order HST - 4th Order HST 258

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

4th order TST. 275

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

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

those of the HST. Even when similar assemblages were found in the TST deposits, the 278

preservation of multi-element skeletons was of lesser quality, although overall diversity was 279

higher in the TST. This has been attributed to both winnowing and a lack of burial during 280

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

This end-member is exemplified by the best studied “butter shale” from the Cincinnatian, 282

the Harpers Run submember (Aucoin and Brett 2016), formerly termed Treptoceras duseri or 283

Trilobite shale (Frey 1987a, 1987b). The shale is situated within the Fort Ancient Member of the 284

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

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

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

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

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

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

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

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

Comment [??1]: I do not really like the

addition of this new abbreviation system. If we do that it should be carefully explained;

otherwise it is just a confusing form of new

jargon.

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

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

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

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

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

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

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

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

3rd

Order FSST - 4th

Order TST and HST 301

A 4th order transgression superimposed on a 3

rd order falling would setup conditions in 302

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

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

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

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

packstones. As in the 3rd order FSST 4

th order TST scenario, “butter shale” formation would be 307

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

th order 308

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

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

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

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

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

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

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

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

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

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

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

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

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

Non-“butter shale” scenarios 322

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

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

3rd Order TST - 4th Order TST 325

The 4th order TST superimposed on the 3

rd order TST, presumably by amplified warming 326

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

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

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

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

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

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

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

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

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

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

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

3rd

Order TST - 4th

Order HST 338

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

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

rd order TST, permitting 340

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

3rd

Order FSST - 4th

Order FSST 356

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

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

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

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

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

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

possibility of “butter shale” formation. 363

Discussion 364

A Waynesville Formation succession 365

The Waynesville Formation (Fig. 6) of the Richmond Group of the Cincinnati Arch 366

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

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

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

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

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

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

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

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

grainstones separated by brachiopod rich shales represents the 3rd order HST 4

th order TST. 375

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

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

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

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

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

Clarksville submember composed of shale with thin limestone interbeds. 381

Exploring the HST 382

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

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

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

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

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

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

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

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

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

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

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

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

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

this silty environment. 396

Beyond the Waynesville 397

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

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

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

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

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

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

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

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

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

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

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

style Lagerstätten in the Cincinnatian. The sequestering of the majority of the coarse siliciclastics 409

near the orogen is important for butter shale development. 410

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

generalized to numerous other small scale sequences in mixed siliciclastic-carbonate successions 432

(also see Brett et al., 2011). 433

Conclusions 434

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

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

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

the local system by fine-grained siliciclastic material. 438

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

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

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

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

optimal conditions for "butter shale" generation. 443

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

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

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

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

448

449

450

Acknowledgements 451

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

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

Association of Petroleum Geologists Grant (CDA), the Clay Mineralogical Society Student 454

Grant (CDA) and an American Chemical Society Petroleum Research Fund Grant 528 # 55225-455

UR8 (BFD). The authors would like to thank Dan Cooper who has repeatedly granted us access 456

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

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

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

Revolution. 460

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616

617

618

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

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

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

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

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

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

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

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

(B) Close up view of the Harpers Run submember “butter shale” from St Leon, Indiana. Image 627

clearly shows the lack of distinct bedding within the claystone. (C) In situ Flexicalymene 628

trilobite from the Harpers Run submember. 629

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

nesting. 631

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

5th and 3

rd order nested systems tracts. 633

Fig. 5. Schematic stratigraphic column representing theoretical 4th / 5

th and 3

rd order nested 634

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

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

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

4th and 5

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

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

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

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

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

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

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

Aucoin and Brett (2016) 645

646

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

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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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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.

179x90mm (300 x 300 DPI)

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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 and 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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