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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
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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
*Corresponding author (C.D. Aucoin) 10
500 Geology Physics Building 11
University of Cincinnati 12
Cincinnati OH 45221-0013 13
Email: [email protected] 14
15
16
17
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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
44
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45
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
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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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