Canadian Journal of Earth Sciences
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;Draft 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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1 Sequence stratigraphic model for repeated “butter shale” Lagerstätten in the Ordovician
2 (Katian) of the Cincinnati region, USA
3 Christopher D. Aucoin 1* , Carlton E. Brett 1, Benjamin F. Dattilo 2 , James R. Thomka 3
4 1Department of Geology, University of Cincinnati, Cincinnati, Ohio, 45221
5 [email protected], [email protected]
6
7 2Geoscience Department, Indiana University Purdue University Fort Wayne, [email protected]
8 3Department of Geosciences, University of Akron, Akron, Ohio 44325, USA;
9 [email protected] Draft
10 *Corresponding author (C.D. Aucoin)
11 500 Geology Physics Building
12 University of Cincinnati
13 Cincinnati OH 45221-0013
14 Email: [email protected]
15
16
17
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18 Sequence stratigraphic model for repeated “butter” shale Lagerstätten in the Ordovician
19 (Katian) of the Cincinnati region, USA
20 Christopher D. Aucoin 1* , Carlton E. Brett 1, Benjamin F. Dattilo 2 , James R. Thomka 3
21
22 Abstract:
23 The “butter shale” Lagerstätten of the Cincinnati Arch have produced an abundance of
24 articulated trilobites, along with assorted bivalves and cephalopods. These bluish-gray shales are
25 rich in clay, poorly calcified, and show vague internal bedding in outcrop. “Butter shales” form a 26 repetitive motif with similar lithological and paleontologicalDraft characteristics suggesting 27 conditions existed that can be explained by the interference between different orders of sequence
28 stratigraphic cyclicity. The characteristics that define "butter shales" include: rarity of coarser
29 interbeds, homogenous, fine grain-size, and abundance of burial horizons. The overriding control
30 is siliciclastic sediment supply. During 3rd order transgressions sediment supply to the basin is
31 too low to produce thick shale-prone intervals. Conversely, during third-order falling stages
32 sediment supply is generally too high to favor "butter shale" deposition. “Butter shales” formed
33 preferentially during 3 rd order HST and two subtly different variants resulted from the
34 superimposed effects of higher order cycles. Highstands moderated by small-scale transgressions
35 are characterized by lower background sedimentation and fewer/thinner mud deposition events.
36 Superposition of small-scale sea level fall on highstands produced increased background
37 sedimentation, higher silt, and patchy fossil occurrences. Juxtaposition of various scaled HSTs
38 provided the optimal “butter shale” conditions, characterized by elevated mud influx and
39 frequent episodic burial events, leading to abundant, articulated trilobites and associated fauna.
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40 In these scenarios, episodic events provide sufficient mud to smother local faunas and create a
41 soft, fine-grained substrate that prohibited recolonization by taxa adapted to firm substrates. Each
42 scenario differs from the others with respect to sedimentology and faunal composition.
43 Keywords: Claystone, Mixed siliciclastic, Trilobite, Waynesville Formation, Lithofacies 44
Draft
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45 46 Introduction
47 Konservat-Lagerstätten, deposits containing exceptionally well-preserved fossils, occur
48 repeatedly in the fossil record (Seilacher et al. 1985; Nudds and Selden 2008). Many of these
49 deposits, commonly genetically related to obrution (rapid burial) events, are famous for their
50 preservation of articulated multi-element skeletons (i.e., intact, delicate echinoderm and
51 arthropod remains) and, fittingly, considerable work has been done on the taphonomy of these
52 assemblages (e.g., Brett et al. 1997; Brett and Seilacher 1991). These deposits are unusually
53 valuable for the reconstruction of whole skeletal anatomy, permitting recognition of life and
54 mortality postures, and enabling interpretation of original community density and structure. To
55 preserve such readily disarticulated organisms in an articulatedDraft state and/or in life position
56 requires that the organism be buried rapidly enough to keep the skeleton intact and sufficiently
57 deep to prevent exhumation or scavenging. Hence, the depositional processes associated with
58 these Lagerstätten are restricted to certain paleoenvironments, and therefore may recur
59 predictably within stratigraphic sequences (Brett and Baird 1986).
60 The Upper Ordovician (Katian; Edenian-Richmondian) of the Cincinnati Arch region of
61 North America contains numerous obrution Lagerstätten referred to informally as “butter shales”
62 or ‘trilobite shales’ (Fig. 1). These shales derive this name from their soft homogenous, fine
63 grained nature and are sought by collectors for articulated trilobites, as well as extraordinarily
64 preserved echinoderms, bivalves, nautiloids and other fossils (Frey 1987a, 1987b; Schumacher
65 and Shrake 1997; Hunda et al. 2006; Aucoin et al. 2015). The shales are typically one to three
66 meters thick and are characterized by a bluish-green coloration, soft, sticky claystone
67 consistency, and a scarcity of interbedded limestones (Brandt Velbel 1984; Frey 1987a, 1987b;
68 Hunda et al. 2006, Aucoin et al. 2015) (Fig. 2). Thus, they form lithological motif as well as a
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69 suite of distinctive taphofacies (sensu Brett and Baird 1986). In this paper we explore the
70 possibility of a sedimentological control, linked to eustatic fluctuations, over the distribution of
71 “butter shales” and the composition and preservation of their faunas. In particular, we explore
72 the possible variations in siliciclastic sediment input that control obrution occurrences and may
73 result from the constructive and destructive interference between cyclic climatic/sea-level
74 oscillations of different scales, thereby allowing the distribution of “butter shales” to be modeled
75 and predicted based on sequence stratigraphy. This research may lead to a better understanding
76 not only of “butter shale” type obrution deposits but of a variety of other taphofacies (cf. Brett
77 1995).
78 Geologic setting Draft 79 During the Late Ordovician the present-day Ohio, Kentucky, and Indiana tri-state region,
80 east-central USA, was covered by a shallow epicontinental sea. The region had a ramp geometry
81 with shallow "lagoonal" to peritidal environments in south-central Kentucky deepening gradually
82 north-northwestwardly into southern Ohio and Indiana (Brett and Algeo 2001; Meyer and Davis
83 2008; Brett et al. 2015). At this time, the Cincinnati Arch region was located approximately 20 oS
84 of the equator and Laurentia was rotated clockwise 45 o relative to present orientation (Holland
85 1993; Brett and Algeo 2001; Holland and Patzkowsky 2007). Upper Ordovician depositional
86 sequences in the Cincinnati region exhibit mixed carbonate-siliciclastic facies with transgressive
87 systems tracts being dominated by carbonates, and highstand and falling stages predominantly
88 siliciclastic muds and silts sourced from the Taconic Mountains to the east.
89 Characteristics of “butter shales” and a predictive model for their occurrence
90
91 “Butter shale” taphofacies and biofacies
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92 As noted, “butter shales” are thicker than average intervals of soft, poorly calcareous
93 claystone, mainly illitic with low total organic carbon and a typically bluish-gray. They contain
94 abundant pyrite and may show very slender pyritic burrows. Small carbonate concretions (~5
95 cm in diameter) may occur at particular horizons, as do very thin shell hash beds and minor
96 calcareous siltstones, but overall these intervals may be nearly pure clay with few fossils except
97 in certain levels (Aucoin et al. 2015).
98 In terms of taphonomy, typical features of "butter shale" taphofacies include: a) abundant
99 articulated, closed or butterflied bivalves, typically as robust composite molds and with black
100 periostracal films preserved; b) three-dimensionally preserved nautiloid cephalopods and 101 gastropods, commonly with calcitic chamber fills; c)Draft abundant and articulated trilobites, both as 102 prone, and typically inverted, carcasses and as enrolled specimens (Hunda et al. 2006). Other
103 fossils, including intact and disarticulated crinoids, brachiopods and disarticulated bivalves,
104 "hash" of trilobite exuviae, and shell debris occur on individual bedding planes (e.g.,
105 Schumacher and Shrake 1997).
106 The faunas and paleoecology of three distinct "butter shale" intervals have been well
107 documented (Frey 1987a, 1987b; Schumacher and Shrake 1997; Hunda et al. 2006; Aucoin et al.
108 2015). All of these shales in the Cincinnatian share common features with respect to faunas.
109 These include: a) lower abundance of otherwise typical shelly, suspension-feeing epibenthos
110 (e.g., articulate brachiopods, bryozoans, crinoids) compared to other intervals of the type
111 Cincinnatian (see Holland and Patzkowsky 2007), and b) a relatively high abundances of vagrant
112 to slightly mobile organisms such as endobyssate and burrowing bivalves, gastropods, nautiloids,
113 and trilobites. In addition, cryptic bioturbation may be pervasive in the mudstones and common
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114 scolecodonts in some intervals points to an originally abundant soft-bodied infauna including
115 polychaete worms (Eriksson and Bergman 2003).
116 “Butter shales”: basic conditions and assumptions
117 Any model to explain the recurrent "butter shales" must consider both taphonomic and
118 paleoecological aspects of these deposits. Three basic factors are considered essential for the
119 development of any “butter shale” deposit. The first is background sediment influx, which
120 controls both substrate consistency (i.e., inhibiting shell bed growth; preventing hardground
121 development via mobilizing redox boundaries in response to migration of the sediment-water
122 interface) and governing faunal composition (i.e., precluding turbidity-intolerant epifaunal taxa;
123 increasing abundance of mobile, turbidity-tolerant, and/orDraft infaunal taxa). Many sedentary
124 suspension feeders, such as crinoids, require firm substrates on which to attach, at least initially
125 (Brett et al. 2008). Strong pulses of mud from the distal outfall of storms and other events create
126 soft substrates prohibiting most crinoids and many brachiopods from colonizing. In areas where
127 the mud has been winnowed or pauses in episodic events have occurred, harder substrates are
128 expected to return and crinoids and brachiopods may be more prevalent.
129 Related to the substrate issue is evidence for a relatively high rates of mud sedimentation.
130 The associated high turbidity and soft, fluid substratum would inhibit colonization by certain
131 groups of organisms. Although mobile epifauna, such as gastropods, nautiloids, and trilobites
132 were better equipped to handle higher background sedimentation and softer substrates, it is more
133 likely that the rate of sedimentation would have to have been more moderate for filter feeding
134 semi-infaunal bivalves to be successful. It appears that these organisms may have been more
135 turbidity tolerant than a majority of brachiopods. High sedimentation rates do not explain the
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136 occasional limestone hash beds with brachiopods and crinoids that occur within the shale and
137 thus substrate controlled by episodic events appears more probable. In the context of this model,
138 the skeletal debris beds would be expected to form in areas of increased winnowing, exposing
139 harder substrates for colonization. Subsequent storm events would continue to winnow in these
140 areas of reduced mud leading to an accumulation of shell hash over multiple generations (Dattilo
141 et al. 2008).
142 The next factor is the fine grain size of these deposits. Cincinnatian “butter shales” are
143 composed primarily of clays largely undiluted by carbonate material, which gives them their
144 soft, butter-like consistency. In addition the lithological contrast with indurated limestone beds 145 makes these intervals stand out sharply, both lithologicallyDraft from the shelly carbonates that 146 comprise much of the Cincinnatian Series (Brett and Algeo 2001; Brett et al. 2008).
147 In addition, the accumulation of soft, clay-rich substrates had ecological effects. Fine-
148 grained sediment is comparatively low in permeability, potentially promoting subsurface anoxia
149 close to the sediment-water interface. Low oxygen within the sediments may have inhibited deep
150 infaunal burrowing and reduced rates of decay. This is supported by large quantities of pyrite
151 framboids, pyritic fossils and pyritic burrows retrieved during disaggregation of the butter shales
152 for microfossil extraction. On the other hand, fine particulate organic detritus accumulates
153 preferentially in muds, making them a rich food source for deposit feeders that tolerate low
154 oxygen conditions (Rhoades and Morse, 1971).
155 A third factor required for development of certain “butter shale” taphofacies is episodic
156 rapid burial by fine-grained siliciclastic sediment. Although background sedimentation can be
157 sufficient to preserve organisms as fossils, the majority of the fossils preserved during low-
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158 sedimentation, quiescent intervals represent individuals that died, became disarticulated, and
159 remained exposed on the sea floor during a significant residence time in the taphonomically
160 active zone (Speyer and Brett 1991). To preserve the abundant articulated multi-element
161 skeletons, commonly observed in “butter shales”, episodic events, which dramatically increase
162 sedimentation beyond normal background conditions, are required to bury organisms to the point
163 where they are smothered and preserved intact. For sessile organisms, such as byssate bivalves,
164 the increase in sedimentation rate associated with burial does not need to be as high as for mobile
165 fauna because they cannot disinter themselves as mobile fauna can and they are more commonly
166 preserved intact. Further, the resultant softer substrate would prohibit a resurgence of these 167 sessile fauna (i.e., inhibitory taphonomic feedback; KidwellDraft and Jablonski 1983; Freeman et al. 168 2013). As noted, a majority of the organisms preserved in "butter shales" were at least mobile to
169 some extent and thus higher rates of event sedimentation and/or other sources of mortality would
170 be required to entomb their intact remains. In fact, many enrolled or partially enrolled trilobites
171 in "butter shale" intervals occur at random orientations with respect to bedding, suggesting that
172 many of the best-preserved horizons may reflect entrainment of organisms in bottom flows (type
173 2 obrution deposits of Brett et al. 2012). This implies episodic input of viscous mudflows as
174 distal tempestites or mud turbidites.
175 These three sedimentation characteristics (rate of background sedimentation, grain size of
176 sediment, and thickness of event deposits) may ultimately be modulated by sea-level
177 fluctuations. Sequence stratigraphy can provide a predictive model for the occurrence of various
178 facies including “butter shales” that qualify as trilobite Lagerstätten (Brett 1995). To better
179 understand the occurrence of "butter shale" taphofacies within the context of sequence
180 stratigraphy it is important to recognize that each of the 3rd order sequences generally accepted
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181 for the greater Cincinnati Arch region (Holland and Patzkowsky 1996, and subsequent papers) is
182 composed of a hierarchy of smaller cyclic sedimentary units that display properties mirroring
183 those of larger sequences. These small-scale cycles comprise condensed limestone-rich
184 transgressive intervals overlain by shalier highstand intervals) and range from meter- to
185 decameter-scale (Dattilo et al. 2012). For the present purposes we have simplified this
186 discussion by using two orders of cyclicity: that are termed 3rd and 4 th order sequences.
187 Interactions of cycles at these two scales may have the net effect of amplifying or dampening
188 particular sedimentary processes related to development of “butter shales”. For the present
189 model we are most concerned with factors that may enhance or suppress offshore mud 190 sedimentation. The precise manner in which nested cyclesDraft may interact is complex but is most 191 readily considered from the perspective of small scale oscillations at shorter time scales (e.g. a
192 few 100 kyr) occurring during times when base-level as a whole was rising or falling. For
193 example, a higher-order transgression occurring during a time of lower-order (larger-scale) high
194 and rising sea level might have the effect of particularly strong mud sequestration in estuarine
195 areas-leading to intensified mud starvation offshore (i.e., amplification of transgression).
196 Conversely, a short term regression superimposed on overall low sea level might lead to stronger
197 than average progradation and elevated deposition of mud and/or silt in offshore areas.
198 For the purposes of a simplified conceptual model we consider six combinations of two
199 "states" of lower order cycles (transgression and highstand/regression) superimposed on three
200 different phases of higher order cycle: transgression, highstand, and falling stage (terminology of
201 Catuneaunu 2006) as might occur during a third order sequence. The lowstand systems tract
202 (LST) was not factored into our analyses because during this systems tract, sea-level is at its
203 lowest causing much of the epeiric basin under study to experience erosion rather than
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204 deposition. For this reason, the LST is mostly absent in the Cincinnatian, with sequence
205 boundaries generally representing co-planar sequence boundaries and transgressive surfaces
206 (Holland 1993; Schramm 2011).
207 Predictive models for “butter shales”
208 A conceptual model linking sequence stratigraphy and taphofacies can be summarized by
209 looking at two nested sea-level curves (Fig. 3). On the rising limb of the diagram, enhanced sea
210 level rise can be caused by constructive inference of a smaller scale sea level rise. It is during
211 this interval that beds enriched in carbonate and phosphatized skeletal debris should accumulate.
212 Near the top of the curve, extending from the very end of the higher order TST to the start of the
213 higher order FSST and encompassing the entire HST,Draft is the interval of maximum potential for
214 "butter shale" development. This interval is characterized by maximum shale/mudstone
215 development and significantly less carbonate and it may also be modulated by the phases of
216 smaller scale cycles. Finally, on the falling limb of the curve, the period of maximum sea level
217 fall is characterized by low shell content, and high silt and mud deposition, but again, this may
218 be modified by effects at smaller scale.
219 Figure 4 shows a comparison of larger 3rd order and smaller (4th or 5 th order) systems
220 tract combinations with respect to the three environmental parameters discussed above. For
221 purposes of this model we utilize 3 rd order to indicate the larger depositional sequences of
222 approximately 0.5-2 million year durations, modified from the C1 to C5 sequences originally
223 recognized by Holland and Patzkowsky (1996); we use 4 th order to imply major subdivisions of
224 these intervals with durations of about 100-400 Kyr. In the following sections, we explore the
225 various nested combinations of cycle phases of the two nested scales of sequences as depicted in
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226 Figures 3, 4 and 5. Through this discussion we will attempt to demonstrate how a “butter shale”
227 style deposit may be expressed, or not, in different sequence pairings. "Butter shales" can
228 actually form in multiple sequence pairings, which will alter the way in which the mudstones are
229 expressed faunally and sedimentologically, although there is an optimal set of conditions for
230 thicker shale formation.
231 “Butter shale” scenarios
232 3rd Order HST - 4th Order TST
233 This case represents superimposition of the lower order TST on the higher order HST. 234 During this pairing the rapid sea level rise of the 4th orderDraft TST will cause a slightly higher than 235 normal rate of sea level rise overall for the HST (Fig. 4) along with a slightly higher maximum
236 landward progression of water. Despite the higher sea level rise and rate, siliciclastic
237 sedimentation would still be predominant and fine-grained siliciclastics would be deposited in
238 the basins at a low rate. Episodic events would create a patchwork of soft and hard substrates. In
239 this scenario, although mixed mobile and sessile faunas would be expected, there would likely be
240 a dominance of sessile suspension feeding organisms. Trace fossils may be present in small
241 quantities, but low sedimentation rates may lead to depletion of the detrital organic matter in the
242 sediment, inhibiting deposit feeders. This pairing may produce shales with occasional limestone
243 or siltstone interbeds (Fig. 5). The thickness of the shale will vary depending on the level of
244 sequestration produced by the TST.
245 As the lower 4th TST passes into the 4th order HST, i.e., there is an additive slowing of
246 the rate of rise, the system approaches the true maximum flooding surface causing a relatively
247 brief period of starvation to occur. During this transition, a buildup of carbonate material with
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248 intercalated shale is expected. This is similar to the conditions of the lower order TST-HST
249 transition.
250 An example of this type of interval is the " Reteocrinus bed", an interval of compact
251 mudstones and ledge-forming limestones that overlies the Upper G. insculpta submember of the
252 Liberty Formation. The interval is very consistent over Ohio, Indiana, and northern Kentucky
253 with a thickness of about 11’ (3.35m) from Waynesville to Madison. Greenish-gray shales in the
254 lower 6’ (1.8m) yielded the most diverse crinoid fauna of the Cincinnatian (including species of
255 Cincinnaticrinus, Paradendrocrinus, Reteocrinus, Glyptocrinus, Compsocrinus and
256 Canistrocrinus ) in creeks in Warren and Clinton County, Ohio (Austin 1927; Morris and Felton 257 1993). Articulated bivalves and nautiloids are also typicalDraft of this interval.
258 3rd Order HST - 4th Order HST
259 The HST-HST pairing is considered to represent the ideal situation for deposition of
260 thicker "butter shale" intervals as it favors conditions that fulfill all three requirements for
261 deposition of muds with relatively little skeletal debris. At the beginning of the HST, sea level
262 has reached its farthest landwards extent and much of the terrigenous sediment is still being
263 sequestered in nearshore and coastal plain settings; however, as defined by Catuneaunu (2009)
264 the highstand is also characterized by sedimentation rates, which exceed those of base level rise,
265 thus allowing sediment to be deposited offshore in an aggradational to progradational pattern.
266 Continued nearshore sequestration in filling estuaries still traps the majority of the coarser
267 material but allows abundant fine-grained sediment to move offshore. Episodic storm turbulence
268 would resuspend this material allowing for burial and smothering of the existing fauna in a
269 “butter shale” style lagerstätten. This scenario would be expressed similarly to that of the 3rd
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270 order FSST 4 th order HST with small modifications. The HST-HST should have a larger quantity
271 of clay-sized sediment and less silt than would be anticipated in the FSST-HST. The
272 homogeneity of sediments would tend to obscure trace fossils, because of a lack of contrast
273 between burrow fillings and matrix. Offshore environments should contain a mixture of sessile
274 and mobile fauna with a greater number of mobile organisms than present in the 3rd order HST
275 4th order TST.
276 Carlucci and Westrop (2014) provide empirical data from the Bromide Formation in
277 Oklahoma, which indicate that deposits yielding quality trilobite preservation were typically
278 those of the HST. Even when similar assemblages were found in the TST deposits, the 279 preservation of multi-element skeletons was of lesserDraft quality, although overall diversity was 280 higher in the TST. This has been attributed to both winnowing and a lack of burial during
281 formation of the time-averaged skeletal accumulations of the TSTs.
282 This end-member is exemplified by the best studied “butter shale” from the Cincinnatian, Comment [??1]: I do not really like the addition of this new abbreviation system. If we do that it should be carefully explained; 283 the Harpers Run submember (Aucoin and Brett 2016), formerly termed Treptoceras duseri or otherwise it is just a confusing form of new jargon. 284 Trilobite shale (Frey 1987a, 1987b). The shale is situated within the Fort Ancient Member of the
285 Waynesville Formation and has been correlated well over 100km. Contained within this unit are
286 the trilobites Flexicalymene and Isotelus¸ the nautiloids Treptoceras duseri, Manitoulinoceras ,
287 molluscan bivalves Ambonychia , Cuneamya miamiensis, Caritodens, Modiolopsis concentrica,
288 Orthodesma curvatum , Lyrodesma the bryozoans Cyphotrypa clarksvillensis, Spatiopora , the
289 corals Tetradium, Labechia , the stromatoporoid Stromatocerium , lingulid brachiopod, crinoids,
290 graptolites, conodonts, gastropods Clathrospira and Sinuites and the ichnofossils Chondrites
291 (Foerste 1908; Austin 1927; Wolford 1930; Frey 1987a, 1987b, etc). This shale tends to be about
292 2m thick with very thin lenses of limestone throughout. The shale also contains discontinuous
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293 lenses of skeletal pavements. The Harpers Run submember is good example of an HST-HST
294 scenario where there environment is extremely muddy and dominated by mollusks and trilobites,
295 with only occasional brachiopods, trepostome bryozoans, and crinoids. However, diastems are
296 recorded by horizons of corroded and bored stromatoporoids and Tetradium corals (Frey 1987a,
297 1987b). These suggest deposition in relatively shallow water settings wherein pauses in
298 sedimentation permitted temporary colonization of the seafloor. Additional examples of HST-
299 HST butter shales include the Oldenburg submember of the Waynesville (Aucoin et al. 2015)
300 and the Mt Orab shale of the Arnheim Formation (Hunda et al. 2006).
301 3rd Order FSST - 4th Order TST and HST
302 A 4 th order transgression superimposed on a 3 Draftrd order falling would setup conditions in
303 which moderate sedimentation would occur as the general drop in sea level that would be
304 expected from an FSST would be temporarily slowed (Fig. 4). The majority of the sediment
305 deposited would be fine-grained siliciclastics. However, coarser silt- or even sand-sized
306 sediment could be mixed with carbonates forming calcareous, shelly siltstones or silty
307 packstones. As in the 3rd order FSST 4th order TST scenario, “butter shale” formation would be
308 possible, but thicker and siltier shales would be expected during the 3rd order FSST 4th order
309 HST pairing. Although silty beds are not unexpected in the other “butter shale” setups, this tract
310 pairing would produce greater thickness of siltstone beds (Fig. 5). This "butter shale" would still
311 likely contain a mixture of mobile and non-mobile fauna; however, the higher rate of
312 sedimentation and softer substrate would create a preference for mobile fauna. This higher
313 propensity for mobile fauna combined with the chance of slightly coarser material, would likely
314 create a deposit wherein trace fossils would be abundant and relatively well defined.
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315 For example, in the Liberty Formation (Fig. 6), there is a "butter shale", informally
316 known as the Minuens shale (named for the small species of Flexicalymene, F. minuens which is
317 found in abundance). The main body of the shale is the typical HST-HST 2m thick clay shale.
318 Just above the shale, is a series of stacked siltstone beds with interbedded clay shale. Another
319 excellent example of a 4th order TST and 3 rd order FSST combination is seen in the extraordinary
320 Glyptocrinus bed of the Maysville area where pockets of perfectly preserved crinoids occur
321 overlying scoured siltstone beds and buried in silty mudstones (Brett et al. 2008; Milam 2013).
322 Non-“butter shale” scenarios
323 Although the following scenarios fail to produce “butter shale” deposits, brief discussions of
324 their conditions are provided to contrast the “butter shale”Draft examples.
325 3rd Order TST - 4th Order TST
326 The 4 th order TST superimposed on the 3 rd order TST, presumably by amplified warming
327 during a longer warm interval, causes an accelerated rate of sea level rise by constructive
328 interference and thus accommodation would have strongly outpaced the rate of sedimentation.
329 During this time, siliciclastic sediments are sequestered in the estuaries and rivers (Fig. 4). This
330 excludes the deposition of fine-grained siliciclastic material in far offshore areas, although minor
331 carbonate mud may be produced locally. Sediment starvation in the downramp environments
332 would mean there would be little sediment for storm disturbances to resuspend and redeposit.
333 Instead, storms would winnow what sediment was present and break up and rework the shell
334 material (Brett et al. 2008). It is during this interval that the maximum carbonate buildup (Fig. 5),
335 consisting of reworked shelly material, often broken and variably biased, along with
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336 accumulation of phosphatic steinkerns, would build up in the absence of dilution (Brett et al.
337 2008; Dattilo et al. 2008, 2016).
338 3rd Order TST - 4th Order HST
339 A shorter-term cooling during a time of rising sea level may result in a slowing of the
340 general rate of rise, i.e., the superimposing of a 4th order HST on a 3rd order TST, permitting
341 some increased offshore sediment progradation. Such a situation sets up conditions, similar to
342 the TST-TST where sea level is higher than usual (Fig. 4). However, the sedimentation rate is
343 also increased due to the influence of the slowing rate of rise, which allows possible movement
344 of fine-grained sediment into the basin. This creates a transitional lithology of thinly interbedded
345 limestones and shales. The fine-grained sediment in theDraft basin would be available for
346 resuspension during storm events although the amount of terrigenous mud would still be minor.
347 These deposits are likely to be relatively thin (Fig. 5).There are occasional obrution deposits
348 within these intervals and the low net rate of sedimentation may allow a stacking of obrution
349 layers, as the only siliciclastic sediments to accumulate are those of extremely large storm
350 disturbances, which export fine-grained sediments offshore. However, these beds will not
351 resemble "butter shales". Rather, they will consist of thin layers of mudstone containing more
352 shelly material and may overlie encrusted hardgrounds. These mud layers will tend to
353 incorporate remains of organisms, such as bryozoans, edrioasteroids or crinoids, that thrive
354 during times of lowered sedimentation and turbidity. Examples include well described
355 edrioasteroid beds of the Grant Lake Formation (Meyer 1990; Shroat-Lewis et al. 2011).
356 3rd Order FSST - 4th Order FSST
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357 Lastly, a nested FSST and FSST package would have constructive interference, which
358 would act to increase the rate of forced regression and the already rapid sedimentation rate
359 expected for a FSST, excluding most organisms from living in such an environment. When
360 present, this pairing would create thick, commonly deformed deposits of silty to sandy beds. One
361 might expect to find extensive discrete trace fossils on some bedding planes in this package. The
362 FSST-FSST and TST-TST pairings represent extreme end members, which largely exclude the
363 possibility of “butter shale” formation.
364 Discussion 365 A Waynesville Formation succession Draft 366 The Waynesville Formation (Fig. 6) of the Richmond Group of the Cincinnati Arch
367 provide the prime example of the expression of systems tract pairs (Fig. 7). At the base of the
368 Waynesville Formation is a bed, about 50cm thick, known as the South Gate Hill submember
369 (SGH), a pack-grainstone made up of Cincinnetina brachiopods (Jin 2012), bivalves and other
370 shells (Aucoin and Brett 2016). This bed represents the TST-TST condition, i.e., accelerated rate
371 of rise produced by constructive interference. Above the SGH is a 6m bed of clay rich barren
372 shale with occasional interbeds of limestone and siltstone. This package, called the Lower Fort
373 Ancient shale or "Barren shale", represents the 3rd order TST 4th order HST. and basal 3rd order
374 HST 4th order TST. The Bon Well Hill submember (BWH), a series of brachiopod rich pack-
375 grainstones separated by brachiopod rich shales represents the 3 rd order HST 4th order TST.
376 Lastly we have the HST-HST represented by the Harpers Run submember (Aucoin and Brett
377 2016). This shale is a 1-3m package of clay rich material containing interbeds of calcisiltite and
378 abundant trilobites, bivalves and cephalopods. Above this level the pattern is repeated, with the
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379 Stony Hollow Creek submember (Aucoin and Brett 2016) representing the TST-TST of the next
380 high order package. Above that, the TST-HST and the HST-TST is represented by the Middle
381 Clarksville submember composed of shale with thin limestone interbeds.
382 Exploring the HST
383 In all three highstand related combinations (HST-TST, HST-HST, FSST-HST), “butter
384 shale” deposits can form and are even likely to form. The 3rd order HST 4th order TST and 3rd
385 order FSST 4th order HST both act as end members for the “sweet spot” zone but what do those
386 end members really mean? The TST-HST scenario results in a slightly coarser substrate allowing
387 for benthic fauna such as crinoids, brachiopods, bryozoans and even corals to thrive. Trilobites
388 and bivalves are still present in this setting, although Draftcompetition from crinoids, brachiopods and
389 bryozoans makes them relatively less abundant. The HST-HST scenario marks the optimal
390 combination for the “butter shale” zone. With a greater influx of clay-sized sediment, the softer
391 substrate causes suspension feeders to decline. Trilobites, bivalves, lingulids and other organisms
392 that are more adapted for muddy substrates proliferate. Thus, these organisms increase relative to
393 sessile suspension feeders. In the FSST-HST scenario mud is replaced by silt as the primary
394 sediment and any non-mobile fauna, such as attached brachiopods, exist in small numbers.
395 Bivalves, trilobites, cephalopods and even lingulids, as well as soft-bodied burrowers, dominate
396 this silty environment.
397 Beyond the Waynesville
398 The primary examples of butter shales presented in this paper include those from the
399 Arnheim, Waynesville and Liberty Formations of the early Richmondian. However the question
400 could be posed, are there butter shales elsewhere in the Cincinnatian, and are there examples of
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401 butter shales elsewhere temporally and geographically? There are a number of other Cincinnatian
402 butter shale examples not discussed in this paper including the “granulosa” shale from the Kope
403 Formation (Gaines et al. 1999; Hughes and Cooper 1999), and the little studied Moranburg, and
404 Western Hill Trilobite shales from the Corryville. There are also number of Isotelus rich beds
405 known from Bromley Member of the Kope Formation as well as the Elkhorn Formation.
406 Although poorly studied, this beds may also be considered potential butter shales.
407 The Cincinnati Arch’s location relative to the Taconic orogen and consequent mixed
408 siliciclastic-carbonate system is a major contributing factor for the prevalence of butter shale
409 style Lagerstätten in the Cincinnatian. The sequestering of the majority of the coarse siliciclastics 410 near the orogen is important for butter shale development.Draft
411 Examples of butter shale style Lagerstätten persist beyond the Cincinnatian. The Silurian
412 (Wenlock) Waldron Shale from Indiana and the Rochester Shale from New York provide
413 excellent examples. Although more calcareous than the Cincinnatian butter shales, the Rochester
414 and Waldron formations both show analogues of "butter shales". These are intervals of soft,
415 rather sparsely fossiliferous mudstone and they may contain obrution beds with abundant
416 articulated trilobites and small crinoids that record HST-HST deposits, whereas TSTs are
417 typified by dense bryozoan and brachiopod packstones (Taylor and Brett 1988, 1999; Peters and
418 Bork 1998; Brett 2015). Limited obrution beds with diverse echinoderm faunas and some
419 trilobites occur in thinner mudstone intervals interpreted as HST-TST or TST-HST pairs (Brett
420 2015). Inferred falling stage deposits consist of alternating silty dolomitic mudstones and
421 calcisiltites, which are generally sparsely fossiliferous, but contain rare well-preserved crinoid-
422 trilobite beds, probably associated with brief transgressions superimposed upon the general
423 forced regression (FSST- TST or HST combinations).
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424 Further examples include the Devonian Hamilton Group of New York, and Ontario
425 (Speyer and Brett 1986; Brett et al. 1986; Miller et al. 1988; Brett 1999; Tsujita et al. 2006).
426 Excellent analogs of Cincinnatian "butter shales" occur in the equivalent Silica Shale of Ohio. In
427 particular, unit 9 consists of 1-2 m of pure claystone which yields abundant, commonly pyritized
428 fossils including enrolled Eldredgeops trilobites (Kesling and Chilman 1975). These beds are
429 quite reasonably interpreted as HST-HST. A very good example of an TST-HST pair is unit 13
430 which shows obrution beds of complete crinoids in mudstones overlying skeletal debris
431 packstones. These examples suggest that the preliminary model presented here may be readily
432 generalized to numerous other small scale sequences in mixed siliciclastic-carbonate successions 433 (also see Brett et al., 2011). Draft 434 Conclusions
435 • Three primary environmental parameters are required in order for the generation of
436 "butter shale" type deposits: background sedimentation rates must be moderate to low,
437 deposition of predominantly fine-grained siliciclastic sediment, and episodic, rapid burial
438 the local system by fine-grained siliciclastic material.
439 • Sequence stratigraphy can be used to generate predictive models for the optimal
440 generation of "butter shale" as well as other facies.
441 • Although there is a spectrum of possible nested systems that can produce butter shale
442 style deposits, a combination of nested lower and higher order highstands represents the
443 optimal conditions for "butter shale" generation.
444 • “Butter shales” are found throughout the Cincinnatian as well during the Silurian and
445 Devonian. It is very likely that additional examples can be found elsewhere. Previous
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446 case studies largely conform to the model developed herein, with the analogs of
447 Cincinnatian "butter shales" occurring at in highstand combinations.
448
449
450
451 Acknowledgements
452 This project was funded by the 2013 Dry Dredgers Paleontological Research Grant (to CDA),
453 the 2014 Association of Applied Paleontological Sciences Grant (CDA), the American
454 Association of Petroleum Geologists Grant (CDA), the Clay Mineralogical Society Student 455 Grant (CDA) and an American Chemical Society PetrolDrafteum Research Fund Grant 528 # 55225- 456 UR8 (BFD). The authors would like to thank Dan Cooper who has repeatedly granted us access
457 to his trilobite quarries. Also we would like to thank Steve Westrop, Jinsu Jin, and the
458 anonymous reviewer that helped greatly improve this paper. This is a contribution to the
459 International Geoscience Programme (IGCP) Project No. 591 – The Early to Middle Paleozoic
460 Revolution .
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593 Ecological, and Evolutionary Implications. Edited by Brett, C.E., and Baird, G.C.
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615 The Ohio Journal of Science, 30 : 301-308.
616
617
618
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619 Fig. 1. Diagram showing the Cincinnatian strata of the tri-state region with some of the “butter
620 shale” horizons recorded. A =Moranburg Shale; B = Western Hill Trilobite Shale; C = Dent
621 Trilobite Shale; D = Mt Orab Shale; E = Harpers Run submember Shale; F = Oldenburg
622 submember; G = Roaring Brook submember; H = Minuens Shale. Sequence stratigraphy is
623 modified from Holland and Patzkowsky (1996) as presented in Aucoin and Brett (2016).
624 Fig. 2. Images of various Waynesville Formation “butter shales”. (A) Polished slab of Oldenburg
625 submember “butter shale” from Oldenburg, Indiana. Visible are two lenses limestone interbeds
626 and highly bioturbated claystone. Scale bar is 1 cm. Image modified from Aucoin et. al., 2015.
627 (B) Close up view of the Harpers Run submember “butter shale” from St Leon, Indiana. Image 628 clearly shows the lack of distinct bedding within the Draftclaystone. (C) In situ Flexicalymene 629 trilobite from the Harpers Run submember.
630 Fig. 3. Composited sea level curve showing the result of higher and lower order sequence
631 nesting.
632 Fig. 4. This diagram shows expected sea level changes and lithologic expression of specific 4 th /
633 5th and 3 rd order nested systems tracts.
634 Fig. 5. Schematic stratigraphic column representing theoretical 4 th / 5 th and 3 rd order nested
635 systems tracts. CL = Clay, Z = Silt, CS = Calcisiltite, SB = Shelly Bed
636 Fig. 6. Diagram showing some of the submember subdivisions in the Waynesville and Liberty
637 Formations along with revised 3 rd order cycles as per Aucoin and Brett (2016). Colors indicate
638 4th and 5 th order systems tracts. Orange is TST, blue is HST and green is FSST.
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639 Fig. 7. Photographs showing actual Waynesville Formation units. (A) Cincinnetina meeki
640 grainstone of South Gate Hill submember from St Leon, Indiana. (B) South Gate Hill
641 submember at the base, overlain by the Lower Fort Ancient Shale and capped but the
642 Cincinnetina meeki grainstone of the Bon Well Hill submember at the top. Succession from St
643 Leon, Indiana. (C) Cincinnetina meeki grainstone of the Bon Well Hill submember from
644 Brookville, Indiana. (D) Harpers Run submember from St Leon, Indiana. All terminology from
645 Aucoin and Brett (2016)
646 Draft
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Draft
Diagram showing the Cincinnatian strata of the tri-state region with some of the “butter shale” horizons recorded. A =Moranburg Shale; B = Western Hill Trilobite Shale; C = Dent Trilobite Shale; D = Mt Orab Shale; E = Harpers Run submember Shale; F = Oldenburg submember; G = Roaring Brook submember; H = Minuens Shale. Sequence stratigraphy is modified from Holland and Patzkowsky (1996) as presented in Aucoin and Brett (2016). 89x96mm (300 x 300 DPI)
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Draft
Images of various Waynesville Formation “butter shales”. (A) Polished slab of Oldenburg submember “butter shale” from Oldenburg, Indiana. Visible are two lenses limestone interbeds and highly bioturbated claystone. Scale bar is 1 cm. Image modified from Aucoin et. al., 2015. (B) Close up view of the Harpers Run submember “butter shale” from St Leon, Indiana. Image clearly shows the lack of distinct bedding within the claystone. (C) In situ Flexicalymene trilobite from the Harpers Run submember.
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Draft
Composited sea level curve showing the result of higher and lower order sequence nesting. 209x382mm (300 x 300 DPI)
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Draft
This diagram shows expected sea level changes and lithologic expression of specific 4th / 5th and 3rd order nested systems tracts. 85x106mm (300 x 300 DPI)
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Draft
Schematic stratigraphic column representing theoretical 4th / 5th and 3rd order nested systems tracts. CL = Clay, Z = Silt, CS = Calcisiltite, SB = Shelly Bed
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Diagram showing some of the submember subdivisions in the Waynesville and Liberty Formations along with revised 3rd order cycles as per Aucoin and Brett (2016). Colors indicate 4th and 5th order systems tracts. Orange is TST, blue is HST and green is FSST. 179x90mmDraft (300 x 300 DPI)
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Draft
Photographs showing actual Waynesville Formation units. (A) Cincinnetina meeki grainstone of South Gate Hill submember from St Leon, Indiana. (B) South Gate Hill submember at the base, overlain by the Lower Fort Ancient Shale a nd capped but the Cincinnetina meeki grainstone of the Bon Well Hill submember at the top. Succession from St Leon, Indiana. (C) Cincinnetina meeki grainstone of the Bon Well Hill submember from Brookville, Indiana. (D) Harpers Run submember from St Leon, Indiana. All terminology from Aucoin and Brett (2016)
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