Palaeontological and Sedimentological Effects of Micro&

Home , Osgood Formation, Waldron Shale, Wise Lake Formation

Palaeontological and sedimentological effects of micro-bioherms in the Middle Silurian Massie Formation of southeastern Indiana, USA

JAMES R. THOMKA AND CARLTON E. BRETT

Thomka J.R., Brett, C.E. 2015: Palaeontological and sedimentological effects of micro- bioherms in the Middle Silurian Massie Formation of southeastern Indiana, USA. Lethaia, Vol. 48, pp. 172–187.

Small build-ups composed primarily of micrite and benthic skeletal remains, termed ‘micro-bioherms’, have been recognized within Silurian strata of eastern and midcontinental United States for well over 75 years; however, previous research has focused nearly entirely on such structures within the upper Wenlock (Homerian) Waldron Shale. An undolomitized section of the lower Wenlock (Sheinwoodian) Massie For- mation in Ripley County, southeastern Indiana, was studied to assess the influence of micro-bioherms on palaeoecological, taphonomical and sedimentological patterns. Increased baffling of fine-grained material by organisms composing and/or encrusting build-ups is evidenced by muddy sediment containing pascichnial traces surrounding micro-bioherms. Pelmatozoan attachment structures densely encrust micro-bioherms, but are swollen by secondary stereomic overgrowths reflecting some form of antagonistic interaction or investment in strong affixation to elevated substrates. Clusters of bumastine trilobite material occur in ‘pockets’ related to cavities within build-ups, and otherwise rare spathacalymenid trilobites, often exceptionally preserved, are found in muds in the vicinity of partially buried micro-bioherms. Coeval sections nearby are nearly unfossiliferous as result of dolomitization, but contain recognizable skeletal material in greatest abundance in micro-bioherm flank beds. The occurrence of these bodies within the Massie Formation is genetically linked to a major transgressive episode, but also reflects a mid-Silurian climatic/palaeoceanographic change. □ Build- ups, bumastine trilobites, Echinodermata, holdfasts, Napoleon quarry.

James R. Thomka [[email protected]], and Carlton E. Brett [brettce@uc- mail.uc.edu], Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA; manuscript received on 13/04/2014; manuscript accepted on 03/06/2014.

Build-ups are common features of the stratigraph- fauna in relatively clear water (Archer & Feldman ical record, and a wide variety of such sedimen- 1986). tary bodies have been described from deposits Although documented from a variety of temporal spanning the Phanerozoic and extending well into and geographic settings, micro-bioherms have per- the Precambrian (Wood 1999; Stanley 2001; Rid- haps been best studied in the Silurian of eastern- ing 2002; Kiessling et al. 2002). Modern coral midcontinental North America. This is not surpris- reefs have attracted considerable attention and ing, as the Silurian Period as a whole was an interval have historically served as a template for compar- of extensive reef and carbonate mound development, ative analysis of ancient build-ups; however, the and the Great Lakes and Cincinnati Arch regions deep-time record of build-ups includes many expose numerous build-ups and associated carbon- structures for which modern analogues are not ate lithofacies (Brunton et al. 1998; Copper 2002). available. Conspicuous among these non-actualis- Nevertheless, micro-bioherms are absent from shal- tic deposits are micro-bioherms, relatively low- low-water lithofacies on the rim of the Michigan relief build-ups composed of micrite, presumably Basin, where sizeable carbonate mounds, in some of microbial origin, and the remains of skeleton- cases extending for more than a kilometre in diame- ized benthic organisms, commonly tabulate corals, ter, occur (Cumings & Shrock 1928; Lowenstam stromatoporoids, laminar bryozoans and pelmato- 1957; Droste & Shaver 1985; Mikulic 1987; Shaver & zoans (Archer & Feldman 1986). Micro-bioherms Sunderman 1989; Mikulic & Kluessendorf 1999). are interpreted as representing the combined Likewise, bioherms of any size are absent from silici- effects of bafﬂing by suspension-feeding macro- clastic-dominated Silurian successions to the east– benthos, in situ micrite production by algae and/ southeast, towards the axis of the Appalachian or microbes and binding by algae and encrusting Foreland Basin (McLaughlin et al. 2008).

True micro-bioherms developed in intermediate settings of Middle Silurian (Wenlock) age, specifi- A cally in mid- to outer-ramp settings in the Cincin- nati Arch region and the adjacent Wabash Platform (Brett et al. 2012a; Ettensohn et al. 2013), as well as m coeval sections in western New York (Brett et al. 1990). These settings, which are dominated by argillaceous carbonates and thin mudrock intervals, represent depositional environments that were too deep and/or turbid, at least intermittently, to support B growth of true reefs or large mounds during the Wenlock. Micro-bioherm-bearing deposits are most readily studied in the southeastern Indiana-southwestern Ohio-northern Kentucky tristate region, where these bodies display sub-metre relief and m diameters and are not laterally continuous with each other (Fig. 1). Unfortunately, late diagenetic dolomitization has strongly influenced preservation of the Middle Silu- rian succession of the Cincinnati Arch region, resulting in altered sedimentary fabrics and sparsely fossiliferous sections in units known to represent C normal marine environments (McLaughlin et al. 2008; Ettensohn et al. 2013). Dolomitized micro- bioherms can easily be recognized due to their irregular shape and clumpy, massive texture (Fig. 1A–C), as well as the effects of anisotropic compaction of m overlying sediments around the solid carbonate masses (Fig. 1D). In spite of their prominence in tristate area strata, micro-bioherms cannot provide detailed data on microstructure, lithology, faunal composition and internal fabric after extensive dolomitization. Likewise, unique encrusting faunas or palaeoecological patterns cannot be documented D after alteration via dolomitization. A few undolomitized micro-bioherm-bearing sections have been documented in the Middle Silurian m of southeastern Indiana, but previous research has focused entirely on build-ups within the Homerian (upper Wenlock) Waldron Shale and at the contact Fig. 1. Micro-bioherms in the lower Massie Formation at sev- between the Waldron and underlying Laurel Lime- eral localities in northern Kentucky. The approximate core of stone (Foerste 1898; Kindle & Barnett 1909; Halleck micro-bioherms is marked by the ‘m’. All micro-bioherms shown here are strongly dolomitized and represent the typical 1973; Ausich 1975; Archer & Feldman 1986; Feld- state of such structures in the Cincinnati Arch region. Scale man 1989). Micro-bioherms have recently been rec- bars = 30 cm. A, typical domal micro-bioherm at roadcut on I- 265 exit 19, Jeffersontown, Kentucky (N38°08052.8100, ognized in the Sheinwoodian (lower Wenlock) 0 00 W85°32 33.05 ). B, massive, amorphous micro-bioherm in Massie Formation in this region (McLaughlin et al. roadcut on KY-329, Crestwood, Kentucky (N38°20009.0700, 2008; Brett et al. 2012a; Thomka & Brett 2014a,b), W85°28026.0600). The head of the hammer is resting on the but have not previously been studied with regard to hardground surface upon which the micro-bioherm grew. C, large, massive micro-bioherm underlain by thin interval of their influences on the palaeontology and sedimen- mudstone. Note that the top of the micro-bioherm is truncated tology of the unit containing them. by the erosive base of the upper calcarenitic facies of the Massie Thus, the goals of this study are as follows: (1) to Formation. Same locality as in 1B. D, narrow, indistinct micro- bioherm in roadcut on I-71S, Oldham County, Kentucky present the stratigraphical setting of micro-bioherms (N38°20018.9700, W85°31017.2100). Note that the micro-bioherm in the Massie Formation of southeastern Indiana can be detected primarily because of compactional warping of and interpret the factors controlling their overlying calcareous mudstone. 174 J. R. Thomka & C. E. Brett LETHAIA 48 (2015) occurrence; (2) to document the influences that unit, the Osgood Formation (Foerste 1897), micro-bioherms exerted on the sedimentology, pal- which Pinsak & Shaver (1964) later downgraded aeoecology and taphonomy of the Massie Forma- to the lower member of the Salamonie Dolomite, tion; and (3) to comment on the significance of overlain by the Laurel Member. However, in the these build-ups to Middle Silurian stratigraphical light of recent high-resolution stratigraphical and palaeoceanographic events. research that has demonstrated persistence of easily recognizable sedimentary bodies throughout the greater Cincinnati Arch region, an updated Locality and stratigraphy lithostratigraphical terminology has been applied to southeastern Indiana (Brett et al. 2012a). The One of the few localities that exposes undolomitized term Salamonie Dolomite was rejected entirely lower Wenlock strata in the Cincinnati Arch region within Ripley County and the ‘Laurel Member’ is the New Point Stone quarry, east of the town of was returned to full formational status (Fig. 3A). Napoleon in Ripley County, southeastern Indiana The former ‘Osgood Member’ was split into (N39°12031.3900, W85°18053.7400; Fig. 2). This local- three individual formation-rank units. The term ity, which is generally known as the Napoleon Osgood Formation was reestablished and quarry, has historically been a focus for Middle Silu- restricted to the former ‘lower Osgood shale’ rian palaeontological research owing to the diverse, (Fig. 3A), and the former ‘upper Osgood carbon- abundant and well-preserved macroinvertebrate ate’ minus the uppermost bed was renamed fauna that has been recovered from units that are using Ohio lithostratigraphical terminology to devoid of identifiable fossils in nearly all other expo- become the Lewisburg Formation (Fig. 3). Like- sures (e.g. Frest et al. 1999, 2011). The stratigraphi- wise, the former ‘upper Osgood shale’ plus the cal succession at the Napoleon quarry is dominated underlying carbonate bed was termed the Massie by clean skeletal carbonates, argillaceous, sparsely Formation (Fig. 3). fossiliferous micrites and calcareous mudstones The sharp contact separating the basal carbonate deposited in upper ramp settings on the transition and overlying mudstone interval of the Massie For- from the western margin of the Appalachian Fore- mation (i.e. the former contact between the ‘upper land Basin to the Wabash Platform (Spengler & Read Osgood carbonate’ and ‘upper Osgood shale’) repre- 2010). sents a major flooding surface that can be traced Much of the stratigraphy of the Napoleon throughout the Cincinnati Arch region (McLaughlin quarry has traditionally been classified as a single et al. 2008; Brett et al. 2012a). At the Napoleon

New Point Stone Quarry

Napoleon IN-48

Osgood Indiana US-421 US-50 Versailles Ripley Silurian outcrop County

Fig. 2. Location of New Point Stone quarry, ca. 1 km east of Napoleon, Indiana. Modiﬁed from Thomka & Brett (2014a). LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 175

Fig. 3. Stratigraphy of the study site. A, stratigraphical column A 5m of a portion of strata exposed at the quarry, with the location of the micro-bioherm-bearing interval marked by the arrow. The full thickness of the Laurel Formation is not shown. Modiﬁed from Thomka & Brett (2014a). B, quarry wall showing in situ micro-bioherms. Note that micro-bioherms occur at a sharp ﬂooding surface and are overlain and ‘onlapped’ by dark muds. Scale bar = 1m.

4m quarry, this surface represents a hardground that is densely encrusted by pelmatozoan attachment

Massie structures and laminar bryozoans (Thomka & Brett 2014a,b) and serves as the source horizon from which micro-bioherms grew (Fig. 3B). The nature of surface and associated fauna and sediments consti- tutes a primary focus of this study. 3m Methods

Micro-bioherms at the Napoleon quarry were Lewisburg inspected in the ﬁeld for physical characteristics; the identity of the fauna comprising the main body of 2m the structure itself; and the presence, identity, and relative abundance of encrusting organisms. Mea- surements of height and diameter were taken on several build-ups, although this was carried out primarily to determine typical measurements, and quantitative results are not presented here. One large, complete specimen and several incomplete 1m specimens were collected and cut open to expose Osgood interior fabrics. The large micro-bioherm was serial- slabbed, producing multiple faces for study. The portion of the quarry where build-ups are common was recently dug up by a bulldozer, resulting in extensive rubble piles composed of complete and partial micro-bioherms and irregular chunks of 0m immediately surrounding material. This area was searched for fossil material that is not found on the B Napoleon hardground where build-ups are absent. Comparisons between taphonomical state, relative abundance and morphology were made between taxa in association with micro-bioherms and on the hardground surface. Faunal counts for quantitative Massie analyses were not taken in part because of the difﬁ- culties of obtaining accurate abundance data on disarticulated pelmatozoan echinoderms and trilobites, which are dominant elements in this faunal assemblage. Lewisburg Several micro-bioherms are preserved in situ at a benched-off surface, corresponding to the hardground under study, at the northern end of the quarry, which permitted direct observation of hori- Osgood zontal cross-sections of the micro-bioherms and sediment immediately surrounding the build-ups. 176 J. R. Thomka & C. E. Brett LETHAIA 48 (2015)

Based on comparisons with relatively complete micro-bioherms discovered as rubble, it is apparent A that much of the middle to upper portions of in situ build-ups was destroyed during removal of over- burden. As a result, encrustation patterns are not preserved. In spite of the vertical truncation, micro- bioherms on the benched-off surface nevertheless provided valuable information on the character of lateral margins and immediately surrounding material. Given that the objectives of this study are to document the stratigraphical setting and palaeontological and sedimentological inﬂuences of micro-bioherms on the Massie Formation, petrographic descriptions B are not presented here. Analysis of thin sections of micro-bioherms will be presented in a forthcoming paper that focuses on composition and genesis of these build-ups.

Description of micro-bioherms

Micro-bioherms at the Napoleon quarry are roughly domal in cross-sectional shape (Figs 3B, 4A) and typically 20–30 cm high and 50–70 cm in diameter (Fig. 4A, B). Where observed in situ, individual build-ups may be separated from each other by sev- C eral metres but are more commonly in close proxim- hft ity. Adjacent micro-bioherms are never laterally hft linked. The exterior surface is dominated by a crin- kled texture with unevenly distributed rod- or pillar- hft like pustules (Fig. 4A). This reflects the abundance of fistuliporoid bryozoans, likely Fistulipora, in the composition of build-ups (Perry & Hattin 1960). Fig. 4. Micro-bioherms from rubble at the Napoleon quarry. A, The exterior surfaces of micro-bioherms are densely B, complete micro-bioherm showing domal structures and fistu- encrusted by radicular pelmatozoan attachment liporoid bryozoan-dominated external fabrics. Scale = structures, commonly in clusters (4C). No borings bars 5 cm. C, close-up of external surface of micro-bioherm with multiple pelmatozoan echinoderm holdfasts (hft) that are have been observed into either build-ups or the swollen with secondary stereom. Scale bar = 1 cm. hardground from which they grew (Thomka & Brett 2014b). Skeletal elements visible on micro-bioherm exteriors occur alongside ‘patches’ of siliciclastic siliciclastic mud and rare articulate brachiopods mud. In some instances, mud appears to have been (Fig. 5). Small calcite-filled void spaces of enigmatic bound by pelmatozoan attachment structures and/or origin are also present, possibly representing bur- bryozoan laminae. Based on size, faunal composition rows (Nose et al. 2006). and character of sediment, these build-ups appear to be equivalent to bryozoan crust mounds of the classification system of Cuffey (1985; see also Gibson Sedimentological effects et al. 1988). The interior fabric of micro-bioherms is domi- The matrix of the hardground from which micro- nated by olive-green micrite, massive bryozoan bioherms at the Napleon quarry grew is generally material and fistuliporoid bryozoan laminae, which well winnowed, being a packstone to grainstone with are more prominent towards the margins of build- slightly muddier sediment in topographically nega- ups (Fig. 5). Subordinate elements include radicles tive areas (Thomka & Brett 2014a,b). However, of pelmatozoan attachment structures, dark-grey immediately surrounding build-ups is sediment LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 177

rad rad

lam rad

scm

rad rad lam

brc

Fig. 5. Internal fabric of micro-bioherms that were slabbed perpendicular to growth orientation. brc = brachiopod valve, lam = fistuliporoid bryozoan laminae, rad = radicles of pelmatozoan attachment structures, scm = siliciclastic mud. Scale bars = 1 cm. A, fabric dominated by green micrite, bryozoan laminae and radicles of pelmatozoan echinoderm attachment structures. Note that the two sets of radicles on the left had encrusted a lower horizon and are attributable to the crinoid Eucalyptocrinites whereas the set of radicles furthest to the right is associated with the rhombiferan Caryocrinites that had encrusted the upper micro-bioherm surface. B, fabric of portion of a thick micro-bioherm displaying amorphous texture and a large Caryocrinites radix. characterized by significantly increased siliciclastic relationships of micro-bioherms may lead to flawed mud and micrite contents (Fig. 6A, B). Based on interpretations about the otherwise well-winnowed observations of bedding plane exposures of the hard- carbonate deposit. ground surface, it was determined that this lithology An additional sedimentological effect of build-ups is present only in narrow rings (generally 30–50 cm in the Massie Formation is the increased abundance wide) surrounding micro-bioherms. The increased of large-diameter pelmatozoan pluricolumnals in fine-grained detrital component of these micro-bio- micro-bioherm-marginal deposits (Fig. 6C). These herm-marginal deposits likely resulted from: (1) the represent bioclasts sourced from the echinoderm baffling effects of suspension-feeding organisms fauna that encrusted micro-bioherms, as evidenced comprising and/or encrusting the build-ups (Mc- primarily by the similarity of columnal morphologi- Kinney et al. 1987; McKinney & Jaklin 2001); and es between both substrata. In addition, field (Meyer (2) the increased production of micrite in micro- & Meyer 1986) and laboratory (Blyth Cain 1968; but bioherm cores that was shed and accumulated in see Savarese et al. 1997; Gorzelak & Salamon 2013) surrounding areas (see also Watts 1988; Nose et al. studies demonstrated the difficulty in transporting 2006; Kershaw et al. 2007). Recognition of this large crinoid bioclasts far from crinoid living sites, build-up-margin sediment is important for properly and no concentrations of large pluricolumnals are reconstructing paleoenvironmental parameters of present beyond micro-bioherm margins in spite of the basal lithofacies of the Massie Formation, as the abundance of echinoderm material on the hard- study of isolated slabs of this material or sections ground surface (Thomka & Brett 2014b). The adja- that do not permit documentation of lateral cent, topographically positive micro-bioherms seem 178 J. R. Thomka & C. E. Brett LETHAIA 48 (2015)

Fig. 6. Sediment from micro-bioherm flanks. A, muddy, pluricolumal-rich flank sediment with in situ micro-bioherm to the left. Scale bar = 1 cm. B, slab of flank sediment collected from Napoleon hardground showing abundance of large pelmatozoan bioclasts and dis- tinctly muddy composition. Scale bar = 1 cm. C, close-up of slab in Figure 6B showing abundance of large pluricolumnals in muddy matrix. The inset is a close-up of a bored pluricolumnal. Scale bar = 0.5 cm. the most likely source area for accumulations of with the common hemicosmitid rhombiferan Cary- large pluricolumnals. The increased abundance of ocrinites (Brett 1978, 1984), and tetralobate lumen- large calcareous bioclasts appears to have resulted in bearing holdfasts cannot be definitively associated decreased physical stability of sediments surround- with a specific pelmatozoan taxon (Thomka & ing build-ups, as inferred by the total absence of Motz 2014). pelmatozoan attachment structures or encrusting The density of encrustation by pelmatozoans laminar bryozoans in such deposits. Although the (Figs 4C, 7A, B) clearly indicates that micro-bio- increased influx of fine-grained sediment may have herms represented favourable substrata. The most played a role, muddy and presumably unlithified likely explanation for this is the increased current substrata in topographically negative areas of the velocities associated with elevated and topographi- adjacent hardground are encrusted. This suggests cally irregular surfaces and the decreased likelihood that the coarse, rubbly substrate surrounding micro- of burial during episodic depositional events. Inter- bioherms precluded permanent colonization by ses- estingly, the taxa that encrusted build-ups are those sile organisms, including crinoids with complex dis- with some of the longest columns of the Massie For- tistelar attachment strategies that were capable of mation echinoderm fauna (Brett 1984; Frest et al. occupying other shifting, unstable substrates (Brett 1999) rather than short-stemmed or thecally 1981, 1984). attached taxa (e.g. holocystitid diploporites, edrioas- teroids and cyclocystoids) that would be expected to benefit most from occupation of the highest Palaeontological effects increased substrate on the seafloor (though Frest et al. 2011 reported diploporite thecal attachments Influences on echinoderm palaeoecology, pelamtozoan on micro-biohermal masses, apparently from other encrustation of micro-bioherms. – Thirteen pelma- localities). The unusual nature of this distribution is tozoan attachment structures, attributable to cri- further highlighted by the partitioning of attachment noids, rhombiferans and diploporites, are present structure morphotypes on the hardground surface on the micro-bioherm-bearing hardground at the where build-ups are absent: in these areas, holdfasts Napoleon quarry, and three of these are unique to attributable to long-stemmed taxa are restricted to build-ups. Micro-bioherm exteriors are densely low areas, whereas increased areas are encrusted encrusted by dendritic radix structures (sensu Brett exclusively by attachment structures associated with 1981; Figs 4C, 7A, B) characterized by lumina with shorter-stemmed taxa. trilobate, tetralobate and pentalobate configura- One explanation for this pattern is that micro- tions. Pentalobate lumen-bearing holdfasts are bioherms served as isolated ‘islands’ of hard associated with the common monobathrid crinoid substrata during initial burial of the hardground by Eucalyptocrinites (Halleck 1973; Brett 1981, 1984), siliciclastic mud of the overlying mudstone lithofa- trilobate lumen-bearing holdfasts are associated cies of the Massie Formation (Fig. 3). That is, the LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 179

that build-ups were occupied by the same (long- A stemmed) taxa throughout their growth. An alternative explanation for this distribution is that pelmatozoan taxa that initially settled on micro- bioherms established and maintained populations on this substrate to the exclusion of others. This is supported by monotypic clustering of radix morphotypes (Figs 4C, 7B), suggesting preferential set- tling of larvae near members of the same taxon, possibly aided by chemosensory data (Donovan et al. 2007; Donovan & Harper 2010; Jagt et al. 2010). Further, this mechanism accounts for the persistence of the same taxa as encrusters during the entire growth history of build-ups, as well as the B identity of encrusters as geographically widespread, eurytopic pelmatozoans (Brett 1984).

Swelling of attachment structures. – One of the most striking features of pelmatozoan holdfasts encrusting micro-bioherms is the severe swelling and thickening of Eucalyptocrinites and Caryocrinites radices by secondary stereom (Fig. 7A, B). Stereomic overgrowths are massive and result in amalgamation of the most proximal portions of radicles and, in some cases, dis- C tortion of the symmetry of lumina (Fig. 7A). Some swollen attachment structures are overgrown by amorphous stereom so severely that no radicles are visible, giving the appearance of conical, crustose holdfasts (Fig. 7B), whereas others display tapering, dichotomously branching distal radicles (Fig. 7A). Dendritic radix structures attributable to taxa encrusting both micro-bioherms and the surrounding hardground are thickened by secondary stereom on the former (Fig. 7A) and characterized by slender radicles on the latter (Fig. 7C). The exact reason(s) for this swelling response on micro-bioherms is Fig. 7. Selected pelmatozoan attachment structures from the unknown. Seilacher & MacClintock (2005) inter- Napoleon quarry. A, severely swollen holdfast of Eucalyptocrinites preted development of a thick secondary stereomic showing amalgamation of proximal radicles, more distal tendril- cortex as a response to increased hydrogen sulphide like radicles and distorted pentalobate lumen. Scale bar = 0.5 cm. B, external portion of micro-bioherm encrusted in sediments occupied by holdfasts; however, we see by numerous attachment structures (marked by arrows) swollen no evidence for adverse geochemical conditions on by secondary stereom growth. Scale bar = 1 cm. C, attachment micro-bioherm exteriors (Thomka & Brett 2014b). structure of Eucalyptocrinites on the hardground surface surrounding micro-bioherms. Note the absence of thickening and The swelling does also not appear to be a reflection the clearly visible plate sutures, which are concealed on holdfasts of very old ages for afflicted taxa, as radices of differ- encrusting micro-bioherms. Scale bar = 1 cm. ent sizes but belonging to the same taxa are swollen to the same extent. A more likely explanation is that swelling was induced as a response to antagonistic upper surfaces of build-ups may have been tempo- interaction between encrusting pelmatozoans and rally disconnected from the surrounding hard- other organisms, perhaps microbes on the surface of ground, having been encrusted by long-stemmed build-ups. The small amount of viscera within pelmatozoans after the hardground was partially or pelmatozoan lumina (see recent discussion in Dono- completely buried (Thomka & Brett 2014b). How- van et al. 2010) would have been easily protected by ever, micro-bioherms that were cut open have excessive stereomic overgrowths, and damage to branching radicles identical to those belonging to overgrowths would not likely have significantly the holdfasts on exterior surfaces (Fig. 5), indicating diminished the health of the organism. 180 J. R. Thomka & C. E. Brett LETHAIA 48 (2015)

particulate organic matter. However, in areas A immediately surrounding micro-bioherms, a trace characterized by dark-grey mud fill and a horizon- tally meandering configuration is present (Fig. 8). Although the ichnotaxonomic identity of this trace is currently unclear, we believe that the best candi- date for this ichnogenus is a non-looped form of Gordia, which appears a more accurate identification than the superficially similar Planolites and Helm- inthopsis. Regardless of ichnogeneric classification, the occurrence of these biogenic sedimentary structures has palaeoenvironmental significance. Meanders represent strong evidence of deposit-feeding/grazing behaviour rather than simple locomotion, making these traces pascichnia rather than repichnia. The B paucity of particulate organic matter precluded grazing behaviour on the sediment-starved surface except in the immediate vicinity of micro-bioherms, suggesting a locally increased influx of detrital material surrounding build-ups. Only in the muddy micro-bioherm-margin deposits could organisms adopt a deposit-feeding strategy, whereas elsewhere on the Napoleon hardground, the relatively pure carbonate substrate allowed only suspension feeders to thrive.

Influences ichnology, bioerosion structures. – The Fig. 8. Pascichnial traces in micro-bioherm flank sediment. Napoleon quarry hardground and associated micro- = Scale bars 1 cm. A, multiple meandering traces displaying bioherms are surprisingly devoid of borings intersections and dark sedimentary fills. B, most of a single trace displaying several meanders and strong contrast to locally light- (Thomka & Brett 2014b). The abundant large coloured sediment. pelmatozoan pluricolumnals that accumulated adjacent to build-ups dramatically increased the number of bioerosion structures, namely the simple boring Trypanites (Fig. 6C). It seems unlikely that An intriguing alternative explanation for the this distribution was controlled by palaeoenviron- swelling of pelmatozoan attachment structures on mental parameters other than substrate (i.e. Trypa- micro-bioherms at the Napoleon quarry is that nites-producing organisms almost certainly did not this represents purposeful growth of skeletal mate- prefer the increased turbidity of micro-bioherm rial to maximize the strength of fixation to margins). Rather, thick pluricolumnals (Fig. 6) increased substrata. Pelmatozoans that occupied were ideal substrata for boring organisms, resulting build-ups were in a position to benefit from in increased abundance of bioerosion structures increased food and decreased respiratory stress, and fidelity of the record of biotic interactions at permitting these taxa to divert significant amounts the study site. of energy away from vital functions and towards growth of larger, stronger holdfasts. Thus, the Influences on trilobite taphonomy, bumastine clus- severe thickening of holdfasts may represent bio- ters. – Localized clusters composed entirely of bu- logical investment in skeletal cement precipitated mastine pygidia, and slightly less abundant cranidia to prevent dislodgement from atop micro- (Fig. 9A, B), are relatively common within the cores bioherms. of micro-bioherms. Determining the precise size of these clusters is difficult because they are rarely pre- Influences on ichnology, pascichnial traces. – Trace served intact as three-dimensional blocks; rather fossils at the Napoleon quarry hardground comprise they are typically preserved as fragments in the por- domichnia (Thalassinoides galleries, minute Trypa- tion of the quarry where micro-bioherms were nites in large bioclasts) reflecting a lithology poor in exposed by quarrying activity. Trilobite material is LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 181

CD E

Fig. 9. Distinctive trilobite material from Napoleon micro-bioherms. Scale bars = 1 cm. A, B, clusters of bumastine material composed primarily of pygidia. C, articulated Spathacalymene from muds immediately overlying the Napoleon hardground where near micro-bioherms. D, articulated Calymene from same sedimentological setting as Figure 9C. This taxon is less common than spathacalymenids in these deposits. E, cluster of two articulated spathacalymenids and one Calymene from sedimentological setting as C, D. commonly fractured and otherwise damaged, Northern Appalachian Basin, where concentrations although this may reflect modern weathering of phacopid material have been documented (Speyer processes. Trilobite elements are densely concen- & Brett 1986). trated and commonly display complementary, Some issues exist with the otherwise parsimonious nested stacking, especially in large clusters (Fig. 9B). interpretation that these features represent protected Nested pygidia/cranidia are the most commonly ori- cavities where bumastines moulted (summarized in ented concave-up, although it is difficult to deter- Mikulic 1976), but the potential role of moulting mine orientations for many examined clusters. Some serves as an important starting point for understand- clusters are entirely grain-supported, but most are ing the nature of clusters of bumastine material in characterized by trilobite grains ‘floating’ in non-fos- the Napoleon quarry. Discarded exuvial elements siliferous sediment. Skeletal material in these clusters would have been readily transported by even low- occurs within light grey, very argillaceous, otherwise velocity currents (Mikulic 1990), although episodic unfossiliferous micrite (Fig. 9A, B). This sediment is storm events seem a more likely explanation for the clearly distinct from that of micro-bioherm margins, nested, potentially edgewise orientation of bioclasts and this fine-grained material occurs in the cores of and disarticulated state of trilobites. The paucity of build-ups. Elements of these large trilobites are rare other clasts of similar size and shape and dominance on the surface of the hardground in between micro- of pygidia/cranidia suggests selective hydrodynamic bioherms (although they are abundant in immedi- sorting by density (Mikulic 1990). Some behavioural ately overlying muds), and clusters are restricted to aspect of bumastine trilobites may have also contrib- the mounds (Mikulic 1999; D. Bissett, personal com- uted to formation of these clusters, but this behav- munication 2013). iour remains unknown. The most logical interpretation is that these clusters represent accumulations within irregular and Influences on trilobite taphonomy, articulated calyme- restricted cavities and/or material swept into under- nids. – One of the most conspicuous palaeontologi- cut platform-like surfaces in micro-bioherms. These cal associations at the Napoleon quarry occurs are genetically linked to palaeoecological and/or sed- between micro-bioherms and articulated calymenid imentary processes in carbonate build-ups, as similar trilobites (Fig. 9C–E). This fauna includes the wide- features have been described from coeval build-ups ranging genus Calymene (Fig. 9D) and the rare, dis- in the Irondequoit Limestone-Rochester Shale of tinctively ornamented spathacalymenids (Fig. 9C, New York and Ontario (Sarle 1901; Mikulic 1979; E), which are found only in the Massie Formation. Cuffey & Hewitt 1989; Brett 1999), large mounds on The distribution of trilobites is patchy, but articu- the rim of the Michigan Basin (Mikulic 1976, 1979, lated specimens occur only in the vicinity of micro- 1987), and in Middle Devonian build-ups of the bioherms, in siliciclastic muds that immediately 182 J. R. Thomka & C. E. Brett LETHAIA 48 (2015) overlie the Massie Formation hardground. Articu- order late transgression-early highstand, character- lated calymenids are significant for three reasons. ized by increased influx of siliciclastic mud to epeiric Firstly, calymenid trilobites can be rare in some ramp settings via episodic storm deposition (Brett Silurian environments, but seemingly not in level- 1995; Brett et al. 2012a), leading to burial of the bottom settings (Mikulic 1999). Secondly, their hardground and, eventually, micro-bioherms. The restriction to the proximity of build-ups indicates obrution events that entombed spathacalymenids that these features exerted some control over would not have resulted in preferential concentra- trilobite distribution, even after partial burial. tion of specific trilobite taxa near build-ups, but Thirdly, exceptionally preserved trilobites indicate rather would preserve elements of primary spatial rapid burial events and allow micro-bioherms to relationships (Brett & Baird 1986; Feldman 1989; serve as markers for the most likely location of com- Brett et al. 1997). Consequently, the hypothesis that plete specimens in the Massie Formation (although calymenids, particularly spathacalymenids, preferen- not necessarily in other Middle Silurian deposits; tially lived near build-ups is supported. Studies of Mikulic & Kluessendorf 2001, 2006; Kleffner et al. micro-bioherms in other comparable settings repre- 2012). sent a promising prospect for discovery of rare and/ The role of carbonate build-ups in development or unusual trilobite faunas. of unique trilobite biofacies is well documented, with previous studies having shown that bumastines represent the dominant taxa in large, shallow-water Discussion mudmounds, with calymenids comprising a surprisingly significant portion of trilobite diversity (Miku- Stratigraphical setting of Napoleon micro- lic 1981, 1999). Data from the Napoleon quarry have bioherms a somewhat similar pattern small build-ups in deeper, level-bottom settings. Spathacalymenids are Development of micro-bioherms at the hardground present in the interval immediately overlying the within the Massie Formation is related to a termina- studied hardground only where micro-bioherms are tion of clastic sedimentation and static redox bound- onlapped by siliciclastic mud (e.g. Fig. 3B), being aries during the most rapid rate of transgression absent in a recognizable form in intervening sedi- within a third-order sequence (McLaughlin et al. ments. Although Calymene remains are present in 2008; Brett et al. 2012a; Thomka & Brett 2014b). carbonate sediments of the Lewisburg Formation Low turbidity and the availability of hard substrata and the basal interval of the Massie Formation, they during this phase of relative sea-level rise permitted are not common. The specific palaeoecological establishment of a biofacies dominated by sessile aspect of calymenid trilobites that dictated their con- suspension feeders (see Brett 1995, 1998), including sistent occurrence with build-ups is unclear, taxa encrusting the hardground surface and incipi- although occupation of micro-bioherm margins or ent build-ups. An abundance of photosynthetic immediately surrounding sediments likely offered microbes responding to favourable, sediment- several benefits. For instance, an increased influx of starved conditions likely enhanced upward growth particulate organic matter or carcasses of micro-bio- of micro-bioherms through increased micrite pro- herm biota shed from build-ups, even partially bur- duction (Archer & Feldman 1986; McLaughlin et al. ied ones, could facilitate deposit/detritus-feeding, 2008). Micrite almost certainly accumulated close to scavenging or predation on soft-bodied deposit- the site of production (Gischler et al. 2010), having feeders. been bound by accreting laminar bryozoans and The disarticulated state of trilobites on the hard- encrusting pelmatozoans and brachiopods, the skele- ground surface surrounding micro-bioherms is due tal remains of which also contributed to upward to the long exposure time of skeletal material at the growth (Archer & Feldman 1986). sediment-water interface, reflecting sediment- The encrusted hardground and associated micro- starved conditions (Thomka & Brett 2014b). The bioherms mark the surface of maximum sediment contrasting, articulated state of calymenid trilobites starvation in siliciclastic-carbonate sequences in (near build-ups) in immediately overlying muds was epeiric seas (Brett 1995, 1998; Schmid et al. 2001; made possible only by rapid burial. This mode of McLaughlin et al. 2008). Consequently, micro-bio- preservation reflects episodic burial of trilobites in herms and similar build-ups commonly occur at the situ by fine-grained sediment during obrution sharp contact between underlying dense carbonates, events, likely generated by storm activity (Speyer & representing early transgressive conditions, and silic- Brett 1986; Brett et al. 1997; and many others). iclastic mudrocks, representing late transgressive to These depositional events occurred during a third- early highstand conditions (Fig. 2). Build-ups pro- LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 183 trude upward into the muds, which onlap the ton Formation from the overlying highstand muds topographically positive and palaeoecologically dis- of the basal Osgood Formation (latest Telychian- tinct features (Walker & Alberstadt 1975; Brett earliest Sheinwoodian) in the Cincinnati Arch 1995). This relationship can also be observed in coe- region. This is unusual because this contact repre- val sections at the contact between the Irondequoit sents a lithologically/stratigraphically analogous set- Limestone and overlying Rochester Shale in western ting to the Massie Formation hardground. Further, New York and Ontario (Sarle 1901; Cuffey & Hewitt this surface represents a eustatic event that is 1989; Brett 1999) as well as within the Middle Devo- largely recognized as the most significant highstand nian succession of New York (Speyer & Brett 1986; of the Silurian Period (Ross & Ross 1996; Johnson Brett 1995). 2006; Loydell 2007). Hence, the conditions respon- Micro-bioherms within the Massie Formation sible for micro-bioherm development were not occur at a lithologically and sequence stratigraphi- strictly dictated by third-order scale stratigraphic cally analogous position to slightly younger build- variations. ups at the hardground contact between the Laurel The concept of time-specific facies (sensu Brett Limestone and overlying Waldron Shale in the Cin- et al. 2012b) provides a conceptual comparative cinnati Arch region (Thomka & Brett 2014b; see also framework within which distinctive lithological fea- Halleck 1973; Ausich 1975; Archer & Feldman tures and faunas can be analysed. The occurrence of 1986). Although many similarities between the Mas- micro-bioherms at the flooding surfaces between the sie and Waldron micro-bioherms have been basal carbonate and middle mudstone lithofacies of described in this study, two major differences exist. the Massie Formation (and coeval Irondequoit- Firstly, beyond small fenestrae, there is no evidence for late-stage dissolution (e.g. stylolites, recrystal- lized fossils) in Massie micro-bioherms, in spite of the large proposed role of dissolution in the genetic A model for Waldron micro-bioherms developed by Archer & Feldman (1986). Secondly, no tabulate corals or stromatoporoids are present within Massie micro-bioherms, in contrast to Waldron micro-bioherms (Archer & Feldman 1986) and to large Wen- lock-age carbonate mounds that grew in shallower water than that interpreted for the Napoleon quarry. Examples of these shallow-water mounds include build-ups within the Hogklint€ Formation of Gotland (Watts & Riding 2000), the Muksha Formation of western Ukraine (Jarochowska et al. 2014) and multiple units on the rim of the Michigan Basin (Miku- B lic 1987; Mikulic & Kluessendorf 1999). These differences appear to reflect bathymetric effects, as growth of large, colonial framework elements and diagenetic dissolution are more significant in up- ramp settings, whereas micrite precipitation, siliciclastic mud trapping and organismal binding were more dominant in deeper environments.

Implications for Middle Silurian palaeoceanographical events

Although micro-bioherm growth can be fostered Fig. 10. Micro-bioherm-margin sediment from strongly dolomi- by purely sedimentological/stratigraphical phenom- tized strata of the Cemex quarry, Greene County, southwestern ena, similar build-ups do not occur at every major Ohio. The Massie Formation is nearly barren here except for sediment surrounding micro-bioherms. A, slab containing abundant flooding surface in the Middle Silurian succession large bioclasts, particularly pelmatozoan pluricolumnals and dis- of eastern North America. Notably, micro-bio- tistelar attachment structures. Scale bar = 5 cm. B, close-up of herms are not present at the sharp flooding con- pluricolumnal from the slab in Figure 10A showing multiple minute Trypanites borings, characteristically abundant in pelma- tact that separates late transgressive carbonate tozoan material from micro-bioherm flank deposits. Scale sediments of the Telychian (late Llandovery) Day- bar = 0.5 cm. 184 J. R. Thomka & C. E. Brett LETHAIA 48 (2015)

Rochester contact) and between the Laurel and quarry in Greene County, Ohio (N39°46052.8500, Waldron formations, but not at the analogous W83°57038.8600). position between the Dayton and Osgood forma- In all instances, micro-bioherm-margin sediments tions, indicates a previously unrecognized aspect of contained recognizable fossil material in greatest time specificity. Both the Massie Formation flooding abundance (Fig. 10A) compared to other lithologies. surface and the Laurel-Waldron contact represent Further, some of the palaeoecological and tapho- micro-biohermal intervals that correspond to glob- nomical patterns documented at the Napoleon ally recognized, positive carbon isotope excursions, quarry were present in dolomitized sections. namely the lower Sheinwoodian (‘Ireviken’) and Notably, pelmatozoan pluricolumnals are relatively lower Homerian (‘Mulde’) excursions, respectively common surrounding micro-bioherms, and these (Cramer et al. 2006; McLaughlin et al. 2012). bioclasts display abundant bioerosion structures Recently, Thomka et al. (2012) proposed a model (Fig. 10B), allowing recognition of evidence for bio- for the lower Sheinwoodian excursion and related tic interactions in deposits where such palaeoecolog- biological, stratigraphical and geochemical events ical data are otherwise impossible to document. We that linked a brief episode of cooling to changes in suggest that, when attempting to compile faunal data eustatic sea level and locally increased palaeoproduc- from diagenetically altered sections, sediments tivity. This ultimately led to a major disruption to immediately surrounding micro-bioherms represent the global carbon cycle (manifest in the carbon iso- the best options for yielding identifiable macrofos- tope excursion) and development of a palaeoecologi- sils. cally unusual echinoderm fauna. We submit that the occurrence of micro-bioherms characterized by a micrite-dominated composition Conclusions is an additional, far-field reflection of this palaeoclimatic perturbation. Increased micrite production, 1 An undolomitized Middle Silurian (Wenlock: although commonly associated with warming epi- Sheinwoodian) section at the New Point Stone sodes (Holland & Patzkowsky 1996), would have quarry near Napoleon, Indiana, permitted been fostered by increased productivity even during detailed study of the influence of micro-bioherms cool intervals, favoring mud-dominated build-ups on the sedimentology, palaeoecology and over skeletal framework-based build-ups (e.g. Riding taphonomy of a transgressive seafloor. Micro-bio- 2009). Hence, we suggest that micro-bioherms in the herms, which display a micrite and fistuliporoid- Massie Formation are genetically related to altered dominated fabric, grew upward from a sediment- palaeoceanographic conditions during the early starved hardground surface during an episode of Sheinwoodian. rapid sea-level rise. 2 Micro-bioherms are surrounded by material char- Regional faunal patterns acterized by increased mud content as well as coarse skeletal rubble. This reflects the baffling Deposits that are equivalent to the micro-bioherm- effects of abundant suspension-feeding organisms bearing unit at Napoleon represent intervals that and increased micrite production, as well as an hold the greatest potential for preserving an inverte- increased influx of large-diameter pelmatozoan brate fossil fauna in sections that have undergone pluricolumnals shed from build-ups. Mud influx dolomitization. Whereas most of these sections are was significant enough to encourage grazing nearly or completely unfossiliferous, the coarse behaviour among vagile benthos, resulting in bioclastic particles and increased argillaceous com- pascichnial traces. The abundance of pluricolum- ponent characteristic of micro-bioherm margin sedi- nals resulted in abundant bioerosion structures. ments may make such deposits more resistant to 3 Pelmatozoan attachment structures that thorough dolomitization. As a test, several dolomi- encrusted micro-bioherms comprise dendritic tized, micro-bioherm-bearing sections were searched radix structures attributable to long-stemmed for remnant fossils, with the lithology and position camerate crinoids and rhombiferans. Holdfasts relative to build-ups noted. Sections studied included are swollen by secondary stereom reflecting either roadcuts at Madison, Indiana (N38°47000.0200, ° 0 00 a response to some antagonistic interaction or W85 22 10.17 ), Crestwood, Kentucky (Fig. 1B, C), biological investment in strong anchorage to the Jeffersontown, Kentucky (Fig. 1A), Mt. Washington, ° 0 00 ° 0 00 advantageous position atop micro-bioherms. Kentucky (N38 04 01.65 , W85 32 52.32 ), on I-71 4 Clusters of bumastine trilobite material, possibly in Oldham County, Kentucky (Fig. 1D) and a related to moulting, are known from pockets LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 185

(cavities or undercut pockets?) within micro-bio- Boucot, A.J. & Lawson, J.D. (eds): Paleocommunities: A Case Study from the Silurian and Lower Devonian, 592–637. Cam- herms. Articulated calymenid trilobites, including bridge University Press, Cambridge. spathacalymenids, are restricted to fine-grained Brett, C.E. & Baird, G.C. 1986: Comparative taphonomy: A key siliciclastic muds associated with partially buried to paleoenvironmental reconstruction based on fossil preservation. Palaios 1, 207–227. build-ups. Brett, C.E., Baird, G.C. & Speyer, S.E. 1997: Fossil lagerst€atten: 5 The occurrence of micro-bioherms in the Massie Stratigraphic record of paleontological and taphonomic Formation is genetically linked to sedimentary events. In Brett, C.E. & Baird, G.C. (eds): Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications, conditions associated with a major transgressive 3–40. Columbia University Press, New York. episode; however, time-specific palaeoclimatic/ Brett, C.E., Cramer, B.D., McLaughlin, P.I., Kleffner, M.A., palaeoceanographic aspects restricted build-ups Showers, W.J. & Thomka, J.R. 2012a: Revised Telychian- Sheinwoodian (Silurian) stratigraphy of the Laurentian to certain intervals. Beyond the Napoleon quarry, mid- continent: Building uniform nomenclature along the micro-bioherms are too badly dolomitized to Cincinnati Arch. Bulletin of Geosciences 87, 733–753. provide detailed faunal and sedimentologic data, Brett, C.E., Goodman, W.M. & LoDuca, S.T. 1990: Sequences, cycles, and basin dynamics in the Silurian of the Appalachian but sediments surrounding build-ups contain rec- Foreland Basin. Sedimentary Geology 69, 191–244. ognizable macrofossils in great enough abundance Brett, C.E., McLaughlin, P.I., Histon, K., Schindler, E. & Ferretti, to be preserved. These fossils contain abundant A. 2012b: Time-specific aspect of facies: State of the art, examples, and possible causes. Palaeogeography, Palaeoclimatology, bioerosion structures, suggesting that palaeoeco- Palaeoecology 367–368,6–18. logical patterns documented at the Napoleon Brunton, F.R., Smith, L., Dixon, O.A., Copper, P., Nestor, H. & quarry were persistent prior to dolomitization. Kershaw, S. 1998: Silurian reef episodes, changing seascapes, and paleobiogeography. New York State Museum Bulletin 491, 265–282. Acknowledgements. – Funding for this study was provided by a Copper, P. 2002: Silurian and Devonian reefs: 80 million years of global greenhouse between two ice ages. In Kiessling, W., Dry Dredgers Paul Sanders Award and Paleontological Research € – Award to JRT. The owners and management of the New Point Flugel, E., Golonka, P. (eds): Phanerozoic Reef Patterns, 181 Stone quarry at Napoleon, Indiana, graciously permitted access 238. 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