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 midcon- tinental 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 antago- nistic 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 epi- sode, 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). DOI 10.1111/let.12097 © 2014 Lethaia Foundation. Published by John Wiley & Sons Ltd LETHAIA 48 (2015) Massie Formation (Silurian) micro-bioherms 173 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 argil- laceous carbonates and thin mudrock intervals, rep- resent 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-south- western 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, result- ing 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 irreg- ular 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 tri- state area strata, micro-bioherms cannot provide detailed data on microstructure, lithology, faunal composition and internal fabric after extensive dolo- mitization. Likewise, unique encrusting faunas or palaeoecological patterns cannot be documented D after alteration via dolomitization. A few undolomitized micro-bioherm-bearing sec- tions 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 eas- ily 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.