The Silurian of central Kentucky, U.S.A.: Stratigraphy, palaeoenvironments and palaeoecology

FRANK R. ETTENSOHN, R. THOMAS LIERMAN, CHARLES E. MASON, WILLIAM M. ANDREWS, R. TODD HENDRICKS, DANIEL J. PHELPS & LAWRENCE A. GORDON

ETTENSOHN, F.R., LIERMAN, R.T., MASON, C.E., ANDREWS, W.M., HENDRICKS, R.T., PHELPS, D.J. & GORDON, L.A., 2013:04:26. The Silurian of central Kentucky, U.S.A.: Stratigraphy, palaeoenvironments and palaeoecology. Memoirs of the Association of Australasian Palaeontologists 44, 159-189. ISSN 0810-8889.

Silurian rocks in Kentucky are exposed on the eastern and western flanks of the Cincinnati Arch, a large-wavelength cratonic structure separating the Appalachian foreland basin from the intracratonic Illinois Basin. The Cincinnati Arch area experienced uplift during latest Ordovician-early Silurian time, so that the exposed Silurian section is relatively thin due to onlap and post- Silurian erosional truncation on the arch. On both flanks of the arch, dolomitic carbonates predominate, but the section on the eastern side reflects a more shale-rich ramp that faced eastern Appalachian source areas. In the Silurian section on the western side of the arch, which apparently developed across a platform-like isolation-accommodation zone, shales are rare except dur- ing some highstand episodes, and rocks in the area reflect deposition across a broad, low-gradient shelf area, interrupted by structurally controlled topographic breaks. Using the progression of interpreted depositional environments and nearshore faunal communities, a relative sea-level curve, which parallels those of previous workers, was generated for the section in Kentucky. While the curve clearly shows the influence of glacial eustasy, distinct indications of the far-field, flexural influence of Taconian and Salinic tectonism are also present. In fact, at times, regional tectonic subsidence seems to have overwhelmed the effects of glacio-eustasy. Regional angular truncations in the section, as well as overlying bentonitic shales and a dysaerobic fauna in the deepest-water part of the section (Estill Shale), are best explained in terms of far-field tectonic subsidence accompanying the first tectophase of the Salinic Orogeny in the Appalachian area.

Frank R. Ettensohn [[email protected]], Department of Earth & Environmental Sciences, University of Kentucky, Lexington, KY 40506, U.S.A.; R. Thomas Lierman [[email protected]], Department of Geography and Geology, Eastern Kentucky University, Richmond, KY 40475, U.S.A.; Charles E. Mason [[email protected]], Department of Earth & Space Sciences, Morehead State University, Morehead, KY 40351, U.S.A.; William M. Andrews [[email protected]], Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506, U.S.A.; R. Todd Hendricks, Department for Environmental Protection, Frank fort, KY 40601, U.S.A.; Daniel J. Phelps, Kentucky Paleontological Society, University of Kentucky, Lexing- ton, KY 40506. U.S.A.; Lawrence A. Gordon, Total Environmental, Tallahassee, FL 32312, U.S.A. Received 24 January 2013.

Keywords: Silurian, Kentucky, basement structures, Cincinnati Arch area, depositional environments, faunal communities, glacial eustasy, far-field tectonics, Appalachian Basin, Taconian Orogeny, Salinic Orogeny

THE SILURIAN rocks of Kentucky are not well exposed the section thickens considerably into Kentucky parts of the or very extensive. In fact, nearly all exposed Silurian rocks Illinois (>207 m; 679 ft) and Appalachian (>335 m; 1100 ft) in this area crop out on the margin of an axis culmination of basins (Shaver 1985; Patchen et al. 1985). the Cincinnati Arch, the Jessamine Dome. In this study, we The Silurian System was first formally recognised in will examine only the better exposures along the flanks of the Kentucky by Owen (1856), but all of his “Lower Silurian” Jessamine Dome in central Kentucky. The Commonwealth would actually be included in the Ordovician System today, of Kentucky sits astride the prominent north-south-oriented whereas his “Upper Silurian” included both Silurian and Cincinnati Arch (Fig. 1). Except for its extreme western Devonian carbonates. Nonetheless, these “Upper Silurian” portion, the Commonwealth is underlain by lower to upper rocks were part of his second major formation in the Palaeozoic rocks that crop out in bands controlled by the Commonwealth, called either the Gray Limestone or Cliff Jessamine Dome (Fig. 1), as well as by the Appalachian Limestone (Collins 1924), but only the lower parts of this foreland basin to the east and the intracratonic Illinois formation, which he called the “Magnesian and Chain-Coral Basin to the west. Silurian rocks crop out in a narrow band, limestones” (Owen 1856; Collins 1924), are units that would ranging from about 1.6-18.3 km (1-11 miles) in width, on currently be included in the Silurian System. the eastern and western flanks of the Jessamine Dome (Fig. Although the stratigraphy and age of these units are 1) and are a part of the North American “Dolomite Suite” of now much better known than in Owen’s time, what these Berry & Boucot (1970). Along the southern margin of the units represent in terms of depositional environments and dome, Silurian rocks are cut out or overlapped by Middle to palaeoecology is still incompletely known. This lack of Upper Devonian rocks (Fig. 1). The Silurian section along knowledge in part reflects the thin, poorly exposed nature the exposure belt is relatively thin, ranging from 0–111 m of the Silurian units and the fact that most have been (0–364 ft), due to depositional thinning across the Cincinnati pervasively dolomitised. In addition, the thinned nature of Arch area and later Middle and Late Devonian erosion, but these units not only reflects deposition across structures 160 AAP Memoir 44 (2013)

Figure 1. Location map showing the distribution of Silurian outcrop belts in Kentucky and related structural features. JK=Jeptha Knob cryptovolcanic structure. in the Cincinnati Arch area, where they were already thin, but also subsequent episodes of intense Early, Middle and Late Devonian erosion, which destroyed large parts of the system in Kentucky (e.g. Ettensohn et al. 1988; Brett et al. 2004). Dolomitisation, which may have also been related to the Cincinnati Arch (Gordon 1980), has obscured many sedimentary textures and fossils and, in the subsurface, has made it so difficult to differentiate Silurian and Devonian carbonates that they are commonly grouped together as the “Corniferous” (Currie 1981). Nonetheless, in the last 30 years a number of local studies and theses from universities in Kentucky have attempted to understand exposed parts of Kentucky’s Silurian section. In this report, we begin to integrate those studies with more recent information to develop a comprehensive model of Silurian deposition and palaeoecology for preserved parts of the section in the central Kentucky, Cincinnati Arch region.

PALAEOGEOGRAPHIC, PALAEOCLIMATIC AND OCEANOGRAPHIC FRAMEWORK Figure 2. Early Silurian (Llandovery) paleogeography, emphasising The Silurian global record represents approximately 28 the convergence of Baltica and Laurentia to form Laurussia and Ma of transition from a period of Late Ordovician global the Caledonian and Salinic mountains. G=Ganderia; K=Kentucky cooling into warmer late Silurian and Devonian greenhouse (adapted from Scotese 2001). AAP Memoir 44 (2013) 161

Figure 3. Chronostratigraphy and stratigraphy of Silurian units in Kentucky with columns showing Gondwanan glaciations, Appalachian tectophases, Appalachian stratigraphic sequences, oceanic events and relative sea-level curves for Kentucky. Diagonally lined areas represent lacunae. ICS chronostratigraphy and absolute dates from Ogg et al. (2008); North American chronostratigraphy from Cramer et al. (2011); L=possible Lilley Formation equivalent in Bisher Fm.; P=possible Peebles Dolostone equivalent in Bisher Fm. conditions (e.g. Fisher 1984). During this time, central by a shallow cratonic sea, but this sea was rimmed on the Kentucky was located on southeastern parts of Laurentia/ southeast margin by eroding Taconic and Salinic highlands Laurussia at about 25° south latitude in the subtropical trade- and mountains (Fig. 2), which shed clastic sediments wind belt (Scotese 2007) (Fig. 2). The warm evaporative westward into a subsiding foreland basin. Lithofacies trends conditions would have been conducive to major carbonate indicate that by latest Ordovician time, the Cincinnati Arch deposition (Lees 1975), which indeed characterises most of area had become a relatively continuous feature (Borella & the section in Kentucky. In fact, Spengler & Read (2010) Osborne 1978; Weir et al. 1984), but not like the present suggested that the Kentucky Silurian may have developed arch which is an Alleghanian feature. Pronounced thinning around a coastal desert that occupied the Cincinnati Arch in the lower Silurian Brassfield Formation toward the arch area. Because of Late Ordovician-early Silurian glaciation, indicates that an uplifted area was well established, though mean early Silurian global temperatures were relatively probably not emergent, by early Silurian time (Gordon cool, perhaps in the 13–14°C range (Nardin et al. 2011), 1980). Hence, it is likely that Silurian units in east-central but by middle and late Silurian time, temperatures on parts Kentucky were deposited on a ramp gently dipping eastward of Laurussia may have reached 25°C and higher (Scotese off the Cincinnati Arch area into the Appalachian foreland 2007; Losey & Benison 2000). Sea-level curves for the time basin, whereas Silurian units in the west seem to reflect vary considerably, but most workers would agree that a deposition on a platform-like area developed on the back prominent cyclicity is present and that Silurian sea levels rose side of the arch. Carbonate units on the eastern side of the from an early Silurian lowstand related to latest Ordovician arch become thicker and more clastic-rich as they progress (Hirnantian) glaciation to a peak in late Llandovery–early eastward into the foreland basin, whereas those on the Wenlock time (Fig. 3) and then slowly declined into Early western flank merely thicken into the Illinois Basin without Devonian (Pragian) time (e.g. Haq & Schutter 2008; Ross major lithologic changes. & Ross 1996). Despite the brevity of the Silurian Period, it not only At this time, most of Laurentia/Laurussia was inundated represents a time of shifting climates, but also a time of 162 AAP Memoir 44 (2013)

Figure 4. Schematic southwest-northeast cross section across the Appalachian Basin, showing relationships between Silurian Appalachian Basin and eastern Kentucky units relative to Salinic tectophases (from Ettensohn 2008). tectonic change from a more open arrangement of Laurussian Kentucky began in latest Ordovician–early Silurian time continents in early Palaeozoic time to a more compact, with the convergence of Ganderia (Fig. 2), a peri-Gondwanan amalgamating arrangement of continents prefiguring the microcontinent and intervening arc, and Laurentia in the area late Palaeozoic development of Pangaea (e.g. Ziegler of the St Lawrence promontory in Maritime Canada (van 1989). Along with these developments came a series of Staal et al. 1996, 1998, 2009). The event is called Taconic ocean-atmosphere-biosphere changes and crises that reflect 3 by van Staal et al. (2009) or the third tectophase of the volatility in Silurian oceanic turnover and global carbon Taconian Orogeny by Ettensohn & Brett (2002) and Ettensohn cycling (Cramer & Saltzman 2005; Calner 2008). Many (2008). This collisional event was partially responsible for of these changes are reflected in a series of events (Fig. 3) the Cherokee unconformity at the Ordovician-Silurian that record species extinctions and originations and other boundary (Fig. 3) throughout most of the Appalachian Basin changes, which were first modelled by Jeppsson (1990, and an early Silurian (Rhuddanian–Aeronian) sedimentary 1998). These events are now routinely interpreted via carbon- tectophase cycle in parts of the Appalachian Basin with isotope excursions, and geochemical evidence supporting Silurian equivalents in central Kentucky (Ettensohn 1994, several of them has been reported from the Silurian of 2008; Ettensohn & Brett 2002). Far-field forces from this Kentucky (e.g. Cramer 2009; McLaughlin et al. 2012). tectophase may have also contributed to initial elevation of the Cincinnati Arch. STRUCTURAL-TECTONIC FRAMEWORK By early Silurian (Telychian) time, the continent of The Silurian was a time of extensive subduction zones Baltica had begun to collide with the northeastern margin (Scotese 2001) (Fig. 2), high plate-drift velocities (Gurnis of Laurentia, closing the Iapetus Ocean and combining & Torsvik 1994), and major terrestrial weathering coupled Laurentia with Baltica to generate the amalgamated continent with uplifted collisional fronts (Cramer et al. 2011), many of Laurussia (Fig. 2) during the Scandian Orogeny of the of which were related to tectonic events along the eastern Caledonian orogenic cycle (e.g. Ziegler 1989; Cocks & margin of Laurentia. In fact, after a Late Ordovician (mid- Torsvik 2002; Dewey & Strachan 2003). Although this Caradoc) east-to-west change in the vergence of Taconic orogeny was mostly concentrated on the northeastern subduction (e.g. Karabinos & Hepburn 2001), latest margin of Laurentia between present-day Greenland and Ordovician through Silurian tectonism along the eastern Scandinavia, a southern extension of the orogeny in the margin of Laurentia involved westward subduction and Maritime Canada-New England area has been called the the convergence of Baltica and various peri-Gondwanan Salinic Orogeny and represents the oblique convergence of terranes with Laurentia during the closure of the Iapetus Avalonian terranes in the “Reading Prong” of Baltica with the Ocean and the formation of Laurussia, while at the same time northern Appalachian margin (Boucot 1962; McKerrow & opening the Rheic Ocean (Fig. 2). Nearly all of the Silurian Ziegler 1972; Scotese & McKerrow 1990) (Fig. 2). This more convergence on southeastern parts of the Laurentian margin southerly equivalent of the Scandian Orogeny occurred in two was relatively “soft”, having involved oblique convergence tectophases and had sedimentary and tectonic implications or transpression rather than direct head-on collision (van as far south as the central Appalachians and as far west as Staal & de Roo 1995; Dewey & Strachan 2003; Cramer et al. central Kentucky (Ettensohn 1992a, 1994, 2008; Ettensohn 2011). Moreover, all of the Silurian convergence that would & Brett 1998, 2002; Dennis 2007). In the Appalachian affect parts of the Appalachian Basin and nearby areas — Basin, each of these tectophases generated a loading-related, even as far west as central Kentucky — was concentrated flexurally controlled tectophase sequence that began with in the Maritime region of Canada and adjacent parts of New an episode of subsidence and transgression, manifest as England (Zagorevski et al. 2008; van Staal et al. 2009, 2012). dark-shale deposition, and ended with a regressive, clastic- Silurian collisional processes that would affect central rich highstand wedge (Fig. 4). Even though these flexural AAP Memoir 44 (2013) 163 sequences are largely restricted to foreland basins (Fig. 4), Paint Creek and Kentucky-Ohio (Vanceburg-Ironton) fault the overall transgressive-regressive scenario inherent in these zones (Figs 1 and 5) cut parts of the Cincinnati Arch and sequences is also typically apparent in platform and ramp Silurian outcrop belt at high angles. The Kentucky River sequences on adjacent parts of the craton like the Cincinnati Fault System forms a complex of northern bounding faults Arch region (Ettensohn 2008). on a late Precambrian–early Cambrian, Iapetan rift called the In the Appalachian Basin, sedimentary response to Salinic Rome Trough, a series of down-to-the-south basement faults Orogeny continued until latest Silurian–Early Devonian time and half-graben systems that extend northeastward from (Ettensohn 2008), but around the Cincinnati Arch region, eastern Kentucky into West Virginia, Pennsylvania and New Middle and Late Devonian erosion associated with later York; the Rome Trough faults were periodically reactivated uplift during the Acadian Orogeny destroyed any indication throughout Palaeozoic time (Ettensohn & Pashin 1992). of these sediments on and near the arch (Fig. 3). Hence, in The Irvine-Paint Creek Fault Zone, on the other hand, is an our study area middle Silurian carbonates are unconformably intra-rift fault zone, whereas the southern boundary of the overlain by Middle and Upper Devonian carbonates and rift is defined by a series of faults that are not apparent at the shales. surface (Fig. 1). Evidence from the surface and subsurface The Silurian outcrop belt in central Kentucky coincides indicates that structures associated with the rift, especially with the Cincinnati Arch area (Fig. 1). The arch itself the Kentucky River Fault Zone, were periodically reactivated has been episodically active from latest Ordovician to throughout Palaeozoic time (Dever et al. 1977; Ettensohn Pennsylvanian time, with peak activity during Silurian time & Pashin 1992), and isopach maps from eastern Kentucky (Kolata & Nelson 1990). In fact, Foerste (1906, 1931, 1935) clearly show that Upper Ordovician, Silurian, and Devonian noted dissimilar faunas in post-Brassfield, Silurian rocks on strata thicken into the trough, indicating that subsidence was either side of the arch, whereas Nicoll & Rexroad (1968) not only occurring during Silurian time, but also in pre- and recognised a pronounced provinciality between Wenlock post-Silurian times (Freeman 1951; Currie 1981; Ettensohn conodonts on the west side of the arch and those from et al. 1988). the Niagara Gorge area in New York. Moreover, Freeman Iapetan rifting in central Kentucky was apparently (1951) noted that the arch probably acted as a barrier to deflected westward by Grenville-front faults and reactivated sedimentation following early Silurian Brassfield deposition parts of the East Continental Rift Basin, and it resumed in based on dissimilar Silurian lithologies on either side of the west-central Kentucky as the west-trending, Iapetan, Rough- arch. Based on the presence of small Silurian outliers near the Creek Graben, a series of down-to-the-north basement faults crest of the Cincinnati Arch (Nelson 1962; Cressman 1975, and half-graben systems that extend into western Kentucky 1981), it is clear that at least the Brassfield was deposited (Fig. 5). Together the Iapetan rift systems in central Kentucky across the Cincinnati Arch, but the unit pinches out (e.g. represent non-overlapping, opposing, half-graben systems McFarlan 1943; Taylor & Lewis 1971) or thins substantially (Ettensohn & Pashin 1992), with an intervening, platform- to the south and west as it crosses the arch area (Gordon 1980; like area that acted as an isolation accommodation zone to Gordon & Ettensohn 1984), and the depositing seas seem to accommodate movement on graben systems with opposite have been at least locally time-transgressive (Andrews 1997). polarity (Frostick & Reid 1987; Ettensohn & Pashin 1992). It Thinning of the Brassfield associated with the Cincinnati was this isolation accommodation zone, largely representing Arch becomes especially dramatic to the south and west of non-overthrust parts of the East Continental Rift Basin Berea (Fig. 1). However, what has been called the Cincinnati that was initially inverted (Drahovzal et al. 1992; Rast & Arch for workers in the Silurian and in later systems has Goodman 1994) to become a broad, platform-like, Cincinnati differed from worker to worker and from time to time. In Arch area during the latest Ordovician and Silurian. The arch fact, what we suggest in following parts of this section is area only began to uplift in the latest Ordovician (Weir et that the Silurian “Cincinnati Arch” was a broad, platform- al. 1984) and early Silurian (McDowell 1983) in response like area (Fig. 5) with local centres of uplift related to the to far-field, flexural forces generated by the Taconian and early interaction of Keweenawan, Grenvillian and Iapetan Salinic orogenies, the first compressional events to affect basement structures with far-field forces generated during eastern Laurentia after Rodinian breakup and Iapetan rifting. Silurian orogenies on the eastern margin of Laurentia. These Hence, the position of Silurian exposures in eastern Kentucky structures will be briefly discussed below. and adjacent parts of Ohio show that the rocks therein were The earliest of the involved structures is the Keweenawan, deposited some distance from the Cincinnati Arch area, and East Continental Rift Basin, which developed from 1.4–1.0 it is not until the outcrop belt approaches the Grenville front Ga ago (Drahovzal et al. 1992) and is represented in part or platform margin of the Cincinnati Arch area, south and in Figure 5 by the shaded series of downthrown basement west of Berea, that the influence of the “arch” is observed. blocks, extending from north-central to southwest-central The various Iapetan basement faults in east-central Kentucky. This rifting event was apparently terminated by Kentucky (Kentucky-Ohio Boundary, Kentucky River and emplacement of the Grenville allochthon at about 990–880 Irvine-Paint Creek faults; Fig. 5) would have also been Ma, which overrode parts of the East Continental Rift Basin reactivated by the same Silurian and post-Silurian far- (Drahovzal et al. 1992) and is represented by the so-called field forces, including bulge moveout and uplift. Hence, Grenville Front, a zone of remnant basement thrust faults, the presence of thinned or truncated Silurian sections and running through west-central Ohio and east-central Kentucky intra-Silurian unconformities should be expected at places (Fig. 5). This thrust-fault zone closely parallels the modern where these faults cross the Silurian outcrop belt (e.g. Hoge (Alleghanian) Cincinnati Arch. et al. 1976; McDowell 1983; Ettensohn et al. 1992; Mason Subsequent to Grenville events, the area experienced et al. 1992b). major rifting and strike-slip fault movement associated with In contrast, the Silurian exposure belt on the western latest Precambrian–early Cambrian, Iapetan rifting from side of the arch largely coincides with the isolation 570–535 Ma ago (Drahovzal et al. 1992; Ettensohn 2008). accommodation zone (Fig. 5), or the broad platform-like In eastern Kentucky, the Iapetan Kentucky River, Irvine- area with more uniform formation thicknesses that we call 164 AAP Memoir 44 (2013)

Figure 5. Map of central Kentucky showing the location of major basement structures. Faults from Keweenawan East Continental Rift Basin are coloured red. This rift basin apparently acted as an isolation accommodation zone between Iapetan fault zones of opposite polarity and itself became inverted in latest Ordovician–Silurian time to form a platform-like area of deposition for Silurian rocks of the western outcrop belt. The barbed line bounding the accommodation zone on the east is the Grenville Front. Silurian rocks on the eastern outcrop belt were deposited on a ramp dipping eastward off of the high generated by the front. Reactivation of Iapetan fault zones in the east generated many stratigraphic anomalies in Silurian rocks crossing the faults. the “Cincinnati Arch area.” Although the entire zone was northward toward the Louisville High and Ripley Island. apparently inverted and uplifted during latest Ordovician However, by the time of Louisville Limestone deposition, and Silurian times (Drahovzal et al. 1992; Rast & Goodman the area of greatest thickening had moved northward to the 1994) to form this “area,” any of the individual faults in Louisville High (Phelps 1990) (Fig. 1). In fact, thickness, this zone may have been reactivated at different times and lithologic and faunal variations are clearly associated with places as transfer faults to accommodate movements of each structural feature in west-central Kentucky, indicating different polarities on either side of the zone (Ettensohn & that each feature possessed significant topographic relief Pashin 1992). Hence, the Cincinnati Arch area may have during Silurian deposition. In contrast to the eastward- had different loci of movement at different times and places. dipping ramp that characterised Silurian deposition on the Other structures in west-central Kentucky shown in eastern side of the Cincinnati Arch, deposition west of the Figures 1 and 5, like the Louisville High, Ripley Island, arch apparently consisted of a broad, low-gradient shelf area, Lyndon Syncline, Bardstown Monocline and Servant interrupted by structurally controlled topographic breaks at Run lineament, show control over Silurian thickness and each structure. facies distribution and are likely related to Precambrian (Keweenawan) faults, uplifts and rifts associated with METHODS the underlying East Continental Rift Complex that were Approximately 87 sections were selected from outcrop periodically reactivated as transfer faults (Black 1986; belts on both sides of the Cincinnati Arch based on quality Hendricks 1996; Andrews 1997; Stark 1997a, b). In general, of exposure, stratigraphic completeness and accessibility. the earlier Silurian units in west-central Kentucky tend Each section was measured and described, and where to thicken south of the Bardstown monocline and thin possible, bedding type, lithology, colour, grain size, AAP Memoir 44 (2013) 165 sedimentary structures, and fossil content were noted; the Cincinnati Arch and that yet higher Brassfield members the biostratigraphy of the respective units was also apparently overlap parts of the arch into the western outcrop examined for placement in a correct chronostratigraphic belt (Andrews 1997), along with older palaeontological and framework. Representative samples were collected from biostratigraphic data (Rexroad 1967), have suggested to each lithostratigraphic unit for the preparation of polished some that the Brassfield was the product of northwestward slabs and for petrographic analyses. Stratigraphic cross to westward transgression by early Silurian seas across the sections were compiled parallel to the outcrop belts for Cincinnati Arch (Rexroad 1967; Gordon 1980; Gordon & each unit to discern stratigraphic relationships among units Ettensohn 1984; Andrews 1997). Although the possibility and lithofacies and the possible influence of structures. of such a transgression cannot be excluded, it would have A synthesis of the literature, outcrop descriptions, slab been a relatively brief event limited to about the five million examination, fossil content and petrographic analyses years represented by the Distomodus kentuckyensis Zone. was then used to infer depositional environments for each On the eastern side of the arch, the Brassfield grades unit or lithofacies. The palaeogeographic, palaeoclimatic, upward into the Crab Orchard Formation, a mid-Llandovery tectonic and glacio-eustatic frameworks for the times and to lowest Wenlock (upper Rhuddanian– lower Sheinwoodian) area were also noted, and together, became the bases for unit up to 48 m thick, composed largely of greenish-grey to respective inferences. Where possible, the fossil content maroon, clay shales with two prominent dolostone intervals, from each unit or lithofacies was also analysed relative the Oldham and Waco members, each 1–4 m thick. The to the Silurian community structures of Ziegler (1965), Waco Member has been equated with the Dayton Dolostone Ziegler et al. (1968), Ziegler & Boucot (1970), Anderson further to the north in Ohio (Foerste 1935; McDowell (1971), Boucot (1975, 1999) and Cocks & McKerrow 1976), but recent work by Sullivan et al. (2012) has shown (1978) for appropriate analogies. The nature of interpreted that they are not equivalent; rather, the Dayton is a slightly environmental sequences and depth-related communities in younger unit represented by a glauconitic horizon within part became the basis for development of relative sea-level the overlying shales of the lower Estill Member (Fig. 3). curves (Fig. 3). Detailed outcrop descriptions and related Dolostone units like the Oldham and Waco divide shaly parts interpretations can be found in theses by Gordon (1980), of the Crab Orchard into the Plum Creek, Lulbegrud and Phelps (1990), Hendricks (1996), Andrews (1997), and in Estill members. Various workers have attempted to divide field-trip articles by Ettensohn (1992a, b), Ettensohn et al. the Crab Orchard Formation, with or without the Brassfield, (1992) and Mason et al. (1992a, b). into several different units, including the Indian Fields and Alger formations (Foerste 1906; McDowell 1983), the STRATIGRAPHIC FRAMEWORK Noland Formation (Rexroad et al. 1965; Rexroad & Kleffner General stratigraphy and biostratigraphy 1984), and the Drowning Creek Formation (McDowell The Silurian section from outcrop on both sides of the 1983). However, we continue to use the terms Brassfield and Cincinnati Arch exhibits three to five formations but is Crab Orchard here as they were originally defined (Linney incomplete and eroded at the top (Fig. 4). On both sides 1882; Foerste 1906) and subsequently used during the U.S. of the arch, the basal Silurian formation is mapped as the Geological Survey-Kentucky Geological Survey Mapping Brassfield Formation, a lower to upper Llandovery unit of Program from 1964–1978 (e.g. McDowell 1976). Overlying predominantly dolostone which ranges from 3–9 m thick. the Crab Orchard Formation on the eastern side of the arch is Nicoll & Rexroad (1968) indicated that the Brassfield the Bisher Dolostone. It occurs only as a few dozen erosional everywhere in Kentucky and Indiana, except for the Lee remnants, no more than 10 m thick (Fig. 3). Creek Member on the western outcrop belt, occurred On the western side of the Cincinnati Arch, the so-called within the “Icriodina regularis” Zone, which is equivalent Brassfield is younger than that on the eastern side (Fig. to the current Distomodus kentuckyensis Zone. This being 3). Using the Lee Creek Member (=Waco Member on the the case, then there is no significant change in conodont east) as an upper constraint, Andrews (1997) correlated faunas that would indicate different ages for the Brassfield the underlying two sequences that comprise the western on either side. Hence, conodont biostratigraphy indicates Brassfield with the underlying two sequences below the that most of the Brassfield on eastern and western sides of Waco Member on the eastern side (Fig. 6). This correlation the outcrop belt is mid-Rhuddanian to early Aeronian in suggests that only the upper massive member of the eastern age. The major exception is the Lee Creek Member of the Brassfield has a correlative on the west, and that most of Brassfield in western Kentucky which, based on physical the western Brassfield is equivalent to the Plum Creek and stratigraphy and conodont biostratigraphy, is early- to mid- Oldham members of the Crab Orchard Formation, any Telychian in age (Pterospathodus eopennatus Zone) and Lulbegrud equivalents having probably been eroded away unconformably overlies parts of the Brassfield (Nicoll & on the unconformity below the Lee Creek Member (Fig. 3). Rexroad 1968; Cramer 2009; Kleffner et al. 2012a; Brett et This means that the western Brassfield, with the exception al. 2012) (Fig. 3). Brett et al. (2012) even suggested that the of the Lee Creek Member, is wholly contained within the Lee Creek Member should be separated from the Brassfield Distomodus kentuckyensis Zone and is most likely middle as a formation in Indiana and Ohio. In Kentucky, however, Aeronian or younger in age (Fig. 3). it is still considered to be a part of the Brassfield Formation The overlying Osgood Formation is predominantly a on the western outcrop belt. medium grey to maroon, silty, clay shale or mudstone, which In extreme northeastern parts of the eastern outcrop is 5–15 m thick. Conodont data from the Osgood and Estill belt, an argillaceous dolostone unit called the Belfast shales (Nicoll & Rexroad 1968; Rexroad & Nicoll 1972; Member (Foerste 1896) is locally present at the base of the Cramer 2009) suggest that lower parts of the Osgood Shale Brassfield (Fig. 3), but it pinches out southward along the may be the same age as the upper Estill Shale Member, but Cincinnati Arch (Gordon 1980; Gordon & Ettensohn 1984; that the Osgood is largely younger than the Estill. Overall, Ettensohn 1992b). The facts that overlying parts of the the character of the Osgood is similar to that of the Estill eastern Brassfield clearly overlap the Belfast Member onto except that beds of limestone and dolostone are more 166 AAP Memoir 44 (2013)

Middle –Late Devonian (Taghanic unconformity) uplift and erosion (Fig. 3) accompanying the first, second, and third tectophases of the Acadian orogeny (Ettensohn et al. 1988; Ettensohn 2008; Brett et al. 2004). On the eastern side of the Cincinnati Arch, Silurian Bisher dolostones are overlain by Middle Devonian carbonates of the Boyle Formation or by Upper Devonian Olentangy, New Albany, Chattanooga, or Ohio shales. In places, this unconformity is angular and exhibits nearly 6 m of relief (McFarlan 1938; McFarlan et al. 1944) or may be a palaeokarstic surface on Bisher carbonates (McDowell 1983; Mason et al. 1992b). On the western side of the arch, the unconformity is commonly a paraconformity and is very difficult to recognise. The Middle Devonian Sellersburg (Beechwood) or Jeffersonville limestones overlie the Silurian Louisville Limestone along a bedding-plane-like surface that represents the paraconformity (Conkin & Conkin 1980; Conkin et al. 1992). Along this surface, Louisville fossils may be truncated along a surface overlain by a thin lag horizon containing phosphorite nodules and reworked Louisville clasts.

Ordovician-Silurian (Cherokee) unconformity Based on missing conodont zones (e.g. Rexroad & Kleffner 1984), it has been well known that throughout most of the east-central United States a major disconformity separates Ordovician and Silurian strata (Figs 3, 7). In fact, work with chitinozoans from both sides of the interval suggests that up to six graptolite zones, ranging from the Late Figure 6. Comparison of stratigraphic sequences from lower Ordovician (Katian) Dicellograptus complanatus to the parts of Silurian sections on eastern and western outcrop belts Llandovery (late Rhuddanian) Coronograptus cyphus (from Andrews 1997). Sequence A may not have been deposited, zones, are missing along this unconformity (Grahn & or was eroded away, or included as a lag at the base of the lower Bergström 1985), representing approximately 8.5 Ma (Ogg massive unit on the western outcrop belt. Possible equivalents of et al. 2008). This prominent break was called the Cherokee the Lulbegrud or upper Oldham on the western outcrop belt may Discontinuity by Dennison & Head (1975) and, because of not have been deposited, may be included in the grainstone unit or its worldwide nature (McKerrow 1979) and its association were eroded on the surface below the Lee Creek. with Gondwanan glaciation (e.g. Dennison 1976; Caputo 1998), this unconformity has been largely attributed to a glacio-eustatic drawdown called the Hirnantian or Ashgill abundant (Hendricks 1996). The pervasively dolomitised lowstand (e.g. Dennison 1976). In northeastern parts of the Laurel Dolostone overlies the Osgood Formation and is no Appalachian Basin, however, this unconformity becomes more than 16 m thick in the outcrop belt. The unit is wholly increasingly angular, suggesting tectonic influence (Rodgers Wenlock in age and contains a diverse series of facies and 1971; Liebling & Sherp 1982; Ettensohn 1994). Ettensohn fossils. Conformably overlying the Laurel is the Waldron & Brett (2002) also pointed out the presence of a typical Shale, a grey, dolomitic, clay shale interbedded with thin tectophase cycle associated with the unconformity in the beds of argillaceous dolostone. Natural exposures are rare same parts of the basin, whereas Ettensohn (2008), based and the unit typically ranges from 3–4 m thick. Fossils are on timing and location, suggested that tectonic components locally common but poorly preserved. The preserved Silurian of the unconformity were most likely related to the third section in Kentucky concludes with the Louisville Limestone Taconian tectophase involving the accretion of Ganderia to which, despite the formation name, is another pervasively Laurentia in the Maritime area of Canada (van Staal et al. dolomitised unit. The unit also contains a diversity of facies 1996, 1998, 2009). Hence, the Cherokee unconformity — and fossils, but not more than 24 m of the unit is preserved even in Kentucky — probably reflects both glacio-eustatic along the outcrop belt. and tectonic components. In the subsurface on either side of the Cincinnati During the generation of this unconformity, north- Arch area, the Silurian section is more complete and is central Kentucky was apparently impacted by a meteorite, conformably or unconformably overlain by uppermost for lower and medial Silurian strata in the circular Jeptha Silurian and lowermost Devonian Helderberg (Ebert & Knob deformed area (Fig. 1, JK) lay unconformably, and Matteson 2003; Kleffner et al. 2009) units like the Flat nearly horizontally, atop deformed Upper Ordovician strata Gap and “Helderberg” limestones. However, on the central (Cressman 1975, 1981). Bucher (1925) first mapped and Kentucky outcrop belts (Fig. 1), the youngest Silurian units described the structure as cryptovolcanic in origin, but later exposed are Ludlow in age (Fig. 3), and the absence of work by Seeger (1968) and Thompson (2005) has suggested any younger Silurian and Devonian units is the product of a meteorite-impact origin. Although the Brassfield Formation several episodes of Devonian erosional truncation on the clearly rests on the unconformity in this area, overlying Cincinnati Arch. In fact, the unconformity atop the Silurian medial Silurian units as young as the Louisville Limestone is compound and represents periods of Early Devonian (Fig. 3) may also be present (Bucher 1925; Foerste 1931). (Walbridge unconformity), Early–Middle Devonian and AAP Memoir 44 (2013) 167

Figure 7. View of the Cherokee unconformity between the Brassfield and Drakes formations (eastern outcrop belt), south of Owingsville, Kentucky (Fig. 1), near I-64, just before the pinchout of the Belfast Member (see Fig. 8)

EASTERN KENTUCKY OUTCROP BELT tract. The overlying Belfast Member of the Brassfield Brassfield Formation (Foerste 1896), which occurs only in northeastern Kentucky The Brassfield Formation is recognised at the base of the (Figs 7, 8), contains thin-bedded, argillaceous, dolomitic Silurian System throughout the Cincinnati Arch area as a mudstones and interbedded shales. In Kentucky, the unit distinctive carbonate unit that onlaps Upper Ordovician strata attains a maximum thickness of 3 m, but apparently pinches across the arch from the southeast to the northwest (Rexroad out by erosion near Owingsville, Kentucky (Figs 1, 3, 7, et al. 1965; Berry & Boucot 1970; Boucot 1975). The unit 8). Conodonts and chitinozoans suggest that the unit is was equated with the Clinton in New York (Foerste 1906) probably late Rhudannian in age (Cooper 1975; Grahn & and grades eastward into the shales and sandstones of the Bergström 1985). Bioturbation in the unit is intense and only Clinch or Tuscarora sandstones. Foerste (1906) described a few strophomenid brachiopods have been found. Gordon the unit as containing thick- to thin-bedded, fossiliferous, & Ettensohn (1984) and Ettensohn (1992b) suggested that rusty, yellowish-brown “limestones” with interbedded many of the beds in the unit represent tempestites deposited shales toward the top. However, later petrographic and in a lagoonal setting at the front of the transgression. Gray & geochemical data make it clear that the Brassfield is a Boucot (1972) suggested that the unit contains equivalents dolostone in Kentucky and becomes progressively more of the Lingula Community or Benthic Assemblage (B.A.) 1. limestone-rich northward into Ohio (Stith & Stieglitz 1979; Gordon 1980; Gordon & Ettensohn 1984). Gordon (1980) Lower massive lithofacies. The overlying lower massive and Gordon & Ettensohn (1984) recognised five distinct lithofacies or “cherty” Brassfield (Figs 7, 8) may be lithofacies or lithologic subunits in the Brassfield (Figs 3, up to 2.5 m thick and is composed of massive, vuggy 8) that represent parts of two early Silurian transgressive dolostones with prominent white chert stringers throughout. events. Each lithofacies is briefly discussed below. Dolomitisation has destroyed most of the original fabric, but petrographic examination of the cherts indicated that Belfast Member. The base of the Silurian section everywhere the unit was originally composed of bryozoan/pelmatozoan in the area exhibits a thin conglomeratic lag consisting grainstones. Rippled bedding surfaces, scours, bar-like of quartz grains, reworked Ordovician fossils and clasts, features, crossbedding, intraformational conglomerates, and phosphorite nodules and glauconite, and represents a typical the dominance of a thick-shelled fauna suggest a high-energy, transgressive surface at the base of a transgressive systems sandbelt environment just seaward of the Belfast lagoon. 168 AAP Memoir 44 (2013)

Figure 8. Schematic northeast-southwest cross section along the east-central Kentucky outcrop belt showing the relationships among major Brassfield lithofacies (adapted from Gordon 1980).

On one hardground in the unit, crinoids, cystoids, asteroids, represents a shallow, open marine setting just below normal encrusting rugose corals, brachiopods and bryozoans were wave base that was periodically subject to major storms. The found (see Blake & Ettensohn 2009). This faunal association pelmatozoan-bryozoan-coral communities are very similar to and the Cryptothyrella Community (Boucot 1975; Berry the coral-stromatoporoid communities described by Johnson & Boucot 1970) found in this lithofacies equate fauna in (1980) from the lower Silurian of Iowa and would probably the sandbelt environment with the Eocoelia Community of equate with the B.A. 3 Eocoelia or Pentamerus communities Ziegler (1965) and Ziegler et al. (1968) or B.A. 2 of Boucot (Ziegler 1965; Ziegler et al. 1968; Anderson 1971; Boucot (1975). However, Foerste (1931) reported the brachiopod 1975, 1999; Johnson 1980). Platymerella from the unit, suggesting a B.A. 3 position (Boucot, 1975, 1999) for some parts of the unit and an age Upper shaly lithofacies. The overlying upper shaly lithofacies of late Rhuddanian to early Aeronian (Berry & Boucot 1970). (Figs 8, 9B), no more than 0.5–1.5 m thick, is composed mostly of shale with a few thin bioturbated dolostone beds Middle thin-bedded lithofacies. The lower massive lithofacies that were originally poorly sorted wackestones or mudstones. grades upward into the middle thin-bedded lithofacies (Figs A diverse fauna, consisting of rugose and tabulate corals, 8, 9B), which consists of 1–3.5 m of thin-bedded dolostones cystoids, crinoids, gastropods, trilobites, bryozoans and and interbedded shale. The individual beds of dolostone tentaculitids is present, but brachiopods predominate and were formerly grainstones and wackestones, and exhibit include dalmanellids, leptaenids, plectorthids, strophomenids amalgamated beds with sharp erosional bases, hummocky and triplesiids. The unit also locally contains the peculiar crossbedding and ripples. Bioturbation is common “cogwheel” crinoid columnals or “beads”, which may be and diverse pelmatozoan-bryozoan-coral communities assignable to the stem genus Floricolumnus Donovan & established themselves on the bedding surfaces, and most Clarke, 1992 (Fig. 9A). The brachiopod associations are organisms exhibit firm attachments to the substrate. The very similar to the Linoporella Community of Rubel (1970), tabulate corals Favosites, Halysites, Syringopora, Aulopora which Boucot (1975, 1999) equated to the B.A. 3 Pentamerus and Heliolites, as well as individual rugose corals and flat Community. The environment is interpreted as representing stromatoporoids are also locally common. Major brachiopod a deeper open marine setting well below normal wave base communities, however, are absent. Current structures, that was the culmination of an early Silurian (Rhuddanian) petrographic textures and fauna indicate periods of intense transgression (Silurian transgression 1 of Johnson 1996, energy, but the interbedded shales and stratigraphic position 2006, and Johnson et al. 1998) (Fig. 3). Although this suggest an environment that was generally below wave base. transgression may represent a period of interglacial highstand Hence, we suggest that the middle, thin-bedded lithofacies (Grahn & Caputo 1992; Caputo 1998), a regional tectonic AAP Memoir 44 (2013) 169

Figure 9. A. Cogwheel or Bead Bed, Upper Shaly member of the Brassfield at locality shown in B;B . Partial Brassfield section south of Owingsville, Kentucky (Fig. 1), near I-64, showing the thin-bedded, upper shaly, and upper massive members of the Brassfield Formation. component was probably also present (Ettensohn & Brett crinoid-stem beads in the cogwheel bed may have been 2002). reworked from the unit below, but are even present where the upper massive facies thins and disappears. The upper Upper massive lithofacies. The top of the Brassfield is massive lithofacies has been interpreted as representing the defined by the upper massive lithofacies (Figs 8, 9B), about return to a high-energy sandbelt environment with abrupt 2.5 m of massive, dolomitic, oolitic and skeletal grainstone. shallowing (Foerste 1906; Rexroad et al. 1965; Gordon & Megaripples and crossbeds are common throughout. In Ettensohn 1984). Conodonts and chitinozoans (Rexroad et northernmost parts of the outcrop, the unit pinches out or al. 1965; Grahn & Bergström 1985) indicate that the lower becomes so thin that it is difficult to recognise (Fig. 8); four Brassfield lithofacies represent early Silurian (late and it becomes difficult to define the top of the Brassfield Rhuddanian–early Aeronian) transgression 1 (Figs 3, 6A) (McDowell 1983; Gordon & Ettensohn 1984; Ettensohn into the Cincinnati Arch area. This transgressive sequence 1992b). The upper massive lithofacies truncates the was abruptly interrupted by a drop in sea level and erosion underlying shales and at its base contains a lag horizon of represented by the minor unconformity or discontinuity at glauconite, phosphorite nodules, and reworked clasts from the base of the upper massive lithofacies. The upper massive below. Diversity is low in this unit and the few fossils include lithofacies and overlying Plum Creek Shale Member of the massive encrusting bryozoans, large beaded “cogwheel” Crab Orchard Formation (Figs 3, 6, 8), appear to represent crinoid stems, rugose corals and the large brachiopod mid-Aeronian transgression 2, which may record subsidence Cryptothyrella (see Gauri & Boucot 1970); this sparse accompanying the accretion of Ganderia during the third community typically occupied higher energy sands and is Taconian tectophase (Ettensohn & Brett 2002; Ettensohn equivalent to the Eocoelia Community of Ziegler (1965) 2008). and Ziegler et al. (1968), the Cryptothyrella Community of Related to this transgression is the fact that upper parts Cocks & McKerrow (1978) or B.A. 3 pentamerid community of the upper massive lithofacies locally contain so much of Boucot (1975,1999). This lithofacies also contains the ferruginous and haematitic dolostone that they have been cogwheel or bead-like crinoid columnals and has been called mined as the Rose Run iron ore (Foerste 1906). Although the “cogwheel bed” or “bead bed” or the Cryptothyrella much of this lithofacies is ferruginous, the major haematite horizon (Whitfieldella horizon of Foerste 1906); it has and limonite deposits (0.6–1.2 m thick) in the unit are located been used to recognise the top of the Brassfield. The thick in Bath County just north of the Kentucky River Fault Zone 170 AAP Memoir 44 (2013)

(Fig. 1) and they thicken toward the fault zone (Stokley or the Brassfield and lower Crab Orchard are treated as a 1948). Similar deposits are known from subsurface Silurian single unit which McDowell referred to as the Drowning deposits to the east, even to the point where Brassfield Creek Formation, though this nomenclature has not been carbonates grade into Clinch shales, siltstone and sandstones widely used. (Freeman 1951). These deposits are similar in composition and occurrence to the Lower Clinton iron ores of New York, Crab Orchard Formation Alabama and Georgia (Chowns 1996; Brett et al. 1998), The Crab Orchard Formation was named by Linney and like the Lower Clinton ores, are mid-Aeronian in age (1882) for a section of interbedded dolostone and shale (Rickard 1975; LoDuca & Brett 1994; Brett et al. 1998). between Medina “sandstones” (Brassfield Formation) and These deposits are concentrated near the top of the upper Corniferous (Devonian) limestones in Lincoln County, massive lithofacies, and are largely composed of haematite Kentucky; his unit may have originally included upper and limonite, which replace and coat fossils, as well as parts of the Brassfield (McDowell 1983). Overall the unit forming the enclosing matrix. Although haematitic ooids contains 35–70 m of alternating carbonates and shales with are common, indicating deposition in an environment with carbonates decreasing up-section. This pattern suggests some degree of agitation, the upper surface of the unit may progressive deepening through time with periodic halts in be encrusted with laminae of haematite and limonite, which depth increase, clastic sedimentation or both (McDowell suggests quieter conditions. Chamosite is also present in some 1983). Three shale members and two intervening dolostone replacements and ooid laminae, suggesting that chamosite members are included in the unit (Fig. 3). may have been precipitated originally and was subsequently altered to limonite and haematite (Stokley 1948). This iron Plum Creek Shale Member. Following a slight break at mineralisation of the upper massive lithofacies appears to the top of the upper massive lithofacies of the Brassfield be related to a maximum flooding surface at its top and an during a major flooding event, transgression continued accompanying period of sediment starvation or condensation with the greenish-grey, clay shales of the Plum Creek (e.g. Brett et al. 1998). Deepening associated with the third Shale Member of the Crab Orchard Formation. The unit Taconian tectophase (Fig. 3) had already begun as the upper ranges from 1.2–1.7 m thick and contains intercalations of massive sandbelt was being deposited, because chamositic dolomitic mudstones and wackestones near the base, which or haematitic ooids and haematitic replacements of fossil grade into a monotonous sequence of shales in upper parts fragments are constituents of the crossbedded sands. This of the member. Although this member is only sparsely suggests that iron-rich waters or waters transporting iron- fossiliferous, the fauna in its lower parts are typically rich clays were being forced up onto the shoals. Later, as dominated by thin-shelled strophomenid brachiopods deepening proceeded, laminar deposits of haematitic and like Leptaena, Strophonella, Coolinia, small orthids, limonitic precursor minerals (chamosite?) were apparently dalmanellids, rhynchonellids and delicate ramose bryozoans directly deposited on the surface in sediment-starved, like Helopora. The fauna is clearly more closely related to mildly reducing waters. Likely sources of iron to the east that of the underlying Brassfield than to that of overlying are thought to reflect the deep, subtropical weathering of Crab Orchard units (Foerste 1931). The environment has Taconian highlands during the Cherokee unconformity been interpreted as shallow, open marine, muddy conditions interval (e.g. Cecil et al. 2004). The resulting iron-rich clays at the base, becoming deeper, open marine toward the top and dissolved iron were then transported into warm, shallow (Gordon 1980; Gordon & Ettensohn 1984); the fauna and seas where they were deposited as red muds or precipitated quiet-water, clayey substrate suggest comparison with the from iron in solution as fossil replacements, matrix material Stricklandia Community of Ziegler et al. (1968) and Cocks or ooids (Folk 1960; Ziegler & McKerrow 1975; Chowns & McKerrow (1978) or B.A. 4 of Boucot (1975, 1999). & McKinney 1980). The proximity and thickening of these The unit has at times been regarded as a distal equivalent deposits toward the Kentucky River Fault Zone (Fig. 1) of the Cabot Head Shale (Calvert 1968), but is more likely suggests an alternative mechanism of iron transport into the equivalent to the mid-Aeronian Maplewood and Neahga area. As already indicated, the Kentucky River Fault Zone shales of New York (Brett et al. 1998). This unit reflects the formed the northern boundary of the Rome Trough, which culmination of Silurian transgression 2 (Johnson 1996, 2006; opened up to eroding Taconian highlands as far northeast as Johnson et al. 1998) and together with the upper massive New York (Ettensohn & Pashin 1992). There may have been member of the Brassfield, probably reflects glacio-eustasy enough relief in this subsiding trough during Silurian time (Grahn & Caputo 1992; Caputo 1998), although the lingering that it could have funneled iron-rich water and sediments effects of Taconian tectonism cannot be excluded. into the area. This structural setting, combined with winds and currents that permitted quasi-estuarine circulation and Oldham Dolostone Member. Sharply overlying the Plum periodic upwelling onto the northern side of the fault zone Creek Shale is 1 to 4.6 m of interbedded, greenish-grey, clay (see Ettensohn 2010, p. 168–169), would have allowed shale and fine- to coarse-grained, thin-bedded dolostones, dissolved iron to upwell and mix with oxygenated surface included in the Oldham Dolostone Member of the Crab waters, explaining those occurrences of the Rose Run Orchard (Foerste 1906). The dolostones were largely (Brassfield) and equivalent iron ores in proximity to the wackestones and packstones and decrease in number and Rome Trough fault system. thickness upward in section. Ferruginous dolostones occur The upper massive lithofacies, as well as the dolomitic locally. The unit is delineated from other dolostones in the units in the Crab Orchard Formation (Oldham and Waco Crab Orchard Formation by the presence of the smooth members), eventually pinch out or become too thin to pentamerid Microcardinalia norwoodi, which occurs with recognise in parts of northern Kentucky (McDowell 1983; thin-shelled strophomenids like Leptaena and Strophonella, Ettensohn 1992b; Kleffner et al. 2012a) (Fig. 8). Where this as well as small orthids and dalmanellids. Calymenid happens, the Brassfield and lower part of the Crab Orchard trilobites, small rugose corals, the tabulates Halysites are merely mapped together as a single unit (e.g. Peck 1967) and Heliolites, and delicate bryozoans like Helopora and AAP Memoir 44 (2013) 171

Lulbegrud Shale Member. Conformably overlying the Oldham Dolostone is 2.4–4.2 m of greenish-grey to bluish- grey, pyritic clay shales that comprise the Lulbegrud Shale (Foerste 1906) (Figs 3, 10). Locally, the unit may contain a few discontinuous beds of dolomitic mudstone and wackestones less than 3-cm thick, and rare fossils (McFarlan 1943; Rexroad & Kleffner 1984). The unit is approximately equivalent to the Sodus Shale of Sequence II in New York (Brett et al. 1990, 1998) and appears to represent the early Telychian Silurian highstand 3 of Johnson (1996, 2006, 2010) and Johnson et al. (1998), which reflects a glacio-eustatic highstand between two South American glaciations (Grahn & Caputo 1992; Caputo 1998; Ettensohn & Brett 1998) (Fig. 3). The Waco-Lulbegrud transition also coincides with the Valgu Event (Fig. 3). Deposition of the unit probably occurred in a deep open marine setting, generally below storm wave base, where the soft, muddy substrates and dysoxic conditions were not conducive for benthic community development. A thin, iron-stained clayey unit containing an abundance of smectitic clays and embayed phenocryst-like quartz grains of probable volcanic origin (Folk 1968) was recently found in the upper Lulbegrud Shale about 15-cm below the contact with the Waco Dolostone on KY Highway 52 near Drowning Creek; locally, it is called the Bybee bentonite (Fig. 10).

Waco Dolostone Member. The Waco Dolostone (Foerste 1906), erroneously known as the “Dayton Dolostone Member” (Foerste 1935; McDowell 1976) in northeastern Kentucky and the “Dayton Formation” northward into Ohio (e.g. McLaughlin et al. 2008; Kleffner et al. 2012a), consists of a basal dolostone (Figs 3, 10), 0.2–1.0 m thick, grading upward into a 2–5 m-thick interval of discontinuous, fossiliferous dolostone beds (up to 5-cm thick) interbedded Figure 10. Exposure on KY State Route 52, east of Bybee, Estill with greenish-grey clay shales. The fine-grained dolostones County, Kentucky, showing the Lulbegrud and Waco members of probably represent former wackestones and packstones the Crab Orchard Formation. The Waco is unconformably overlain deposited as tempestites and have been intensely bioturbated. by the Middle Devonian Boyle Dolostone so that only the lower In northeastern Kentucky, the upper interval of discontinuous Waco Dolostone is present. beds disappears, generating an unconformity between the lower Waco and the overlying Estill Shale. Hence, in most Hallopora are also present (Foerste 1931). The environment places throughout eastern Kentucky only the thin basal has been interpreted as a shallow open marine, muddy, Waco dolostone (McDowell 1976, 1983) (Fig. 10) or a carbonate substrate with a fauna very similar to the thin, pelletal glauconite bed equal to the Dayton Dolostone Stricklandia Community of Ziegler et al. (1968) and Cocks in Ohio (see Sullivan et al. 2012) separates the Lulbegrud & McKerrow (1978) or B.A. 4 of Boucot (1975, 1999). and Estill shales. Both the dolostones and shales are very The Oldham has been interpreted as a cratonic, lowstand fossiliferous, but the fossils appear to reflect a largely new equivalent of early transgressive parts of Silurian Sequence fauna dominated by corals. In particular, the unit can be II of Brett et al. (1990, 1998)(Fig. 3), inferences from which distinguished from other Crab Orchard dolostones by the have recently been used for possibly equivalent units in Ohio presence of the rugose coral Arachnophyllum, and at least and Kentucky (McLaughlin et al. 2008). The few conodonts 18 other coral species, nine brachiopod species (including known from the unit and brachiopod distribution suggest that pentamerids), two gastropod species, three trilobite species, it could be as young as mid-Aeronian and reflect uppermost and two delicate bryozoan species characterise the unit parts of the Distomodus kentuckyensis Zone (Rexroad 1967; (Foerste 1906, 1931); this fauna probably reflects B.A. 3 Nicoll & Rexroad 1968). If so, the Oldham is probably of Boucot (1975, 1999). Foerste (1906, 1931) originally equivalent to parts of the Reynales Limestone in Sequence thought that the unit was approximately equivalent to the II of New York (Brett et al. 1990, 1998; McLaughlin et al. Rochester Shale of New York, but that the fossils showed 2008). Like the Reynales, it represents shallower facies greater affinities to a fauna in Gotland, Sweden, than to than adjacent shales, but the sharp basal contact probably those in the Rochester Shale. The Waco is now equated represents the sea-level drawdown associated with glaciation with the Merritton Limestone in New York, which is now on South America (Grahn & Caputo 1992; Caputo 1998), associated with parts of transgressive systems tract III (Fig. whereas the fossiliferous dolostones of the Oldham itself 3). The unit is interpreted to reflect a shallow open marine, appear to represent the subsequent initial transgression. muddy-bottom environment that becomes deeper open Goodman & Brett (1994) suggested local tectonic influences marine with time; water depth, however, apparently never for the equivalent sequence in the northern Appalachian exceeded storm wave base. Basin. The Waco and its equivalents appear to represent a mid- Telychian (Cramer 2009), lowstand carbonate environment 172 AAP Memoir 44 (2013)

Figure 11. Schematic east-west section across the Cincinnati Arch area, showing projected stratigraphic relationships among Lower and medial Silurian units. Note the regional truncation of units from east to west during Waco and “Dayton” deposition, which suggests uplift in the Cincinnati Arch area related to a possible Salinic tectophase (adapted from Lukasik 1988). Undulating lines represent unconformities. that deepens upward over a short interval; however, its Rockway continuum to the east. deposition is not easily explained by a Silurian glacial event on South America (Grahn & Caputo 1992; Caputo 1998; Estill Shale Member. The Estill Shale Member (Foerste Ettensohn & Brett 1998) or by one of Johnson’s (2006, 1906) is the thickest member of the Crab Orchard Formation, 2010) sea-level highs (Fig. 3). Instead, it is equated with ranging from 18 to 51 m (McDowell 1983). The unit is the base of Silurian sequence III transgression of Brett et al. the most persistent and recognisable member of the Crab (1990, 1998) (Fig. 3). In the past, the Waco was associated Orchard and thickens to the east away from the Cincinnati with the base of the Silurian sequence IV transgression, Arch area. Lithologically, the Estill consists predominantly which was interpreted to mark the beginning of the Salinic of mudstones with thin, discontinuous beds of silty dolostone Orogeny and the convergence of Avalonia with Laurussia (Fig. 12). The mudstones are poorly fissile and medium- (Fig. 2). However, recent work suggests that sequence IV to light-grey in colour, but toward the north a number of in this part of Kentucky actually begins near the base of the distinctive, maroon beds are present, which are probably Estill Shale with a pelletal glauconite that represents the related to periodic influxes of oxidised iron and iron-rich clays “genuine” Dayton Dolostone (Fig. 3) to the north (Sullivan from a source area to the east. Clay mineralogy indicates that et al. 2012). In fact, the interbedded dolomitic limestones the Estill mudstones are composed of up to 89% mixed-layer and shales of the upper Waco are separated from the main illite-smectite, and when wet the mudstones expand to form a Estill Shale by this glauconite-rich bed that has been equated popcorn-like crust, all of which suggests that the mudstones with the Dayton (Sullivan et al. 2012; Kleffner et al. 2012a). contain a substantial amount of expandable clays that reflect The Waco is supposedly underlain by a late Telychian a former bentonitic component (Lierman & Manley 1991; erosion surface in the east that opens up on the western Mason 2002). Similarly, McLaughlin et al. (2008) reported outcrop belt as a subtle disconformity or regionally angular thin bentonites from the Estill Shale in southwestern Ohio. unconformity (Lukasic 1988; Brett et al. 1990; Goodman Higgins (1992) examined the maroon and grey shales & Brett 1994; Ettensohn 1992a, 1994; Ettensohn & Brett for differences in macrofossil content and their mode 1998; McLaughlin et al. 2008) (Fig. 11). Although there is of preservation. However, no significant differences little doubt about disconformable relationships in the west were observed in either macrofossil diversity or mode (Rexroad 1967, 1980; Nicoll & Rexroad 1968; Rexroad & of preservation. The shales are heavily bioturbated to Kleffner 1984), evidence for the unconformity in the east completely homogenised (6 on the ichnofabric Index of is less definitive (Fig. 10) as both the Lulbegrud and lower Bottjer & Droser 1994); thus, physical sedimentary structures Waco occur within the same conodont zone (Pterospathodus and discrete trace fossils are rare to absent. The thin (less than eopennatus Zone). Typically, a disconformity that becomes 1 to 5 cm) dolostone beds are silty dolomicrites that are finely regionally angular would suggest a tectonic origin. This crystalline and slightly calcareous with a medium-grey fresh could mean that the Waco and its sequence III equivalents colour; these beds have sharp erosional bases which grade elsewhere in the Appalachian Basin represent an early, upward into overlying shales. Their soles exhibit preserved unrecognised Salinic tectophase, a period of uplift across the tool marks, while the tops have a hummocky appearance. broad Cincinnati Arch accommodation zone (Fig. 2) and/or Internal sedimentary structures (as well as fossils) have a previously unrecognised transgressive sequence between mostly been destroyed by dolomitisation; however, a few Johnson’s (2006, 2010) sea-level highstands 3 and 4 (Fig. beds show fine parallel laminae as well as rare, wavy or 3). Although none of Johnson’s sea-level curves show a hummocky cross laminae (Ettensohn et al. 2009). similar mid-Telychian transgressive event, shallowing and McFarlan (1943) reported that the Estill was relatively accompanying transgression are shown on the global sea- unfossiliferous, whereas Foerste (1931) reported that the level curve of Haq & Schutter (2008). Whatever the origin upper 30 ft (9.1 m), which he called the Ribolt Member in of this thin Waco sequence, it represents a brief return to northernmost parts of the Kentucky outcrop belt (Lewis shallow, open marine environments (B.A. 3) and a short- County), contained abundant trilobites (Calymene and lived transgression, before major Salinic subsidence set the Trimerus) and fossils belonging to the Mastigobolbina typus stage for the deep, basinal environments in the overlying (ostracod) fauna. In fact, along most of the Crab Orchard Estill Shale (Fig. 3) and its equivalents in the Williamson- outcrop belt, the Estill is poorly exposed and surface finds of AAP Memoir 44 (2013) 173

Figure 12. Badlands topography developed on the thick Estill Shale at Knob Lick near Peasticks, Kentucky. fossils are rare. However, a few natural outcrops occur in the (Kammer et al. 1986). form of badlands topography in Bath and Lewis counties (Fig. Overall, the Estill Shale is interpreted to represent deeper, 12), and in these thicker natural exposures, fossils weather open marine deposition at or below storm wave base. The out. Moreover, it should be noted that fossils on the surface interspersed dolostone layers contain storm-related features of these exposures are not readily apparent because most are and are interpreted as distal storm deposits or tempestites juveniles of very small size. Both the micro- and macro-fauna (Mason et al. 1992a; Mason 2002; Ettensohn et al. 2009). collected from the Estill Shale are representative of an open The Estill Shale represents the deepest of early Silurian marine, dysoxic fauna. The common macro-invertebrates environments present in either Kentucky outcrop belt (Fig. include brachiopods, bryozoans, crinoid columnals, corals, 3). It also reflects the deepest part of sequence IV of Brett gastropods, bivalves and trilobites. Rare forms include et al. (1990, 1998) and reflects highstand 4 on the master nautiloids and blastoids. These fossils are commonly curves of Johnson (1996, 2006) and Johnson et al. (1998). preserved as pyrite replacements, moulds or carbonate The uppermost parts of the unit, the Ribolt member of Foerste recrystallisation. Also, complete graptolites, preserved by (1931), if it is included in the Estill, apparently represents carbonisation, have been recovered from the Estill (Mason the final regression of the falling stage before the lowstand et al. 1992a). Microfauna commonly includes agglutinated at the base of sequence V and the initiation of highstand 5 foraminifera, conodonts, ostracods, scolecodonts and (Fig. 3). This Llandovery–Wenlock transition between the chitinozoans. The most abundant macrofossils collected Estill and Bisher, between sequences IV and V, and between from the Estill Shale are brachiopods, bryozoans and highstands 4 and 5 not only occurs at a time of major crinoids, which are sessile filter-feeding forms rather than Gondwanan glaciation (Caputo 1998), but also coincides the vagile, scavenging or predatory forms like gastropods with the Ireviken Event (Fig. 3). and cephalopods that traditionally dominate Devonian and Highstand 4, which coincides with nearly all Estill post-Devonian dysoxic environments (Kammer et al. 1986). deposition, most likely reflects a combination of glacio- Hence, the upper part of the Estill Shale is interpreted to eustatic and tectonic influences (Ettensohn & Brett 1998), represent a deeper-water, dysoxic environment (Mason et but because coeval Salinic tectonism was so proximal al. 1991). Evidence supporting this interpretation includes: and affected northern parts of the Appalachian Basin and 1) its low diversity for an open marine fauna; 2) the adjacent parts of the craton (Fig. 4), it is probable that diminutive, apparently juvenile state of the fauna; and 3) the tectonic subsidence greatly overprinted any glacio-eustatic preservation of most calcareous skeletons with pyrite or as contribution. In fact, the Mastigobolbina typus ostracods pyritic internal moulds. This fauna may represent the earliest and conodonts (Cramer 2009) show that at least the upper reported dysaerobic fauna, but shows major differences in Estill Shale (Ribolt Member) is equivalent to the Williamson- both dominance and trophic structure compared to faunas Rockway succession of New York in the Appalachian Basin of Devonian and post-Devonian dysoxic environments (Fig. 4), which is reported to represent tectonic subsidence 174 AAP Memoir 44 (2013) related to the first Salinic tectophase (Ettensohn 1994, 2008; in which individual beds (proximal and distal tempestites) Goodman & Brett 1994; Ettensohn & Brett 1998). Hence, formed below normal wave base as storm scours washed highstand 4 in both New York and Kentucky probably reflects out bottom faunas and associated sediments in waning- a major Salinic tectonic component of subsidence, and the flow conditions. The faunal diversity suggests comparison coincident ash-fall sedimentation (bentonitic shales) in the with the Clorinda and Stricklandia communities of Ziegler Estill supports a volcanic influence and tectonism to the (1965), Ziegler et al. (1968) and Anderson (1971), but the east. Late Telychian transgression or highstand 4 marks the community may be more comparable with the Coelospira- largest of the Silurian global sea-level rises (e.g. Johnson PacificocoeliaCommunity (B.A. 5) of Boucot (1982, 1999) et al. 1998; Calner 2008; Johnson 2010) (Fig. 3), and the in its environmental setting. The gradational nature of this tectonic regime in the Appalachian and central Kentucky Bisher facies with the underlying Estill Shale suggests areas clearly overprinted and accentuated that deepening. overall shallowing-upward conditions from deep basinal The very scant, diminutive faunas found in the Estill conditions in the Estill to shallower storm-shelf conditions seem to indicate community conditions deeper than those in the lower Bisher. recognised by Ziegler (1965), Ziegler et al. (1968) and The next youngest Bisher lithofacies has been called Anderson (1971). Although not truly a recognised benthic the sandbelt lithofacies by Mason et al. (1992b) and is the community, these faunas reflect relatively deep-water most typical of the Bisher (McDowell 1983). Although assemblages similar to the “pelagic community” or B.A. 6 the lithofacies always occurs in erosional contact with the of Boucot (1975) and the “graptolite assemblage” of Cocks underlying tempestite lithofacies, this contact is interpreted & McKerrow (1978). as representing a brief lacuna generated by the high-energy, erosional nature of the sandbelt facies and does not represent Bisher Dolostone a substantial amount of time. Although McLaughlin et al. The Bisher Dolostone was named by Foerste (1917) for (2008) indicate the presence of a widespread disconformity exposures in west-central Ohio, and by 1931, Foerste had at this boundary in Ohio, we suggest that the break in begun using the term for rocks in northeastern Kentucky Kentucky represents a minor discontinuity and is effectively at a similar stratigraphic position, above the Crab Orchard “gradational” as suggested by Foerste (1906, p. 60) and Shale but below Devonian dolostones or shales. The outcrop McDowell (1983). belt from Ohio can only be traced about 25 km to pinchout The sandbelt facies is characterised by medium to thick in northeastern Kentucky, but the unit reappears as thin beds of bioclastic, arenaceous dolarenite and bioclastic erosional remnants below the Devonian Wallbridge and dolomicrite that probably represented former packstones and Taghanic unconformities (Fig. 3) up to 50 km farther south grainstones. Internal stratification includes parallel laminae, in east-central Kentucky (Hoge et al. 1976; McDowell trough cross-laminae and hummocky cross-laminae; also 1975). The unit attains its maximum thickness of about 30.5 present are scours, rip-up clasts, fossil lags, ripple marks, m along the Ohio River in northeastern Kentucky (Peck & and crude graded bedding. An open marine fauna is present Pierce 1966), whereas erosional remnants to the south vary throughout, but is especially abundant in the form of broken from 2.1 to 18.3 m thick (Hoge et al. 1976; McDowell 1975). and abraded fragments about 3 m above the base of the unit The Bisher is probably the most poorly understood of in what has been called variously the Whitfieldella bed, the Silurian units in Kentucky, as internal stratigraphic Cryptothyrella bed, Cryptothyrella lithofacies or Meristina relationships and relationships with the underlying Estill Community (e.g. Foerste 1931; McFarlan 1943; McLaughlin Shale are not everywhere certain. In northern Kentucky, et al. 2008). The brachiopod Meristina cylindrica is the Bisher has been reported to exhibit three different especially abundant, but other faunal elements include lithofacies, although it is likely that some of the lithofacies tabulate and rugose corals, stromatoporoids, bryozoans, other mapped as Bisher in Kentucky may actually be equivalent brachiopods, nautiloids, as well as disarticulated crinoid and to two or more different stratigraphic units in Ohio to the trilobite debris. Trace fossils are rare and poorly developed. north (e.g. McFarlan et al. 1944; Peck & Pierce 1966). The predominance of coarse-grained, bioclastic sediments, The stratigraphically lowest of these lithofacies, called the an open marine fauna, and the noted sedimentary structures tempestite lithofacies by Mason et al. (1992a), is gradational suggest a high-energy sandbelt or shoal environment with the underlying Estill Shale Member of the Crab with tidal influence at or above wave base. The fauna Orchard Formation; this facies was probably included by was probably similar to the Pentamerus Community of Foerste (1931) in his Ribolt Member of the Estill Shale. This Ziegler (1965), Ziegler et al. (1968) and Anderson (1971) lithofacies crops out in the Charters area of Kentucky (Morris or the Meristina Community (B.A. 3) of Watkins & Boucot 1965) and includes 6.1–9.1 m of thin- to medium-bedded, (1975) and Boucot (1999). This environment indicates the argillaceous dolomicrites to dolarenites (former wackestones further shallowing of Bisher seas, perhaps to the point that and packstones) interbedded with light-grey clay shales. underlying storm-shelf sediments were eroded away in the Each dolostone bed has an erosional base with tool marks and resulting high-energy conditions. commonly exhibits parallel laminae or small-scale trough The third Bisher lithofacies has been called the biostromal cross laminae, crude graded bedding, rip-up clasts, fossil lithofacies by Ettensohn et al. (1992) and occurs in a single lags, ripple marks, gutter casts or hummocky crossbedding. exposure with no clear relationship to the other Bisher Bryozoans, brachiopods, cephalopods, conulariids, lithofacies. It consists of seven fining-upward cycles, each gastropods, bivalves, crinoid debris and trilobite fragments about a metre thick, and each containing up to a dozen occur largely as transported fauna at the bases of beds or as individual layers of overturned and broken stromatoporoids local communities on top of the beds. Trace fossils belonging along with fragmented tabulate and solitary rugose corals, to the Cruziana ichnofacies are abundant and may include and trepostome bryozoans in a muddy matrix with differential forms like Bifungites, Chrondites, Cruziana, Diplocraterion, compaction and bioturbation. Fragmented brachiopods, Fascifodina and Rusophycus. The body fossil and trace fossil gastropods, crinoids and trilobites are also present, but they assemblages suggest a deeper to shallow, open marine setting were apparently transported into a low-diversity community AAP Memoir 44 (2013) 175 dominated by a single stromatoporoid species, a single 1998) in which the Rochester Shale represents the major tabulate species, and a single trepostome bryozoan species, phase of tectonic subsidence and culmination of tectophase very similar to the Coral-Stromatoporoid Community of transgression (Goodman & Brett 1994; Ettensohn 1994; Johnson (1980) and the Laminar Stromatoporoid Community Ettensohn & Brett 2002). The middle Bisher is correlated (B.A. 3) of Boucot (1975, 1999). Each of the smaller layers with the Rochester, based on fauna (Foreste 1931) and begins with dolomitic fossil-fragment rudstones at the base, sequence stratigraphy (Brett et al. 1998; McLaughlin et al. grades upward into floatstones, and finally into mudstones. 2008). The relative abundance of muds, muddy matrix, and the If a Lilley equivalent is present in the Bisher of restricted fauna suggest that this facies may represent a northeastern Kentucky, based on conodonts from the unit platform-lagoonal setting that was repeatedly disrupted in Ohio (Cramer et al. 2010), it would probably represent by large tropical storms. Exactly how this facies relates to middle Sheinwoodian (Sequence VI) deposition during the the others is uncertain, but it could represent a protected brief regressive event in the middle of Silurian transgression lagoonal facies behind the sandbelt noted above, or it more 5 of Johnson (1996, 2006, 2010) and Johnson et al. (1998). likely represents a completely different unit separated Possible Peebles equivalents, on the other hand, would from underlying Bisher facies by an unconformity. In fact, likely represent middle to late Sheinwoodian (Sequence VI) the unit is very similar to the Lilley Dolostone described deposition (McLaughlin et al. 2008) during the second of the from the subsurface in eastern Kentucky by McFarlan smaller highstands that comprise Silurian transgression 5. (1938, 1943) and McFarlan et al. (1944) and to parts of the Although upper Wenlock, Ludlow and Pridoli rocks are Lilley described from Ohio (McLaughlin et al. 2008). The preserved in the subsurface of eastern Kentucky (e.g. Freeman presence of Lilley equivalents in the Bisher of Kentucky 1951; Currie 1981), units above the lower Wenlock Bisher has also been suggested by Peck (in McDowell 1983, p. Formation have been eroded away on the eastern outcrop 17). Our preferred explanation is that the biostromal Bisher belt. Even the Bisher itself has been substantially thinned lithofacies is a partial Lilley equivalent, which is probably along intra- and post-Silurian unconformities, removed separated from the Bisher by an unconformity (Fig. 3). If across a broad area in east-central Kentucky, and shows signs so, this unconformity is likely the same one that marks the of condensation and abnormal shallowing in the few known basal Lockport Group (Sequence VI) in New York (Brett & exposures. Inasmuch as most Bisher exposures occur near Ray 2006), and this is the first reported exposure of Lilley- basement faults associated with the Ohio-Kentucky border equivalent rocks in eastern Kentucky. fault, the Waverly Arch basement fault, or faults associated According to Foerste (1931), based on included faunas, with the Rome Trough (Figs 1, 5), we suggest that Silurian equivalents of the Peebles (Foerste 1929) and Greenfield and post-Silurian reactivation of these structures must have (Orton 1871) dolostones also occur in Lewis County, influenced unit deposition and subsequent preservation. Kentucky, on and near the Ohio River (Fig. 3). If they are Ordovician, Devonian and Carboniferous movements along present, they are currently included in the mapped Bisher these structures are well known and documented (e.g. Dever Formation, and the present lithologies are of the correct type et al. 1977; Ettensohn et al. 1988, 2002; Ettensohn 1992c), and thick enough to include at least a Peebles equivalent and it is likely that far-field forces generated by the Salinic (Morris 1965, 1968; Morris & Pierce 1967). As for the orogeny to the east similarly reactivated these structures. Greenfield equivalent, Foerste (1931) reported a single However, it was probably Acadian reactivation of these exposure along the bank of the Ohio River at Vanceburg, structures that influenced most of the erosion from above, Kentucky, that contained abundant Leperditia ohioensis, and in some cases, the local preservation of Silurian units. ostracods typical of this unit in Ohio. Based on current mapping in the area (Morris & Pierce 1967), this exposure WESTERN KENTUCKY OUTCROP BELT is no longer present. Clearly, more work will be required Brassfield Formation to determine if equivalents of either unit are present in the The Brassfield Formation in western Kentucky outcrop belt Bisher Formation of Lewis County, Kentucky. (Fig. 1) appears to be the same reddish-brown dolostone Bisher facies are everywhere shallower than those that characterises the Brassfield in the eastern outcrop belt of the underlying Estill Shale, and based on conodonts (Foerste 1906; Peterson 1981; McDowell 1983); however, (Kleffner 1987; McLaughlin et al. 2008; Cramer 2009), the western Brassfield is glauconitic throughout, contains apparently represent an early Wenlock (early to early-middle almost no shale and is in part younger than that in the east. Sheinwoodian) sea-level lowstand that reflects the onset of Except for the uppermost Lee Creek Member of the Silurian transgression 5 (Johnson 1996, 2006; Johnson et Brassfield of the western outcrop belt, all underlying al. 1998). Inasmuch as Bisher deposition largely coincides Brassfield units appear to occur within the Distomodus with a period of glacio-eustatic lowstand (Johnson 1996, kentuckyensis Zone (Rexroad 1967; Nicoll & Rexroad 2006; Johnson et al. 1998) that followed the last Silurian 1968), meaning that the “western” Brassfield (excluding (early Wenlockian) glaciation in the Amazon and Paraná the Lee Creek Member) could represent all of the “eastern” basins of South America (Grahn & Caputo 1992; Caputo Brassfield and lower parts of the Crab Orchard Formation 1998; Ettensohn & Brett 1998), glacio-eustasy may well be through the Oldham Member. In effect, the Brassfield on involved in this lowstand. However, early Bisher deposition, the western flank of the Cincinnati Arch area appears to largely equivalent to deposition of the Irondequoit Limestone represent a condensation of some of the units included in in the northern Appalachian Basin (McLaughlin et al. 2008), the Brassfield-Waco section on the eastern side of the arch is also coeval with initiation of the second Salinic tectophase (Fig. 13). In an attempt to correlate units from one side of to the east, and the Bisher shallow-water carbonates on top the arch to the other, Andrews (1997) used stratigraphy, of a regional disconformity may similarly reflect bulge palaeontology and sedimentology to determine depositional moveout and initiation of transgression during the tectophase environments and sequences in the “western” Brassfield (Fig. (Ettensohn & Brett 2002) (Fig. 3). In fact, this tectophase 6). Using the unconformable base of the Lee Creek Member, is represented by Silurian Sequence V of Brett et al. (1990, an equivalent of the Waco Member in the eastern section 176 AAP Memoir 44 (2013)

Figure 13. Schematic north-south cross section along the west- central Kentucky Silurian outcrop belt, showing relationships among Brassfield lithofacies relative to structures (adapted from Andrews 1997).

(Nicoll & Rexroad 1968), as a datum, he then correlated time) to fully onlap the accommodation zone. Whatever sequences downward in the section from the western outcrop happened with early Silurian seas during the time represented belt to the eastern belt (Fig. 6). This correlation suggests by sequence A on the western outcrop belt, it happened that the grainstone unit, or sequence C, is equivalent to the relatively fast (3–4 Ma) and wholly within the Distomodus Oldham–Lulbegrud continuum and represents Sequence II kentuckyensis Conodont Zone. and transgression 3 in Figure 3. Biostratigraphy certainly Dolomitisation in the west has been generally more does not preclude the equivalence of the grainstone unit pervasive and intense, so that most fossils and depositional with the Oldham, although nothing currently supports the textures have been obscured. Nonetheless, four lithofacies presence of a Lulbegrud equivalent in the grainstone unit. have been distinguished (Figs 6, 13), and it is apparent that the Either the Lulbegrud equivalents were eroded away below distribution of these facies was influenced by synsedimentary the Lee Creek Member, or they have not yet been sampled. structural activity on the Servant Run Lineament, Bardstown Similarly, moving down section, the sequence represented Monocline and Lyndon Syncline (Andrews 1997) (Figs 1, by the lower massive-vuggy continuum (sequence B) in 14). Each lithofacies is briefly discussed below. the western outcrop appears to correlate with the upper massive-Plum Creek sequence continuum (part of Sequence Lower massive lithofacies. This lithofacies is 0–2.4 m in I; transgression 2) on the eastern outcrop (Fig. 6). Underlying thickness and consists of laminated to massive, light-grey Silurian sequences, however, appear to be absent on the to light-brown, argillaceous and glauconitic dolosiltite to western outcrop belt, as there appears to be no equivalent dolarenite. Regular and irregular laminae are present and to sequence A, the lower four units of the Brassfield on the some of these have been phosphatised. The unit is intensely east (Fig. 6). Although we cannot exclude the presence of bioturbated, but body fossils are sparse, including a few a condensed sequence A in the lower massive unit of the solitary rugose corals, thin-shelled brachiopods, and crinoid western Brassfield, this seems unlikely in the few metres debris. For the most part, the unit is preferentially preserved of uniform section available. This situation suggests south of the Bardstown Monocline in what appears to have three possibilities: either sequence A was deposited in been a small basin between the Bardstown Monocline and the west and then removed, possibly due to uplift-related Servant Run Lineament (Fig. 13). erosion; sequence A is represented wholly by a very thin, The presence of laminae, intense bioturbation, glauconite phosphatic omission facies or transgressive ravinement and phosphorite suggests slow sedimentation in a quiet, that occurs locally at the base of the Brassfield (Laferriere somewhat restricted marine environment. The scarcity et al. 1986; Andrews 1997); or that an uplifted Cincinnati of fauna may reflect some degree of restriction, as in the Arch accommodation zone (Fig. 5) created a barrier to Eocelia Community (Ziegler 1965; Ziegler et al. 1968; transgressing early Silurian seas such that it took westwardly Anderson 1971) or B.A. 2 (Boucot 1975), or may be a transgressing seas 3–4 Ma (late Rhuddanian–early Aeronian preservation effect of intense dolomitisation. Erosional AAP Memoir 44 (2013) 177

Figure 14. Schematic cross section along the central Kentucky Silurian outcrop belts, showing inferred relationships between Brassfield lithofacies on the east (Gordon, 1980) and those on the west (Andrews 1997); lm=lower massive unit; v=vuggy unit (adapted from Andrews 1997). remnants of the unit to the north suggest that it was deposited (Fig. 14). It consists of yellowish-white to grey calcarenites across the entire area, and subsequently eroded north of and calcisiltites in the north and light-brown, cherty the Bardstown Monocline (Fig. 13). Sequence stratigraphy dolarenites with shale partings in the south. Glauconite, (Fig. 6) suggests that the unit is equivalent to the upper bioturbation, ripples, intraclasts, cross bedding and laminae massive and Plum Creek lithofacies on the eastern outcrop are present throughout the unit. Small solitary rugose corals, belt (Andrews 1997). Together, these indicators suggest that bryozoans, small brachiopods and crinoid debris are locally the unit represents a deepening-upward, platform-lagoonal common. The unit is interpreted as representing a high- deposit that was restricted to the west of a rising Cincinnati energy skeletal sandbelt environment, largely at or above Arch and/or a carbonate sandbelt equivalent to the upper wave base with the finer grained constituents reflecting massive lithofacies to the east. Uplift on faults controlling slightly deeper, intervening lagoons between the migrating the Bardstown Monocline and Lyndon Syncline before shoals, and in the case of the southern part of the area where deposition of the next unit, resulted in erosion of this unit finer-grained materials are more abundant, deepening- from northern parts of the outcrop belt (Fig. 14). upward conditions. Fossils probably reflect thePentamerus or Stricklandia communities of Ziegler (1965), Ziegler et al. Vuggy lithofacies. This lithofacies occurs only south of the (1968) and Anderson (1971) (BA 3 or BA 4 of Boucot 1975, Bardstown Monocline and appears to infill a small basin 1999). Biostratigraphy and sequence stratigraphy suggest between the Servant Run Lineation and the Bardstown that this lithofacies is equivalent to the Oldham Member and Monocline (Fig. 13). The unit is 0–1.8 m thick and consists perhaps parts of the Lulbegrud Member of the Crab Orchard of glauconitic, greyish-orange to olive-grey, sharp-edged Formation on the eastern outcrop belt (Andrews 1997) to indistinct dolosiltite clasts in a porous, vuggy matrix (Figs 6, 14). Despite the transgressive, deepening-upward of dolosiltite to dolarenite. Ripples, laminae and fossils conditions indicated by the equivalent Oldham-Lulbegrud are generally sparse. The unit matrix suggests low-energy continuum, these conditions are only suggested in southern conditions, whereas the clasts apparently reflect a high- parts of the grainstone lithofacies. The fact that the grainstone energy influence. Our preferred origin for the unit suggests facies appears to reflect largely high-energy lowstand that the clasts reflect erosion of the lower massive lithofacies conditions throughout deepening-upward conditions to the from the northern part of the outcrop belt beyond the east and south, again suggests persistent synsedimentary Bardstown monocline, prior to high-energy, lowstand uplift on faults associated with the Bardstown monocline. conditions associated with the grainstone lithofacies, and transportation into a small basin between the Servant Run Lee Creek Member. The Lee Creek Member (Nicoll & Lineation and Bardstown Monocline (Andrews 1997) (Fig. Rexroad 1968) is a greenish-grey to brownish-grey, cherty, 13). This erosion and removal of early Brassfield sediments glauconite-rich dolosiltite, 0.3–2.5 m thick. Fossils are north of the Bardstown Monocline also implies likely generally scarce, but their preservation locally in chert structural movement on faults associated with the Bardstown indicates the presence of bioturbation, brachiopods, rugose Monocline. Sequence stratigraphy suggests that the unit is at corals, and crinoid debris. The unit preserves intraclasts at least partially equivalent to the Plum Creek Shale Member the base, and laminae, as well as localised mudcracks and on the eastern outcrop belt (Figs 6, 14). crossbeds. Biostratigraphy and sequence stratigraphy equate the Lee Creek Member with the basal Waco Member in the Grainstone lithofacies. The grainstone lithofacies occurs east (Rexroad 1967, 1980; Rexroad & Kleffner 1984) and throughout the area and ranges in thickness from 0.3 to 3.0 m indicate that it overlies the grainstone facies in the west with 178 AAP Memoir 44 (2013) a disconformity or subtle, regionally angular unconformity The original Osgood of Foerste (1897) is largely equivalent (Lukasik 1988; Andrews 1997; McLaughlin et al. 2008) to the upper Estill Shale as well as to the lower Bisher and (Figs 6, 11). possibly Lilley equivalents in the east (McLaughlin et al. As with the Waco Member, the unconformity below 2008; Cramer 2009; Kleffner et al. 2012a; Brett et al. 2012) the Lee Creek may reflect far-field bulge moveout and (Fig. 3). The major unconformity between the Osgood and uplift associated with an earlier, previously unrecognised the Lee Creek Member of the Brassfield (Figs 3, 6, 11) Salinic tectophase or broad uplift across the Cincinnati apparently represents a period of uplift and erosion related Arch accommodation zone. Also like the Waco, the unit is to initiation of the first Salinic tectophase, and the fact that a lowstand to early transgressive carbonate that reflects the the erosion was so widespread and long may reflect uplift initiation of subsidence. The Lee Creek apparently represents across the entirety of the broad Cincinnati accommodation a progression of high- to low-energy environments, ranging zone. Parts of the Osgood below the upper carbonate from near exposure to deeper open marine environments, appear to represent regional subsidence and transgression in a very small package. The unconformity at the top of the associated with the first Salinic tectophase and highstand Lee Creek (Figs 3, 11) apparently represents a transgressive 4 of Johnson (1996, 2006) and Johnson et al. (1998) (Fig. surface and a time of sediment starvation as sediments from 3). Major subsidence occurred early in Osgood deposition the east made their way across the Cincinnati Arch region. and is represented by the lower carbonate and lower shale Brett et al. (2012) elevated the Lee Member in southeastern subunits, which reflect deeper, open marine environments Indiana and adjacent parts of Kentucky from member status with muddy, unstable substrates. Bioturbation was intense, in the Brassfield to formational status. and along with the soft substrates, probably did not allow widespread colonisation by epifaunal benthos. Osgood Formation The unconformity that separates early and late (upper The Osgood Formation (Foerste 1897; Butts 1915) carbonate and shale subunits) Osgood deposition apparently disconformably overlies the Brassfield Formation (e.g. Butts reflects sea-level drawdown related to glaciation and 1915) (Figs 3, 6, 11), except in the area of Ripley Island (Fig. inception of the second Salinic tectophase (Fig. 3). Hence, 1) where the Brassfield is absent. As originally defined, the the upper Osgood represents initiation of Sequence V and formation consists of 5.0–15.0 m of grey and maroon clay transgression 5 and is equivalent to the lower Bisher in shale or mudstone and light-grey, argillaceous dolosiltite, in eastern Kentucky, as well as to the Irondequoit and parts four persistent, cyclic subunits: a lower carbonate, a lower of the Rochester, and perhaps overlying units, in New York shale, an upper carbonate and an upper shale, all of which are (McLaughlin et al. 2008; Kleffner et al. 2012a). The initial sparsely fossiliferous. Later, however, Foerste (1917, 1929, decline in sea level at the beginning of Sequence V seems 1935) presented several different interpretations about which to have permitted shallower, open marine environments of these subunits were included in the Osgood and overlying with coarser, firmer substrates to develop, so that faunas Laurel formations, and even the U.S.G.S. in mapping western similar to the Stricklandia Community (B.A. 4) of Ziegler Kentucky included parts of Foerste’s original Osgood within (1965), Ziegler et al. (1968) and Anderson (1971) became the Laurel (Peterson 1981). Cramer (2009), Kleffner et locally common during late Osgood deposition. Osgood al. (2012a) and Brett et al. (2012) resolved many of these deposition coincides almost wholly with the Ireviken Event issues and suggested major changes in the nomenclature of (e.g. McLaughlin et al. 2012). this unit based on physical stratigraphy, biostratigraphy and chemostratigraphy in southwestern Ohio and southeastern Laurel Dolostone Indiana; although we will mention the suggested changes, The Osgood Formation grades upward across an interval of we will continue to use Foerste’s original definition for about a metre into the Laurel Dolostone (Foerste 1896), a Kentucky. bluish to light-grey, fine-grained, pervasive dolostone. The The Osgood is thickest south of the Bardstown Monocline unit varies in thickness from 10.6 to 19.8 m, with the greatest and thins northward, but all the maroon shales and mudstones thicknesses occurring south of the Bardstown Monocline. are found only in the lower shale unit, although the colour Fossils are scarce, but scarcity may reflect the pervasive is only present south of the Lyndon Syncline (Hendricks dolomitisation. Four informal lithofacies are recognised: 1996). The lower two subunits, which are included in the lower vuggy, quarrystone, upper vuggy and oolite the reconstituted Osgood Formation of Cramer (2009), (Hendricks 1996). Kleffner et al. (2012a) and Brett et al. (2012), are intensely bioturbated but show almost no evidence of a benthic Lower vuggy lithofacies. The lower vuggy lithofacies consists epifauna. Petrographic evidence suggests that they were predominantly of greyish-blue, finely crystalline, vuggy, largely carbonate and clay-rich mudstones. porous, massive fossiliferous dolostone (Fig. 15A). It ranges The upper carbonate, included in the Lewisburg in thickness from 4.9 to 7.6 m, and its most characteristic Formation of Cramer (2009), Kleffner et al. (2012a) and feature is the presence of light-grey, interconnected, dolomite- Brett et al. (2012), may unconformably overlie the lower lined vugs, 3 to 4 cm in width that occur in distinct horizons shale (McLaughlin et al. 2008) (Fig. 3). This carbonate up to metres in length. Intercrystalline, vuggy and mouldic exhibits wackestone and packstone textures and is relatively porosity are present throughout. Thin-section petrography fossiliferous, containing the brachiopods Atrypa, Meristina, indicates that argillaceous carbonate mudstones and Cryptothyrella, Leptaena, Dolerorthis and Eospirifer, wackestones are most common near the base and at the top along with rugose and tabulate corals, crinoids, bryozoans, of the unit, but that pelmatozoan packstones and grainstones gastropods and trilobites (B.A. 3). Even the upper shale, predominate throughout the remainder (Hendricks 1996). which was included in the Massie Formation of Cramer Crinoid and cystoid debris (Caryocrinites, Pisocrinus and (2009), Kleffner et al. (2012a) and Brett et al. (2012), Eucalyptocrinites) is the most common faunal element, contains fossils like these in thin dolostone lenses scattered and though less common, brachiopods like Meristina, throughout the unit. Dolerorthis, Atrypa, Antirhynchonella, Eospirifer, Leptaena, AAP Memoir 44 (2013) 179

Figure 15. Laurel lithologies: A. Lower Vuggy lithofacies with meter stick for scale; B. Quarrystone lithofacies with metre stick for scale; C. Close-up view of the Oolite lithofacies; diameter of coin is 23 mm.

Platystrophia, Plectatrypa, Protomegastrophia, Uncinulus Dolostone (McLaughlin et al. 2008; Cramer 2009; Kleffner and Whitfieldia are locally common. Dalmanitid trilobites, et al. 2012a; Brett et al. 2012) contains not only relatively rugose and tabulate corals, gastropods, cephalopods, abundant Gravicalymene celebra, but also abundant fenestrate and ramose bryozoans, and receptaculitid algae are pentamerid brachiopods that are lacking in the quarrystone also present, but not common. The unit has been interpreted lithofacies (Mikulic et al. 2012). This may suggest that parts as representing a shallow, open marine environment below of the Springfield in southwestern Ohio represent higher- normal wave base containing a fauna that is consistent with energy B.A.3 environments on the rim of the accommodation a B.A. 3–4 assignment. Hendricks (1996) has also shown zone compared with the lower-energy B.A. 4 environments that the unit contains facies variations apparently controlled recorded from the western flank of the accommodation zone by local structure. in western Ohio. The lower vuggy lithofacies is probably in large part The quarrystone represents the deepest-water lithofacies equivalent to the Euphemia Formation of Foerste (1917, in the Laurel and coincides with the late part of highstand 1929, 1935) or the Euphemia Member of the Laurel five of Johnson (1996, 2006) and Johnson et al. (1998); it Formation (McLaughlin et al. 2008; Cramer 2009; Kleffner probably reflects regional early Homerian subsidence and et al. 2012a; Brett et al. 2012) in southwestern Ohio and transgression that coincides with later parts of Sequence VI southeastern Indiana. The Euphemia occurs above the top (Goat Island Dolostone of New York) of Brett et al. (1990, of the Kockelella walliseri Zone (Kleffner et al. 2012a), 1998) and the second tectophase of the Salinic orogeny which means that it and the lower vuggy lithofacies represent (Fig. 3). latest Sheinwoodian deposition during Sequence VI, during or just after the minor regression superimposed on Silurian Upper vuggy lithofacies. The upper vuggy lithofacies highstand 5 and also approximately coincident with the Boge is composed of light-grey, finely crystalline, vuggy, Event (Johnson 2006; Calner 2008). porous dolostone. Petrographically, carbonate mudstones predominated throughout the unit, except near the top Quarrystone lithofacies. This lithofacies is represented by where former wackestones and packstones become more a relatively uniform, light-grey, finely crystalline dolostone common. Extensive burrow mottling has destroyed nearly (Fig. 15B) that is typically 4.6–6.7 m thick. Petrographically, all indications of bedding, and most sections of the unit the unit appears to have been composed largely of laminated weather to a single massive bed. Fossils are scarce, but carbonate mudstones and wackestones, but bioturbation brachiopods, gastropods and trilobites occur locally; the has been intense and destroyed most original structures. brachiopod Pentamerus is locally common south of the Stained firmgrounds occur locally throughout and chert Bardstown Monocline. The unit thickens toward the south is rare in the unit south of Indiana. Other than small and is not present north of the Lyndon Syncline. pelmatozoan fragments, fossils are generally rare, but a few This lithofacies is interpreted as representing a shallow, small brachiopods like Lingula, Atrypa, Antirhynchonella, open marine environment generally below normal wave Meristina and Plectatrypa, along with a few rare crinoids, base, but it occasionally may have experienced rough-water rugose and tabulate corals, bryozoans and dendroid conditions related to storms. The unit is part of a shallowing- graptolites also occur. The trilobite Gravicalymene celebra upward trend in parts of the Laurel above the quarrystone is locally common. This facies has been interpreted as lithofacies, and fauna from the unit are most closely related representing a deeper, open marine environment with to the Pentamerus Community of Ziegler (1965), Ziegler et probable equivalents in the Clorinda Community or with al. (1968) and Anderson (1971) (BA3). the Dicoelosia-Skenidioides Community (B.A. 4) of Boucot (1975, 1999). The overall scarcity of epifauna may reflect Oolite lithofacies. The oolite lithofacies consists of 0.5–1.8 m the soft substrates and intense bioturbation. In contrast, in of light- to medium-grey, finely crystalline oolitic dolostone southwestern Ohio, the quarrystone-equivalent Springfield (Fig. 15C). Locally, the unit may intertongue with the 180 AAP Memoir 44 (2013) overlying Waldron Shale. Petrographically, the unit is the the fauna is locally concentrated near the base of the unit. most diverse in the Laurel, exhibiting mudstones, skeletal The most common echinoderm fossils in the Waldron are and oolitic wackestones, packstones, and grainstones, the camerate crinoid Eucalyptocrinites and the cystoid biohermal boundstones and shales—at places all in the Caryocrinites. Bryozoans and gastropods occur locally and same exposure (Hendricks 1996). Common sedimentary some brachiopods, including Atrypa, various rhynchonellids features include current ripples, herringbone and hummocky and strophomenids, are also locally common. According to crossbedding, hardgrounds and intraclasts. This facies Feldman (1989), the Waldron represents a shallow, open also contains the most diverse and abundant faunas of all marine to deeper open marine depositional setting, with Laurel lithofacies in Kentucky. The most common fossil continued deepening toward the top of the unit, which may constituent is pelmatozoan debris, and the camerate crinoids contain a maximum flooding surface (Hendricks 1996). Eucalyptocrinites, Calliocrinus and Periechocrinites, Initial communities in the unit developed on hardgrounds the flexible crinoid Lecanocrinus, as well as the cystoid formed on the lithified upper surface of the Laurel Dolostone Caryocrinites, have been recognised. Up to seven species and included encrusting Eucalyptocrinites and auloporid of brachiopods are common near bioherm flanks, especially corals (Halleck 1973). However, with deepening, a soft- clusters of Coolinia subplana and Eospirifer eudora. substrate, muddy community began to develop, based on Tabulate and rugose corals, bivalves, bryozoans, sponges, the globose brachiopods, Sphaerirhynchia, Homeospira trilobites and receptaculitid algae are also present. Bioherms and Eospirifer, and the flat brachiopod Leptaena, all of are typically no more than a metre thick and are normally which served as attachment sites for crinoids and favositid contained wholly within the facies, but in a few localities they corals (Halleck, 1973). This soft-mud community has been may extend from the base of the lithofacies into the overlying equated with the Sphaerirhynchia Community of Cocks & Waldron Shale. The bioherms contain abundant fistuliporid McKerrow (1978) (BA4). Soft substrates and/or dysoxic and fenestrate bryozoans with pelmatozoans and brachiopods conditions may have created restrictive conditions during embedded in a dolomitic mud matrix; small muddy flank parts of Waldron deposition. beds intertongue with surrounding oolitic grainstones. In Tennessee (Cramer et al. 2006; Cramer 2009), and Many of the bioherms have been bored, indicating that they subsequently in Indiana and Ohio (Kleffner et al. 2012b), were already lithified before final burial. The bioherms are the double positive carbon isotope peak of the Mulde Event especially well developed near the Bardstown Monocline, was discovered in the Waldron Shale or in the equivalent where they form linear mounds and chains that parallel the Cedarville-Greenfield transition. This event largely coincides monocline, suggesting structural influence (Hendricks 1996). with highstand 5A (Johnson 2006; Calner 2008) and would The oolite lithofacies is best developed south of the be included in Sequence VI. Hence, the Waldron Shale is Lyndon syncline and thickens south of the Bardstown probably mid-Homerian in age (Fig. 3) and is probably Monocline. This lithofacies may unconformably overlie equivalent to the Greenfield Formation in southwestern Ohio the upper vuggy lithofacies and may be partially equivalent (Brett et al. 2012; Kleffner et al. 2012b). to the Limberlost Dolostone below the Waldron Shale in northern Indiana (Droste & Shaver 1976). Together with the Louisville Limestone upper vuggy lithofacies, the Oolite lithofacies represents an The Louisville Limestone (Foerste 1897) is the youngest early Homerian period of declining sea level and lowstand of exposed Silurian units in Kentucky. Butts (1915) between Silurian highstands 5 and 5A (Fig. 3); these two subsequently divided it into 17 lithostratigraphic units and facies are also largely equivalent to the Cedarville Dolostone gave them names based on the ledge terminology of the of southwestern Ohio (McLaughlin et al. 2008; Brett et al. old quarrymen, whereas Conkin et al. (1992) recognised 2012). Both lithofacies also coincide with inception of the an additional seven units below Butts’ 17 units. The Mulde Event (Johnson 2006; Calner 2008). Louisville is a largely regressive carbonate unit composed The presence of ooids and other high-energy sedimentary of light-grey, finely crystalline, thick-bedded dolostone structures indicate that this facies was largely deposited or dolomitic limestone that ranges in thickness from 12.2 in a well agitated, tidally influenced, migrating sandbelt, to 30.5 m. The greatest thicknesses of the unit, however, generally above normal wave base. The presence of already occur on the Louisville High (Fig. 1) and in the area of the lithified bioherms also suggests a high-energy setting, but this Servant Run Lineament; the unit thins over the Bardstown setting may also reflect structural uplift of the area on and Monocline. Compared to some of the previous Silurian near the Bardstown Monocline, on which the bioherms are units, the Louisville has a rather diverse and abundant best developed. The abundance of more muddy lithologies fauna dominated by corals and stromatoporoids. Most of in the facies suggests that the facies also included slightly the stromatoporoids have been completely dolomitised or deeper, shallow, open marine, as well as quiet-water inter- silicified, making identification difficult, but more than 60 shoal, environments. Boucot (1999) introduced the term species of corals have been identified (Butts 1915; Miller Janius Community for such a facies. 1919; McFarlan 1943). The fauna also includes 60 species of brachiopods, dominated by large forms like Pentamerus Waldron Shale and Conchidium, as well as crinoids, cystoids, gastropods, The Waldron Shale (Elrod 1883) conformably overlies the bryozoans, sponges, cephalopods and trilobites. Parts of the Laurel Dolostone and consists of 2.9–4.1 m of medium-grey, Louisville also contain bioherms up to 2 m thick. Phelps dolomitic clay shale with finely crystalline, argillaceous (1990) identified seven stacked lithofacies in the unit, of dolomitic mudstones intertonguing near the top and bottom which the last three may be absent in many places due to of the unit. Dolomitic bioherms occur locally but typically Devonian erosion. near the top of the unit. Although the Waldron Shale is very In Kentucky, the Louisville conformably overlies the fossiliferous in Tennessee (Wilson 1990) and Indiana (Hall Waldron Shale with a relatively sharp contact, although it 1879, 1882), it is only sparsely fossiliferous in Kentucky is locally gradational through as much as 30 cm (Peterson and the quality of preservation is typically poor. Most of 1981; Phelps 1990). However, in parts of Indiana, Waldron AAP Memoir 44 (2013) 181 and Louisville equivalents may be gradational through of this lithofacies. Flank beds are composed largely of as much as 12 m, and lower parts of the Louisville were pelmatozoan debris with fragmented corals, brachiopods and commonly grouped with the Waldron. The ambiguity of stromatoporoids that are crudely graded and amalgamated. establishing a Waldron-Louisville contact has now been The largest bioherms appear to have developed along the alleviated by making Waldron and Louisville equivalents flanks of the Louisville High (Fig. 1), suggesting structural parts of the Pleasant Mills Formation in northern Indiana influence. The lithofacies has been interpreted as representing (Droste & Shaver 1982). deposition on a shallow, open marine storm shelf, between The basal dolarenite lithofacies sharply overlies the normal and storm wave bases, where storms played a key Waldron and consists of about 0.3 m of light-grey calcarenite role in the origin and maintenance of the bioherms (Phelps to dolarenite that was probably originally packstones and 1990). Apparently, storms produced irregular hardgrounds grainstones. A sparse fauna includes stromatoporoids, on the sea floor, which were focal points for algal, bryozoan, tabulate corals and brachiopods. The unit exhibits abundant coral and stromatoporoid attachment and the initiation of sedimentary structures like hummocky crossbeds, swaley bioherms. The transport of sands and muds around the bedding, gutter casts, scours, intraclasts and crude graded bases of the bioherms would have forced upward growth. bedding with erosional bases, all of which suggest a This growth, however, was probably periodically disrupted high-energy, storm (tempestite) origin for the lithofacies. by storms, cleaning off the tops and allowing for the fresh Compared with the underlying Waldron Shale, inception growth of binding organisms like algae, stromatoporoids of Louisville deposition reflects a return to shallowing and sponges and later recolonisation by sessile filter feeders conditions and the establishment of storm shelf throughout (Phelps 1990). the area. The overlying massive dolosiltite lithofacies occurs only The vuggy beds lithofacies is bounded by thin shale north of the Lyndon Syncline and consists of light grey, partings and is typically 0.3–1.6 m thick. The lithofacies medium-bedded to massive dolosiltites, 0–6.6 m thick, that is composed of light-grey dololutite to dolarenite, with the were likely wackestones, packstones and grainstones. Some coarser-grained dolarenites occurring more commonly. As high-energy structures like scours, gutter casts, ripples and in the basal dolarenite lithofacies, fossils are scarce, and the graded bedding are present, and the fauna, though generally same set of high-energy sedimentary structures is present. scarce, consists of laminar stromatoporoids, rugose corals, In contrast, however, many of the vuggy beds are nodular, tabulate corals like Halysites, and the pentamerid brachiopod whereas others are coated with phosphorite and iron and Conchidium that lived on firmgrounds or hardgrounds. The manganese oxides, suggesting firmground or hardground presence of high-energy structures and fauna, as well as development. The surfaces are commonly encrusted with coarser-grained rocks, suggests a return to a shallow-open corals and stromatoporoids, and the presence of nodular beds marine, storm-shelf setting above storm wave base. The and vugs is probably related to bioturbation. Small bioherms, fauna here is comparable to the Pentamerus Community of no more than 0.6 m thick, are also common throughout Ziegler (1965) and Ziegler et al. (1968) (BA3). the unit and typically show an initiating thrombolitic core, The yellowish-grey, massive dolarenite lithofacies followed by a concentration of fossil debris that includes conformably overlies the massive dolosiltite facies and fenestrate bryozoans, pentamerid brachiopods, tabulate is similar in appearance, except that massively bedded corals, crinoids and cystoids; flank beds are composed largely dolarenites and calcarenites predominate. The unit is 0–13.2 of pelmatozoan debris. The vuggy beds, like the dolarenite m thick and exhibits scours, intraclasts and graded beds lithofacies, is thought to reflect storm deposition, except that with sharp basal contacts. This fauna is of low diversity and storm frequency was apparently less than in the dolarenite largely consists of laminar stromatoporoids, the tabulate coral facies, generating periods of little deposition during which Halysites, two colonial rugose corals, Entelophyllum and firmgrounds and hardgrounds could develop, bioturbation Strombodes, a large turbiform gastropod and pelmatozoan could proceed largely uninterrupted, and bioherms had debris, all of which commonly occur on firmgrounds or sufficient time to develop relief above the seafloor to avoid hardground throughout the unit. The coarseness of the burial and enhance nutrient collection in higher-level waters. sediment and high-energy structures suggest proximal A thin, grey, laminated, unfossiliferous shale lithofacies, blanket sands on a storm-shelf, sandbelt environment above 2.5–15-cm thick, occurs everywhere between the vuggy normal wave base (B.A. 3). The large turbiform gastropod and biohermal lithofacies. This facies apparently reflects an as well as laminar stromatoporoids and corals also support abrupt return to calm, deeper-water conditions, suggesting a such a setting (Peel 1977; Nestor 1984; Graus et al. 1977). brief period of eustatic sea-level rise that indicates initiation The youngest Louisville lithofacies is the blue dolosiltite of early Ludlow (Gorstian) highstand 6 of Johnson (2006) and it consists of 0–2.0 m of massively bedded, light grey (Fig. 3). The thinness of the unit may be related to erosion dolosiltite with a bluish cast when weathered. Laminae, accompanying deposition of storm beds in the overlying vertical burrows and possible mud cracks are present. biohermal lithofacies. Fossils are very rare and include a few tabulate corals and In the overlying biohermal lithofacies, high-energy laminar stromatoporoids, which may have been transported. sedimentary structures like those in the dolarenite and The environment has been interpreted as representing very vuggy lithofacies are present in light-grey dolarenites and shallow lagoonal to low, intertidal conditions (Phelps 1990), dolosiltites up to 8.3 m thick. The bioherms are present as and it represents the shallowest of Louisville environments lenticular bodies, 0.3–2.0 m thick, and up to 4.0 m wide. and the shallowest part of a Ludlow (late Gorstian– Unlike the medium-bedded tempestite beds in which they Ludfordian) lowstand between highstands 6 and 7 (Fig. 3). occur, the bioherms are massively bedded and pervasively Although Rexroad et al. (1978) suggested that this facies dolomitised. The bioherms themselves appear to have begun of the Louisville was probably equivalent to parts of the with a massive stromatolitic or thrombolitic core with Mississinewa Shale Member of the Wabash Formation in laminar stromatoporoids and platy corals. The brachiopod eastern and northern Indiana, Conkin et al. (1992) indicated Pentamerus cylindricus is relatively common in parts that this was an erroneous correlation based on the presence 182 AAP Memoir 44 (2013) of a misidentifiedKirkidium brachiopod. reactivation of these structures by Taconian, Salinic and Although higher units are not preserved in exposure, Acadian far-field forces generated locally thinned sections, uppermost parts of the Louisville Limestone in the condensed profiles and anomalous Silurian and post-Silurian subsurface are abruptly overlain by the Dixon Shale (Fig. erosion. The juxtaposition of subsiding basin and rising arch 3) which probably reflects the late Ludlow (Ludfordian) area generated a slightly steeper ramp gradient on the east highstand 7 of Johnson (1996, 2006) and Johnson et al. side that gave rise to a distinct pattern of lateral shoreline (1998). Elsewhere, the Louisville is unconformably overlain environments relative to wave base (Shaw 1964; Irwin by the Middle Devonian Jeffersonville Limestone or the 1965; Anderson 1971) and a related pattern of shoreline Beechwood Member of the Sellersburg Formation. communities that clearly parallel those of Ziegler (1965), Ziegler et al. (1968) and Boucot (1975, 1999). The Kentucky DISCUSSION AND CONCLUSIONS River Fault System also seems to have briefly altered this The exposed Silurian section in central Kentucky is no more setting during late Brassfield deposition, when movement on than 64 m thick and includes rocks that are late Rhuddanian the fault apparently allowed iron-rich waters and sediments to early Ludfordian (early Llandovery–late Ludlow) in age. easy ingress to the area to precipitate the Rose Run iron ore The nature of the succession now, and as it formed during just north of the fault (Fig. 1). the Silurian, was in large part controlled by the basement Despite environmental differences on either side of the structures that generated the Cincinnati Arch area, a large Cincinnati Arch, the Silurian rocks on either side faithfully cratonic structure that defines the western flank of the support relative sea-level curves (Fig. 3) very similar Appalachian foreland basin (Figs 1, 5). The uplifted arch to the master curve of Johnson (1996, 2006, 2010) and area began to grow in latest Ordovician–early Silurian time Johnson et al. (1998) based on lithologies and depth-related and clearly influenced the nature of Silurian deposition on communities. On the eastern side of the arch, evidence for either side. Compared to Silurian sections in the Appalachian highstands 1–5 is apparent (Fig. 3), whereas on the west side and Illinois basins, the exposed section in the Cincinnati of the arch, evidence supports the presence of highstands Arch area is a more condensed, glauconite-rich succession, 2–6 (Fig. 3). So overall, Silurian rocks on the western side because sedimentation was distal from sources and slow. of the arch are younger than those on the eastern side, and Not only do Silurian units thin as the exposure approaches these age differences are supported by biostratigraphy the arch, but depositional environments on either side were (e.g. Rexroad & Kleffner 1984). These age differences in different, although partly contemporaneous. part reflect the fact that younger rocks are preserved in the During Silurian deposition, the modern Cincinnati Arch west, but also the fact that Silurian deposition only began had not yet fully developed. The arch area comprised a in mid-Aeronian time in the west and in late Rhuddanian large isolation-accommodation zone (Fig. 5) that served as time in the east. Although parts of the Brassfield below a platform-like area for Silurian deposition on the western the Lee Creek Member in the west and parts of the Crab outcrop belt. Silurian rocks are carbonate-rich and represent Orchard and Brassfield below the Waco Member in the east deposition on a uniform shelf-platform setting that at times all occur within the single Distomodus kentuckyensis Zone, was interrupted by a step-like series of faults that cut the sequence-stratigraphic analysis of Brassfield lithologies platform at high angles (Figs 1, 5). In fact, in contrast to the in the west (Andrews 1997) indicates that the lowermost eastern side of the arch where Silurian thickness variations upper Rhuddanian–lower Aeronian sequence from the east are more or less parallel to the arch, on the western side of is missing in the west (Fig. 6). Either that sequence was the arch, thickness variations are largely perpendicular to deposited in the west and subsequently eroded away, or more the arch, controlled by the independent, synsedimentary likely, it took early Silurian seas much of late Rhuddanian– movement of the four or five faults that cut the arch area. early Aeronian time (3–4 Ma) to onlap and transgress across Hence, the shoreline communities of Ziegler (1965), Ziegler the isolation-accommodation zone. If this is the case, then et al. (1968) and Boucot (1975, 1999) are not as well much of the western Brassfield is equivalent to the Plum organised or discerned. In addition, after the period of late Creek, Oldham and possibly Lulbegrud members of the Crab Telychian–early Sheinwoodian deepening reflected in the Orchard Formation in the east. Osgood Shale, the entire area west of the arch largely seems It is also clear that the highstands and intervening to have become a high-energy storm shelf dominated by lowstands are nearly synchronous on both sides of the stromatoporoid-coral-pentamerid communities throughout arch and that they reflect both glacio-eustatic and tectonic much of remaining Wenlock time. components (Fig. 3). In fact, an Ordovician–Silurian and In contrast, rocks on the eastern side of the arch are more three Silurian glaciations on South America are coeval with shale-rich, and although distal from Taconian and Salinic four major lowstands (Fig. 3). At the same time, the nature source areas on the eastern side of the Appalachian Basin, of correlations between Kentucky Silurian sequences and the environments they represented still faced toward source tectophase sequences in the Appalachian Basin (Brett et al. areas and were part of the arch-related barrier that prevented 1990; Ettensohn 1994; Goodman & Brett 1994; Ettensohn clastics from readily moving westward. This barrier was & Brett 1998, 2002), make it likely that far-field, flexural, formed largely by the eastern margin of the isolation- tectonic controls, though of lesser intensity, were at work accommodation zone, the Grenville Front (Fig. 5). The in Kentucky as well. In particular, Taconian (tectophase eastern outcrop belt occurs wholly east of the Grenville Front, 3) far-field influence is implicit in the upper Brassfield- and from its margin, Silurian environments were apparently Plum Creek lowstand-highstand sequence (Fig. 3); Salinic part of a gentle ramp that dipped eastward into the subsiding tectophase 1 influence is very implicit in the Dayton-Estill Appalachian Basin. Nonetheless, these ramp environments Shale and lower Osgood lowstand-highstand sequences were apparently disrupted at times by reactivated basement (Fig.11); and Salinic tectophase 2 influence is apparent in structures, both parallel and perpendicular to strike in the lowstand-highstand sequences from the Bisher and upper Kentucky River Fault System, Ohio-Kentucky Border Fault Osgood-Laurel sequences (Fig. 3). An earlier, unrecognised and the Waverly Arch basement fault (Figs 1, 5). Periodic tectophase or local transgression may be represented by AAP Memoir 44 (2013) 183 the Waco and Lee Creek members (Fig. 3). Moreover, if favorable for periodic mixing-zone dolomitisation (e.g. Goodman & Brett (1994) are correct, tectonic influence is Badiozamani 1973) on and near the arch. The fact that present throughout the Appalachian Silurian, and equivalents many of the Silurian dolostones are also ferroan indicates of their sequences I, II, possibly III, IV, V, VI and VII are that a mixing-zone mechanism is not the sole possibility, present in the Kentucky Silurian (Fig. 3). Perhaps most and that several mechanisms may have operated together demonstrative of Silurian far-field tectonic influence is or in succession. late Telychian Sequence IV of Goodman & Brett (1994), Our purpose in this paper has been to point out the equivalent to highstand 4 of Johnson (1996, 2006) and interesting Silurian succession in central Kentucky and Johnson et al. (1998) (Fig. 3). In the Appalachian Basin to contribute toward establishing a general framework this lowstand-highstand sequence is bound by an angular for understanding its stratigraphy, palaeoenvironments unconformity and involves the Williamson Shale and its and palaeoecology. Many older studies of the Silurian of equivalents (Fig. 4); it is the sedimentary manifestation of Kentucky and nearby areas have implied that the Silurian was the first tectophase of the Salinic orogeny (Ettensohn 1994; a relatively brief and stable period of earth history. However, Ettensohn & Brett 1998). According to Brett et al. (1990, many studies in the last decade from the east-central United 1998), the Williamson represents the deepest water deposits States and elsewhere, based on the integration of sequence in the Appalachian Basin, and is coeval with the greatest stratigraphy, biostratigraphy and chemostratigraphy, have of the Silurian flooding events, highstand 4 of Johnson shown instead that the Silurian Period was one of the most (1996, 2006), Kaljo et al. (1996) and Johnson et al. (1998) volatile periods in Phanerozoic time, representing major (Fig. 3). In Kentucky, this same sequence is represented by transitions in global tectonics and in the ocean-atmospheric the Dayton/Estill sequence in the east and lower Osgood system (e.g. Gurnis & Torsvik 1994; Cramer & Saltzman sequence in the west (Fig. 3), and it is similarly bound by 2005, 2007; Calner 2008; Cramer et al. 2011). Without a regionally angular unconformity surface below (Lukasic doubt, the Silurian section of central Kentucky supports 1988; McLaughlin et al. 2008) (Figs 4, 11), indicating the these interpretations and reflects a complex but subtle, array presence of Salinic far-field forces as far west as central of depositional events that were underpinned by dynamic Kentucky. In addition, as there were no proven glacial interactions among palaeogeographic, palaeoclimatic, episodes on Gondwana at this time (Grahn & Caputo 1992; ancient oceanographic and tectonic controls at the time. Much Caputo 1998) (Fig. 3), at least in eastern Laurussia, tectonic of what we report in this paper involves the assimilation of components must have comprised the larger part of this late older studies into a growing body of knowledge generated by Telychian lowstand-highstand pair. Analogously, the Estill more recent integrative approaches. Clearly, a more holistic and lower Osgood parts of this sequence apparently represent framework for the Silurian of Kentucky is emerging, but the deepest parts of the Silurian sequence in Kentucky (Fig. further investigation is still warranted. 3). Clay mineralogy of the Estill Shale also suggests a major volcanic ash component to the shales, most likely from ACKNOWLEDGEMENTS Salinic subduction to the east. A depauperate and diminutive We wish to thank the editors of the volume for their invitation fauna in the shale reflects one of the earliest Palaeozoic to contribute, but we especially thank our reviewers, C.E. dysaerobic faunas. Brett and M.A. Kleffner, for their very helpful comments, The Silurian section in central Kentucky both begins and which vastly improved the paper. Any errors in presentation, ends with major regional unconformities. The Ordovician- however, are wholly the responsibility of the authors. 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