1. Introduction
The Upper Ordovician of eastern North America contains unusually high phosphate concentrations, with several intervals enriched in heavy rare earth elements (HREEs; Emsbo et al. Reference Emsbo, McLaughlin, Breit, du Bray and Koenig2015). Mapping phosphate concentrations and thicknesses and investigating the depositional context of HREE enrichment is underway by multiple research groups. Historically, multiple studies cited the elevated phosphate content in the marine strata of the Late Ordovician Taconic basin covering the modern Appalachian region from Tennessee (United States) in the south, to Quebec (Canada) in the north (e.g. Smith and Whitlatch, Reference Smith and Whitlach1940; Cressman, Reference Cressman1973; Holland and Patzkowsky, Reference Holland, Patzkowsky, Witzke, Ludvigsen and Day1996; Baird and Brett, Reference Baird and Brett2002; Dattilo et al. Reference Dattilo, Freeman, Zubovic, Brett, Straw, Frauhiger, Hartstein and Shoemaker2019; Oruche and Dix, Reference Oruche and Dix2021; Haynes et al. Reference Haynes, Villanueva, Robinet, Leslie and Herrmann2025). Historic and ongoing work also documents over a dozen Upper Ordovician phosphate horizons across an area of more than 1 million km2 in the North American midcontinent from Oklahoma (US), in the south, to Ontario (Canada), in the north (e.g. Witzke, Reference Witzke, Fouch and Magathan1980; Kolata and Graese, Reference Kolata and Graese1983; Black and Carozzi, Reference Black and Carozzi1988; Kolata et al. Reference Kolata, Huff and Bergström2001; Dix et al. Reference Dix, Sharma, Al-Aasm, Coniglio, Linnen, Riva and Achab2010; McLaughlin et al. Reference McLaughlin, Emerson, Witzke, Sell, Emsbo, Miller, Hudak, Wittkop and McLaughlin2011). A few of these phosphate intervals are metres thick and higher grade than the HREE concentrations in many conventional resources (Emsbo et al. Reference Emsbo, McLaughlin, Breit, du Bray and Koenig2015). An essential step in our regional assessment is to temporally map the phosphate horizons into a comprehensive time-stratigraphic (i.e. chronostratigraphic) model to better constrain phosphogenic processes. Best practices dictate building this model from integrated biostratigraphy, chemostratigraphy, geochronology, and facies analysis interpreted within a sequence stratigraphic framework (e.g. Coe et al. Reference Coe, Bosence, Church, Flint, Howell and Wilson2003; Catuneanu, Reference Catuneanu2022). As the North American Upper Ordovician standard, the Cincinnati area features the most highly developed chronostratigraphic model in the region and is therefore the natural anchor point for our regional studies (e.g. Sweet and Bergström, Reference Sweet and Bergström1971). Renowned for its outstanding fossil preservation and abundance of surface exposures, it remains an active research area for investigating Late Ordovician environments and events.
The Upper Ordovician strata of the Cincinnati area have a rich history of study. At the dawn of the twentieth century, investigations focused on characterizing the abundant macrofossil composition (e.g. Foerste, Reference Foerste1903). By the mid-century, research on the successions around Cincinnati shifted to microfossil biostratigraphy (e.g. Sweet et al. Reference Sweet, Turco, Warner and Wilkie1959) and lithostratigraphy (e.g. Weiss and Norman, Reference Weiss and Norman1960). Event stratigraphy and sequence stratigraphy dominated studies of the past few decades (e.g. Holland and Patzkowsky, Reference Holland, Patzkowsky, Witzke, Ludvigsen and Day1996; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020, and references therein). Sequence stratigraphic studies documented sea-level fluctuations on the order of tens of metres and the influence of far-field tectonics from the Taconic orogeny (Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Biotic immigration events also reflect shifting environmental conditions that impacted biodiversity (Stigall et al. Reference Stigall, Edwards, Freeman and Rasmussen2019; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020; Stigall, Reference Stigall2023). In addition, preliminary stable carbon isotope chemostratigraphy was completed for a few sections utilizing brachiopod (e.g. Shields et al. Reference Shields, Carden, Veizer, Meidla, Rong and Li2003) and whole rock carbonates (e.g. Young et al. Reference Young, Saltzman and Bergström2005; Bergström et al. Reference Bergström, Young and Schmitz2010 b). Carbon isotope curves show evidence of several positive excursions of up to 2.5‰ that are tentatively correlated with similar excursions documented in the intracratonic Baltic Basin (Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Mannik, Nõlvak and Tinn2010; Bergström et al. Reference Bergström, Young and Schmitz2010 b) and on the Laurentian Cordilleran passive margin (Saltzman and Young, Reference Saltzman and Young2005; Jones et al. Reference Jones, Creel and Rios2016; Bergström et al. Reference Bergström, Kleffner and Eriksson2020).
This study adds new high-resolution chitinozoan biozonation and coupled chemostratigraphy to Upper Ordovician data sets on the eastern margin of the Cincinnati area near Maysville, Kentucky. Our objective is to provide a multifaceted chronostratigraphic framework for the Cincinnatian strata in the type area for the North American Maysvillian Stage. With the large road cuts, bountiful fossils and diverse mixed carbonate-siliciclastic-facies succession in and around Maysville, the designation as the type area is well deserved. Chitinozoan organic microfossils are abundant in Upper Ordovician rocks near Maysville (Miller, Reference Miller1976; Velleman, Reference Velleman2016; Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025) and are important components of Upper Ordovician biostratigraphy globally (e.g. Achab, Reference Achab1989; Paris, Reference Paris1990; Vandenbroucke, Reference Vandenbroucke2008 a). They are found across a broad spectrum of shallow-to-deep water-facies and palaeolatitudinal distribution (Vandenbroucke et al. Reference Vandenbroucke, Armstrong, Williams, Paris, Sabbe, Zalasiewicz, Nõlvak and Verniers2010). Carbon isotope chemostratigraphy of carbonates is a complementary tool to increase temporal resolution and reinforce chronocorrelation of sections (e.g. McLaughlin et al. Reference McLaughlin, Emsbo, Brett, Bancroft, Desrochers and Vandenbroucke2019; Rine et al. Reference Rine, McLaughlin, Bancroft, Harrison, Kuglitsch, Caruthers, Ramezani, Kaczmarek and Emsbo2020). Expanding biostratigraphic and chemostratigraphic characterization of the Upper Ordovician in the Cincinnati area more firmly establishes it as a regional chronostratigraphic anchor point in North America for temporal ordering of HREE-enriched phosphorites and Late Ordovician events.
2. Geological setting
2.a. Location and structure
The Cincinnati Arch is an approximately north–south-trending anticlinal structure, extending from Ohio to Tennessee, that brings Upper Ordovician rocks to the surface (Figure 1) (Borella and Osborne, Reference Borella and Osborne1978; Weir et al. Reference Weir, Peterson and Swadley1984). The structure is divided into the Jessamine Dome (north) and the Nashville Dome (south). The Cincinnati area, as referred to in this work, is largely synonymous with the Jessamine Dome and also includes adjacent features in the shallow subsurface of northern Kentucky, southwestern Ohio and southeastern Indiana. The dip of strata is very low across much of the dome and steepens at the margins. The oldest rocks at the surface in the Cincinnati area are Upper Ordovician and are located in central Kentucky.
Location maps. (a) Map of the USA highlighting the general study area. (b) Map of the study area in the states of Indiana, Ohio and Kentucky. (c) Generalized geologic map of the Cincinnati area (modified from Coogan, Reference Coogan, Feldmann and Hackathorn1996) showing studied sections and reference localities. Stars mark the MY-14-01 and the OGS-2682 drill cores studied here. Other sections mentioned in the text are marked by a circled number: 1, Jacksonville outcrop. 2, Maysville section. 3, Clifton Avenue section, Cincinnati. 4, New Point/Indiana Geological Survey Drill Hole no. 124. 5, China/Dry Fork section. 2, 3, 4 and 5 are sections previously studied by Miller (Reference Miller1976). 6, Covington section. 7, Minerva CA-38 core. 8, Frankfort section. 6, 7 and 8 are successions studied by Knabe (Reference Knabe1980). 9, Tollesboro section. 10, Brush Creek section. Locations 9 and 10 were studied by Grahn and Bergström (Reference Grahn and Bergström1985). 11, Middletown (OGS-860) core. 12, Elkhorn core. 13, Sand Creek stream cut, studied by Farnam et al. (Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026).
Upper Ordovician strata in North America are divided into the Mohawkian Series and Cincinnatian Series (e.g. Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020). The Mohawkian Series is divided into Turinian and Chatfieldian stages. The Cincinnatian Series is divided into Edenian, Maysvillian, Richmondian and Gamachian stages. These designations date back to the early twentieth century, with some later updates, and are defined primarily on lithologic and macrofossil criteria (e.g. Sweet, Reference Sweet1979 a; Bergström in Ross et al. Reference Ross, Adler, Amsden, Bergstrom, Bergström, Carter, Churkin, Cressman, Derby, Dutro, Ethington, Finney, Fisher, Fisher, Harris, Hintze, Ketner, Kolata, Landing, Neuman, Sweet, Pojeta, Potter, Rader, Repetski, Shaver, Thompson and Webers1982). The only Turinian strata exposed at the surface in the Cincinnati area are upper Turinian. By contrast, Chatfieldian through Richmondian age strata are abundantly exposed and penetrated by many cores (Figure 2). Gamachian age strata are considered to be largely missing from the Cincinnati area (Bergström in Ross et al. Reference Ross, Adler, Amsden, Bergstrom, Bergström, Carter, Churkin, Cressman, Derby, Dutro, Ethington, Finney, Fisher, Fisher, Harris, Hintze, Ketner, Kolata, Landing, Neuman, Sweet, Pojeta, Potter, Rader, Repetski, Shaver, Thompson and Webers1982; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020; Farnam et al. Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026; Waid et al. Reference Waid, Farnam, Hogancamp and Brett2026). The exact relationship of these regional stages to the global Sandbian, Katian and Hirnantian stages remains a matter of debate that is advancing with the generation of new regional data sets, including chitinozoan biostratigraphy (e.g. Vandenbroucke et al. Reference Vandenbroucke, Rickards and Verniers2005, Reference Vandenbroucke, Ancilletta, Fortey and Verniers2009; Vandenbroucke, Reference Vandenbroucke2008 a; Meidla et al. Reference Meidla, Ainsaar, Hints, Radzevičius, Harper, Lefebvre, Percival and Servais2023).
General geologic setting of the type area for the Maysvillian Regional Stage on the eastern side of the Cincinnati area. (a) Overview image of the large road cut on US Route 68 just north of Maysville, Kentucky, spanning the Kope to Grant Lake formations. (b) Close-up images of the Grewingkia coral bed mapped in the Maysville area. (c) Representative images of lithostratigraphic units in the MY-14-01 core.
2.b. Lithostratigraphy and palaeogeography
The lithostratigraphic framework of the Cincinnati area has received multiple revisions throughout the past century. During the 1960s to 1980s, the US Geological Survey and the Kentucky Geological Survey mapped all the Upper Ordovician rocks of Kentucky at a 1:24 000 scale, covering nearly a third of the state (c. 24 000 km2). This effort included documenting lithologic variation and macrofossil distributions. A spectrum of facies was interpreted, ranging from marginal marine to deep subtidal settings, and as a result a complex lithostratigraphic framework with interfingering units was erected (Figures 2c and 3; Cressman, Reference Cressman1973; Weir et al. Reference Weir, Peterson and Swadley1984). The regional lithostratigraphic framework was built on the superposition of units relative to widespread datums, such as the Millbrig and Deike K-bentonites in the interval just below the Lexington Limestone. The lithostratigraphy also incorporated, to limited extent, the occurrence of widespread macrofossil epiboles, such as the mass occurrence of the small strophomenid brachiopod Sowerbyella near the base of the Kope Formation (Cressman, Reference Cressman1973) and the rugose coral Grewingkia near the middle of the Bull Fork Formation (Weir et al. Reference Weir, Peterson and Swadley1984; Figure 2b). Regional synthesis of these lithostratigraphic studies was utilized to reconstruct the palaeogeographic evolution of the Cincinnati area on the distal margin of the Taconic foreland basin during the Late Ordovician.
Chronostratigraphic framework for the Upper Ordovician in the Cincinnati area. Graptolites are integrated from Bergström and Mitchell (Reference Bergström and Mitchell1986), Goldman and Bergström (Reference Goldman and Bergström1997) and Maletz (Reference Maletz2021). Conodonts are integrated from Rexroad et al. (Reference Rexroad, Branson, Smith, Summerson and Boucot1965), Cooper (Reference Cooper1975), Sweet (Reference Sweet1979 a, b, Reference Sweet and Bruton1984) and Richardson and Bergström (Reference Richardson and Bergström2003). Lithostratigraphy is after Cressman (Reference Cressman1973) and Weir et al. (Reference Weir, Peterson and Swadley1984). Phases of the Richmond Invasion are after Stigall (Reference Stigall2023). Sequence stratigraphy is from Holland and Patzkowsky (Reference Holland, Patzkowsky, Witzke, Ludvigsen and Day1996), Pope and Read (Reference Pope and Read1997) and Brett et al. (Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Chemostratigraphy is modified from Bergström et al. (Reference Bergström, Young and Schmitz2010 b). Species names in grey and discontinuous lines reflect the uncertainties about the recognition of some biozones and their limits in the Cincinnati area. Abbreviations and acronyms: SIL = Silurian; GSt = Global Stage; Sa = Sandbian; R = Rhuddanian; RS = Regional Stage; T = Turinian; Atl. = Atlantic Faunal Region; A. = Amorphognathus.
During the Late Ordovician, a shallow epicontinental sea covered much of Laurentia, and the Taconic orogeny was active along the southern margin of the palaeocontinent (>400 km from the Cincinnati area; Rowley and Kidd, Reference Rowley and Kidd1981; Stanley and Ratcliffe, Reference Stanley and Ratcliffe1985; Macdonald et al. Reference Macdonald, Karabinos, Crowley, Hodgin, Crockford and Delano2017). The Cincinnati area was situated at approximately 20 to 25° S latitude along the distal margin of the Taconic foreland basin (Scotese and McKerrow, Reference Scotese and McKerrow1990). Throughout the Late Ordovician, large volumes of siliciclastic sediments were eroded from the Taconic uplifts and shed into the surrounding basins (MacDonald et al. Reference Macdonald, Karabinos, Crowley, Hodgin, Crockford and Delano2017). The proximal foreland basin transitioned from graptolitic black shales to terrestrial red beds through the upper Chatfieldian to the upper Maysvillian/Lower Richmondian stages (e.g. Dennison, Reference Dennison and Bassett1976; Mitchell and Bergström, Reference Mitchell, Bergström, Barnes and Williams1991; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Importantly, quartz sand and abundant silt in the Cincinnati area increase to the southeast towards Virginia, demonstrating a spatially restricted corridor between foredeeps providing a direct connection to the mountain belt (Pope et al. Reference Pope, Read, Bambach and Hofmann1997). The Cincinnati area received a mix of these transported siliciclastic sediments and autochthonous carbonates (e.g. Weir et al. Reference Weir, Peterson and Swadley1984; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). The dominant oceanographic feature in the Cincinnati area was a north-dipping (modern orientation) ramp that formed at a high angle to the rest of the Taconic foreland basin. It was not until the late Richmondian that terrestrial red beds migrated fully across the Taconic basin to be deposited in the Cincinnati area on the distal basin margin. In addition to lithostratigraphic superposition, the chronology of these regional environmmental changes was enabled through incorporating the findings of biostratigraphic studies.
3. Time-rock calibration
3.a. Conodont and graptolite biostratigraphy
Upper Ordovician studies in the Cincinnati area traditionally utilized conodonts and graptolites for regional and intercontinental biostratigraphic correlations. In general, zonally diagnostic conodonts were recovered from carbonate-dominated successions, whereas graptolites were recovered from shale-dominated successions (e.g. Bergström and Mitchell, Reference Bergström and Mitchell1990; Mitchell and Bergström, Reference Mitchell, Bergström, Barnes and Williams1991). Numerous conodont studies were conducted in the abundant outcrops and cores of the Cincinnati area, overlapping with numerous geologic mapping projects in Kentucky (e.g. Bergström and Sweet, Reference Bergström and Sweet1966; Rexroad, Reference Rexroad1967; Sweet, Reference Sweet1979 a). This research spans the intellectual shift from individual morphospecies to apparatus reconstructions, and not all of the early work was updated, leaving some uncertainties. Diagnostic species of both the North Atlantic and Midcontinent conodont zonal schemes are present in the Cincinnati area (Figure 3). Linking the North Atlantic conodont faunas (e.g. the Appalachian region, Great Britain, Baltic region) with the diverse US midcontinent faunas (Bergström, Reference Bergström1970; Sweet, Reference Sweet and Bruton1984) and the Laurentian and Oceanic graptolite faunas (sensu Goldman and Bergström, Reference Goldman and Bergström1997) was in part resolved by finding areas of overlap near Cincinnati. The Middletown core, located in the northern part of the Cincinnati area (c. 40 km north–northeast of Cincinnati; Figure 1c, locality 11), is one of the most informative sections (Mitchell and Bergström, Reference Mitchell, Bergström, Barnes and Williams1991; Richardson and Bergström, Reference Richardson and Bergström2003). It shows that the base of the Amorphognathus superbus conodont Biozone occurs near the middle of the Plectodina tenuis conodont Biozone and is just slightly younger than the base of the Diplacanthograptus spiniferus graptolite Biozone near the middle of the Lexington Limestone and lateral equivalents (Finney Reference Finney1986; Mitchell and Bergström, Reference Mitchell, Bergström, Barnes and Williams1991). The base of the Geniculograptus pygmaeus graptolite Biozone is located about 25 m above the base of the Kope Formation at Cincinnati. Nearby sections in southeastern Indiana demonstrate that the base of the Amorphognathus ordovicicus conodont Biozone is within the upper Arnheim Formation (Bergström and MacKenzie, Reference Bergström and MacKenzie2005; approximately coincident with a level near the middle of the Aphelognathus grandis conodont Biozone). The base of the Paraorthograptus manitoulinensis graptolite Biozone is projected to a similar level about 80 m above the base of the G. pygmaeus Biozone near Cincinnati (Goldman and Bergström, Reference Goldman and Bergström1997), close to the base of the Arnheim Formation. Dicellograptus complanatus, index species of the Dice. complanatus graptolite Biozone, has not been recovered from the Cincinnati area. Nonetheless, the uppermost Whitewater Formation (upper Richmondian Stage) is often suggested to belong to that zone (e.g. Bergström et al. Reference Bergström, Young and Schmitz2010 b, Reference Bergström, Kleffner and Eriksson2020; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Correlation of these biozonal boundaries from the northern part of the Cincinnati area to the southeastern part near Maysville was established primarily using litho-and sequence stratigraphy (e.g. Holland, Reference Holland1993; Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020).
3.b. Chitinozoan studies
Historically, Upper Ordovician chitinozoan biostratigraphic studies in the Cincinnati area were typically limited. A single peer-reviewed chitinozoan study was published for the Ordovician-Silurian boundary interval with only seven species recovered (Grahn and Bergström, Reference Grahn and Bergström1985). Additionally, a few master’s theses were completed for portions of the Upper Ordovician in sections across the Cincinnati area (Miller, Reference Miller1976; Knabe, Reference Knabe1980). Recently, a preliminary study of the chitinozoan assemblages of the MY-14-01 core drilled near Maysville, Kentucky (Figure 1), recovered abundant, well-preserved and diverse taxa (Velleman, Reference Velleman2016). An expanded study on the systematic palaeontology and ranges of the chitinozoans from Maysville was published, with several new species described (Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025). Chitinozoan studies in the Cincinnati area benefit from comparison with published works from other parts of Laurentia, including Anticosti Island and other parts of eastern Canada (e.g. Martin, Reference Martin1983; Achab, Reference Achab1989; Achab et al. Reference Achab, Asselin, Desrochers and Riva2013), the Western USA (Soufiane and Achab, Reference Soufiane and Achab2000 b) and Scotland (Vandenbroucke et al. Reference Vandenbroucke, Verniers and Clarkson2003; Lissens et al. Reference Lissens, Vancoppenolle, Ingham and Vandenbroucke2026). These studies described diverse Upper Ordovician taxa and established biozonations. Unlike graptolites and conodonts, chitinozoans are often highly abundant in shale samples across shallow-to-deep marine palaeoenvironments (e.g. Hints et al. Reference Hints, Killing, Männik and Nestor2006; Vandenbroucke et al. Reference Vandenbroucke, Armstrong, Williams, Paris, Sabbe, Zalasiewicz, Nõlvak and Verniers2010) – features that make this fossil group ideal for advancing high-resolution biostratigraphy in the Cincinnati area.
3.c. Chemostratigraphy
Chemostratigraphic studies represent the most recent phase of relative age dating in the Cincinnati area. Most studies utilize stable carbon isotopes from carbonate (δ13Ccarb). Several studies have focused on the Guttenberg carbon isotope excursion (GICE), which spans the regional Turinian–Chatfieldian stage boundary, considered to be close in age to the Sandbian–Katian global stage boundary (Figure 3; e.g. Young et al. Reference Young, Saltzman and Bergström2005; Coates et al. Reference Coates, Ettensohn, Rowe, Finney and Berry2010; Bergström et al. Reference Bergström, Schmitz, Saltzman, Huff, Finney and Berry2010 a). Chemostratigraphic study of the Ordovician–Silurian boundary interval, featuring the Hirnantian carbon isotope excursion (HICE), has also received a great deal of attention in surrounding parts of eastern North America (e.g. Bergström et al. Reference Bergström, Saltzman and Schmitz2006, Reference Bergström, Kleffner, Schmitz and Cramer2011 a, b; Farnam and Brett, Reference Farnam and Brett2023). The stratigraphic interval between these two large positive δ13Ccarb excursions (GICE and HICE) has received far less attention in the region. Bergström et al. (Reference Bergström, Young and Schmitz2010 b) generated new δ13Ccarb results for two cores near Maysville (Kentucky) and Richmond (Indiana). These authors combined this data with a published record from the Frankfort section (Kentucky; Figure 1c, locality 8) to create a composite δ13Ccarb curve spanning the entire Chatfieldian through Richmondian succession preserved in the Cincinnati area (summarized in Figure 3). Comparison of Cincinnati area chemostratigraphy with that of biostratigraphically well-characterized Upper Ordovician sections from other parts of North America, including Anticosti Island (McLaughlin et al. Reference McLaughlin, Emsbo, Desrochers, Bancroft, Brett, Riva, Premo, Neymark, Achab, Asselin and Emmons2016) and Nevada and Utah (Jones et al. Reference Jones, Creel and Rios2016, reinterpreted in Bergström et al. Reference Bergström, Kleffner and Eriksson2020), provides an opportunity for further refining relative age interpretations.
3.d. Sequence stratigraphy
Multiple sequence stratigraphic studies in the Cincinnati area integrated previous litho-, bio-and chemostratigraphic interpretations into a temporal framework of relative sea-level oscillations based on facies analysis and mapping of erosional sequence boundaries, flooding surfaces and other surfaces of stratigraphic importance (Figure 3). Interpretations have varied in terms of the position of sequence boundaries and the number of third-order depositional sequences (e.g. Holland and Patzkowsky, Reference Holland, Patzkowsky, Witzke, Ludvigsen and Day1996; Pope and Read, Reference Pope and Read1997). Recently, the sequence stratigraphic interpretation of the Cincinnati area was updated through study of classic exposures, cores and many recently created outcrops (Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Utilizing numerous marker horizons (e.g. faunal epiboles, seismites, tephras, etc.), correlation across depositional dip (south to north) in the Cincinnati area resulted in a shift in the position and number of interpreted sequence boundaries and delineation of several fourth-order sequences.
4. Materials and methods
Our study of Upper Ordovician strata in the Cincinnati area integrates a variety of biostratigraphic and chemostratigraphic techniques that complement previous studies. The primary focus was study of the MY-14-01 core drilled by Carmeuse Company in 2014 on the eastern outskirts of Maysville, Kentucky (38.621461° N, 83.671719° W), approximately 90 km southeast of Cincinnati (Figures 1c and 2c). The total core length exceeds 500 m (1640 ft), and the upper 224 m (735 ft) were donated for this study (Figure 4; archived at Illinois State Geological Survey (ISGS)). The potential for follow-up studies is high as the Kentucky Geological Survey archives another 23 cores from this mine, with some exceeding 400 m in thickness. Based on local geologic mapping, the Ordovician-Silurian boundary is estimated to occur ∼24 m (80 ft) above the top of the succession preserved in the MY-14-01 core (Schilling and Peck, Reference Schilling and Peck1967). To gain a better understanding of this upper part of the local Ordovician succession, we also studied the Jacksonville outcrop (38.888647° N, 83.454508° W; Figure 1c, locality 1) and the OGS-2682 core (39.378372° N, 83.572268° W; Figure 1c) from nearby southern Ohio.
Lithostratigraphy and chemostratigraphy of the MY-14-01 core. Graphic log shows general litho-facies (marked in feet; 10 m = 33 ft) with lithostratigraphic formation picks in black text following criteria in Weir et al. (Reference Weir, Peterson and Swadley1984). Relative position of other lithostratigraphic unit names used in Maysville outcrop studies is shown in grey text. Profiles of δ13Ccarb results show all data (light gray) and 5-point moving average (dark gray). Red arrows mark third order sequence boundaries of Brett et al. (Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Select elemental concentrations from pXRF analysis with 5-point moving averages plotted showing high lithologic heterogeneity and relatively subtle compositional changes at formation boundaries (see Supplementary Table S1 for additional profiles and cross plots). LITHO = Lithostratigraphy; def = soft-sediment deformation.
Benchtop study of the MY-14-01 core began with the documentation of the lithologic and macrofossil succession to provide links to the extensive mapping and conodont studies previously completed for the area around Maysville. The lithofacies study included non-destructive elemental analysis on the MY-14-01 core every 15 cm (6 in) using a Thermo Niton XLt3 GOLD+ portable X-ray fluorescence (pXRF) analyser in Test All Geo mode with a 75 s read time (N = 1470) at the Indiana Geological Survey (Supplementary Table S1). Four certified carbonate and shale standards were run every 50 samples.
A total of 66 samples were collected from the MY-14-01 core, to be studied for chitinozoan biostratigraphy following a protocol based on classic palynological procedures (e.g. Paris, Reference Paris1981) at the palynological laboratory of Ghent University, Belgium (see De Backer et al. Reference De Backer, Day, Emsbo, McLaughlin and Vandenbroucke2024). The organic residue of each sample was hand-picked for chitinozoans using a Zeiss Discovery V20 Stereomicroscope at c. 50× magnification and mounted for scanning electron microscope (SEM) imagery and identification using a TESCAN field emission TIMA3-X GMU. The systematic palaeontology of the specimens recovered is documented in the study of Esteves et al. (Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025); holotypes mentioned in the latter study are curated in the collections of the Royal Belgian Institute of Natural Sciences in Brussels (RBINS collection numbers b 10042 and b 10050). The remaining specimens, stubs, sample residues and rock samples are stored in the collections of the Department of Geology, Ghent University. Chitinozoan diversity was calculated for the MY-14-01 and displayed graphically (cf. Colmenar and Rasmussen, Reference Colmenar and Rasmussen2018). Sample-based diversity consists of the number of different species present in each sample. For constructing the cumulative diversity curve, the complete range of the taxa has been considered (i.e. from lowest to highest occurrence (HO)), even if that specific taxon does not occur in the studied sample.
Stable carbon and oxygen isotope analysis was conducted on whole rock calcareous powders collected from MY-14-01 at 60 cm (2 ft) intervals (N = 363) using a Thermo Scientific Delta V isotope ratio mass spectrometer coupled to a GasBench II device at the ISGS Basin Analysis Lab (Supplementary Table S2). Stable isotope analyses of calcareous powders collected approximately every 30 cm (1 ft; N = 35) from the OGS-2682 core were conducted at the University of Kansas Keck Paleoenvironmental and Environmental Stable Isotope Lab on a Thermo Scientific MAT 253 with a Kiel III carbonate device (Supplementary Table S2). Samples were carefully selected preferentially from fine-grained lithologies, avoiding skeletal material whenever possible. Sampling never penetrated vugs or veins. Reproducibility of carbon isotope results from the analysis of standards was typically better than 0.05‰ (PDB) for both labs.
5. Results
5.a. Lithologic composition
General litho-and bio-facies logging of the MY-14-01 core showed a succession highly similar to that exposed in the large road cuts around Maysville, Kentucky (Figure 2), and likewise, in Ohio, the OGS-2682 core showed a succession highly similar to the Jacksonville road cut (Figure 5). The MY-14-01 core contains a succession of rapidly alternating shales and skeletal limestones with subtle changes in their relative proportions and some phosphatic horizons. The studied portion of the OGS-2682 core shows a very different succession with relatively pure shales at the base, sharply overlain by silty calcareous mudstones, and capped by a relatively pure skeletal limestone package.
Ordovician-Silurian boundary interval in southeastern Ohio. (a) Stratigraphic succession at Jacksonville road cut on the western side of Ohio State Route 41, 0.3 km south of the bridge over Ohio Brush Creek. Lithostratigraphic units and Ozarkodina hassi and Distomodus kentuckyensis conodont biozones are as in Farnam et al. (Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026). (b) Images of OGS-2682 drill core, showing the Preachersville Member of the Drakes Formation through Brassfield Formation. (c) Graphic log and δ13Ccarb profile for the OGS-2682 core displaying the Elkhorn Excursion within the Preachersville Member (Drakes Formation) and the end of the HICE in the Whippoorwill Formation. Abbreviations and acronyms: Sys. = System; GSt = Global Stage; RSt = Regional Stage.
The results of pXRF elemental analysis on the MY-14-01 core provide quantification of the highly variable lithologic succession with a total of 1470 analyses yielding informative results for 13 elements (Figure 4; Supplementary Table S1). Notably, within this data set, high calcium (Ca) values and low potassium (K) indicate clean limestones, and the reverse patterns are indicative of relatively pure clay shales (primarily composed of illite and chlorite). Aluminium, iron, titanium and silicon (Si) have a high covariance with K (R 2 > 85%). Zirconium (Zr, R 2 > 50%) and phosphorus (P, R 2 > 35%) also show some covariance with K, indicating a partial detrital source. A high ratio of Si to Al corresponds closely with observation of quartz silt abundance in the section. Variations in these elements represent changes in detrital inputs to this part of the distal foreland basin margin from the hinterland. Phosphorus (P) concentrations ranged from 0 to 2.48 wt% with an average concentration of 0.28 wt%. Converted to oxide, the maximum concentration was 5.67 wt%, and the average is 0.64 wt% (nearly 5× the North American shale composite weighted average value of 0.13 wt%; Gromet et al. Reference Gromet, Dymek, Haskin and Korotev1984). The minimum P values were in skeletal grainstones, and the maximum values were at horizons showing macroscopic authigenic phosphate in the form of crusts, amorphous grains and steinkerns, typically in wackestones and packstone, but occasionally concentrated in shales.
5.b. Chitinozoan record
All the biostratigraphic samples studied from the MY-14-01 core were productive and generally yielded assemblages of abundant and well-preserved chitinozoans. A total of 50 different taxa are identified (Figure 6; also see Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025). Co-occurring with the chitinozoans in the organic residues are acritarchs, graptolite fragments, melanosclerites and scolecodonts. Only the chitinozoans in these residues were used to extract biostratigraphic information. Diversity trends shown by chitinozoans of the MY-14-01 core are generally consistent with global marine invertebrate diversity estimates for the same period, with decreasing diversity from the lower Katian (∼Chatfieldian-Edenian) onwards (Rasmussen et al. Reference Rasmussen, Kröger, Nielsen and Colmenar2019).
Log of the MY-14-01 core showing the stratigraphic positions of the chitinozoan samples studied, the ranges of selected species (closed and open circles represent certain and uncertain identifications, respectively), the proposed biozonation and species diversity curves, modified from Esteves et al. (Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025). Abbreviations and acronyms: def = deformed beds; FAD = first appearance datum; B. = Belonechitina; H. = Hercochitina; Pl. = Plectochitina; Ac. = Acanthochitina; K. = Kalochitina; Pt. = Pterochitina; Co. = Conochitina; D. = Desmochitina; Anc. = Ancyrochitina.
5.c. Stable δ13Ccarb stratigraphy
In the MY-14-01 core, δ13Ccarb values range from −2.0 to +1.3‰ (Figure 4; Supplementary Table S2). The 363 stable isotope results for the 224 m (735 ft) mixed carbonate–siliciclastic succession in the MY-14-01 core have a relatively high degree of scatter with the distance between adjacent sample horizons ranging up to 1.8‰ with an average of 0.4‰. This scatter in values is not strongly correlated with composition (e.g. Ca vs δ13Ccarb, R 2 = 0.28). Smoothing of the δ13Ccarb curve using a 3 pt moving average reveals three broad oscillations in values (Figure 4). These oscillations span approximately 60 m (200 ft) of stratigraphy each, with peaks in the Kope, Grant Lake and Bull Fork formations.
Stable isotope results for the OGS-2682 core show δ13Ccarb values that range from −1.0 to +1.7 (Figure 5; Supplementary Table S2). The 35 results for this 12 m (40 ft) interval show less scatter than the MY-14-01 core, with the distance between adjacent samples ranging up to 1.1‰ with an average of 0.3‰. The most positive values are found in the lower part of the Centerville Member of the Whippoorwill Formation.
6. Discussion
6.a. Lithostratigraphy
Lithostratigraphic formation picks were interpreted based on distinctive stratigraphic features defined in previous studies and less so on the lithologic succession (quantified via pXRF). These lithostratigraphic features are often related to lowest appearances or changes in abundance of brachiopod genera or macrofaunal assemblages as the high-frequency oscillation between limestone and shale renders a purely lithologic basis for picking formation contacts highly uncertain (cf. Supplementary Table S1). Within the MY-14-01 core the top of the Millbrig K-bentonite, marking the base of the Chatfieldian Stage, occurs 8 cm below a hardground at the base of the Lexington Limestone (310.8 m, 1019.8 ft). Our study interval begins 71.5 m (236 ft) higher, near the top of the Point Pleasant Formation (239.3 m, 785 ft), and extends upwards to 15.2 m (50 ft) below the land surface (Figure 4). The contact between the Point Pleasant Formation and the overlying Kope Formation is picked at 209.7 m (688 ft). This interpretation is based primarily on previously documented patterns of abrupt macrofauna change at the base of the Kope Formation, which includes the epibole of the distinctive crinoid Merocrinus (McLaughlin and Brett, Reference McLaughlin and Brett2007). The similarity of elemental geochemical patterns on either side of this horizon reflects the subtle change in lithologic composition between these formations in northern Kentucky and adjacent Ohio that led some mappers to lump the two units together within the Clays Ferry Formation (cf. Weir and Green, Reference Weir and Green1965). The contact between the Kope and Fairview formations (452 ft/137.8 m) in the core is picked based on the abrupt increase in carbonate and the appearance of brachiopod coquinas (cf. Brett et al. Reference Brett, Algeo, McLaughlin and Harries2008). The middle part of the Fairview Formation contains abundant quartz silt, and the upper part features multiple horizons of soft-sediment-deformation capped by cross-bedded calcarenite (Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). The contact between Fairview and Grant Lake formations (104.2 m, 342 ft) is picked with confidence based on the abrupt drop in quartz silt and return of brachiopod coquinas featuring the large orthid Vinlandostrophia ponderosa. The contact between the Grant Lake and Bull Fork formations (69.8 m, 229 ft) is marked by a decrease in carbonate, an increase in shale, and a change in the dominant macrofauna. The lowest horizon of Grewingkia rugose corals (Figures 2b and 4), used in local mapping, is present within the core at 50.3 m (165 ft). Similar to the findings of the outcrop-based mapping studies, no identifiable graptolites were recovered during facies analysis and biostratigraphic study of the MY-14-01 core.
The lithostratigraphic successions spanning the Ordovician–Silurian boundary at the Jacksonville outcrop and within the OGS-2682 core are highly similar and show an upsection shift from siliciclastic to carbonate-dominated facies (Figure 5). The Preachersville Member of the Drakes Formation is dominated by red shales and thin calcisiltites with occasional wackestones and packstones containing low diversity macrofaunas. The contact with the overlying Centerville Member of the Whipporwill Formation (sensu Farnam et al. Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026) at the Jacksonville outcrop is sharp and marked by an undulating surface. Within the OGS-2682 core, the contact is also sharp and overlain by stromatolitic carbonate mudstone at the base of the Centerville Member. The remainder of the Centerville Member at both locations is composed of alternating shales and calcareous siltstones. The contact with the Belfast Member A is also sharp and marks a change to massive, Thallasinoides-burrowed, argillaceous silty calcisiltite. The Belfast A is offset from the Belfast Member B by a sharp contact, and the latter is composed of cherty skeletal wackestone to packstone similar to the overlying Brassfield. The contact with the base of the Brassfield Formation is marked by a change to more massive bedding and more abundant grainstones.
6.b. Chitinozoan biozonation
In North America, chitinozoan biozonations were previously constructed for Chatfieldian through Gamachian strata in some areas, but none have been applied to the Cincinnati area. Originally, five chitinozoan biozones were recognized for the Viola Limestone and Fernvale Limestone of Oklahoma (Jenkins, Reference Jenkins1969), representing the Chatfieldian through Richmondian stages (Bergström et al. Reference Bergström, Young and Schmitz2010 b). Unfortunately, the taxonomy is now outdated and difficult to correct or correlate without high-quality SEM images (although reassessment of this succession has begun; Lardinois, Reference Lardinois2025). More recently, several biozonations were proposed for eastern Canada (Achab, Reference Achab1989; Achab et al. Reference Achab, Asselin, Desrochers, Riva and Farley2011, Reference Achab, Asselin, Desrochers and Riva2013; McLaughlin et al. Reference McLaughlin, Emsbo, Desrochers, Bancroft, Brett, Riva, Premo, Neymark, Achab, Asselin and Emmons2016), two sections in central Nevada, USA, and one in Arctic Canada (upper Katian–Hirnantian ≃ upper Richmondian–Gamachian; Soufiane and Achab, Reference Soufiane and Achab2000 b), and for out-board Laurentian areas in Scotland (Vandenbroucke et al. Reference Vandenbroucke, Verniers and Clarkson2003; Lissens et al. Reference Lissens, Vancoppenolle, Ingham and Vandenbroucke2026).
Based on the lowest local occurrence of selected species (as similarly done by Achab, Reference Achab1989, and Verniers et al. Reference Verniers, Nestor, Paris, Dufka, Sutherland and Van Grootel1995), chitinozoan biozones are proposed for the MY-14-01 core (Figure 6), with application in the Cincinnati area (see Sections 6.c.2. and 6.c.3.). The description of the relevant species used in the construction of this biozonation is treated elsewhere (Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025). The selection of index species was partially influenced by their dominant roles in the biozonal frameworks defined for eastern Canada (Achab, Reference Achab1989) and the UK (Vandenbroucke, Reference Vandenbroucke2008 a).
6.b.1. The Belonechitina duplicitas Biozone (Achab, Reference Achab1989; adapted)
Definition: This biozone corresponds to the partial-range of B. duplicitas (Figure 7a), from its lowest occurrence (LO) up to the LO of the index species of the overlying biozone.
Scanning electron microscope images of the index species of the chitinozoan biozones of the MY-14-01 core, modified from Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025. (a) Belonechitina duplicitas (Martin, Reference Martin1983; MY-731.9 ft, 223.1 m, Point Pleasant Formation). (b) Acanthochitina cancellata Martin, Reference Martin1983 (MY-761.0 ft, 232.0 m, Point Pleasant Formation). (c) Plectochitina spongiosa (Achab, Reference Achab1977; MY-633.5 ft, 193.1 m, Kope Formation). (d) Conochitina? pygmaea Achab, Reference Achab1987 (MY-653.3 ft, 199.1 m, Kope Formation). (e) Acanthochitina latebrosa Vandenbroucke, Reference Vandenbroucke2008 b (MY-584.6 ft, 178.2 m, Kope Formation). (f) Clathrochitina mangle Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025 (MY-477.0 ft, 145.4 m, Kope Formation, holotype, RBINS b 10050). (g, h) Tanuchitina hooksae Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025 (g, MY-185.0 ft, 56.4 m, holotype, RBINS b 10042/h, MY-114.8 ft, 35.0 m; Bull Fork Formation). (i) Belonechitina senta (Achab, Reference Achab1978; MY-075.0 ft, 22.9 m, Bull Fork Formation). All scale bars = 50 µm, except when another value is indicated in the figure.
Significant chitinozoan occurrences: Belonechitina kjellstromi and Hercochitina spinetum (Figure 6), consistently reported in other assemblages of the Chatfieldian Stage in Laurentia (e.g. Martin, Reference Martin1983; Melchin and Legault, Reference Melchin and Legault1985; Asselin et al. Reference Asselin, Achab and Soufiane2000, Reference Asselin, Achab and Soufiane2004).
Occurrences in the Cincinnati area: Point Pleasant Formation, Chatfieldian Stage, samples MY-778.3 ft (237.2 m) and MY-770.3 ft (234.8 m), MY-14-01 core. The documented LO of the index fossil defining this biozone in the MY-14-01 core likely does not represent its regional first appearance datum (FAD) since it occurs in the lowermost sample studied.
Tentative correlations: This biozone could be equivalent to the ‘Kalochitina multispinata and Hercochitina? duplicitas’ Biozone defined by Achab (Reference Achab1989), based on the first occurrence of both taxa and their distribution in six samples of the Saint-Casimir Formation of the Trenton Group near the city of Neuville, Province of Quebec (Martin, Reference Martin1983). However, in the MY-14-01 core, the LOs of B. duplicitas and K. multispinata are slightly diachronous, with the former occurring at sample MY-778 ft (237.2 m), and the LO of K. multispinata is in sample MY-761.0 ft (232 m). Asselin et al. (Reference Asselin, Achab and Soufiane2004) studied a sample from a K-bentonite bed (Casimir Member of the Neuville Formation), containing a chitinozoan assemblage indicative of the B. duplicitas Biozone. That K-bentonite level was tentatively correlated by Bergström and Mitchell (Reference Bergström, Mitchell and Landing1994) to a level within the Corynoides americanus graptolite Biozone.
Remarks: Achab (Reference Achab1989) considered her biozone to correspond to the C2 microfauna of Martin (Reference Martin1983).
6.b.2. The Acanthochitina cancellata Biozone (Achab, Reference Achab1989)
Definition: This biozone corresponds to the partial-range of Ac. cancellata (Figure 7b), from its LO up to the LO of the index species of the overlying biozone (Achab, Reference Achab1989).
Significant chitinozoan occurrences: B. kjellstromi, Desmochitina minor, Cyathochitina brevis and K. multispinata are characteristic of this biozone in the Province of Quebec (Martin, Reference Martin1983; Achab, Reference Achab1989) and also occur in the MY-14-01 core. The presence of Hercochitina edingerae is relevant (Figure 6).
Occurrences in the Cincinnati area: Point Pleasant Formation, Chatfieldian Stage, samples MY-761.0 ft (232.0 m) to MY-731.9 ft (223.1 m), MY-14-01 core.
Tentative correlations: This biozone was defined by Achab (Reference Achab1989) after being recognized in four samples from the upper Neuville (Trenton Group) and lower Lotbinière (Utica Group) formations, from the bay to the west of Delisle Point (46°41′57″ N, 71°36′29″ W) and the Verreault quarry (46°51′31″ N, 71°12′11″ W), Province of Quebec (Martin, Reference Martin1983). In this region, the Ac. cancellata Biozone covers the gradual transition from the Trenton Group to the Utica Group, characterized by the Orthograptus ruedemanni graptolite Biozone and the lower part of the Dip. spiniferus Biozone (Riva, Reference Riva and Kay1969; Martin, Reference Martin1983).
Remarks: Achab (Reference Achab1989) considered this biozone equivalent to the C5 microfauna of Martin (Reference Martin1983).
6.b.3. The Plectochitina spongiosa Biozone (Achab, Reference Achab1989)
Definition: This biozone corresponds to the partial-range of Pl. spongiosa (Figure 7c), from its LO up to the LO of the index species of the overlying biozone (Achab, Reference Achab1989).
Significant chitinozoan occurrences: Pterochitina hymenelytrum and Ancyrochitina barbescens occur in this biozone, an assemblage comparable to the one used in its definition (Martin, Reference Martin1983; Achab, Reference Achab1989), and to others from Laurentia (Achab, Reference Achab1987). The presence of Hercochitina turnbulli and Hercochitina anningae is also considered relevant (Figure 6). The HOs of B. duplicitas, B. kjellstromi and H. edingerae occur within this biozone.
Occurrences in the Cincinnati area: upper Point Pleasant to lower Kope Formation, lower Edenian Stage (revised, section 6.d.2. below), samples MY-722.6 ft (220.2 m) to MY-672.3 ft (204.9 m), MY-14-01 core. Likely also occurs in other successions of Kentucky: in the upper Lexington Limestone (Minerva CA-38 core and Frankfort section; Figure 1c, localities 7 and 8, respectively), Point Pleasant Formation (Minerva CA-38 core), Clays Ferry Formation (Frankfort and Clays Ferry sections) and lower Kope Formation (Minerva CA-38 core; Covington section, Figure 1c, locality 6), after revision of Knabe (Reference Knabe1980; Ancyrochitina sp. 1 Miller, Reference Miller1976 = Pl. spongiosa).
Tentative correlations: At Cincinnati, the base of the Geniculograptus pygmaeus Biozone is 25 m above the base of the Kope Formation (Bergström and Mitchell, Reference Bergström and Mitchell1986) and should occupy a similar position in the MY-14-01 core based on high-resolution cycle stratigraphy (Brett et al. Reference Brett, Algeo, McLaughlin and Harries2008). In the MY-14-01 core, the Pl. spongiosa Biozone occurs below the estimated FAD of the G. pygmaeus Biozone and therefore could partially correlate with the older Dip. spiniferus Biozone (e.g. Maletz, Reference Maletz2021).
The Pl. spongiosa Biozone is based on three samples from the Delisle Member of the Lotbinière Formation (Utica Group), around Delisle Point (46°41′48″ N, 71°36′09″ W), Province of Quebec (Martin, Reference Martin1983). A similar chitinozoan assemblage to the one attributed to this biozone (Achab, Reference Achab1989) was identified in four samples of the Utica Group of the Bald Mountain Saint-Roch borehole (near Montréal, Province of Quebec; Achab, Reference Achab1987). This assemblage occurs at the base of the G. pygmaeus Biozone (Riva, Reference Riva and Kay1969; Achab, Reference Achab1987), the graptolite biozone above the Dip. spiniferus (e.g. Maletz, Reference Maletz2021). Therefore, the Pl. spongiosa Biozone may be older in the Cincinnati area than in the strata where it was originally defined.
Remarks: Achab (Reference Achab1989) considered the appearance of Pl. spongiosa, Anc. barbescens and Pt. hymenelytrum to mark the base of the C6 microfauna of Martin (Reference Martin1983).
6.b.4. The Conochitina? pygmaea Biozone (Achab, Reference Achab1989; adapted)
Definition: This biozone corresponds to the partial-range of Co.? pygmaea (Figure 7d), from its LO up to the LO of the index species of the overlying biozone.
Significant chitinozoan occurrences: The LO of Hercochitina krafftae, the presence of Hercochitina sp. 2 (sensu Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025) and the HOs of Ac. cancellata and H. turnbulli within this biozone are pertinent to mention (Figure 6).
Occurrences in the Cincinnati area: the Kope Formation, Edenian Stage, samples MY-653.3 ft (199.1 m) to MY-603.5 ft (183.9 m), MY-14-01 core.
Tentative correlations: In the MY-14-01 core, the Co.? pygmaea Biozone partially overlaps with the estimated range of the G. pygmaeus Biozone in the region (Bergström and Mitchell, Reference Bergström and Mitchell1986; Goldman and Bergström, Reference Goldman and Bergström1997; Maletz, Reference Maletz2021).
Achab (Reference Achab1989) defined the Conochitina pygmaea and Hercochitina cristata Biozone with its base corresponding to the LO of the nominal taxa, up to the LO of the index species of the overlying biozone. This biozone was recognized in 12 samples from the upper Utica to lower Lorraine groups, of the Bald Mountain Saint-Roch borehole (near Montréal) and from the Macasty Formation of Anticosti Island (Lowlands Gamache Princeton Lake No. 1, and Lowlands Gamache Carleton Point No. 1, cores, the number of samples not provided), in strata belonging to the G. pygmaeus Biozone (Riva, Reference Riva and Kay1969; Achab, Reference Achab1987). H. cristata has not been identified with certainty in the studied core, but we still consider the Co.? pygmaea Biozone defined here to be partially equivalent to the biozone defined by Achab (Reference Achab1989).
6.b.5. The Acanthochitina latebrosa Biozone (Vandenbroucke, Reference Vandenbroucke2008a; adapted)
Definition: This biozone corresponds to the partial-range of Ac. latebrosa (Figure 7e), from its LO up to the LO of the index species of the overlying biozone.
Significant chitinozoan occurrences: The LOs of Hercochitina tharpae and Belonechitina sp. 2 (sensu Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025), the presence of H. krafftae and the HOs of Cy. brevis and H. anningae within this biozone are also relevant to mention (Figure 6).
Occurrences in the Cincinnati area: the Kope Formation, Edenian Stage, samples MY-584.6 ft (178.2 m) to MY-487.0 ft (148.4 m), MY-14-01 core.
Tentative correlations: In the MY-14-01 core, these levels overlap with the estimated range of the G. pygmaeus Biozone in the region (Bergström and Mitchell, Reference Bergström and Mitchell1986; Goldman and Bergström, Reference Goldman and Bergström1997).
In British Avalonia (England and Wales), Vandenbroucke et al. (Reference Vandenbroucke, Ancilletta, Fortey and Verniers2009) defined an Ac. latebrosa-Ancyrochitina onniensis Biozone based on the same definition as we use herein (also see Vandenbroucke, Reference Vandenbroucke2008 a). The Ac. latebrosa-Anc. onniensis assemblage from the UK was recognized in the Onny Formation, in the type locality of the Onnian British Regional Stage (upper Caradoc), which is stratigraphically consistent with the levels where Ac. latebrosa is occurring in the Maysville core.
Remarks: Anc. onniensis was considered an important accessory index fossil of the biozone in the UK, but it was not found in the MY-14-01 core.
6.b.6. The Clathrochitina mangle Biozone (new)
Definition: This biozone corresponds to the partial-range of Cl. mangle (Figure 7f), from its LO up to the LO of the index species of the overlying biozone.
Type stratum: upper Kope Formation in the MY-14-01 core, Maysville, Kentucky, USA.
Significant chitinozoan occurrences: Hercochitina andresenae and Angochitina cf. capillata are considered accessory chitinozoans (Figure 6).
Occurrences in the Cincinnati area: In the MY-14-01 core, this biozone occurs in the upper Kope Formation and the Fairview Formation, Maysvillian Stage (revised, section 6.d.3. below), in samples MY-477.0 ft (145.4 m) to MY-350.7 ft (106.9 m). Also, Cl. mangle (under a different name, see synonymy discussed in Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025) has been identified from other successions in the upper Clays Ferry Formation and Garrard Siltstone (Clays Ferry section, Kentucky), the upper part of the Kope Formation (Maysville road – Figure 1c, locality 2 – and Covington sections, and Minerva CA-38 core, Kentucky; Clifton Avenue section, Cincinnati, Ohio - Figure 1c, locality 3), the Fairview Formation (Maysville road and Covington sections, and Minerva CA-38 core, Kentucky; Clifton Avenue section, Cincinnati, Ohio), the lower Grant Lake Formation (Maysville road), and the Dillsboro Formation (New Point core, Indiana; Figure 1c, locality 4) (Miller, Reference Miller1976; Knabe, Reference Knabe1980).
6.b.7. The Tanuchitina hooksae Biozone (new)
Definition: This biozone corresponds to the partial-range of T. hooksae (Figure 7g and h), from its LO up to the LO of the index species of the overlying biozone.
Type stratum: Grant Lake Formation in the MY-14-01 core, Maysville, Kentucky, USA.
Significant chitinozoan occurrences: Hyalochitina hyalophrys?, Belonechitina laciniata and Conochitina rudis are considered accessory chitinozoans. The LO of Hercochitina polygonia and the HO of H. andresenae within this biozone are also relevant to mention (Figure 6).
Occurrences in the Cincinnati area: Grant Lake and Bull Fork formations, straddling the Maysvillian–Richmondian boundary, samples MY-333.0 ft (101.5 m) to MY-095.0 ft (29.0 m), MY-14-01 core. Likely also occurs in other successions, in the uppermost Fairview Formation (Clifton Avenue section, Cincinnati, Ohio), the Grant Lake Formation (Maysville road section, Kentucky; Clifton Avenue section, Cincinnati, Ohio), the Mount Auburn and Tanner’s Creek formations (China/Dry Fork section, Indiana; Figure 1c, locality 5) and the Dillsboro Formation (New Point core, Indiana), after revision of Miller (Reference Miller1976; Hy. hyalophrys sensu Miller Reference Miller1976 ∼ T. hooksae Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025 and Hy. hyalophrys? sensu Esteves et al. Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025).
Tentative correlations: The transitional facies between the Grant Lake Formation and Bull Fork Formation are likely equivalent to those of the Arnheim Formation. Near Cincinnati, the base of the Pa. manitoulinensis Biozone occurs close to the base of the Arnheim Formation (Goldman and Bergström, Reference Goldman and Bergström1997). Additionally, the FAD of the conodont A. ordovicicus in the Cincinnati region is found near the top of the Arnheim Formation (Bergström and MacKenzie, Reference Bergström and MacKenzie2005). Using the regional sequence stratigraphic framework of Brett et al. (Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020) we estimate the A. ordovicicus Biozone to start around 200 ft (61 m) in the MY-14-01 core, within the lower part of the Bull Fork Formation, and about the middle part of the T. hooksae Biozone.
In the MY-14-01 core, a species tentatively identified as Hy. hyalophrys occurs in the lower part of the T. hooksae Biozone. Hy. hyalophrys is the index species of a biozone defined by Achab (Reference Achab1989), recognized in one sample from the upper Lorraine Group of the Bald Mountain Saint-Roch borehole (near Montréal; Achab, Reference Achab1987). This sample is above the range of Pa. manitoulinensis (Riva, Reference Riva and Kay1969; Achab, Reference Achab1987), but probably within the Pa. manitoulinensis Biozone. Hy. hyalophrys has also been identified in the Grant Lake Formation in Ohio and Kentucky, usually occurring in strata without other diagnostic biostratigraphic markers (Miller, Reference Miller1976; Knabe, Reference Knabe1980).
6.b.8. The Belonechitina senta Biozone (Achab, Reference Achab1989; adapted)
Definition: This biozone corresponds to the partial-range of B. senta (Figure 7i), from its LO up to the LO of the index species of the overlying biozone, which remains undefined in this section.
Significant chitinozoan occurrences: The HOs of Hercochitina polygonia, Angochitina cf. capillata and Angochitina oklahomensis within this biozone are relevant to mention (Figure 6).
Occurrences in the Cincinnati area: Bull Fork Formation, Richmondian, samples MY-085.0 ft (25.9 m) to MY-055.0 ft (16.8 m; uppermost studied sample), MY-14-01 core. Although Esteves et al. (Reference Esteves, McLaughlin, Velleman, De Backer, De Weirdt, Miller and Vandenbroucke2025) identified B. senta with doubt (one flattened specimen in sample 065.4 ft/19.9 m) in the MY-14-01 core, the reexamination of two samples below (075.0 ft/22.9 m and 085.5 ft/26.1 m) during this study found specimens that can certaintly be attributed to this species.
Tentative correlations: In the NACP borehole drilled in the Anticosti Island, the B. senta Biozone was defined by Achab (Reference Achab1989), based on the first appearance of the index species at the base of the Vauréal Formation. These units have been included within the Pa. manitoulinensis Biozone (McLaughlin et al. Reference McLaughlin, Emsbo, Desrochers, Bancroft, Brett, Riva, Premo, Neymark, Achab, Asselin and Emmons2016). The observations of Eisenackitina cf. ripae and Hercochitina aff. longi at the top of the MY-14-01 core suggest that, if these ‘open nomenclature’ occurrences could be confirmed in the future (which remains an open question), the LOs of these species may be in considerably older strata in Maysville than they are elsewhere (cf. Soufiane and Achab, Reference Soufiane and Achab2000 b; Achab et al. Reference Achab, Asselin, Desrochers and Riva2013).
6.c. Carbon isotope stratigraphy
The δ13Ccarb trends in the MY-14-01 and OGS-2682 cores present a combination of previously documented and newly recognized features. The primary δ13Ccarb reference curve for the Cincinnati area is a composite of three sections (Figure 8; Bergström et al. Reference Bergström, Young and Schmitz2010 b). The basal portion of the composite is from outcrops near Frankfort, Kentucky, that closely replicated patterns documented in Iowa, Missouri, and elsewhere containing the Guttenburg positive excursion (Bergström et al. Reference Bergström, Schmitz, Saltzman, Huff, Finney and Berry2010 a). The middle of the composite is from the Minerva CA-38 core that was drilled only 23 km northwest of the MY-14-01 core. The upper portion of the composite is from the Elkhorn core (Figure 1c, locality 12) drilled in Ohio, near Richmond, Indiana – type area of the Richmondian Stage. Richmond is on the northwestern edge of the Cincinnati area and more than 160 km north-northwest of the MY-14-01 core.
Summary chronostratigraphic diagram for the Cincinnati area with new information added from Maysville (Kentucky) and southern Ohio. Time scale in millions of years plotted using recently published U-Pb CA-ID-TIMS ages for the base of the Chatfieldian Stage (Metzger et al. Reference Metzger, Ramezani, Bowring and Fike2021) and the Ordovician–Silurian boundary (Zhang et al. Reference Zhang, Yang, Sahy, Zhan, Wu, Li, Deng, Huang, Condon, Rong and Li2025), and the CONOP calculated age base of the D. complanatus Biozone at 448.5 Ma (Cooper and Sadler, Reference Cooper, Sadler, Gradstein, Ogg, Schmitz and Ogg2012). Revised positions for the bases of the Edenian, Maysvillian and Richmondian regional stages (as the LOs of the chitinozoans Pl. spongiosa and Cl. mangle and the FAD of the conodont A. ordovicicus, respectively) are indicated with green arrows and lines. Graptolite and conodont biostratigraphy are as shown in Figure 3. Chitinozoan zonation is as developed in this study. Dashed lines indicate uncertainty. Relative age distribution of sequences M4 through C8 and associated lithostratigraphy adapted from Brett et al. (Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020). Comparison of MY-14-01 and OGS-2682 δ13Ccarb trends with Cincinnati area composite (Bergström et al. Reference Bergström, Young and Schmitz2010 b) and O-S boundary interval at Sand Creek stream cut, Greensburg, Indiana (Farnam et al. Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026; Figure 1c, locality 13). Chemostratigraphic profiles rescaled to match depositional sequences. Abbreviations and acronyms: GSt = Global Stage; RSt = Regional Stage; A. = Amorphognathus; D. = Dicellograptus.
The Minerva CA-38 core range of δ13Ccarb values and trend are closely replicated in the MY-14-01 core (Figure 8). Stable carbon isotope data from the Minerva core start in the Lexington Limestone and extend upwards into the Fairview Formation. The δ13Ccarb profile shows a dip in values near the top of the Point Pleasant Formation followed by a broad peak spanning the lower Kope Formation (i.e. the Kope Excursion) and topped with a rise in values in the lower to mid-Fairview Formation (i.e. Fairview Excursion).
By contrast with the Minerva CA-38 core, the Elkhorn core shows several differences from the MY-14-01 core. The Elkhorn core shows low δ13Ccarb values in the Grant Lake Formation followed by a rise in values that peak in the Waynesville Formation (i.e. the Waynesville Excursion; Figure 8). These trends only partially agree with those in the MY-14-01 core. Rather than showng a drop in values in the Grant Lake Formation, the trend in the MY-14-01 core shows a prominent peak followed by a decrease in values near the top of the unit. The average values in the ‘Grant Lake’ in the Elkhorn core are approximately −1.0‰, whereas they are +0.5‰ in the MY-14-01 core. The Bullfork Formation in MY-14-01 also appears to show only a portion of the Waynesville Excursion, as preserved in the Elkhorn core. Given the excellent preservation of the strata in both cores and similarities in sampling approach and analysis, but significant difference in δ13Ccarb values, it is unlikely that the strata assigned to the Grant Lake Formation and Bull Fork Formation (=Arnheim-Waynesville formations) in the two cores are exactly age equivalent (i.e. lithostratigraphic correlations cross-cut chronostratigraphic correlations). To clarify the source of disconnect between the δ13Ccarb record in the MY-14-01 and Elkhorn cores, additional data are required. First, additional work is needed to demonstrate the reproducibility of the positive excursion in the MY-14-01 core Grant Lake Formation. Second, additional biostratigraphic control is needed to establish a more robust chronocorrelation between the two cores.
The OGS-2682 core yields δ13Ccarb results that clarify the age of the Preachersville Member of the Drakes Formation and reinforce recent studies calling for a shift in the position of the Ordovician–Silurian boundary in the Cincinnati area. The Preachersville Member contains δ13Ccarb values that are elevated relative to those at the top of the MY-14-01 core, but closely match the values of the Whitewater and Elkhorn formations in the Elkhorn core (i.e. Elkhorn Excursion, Bergström et al. Reference Bergström, Young and Schmitz2010 b; Figure 8). The sharp contact at the base of the overlying Whippoorwill Formation, known regionally as the Cherokee unconformity, is marked by a positive offset in δ13Ccarb values. The Whippoorwill Formation δ13Ccarb values show a negative slope that is continuous through both the Centerville Member and Belfast A, interpreted as the descending limb of the HICE. The abrupt change in the δ13Ccarb trend at the base of Belfast B likely marks a discontinuity surface. Strata of the Belfast B and Brassfield Formation have a relatively flat δ13Ccarb trend.
The Jacksonville road cut preserves a succession similar to that of the OGS-2682 core and represents one of the best exposures of the Ordovician–Silurian boundary interval in the Cincinnati area of southern Ohio (Figure 5a). The conodont assemblages from this section were recently revised (Farnam et al. Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026; Waid et al. Reference Waid, Farnam, Hogancamp and Brett2026; in their studies, Jacksonville road cut = West Union section). The Centerville Member, Belfast A and lower Belfast B yield conodonts of the O. hassi Biozone, suggesting a late Hirnantian age, and the upper Belfast B and the Brassfield Formation contain conodonts of the Distomodus kentuckyensis Biozone, indicating a Rhuddanian age (Waid et al. Reference Waid, Farnam, Hogancamp and Brett2026). The positive carbon isotope excursion in the Whippoorwill Formation of the OGS-2682 core confirms a latest Hirnantian age for this unit, consistent with recent work by Farnam et al. (Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026). Thus, the base of Belfast B marks the Ordovician–Silurian boundary in the OGS-2862 core, on the eastern side of the Cincinnati area.
6.d. Advancing regional chronostratigraphy
The results of this study provide an opportunity to advance the chronostratigraphic characterization of North American Upper Ordovician regional stages. Although there remain many reasons to carry on intercontinental correlation efforts, advances made in this and other studies (e.g. Cocks et al. Reference Cocks, Fortey and Rushton2010; Meidla et al. Reference Meidla, Ainsaar, Hints, Radzevičius, Harper, Lefebvre, Percival and Servais2023; Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen, Stouge, Harper, Lefebvre, Percival and Servais2023) underscore the importance of continuing to refine the characterization of regional stages and their chronocorrelation.
6.d.1. Chatfieldian Stage
In North America, the Millbrig K-bentonite marks the base of the Chatfieldian Stage (Bergström et al. Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009). This is one of the strongest temporal anchor points in the Ordovician of eastern North America due to the advanced geochemical characterization that this bed has received across multiple locations. Tephrostratigraphic correlation of the Millbrig K-bentonite across broad stretches of eastern North America utilized a variety of geochemical fingerprinting techniques (Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015, and references therein). Geochronology utilizing CA-ID-TIMS U-Pb analysis of Millbrig zircons yields similar ages across eastern North America, providing a preferred eruption age of 453.36 ± 0.14/0.24/0.54 Ma (Metzger et al. Reference Metzger, Ramezani, Bowring and Fike2021; X-Y-Z 2σ error notation). This age suggests that it is slightly older than the Global Stratotype Section and Point for the base of the global Katian Stage at the Black Knob Ridge section, Oklahoma, calculated through Constrained Optimization (CONOP) modelling at 452.75 Ma and defined by the FAD of the graptolite Dipl. caudatus (Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020). Unfortunately, the Millbrig K-bentonite is only known to occur in North America and is not found west of Iowa, limiting its utility to define the base of the stage in western localities (Kolata et al. Reference Kolata, Huff and Bergström1996).
Defining a new biostratigraphic or chemostratigraphic horizon for the base of the Chatfieldian Stage will require additional work. The current base of the Chatfieldian occurs near the base of the Plectodina tenuis conodont Biozone, the base of the δ13Ccarb GICE and the M4-M5 sequence boundary. However, there is some uncertainty about the isochroneity of these horizons regionally relative to the Millbrig K-bentonite (Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015). Ongoing revision of preliminary chitinozoan studies of this interval in cores drilled in Cincinnati, Ohio and western Wisconsin (Velleman, Reference Velleman2016; Bas, Reference Bas2017; De Waele, Reference De Waele2018) may provide additional criteria for redefining the base of the Chatfieldian Stage.
6.d.2. Edenian Stage
The base of the Edenian Stage was designated as the base of the Kope Formation in the Cincinnati area (Bergström in Ross et al. Reference Ross, Adler, Amsden, Bergstrom, Bergström, Carter, Churkin, Cressman, Derby, Dutro, Ethington, Finney, Fisher, Fisher, Harris, Hintze, Ketner, Kolata, Landing, Neuman, Sweet, Pojeta, Potter, Rader, Repetski, Shaver, Thompson and Webers1982; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020, and references therein). This boundary was never defined biostratigraphically, and a lithologic definition is not standard practice for a formal chronostratigraphic unit (Salvador, Reference Salvador1994). The lithologically defined base of the Kope Formation (like other lithostratigraphic units) will shift temporally as the observer moves up and down depositional dip. Only along depositional strike should the age of the Kope Formation remain relatively unchanged.
Based on the findings in this study, the base of the Edenian Stage is redefined as the base of the Pl. spongiosa Biozone (Figure 8), established on the LO of this characteristic index fossil (Figure 7c), at the MY-14-01 core, Maysville, Kentucky (here proposed as the stratotype section). This horizon is also very close to the level of the C1 sequence boundary (Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020) and the onset of the δ13Ccarb Kope Excursion: these features should be used as accessory markers in the absence of chitinozoan data. In the Frankfort section (38.161314°, −84.850235°; Kentucky), the base of Pl. spongiosa Biozone (after revision of Knabe, Reference Knabe1980) also occurs just metres below the onset of the Kope Excursion (P.I. McLaughlin, unpublished data). These findings confirm that the base of Pl. spongiosa Biozone is a reproducible chronostratigraphical horizon that crosscuts-facies changes (cf. Cressman, Reference Cressman1973). By this slightly revised definition of the Edenian Stage, its base still lies above the base of the A. superbus and Belodina confluens conodont biozones and near the top of the Dip. spiniferus graptolite Biozone (Bergström and Mitchell, Reference Bergström and Mitchell1986, Reference Bergström and Mitchell1990, Reference Bergström, Mitchell and Landing1994).
6.d.3. Maysvillian Stage
The base of the Maysvillian Stage was placed at the base of the Fairview Formation in the Cincinnati area (Sweet, Reference Sweet1979 a). This stage occurs within the A. superbus conodont Biozone. The base of the Maysvillian Stage is redefined as the base of the Cl. mangle Biozone (Figure 8), based on the LO of this distinctive index fossil (Figure 7f), at the MY-14-01 core, Maysville, Kentucky (here proposed as the stratotype section). This horizon is 14 m below (∼46 ft) the level of the C2 sequence boundary (Brett et al. Reference Brett, Aucoin, Dattilo, Freeman, Hartshorn, McLaughlin and Schwalbach2020) and between the top of the δ13Ccarb Kope Excursion and the onset of the δ13Ccarb Fairview Excursion; these features are accessory markers to be used in the absence of chitinozoan data. In the Minerva CA-38 core (Kentucky; Figure 1c, locality 7), the base of the Cl. mangle Biozone (after revision of Knabe, Reference Knabe1980) occurs 40 ft/12.2 m below the level of the C2 sequence boundary and between the top and base of the respective Kope and Fairview δ13Ccarb excursions (Bergström et al. Reference Bergström, Young and Schmitz2010 b). These findings confirm that the base of the Cl. mangle Biozone is a reproducible chronostratigraphical horizon.
6.d.4. Richmondian Stage
The base of the Richmondian Stage was considered the top of the Vinlandostrophia ponderosa (brachiopod) beds in the Cincinnati area (Orton, Reference Orton1873 in Sweet, Reference Sweet1979 a), which differs from the position of the last appearance datum/LAD of V. ponderosa (e.g. Stigall, Reference Stigall2023). The top of the V. ponderosa beds is a level slightly above the base of the Arnheim Formation (or equivalent units, such as the lower part of Bull Fork Formation), where an early phase of the Richmondian invasion is recorded (Stigall, Reference Stigall2023, and references therein). However, the isochroneity of this distinctive brachiopod occurrence is not well established and serves as an imprecise stage boundary.
Here we revise the definition for the base of the Richmondian Stage to the base of the A. ordovicicus conodont Biozone. The stratotype is the uppermost Arnheim Formation at the Southgate Hill section, Indiana (after MacKenzie, Reference MacKenzie1993). The FAD of A. ordovicicus is well characterized regionally, occuring near the top of the Arnheim Formation in the Cincinnati area, the top of the Dubuque Formation in eastern Iowa, and the top of the Collingwood Formation in southern Ontario (Bergström and MacKenzie, Reference Bergström and MacKenzie2005; Zhang et al. Reference Zhang, Tarrant and Barnes2011). The FAD of A. ordovicicus is a useful level for global correlation and corresponds to the lower-middle Cautleyan Stage in the UK and the upper Nabala to lowermost Vormsi Stage in Baltoscandia (Nõlvak et al. Reference Nõlvak, Hints and Männik2006; Bergström et al. Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009). The top of the Richmondian Stage in the Cincinnati area is not biostratigraphically well defined and will require additional work.
6.d.5. Gamachian Stage
The striking difference between the macrofauna of the Ellis Bay Formation on Anticosti Island, Quebec, and that of the Richmondian age strata in the Cincinnati area led Twenhofel (Reference Twenhofel1927) to propose the Gamachian Stage above the Richmondian. By definition, the base of this Gamachian Stage is placed at the base of the Ellis Bay Formation, coincident with an abrupt change in conodont fauna (designated Fauna 13; McCracken and Barnes, Reference McCracken and Barnes1981; McCracken and Nowlan, Reference McCracken and Nowlan1988; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020). The Gamachian Stage extends up to the Ordovician-Silurian boundary that was placed at the Ellis Bay-Becscie formational contact. The interpretation that the entire Ellis Bay Formation occurs within the Hirnantian Global Stage is a point of ongoing debate, particularly in regard to the lower members (e.g. Melchin and Holmden, Reference Melchin and Holmden2006; Achab et al. Reference Achab, Asselin, Desrochers, Riva and Farley2011, Reference Achab, Asselin, Desrochers and Riva2013; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020; Zimmt and Jin, Reference Zimmt and Jin2023). However, there seems to be general agreement, due to significant conodont and δ13Ccarb evidence, that the Laframboise Member (top of the Ellis Bay Formation) as well as the basal portion of the overlying Becscie Formation are latest Hirnantian in age (Desrochers et al. Reference Desrochers, Farley, Achab, Asselin and Riva2010; Bergström et al. Reference Bergström, Kleffner, Schmitz and Cramer2011 a; Zimmt et al. Reference Zimmt, Bergmann, Finnegan, Present, Jost and Desrochers2025). Thus, we suggest abandoning the lithostratigraphic definition for the top of the Gamachian Stage at the base of the Becscie Formation and continue to define it as coincident with the Ordovician–Silurian boundary, thus shifting the top of the stage into the lower Bescsie Formation.
Until recently, strata assignable to the Gamachian Stage in the Cincinnati area were considered to be completely absent (e.g. Bergström et al. Reference Bergström, Kleffner and Eriksson2020). Recent work across several portions of eastern North America has discovered thin intervals age equivalent to the later part of the Gamachian Stage. As with the Laframboise Member and the lower Bescsie Formation on Anticosti Island, the Oz. hassi conodont Biozone is coincident with the later part of the HICE δ13Ccarb excursion in the Manitoulin Formation of Ontario (Bergström et al. Reference Bergström, Kleffner, Schmitz and Cramer2011 a; Farnam and Brett, Reference Farnam and Brett2023) and the Wilhelmi-Mosalem interval in Illinois and Iowa (Bergström et al. Reference Bergström, Kleffner and Schmitz2011 b). This global conodont biozone overlaps with the Metabolograptus persculptus graptolite Biozone (e.g. Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020). The top of the Hirnantian δ13C excursion (HICE) occurs in the lowermost part of this conodont biozone (Bergström et al. Reference Bergström, Eriksson, Young, Ahlberg and Schmitz2013; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020). Recently, this combination of features was discovered in the newly designated Whippoorwill Formation in the Cincinnati area (Farnam et al. Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026; Waid et al. Reference Waid, Farnam, Hogancamp and Brett2026).
6.d.6. Cherokee unconformity
Across the Cincinnati area, the Cherokee unconformity separates Richmondian age strata from those now interpreted as latest Gamachian age (Figure 8). The top of the Richmondian age Bull Fork Formation in the MY-14-01 core is assigned to the B. senta Biozone, which on Anticosti Island occurs with graptolites of the P. manitoulinensis Biozone. This now represents the youngest definitive biostratigraphic age in the Richmondian Stage of the Cincinnati area. As noted above, previous chitinozoan study of the Drakes and Whippoorwill formations did not yield biozonally diagnostic taxa (Grahn and Bergström, Reference Grahn and Bergström1985). The late Gamachian age interpretation of the Whippoorwill Formation suggests that it belongs to the Ancyrochitina ellisbayensis Biozone (Soufiane and Achab, Reference Soufiane and Achab2000 a). These chitinozoan biozones span the Dice. complanatus through M. extraordinarius graptolite biozones. Incorporating recent revisions to the age of the base of the Chatfieldian Stage (Metzger et al. Reference Metzger, Ramezani, Bowring and Fike2021) and the age of the O–S boundary as ∼442.2 Ma (Zhang et al. Reference Zhang, Yang, Sahy, Zhan, Wu, Li, Deng, Huang, Condon, Rong and Li2025) as well as the CONOP calculated ages of the base of the D. complanatus graptolite Biozone (Cooper and Sadler, Reference Cooper, Sadler, Gradstein, Ogg, Schmitz and Ogg2012; Goldman et al. Reference Goldman, Sadler, Leslie, Gradstein, Ogg, Schmitz and Ogg2020), the Cherokee unconformity in the Cincinnati area could represent a 6 to 7 million year gap in the Late Ordovician record (Figure 8). However, this large number is more than half the time represented by the global Katian and Hirnantian stages combined. This calculated gap is far larger than that proposed by previous authors. Indeed, Farnam et al. (Reference Farnam, Brett, Shoemaker, Jin, Elias and Waid2026) suggest that the Cherokee unconformity may represent a gap of just a few hundred thousand years. The extreme difference between these interpretations emphasizes the pressing need for additional biostratigraphic and geochronologic constraints in the Upper Ordovician of eastern North America.
Conclusions
New bio-and chemostratigraphic studies of the Upper Ordovician succession in the Cincinnati area strengthen its role as a regional chronostratigraphic anchor point, an essential step in temporally ordering the regionally extensive Upper Ordovician strata in eastern North America. The results of this study:
-
(1) Establish a new chitinozoan biozonation with eight subdivisions for the Cincinnati area. In part, this framework is an expansion of biozones previously defined and utilized in other parts of Laurentia and British Avalonia.
-
(2) Nearly double the published δ13Ccarb data for the Cincinnati area. The new δ13Ccarb curve closely replicates previously published upper Chatfieldian to lower Maysvillian age strata in the Cincinnati area. However, discrepancies in the middle Maysvillian through Richmondian age portions of the curves require further investigation.
-
(3) Modify the primary lower boundary criteria of the Edenian, Maysvillian and Richmondian regional stages to biozone boundaries and assigns basal stratotypes, for a better age constraint and more accurate utilization of the regional chronostratigraphy.
-
(4) Demonstrate that the chronostratigraphic gap at the Cherokee unconformity is significant and could be up to 7 Ma in duration. Additional work is needed to establish the exact biostratigraphic age of the top of the Richmondian Stage in its type area and increase geochronologic control of the interval between the globally recognized GICE and the HICE.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756826100788
Acknowledgements
This research was completed in large part as the doctoral study of CJPE at the University of Ghent and supported by a ‘Fundação para a Ciência e a Tecnologia (FCT)’ PhD grant (SFRH/BD/144840/2019) and also by the Research Foundation–Flanders (research grant G038722N). Travel was supported via the King Baudouin Foundation (Professor T. Van Autenboer Fund). Chitinozoan SEM images were generated using research infrastructure funded through FWO grant I013118N. S. Van Cauwenberghe and A.-E. Debeer are thanked for their assistance with palynology lab protocols and SEM imaging. CJPE thanks J. Colmenar, S. Pereira, and J. Verniers for productive discussions regarding Upper Ordovician stratigraphy and chitinozoan biozonation. PIM was supported by the US National Cooperative Geological Mapping Program and the US Geological Survey Earth Mapping Resources Initiative, grant number G21AC10496-00. Appreciation to the Carmeuse Company for donating the MY-14-01 core and to the Ohio Geological Survey for permission to sample the OGS-2682 core. Thanks to C. Brett and C. Aucoin for discussions of MY-14-01 stratigraphy; C. Wade and C. Farnam for discussion of the OGS-2682 core chemostratigraphy and conodont biostratigraphy of the Ordovician-Silurian boundary interval in the eastern USA; and A. Bancroft for helpful discussion of conodont biostratigraphy. Thanks to O. Hints and C. Brett for thoughtful and constructive reviews. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government. This is a contribution to IGCP 735: ‘Rocks and the Rise of Ordovician Life’.
Competing interests
The authors declare none.
Open access
