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GFF Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/sgff20 Anatomy of a compound sequence boundary: a basal karstic unconformity in the Cincinnati Arch region Carlton E. Bretta, James R. Thomkaa, Nicholas B. Sullivanb & Patrick I. Mclaughlinb a Department of , University of Cincinnati, Cincinnati, OH45221, USA; , b Wisconsin Geological and Natural History Survey, Madison, WI 53705, USA; , Published online: 19 Feb 2014.

To cite this article: Carlton E. Brett, James R. Thomka, Nicholas B. Sullivan & Patrick I. Mclaughlin (2014) Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region, GFF, 136:1, 42-47, DOI: 10.1080/11035897.2014.882978 To link to this article: http://dx.doi.org/10.1080/11035897.2014.882978


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Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region


Brett, C.E., Thomka, J.R., Sullivan, N.B. & Mclaughlin, P.I., 2014: Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region. GFF, Vol. 136 (Pt. 1, March), pp. 42–47. q Geologiska Fo¨reningen. doi: http://dx.doi.org/10.1080/11035897.2014.882978. Abstract: The –Silurian contact in parts of the Cincinnati Arch region is an irregularly pitted reflecting karstic weathering during a lower-mid Silurian (Aeronian) lowstand, superimposed on the Hirnantian–Rhuddanian Cherokee Unconformity. This irregular unconformity is overlain by a unit termed the “golden Brassfield”, which grades laterally into strata identified as the “red Brassfield”, a probable equivalent of the Oldham to the east–southeast. These units are Aeronian or early Telychian, considerably younger than the Rhuddanian–early Aeronian Brassfield Formation sensu stricto. Preservation of erosional topography, resulting from rapid flooding and burial of karstic surfaces, is typical of unconformities representing moderate durations. Keywords: golden Brassfield; sinkholes; Noland Formation; karstification.

1Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA; [email protected], [email protected] 2Wisconsin Geological and Natural History Survey, Madison, WI 53705, USA; [email protected], [email protected] Manuscript received 5 June 2013. Revised manuscript accepted 8 January 2014.

Introduction Unconformities display a variety of distinctive features that there are cases where the base of the Brassfield displays some relate to their mode of origin and the duration of sea-level rise broad undulations (Foerste 1885, 72; K. Hartshorne, personal following a period of exposure and erosion. Paradoxically, many communication, 2013). At certain localities in southern Ohio, of the largest unconformities, such as the boundaries of northern Kentucky and Indiana, the Ordovician–Silurian megasequences defined by Sloss (1963), are flat, nearly unconformity exhibits considerable relief of up to 3 m. At featureless horizons that may nevertheless represent periods of some exposures around Crestwood, Kentucky, the basal Silurian up to several millions of years of missing record. A notable unit, previously termed “Brassfield”, only occurs in local example is the Wallbridge Unconformity, also known as the pockets 5–20 m wide, whereas at other nearby localities, this Louisville Paraconformity in the vicinity of Louisville, unit is absent altogether. Downloaded by [University of Cincinnati Libraries] at 07:21 09 May 2014 Kentucky, which separates Silurian and strata and Consideration of these dual unconformities reveals several locally encompasses as much as 35–40 million years of elapsed important new insights. First, the irregular Ordovician–Silurian time; yet, this discontinuity is a planar surface between similar, contact is, in fact, not the basal Silurian Cherokee Unconformity -rich that, in spite of its temporal significance, is at all, but rather a modification of this surface that occurred difficult to identify in many outcrops (Conkin & Conkin 1976). during at least three later episodes of erosion and dissolution, Likewise, the basal Silurian unconformity, frequently termed wherein hollows were developed in underlying Ordovician the Cherokee Unconformity throughout much of eastern North carbonates. Second, the dolomitic limestone filling these pockets America (Dennison & Head 1975), is a surface that may be is not the true Brassfield Formation; it is a younger unit that has equally cryptic. In most exposures within the Cincinnati Arch been misidentified. Third, regional mapping of this surface region in the Kentucky, Indiana and Ohio tri-state area, this indicates that this particular unconformity, with its distinctive contact occurs between Upper Ordovician dolomitic shales of irregular erosion surfaces, is traceable northeastward into an the Drakes Formation and fossiliferous dolostones of the internal unconformity within the so-called “Brassfield For- Silurian Brassfield Formation (Gordon & Ettensohn 1984; mation” in west-central Ohio, which is therefore a composite Ettensohn et al. 2013). In general, the basal contacts of the unit. This irregular disconformity may also be coextensive with Brassfield, including the so-called Centerville, Belfast and an erosion surface that locally removes the true Brassfield massive/cherty members – all of which rest locally on Upper Formation or reduces it to a series of local cobbles or blocks Ordovician (Richmondian) strata – are nearly planar. However, such as those commonly found in the vicinity of Ripley County, GFF 136 (2014) Brett et al.: Anatomy of a compound sequence boundary 43

Indiana (Foerste 1904, 1935), and which has produced local are uncertain whether these are actually encrusting the surface or knobs of Ordovician limestone that appear to have been simply platy bryozoans that rest on the surface. Elsewhere, developed as small pedestals or “sea stacks” on the mid-Silurian however, oolitic, crinoidal of the “golden Brassfield” seafloor (Mikulic et al. 2012). has been piped into the borings (Trypanites) in the topmost surface of the underlying carbonates. Description and correlation of the In local exposures along I-71 (Fig. 1), an additional complexity is observed wherein the pockets on the upper “golden Brassfield” unconformity surface of the Saluda Dolostone appear to be filled with a pale Where best exposed – in cuts along Route 329 near Crestwood, gray-weathering grainstone of unknown affinities, which is, in 0 00 0 00 Kentucky (N38820 36.29 , W85829 26.35 ) and adjacent sec- turn, sharply cross-cut by the “golden Brassfield” (Fig. 3D). 0 00 tions on I-71 just south of mile-marker 12.5 (N38820 11.27 , This unit is apparently a remnant of an earlier carbonate fill, 0 00 W85831 21.87 ), southward to cuts on Route 31 near Mt. which may be a distinct facies of Aeronian age, a remnant of the 0 00 0 00 Washington, Kentucky (N38804 01.65 , W85832 52.32 ; Fig. 1) true Brassfield, or even a Late Ordovician unit that filled an – the basal Silurian unconformity displays lenses of dolomitic earlier set of erosional pockets on the Saluda surface. This grainstone that occupy channel-like areas along the contact suggests the possibility that at least some of the “golden with thinly laminated dolomitic siltstones of the underlying Brassfield” pods might have accumulated in re-excavated Upper Ordovician Saluda Formation (Katian, Richmondian). cavities of an earlier erosion episode. The cross-cutting surfaces In intervening areas, there is a cryptic, planar unconformity that separate the gray limestone from the “golden Brassfield” separating the Saluda from superficially similar dolostones of are as sharp and steep-sided as those which occur on the Saluda the mid-Silurian (Llandovery, Telychian) Lee Creek Formation Formation (Fig. 3). (sensu Brett et al. 2012; see Fig. 2 and discussion below). Between these pods, the “golden Brassfield” is locally absent These pods, which are commonly , 1.25 m thick and , 5m and the unconformity between Ordovician and Silurian units is wide but can reach up to 20 m in width and 3 m of thickness, essentially flat (Fig. 3C). This planar unconformity is confluent exhibit sharp oblique to vertical or even undercut channelized with the contact between the “golden Brassfield” and an surfaces within the top of the Saluda Formation (Fig. 3A–C). overlying dolostone, formerly referred to as the “Lee Creek These lenses are composed of pale, yellow-orange, dolomitic, Member of the Brassfield” but recently elevated to formational locally oolitic, crinoidal pack- to grainstone that contains rugose status (Brett et al. 2012). This unit is a much younger and tabulate (Favosites, Halysites), brachiopods, gastro- sedimentary body, unrelated to either the “golden Brassfield” or pods and other fossils; this sediment is typical of a distinctive unit the true Brassfield Formation (Fig. 2). referred to as the “golden Brassfield” (McLaughlin et al. 2008). To the northeast, the irregular erosion surface beneath the At this locality, there are no borings on the irregular erosion “golden Brassfield” becomes more pronounced locally. At surface; possible encrusting bryozoans may be present on the Napoleon Quarry, in Ripley County, southeastern Indiana surface (K. Hartshorne, personal communication, 2013), but we (N39812031.3900, W85818053.7400; Fig. 1), irregular masses of pale cream-colored to purplish micritic limestone with Rafinesquina and other Ordovician brachiopods extend up to 2 m into the “golden Brassfield”. These Ordovician limestones appear to be conformable with underlying Upper Ordovician (Elkhorn Formation) strata, although their exact age relation- Piqua Ohio Trypanites quarry ships are presently under study. borings in the, as yet unidentified Ordovician limestone unit, have been infilled with Dayton crinoidal of the “golden Brassfield” (Mikulic et al. 2012). The steep-sided irregular masses have been interpreted as sea stacks buried by “golden Brassfield” sediments (Mikulic Cemex et al. 2012). Hence, this is a local manifestation of the same Napoleon quarry

Downloaded by [University of Cincinnati Libraries] at 07:21 09 May 2014 irregular unconformity. Elsewhere in Ripley County, the true quarry Brassfield may be represented by a series of blocks and cobbles Cincinnati along the Ordovician–Silurian unconformity (Foerste 1904, Indiana 1935); this may be an extreme example of the degree of corrosional degradation along the mid-Aeronian unconformity. Finally, the probably contemporaneous “red Brassfield” shows I-71 roadcuts an irregular unconformity on underlying true Brassfield. For example, in the limestone quarry at Piqua, Ohio (N40807018.6200, Crestwood 8 0 00 Louisville Kentucky W84 13 34.56 ; Fig. 1), north of Dayton, the “red Brassfield” fills channel-like features incised into the lower, massive Brassfield, and at the Cemex quarries at Fairborn, Ohio Mt. Washington (N39846052.8500, W83857038.8600; Fig. 1), east of Dayton, an irregular erosion surface at the base of the “red Brassfield” cuts 0 km 100 km into and locally removes a marker bed rich in mollusks Fig. 1. Map of the Indiana–Kentucky–Ohio tri-state area of eastern- (“cephalopod bed” of McDonough 2006). Hence, irregular relief midcontinental United States with all localities mentioned in the text appears to be a “fingerprint” of the mid-Aeronian unconformity labeled. Latitude and longitude coordinates for localities are given in and the degree of relief and extent of erosion beneath this unit the text. appear to increase to the northwest. 44 Brett et al.: Anatomy of a compound sequence boundary GFF 136 (2014)

Fig. 2. Chronostratigraphic relations between Upper Ordovician–Lower Silurian units discussed in text. Note the lateral equivalency of the “golden Brassfield”, Oldham Member (Noland Formation), and “red Brassfield”, the truncation of these units beneath a regionally angular unconformity at the base of the Waco Member/Lee Creek Formation, and the separation between these units and the underlying true Brassfield. Time scale, biostratigraphic and chemostratigraphic data largely from Melchin et al. (2012) and unpublished data.

In most of these localities, the “golden Brassfield” is overlain displays a series of widespread units including (1) a lower, Downloaded by [University of Cincinnati Libraries] at 07:21 09 May 2014 at a sharp, planar contact by a pale orange-weathering, glauconitic, silty dolostone and shale-the Belfast Member; (2) bioturbated, glauconitic, argillaceous dolostone of the Lee a massive, typically cherty member; (3) a thinly bedded to Creek Formation. Locally, at localities displaying evidence for shaly succession with distinctive, large crinoid columnals karstic erosion, this dolostone rests directly on the superficially (Floricolumnus or “beads”) and (4) an uppermost reddish, similar dolostones of the Upper Ordovician Saluda Formation at hematitic grainstone (Gordon & Ettensohn 1984; Ettensohn et al. a cryptic, but much larger unconformity (Fig. 3). This later 2013). None of these units have been correlated with the “golden disconformity appears to be a widespread unconformity that Brassfield” of the Louisville area, which overlies an irregular underlies the Waco Member of the Noland Formation and unconformity that has locally removed the true Brassfield locally removes a series of older Aeronian units, including the completely and cut downward through the older Cherokee “golden Brassfield” and probably equivalent Oldham Member Unconformity to form irregular hollows. The unit identified as (see below) as well as overlying units (Fig. 2). “golden Brassfield” has recently been traced to the northwest into a distinctive reddish- to greenish-colored grainstone and bioherm-bearing argillaceous unit informally identified as the Age of the “golden Brassfield” and associated “red Brassfield” (McLaughlin et al. 2008). This is younger than units the true Brassfield in its type area in south-central Kentucky, The “golden Brassfield” along the western side of the Cincinnati which is thought to be of Rhuddanian to early Aeronian age. Arch is lithologically distinct from the true Brassfield, which In fact, these local ferruginous grainstones probably record GFF 136 (2014) Brett et al.: Anatomy of a compound sequence boundary 45

Ohio, strongly suggests that this unit is a lateral equivalent of the Oldham Limestone Member of the Noland Formation exposed in the vicinity of Madison, Estill and Powell Counties, Kentucky (Boucot & Ehlers 1963; Rexroad et al. 1965; McLaughlin et al. 2008). The presence of the conodonts Distomodus staurog- nathoides and Icriodina n. sp. A of Rexroad et al. (1965) in the Oldham and the “red Brassfield” suggests a late Aeronian age for both units (Fig. 2). This is consistent with reports of conodonts in the “golden Brassfield” in Indiana (Rexroad et al. 1965; Berry & Boucot 1970). In most localities in south-central Kentucky, the Oldham rests sharply on the underlying Plum Creek Shale Member at a nearly flat contact that probably represents a minor discontinuity. Both units have been traced tentatively as far north as Adams County, Ohio, where they are truncated by an unconformity at the base of the Waco Member (Sullivan 2013). However, the lateral equivalent of the Oldham, the “red Brassfield”, reappears to the northwest beneath the Dayton Formation, occurring near Dayton, Ohio, and extending westward to the Indiana state line. Here, this unit appears to grade into the lighter, yellowish dolo-grainstones of the “golden Brassfield” (McLaughlin et al. 2008). The Lee Creek Formation, which sharply and unconformably overlies the “golden Brassfield”, is of mid-Telychian age, as indicated by conodonts of the Pterospathodus eopennatus Superzone (Ma¨nnik 2007; McLaughlin et al. 2008; Kleffner et al. 2012). It appears to be a remnant of the formerly extensive Waco Member, also probably of the P. eopennatus Superzone (Kleffner et al. 2012), which oversteps units of the Noland and Brassfield formations along a regionally angular unconformity (Lukasik 1988; Brett & Ray 2005; Sullivan 2013).

Interpretation A mid-Silurian (late Aeronian) transgression, probably equiv- alent to the S-2 deepening of Johnson (1996, 2006) and the base of sequence S-IIB (Brett et al. 1990; Brett & Ray 2005), resulted in flooding of an irregular erosion or corrosion surface. This irregular, karstic unconformity formed during a prolonged interval of time following subaerial emergence along a proto- Cincinnati Arch during early Aeronian time in the vicinity of the present-day western Ohio, southeastern Indiana and west north- central Kentucky. It was superimposed upon, and in most areas Fig. 3. Outcrop photographs of “pods” of “golden Brassfield” from

Downloaded by [University of Cincinnati Libraries] at 07:21 09 May 2014 cut through, the pre-existing and better known Cherokee Crestwood, Kentucky, and nearby sections on I-71. Scale bars in B and Unconformity, which separates the basal “true” Brassfield (early C ¼ 0.5 m; scale bar in D ¼ 5 cm. GB, golden Brassfield; GU, unnamed gray unit. A. Irregular, relatively small “pod” standing in sharp contrast Rhuddanian; McLaughlin et al. 2008) from underlying Katian to surrounding dolomitic carbonates. Person (1.65 m) for scale. B. (Ashgillian in older terminology) Saluda or Hitz (upper Large, channelform body of “golden Brassfield” clearly showing Whitewater-Elkhorn) strata (Fig. 2). Hence, the unconformity truncation by overlying Lee Creek Formation. C. Pinched-out edge of underlying the “golden Brassfield” is a composite erosion “pod”, allowing Ordovician strata to be directly overlain by the Lee surface, but the irregular topography of the surface was probably D Creek Formation. . View of un-named gray carbonate unit (remnants produced in post-Brassfield (early Aeronian) time as the of true Brassfield?) found within pockets cut into the Saluda and truncated by the “golden Brassfield”, which is, itself, truncated by the Cherokee Unconformity beneath the Brassfield sensu stricto is Lee Creek (not shown). The presence of this extra unit indicates a more essentially planar wherever observed. complex pocket-filling history than previously recognized. The presence of undercut cavities and, in places, networks of crack or fissure filling down into the true Brassfield or Upper Ordovician, suggests karst dissolution of the older carbonates. remnants of a totally different late Aeronian unit, which is The irregular erosion surface was immersed in a submarine exposed below a major, regionally angular unconformity to the environment and was colonized locally by boring worms southeast in central Kentucky. (sipunculids?) that produced Trypanites as dwelling traces, Occurrence of the distinctive stricklandiid brachiopod likely in an intertidal to shallow subtidal zone. This surface was Microcardinalia norwoodi in the “red Brassfield” at Dayton, subsequently rapidly buried by skeletal debris that formed the 46 Brett et al.: Anatomy of a compound sequence boundary GFF 136 (2014)

matrix of the “golden Brassfield”, which infilled steeply sided or rapid burial and infilling of relief by sediments. This occurred even slightly undercut pockets of Saluda Formation. during transgressions in which high-energy erosive shoreface There is evidence that this same irregular topography occurs erosion did not linger for prolonged time on a given surface; beneath the “golden Brassfield”, and its lateral equivalents over otherwise, the surface would have been abraded down to a more a substantial area. Considering that a co-extensive erosion nearly planar contact typical of most unconformities. surface exists perhaps as far northeast as Dayton, Ohio, as far Acknowledgements – Funding for this research was provided by NSF Grant EAR 0819820 northwest as Napoleon, Indiana and at least as far southwest as to CEB, a Dry Dredgers Paul Sanders Award to JRT, and Society for Sedimentary Geology (SEPM) and Geological Society of America (GSA) student research grants awarded to NBS. Mt. Washington, Kentucky (Fig. 1), this irregular erosion Patrick Cullen and Daniel List (University of Cincinnati) provided field assistance. Kyle surface may cover several thousand square kilometers at least. Hartshorne (Dry Dredgers) assisted significantly with research into Brassfield stratigraphy. Previous versions of this article were improved by reviews by Bradley D. Cramer Thus, in this particular instance, preservation of original (University of Iowa) and Markes E. Johnson (Williams College). This is a contribution to the International Geoscience Programme (IGCP) Project No. 591 – The Early to Middle erosional relief of the seafloor is similar to that of buried rocky Paleozoic Revolution. shoreline occurrences described in detail by Johnson (1988), Johnson et al. (1998) and Johnson & Baarli (2012), among others. Why erosional topography has been preserved at this References particular surface remains unclear, but rapid emersion and burial Berry, W.B.N. & Boucot, A.J., 1970: Correlation of the North American Silurian rocks. Geological Society of America Special Paper 102, 1–289. associated with rapid transgression are almost certainly required Boucot, A.J. & Ehlers, G.M., 1963: Two new genera of stricklandid brachiopods. for preservation of original relief. The burial sediments are well- University of Michigan Museum of Paleontology Contributions 18, 47–66. Brett, C.E., Cramer, B.D., McLaughlin, P.I., Kleffner, M.A., Showers, W.J. & sorted skeletal/oolitic grains reworked in a high-energy “shoal” Thomka, J.R., 2012: Revised Telychian–Sheinwoodian (Silurian) stratigra- environment. We postulate that, as this shoal environment phy of the Laurentian mid-continent: building uniform nomenclature along the migrated rapidly over the relict surface, skeletal sands were Cincinnati Arch. 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