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A Basal Silurian Karstic Unconformity in the Cincinnati Arch Region Carlton E This article was downloaded by: [University of Cincinnati Libraries] On: 09 May 2014, At: 07:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK GFF Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/sgff20 Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region Carlton E. Bretta, James R. Thomkaa, Nicholas B. Sullivanb & Patrick I. Mclaughlinb a Department of Geology, 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 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions GFF, 2014 Vol. 136, No. 1, 42–47, http://dx.doi.org/10.1080/11035897.2014.882978 Article Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region CARLTON E. BRETT1, JAMES R. THOMKA1, NICHOLAS B. SULLIVAN2 and 2 PATRICK I. MCLAUGHLIN 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 Ordovician–Silurian contact in parts of the Cincinnati Arch region is an irregularly pitted erosion surface 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 Limestone 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 Devonian 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 coral-rich limestones 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 sediment 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,
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