Estonian Journal of Earth Sciences, 2015, 64, 1, 19–23 doi: 10.3176/earth.2015.04 Calibrating water depths of Ordovician communities: lithological and ecological controls on depositional gradients in Upper Ordovician strata of southern Ohio and north-central Kentucky, USA Carlton E. Bretta, Thomas J. Malgieria, James R. Thomkab, Christopher D. Aucoina, Benjamin F. Dattiloc and Cameron E. Schwalbacha a Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA; [email protected], [email protected], [email protected], [email protected] b Department of Geosciences, University of Akron, Akron, Ohio 44325, USA; [email protected] c Department of Geoscience, Indiana University-Purdue University-Fort Wayne, Fort Wayne, Indiana 46805, USA; [email protected] Received 2 July 2014, accepted 13 October 2014 Abstract. Limestone and shale facies of the Upper Ordovician Grant Lake Formation (Katian: Cincinnatian, Maysvillian) are well exposed in the Cincinnati Arch region of southern Ohio and north-central Kentucky, USA. These rocks record a gradual change in lithofacies and biofacies along a gently northward-sloping ramp. This gradient spans very shallow, olive-gray, platy, laminated dolostones with sparse ostracodes in the south to offshore, nodular, phosphatic, brachiopod-rich limestones and marls in the north. This study uses facies analysis in outcrop to determine paleoenvironmental parameters, particularly those related to water depth (e.g., position of the photic zone and shoreline, relative degree of environmental energy). Within a tightly correlated stratigraphic interval (the Mount Auburn and Straight Creek members of the Grant Lake Formation and the Terrill Member of the Ashlock Formation), we document the occurrence of paleoenvironmental indicators, including desiccation cracks and light-depth indicators, such as red and green algal fossils and oncolites. This permitted recognition of a ramp with an average gradient of 10–20 cm water depth per horizontal kilometer. Thus, shallow subtidal (“lagoonal”) deposits in the upramp portion fall within the 1.5–6 m depth range, cross-bedded grainstones representing shoal-type environments fall within the 6–18 m depth range and subtidal, shell-rich deposits in the downramp portion fall within the 20–30 m depth range. These estimates match interpretations of depth independently derived from faunal and sedimentologic evidence that previously suggested a gentle ramp gradient and contribute to ongoing and future high-resolution paleontologic and stratigraphic studies of the Cincinnati Arch region. Key words: paleobathymetry, Cincinnatian, faunal gradients, microendoliths, water depth. INTRODUCTION classic Cincinnati Arch region of northern Kentucky and southern Ohio, USA. We also determine the The problem of absolute depth in ancient depositional orientation and gradient of an ancient, gently dipping environments is a difficult one. While it is commonly carbonate ramp. possible to determine the relative depth of a given facies or fossil community, it is far more difficult to assign depths in terms of meters below sea level. Given the GEOLOGIC HISTORY AND DEPOSITIONAL ecological importance of depth-related facies (Patzkowsky SETTING & Holland 2012), it is important to attempt assignment of fossil assemblages to quantitatively defined depth Strata assigned to the upper Katian (Cincinnatian: zones. Estimates of absolute depth can be made based uppermost Maysvillian to lower Richmondian) Mount upon integrated analysis of a variety of distinctive Auburn Member of the Grant Lake Formation in sedimentary structures related to exposure, indicators of southwestern Ohio and its lateral equivalents, the Terrill position with respect to shoreline, normal wave base Member of the Ashlock Formation in central Kentucky and storm wave base, and evidence of light-related zones and the Straight Creek Member of the Grant Lake (Brett et al. 1993). Limestone (sensu Schumacher et al. 1991) of southern In the present study, we attempt to calibrate absolute Ohio and northern Kentucky, crop out along the periphery depths of lithofacies and biofacies in a well-constrained of the Cincinnati Arch (Fig. 1). The upper Katian of the stratigraphic interval in the Upper Ordovician of the Cincinnati Arch region is made up of mixed carbonate 19 Estonian Journal of Earth Sciences, 2015, 64, 1, 19–23 A B Fig. 1. Paleobathymetry of an Upper Ordovician ramp in the Cincinnati Arch region. A, map showing the distribution of the Ordovician outcrop, localities mentioned in the text and approximate inferred depths below the sediment–water interface. The 0-m line marks the northernmost occurrence of desiccation cracks, the 10-m line marks the northernmost occurrence of micritic, pale green mudstones, the 20-m line marks the northernmost occurrence of oncoids and abundant algae. “MW” = Mt Washington. B, idealized cross-section of the ramp showing lateral lithofacies relationships. Shoal topography is exaggerated, although some areas may have built up locally to a few meters in height. Insets show examples of certain aspects of lithofacies. Note the desiccation cracks in the dolomitic mudstone associated with the tidal flat facies, the carbonate–dark shale rhythms in the lagoon facies, the large stromatoporoids in the shoal facies and the muddy wackestones and shelly packstones of the shelf facies. “NWB” = normal wave base. 20 C. E. Brett et al.: Ordovician paleobathymetry and siliciclastic rocks that accumulated in the distal FACIES AND DEPOSITIONAL portion of the Taconic foreland basin. Most strata ENVIRONMENTS accumulated along a north-sloping ramp known as the Lexington Platform (Cressman 1973). This ramp contains The position of the paleo-shoreline was identified by the lithofacies ranging from shallow peritidal deposits in the appearance of desiccation cracks in the lower Terrill southern depositional limit to shoal and outer ramp Member. This was identified as the supratidal zone deposits in the north. Siliciclastic sediments were derived where carbonate mud-flats were exposed to alternating from tectonic source areas to the east that were uplifted episodes of wetting and more prolonged drying; hence, during the Taconic Orogeny. Carbonates accumulated this represents a setting above average sea level, assuming largely in situ, with micritic wackestones in shallow- a microtidal coast, typical of most epicontinental seas water areas, skeletal, oncoid- and stromatoporoid-bearing (Allison & Wells 2006). The northernmost appearance grainstone facies in mid-ramp settings and muddy, of desiccation cracks in age-equivalent rocks, recording nodular, brachiopod-dominated packstones with minor tidal flat facies (Terrill Member), occurs just north of grainstone beds in areas below normal wave base to Richmond, Kentucky, and constrains the position of the the north. shoreline (Fig. 1A). The width of the mudcracked, argillaceous mudstone facies, approximately 20 km, gives a sense of the width of the supratidal zone during METHODS deposition of this interval. In general, desiccation cracks range from small, vaguely defined polygons in northern- Our approach to reconstructing absolute depths along most crack-bearing outcrops to larger prism cracks in the ramp involved first establishing a precisely correlated, outcrops to the south (Fig. 1B). thin stratigraphic interval (10–20 m thick) and then These facies pass laterally northward into heavily identifying benchmark positions at which the depth bioturbated, pale green, argillaceous, micrite facies that range could be reasonably well-constrained. The Mount carry in situ lingulid brachiopods and mollusks; these Auburn Member of the Grant Lake Formation and its are inferred to record shallow lagoonal settings. The lateral equivalents in the Terrill Member of the Ashlock presence of carbonized dasyclad algae suggests water Formation was chosen because: 1) regional stratigraphic depths < 15 m (Brett et al. 1993). Such facies pass correlations were constrained to decimeter scale using abruptly into skeletal packstones and grainstones, mapped a combination of key surfaces and marker beds (e.g., locally as either the Straight Creek Member of the Grant hardgrounds, erosional surfaces, major lithofacies offsets), Lake Limestone or the Mount Auburn Member of the as well as faunal epiboles in closely spaced outcrops Grant Lake Formation (Schumacher et al. 1991), which along both sides of the Cincinnati Arch and 2) this can contain abundant oncoids and stromatoporoids interval extends laterally from desiccation-cracked, shaly (Labechia) up to 1 m in diameter (Fig. 1B). Evidence of facies to nodular, fossiliferous facies representing environ- strong winnowing and reworking of grains suggests ments below normal wave-base that have been previously a high-energy shoal environment. The deepest-water examined for microendoliths, traces of light-sensitive environments of the studied transect, exposed near endolithic cyanobacteria and green and red algae (Vogel Hamilton, Ohio, comprise nodular-bedded, phosphatic & Brett 2009). To reconstruct our absolute depth limestone facies (Fig. 1B) dominated faunally by the gradient, we needed to establish three points of reference: robust orthid brachiopod Vinlandostrophia ponderosa. 1) depositional strike, the orientation of facies belts Sediments of these facies were deposited below normal relative to shoreline, which allowed determination of wave base but within the shallow euphotic zone, as depositional dip, the direction of most rapid change