JOURNAL OF SEDIMENTARY RESEARCH,VOL. 68, NO. 4, JULY, 1998, P. 684–699 Copyright  1998, SEPM (Society for Sedimentary Geology) 1073-130X/98/068-684/$03.00 SEQUENCE STRATIGRAPHY AND RELATIVE SEA-LEVEL HISTORY OF THE MIDDLE AND UPPER ORDOVICIAN OF THE NASHVILLE DOME, TENNESSEE S.M HOLLAND 1 AND M.E. PATZKOWSKY 2 1 Department of Geology, University of Georgia, Athens, Georgia 30602-2501, U.S.A. 2 Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802-2714, U.S.A. ABSTRACT: Eleven depositional sequences are present in the Middle and Upper Ordovician of the Nashville Dome. All but one lack low- stand systems tracts, and many display evidence of subaerial exposure such as paleokarst or regional erosional truncation at their sequence boundaries, which are combined with transgressive surfaces. Features formed during the transgression, such as hardgrounds, are more prom- inent at most sequence boundaries than are those features related to subaerial exposure. All sequences have maximum flooding zones with aggradational stacking rather than discrete maximum flooding sur- faces. Condensation features are much more common at transgressive surfaces than within the maximum flooding zone. The expression of these parasequences and sequences changes with the onset of the Ta- conic orogeny. Although parasequences from equivalent depositional environments can be recognized before and after the onset of the orog- eny, their facies composition changes completely. Sequences following the orogeny display fewer paleokarst features, more pyritized and iron- stained hardgrounds at flooding surfaces and transgressive surfaces, and more phosphatic lags and mineralization at transgressive surfaces than those preceding the orogeny. A relative sea-level curve for the Nashville Dome has a long-term ( 5 m.y.) component presumably dominated by subsidence and a short-term (1–3 m.y.) component con- sisting of 10–50 m fluctuations that is primarily the result of eustasy. INTRODUCTION The emplacement of thrust loads is well known to control the strati- graphic architecture of the proximal parts of foreland basins (e.g., Read 1980; Van Wagoner 1995). Many studies have also pointed to the role of thrust loading in shaping the stratigraphy of distal parts of foreland basins, including the peripheral bulge (Dorobek and Read 1986; Flemings and Jordan 1990). We have previously documented several effects of the Mid- dle Ordovician Taconic orogeny on the stratigraphy of mixed carbonate– clastic deposits of the Nashville Dome in central Tennessee (Holland and Patzkowsky 1997). The onset of this orogeny is accompanied there by a shift from tropical-type to temperate-type carbonates, the initiation of wide- spread phosphate deposition, the introduction of large amounts of terrige- nous silt and clay, the occurrence of widespread seismically induced soft- sediment deformation, and a change from a carbonate shelf to a ramp (Fig. 1). The onset of the orogeny had no immediate effects on accommodation rates on the Nashville Dome, but 4 million years following the onset, ac- commodation rates underwent a fourfold drop. Changes in accommodation rates and changes in sediment supply should be expected to leave a distinct change in sequence stratigraphic architecture, and this study documents a number of these changes. This study also uses depositional sequences to establish a relative sea-level history for the Middle and Upper Ordovician of the Nashville Dome. This paper is a companion to Holland and Patz- kowsky (1997), and readers are referred to that paper for additional mea- sured sections as well as details on the lithologic changes and the tectonic oceanographic evolution of the Nashville Dome. Regional Tectonic and Climatic Background The Nashville Dome and the Cincinnati Arch (Fig. 2) are widely con- sidered to be peripheral bulges, or flexural uplifts driven primarily by thrust loads to the east of the Appalachian Foreland Basin (Beaumont et al. 1988). Although the Nashville Dome and Cincinnati Arch experienced thrust-in- duced flexural uplift throughout much of the Paleozoic, they were first uplifted during the Middle Ordovician during two orogenic events, the Blountian and Taconic orogenies (Stearns and Reesman 1986). The Bloun- tian orogeny occurred during the early Middle Ordovician (ca. 463 Ma) and was centered in easternmost Tennessee, whereas the Taconic orogeny occurred during the late Middle Ordovician (ca. 454 Ma) and was centered on northeastern Virginia, and easternmost Pennsylvania and New York. The Taconic orogeny was driven by the attempted and aborted subduc- tion of the eastern edge of North America under either a small microplate or terrane (Delano et al. 1990), a series of island arcs (Rowley and Kidd 1981), or possibly even western Argentina (Dalla Salda et al. 1992). As the North American continent approached the subduction zone, loading of the eastern edge of North America formed a peripheral foreland basin and uplifted the Nashville Dome and Cincinnati Arch (Beaumont et al. 1988). Flexural uplift along the margins of the Michigan, Illinois, and Black War- rior Basins transformed the Nashville Dome and Cincinnati Arch into a series of domes and arches rather than a single linear peripheral bulge (Beaumont et al. 1988). During the Middle and Late Ordovician, the Nashville Dome lay in sub- tropical latitudes at approximately 25°S (Scotese and McKerrow 1991). The trade winds that would have formed at this latitude would have blown towards the west–northwest. Evaporites are absent from the Nashville Dome region, suggesting less than arid conditions. Yet, prominent paleo- karst is absent as well, suggesting no significant rainfall. On the basis of tidal-flat deposits, the adjacent central Kentucky region has been interpreted as semi-humid (Pope and Read 1993). The Middle to Late Ordovician was a time of greenhouse climate, and oxygen isotopic evidence suggests that end-Ordovician glaciation was limited to a half-million-year interval nearly at the end of the Ordovician (Brenchley et al. 1994). There are no rocks of this age on the Nashville Dome, indicating that all of the Ordovician rocks of the Nashville Dome were deposited in a greenhouse climate prior to any significant continental glaciation. FACIES Five major depositional environments are identified in the Ordovician of the Nashville Dome on the basis of grain types, physical and biogenic sedimentary structures, and vertical facies relationships. These include peri- tidal, shallow subtidal, sand shoal, deep subtidal, and offshore environ- ments. These five are represented by 18 lithofacies (Tables 1–5). Despite differences in tropical-type versus temperate-type carbonates as well as percent shale, facies from corresponding environments share many simi- larities before and after the onset of the Taconic orogeny. In the study area, peritidal facies are characterized by typically low levels of bioturbation (ichnofabric index or ii 1–2, Droser and Bottjer 1989), the presence of desiccation cracks, laminar to tubular fenestral fabrics, and very thin to thin bedding. Somewhat deeper and less restricted peritidal facies display more bioturbation (ii 2–3), fewer desiccation features, common tubular to bubble fenestrae, and evidence of more frequent reworking in the form of erosionally based skeletal lags. In this study, shallow subtidal facies are highly variable, but contain abundant peloids or skeletal material, are nearly everywhere highly biotur- bated (ii 4–5), and are very thickly to massively bedded. Shallow subtidal