Integrated Silurian conodont and carbonate carbon isotope stratigraphy of the east-central Appalachian Basin

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Highlights

  • A high amplitude Lau Excursion was not observed in our chemostratigraphic data.

  • Data indicate a majority of the Ludlow is absent.

  • Tonoloway strata at Moorefield, West Virginia, are now assigned as Wills Creek.

  • The δ13Ccarb record is more negative than typical global data.

  • We have identified both the Ireviken Excursion and the Mulde Excursion.

Abstract

The eastern and southeastern margins of the Appalachian Basin contain extensive exposures of Telychian through Pridoli (upper Silurian) strata. Litho- and biostratigraphic analyses of these strata over the last century provided a general means for their chronostratigraphic correlation to global series and stage boundaries. However, a rarity of zonally diagnostic fauna limited precise correlation both within and beyond the Appalachian Basin. Recent conodont biostratigraphic studies substantially revised the chronostratigraphic position of these strata by more than a series in some cases. Much of the interval, however, remained biostratigraphically unzoned and the precise position of series and stage boundaries remained tentative.

To improve the chronostratigraphic correlation of these strata to global series and stage boundaries we provide new integrated conodont and high-resolution δ13Ccarb chemostratigraphic analyses from two outcrops located in West Virginia and Virginia, spanning the upper Rose Hill through Tonoloway formations (upper Telychian through Pridoli). These new data show that the Silurian carbon isotope record for this portion of the Appalachian Basin is shifted to more negative δ13Ccarb values than typical global data. However, the overall isotopic records appear correlative at least within the Appalachian Basin thereby providing a means for future correlation within the basin for intervals in which biostratigraphy is traditionally lacking.

These coupled data demonstrate that the Ireviken and Mulde excursions are contained within our sections. The high-amplitude Lau Excursion was, however, not observed and the present biostratigraphic data suggest that a majority of the Ludlow Series was either not recorded or was later removed by erosion. These coupled analyses therefore indicate that the Ludlow-Pridoli boundary resides within the Wills Creek Formation and we propose that the overlying isotopic record presented here records the Late Ludlow Carbon Isotope Excursion (LLCIE) and Silale Low, previously identified only within the Baltic Basin.

Introduction

The Silurian was arguably one of the more dynamic periods within the Paleozoic with numerous global biogeochemical perturbations affecting both the carbon cycle and marine organisms, in particular, nekto-benthic conodont and planktonic graptolite marine fauna. The intense global study of these events over the past two decades helped to produce a high-resolution carbonate carbon isotopic curve (δ13Ccarb) and well-constrained conodont and graptolite zonations for the Silurian System (e.g., Jeppsson and Aldridge, 2000; Jeppsson and Calner, 2003; Munnecke et al., 2003; Calner and Eriksson, 2006; Cramer and Saltzman, 2007; Lehnert et al., 2007; Calner, 2008; Cramer et al., 2010b; Munnecke et al., 2010; Lehnert et al., 2010; Melchin et al., 2012; Cramer et al., 2012; Vandenbroucke et al., 2013; Sullivan et al., 2016; McAdams et al., 2017; Radzevičius et al., 2014a, Radzevičius et al., 2014b; Waid and Cramer, 2017a, Waid and Cramer, 2017b; Mergl et al., 2018; Caruthers et al., 2018; McAdams et al., 2019; Rose et al., 2018; Danielsen et al., 2019; Bancroft and Cramer, 2020).

Of the seven major Silurian biogeochemical perturbations, four occurred within the upper Llandovery—lower Pridoli interval and include the Ireviken, Mulde, Linde, and Lau events (Fig. 1). These events have been extensively investigated throughout central and eastern Laurentia (Saltzman, 2001; Cramer et al., 2006a, Cramer et al., 2006b; Barrick et al., 2009, Barrick et al., 2010; Cramer et al., 2010a; Brett et al., 2012; McLaughlin et al., 2012, McLaughlin et al., 2013; Bancroft et al., 2015, Bancroft et al., 2016; Sullivan et al., 2016; McAdams et al., 2019; Danielsen et al., 2019), tremendously improving the chronostratigraphic correlation of Laurentian strata to the global Silurian series and stage boundaries. However, this interval has remained largely under-evaluated in the southern and east-central part of the Appalachian Basin despite the fact that it contains some of the most extensive and accessible Silurian strata in North America.

Numerous studies conducted within the eastern and south-central margins of the basin have described, and in some cases, established biozonations or provided summaries for late Llandovery through Pridoli marine fauna, including, but not limited to, ostracods (Swartz, 1923; Ulrich and Bassler, 1923; Butts, 1940; Woodward, 1941), brachiopods (Bowen, 1967; Berry and Boucot, 1970), and conodonts (Helfrich, 1972, Helfrich, 1975, Helfrich, 1978, Helfrich, 1980; Sartain, 1981; Denkler and Harris, 1988a-b). These studies provided a general means for chronostratigraphic correlation to global series and stage boundaries. However, a rarity of zonally diagnostic conodonts and long ranging occurrences and provinciality in ostracod and brachiopod fauna limited precise correlation both within and beyond the Appalachian Basin. Recent taxonomic analyses and establishment of a revised biozonation for the conodont collection of Helfrich, 1972, Helfrich, 1975, Helfrich, 1980 by Bancroft and Cramer (2020) greatly improved our ability to constrain these units chronostratigraphically, and in doing so, substantially revised the chronostratigraphic position of some units by more than a series. The exact stratigraphic positions of several series and stage boundaries remain tentative as much of the interval was biostratigraphically unzoned. The aim of this investigation is to refine the chronostratigraphic precision provided by the recently revised biostratigraphic framework of the east-central Appalachian Basin through new integrated analyses of conodont biostratigraphy and carbonate carbon isotope chemostratigraphy. Here, we provide new, high-resolution δ13Ccarb analyses of Telychian through Pridoli strata from two sections located on the eastern margin of the Appalachian Basin (Fig. 2), in addition to new conodont data to improve the chronostratigraphic correlation of these strata to global series and stage boundaries, as well as to other sections within the Appalachian Basin and the Laurentian paleocontinent.

Section snippets

Geological setting

Silurian strata analyzed in this study were deposited along the southeastern margin of the Taconic foreland basin (i.e., eastern margin of the modern Appalachian Basin) within the southern arid trade-wind belt at approximately 25–30°S latitude (Cocks and Torsvik, 2011). The succession of collisional events along the present day eastern margin of the paleocontinent of Laurentia from the Ordovician to the Devonian, which are variably termed the Taconic, Salinic, and Acadian orogenies, provided

Methods

For this investigation we conducted high-resolution bio-chemostratigraphic analyses of outcrops from Crabbottom, Highland County, Virginia (east of Bluegrass, Virginia, at the intersection of State Road 642 and US Highway 220), and Highway 48 (Corridor H), west of Moorefield, West Virginia (Latitude: 39.129953° N, Longitude: 79.038805° W). The exact location of the lowest east-dipping stratigraphic unit at Crabbottom is at latitude: 38.485404° N, longitude: 79.520345° W. Strata analyzed include

Results

For this study, we sampled intervals RH3 through T4A* of the Helfrich, 1972, Helfrich, 1975, Helfrich, 1980 Crabbottom locality (cross-correlations from this study and Helfrich, 1972, Helfrich, 1975 are provided in Fig. 4). Conodont samples collected by Helfrich, 1972, Helfrich, 1975, Helfrich, 1980 from Crabbottom, were reported in lithological divisions with varying thicknesses throughout the outcrop rather than by individual footages. Consequently, the exact placement of individual samples

Revision of nomenclature

The strata previously identified as “upper” Tonoloway (Haynes et al., 2015b, Stop 2–2, p. 41) at the Moorefield, West Virginia, locality or in nearby sections (“lower through upper” of Woodward, 1941, p. 190; see Fig. 2 herein) are, in-fact, correlative with the Wills Creek Formation of Virginia and should be redefined as belonging to this unit. Near Pinto, Maryland, (i.e., the neostratotype and principal reference sections of these units) the Wills Creek-Tonoloway contact is abrupt; sands and

Conclusions

The integrated conodont and carbon isotope bio-chemostratigraphy presented here provide important new insights into the global chronostratigraphic correlation of Silurian strata within the Appalachian Basin in addition to a new carbon isotope record for correlation within the basin itself for intervals in which biostratigraphy has been traditionally lacking. We have been able to reassign strata at the Moorefield, West Virginia, locality that were previously classified as the upper Tonoloway

Acknowledgments

We thank Andy Stacey, a former employee of the University of Iowa Department of Earth and Environmental Sciences, for assisting in the collection of samples at Crabbottom, Virginia, and Moorefield, West Virginia, over the summer of 2017. The 2017 field season involved extensive amounts of physical trenching, often in locations covered in poison sumac, and we thank Andy for always being in high spirits and for providing great conversation. This work was partially supported by the National

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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