The diagenetic origin and depositional history of the Cherry Valley Member, Middle Devonian Marcellus Formation
Introduction
The Middle Devonian Marcellus Formation (Fm.) has received extensive attention as one of the largest shale plays in North America (e.g., Engelder and Lash, 2008). Despite numerous studies documenting that much of the Marcellus Fm. was deposited under pervasive anoxic to euxinic conditions (e.g., Werne, 2002; Sageman et al., 2003; Lash and Engelder, 2011; Kohl et al., 2014; Lash and Blood, 2014), debate continues over the temporal and spatial evolution of the paleo-depositional environment because a basin-wide carbonate member occurs within the mudstone succession. This widespread dark-colored, bedded, clay-rich, nodular marlstone or fine-grained carbonate interbed is recognized as the Cherry Valley Member (Mbr., Ver Straeten et al., 1994) and is thought to indicate a receding oxygen-minimum zone associated with base level drop (e.g., Kohl et al., 2014; Lash and Blood, 2014). Unlike the overlying Oatka Creek Mbr. (e.g., Werne, 2002) and the underlying Union Spring Mbr. (e.g., Wendt et al., 2015), the deposition model of the Cherry Valley Mbr. has received little attention. The lithology of the Cherry Valley Mbr. evolves from sandstone and carbonate in the north and east sector of the Appalachian basin to pyritiferous carbonate with abundant barite nodules in the southwest exhibiting large spatial and temporal heterogeneity. The depositional environment of the Cherry Valley Mbr. is of special interest to the understanding of paleo-bathymetric evolution of the Appalachian Basin during Marcellus deposition. From the perspective of shale gas production, the Cherry Valley Mbr. is often regarded as a “fracking barrier” and a calibration layer for geosteering (Kaufman et al., 2013; Soeder, 2017). The presence of barite nodules within the lower Cherry Valley Mbr. could cause drilling incidents (e.g., bit jamming) and has raised environmental concerns (e.g., the potential release of excessive barium into fracking flowback and produced water; Rowan et al., 2011; Chapman et al., 2012; Warner et al., 2012; Shih et al., 2015; Renock et al., 2016). A thorough understanding of the origin, distribution, and paleoenvironment of the Cherry Valley Mbr., therefore, holds scientific, engineering, and environmental interests.
The occurrence of decimeter- to meter-thick carbonate beds within predominant mudstone sequences has long intrigued geologists (e.g. Raiswell, 1988). Hypotheses for their origins have gravitated towards three different views: 1) carbonate formation due to the sequential change of depositional environment, such as shallow marine environment because of sea level drop (e.g., Arthur et al., 1984; Bottjer et al., 1986), 2) carbonate deposition associated with hydrothermal vents or submarine springs (e.g., Kauffman et al., 1996), and 3) authigenic carbonate precipitation during early and late diagenesis (e.g., Munnecke and Samtleben, 1996). In order to understand the origin of the carbonate in the Cherry Valley Mbr. and arrive at an overall depositional history, we examine δ13C, δ18O, δ34S, morphology of pyrite, and selected trace metal contents spanning from the upper Union Spring Mbr. to the lower Oatka Creek Mbr. of the Marcellus Fm., and perform detailed petrological and isotopic characterization of barite nodules in the lower unit of the Cherry Valley Mbr. from two drilled cores. Deep burial diagenesis and its impact on the chemical compositions of the Cherry Valley Mbr. have also been evaluated by fluid inclusion data. Additionally, we present the thickness and density isochore maps compiled from 536 wells to assess the distribution of the Cherry Valley Mbr. and barite nodule occurrence. This information is integrated to construct a depositional model. In this study, we focus on answering three main questions: 1) the origin(s) of CaCO3 for the Cherry Valley Mbr.; 2) the mechanism(s) that caused the deposition of barite nodules; and 3) the evolution of redox condition during the deposition of the Cherry Valley Mbr. Overall, we find that authigenic carbonate comprises a volumetrically significant portion of the Cherry Valley Mbr., particularly in the proximal part/producing zone of the basin. This study could provide an effective model for the depositional environment of the Cherry Valley Mbr. enhancing the understanding of Marcellus sequence stratigraphy and shed light on the development of predominantly mudstone successions with interspersed carbonate units and related engineering issues.
Section snippets
Background: carbon, oxygen, sulfur isotopes and redox proxies
While most Phanerozoic carbonates are biological in origin, recent studies have found that the precipitation of authigenic calcium carbonate may have contributed significantly in global carbonate burial flux (Higgins et al., 2009; Schrag et al., 2013; Sun and Turchyn, 2014). Here, we first provide a brief review of selected proxies for the investigations of the authigenic carbonate minerals and the evolution of redox conditions.
In the modern ocean, most authigenic carbonate is formed during
Geological setting
The Marcellus Fm. was deposited during the Middle Devonian in the Appalachian basin, which was located approximately 25–35° south of the equator (Fig. 1, Scotese and McKerrow, 1990). The basin was bounded by the Acadian highlands (E and SE) and the Findlay-Algonquin Arch (W and NW) connecting with the Rheic ocean to the SW as a narrow seaway (Fig. 1-C; Ettensohn, 1985, Ettensohn, 1987). The Acadian orogeny, which began in the Middle Devonian and climaxed in the late Devonian, was the major
Sample preparation and petrological analysis
The Bald Eagle core and the Snow Shoe core were split and processed in the ABBS Core Lab at Penn State. A total of 53 samples spanning the Cherry Valley Mbr. of Marcellus shale from Bald Eagle (n = 26) and Snow Shoe (n = 27) sections were drilled into powders (2– 4 g) with a low-speed, micro-drill equipped with a 0.7 mm stainless steel bit for CaCO3 (wt%) measurement and carbon, oxygen, sulfur isotope analyses. Drilling sites were carefully selected to reflect the bulk matrix of carbonate and
Petrological descriptions
Detailed petrological descriptions of the two studied cores and other available cores reveal that the Cherry Valley Mbr. does not comprise solely limestone explicitly outlining upper and lower mudstone boundaries as one may expect from well logs (Fig. 1-E). Instead, it is a complex lithofacies package consisting of dark-grey nodular calcareous mudstone, fossil-fragment-bearing limestone, and silty micrite with gradational boundaries. Based on its lithological and textural features, we divided
The source of the Cherry Valley carbonate rocks
Although the entire Cherry Valley Mbr. is generally interpreted as LST deposition under oxygenated conditions, only the middle unit exhibits feature of oxygenated shallow-water carbonate sediments (e.g., with fossil fragments). The δ13C values of the middle unit average around −1% and −3% for the Bald Eagle and Snow Shoe cores respectively, which are slightly lower but in general agreement with the range of Middle Devonian seawater δ13C and likely reflect a primary marine carbonate source (e.g.
Conclusion and the depositional model
Together with petrological and chemical observations, the spatial and temporal distribution of an oxygenated water column during the deposition of the Cherry Valley Mbr. may have been overestimated by previous studies. While the Cherry Valley Mbr. has been identified as LST deposition in a 3rd order sea-level oscillation (e.g. Kohl et al., 2014), most of shallow-water marine deposits occurred on the northeastern edge of the basin on inherited basin topography. In the center and southwestern
Declaration of competing interest
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.
Acknowledgements
We are grateful to L. Kump, M. Patzkowsky, R. Slingerland, T. Engelder, and X. Gu for discussions that improved the study, and D. Walizer, T. Sowers, Y. Chen, and R. Bodnar for help with laboratory analyses. We also thank Huan Cui and two anonymous reviewers for thoughtful comments and suggestions, and Editor Michael Böttcher for handling the paper. This work was funded by the Appalachian Basin Black Shale Group in the Department of Geosciences, which was supported by Chesapeake Energy
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