Comment: Fossil tabulate corals reveal outcrops of Paleozoic sandstones in the Atlantic Coastal Plain Province, southeastern USA

Christopher S. Swezey1, Bradley A. Fitzwater2, and G. Richard Whittecar3

1 U.S. Geological Survey, Florence Bascom Geoscience Center, 12201 Sunrise Valley Drive, MS 926A, Reston, VA 20192
2 Old Dominion University, Dept. of Ocean, Earth, and Atmospheric Sciences, Norfolk, VA 23539 [Present address: Draper Aden Associates, 1030 Wilmer Avenue, Richmond, VA 23227]
3 Old Dominion University, Dept. of Ocean, Earth, and Atmospheric Sciences, Norfolk, VA 23539

Landmeyer et al. (2019) have constructed an unlikely story about the geologic history of the southeastern United States on the basis of a misidentification of their studied fossil specimens. The fossils shown in their Figure 4 are pieces of Cretaceous petrified wood (the bark of conifers), not poorly preserved Ordovician to Silurian tabulate corals (Syringophyllidae) that are inferred to lack septa because of diagenetic processes. Thus, their study does not “extend the distribution of the Syringophyllidae into North America where they were not previously recognized or recorded.”

In their study area (Chesterfield County, South Carolina, USA), fossil specimens like those shown in Landmeyer et al. (2019) are widely distributed within units of slightly-indurated sandstone and pebbly sandstone that extend along the inner Atlantic Coastal Plain Province, and they are not restricted only to iron-cemented pebbly sandstone where the specimens of Landmeyer et al. (2019) were found. Traditionally, these lithologies in Chesterfield County are mapped collectively as the Middendorf Formation (e.g., Swift and Heron, 1969; Cohee and Wright, 1976; Swezey et al., 2016). Plant fossils in the Middendorf Formation have been known for more than 100 years and they form the basis for the Late Cretaceous age assignment of these strata (e.g., Berry, 1914). We have found examples of specimens like those shown by Landmeyer et al. (2019) in outcrops of slightly-indurated sandstone (not cemented by iron) at numerous locations within their study area, including: (1) the east side of the Middendorf Formation type locality where a powerline crosses the railroad tracks; (2) along railroad tracks near the town of Patrick, in association with obvious petrified tree trunks and branches; and (3) in numerous sand pits within the Carolina Sandhills National Wildlife Refuge. Similar fossils from the Middendorf Formation have been examined in thin-section by paleobotanists and identified as conifers (G. Robert Ganis, Consulting Geologist-Southern Pines, North Carolina; personal communication, 2019). Although preservation is sometimes poor, the gross morphology of this material is easily recognized as being tree-related. A good photograph of one such petrified wood fragment may be found in Fitzwater (2016, p. 35).

In addition, it is unlikely that Ordovician to Silurian sandstone in this area might have survived the various Appalachian orogenic events and remained unmetamorphosed. Greenschist facies metasedimentary rocks of Cambrian age are exposed in outcrop ~20-30 km west of the Landmeyer et al. (2019) fossil localities (Wynn and Luce, 1984). These metamorphic rocks of the Piedmont Province extend east beneath the Coastal Plain surface and are present throughout the study area of Landmeyer et al. (2019) at just a few tens of meters depth beneath the Middendorf Formation. For example, the Cheraw core (CTF-081), which was drilled by the U.S. Geological Survey in 1995 ~15 km northeast of the town of Patrick, penetrated ~67 m of slightly indurated (unmetamorphosed) sandstone, pebbly sandstone, and clay (typical Middendorf Formation), before reaching a quartz-pebble lag above Paleozoic metasiltstone/phyllite (Swezey et al., 2016).

Furthermore, as noted by Landmeyer et al. (2019), outcrops of coarse sandstone are not likely places to find coral fossils. Their Locality #6 (Sugarloaf Mountain), for example, is a ~4 m thick unit of pebbly coarse sandstone that displays cross-bedding and is cemented by iron. This pebbly sandstone is the uppermost unit of a ~30 m high outcrop of different units of sandstone and clay that are mapped collectively as the Cretaceous Middendorf Formation and are interpreted as fluvial deposits (Sloan, 1904; Swezey et al., 2016). We have not seen evidence of reactivated reverse faults that would separate the iron-cemented sandstone from the rest of the underlying sandstone units. Also, the iron cementation is not really “a matter of considerable debate.” Localized iron-cemented units are ubiquitous throughout the regional extent of the Middendorf Formation. In the Cheraw core and other cores from the area stored at the South Carolina Geological Survey, iron cementation is common in the coarser-grained sandstone units that lack a kaolinite matrix. We interpret the iron cementation in these cores and the iron cementation of the pebbly sandstone at the top of Sugarloaf Mountain as having formed by shallow groundwater diagenetic processes under anaerobic conditions such as are occurring today at <140 m depth ~50-70 km to the east of the Landmeyer et al. (2019) study area (Chapelle and Lovley, 1992; Park et al., 2006, 2009).

References:
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Swezey, C.S., Fitzwater, B.A., and Whittecar, G.R., 2016, Geology and geomorphology of the Carolina Sandhills, Chesterfield County, South Carolina, in: Gold, Structures, and Landforms in Central South Carolina – Field Guides for the GSA Southeastern Section Meeting, Columbia, South Carolina, 2016 (W.R. Doar III, ed.): Geological Society of America Field Guide 42, p. 9–36.

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