My first experience with the Chowan River Formation was during a collecting trip in 2011 to collect Zone 2 Yorktown sediments along the Roanoke River in North Carolina. The paltry fauna that I collected from the two Chowan sites stood in stark contrast to the molluscan-rich Yorktown Rushmere Member exposed further upstream. Since I had been writing about the Upper Pliocene deposits of the Eastern United States, I was looking forward to collecting these youngest Pliocene deposits but with a total of only 13 species, I was completely underwhelmed. So earlier this year when I had the opportunity to collect at the namesake exposures along the Chowan River as well as another on the Wiccacon River, I jumped at the chance.
The Chowan River Formation was named by Blake Blackwelder for shelly, silty sands deposited in northeastern North Carolina found primarily along the Chowan River and a few quarry exposures in southern Virginia. Early authors such as W.C. Mansfield had attributed these sites as Upper Yorktown while acknowledging that the deposits were stratigraphically higher i.e. younger than the traditional Yorktown. The Chowan fauna is notable for the lack of many of the key Yorktown taxa including Chesapecten, Chesacardium, Planicardium, and Ecphora. Sandwiched in between Zone 2 Yorktown and the James City Formation, the Chowan fauna is transitory between Pliocene and Pleistocene containing species from both. A world-wide cooling event occurring around 3 mya dropped sea levels resulting in the formation of the present Arctic ice cap and proved particularly devastating to the Yorktown fauna. When global temperatures warmed once more, the resulting Chowan River deposition contained a subtropical fauna which probably derived from the south. The result is that mollusks from the Chowan River Formation include Yorktown survivors, the first occurrence of the Gelasian fauna and endemics found only in the Chowan (fig. 1).
Figure 1. Mollusks from the Chowan River Formation. Ptychosalpinx chesapeakensis, Martin County, NC and Crucibulum leeanum, Hertford County, NC make last appearance. Diodora pamlicoensis, Hertford County, NC and Crucibulum lawrencei, Bertie County, NC make first appearance. Costaglycymeris hummi, Bertie County, NC endemic to the Chowan River Formation.
The Chowan River itself originates near the North Carolina and Virginia border by the merger of the Blackwater and Nottoway rivers and along with the Meherrin River forms the main drainage basin for the Albemarle Sound. Fossiliferous deposits are found on the right bank of the river although not apparent from the left bank boat launch at US 17 due to the very wide nature of the river. The longest continuous deposits were at Edenhouse and Blackwelder named the lower fossiliferous-most member after the small town. All of those deposits are now graded over, planted and rip-rapped by riverside mansions. The closest sites from the marina are far upstream and can only be reached by boat. Most sites are current deposited sands with collecting consisting of picking mollusks weathered from the bluffs. Locality 1032 however, is siltier representing a gentle environment with many double-valved pelecypods, echinoids and a single great white shark tooth, very rare for this vertebrate poor formation.
The Wiccacon River is very different as it is not much more than a creek that feeds into the Chowan River. The lone site is at water level at the base of a bluff with a lot of overhanging vegetation that limits access to the exposure. The sand here however lends itself well to screen washing and I collected a 5 gallon bucket of ¼ inch sorted sediment which allowed me to collect smaller material from the comfort of home that I would have normally overlooked. Included within the washed material where assorted cheliped from six different crab species which I would guess as never being characterized from the Chowan. Above the sandy layer was a more resistant zone containing larger shells including many endemic Carolinapecten eboreus bertiensis and a single relatively well preserved specimen of Busycotypus concinnum which I put my pick through. This gastropod makes its first appearance in the Chowan River Formation and its last in the Lower Pleistocene James City and Waccamaw Formations. Blackwelder listed this site as basal Chowan River and screen washing did come up with some scrap Yorktown material that was probably redeposited (fig. 2).
Figure 2. Locality 1028 on the Wiccacon River.
The species list below represents my collecting efforts from the youngest Pliocene exposed on the Chowan, Roanoke and Wiccacon Rivers in North Carolina containing 130 different species. The Chowan River Formation is dominated by mollusks but in addition I have collected bryozoans, corals, barnacles, crabs, echinoids and rare vertebrates within these deposits.
Bailey, R. H., 1977: Neogene Molluscan Assemblages along the Chowan River North Carolina. Southeastern Geology 18(3): 173-190.
Blackwelder, Blake W. 1981. Stratigraphy of Upper Pliocene and Lower Pleistocene Marine and Estuarine Deposits of Northeastern North Carolina and Southeastern Virginia. USGS Bulletin: 1502-B.
Blackwelder, Blake W. 1981. Late Cenozoic Stages and Molluscan Zones of the U.S. Middle Atlantic Coastal Plain. Memoir (The Paleontological Society) Vol. 12, Supplement to Vol. 55, no. 5 of the Journal of Paleontology (Sep., 1981), pp. 1-34.
Campbell, Lyle. 1993. Pliocene Molluscs from the Yorktown and Chowan River Formations in Virginia. Virginia Division of Mineral Resources Publication 127.
Petuch, Edward J. 2004. Cenozoic Seas. CRC Press.
Ward, Lauck W. 1993. Pliocene Stratigraphy and Biostratigraphy – Virginia to Florida in the Neogene of Florida and Adjacent Regions. Florida Geological Survey Special Publication No. 37.
Ward, Lauck W. 2008. Synthesis of Paleontological and Stratigraphic Investigations at the Lee Creek Mine, Aurora, NC (1958-2007) in The Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virgina Museum of Natural History Special Publication No. 14.
Ward, L.W. and Blackwelder, B.W. 1980. Stratigraphic Revision of Upper Miocene and Lower Pliocene Beds of the Chesapeake Group-Middle Atlantic Coastal Plain. USGS Bulletin 1482-D.
Ward, L. W., and Blackwelder, B. W. 1987. Upper Pliocene and lower Pleistocene mollusks of the Lee Creek mine, Aurora, North Carolina, in Ray, C. E., editor, Geology and Paleontology of the Lee Creek Mine, North Carolina, ?II: Smithson, Contrib. Paleobiol. 61:113-283.
Ward, L. W., and Gilinsky, N. L. 1993. Molluscan assemblage of the Chowan River Formation, Part A. Biostratigraphic analysis of the Chowan River Formation (Upper Pliocene) and adjoining units, the Moore House Member of the Yorktown Formation (upper Pliocene) and the James City Formation (Lower Pleistocene): Virginia Museum of Natural History Memoir 3, part A., 33 p.