At the end of the Pliocene, two sea level pulses occurred. With the first of these, temperatures cooled and sea level dropped as much as 25 ft, and in the process caused the extinction of approximately 80 percent of the cool-temperate Yorktown molluscan fauna including famed genera such as Chesapecten and Ecphora. When the sea rose once more during the latest Pliocene, temperate conditions returned and a mix of survivors, sub-tropical emigrants, and a few new molluscan species repopulated the area. These deposits, named the Chowan River Formation by Ward and Blackwelder, are limited to southern Virginia and northern North Carolina. Campbell (2010) reports 250 molluscan species in the Chowan River Formation compared to the preceding 519 species in Zone 2 Yorktown sediments.
Most exposures of the Chowan River Formation are river sites and have disappeared to development. I had the opportunity to visit a rare exposure on the Roanoke River in 2010. Unfortunately it was late in the day and I was only able to collect for about 15 minutes. My collection from this site (locality 1010) is composed of a few large shells that I collected and associated small species from the screened sediment (Table 1).
Table 1. Fossil species from Locality 1010, Upper Pliocene Chowan River Formation,
Martin County, North Carolina.
Carolinapecten eboreus bertiensis
Glycymeris subovata hummi
Chesaconcavus cf. proteus
Of interest was a Ptychosalpinx chesapeakensis Campbell 1993 that I found as float (fig. 1). Neither Campbell nor Ward list the genus Ptychosalpinx after the Yorktown extinction however this is not the first time that I found a member of the same genus in Chowan River deposits. In 1997, I found a small Ptychosalpinx altile (Conrad, 1832) (fig. 2) at Lee Creek Mine (locality 171). Petuch (2004) reported a large Chowan River fauna from the Lee Creek Mine however Ward (2008) discounts almost all of it as Lower Pleistocene James City Formation since the Chowan River Formation at Lee Creek Mine is a thin sandstone and only small mollusks are preserved. Since only calcitic fossil shells (pectens, oysters, Ecphora) are preserved in the Yorktown exposures at Lee Creek and the only aragonitic shells are from the Lower Pleistocene James City Formation, it must have come from the Chowan River Formation.
Figure 1. Ptychosalpinx chesapeakensis Campbell 1993 Locality 1010, Chowan River Formation, Martin County, North Carolina. a)apertural view b)abapertural view
Figure 2. Ptychosalpinx altile(Conrad, 1832)Locality 171, Chowan River Formation, Beaufort County, North Carolina. a)apertural view b)abapertural view
After the Chowan ocean high-stand another regressive/transgressive pulse occurred issuing in the previously reported Waccamaw Formation.
Buzas, Martin A. and Culver, Stephen J. 1994. Species Pool and Dynamics of Marine Paleocommunities. Science.264.5164.1439.
Campbell, Lyle. 1993. Pliocene Molluscs from the Yorktown and Chowan River Formations in Virginia. Virginia Division of Mineral Resources Publication 127.
Campbell, Lyle, Campbell, Matthew, and Campbell, Sarah. 2010. Molluscan Diversity of the Yorktown Formation (Pliocene) in Virginia: an Update. Paper No. 77-10. Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting GSA.
Krantz, D.E., 1991. A chronology of Pliocene sea-level fluctuations: The U.S. Middle Atlantic Coastal Plain record: Quaternary Science Reviews, v. 10.
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. Synethesis 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.