If anyone comes away with a message from reading my blog, it should be that things change. The earth is constantly going through periods of warming and cooling which affect not only sea levels and ocean currents, but also organisms adapted to their environment. Profound environmental change leads to extinction, but pushes evolutionary factors to refill empty niches with new species. Rivers in the Albemarle basin of northeastern North Carolina cut through four distinct shell beds that trace molluscan evolution in response to environmental change. I have discussed two of these already; the Upper Miocene Eastover Formation and the Upper Pliocene Chowan River Formation. The other two are within the Yorktown Formation bookended by the Eastover and Chowan River Formations.
One of the giants in American paleontology, Wendell Mansfield, proposed a zonation of the Yorktown based upon different faunas within two biozones. The lower of these was later named the Sunken Meadow Member of the Yorktown Formation by Ward and Blackwelder and is presently classified as Zanclean (Lower Pliocene) in age. The environment was cool temperate, similar to that in the Aleutian Islands. Although water temperatures were colder than presently exist in the Mid-Atlantic region, productivity was high as its shell beds are rich in individuals, albeit lower in diversity than the overlying Zone 2. The Sunken Meadow Member was rich with marine vertebrates as well and is the member of the Yorktown from which shark, bird, whale, dolphin and seal remains are found at the Lee Creek Mine.
I have collected the Sunken Meadow Member along the James River in Virginia and in the Lee Creek Mine and had the opportunity to collect similar deposits during my 2010 North Carolina river trip. Vertebrates were not very common that day along the Meherrin as I suspect enterprising locals keep the area swept clean, however for a fossil shell collector, the typical Zone 1 guide fossils were ever present (fig. 1). I did not spend a lot of time that day collecting Zone 1 as I was more interested in the much more diverse overlying Zone 2 fauna, so the only other shell that I found in the Sunken Meadow Member were fragmental Turritella bipertita Conrad, 1844 from locality 1015.
Figure 1. Guide fossils of the Sunken Meadow Member of the Yorktown Formation. A) Chesapecten jeffersonius (Say, 1824), loc. 1014; B ) Placopecten clintonius (Emmons, 1858), loc. 1014; C) Ostrea compressiostrea Say, 1824, loc. 1014; D) Ecphora quadricostata (Say, 1824), loc. 1015. All from Hertford County, North Carolina.
Campbell, Lyle. 1993. Pliocene Molluscs from the Yorktown and Chowan River Formations in Virginia. Virginia Division of Mineral Resources Publication 127.
Mansfield, W.C., 1943 . Stratigraphy of the Miocene of Virginia and the Miocene and Pliocene of North Carolina in Gardner, Julia ed. Mollusca from the Miocene and Lower Pliocene of Virginia and North Carolina. USGS Professional Paper 199A, p. 1-19.
Petuch, Edward J. 2004. Cenozoic Seas. CRC Press.
Ward, Lauck W. 1992. Tertiary Molluscan Assemblages from the Salisbury Embayment of Virginia. Virginina Journal of Science, Volume 43, no. 1B.
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.