The Yorktown was divided into members by Ward and Blackwelder (1980). These are in ascending order: Sunken Meadow Member, Rushmere Member, Morgarts Beach Member, and Moore House Member.[3] The uppermost Tunnels Mill Member is recognized in Maryland only.
The most diverse paleobiota of the Yorktown Formation has been recovered from the Aurora mine of Beaufort County, North Carolina, where it overlies the intensively-mined Pungo River Formation. As with the Pungo River Formation, this locality has become a prime target for fossil collecting, and some of the most notable fossils from both formations are displayed at the Aurora Fossil Museum.[5]Foraminiferal analyses suggests that this locality belongs the Early Pliocene-aged Sunken Meadow Member.[6] The Yorktown fauna at the Aurora mine is far more diverse than the Pungo River fauna from the same locality, as the Pungo River sediments are mechanically processed for their phosphorite, while the Yorktown sediments are discarded and are thus in better condition for study.[7] This locality was likely deposited in an bay, with older sediments being deposited at a depth of 80 to 100 metres (260 to 330 ft) underwater, while younger sediments were deposited at a depth of 30 metres (98 ft) underwater.[8]
Clear differences are seen in the paleoichthyofauna of the Pungo River Formation and the overlying Yorktown Formation at the Aurora mine, with the Pungo River fauna representing almost exclusively warm-water taxa, while the Yorktown fauna preserves both warm and cooler-water taxa. These fossils provide important evidence for the significant cooling of ocean temperatures between the Miocene and Pliocene. However, the invertebrate fauna from both time periods appears to have cool-temperate affinities.[9]
Hazel (1971) revised the age of the Yorktown from Miocene to Late Miocene to Early Pliocene using ostracodbiostratigraphy.[10] The age was revised by Gibson (1983) to extend into the Middle Pliocene based on foraminifera.[11] Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.[12] More recently, Spivey (2025) dated the Sunken Meadow Member to the Zanclean stage, about 4.8 to 3.8 million years ago[6], while Dowsett et al (2001) dated the Rushmere and Morgarts Beach Members to the early-mid Piacenzian stage, about 3.3 to 3.15 million years ago. The deposition of these latter two members is thought to be linked to a marine transgression caused by the mid-Piacenzian warm period.[13]
Foraminifera, including the biostratigraphic marker species Dentoglobigerina altispira (see Globigerinida), Sphaeroidinellopsis, and Globorotalia puncticulata[15]
Based on Olson & Rasmussen (2001). An extremely high diversity of fossil birds, primarily known from isolated but diagnostic limb bones, is known from the formation. All specimens were collected from the Lee Creek Mine. Most taxonomic assignments were based on rough similarity to living species, hence the "aff." suffix to indicate similarities, and are not intended to be meant as direct taxonomic assignments.[19]
^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, IN Contributions to stratigraphy: U.S. Geological Survey Bulletin, 1482-D, 61 p.
^Hazel, J.E., 1971, Ostracode biostratigraphy of the Yorktown Formation (upper Miocene and lower Pliocene) of Virginia and North Carolina: U.S. Geological Survey Professional Paper, 704, 13 p. https://pubs.er.usgs.gov/pubs/pp/pp704
^Gibson, T.G., 1983, Key Foraminifera from upper Oligocene to lower Pleistocene strata of the U.S. central Atlantic Coastal Plain, IN Ray, C.E., ed., Geology and paleontology of the Lee Creek Mine, North Carolina, I: Smithsonian Contributions to Paleobiology, no. 53, p. 355-454.
^Cronin, T.M., 1991, Pliocene shallow water paleoceanography of the North Atlantic Ocean based on marine ostracodes: Quaternary Science Reviews, v. 10, p. 175-188.
^Rader, E.K., and Evans, N.H., 1993, Geologic map of Virginia; expanded explanation: Virginia Division of Mineral Resources, 80 p.
^Dowsett, H.J., and Wiggs, L.B., 1992, Planktonic foraminiferal assemblage of the Yorktown Formation, Virginia, USA: Micropaleontology, v. 38, no. 1, p. 75-86.
.png)
.png)
.png)
.gif)
.jpg)
.jpg)
.jpeg)
.jpg)
.png)
Coral growing on a Crepidula sp. (slipper snail) from the Yorktown Formation
Rushmere Member of the Yorktown Formation, with abundant Chama bivalves
Sunken Meadow Member of the Yorktown Formation
Balanus sp. (barnacles) from Carters Grove Bluffs outcrop, growing on a bivalve
