Yorktown Formation

Yorktown Formation Stratigraphic range: Late Miocene to Middle Pliocene
Outcrop of the Yorktown Formation at Carters Grove Bluffs along the James River
Type	sedimentary
Unit of	Chesapeake Group
Sub-units	Sunken Meadow Member, Rushmere Member, Morgarts Beach Member, Moore House Member, Tunnels Mill Member
Underlies	Croatan Formation
Overlies	Eastover Formation
Lithology
Primary	sand, clay
Other	shells
Location
Region	Atlantic Coastal Plain of North America
Extent	Maryland, Virginia, North Carolina, South Carolina
Type section
Named for	Yorktown, Virginia
Named by	Clark and Miller, 1906[1]

The Yorktown Formation is a mapped bedrock unit in the Coastal Plain of Maryland, Virginia, North Carolina and South Carolina. It is overconsolidated and highly fossiliferous.

Description

The Yorktown is composed largely of overconsolidated sand and clay with abundant calcareous shells, primarily bivalves.

Stratigraphy

The Yorktown unconformably overlies the Miocene Eastover Formation, and conformably underlies the Pliocene Croatan Formation.

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.^[2] The uppermost Tunnels Mill Member is recognized in Maryland only.

Fossils

• bivalves, including Glycymeris subovata (see Glycymerididae), Chesapecten jeffersonius, Chesapecten madisonius, Mercenaria tridacnoides, Panopera reflexa,^[3] Chama, Ensis, Striarca and Noetia (see Noetiidae), Cerastoderma, Dosinia, Mulinia, Kuphus (Shipworm), Panope (Geoduck), and the oyster Ostrea
• gastropods, including Crucibulum, Calliostoma, Busycon, Turritella, and Crepidula
• Foraminifera, including the biostratigraphic marker species Dentoglobigerina altispira (see Globigerinida), Sphaeroidinellopsis, and Globorotalia puncticulata^[4]
• Scleractinian corals, including Septastrea marylandica, Paracyathus vaughani (see Caryophylliidae), and Astrangia lineata
• Ostracods
• bryozoans
• Barnacles, including Balanus
• Worms
• Sponges
• birds, including the large pelican Pelecanus schreiberi.

Notable Exposures

• Type Section: Yorktown, Virginia on southwest side of York River, York County
• Carters Grove Bluffs, north side of James River^[5] 37°12′33″N 76°38′00″W / 37.20917°N 76.63333°W
• Moore House Bluff, southwest side of York River, York County 37°13′25″N 76°29′14″W / 37.22361°N 76.48722°W (very close to type section)

Age

Hazel (1971) revised the age of the Yorktown from Miocene to Late Miocene to Early Pliocene using ostracod biostratigraphy.^[6] The age was revised by Gibson (1983) to extend into the Middle Pliocene based on foraminifera.^[7] Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.^[8]

Gallery

• 

  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

References

1. Clark, W.B., and Miller, B.L., 1906, Clay deposits of the Virginia coastal plain: Virginia Geological Survey Bulletin, no. 2, pt. 1.
2. 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.
3. Rader, E.K., and Evans, N.H., 1993, Geologic map of Virginia; expanded explanation: Virginia Division of Mineral Resources, 80 p.
4. 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.
5. Post-impact Effects of the Eocene Chesapeake Bay Impact, Lower York-James Peninsula, Virginia, 31st Annual Meeting, Virginia Geological Field Conference, Williamsburg, Virginia, Oct. 19 and 20, 2001, G.H. Johnson et al. (fieldtrip guidebook)
6. 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. http://pubs.er.usgs.gov/pubs/pp/pp704
7. 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.
8. 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.