Yorktown Formation Explained

Yorktown Formation
Type:sedimentary
Prilithology:sand, clay
Otherlithology:shells
Namedfor:Yorktown, Virginia
Namedby:Clark and Miller, 1906[1]
Region:Atlantic Coastal Plain of North America
Unitof:Chesapeake Group
Subunits:Sunken Meadow Member, Rushmere Member, Morgarts Beach Member, Moore House Member, Tunnels Mill Member
Overlies:Eastover Formation
Underlies:Croatan Formation
Extent:Maryland, Virginia, North Carolina, South Carolina
Age:Early to Middle Pliocene
Period:Neogene

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.[2]

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.

Notable exposures

Age

Hazel (1971) revised the age of the Yorktown from Miocene to Late Miocene to Early Pliocene using ostracod biostratigraphy.[5] The age was revised by Gibson (1983) to extend into the Middle Pliocene based on foraminifera.[6] Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.[7]

Fossils

Notes and 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. Pineda-Salgado, G., Schaaf, P., Aguilar-Piña, M., Solís-Pichardo, G., Vega, F.J.. 2016. Contribución al alcance estratigráfico de la Formación Agueguexquite (Mioceno), Veracruz, México. Boletín de la Sociedad Geológica Mexicana. 68. 2. 187–197. 10.18268/BSGM2016v68n2a2. free.
  3. 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.
  4. 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)
  5. 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
  6. 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.
  7. 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.
  8. Rader, E.K., and Evans, N.H., 1993, Geologic map of Virginia; expanded explanation: Virginia Division of Mineral Resources, 80 p.
  9. 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.

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