Yorktown Formation
Encyclopedia
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 fossil
iferous.
and clay
with abundant calcareous shells, primarily bivalves
.
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. The uppermost Tunnels Mill Member is recognized in Maryland only.
to Early Pliocene
using ostracod
biostratigraphy
. The age was revised by Gibson (1983) to extend into the Middle Pliocene based on foraminifera
. Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.
Coastal plain
A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...
of Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
, Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
, North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
and South Carolina
South Carolina
South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...
. It is overconsolidated and highly fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
iferous.
Description
The Yorktown is composed largely of overconsolidated sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
with abundant calcareous shells, primarily bivalves
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
.
Stratigraphy
The Yorktown unconformablyUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
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. The uppermost Tunnels Mill Member is recognized in Maryland only.
Fossils
- bivalvesBivalviaBivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
, including Glycymeris subovata (see GlycymerididaeGlycymerididaeGlycymerididae, previously known as Glycymeridae, common names dog cockles or bittersweets, is a worldwide family of marine bivalve mollusks, related to the ark clams. In this family the shell is generally round in outline, is slightly longer than it is wide, and the external ligament lacks...
), Chesapecten jeffersoniusChesapecten jeffersoniusChesapecten jeffersonius is the state fossil of the State of Virginia in the United States. It is the fossilized form of an extinct scallop, which lived in the early Pliocene epoch between four and five million years ago on Virginia's coastal plain. In 1687, Martin Lister published a drawing of C...
, Chesapecten madisonius, MercenariaMercenariaMercenaria is a genus of edible saltwater clams, marine bivalve molluscs in the family Veneridae, the Venus clams.The genus Mercenaria includes the quahogs, Mercenaria mercenaria, the northern quahog or hard clam, and M. campechiensis, the southern quahog...
tridacnoides, Panopera reflexa, Chama, EnsisEnsisEnsis is a genus of medium-sized edible saltwater clams, littoral bivalve mollusks in the family Pharidae.In the United States, other common names for species in this genus are razor clams or jackknife clams. This is because the long, narrow, and parallel-sided shape of their shells is unusual in...
, Striarca and Noetia (see NoetiidaeNoetiidaeNoetiidae is a family of bivalves related to the ark clams. They are differentiated from the ark clams by the presence of striations on the hinge ligament...
), CerastodermaCerastodermaCerastoderma is a genus of marine bivalves, which includes the common cockle C. edule.-Species:Extant and fossil species include:* Cerastoderma edule - common cockle...
, DosiniaDosiniaDosinia is a genus of marine bivalve molluscs, in the family Veneridae . Dosinia species are almost disc-like in shape and reminiscent of Lucinid bivalves.- Species :* Dosinia abyssicola T. Habe, 1961 Indo...
, MuliniaMuliniaMulinia is a genus of bivalve mollusks or clams.-Species:* Mulinia cleryana* Mulinia coloradoensis - Colorado Delta clam* Mulinia edulis* Mulinia lateralis* Mulinia pallida...
, KuphusShipwormShipworms are not worms at all, but rather a group of unusual saltwater clams with very small shells, notorious for boring into wooden structures that are immersed in sea water, such as piers, docks and wooden ships...
(Shipworm), PanopeGeoduckThe geoduck , Panopea generosa, is a species of very large saltwater clam, a marine bivalve mollusk in the family Hiatellidae.The shell of this clam is large, about to over in length, but the very long siphons make the clam itself very much longer than this: the "neck" or siphons alone can be ...
(Geoduck), and the oyster OstreaOstreaOstrea is a genus of oyster in the family Ostreidae. Evidence of a number of species of this genus in the geological fossil record demonstrates the ancient nature of this genus, and also gives testimony to the species that co-existed with members of the Ostrea genus... - gastropods, including CrucibulumCrucibulum (gastropod)Crucibulum, common name the cup-and-saucer snails, is a genus of sea snails, marine gastropod mollusks in the family Calyptraeidae, the slipper snails and cup-and-saucer snails.-Species:...
, CalliostomaCalliostomaCalliostoma is a genus of small to medium-sized sea snails with gills and an operculum, marine gastropod molluscs within the family Calliostomatidae, the calliostoma top snails . Previously this genus was placed within the family Trochidae...
, BusyconBusyconBusycon is a genus of very large edible sea snails, marine gastropod mollusks in the family Buccinidae. These snails are commonly known in the United States as whelks or Busycon whelks....
, TurritellaTurritellaTurritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. They have tightly coiled shells, whose overall shape is basically that of an elongated cone....
, and CrepidulaCrepidulaCrepidula, common name the "slipper limpets" or "slipper shells", is a genus of sea snails, marine gastropod mollusks in the family Calyptraeidae, the slipper snails and cup-and-saucer snails.... - ForaminiferaForaminiferaThe Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, including the biostratigraphic marker species Dentoglobigerina altispira (see GlobigerinidaGlobigerinidaThe Globigerinida are a common group of foraminiferans that are found as marine plankton . They produce hyaline calcareous tests, and are known as fossils from the Jurassic period onwards. The group has included more than 100 genera and over 400 species, of which about 30 species are extant...
), Sphaeroidinellopsis, and Globorotalia puncticulata - Scleractinian coralsScleractiniaScleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
, including Septastrea marylandica, Paracyathus vaughani (see CaryophylliidaeCaryophylliidaeCaryophylliidae is a family of stony corals. In addition to the tropics they are also found in temperate seas and in very deep water.The coral colonies are often composed of only a few polyps, especially in Catalaphyllia jardinei, living on sandy land in partly cloudy water, associated with partner...
), and Astrangia lineata - OstracodOstracodOstracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
s - bryozoaBryozoaThe Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
ns - BarnacleBarnacleA barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
s, including BalanusBalanusBalanus is a genus of barnacles in the family Balanidae of the subphylum Crustacea, containing the following species:*Balanus albicostatus*Balanus amaryllis Darwin*Balanus amphitrite Darwin, 1854*Balanus aquila Pilsbry, 1907... - WormWormThe term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s - Sponges
Notable Exposures
- Type Section: Yorktown, VirginiaYorktown, VirginiaYorktown is a census-designated place in York County, Virginia, United States. The population was 220 in the 2000 census. It is the county seat of York County, one of the eight original shires formed in colonial Virginia in 1634....
on southwest side of York RiverYork River (Virginia)The York River is a navigable estuary, approximately long, in eastern Virginia in the United States. It ranges in width from at its head to near its mouth on the west side of Chesapeake Bay. Its watershed drains an area including portions of 17 counties of the coastal plain of Virginia north...
, York CountyYork County, VirginiaYork County is a county located on the north side of the Virginia Peninsula in the Hampton Roads metropolitan area of the Commonwealth of Virginia, a state of the United States. Situated on the York River and many tributaries, the county seat is the unincorporated town of Yorktown... - Carters Grove Bluffs, north side of James RiverJames RiverThe James River may refer to:Rivers in the United States and their namesakes* James River * James River , North Dakota, South Dakota* James River * James River * James River...
37°12′33"N 76°38′00"W - Moore House Bluff, southwest side of York RiverYork River (Virginia)The York River is a navigable estuary, approximately long, in eastern Virginia in the United States. It ranges in width from at its head to near its mouth on the west side of Chesapeake Bay. Its watershed drains an area including portions of 17 counties of the coastal plain of Virginia north...
, York CountyYork County, VirginiaYork County is a county located on the north side of the Virginia Peninsula in the Hampton Roads metropolitan area of the Commonwealth of Virginia, a state of the United States. Situated on the York River and many tributaries, the county seat is the unincorporated town of Yorktown...
37°13′25"N 76°29′14"W (very close to type section)
Age
Hazel (1971) revised the age of the Yorktown from Miocene to Late MioceneMiocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
to Early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
using ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
biostratigraphy
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
. The age was revised by Gibson (1983) to extend into the Middle Pliocene based on foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
. Further biostratigraphic work with ostracods and foraminifera was completed by Cronin (1991), which also summarized previous investigations.