Yorktown Formation

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. 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.

Notable exposures

Type Section: Yorktown, Virginia on southwest side of York River, York County
Carters Grove Bluffs, north side of James River
Moore House Bluff, southwest side of York River, York County
Aurora mine

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. Foraminiferal analyses suggests that this locality belongs the Early Pliocene-aged Sunken Meadow Member. 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. This locality was likely deposited in an bay, with older sediments being deposited at a depth of underwater, while younger sediments were deposited at a depth of underwater.
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.

Age

Hazel revised the age of the Yorktown from Miocene to Late Miocene to Early Pliocene using ostracod biostratigraphy. The age was revised by Gibson to extend into the Middle Pliocene based on foraminifera. Further biostratigraphic work with ostracods and foraminifera was completed by Cronin, which also summarized previous investigations. More recently, Spivey dated the Sunken Meadow Member to the Zanclean stage, about 4.8 to 3.8 million years ago, while Dowsett et al 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.

Fossils

Bivalves, including Glycymeris subovata, Chesapecten jeffersonius, Chesapecten madisonius, Mercenaria tridacnoides, Panopera reflexa, Chama, Ensis, Striarca and Noetia, Cerastoderma, Dosinia, Mulinia, Kuphus, Panope, and the oyster Ostrea
Gastropods, including Crucibulum, Calliostoma, Busycon, Turritella, and Crepidula
Foraminifera, including the biostratigraphic marker species Dentoglobigerina altispira, Sphaeroidinellopsis, and Globorotalia puncticulata
Scleractinian corals, including Septastrea marylandica, Paracyathus vaughani, and Astrangia lineata
Ostracods
Bryozoans
Barnacles, including Balanus
Worms
Sponges
Birds, including the large pelican Pelecanus schreiberi.
Whales, including the prehistoric sperm whale Scaldicetus.
Vertebrate paleobiota

As per the Paleobiology Database:

Cartilaginous fishes

Based on Purdy et al. :

Sharks

Rays

Ray-finned fishes