Nanjemoy Formation

The formation is named after Nanjemoy Creek, a left tributary of the Potomac River. Nanjemoy is probably an Ojibwe word, meaning "one goes downward," Potapaco was an early name for Port Tobacco Creek that was named after the Piscataway people == Definition ==

The Nanjemoy Formation was defined by Clark and Martin in 1901, as part of the mapping by the Maryland Geological Survey. The Nanjemoy Formation was divided into two members, the lower or Potapaco, and the upper or Woodstock. The main lithologic distinction is that the lower part of the Nanjemoy is much more clayey than the upper part. In the subsurface, the distinction between the members is less evident than in outcrops, especially in Maryland, so the formation has been left undivided. In the outcrops along the Potomac River near Popes Creek, the contact between the Woodstock and Potapaco at about above the water level. == Extent ==

The geologic map of the Washington West 30' × 60' Quadrangle (containing Maryland, Virginia, and Washington D.C.) maps out the Nanjemoy Formation, represented as Tn, as restricted to the southeastern side of the Potomac Basin. In his thesis, Scott (2005) included a map showing the thin bands of outcrops of both the underlying Marlboro Clay and the Nanjemoy Formation, restricted to the many creeks feeding the Chesapeake Bay. The Nanjemoy Formation (Eocene), the Marlboro Clay (Paleocene), and the Aquia Formation (Paleocene) are present in the westernmost part of the Potomac channel. The outcrop area of the formation is designated Nanjemoy Wildlife Management Area. == Geology ==

Geologically, the area of deposition of the Nanjemoy Formation is part of the Atlantic coastal plain province. The depositional environment of the Nanjemoy Formation is mostly shallow shelf. The more clayey beds suggest an area or time of quiet water, not affected by waves, tides, or current activity; intercalated sandier zones may reflect the higher energy of waves or currents during episodic storms. Its regional dip is eastward at 15–20 ft per mile (3-3.5 m/km). == Paleoecology ==

The floral and faunal assemblage of the Nanjemoy Formation is diverse and provides an insight into the paleobiological and paleoclimatological environment of the early Eocene. Fossils of bivalves, sharks, rays, actinopterygian fishes, reptiles, birds and mammals, and of fruits and seeds are common in the Potapaco Member. More than 2000 vertebrate coprolites from the Potapaco Member at the Fisher/Sullivan Site in Virginia were analyzed by Dentzien Dias et al. (2019). The chemical composition (phosphatic), inclusions and morphology suggest that only carnivorous scats were preserved. All Nanjemoy coprolites were produced by fishes, namely carcharhiniform & lamniform sharks, probably the genus Carcharias. Other morphotypes were produced by actinopterygian fishes. The surface marks and the lack of flatness on most coprolites suggests early lithification of the Potapaco Member. • Woodstock; King George County, VA • Fisher/Sullivan; Stafford County, VA • Evergreen Plantation; Prince Charles County, VA Fisher/Sullivan site, VA In October 1990, Mr. Richard Brezina of the Maryland Geological Society (MGS) discovered an important fossil site east of Fredericksburg, in eastern Stafford County, Virginia. This locality, along an unnamed tributary of Muddy Creek, became known as the Fisher/Sullivan site in recognition of its principle landowners. Brezina immediately realized that the site was exceptional, because it yielded numerous shark teeth and other vertebrate remains from the sands and gravels in the unnamed tributary. Brezina notified other members of the Maryland Geological Society, and together members of the MGS began to screen stream sediments at the site for more shark teeth and other remains. It soon became apparent, from the types of teeth that were being found and from the color and texture of the sediments in the banks of the creek, that the fossils were being reworked from glauconitic ("greensand") horizons of the Lower Tertiary (Paleocene-Eocene) Pamunkey Group. Because the Pamunkey Group previously had yielded only sparse vertebrate remains, it seemed reasonable to suspect that this locality was scientifically important. == Vertebrate paleobiota ==

Mammals Birds Based on Mayr et al (2021): In addition, several indeterminate birds are also known, such as a very large bird the size of a sandhill crane, known only from a single pedal phalanx. Based on: Many invertebrates (such as gastropods, bivalves, corals, and bryozoans) have also been reported from the Nanjemoy Formation. == Paleoflora ==