The Nanjemoy Formation is a geologic formation pertaining to both the Wilcox Group and the Pamunkey Group of the eastern United States, stretching across the states of Virginia, Maryland, and the District of Columbia. The formation crops out east of the Appalachians and dates back to the Ypresian stage of the Eocene epoch, about 55 to 50 Ma or Wasatchian in the NALMA classification. It is roughly contemporaneous with the Wasatch Formation of the Interior West.
The -thick formation crops out in a narrow irregular band in some of the creeks in southern Maryland, and on the southern side of the Potomac River in northern Virginia. The formation was divided into two members by Clark and Martin in 1901; the Potapaco and Woodstock, representing different phases in the basin history. The lower Potapaco Member is much more clayey, described as marl, than the upper Woodstock Member, which is probably characteristic of less storm influences in the shallow shelf sediments.
The formation has provided a wealth of fossils of mainly fish, but also mammals, reptiles, birds and flora. The presence of the sharks Otodus obliquus and Otodus aksuaticus, as well as various other shark and ray species are notable. Crocodylian, snake, turtle, mammal, and bird remains have all been found in the 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," representing the many rivers and creeks in the wet watershed of Chesapeake Bay, an area originally inhabited by the Algonquin-speaking Nanticoke and Powhatan. Potapaco was an early name for Port Tobacco Creek that was named after the Piscataway people
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.
The geologic map of the Washington West 30' ÃÂ 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.
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 15âÂÂ20 ft per mile (3-3.5 m/km).
The Nanjemoy Formation belongs to two geologic groups; the Wilcox Group of the Gulf of Mexico Basin in the southernmost surface expression of the formation and the Pamunkey Group in the northern and central portions from Maryland in the north through Virginia and the Carolinas. Both geologic groups have been dated to the early Paleogene; the Paleocene and Eocene periods, or in the commonly used NALMA classification; Wasatchian, defined by the age-equivalent Wasatch Formation of Wyoming.
This formation is the third-oldest formation in the Pamunkey Group and overlies the Marlboro Clay. The Nanjemoy is partly overlain by the Piney Point Formation and in many areas covered by the Miocene Calvert Formation, separated by an unconformity representing about 34 Ma.
The upper surface reaches an elevation of about and is overlain in most places by the Calvert Formation (Tc). The unit is present only in the southeastern part of the map area of Washington D.C., and it reaches a maximum thickness of about .
The formation comprises glauconitic quartz sand, dark-grayish-green to olive-black (tan to orange where weathered), fine to medium-grained; and dark-greenish-gray silty clay. In places, the sand is very muddy or contains many small quartz pebbles, and the clay is silty or sandy. Both lithologies contain richly fossiliferous beds including abundant mollusk shells.
Robert E. Weems and Gary J. Grimsley (1999) described the geology of the Fisher/Sullivan site in Virginia as:
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.
Main fossil sites of the Nanjemoy Formation are
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.
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.
The paleoichthyofauna of this formation shares close similarities with better-preserved fossils of concurrent lagerstatte from Europe, most notably the London Clay and Monte Bolca.
Based on:
Many invertebrates (such as gastropods, bivalves, corals, and bryozoans) have also been reported from the Nanjemoy Formation.