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.Weems & Grimsley, 1999
Etymology The formation is named after Nanjemoy Creek, a left tributary of the Potomac River.Joseph Norris, TheBayNet, 2016 Nanjemoy is probably an Ojibwe word, meaning "one goes downward," representing the many rivers and creeks in the wet watershed of the Chesapeake Bay, an area originally inhabited by the Algonquin-speaking Nanticoke and Powhatan.Library of Congress - National atlas. Indian tribes, cultures & languages: United States Potapaco was an early name for Port Tobacco CreekUoC, 1938, p.1713 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.McCartan et al., 1995, p.17
In the outcrops along the Potomac River near Popes Creek, the contact between the Woodstock and Potapaco at about above the water level.
Extent Notable sites of the Nanjemoy Formation in the Potomac River basin in northeastern Virginia and western MarylandNC - Nanjemoy Creek, Ws - Woodstock, FS - Fisher/Sullivan site
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.Geologic Map MD; VA; D.C., USGS, 2017 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.Scott, 2005, p.67 The Nanjemoy Formation (Eocene), the Marlboro Clay (Paleocene), and the Aquia Formation (Paleocene) are present in the westernmost part of the Potomac channel.McCartan et al., 1995, p.7 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).
Stratigraphy 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 .
Petrology 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:Weems & Grimsley, 1999, p.5
Formation Member Bed Lithology Thickness (ft) Nanmejoy Potomac B Sand, dominantly quartz, fine-grained, well sorted, micaceous, glauconitic, medium-brown, spa$e wood fragments present and molds and casts of shells 1 Sand, dominantly quartz, dominantly fine-grained but with abundant rounded grains of medium- to coarse-grained quartz and scattered rounded quartz granules and pebbles to in diameter, glauconitic, medium-brown, contains abundant shell casts of Venericardia potapacoensis and abundant teeth and bones 2 Unconformity Nanmejoy Potomac A Sand, dominantly quartz, very fine- to fine-grained, bioturbated and massive, glauconitic, medium-brownish- gray, scattered wood fragments throughout and scattered molds and casts of shells, upper foot bioturbated and burrows filled with matrix from above bed, basal foot contains abundant medium- to coarse-grains and is more glauconitic than sediments above 15 Marlboro Clay Clay, silty, finely micaceous, greasy, sticky, lightgray, upper two feet intensely burrowed and burrows filled with matrix from bed above 8 Total section exposed 26
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.Dentzien Dias et al., 2019
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 Nanjemoy Creek; Charles County, MDNanjemoy Creek at Fossilworks.org Woodstock; King George County, VAWoodstock at Fossilworks.orgWoodstock 2 at Fossilworks.orgWoodstock 3 at Fossilworks.org Fisher/Sullivan; Stafford County, VAFisher/Sullivan 1 at Fossilworks.orgFisher/Sullivan 2 at Fossilworks.org Evergreen Plantation; Prince Charles County, VAEvergreen Plantation 1 at Fossilworks.org
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.Weems & Grimsley, 1999, p.3
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 Based on Weems & Grimsley (1999): +Mammals of the Nanjemoy Formation Taxon Species Fossils Member Notes Images cf. Esthonyxcf. E. sp.A tillodont.aff. Homogalaxaff. A. sp.An isectolophid tapiromorph. cf. Hyopsoduscf. H. sp.A hyopsodontid ungulate relative.Miacidae indet.A miacid carnivoraform of uncertain affinities.Nyctitheriidae indet.A nyctitheriid eulipotyphlan of uncertain affinities.Omomyidae indet.An omomyid primate. cf. Palaeosinopacf. P. sp. A pantolestid.PeradectesP. gulottaiA peradectid metatherian. Type locality of species. Ischyromyidae indet.A potential ischyromyid rodent.
Birds Based on Mayr et al (2021): +Birds of the Nanjemoy Formation Taxon Species FossilsNotes Images ?Caprimulgidae indet.A potential nightjar relative.Charadriiformes indet.sp. A.A shorebird of uncertain affinities.sp. B.?Charadriiformes indet.cf. Coturnipescf C. sp.A messelornithid gruiform.cf. EocypselidaeA potential eocypselid apodiform of uncertain affinities. "Graculavidae" indet.sp. 1A waterbird of uncertain affinities. Potentially a threshkiornithid.sp. 2Halcyornithidae indet.A halcyornithid eufalconimorphan of uncertain affinities.cf. Limnofregatacf. L. sp.A potential frigatebird.Messelasturidae indet.A messelasturid eufalconimorphan of uncertain affinities.cf. MicroenaA bird similar to the putative columbiform Microena from the London Clay.cf. MorsoravisA morsoravid bird.Parvicuculidae indet.A parvicuculid bird. Previously assigned to the Aegialornithidae. Pelagornithidae indet. A pseudotooth bird.?Phoenicopteriformes indet.A potential flamingo relative. Potentially a threshkiornithid instead.?Procellariiformes indet.A potential tubenose.cf. Pumiliorniscf P. sp.A psittacopedid.?Steatornithidae indet.A relative of the oilbird.cf. ThreskiornithidaeA potential ibis. The "graculavid" and "phoenicopteriform" previously reported may be conspecific with this taxon.YpresiglauxY. gulottaiAn extremely small protostrigid owl, one of the smallest owls known to have existed. Type locality of species. Initially described in Eostrix.Zygodactylidae indet.A zygodactylid psittacopasseran of uncertain affinities. 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.
Reptiles