TheDoswell Formation (also known as theDoswell Group) is ageologic unit ofUpper Triassic age, part of theNewark Supergroup. The Doswell Formation was originally named to refer to a geological sequence which forms the lower part of thesedimentary fill of theTaylorsville Basin inVirginia andMaryland. This sequence was deposited by lakes and rivers in the developingrift basin.^[1] A 2016 study argued that several geological layers inPennsylvania as well as the neighboringRichmond Basin ofVirginia also qualified as components of the Doswell Formation.^[2]

The most diverse and fossiliferous component of the Doswell formation is the Vinita member, also sometimes known as the Turkey Branch, Tuckahoe, or Falling Creek Formations in earlier publications, and frequently referred to as theVinita Formation by many authors. The Doswell formation isbiostratigraphically characterized by a fauna including the fishDictyopyge macrurus^[3] and theconchostracanLaxitextella multireticulata.^[4] The Richmond Basin has several notable fossil sites, such as the Tomahawk site which has hundreds of fossils from thecynodontBoreogomphodon,^[5] and theWinterpock site which has an extraordinarily diverse assortment of plants.^[3] The Taylorsville Basin is much more restricted in terms of fossil locales, but it was home to the unusual armored reptileDoswellia.^[6]

The Doswell Formation is among the oldest Triassic formations on the east coast of theUnited States. It is believed to belong to the earlyCarnian (Cordevolian) age of the Triassic based on its fauna and flora,^[4][2] which is distinctly dominated by tetrapods similar togondwanan groups, as well as a high diversity of humidity-loving plants such asferns andcycads. This gives it a distinct disconnect from the younger formations of the Newark Supergroup, which typically have a fauna similar to the Triassic formations of the western United States and a flora including elements such asconifer trees, which are better adapted for drier conditions.^[3]

The Doswell Formation was originally described byUSGS paleontologist Robert E. Weems in 1980. He used it to refer to the entire Triassic geological sequence preserved at the Taylorsville basin, overlying the much olderCarboniferous "Petersburg granite" which predated the basin, and underlying much youngerCretaceous andCenozoicgravel. Weems subdivided the formation into several "members". The oldest of these (middleCarnian in age) was theStagg Creek Member, afluvial (river) deposit ofsandstone andconglomerate.^[7] Weems originally described this layer as lacking fossils, although later studies argued otherwise.^[3]

Overlying the Stagg Creek Member was what Weems called theFalling Creek Member (late middle Carian). As sediments began to slow down the rivers of the Stagg Creek Member,lacustrine (lake) deposits began to form. Rivers still managed to flow into the now dominant lakes from different directions, depositing a diverse assortment of sediments. This allowed the Falling Creek Member to contain a variety of rock types, including sandstone,shale,siltstone, and occasionally evencoal. In addition, it is the most fossiliferous part of the Taylorsville basin, containing the fossils of not only fish and invertebrates, but also reptiles such as the heavily armoredDoswellia.^[7]

The youngest (early late Carnian) and most geographically extensive member was theNewfound Member, named after theNewfound river. Outcrops belonging to this member either contain coarse sandstone and conglomerate, or much finer sandstone and siltstone. This member is believed to have been formed by a vastalluvial fan or delta created by southeastern-flowing rivers, andAraucarioxylon (petrified wood) is occasionally found within the member.^[7]

In 1990, paleontologists Bruce Cornet &Paul Olsen described the Triassic fauna and flora of Virginia in detail. They argued that the Stagg Creek and Falling Creek Members actually coexisted at the same time, based onpalynological correlations. In addition, they noted that the Stagg Creek actually preserved a few fossils, mainly of crustaceans and the abundant fishDictyopyge.^[3]

In 2003,Columbia University geologist Peter LeTourneau became the first geologist to argue that the Doswell Formation was not the only Triassic geological layer in the Taylorsville Basin. He recognized the existence of a younger layer, which he called theKing George Group. The King George Group was an extensive geological interval dominated by sandstone and conglomerate. LeTourneau also elevated the ranking of the Doswell Formation, renaming it to the Doswell Group. In conjunction with this, he also elevated the Falling Creek, and Newfound Members to formations. However, he also placed the Newfound Formation outside of the Doswell Group, instead placing it as the oldest unit of the King George Group.^[1]

LeTourneau also evaluated Cornet & Olsen's claim that the Stagg Creek member coexisted with the Falling Creek Formation. He found that there were actually several distinct units grouped as part of the Stagg Creek unit. The original Stagg creek site which Weems (1980) based the member off of was found to belong to the middle portion of the Falling Creek Formation, therefore making the Stagg Creek Member part of that Formation. The portion of the Falling Creek Formation which was younger than the Stagg Creek Member was designated the Poor Farm Member, while the older portion was designated the Deer Creek Member. LeTourneau also added an additional formation to the Doswell Group, theSouth Anna Formation. This formation, the oldest section in the group, was very similar to the Stagg Creek Member in terms of its geological appearance and thickness, and as such South Anna outcroppings were originally considered to belong to the Stagg Creek Member according to Weems (1980). LeTournea differentiated the two based on their age, as determined by palynological dating.^[1]

In 2016, several paleontologists and geologists cooperated in a project which meant to correlate the individual Triassic basins of theNewark Supergroup with each other. They found that theRichmond and Taylorsville basins were likely deposited at the same time, a suspicion voiced earlier by Cornet & Olsen (1990).^[3] In light of this revelation, they set out to link the formations and members of the Richmond Basin with those of the Taylorsville. They deranked the Doswell Group back to the Doswell Formation, and also found that the differences between the South Anna Formation and the Stagg Creek Member were not statistically significant. As a result, they abandoned the designation of the South Anna Formation, synonymizing it with the Stagg Creek Member as originally considered in 1980. They additionally found that the "barren beds" of the Richmond Basin (and several thinner slivers of strata in other basins) were also synonymous with the Stagg Creek Member.^[2]

The Falling Creek Formation was more thoroughly deconstructed. Weems, Tanner, & Lucas found that this formation was basically identical to theVinita Member (or "Vinita beds") of the Richmond basin. As the Vinita member was named approximately 70 years earlier than the Falling Creek Formation, it was considered to take priority in naming. The Poor Farm and Deer Creek members of the Falling Creek Formation were also abandoned due to being poorly defined by LeTourneau.^[2]

These authors not only found the Doswell Formation to extend to the Richmond basin, but also to several other Triassic basins in Eastern Pennsylvania. For example, the "Irishtown beds" at the base of theGettysburg basin were found to be a young layer of the Doswell Formation (theIrishtown Member) due toconchostracan dating. Lastly, an unusually old section of theStockton Formation was also found to be a young part of the Doswell Formation (as theLahaska Creek Member) due to preserving fossils ofCalamops, atemnospondylamphibian which lived at the same time as the Doswell Formation.^[2]

Genus	Species	Sub-unit	Basin	Abundance	Notes	Images
Boreogomphodon[5]	B. jeffersoni	Vinita member (Tomahawk assemblage)	Richmond	"hundreds of specimens"	Atraversodontidcynodont
Calamops[2]	C. paludosis	Lahaska Creek member	Newark		Atemnospondyl
Doswellia[6]	D. kaltenbachi	Vinita member	Taylorsville, Richmond?	Partial skeleton, isolated bones	Anarchosauriform
Euscolosuchus[5]	E. olseni	Vinita member (Tomahawk assemblage)	Richmond	Armor scutes, vertebrae, and ribs	Anarchosauriform, possibly related toCrocodyliformes
Gomphiosauridion[8]	G. baileyae	Vinita member (Tomahawk assemblage)	Richmond	skull fragments	Aprocolophonid
Idiosaura[9]	I. virginiensis	Vinita Formation (Tomahawk assemblage)	Richmond	Partial dentaries	Akuehneosaurid-likelepidosauromorph withpleurodont dentition.[9]
Lacertilia indet.?[3]		Vinita member (Tomahawk assemblage)	Richmond
Lissamphibia indet.?[3]		Vinita member	Richmond	jaw fragment
Microconodon[5]	M. tenuirostris	Vinita member (Tomahawk assemblage)	Richmond	Teeth and jaw bones	Acynodont
Micromenodon[10]	M. pitti	Vinita Formation (Tomahawk assemblage)	Richmond	Partial maxilla	An earlysphenodontlepidosaur withacrodont dentition.[10]
Phytosauria indet.[5][6]		Vinita member	Richmond, Taylorsville	teeth
Poposaurus	P. gracilis	Vinita member	Taylorsville	vertebrae, partial humerus	A bipedalpoposauridsuchian.[11] Initially considered an indeterminaterauisuchid by Weems (1980).[6]
Rhynchocephalia indet.[5]		Vinita member (Tomahawk assemblage)	Richmond	skull fragments
Uatchitodon[5]	U. kroehleri	Vinita member (Tomahawk assemblage)	Richmond	teeth	A venomousreptile
Vinitasaura[12]	V. lizae	Vinita Formation (Tomahawk assemblage)	Richmond	Jaws	A deep-jawed lepidosauromorph with subthecodont dentition.[12]
Xenodiphyodon[8]	X. petraios	Vinita member (Tomahawk assemblage)	Richmond	partial jaw with teeth	A possibleprocolophonian

Genus	Species	Sub-unit	Basin	Abundance	Notes	Images
Cionichthys[3]	C. meekeri	Vinita member	Richmond, Taylorsville		Aredfieldiid
Coelacanthiformes indet.[3]		Vinita member	Richmond
Dictyopyge[3]	D. macrurus	Stagg Creek member, Vinita member	Richmond, Taylorsville	Abundant	Aredfieldiid
Lissodus	L. sp.	Vinita member (Tomahawk assemblage)	Richmond	teeth	Ahybodontshark
Tanaocrossus[3]	T. sp.	Vinita member	Richmond		Apalaeonisciform

^[3]

^[3]

• Carnian Stage
• Geologic formations of Pennsylvania
• Geologic formations of Maryland
• Geologic formations of Virginia
• Triassic geology of Pennsylvania
• Triassic geology of Virginia

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