{{Short description|Geologic formation in the United States}} {{Infobox rockunit | name = Doswell Formation | image = | caption = | type = Formation | age = Upper Triassic, {{fossilrange|237|234.5|early Carnian}} | period = Carnian | prilithology = | otherlithology = | namedfor = Doswell, Virginia | namedby = Robert E. Weems | region = Virginia, Pennsylvania | country = | coordinates = | unitof = Chatham Group | subunits = Stagg Creek Member, Vinita Member, Irishtown Member, Lahaska Creek Member | underlies = Stockton Formation | overlies = Petersburg Granite, Evangeline Formation | thickness = {{convert|2000|ft|m|abbr=on}} | extent = | area = | map = | map_caption = }} The '''Doswell Formation''' (also known as the '''Doswell Group''') is a geologic unit of Upper Triassic age, part of the Newark Supergroup. The Doswell Formation was originally named to refer to a geological sequence which forms the lower part of the sedimentary fill of the Taylorsville Basin in Virginia and Maryland. This sequence was deposited by lakes and rivers in the developing rift basin.<ref name="LeTourneau">{{cite book|url=https://archive.org/details/greatriftvalleys0000unse|title=The Great Rift Valleys of Pangea in Eastern North America: Volume 2|last=LeTourneau|first=P.M.|publisher=Edinburgh University Press|year=2003|isbn=978-0-231-12676-2|editor=LeTourneau P.M. & Olsen P.E.|chapter=Stratigraphic Architecture and Paleomagnetic Reversal Stratigraphy of the Late Triassic Taylorsville Basin, Virginia and Maryland|access-date=24 November 2012|url-access=registration}}</ref> A 2016 study argued that several geological layers in Pennsylvania as well as the neighboring Richmond Basin of Virginia also qualified as components of the Doswell Formation.<ref name=":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 the '''Vinita Formation''' by many authors. The Doswell formation is biostratigraphically characterized by a fauna including the fish ''Dictyopyge macrurus''<ref name=":1" /> and the conchostracan ''Laxitextella multireticulata''.<ref name="Concho" /> The Richmond Basin has several notable fossil sites, such as the Tomahawk site which has hundreds of fossils from the cynodont ''Boreogomphodon'',<ref name="Tet1994" /> and the Winterpock site which has an extraordinarily diverse assortment of plants.<ref name=":1" /> The Taylorsville Basin is much more restricted in terms of fossil locales, but it was home to the unusual armored reptile ''Doswellia''.<ref name="WRE80" />
The Doswell Formation is among the oldest Triassic formations on the east coast of the United States. It is believed to belong to the early Carnian (Cordevolian) age of the Triassic based on its fauna and flora,<ref name="Concho" /><ref name=":2" /> which is distinctly dominated by tetrapods similar to gondwanan groups, as well as a high diversity of humidity-loving plants such as ferns and cycads. 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 as conifer trees, which are better adapted for drier conditions.<ref name=":1" />
==History and stratigraphy==
=== Weems (1980)'s interpretation === The Doswell Formation was originally described by USGS 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 older Carboniferous "Petersburg granite" which predated the basin, and underlying much younger Cretaceous and Cenozoic gravel. Weems subdivided the formation into several "members". The oldest of these (middle Carnian in age) was the Stagg Creek Member, a fluvial (river) deposit of sandstone and conglomerate.<ref name=":0">{{Cite book|url=https://dmme.virginia.gov/commercedocs/PUB_27.pdf|title=Geology of the Taylorsville basin, Hanover County, Virginia|last=Weems|first=Robert E.|work=Contributions to Virginia Geology- IV|publisher=Department of Conservation and Economic Development|year=1980|pages=23–38}}</ref> Weems originally described this layer as lacking fossils, although later studies argued otherwise.<ref name=":1">{{Cite book|url=https://www.researchgate.net/publication/301677351|title=Early to Middle Carnian (Triassic) Flora and Fauna of the Richmond and Taylorsville Basins, Virginia and Maryland, U.S.A.|last1=Cornet|first1=Bruce|last2=Olsen|first2=Paul E.|publisher=Virginia Museum of Natural History|year=1990|isbn=0-9625801-1-2|pages=1–84}}</ref>
Overlying the Stagg Creek Member was what Weems called the Falling 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 even coal. 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 armored ''Doswellia''.<ref name=":0" />
The youngest (early late Carnian) and most geographically extensive member was the Newfound Member, named after the Newfound 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 vast alluvial fan or delta created by southeastern-flowing rivers, and ''Araucarioxylon'' (petrified wood) is occasionally found within the member.<ref name=":0" />
=== Cornet & Olsen (1990)'s interpretation === 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 on palynological correlations. In addition, they noted that the Stagg Creek actually preserved a few fossils, mainly of crustaceans and the abundant fish ''Dictyopyge''.<ref name=":1" />
=== LeTourneau (2003)'s interpretation === 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 the King 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.<ref name="LeTourneau" />
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, the South 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.<ref name="LeTourneau" />
=== Weems, Tanner, & Lucas (2016)'s interpretation === In 2016, several paleontologists and geologists cooperated in a project which meant to correlate the individual Triassic basins of the Newark Supergroup with each other. They found that the Richmond and Taylorsville basins were likely deposited at the same time, a suspicion voiced earlier by Cornet & Olsen (1990).<ref name=":1" /> 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.<ref name=":2">{{Cite journal|last1=Weems|first1=Robert E.|last2=Tanner|first2=Lawrence H.|last3=Lucas|first3=Spencer G.|date=2016|title=Synthesis and revision of the lithostratigraphic groups and formations in the Upper Permian?–Lower Jurassic Newark Supergroup of eastern North America|url=https://www.researchgate.net/publication/309853816|journal=Stratigraphy|language=en|volume=13|issue=2|pages=111–153 |doi=10.29041/strat.13.2.03 }}</ref>
The Falling Creek Formation was more thoroughly deconstructed. Weems, Tanner, & Lucas found that this formation was basically identical to the Vinita 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.<ref name=":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 the Gettysburg basin were found to be a young layer of the Doswell Formation (the Irishtown Member) due to conchostracan dating. Lastly, an unusually old section of the Stockton Formation was also found to be a young part of the Doswell Formation (as the Lahaska Creek Member) due to preserving fossils of ''Calamops'', a temnospondyl amphibian which lived at the same time as the Doswell Formation.<ref name=":2" />
==Paleobiota== ===Tetrapods===
{| class="wikitable" align="center" width="100%" |- ! Genus ! Species ! Sub-unit ! Basin ! Abundance ! Notes ! Images |- | ''Boreogomphodon''<ref name="Tet1994">{{Cite book|author=Sues|first1=Hans-Dieter|last2=Olson|first2=Paul E.|last3=Kroehler|first3=Peter A.|year=1994|title=Small tetrapods from the Upper Triassic of the Richmond basin (Newark Supergroup), Virginia |url=https://www.ldeo.columbia.edu/~polsen/nbcp/sues_et_al_94.pdf|work=In the Shadow of the Dinosaurs|publisher=Cambridge University Press|pages=161–170}}</ref> | ''B. jeffersoni'' | Vinita member (Tomahawk assemblage) | Richmond | "hundreds of specimens" | A traversodontid cynodont | |- | ''Calamops''<ref name=":2" /> | ''C. paludosis'' | Lahaska Creek member | Newark | | A temnospondyl | |- | ''Doswellia''<ref name="WRE80">{{Cite journal|author=R. E. Weems|year=1980|title=An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A.|url=https://www.researchgate.net/publication/270069424|journal=Transactions of the American Philosophical Society |series=New Series|volume=70|issue=7|pages=1–53|doi=10.2307/1006472|jstor=1006472}}</ref> | ''D. kaltenbachi'' | Vinita member | Taylorsville, Richmond? | Partial skeleton, isolated bones | An archosauriform | 200px |- | ''Euscolosuchus''<ref name="Tet1994"/> | ''E. olseni'' | Vinita member (Tomahawk assemblage) | Richmond | Armor scutes, vertebrae, and ribs | An archosauriform, possibly related to Crocodyliformes | |- | ''Gomphiosauridion''<ref name="Procol">{{Cite journal|author=Sues|first1=Hans-Dieter|last2=Olson|first2=Paul E.|year=1992|title=A new procolophonid and a new tetrapod of uncertain, possibly procolophonian affinities from the Upper Triassic of Virginia |url=https://www.ldeo.columbia.edu/~polsen/nbcp/sues_et_al_94.pdf|journal=Journal of Vertebrate Paleontology|volume=13|issue=3|pages=282–286|doi=10.1080/02724634.1993.10011510}}</ref> | ''G. baileyae'' | Vinita member (Tomahawk assemblage) | Richmond | skull fragments | A procolophonid | |- |''Idiosaura''<ref name=":3">{{Cite journal |last1=Kligman |first1=Ben T. |last2=Sues |first2=Hans-Dieter |last3=Melstrom |first3=Keegan M. |date=2024-06-10 |title=A new lizard-like reptile with unusual mandibular neurovasculature from the Upper Triassic of Virginia |url=https://www.tandfonline.com/doi/full/10.1080/02724634.2024.2353636 |journal=Journal of Vertebrate Paleontology |language=en |doi=10.1080/02724634.2024.2353636 |issn=0272-4634|url-access=subscription }}</ref> |''I. virginiensis'' |Vinita Formation (Tomahawk assemblage) |Richmond |Partial dentaries |A kuehneosaurid-like lepidosauromorph with pleurodont dentition.<ref name=":3" /> | |- | Lacertilia indet.?<ref name=":1"/> | | Vinita member (Tomahawk assemblage) | Richmond | | | |- | Lissamphibia indet.?<ref name=":1"/> | | Vinita member | Richmond | jaw fragment | | |- | ''Microconodon''<ref name="Tet1994"/> | ''M. tenuirostris'' | Vinita member (Tomahawk assemblage) | Richmond | Teeth and jaw bones | A cynodont | |- |''Micromenodon''<ref name=":4">{{Cite journal |last1=Sues |first1=Hans-Dieter |last2=Schoch |first2=Rainer R. |date=2021 |title=A new early-diverging sphenodontian (Lepidosauria, Rhynchocephalia) from the Upper Triassic of Virginia, U.S.A. |url=https://www.cambridge.org/core/product/identifier/S0022336020000876/type/journal_article |journal=Journal of Paleontology |language=en |volume=95 |issue=2 |pages=344–350 |doi=10.1017/jpa.2020.87 |issn=0022-3360|url-access=subscription }}</ref> |''M. pitti'' |Vinita Formation (Tomahawk assemblage) |Richmond |Partial maxilla |An early sphenodont lepidosaur with acrodont dentition.<ref name=":4" /> | |- | Phytosauria indet.<ref name="Tet1994"/><ref name="WRE80"/> | | Vinita member | Richmond, Taylorsville | teeth | | |- |''Poposaurus'' |''P. gracilis'' |Vinita member |Taylorsville |vertebrae, partial humerus |A bipedal poposaurid suchian.<ref>{{Cite book |last1=Lucas |first1=Spencer G. |url=https://books.google.com/books?id=YCKIEAAAQBAJ |title=FOSSIL RECORD 8 |last2=Blodgett |first2=Robert B. |last3=Lichtig |first3=Asher J. |last4=Hunt |first4=Adrian P. |date=2022-08-02 |publisher=New Mexico Museum of Natural History and Science |language=en}}</ref> Initially considered an indeterminate rauisuchid by Weems (1980).<ref name="WRE80" /> |frameless |- | Rhynchocephalia indet.<ref name="Tet1994"/> | | Vinita member (Tomahawk assemblage) | Richmond | skull fragments | | |- | ''Uatchitodon''<ref name="Tet1994"/> | ''U. kroehleri'' | Vinita member (Tomahawk assemblage) | Richmond | teeth | A venomous reptile | |- |''Vinitasaura<ref name=":5">{{Cite journal |last1=Sues |first1=Hans-Dieter |last2=Kligman |first2=Ben T. |date=2020-12-10 |title=A new lizard-like reptile from the Upper Triassic (Carnian) of Virginia and the Triassic record of Lepidosauromorpha (Diapsida, Sauria) |journal=Journal of Vertebrate Paleontology |volume=40 |issue=6 |article-number=e1879102 |bibcode=2020JVPal..40E9102S |doi=10.1080/02724634.2021.1879102 |issn=0272-4634 |s2cid=233805788}}</ref>'' |''V. lizae'' |Vinita Formation (Tomahawk assemblage) |Richmond |Jaws |A deep-jawed lepidosauromorph with subthecodont dentition.''<ref name=":5" />'' | |- | ''Xenodiphyodon''<ref name="Procol"/> | ''X. petraios'' | Vinita member (Tomahawk assemblage) | Richmond | partial jaw with teeth | A possible procolophonian | |- |}
===Fish=== {| class="wikitable" align="center" width="100%" |- ! Genus ! Species ! Sub-unit ! Basin ! Abundance ! Notes ! Images |- | ''Cionichthys''<ref name=":1"/> | ''C. meekeri'' | Vinita member | Richmond, Taylorsville | | A redfieldiid | |- | Coelacanthiformes indet.<ref name=":1"/> | | Vinita member | Richmond | | |- | ''Dictyopyge''<ref name=":1"/> | ''D. macrurus'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | Abundant | A redfieldiid | 200px |- | ''Lissodus'' | ''L. sp.'' | Vinita member (Tomahawk assemblage) | Richmond | teeth | A hybodont shark |- | ''Tanaocrossus''<ref name=":1"/> | ''T. sp.'' | Vinita member | Richmond | | A palaeonisciform | |- |}
===Invertebrates===
{| class="wikitable" align="center" width="100%" |- ! Genus ! Species ! Sub-unit ! Basin ! Abundance ! Notes ! Images |- | ''Darwinula''<ref name=":1"/> | | Vinita member, Stagg Creek member | Richmond, Taylorsville | | Ostracods | |- | Gastropoda indet.<ref name=":1"/> | | Vinita member | Richmond, Taylorsville | | Snails | |- | ''Laxitextella''<ref name="Concho">{{Cite journal|author=Kozur|first1=Heinz W.|last2=Weems|first2=Robert E.|year=2010|title=The biostratigraphic importance of conchostracans in the continental Triassic of the northern hemisphere |url=http://www.terra-triassica.de/wp-content/uploads/2014/01/Kozur-u-Weems-2010-Conchostracans.pdf|journal=Geological Society of London, Special Publications|volume=334|issue=1|pages=315–417|doi=10.1144/SP334.13|bibcode=2010GSLSP.334..315K|s2cid=131224365}}</ref> | ''L. multireticulata'' | Stagg Creek member, Vinita member, Irishtown member | Richmond, Taylorsville, Gettysburg | Abundant | A conchostracan (clam shrimp) | |- | Unionidae indet.<ref name=":1"/> | | Vinita member | Taylorsville | | Clams | |- |}
===Pteridophytes (spore-bearing plants)=== <ref name=":1"/> {| class="wikitable" align="center" width="100%" |- ! Genus ! Species ! Sub-unit ! Basin ! Abundance ! Notes ! Images |- | ''Auriculophora'' | ''A. acrostichoides'' | Vinita member (Winterpock assemblage) | Richmond | | A marattialean fern | |- | ''Cladophlebis'' | ''C. auriculata'', ''C. mexicana'' | Vinita member | Richmond | | A filicalean fern | |- | ''Clathropteris'' | ''C. meniscoides'' | Vinita member (Winterpock assemblage) | Richmond | | A dipterid fern | |- | ''Cyathocaulis'' | ''C. carolinensis'' | Vinita member (Winterpock assemblage) | Richmond | | A cyatheaceaen tree fern | |- | ''Cyathoforma'' (''Asterotheca, Asterocarpus'') | ''C. carolinensis'', ''C. minuta'', ''C. penticarpa'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A large tree fern | |- | ''Danaeopsis'' | ''D. virginiensis'' | Vinita member (Winterpock assemblage) | Richmond | | A marattialean fern | |- | ''Dicranopteris'' | ''D.'' sp. | Vinita member (Winterpock assemblage) | Richmond | | A forked fern | |- | ''Equisetites'' | ''E. richmondensis'', ''E. rogersii'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A horsetail | |- | ''Gleichenites'' | ''G. distans'' | Vinita member (Winterpock assemblage) | Richmond | common | A forked fern | |- | ''Isoetodendron'' | ''I. striata'' | Vinita member (Winterpock assemblage) | Richmond | | A quillwort | |- | ''Leptocyclotes'' | ''L. americana'' | Vinita member (Winterpock assemblage) | Richmond | | A quillwort | |- | ''Lonchopteris'' | ''L. oblonga'' | Vinita member (Winterpock assemblage) | Richmond | | A fern | |- | ''Mertensides'' | ''M. bullatus'' | Vinita member (Winterpock assemblage) | Richmond | common | A fern | |- | ''Neocalamites'' | ''N. virginiensis'',''N. delawarensis'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A horsetail | |- | ''Osmundites'' | ''O. winterpockensis'' | Vinita member (Winterpock assemblage) | Richmond | | An osmunadcean fern | |- | ''Pecopteris'' | ''P. rarinervis'' | Vinita member | Richmond | | A type of leaf referable to several types of plants | |- | ''Phlebopteris'' | ''P. smithii'' | Stagg Creek member | Taylorsville | | A matoniacean fern | |- | ''Pteridocaulis'' | ''P. rhombiformis'', ''P. facialis'' | Vinita member (Winterpock assemblage) | Richmond | | Stem of a tree fern | |- | ''Todites'' (''Acrostichites'') | ''T. linnaeaefolius'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A fern | |- |}
===Gymnosperms=== <ref name=":1"/> {| class="wikitable" align="center" width="100%" |- ! Genus ! Species ! Sub-unit ! Basin ! Abundance ! Notes ! Images |- | ''Macrotaeniopteris'' | ''M. crassinervis'', ''M. magnifolia'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A cycad | |- | ''Plicarizamites'' | ''P. lanceolatus'' | Vinita member (Winterpock assemblage) | Richmond | | | |- | ''Podozamites'' | ''P. lanceolatus'', ''P. tenuistriatus'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | | A type of leaf referable to conifers | |- | ''Primaraucaria'' | ''P. wielandii'' | Vinita member | Richmond | fairly common | An araucariacean conifer | |- | ''Pterophyllum'' | ''P. affinae'', ''P. braunianum'', ''P. inaequale'', ''P. giganteum'', ''P. grandifolium'', ''P. taxinum'', ''P. tenuinervis'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | Very common | A cycadeoid bennetitalean | |- | ''Sagenopteris'' | ''S. rhoifolia'' | Vinita member (Winterpock assemblage) | Richmond | | A seed fern | |- | ''Sphenobaiera'' | ''S. striata'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A ginkgophyte | |- | ''Sphenopteris'' | ''S. sitholeyi'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A seed fern | |- | ''Sphenozamites'' | ''S. rogersianus'' | Stagg Creek member, Vinita member | Richmond, Taylorsville | common | A cycad | |- | ''Stangerites'' | ''S. obliqua'', ''S. planus'' | Vinita member (Winterpock assemblage) | Richmond | | A cycad | |- | ''Taeniopteris'' | ''T. diminuta'' | Vinita member (Winterpock assemblage) | Richmond | common | Leaves of a ''Pentoxylon''-like plant | |- | ''Triassiflorites'' | ''T. grandiflora'' | Vinita member (Winterpock assemblage) | Richmond | | A cone, possibly from a bennetitalean | |- | ''Zamiostrobus'' | ''Z. lissocardius'' | Vinita member (Winterpock assemblage) | Richmond | | A cone from a cycad or conifer | |- | ''Zamites'' | ''Z. powellii'' | Stagg Creek member | Taylorsville | | A cycadeoid bennetitalean | |- |}
==References== {{Reflist}}
{{Geology of the Newark Basin}}
Category:Carnian Stage Category:Geologic formations of Pennsylvania Category:Geologic formations of Maryland Category:Geologic formations of Virginia Category:Triassic geology of Pennsylvania Category:Triassic geology of Virginia