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38-715: 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 and the conchostracan Laxitextella multireticulata . The Richmond Basin has several notable fossil sites, such as

76-407: A faunal assemblage , rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Furthermore, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of

114-497: A temnospondyl amphibian which lived at the same time as the Doswell Formation. Biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. The primary objective of biostratigraphy is correlation , demonstrating that a particular horizon in one geological section represents

152-575: A few meters, up to hundreds of meters. They can also range from local to worldwide, as the extent of which they can reach in the horizontal plane relies on tectonic plates and tectonic activity. Two of the tectonic processes that run the risk of changing these zones' ranges are metamorphic folding and subduction . Furthermore, biostratigraphic units are divided into six principal kinds of biozones: Taxon range biozone , Concurrent range biozone, Interval biozone, Lineage biozone, Assemblage biozone, and Abundance biozone . The Taxon range biozone represents

190-404: A new period, most of the periods we recognize today are terminated by a major extinction event or faunal turnover. A stage is a major subdivision of strata, each systematically following the other each bearing a unique assemblage of fossils. Therefore, stages can be defined as a group of strata containing the same major fossil assemblages. French palaeontologist Alcide d'Orbigny is credited for

228-399: A particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that

266-702: A suspicion voiced earlier by Cornet & Olsen (1990). 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

304-404: A unique association of three or more taxa within it. Abundance biozones are strata in which the abundance of a particular taxon or group of taxa is significantly greater than in the adjacent part of the section. Index fossils (also known as guide fossils , indicator fossils , or dating fossils ) are the fossilized remains or traces of particular plants or animals that are characteristic of

342-534: Is believed to belong to the early Carnian (Cordevolian) age of the Triassic based on its fauna and flora, 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

380-527: 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. 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

418-557: 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 . 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

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456-811: 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 ,

494-519: The Stagg Creek Member according to Weems (1980). LeTournea differentiated the two based on their age, as determined by palynological dating. 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,

532-533: The Stagg Creek actually preserved a few fossils, mainly of crustaceans and the abundant fish Dictyopyge . 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

570-490: 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

608-576: The Tomahawk site which has hundreds of fossils from the cynodont Boreogomphodon , and the Winterpock site which has an extraordinarily diverse assortment of plants. The Taylorsville Basin is much more restricted in terms of fossil locales, but it was home to the unusual armored reptile Doswellia . The Doswell Formation is among the oldest Triassic formations on the east coast of the United States . It

646-452: The appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone. Oppel's zones are named after a particular distinctive fossil species, called an index fossil. Index fossils are one of the species from the assemblage of species that characterize the zone. Biostratigraphy uses zones for the most fundamental unit of measurement. The thickness and range of these zones can be

684-464: 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 . Weems originally described this layer as lacking fossils, although later studies argued otherwise. Overlying the Stagg Creek Member

722-415: 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. The Falling Creek Formation was more thoroughly deconstructed. Weems, Tanner, & Lucas found that this formation

760-506: The early Carnian -aged Doswell Formation in what is now Virginia , United States: Both species inhabited the rift lakes of the Newark Supergroup , which formed due to the tectonic changes that were starting to break up Pangaea . Several possibly unrelated species from the Triassic of Europe are provisionally referred to Dictyopyge (" D. " rhenana , " D. " socialis , " D. " catoptera , " D. " superstes ), while three species from

798-414: The invention of this concept. He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based. In 1856 German palaeontologist Albert Oppel introduced the concept of zone (also known as biozones or Oppel zone). A zone includes strata characterized by the overlapping range of fossils. They represent the time between

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836-413: The isotopes found within fossils via radioactive decay. Current 21st century uses of biostratigraphy involve interpretations of age for rock layers, which are primarily used by oil and gas industries for drilling workflows and resource allocations. Fossil assemblages were traditionally used to designate the duration of periods. Since a large change in fauna was required to make early stratigraphers create

874-481: The known stratigraphic and geographic range of occurrence of a single taxon. Concurrent range biozone includes the concurrent, coincident, or overlapping part of the range of two specified taxa. Interval biozones include the strata between two specific biostratigraphic surfaces and can be based on lowest or highest occurrences. Lineage biozones are strata containing species representing a specific segment of an evolutionary lineage. Assemblage biozones are strata that contain

912-502: The late 18th century the Cambrian and Carboniferous periods were internationally recognized due to these findings. During the early 20th century, advancements in technology gave scientists the ability to study radioactive decay . Using this methodology, scientists were able to establish geological time, the boundaries of the different eras ( Paleozoic , Mesozoic , Cenozoic ), as well as Periods ( Cambrian , Ordovician , Silurian ) through

950-454: The mechanism behind it— evolution . Scientists William Smith , George Cuvier , and Alexandre Brongniart came to the conclusion that fossils then indicated a series of chronological events, establishing layers of rock strata as some type of unit, later termed biozone . From here on, scientists began relating the changes in strata and biozones to different geological eras, establishing boundaries and time periods within major faunal changes. By

988-446: The name of Nicolas Steno was one of the first geologists to recognize that rock layers correlate to the Law of Superposition . With advancements in science and technology, by the 18th century it began to be accepted that fossils were remains left by species that had become extinct, but were then preserved within the rock record. The method was well-established before Charles Darwin explained

1026-461: The organism was incompletely known, and the strata extend the known fossil range. For instance, the presence of the trace fossil Treptichnus pedum was used to define the base of the Cambrian period, but it has since been found in older strata. If the fossil is easy to preserve and easy to identify, more precise time estimating of the stratigraphic layers is possible. The concept of faunal succession

1064-475: The principle of faunal succession, where fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be categorized by its fossil extent. Dictyopyge Dictyopyge is an extinct genus of prehistoric freshwater ray-finned fish that inhabited eastern North America during the early part of the Late Triassic . Two species are recognized, both from

1102-402: The same period of time as another horizon at a different section. Fossils within these strata are useful because sediments of the same age can look completely different, due to local variations in the sedimentary environment . For example, one section might have been made up of clays and marls , while another has more chalky limestones . However, if the fossil species recorded are similar,

1140-832: The species lived. Index fossils were originally used to define and identify geologic units, then became a basis for defining geologic periods , and then for faunal stages and zones. Ammonites , graptolites , archeocyathids , inoceramids , and trilobites are groups of animals from which many species have been identified as index fossils that are widely used in biostratigraphy. Species of microfossils such as acritarchs , chitinozoans , conodonts , dinoflagellate cysts, ostracods , pollen , spores and foraminiferans are also frequently used. Different fossils work well for sediments of different ages; trilobites, for example, are particularly useful for sediments of Cambrian age. A long series of ammonite and inoceramid species are particularly useful for correlating environmental events around

1178-424: The two sediments are likely to have been laid down around the same time. Ideally these fossils are used to help identify biozones , as they make up the basic biostratigraphy units, and define geological time periods based upon the fossil species found within each section. Basic concepts of biostratigraphic principles were introduced centuries ago, going as far back as the early 1800s. A Danish scientist and bishop by

Doswell Formation - Misplaced Pages Continue

1216-404: The western United States and a flora including elements such as conifer trees, which are better adapted for drier conditions. 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

1254-635: The world during the super-greenhouse of the Late Cretaceous . To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short-lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly, such as ammonites, are favored over forms that evolve much more slowly, like nautiloids . Often biostratigraphic correlations are based on

1292-650: 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. LeTourneau also evaluated Cornet & Olsen's claim that

1330-482: 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. These authors not only found the Doswell Formation to extend to

1368-619: Was theorized at the beginning of the 19th century by William Smith . When Smith was studying rock strata, he began to recognize that rock outcrops contained a unique collection of fossils. The idea that these distant rock outcrops contained similar fossils allowed for Smith to order rock formations throughout England. With Smith's work on these rock outcrops and mapping around England, he began to notice some beds of rock may contain mostly similar species, however there were also subtle differences within or between these fossil groups. This difference in assemblages that appeared identical at first, lead to

1406-602: 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

1444-636: 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

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