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29-534: The Atarque Sandstone is a geologic formation in New Mexico . It preserves fossils dating back to the late Cretaceous period . The formation consists of very fine to fine buff fossiliferous sandstone with a maximum thickness of 100 feet (30 m). The beds are flat, massive and have low-angle crossbedding . The formation intertongues with the underlying Rio Salado Tongue of the Mancos Shale and

58-611: A geologic formation goes back to the beginnings of modern scientific geology. The term was used by Abraham Gottlob Werner in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817. The concept became increasingly formalized over time and is now codified in such works as the North American Stratigraphic Code and its counterparts in other regions. Geologic maps showing where various formations are exposed at

87-542: A newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first name applied to a particular formation. As with other stratigraphic units, the formal designation of a formation includes a stratotype which is usually a type section . A type section

116-617: A permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the Morrison Formation , named for the town of Morrison, Colorado , and the Kaibab Limestone , named after the Kaibab Plateau of Arizona. The names must not duplicate previous formation names, so, for example,

145-449: Is not a valid lithological basis for defining a formation. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geologic mapping normally practiced in the region; the thickness of formations may range from less than a meter to several thousand meters. Geologic formations are typically named after

174-426: Is also used informally to describe the odd shapes (forms) that rocks acquire through erosional or depositional processes. Such a formation is abandoned when it is no longer affected by the geologic agent that produced it. Some well-known cave formations include stalactites and stalagmites . Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in

203-560: Is central to the geologic discipline of stratigraphy , and the formation is the fundamental unit of stratigraphy. Formations may be combined into groups of strata or divided into members . Members differ from formations in that they need not be mappable at the same scale as formations, though they must be lithologically distinctive where present. The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata . Formations were at first described as

232-455: Is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional reference sections may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining

261-721: Is overlain by either the Carthage Member of the Tres Hermanos Formation or the Moreno Hill Formation . It was deposited during the early to middle Turonian Age of the late Cretaceous . The formation is interpreted as a coastal barrier sandstone deposited by a regression of the Western Interior Seaway to the northeast. The formation contains fossil pelecypods and plesiosaur vertebra. It also contains fossils of

290-800: The Tendaguru Formation in Tanzania . The age and much of the fauna is similar to the Lourinhã Formation in Portugal. Throughout the western United States, it variously overlies the Middle Jurassic Summerville, Sundance , Bell Ranch, Wanakah, and Stump Formations. At the time, the supercontinent of Laurasia had recently split into the continents of North America and Eurasia, although they were still connected by land bridges. North America moved north and

319-806: The ammonite Spathites rioensis characteristic of the middle Turonian. The unit was first defined as the Atarque Member of the Mesaverde Formation by W.S. Pike in 1947. Pike assigned the lowermost 127 feet (39 m) of the Mesaverde in the San Juan Basin to the unit. S.C. Hook and coinvestigators revised the Cretaceous stratigraphy of the San Juan and Zuni basins in 1983 and assigned both Pike's Atarque Member and beds in

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348-781: The Morrison include the Stockett Bed in Montana, an unofficial sub-unit which contains bituminous coal; the outdated terms Casamero, Chavez, and Prewitt Sandstone for the Brushy Basin, Recapture, and Westwater Canyon, respectively; and the Bullington Member, which has been discarded entirely. Though many of the Morrison Formation fossils are fragmentary, they are sufficient to provide a good picture of

377-755: The Zuni Basin assigned to the lower Gallup Sandstone to the Tres Hermanos Formation . The Atarque Member was redefined as the lower sandstone beds of the Tres Hermanos Formation. Simultaneously, M.W. McLellan and coinvestigators raised the Atarque to formation rank as the Atarque Sandstone in the Salt Lake coal field of west-central New Mexico. Formation (geology) A formation must be large enough that it can be mapped at

406-586: The borders of present-day Nevada and Utah) and deposited in swampy lowlands, lakes, river channels and floodplains , became the Morrison Formation. In the north, the Sundance Sea , an extension of the Arctic Ocean , stretched through Canada down to the United States. Coal is found in the Morrison Formation of Montana, which means that the northern part of the formation, along the shores of

435-550: The essential geologic time markers, based on their relative ages and the law of superposition . The divisions of the geological time scale were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. Geologic formations can be usefully defined for sedimentary rock layers, low-grade metamorphic rocks , and volcanic rocks . Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as lithodemes . "Formation"

464-547: The flora and fauna in the Morrison Basin during the Kimmeridgian. Overall, the climate was dry, similar to a savanna but, since there were no angiosperms ( grasses , flowers , and some trees ), the flora was quite different. Conifers , the dominant plants of the time, were to be found with ginkgos , cycads , tree ferns , and horsetail rushes . Much of the fossilized vegetation was riparian , living along

493-522: The formation is expected to describe the stratotype in sufficient detail that other geologists can unequivocally recognize the formation. Although formations should not be defined by any criteria other than primary lithology, it is often useful to define biostratigraphic units on paleontological criteria, chronostratigraphic units on the age of the rocks, and chemostratigraphic units on geochemical criteria, and these are included in stratigraphic codes. The concept of formally defined layers or strata

522-583: The large body mass of the sauropods has been interpreted as an adaptation to migration in times of drought. Sauropods that have been discovered include Diplodocus (most famously, the first nearly complete specimen of D. carnegii , which is now exhibited at the Carnegie Museum of Natural History , in Pittsburgh, Pennsylvania), Camarasaurus (the most commonly found sauropod), Brachiosaurus , Apatosaurus , Brontosaurus , Barosaurus ,

551-484: The panhandles of Oklahoma and Texas , New Mexico , Arizona , Utah , and Idaho . Equivalent rocks under different names are found in Canada. It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists . Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent

580-472: The river flood plains. Along the rivers, there were fish , frogs , salamanders , lizards , crocodiles , turtles , pterosaurs , crayfish , clams , and mammaliforms . The dinosaurs were most likely riparian, as well. Hundreds of dinosaur fossils have been discovered, such as Allosaurus , Ceratosaurus , Torvosaurus , Saurophaganax , Camptosaurus , Ornitholestes , several stegosaurs comprising at least two species of Stegosaurus and

609-421: The sea, was wet and swampy, with more vegetation. Aeolian , or wind-deposited sandstones, are found in the southwestern part, which indicates it was much more arid—a desert , with sand dunes . The Morrison Formation is subdivided into several members, the occurrence of which are varied across the geographic extent of the Morrison. Members are (in alphabetical order): Other informal or disused designations of

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638-503: The slightly older Hesperosaurus , and the early ankylosaurs , Mymoorapelta and Gargoyleosaurus , most notably a very broad range of sauropods (the giants of the Mesozoic era). Since at least some of these species are known to have nested in the area ( Camptosaurus embryoes have been discovered), there are indications that it was a good environment for dinosaurs and not just home to migratory, seasonal populations. However,

667-643: The surface are fundamental to such fields as structural geology , allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources. The boundaries of a formation are chosen to give it the greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary depositional structures , fossils regarded as rock-forming particles, or other organic materials such as coal or kerogen . The taxonomy of fossils

696-432: The surface or traced in the subsurface. Formations are otherwise not defined by the thickness of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even a heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent bodies of rock. The concept of

725-543: The uncommon Haplocanthosaurus and Supersaurus . The very diversity of the sauropods has raised some questions about how they could all co-exist. While their body shapes are very similar (long neck, long tail, huge elephant-like body), they are assumed to have had very different feeding strategies, in order for all to have existed in the same time frame and similar environment. Locations where significant Morrison Formation fossil discoveries have been made include: The Morrison Formation contains uranium deposits, including

754-713: The western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone , sandstone , siltstone , and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period. It is centered in Wyoming and Colorado , with outcrops in Montana , North Dakota , South Dakota , Nebraska , Kansas ,

783-547: Was a major source of uranium ore. According to radiometric dating , the Morrison Formation dates from 156.3 ± 2 million years old ( Ma ) at its base, to 146.8 ± 1 million years old at the top, which places it in the earliest Kimmeridgian , and early Tithonian stages of the late Jurassic. This is similar in age to the Solnhofen Limestone Formation in Germany and

812-535: Was eroded during exhumation of the Rocky Mountains . It was named after Morrison, Colorado , where some of the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars , a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope . In Colorado, New Mexico, and Utah, the Morrison Formation

841-607: Was passing through the subtropical regions . The Morrison Basin, which stretched from New Mexico in the south to Alberta and Saskatchewan in the north, was formed during the Nevadan orogeny , a precursor event to later orogenic episodes that created the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins , carried by streams and rivers from the Elko Highlands (along

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