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16-758: The Ojo Alamo Formation is divided into two subunits separated by a large unconformity —a gap in the geologic record. The lower Naashoibito member (sometimes considered part of the Kirtland Formation) was deposited during the Maastrichtian age of the Cretaceous period, specifically between about 69-68 million years ago. It overlies the De-na-zin member of the Kirtland formation, though the two are separated by another large unconformity that spans

32-618: A period of geologic time equivalent to 73-69 million years ago. All dinosaur fossils probably come from this unit. Although argon dating suggest the naashoibito member would be dated to 66.5 mya. The upper unit of the Ojo Alamo Formation is the Kimbeto Member, which was deposited mainly during the earliest Cenozoic ( Danian age of the Paleogene period), between 66 and 64 million years ago. Dinosaur remains are among

48-421: Is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record . The significance of angular unconformity (see below)

64-565: Is deposited against older strata thus influencing its bedding structure. A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols , or beds of pebbles derived from the underlying rock. Law of superposition The law of superposition is an axiom that forms one of the bases of the sciences of geology , archaeology , and other fields pertaining to geological stratigraphy . In its plainest form, it states that in undeformed stratigraphic sequences,

80-467: Is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Human-made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes. Some archaeological strata (often termed as contexts or layers) are created by undercutting previous strata. An example would be that

96-614: The fossils that have been recovered from the formation, though all dinosaur remains come from the lowest part of the formation, the Naashoibito member (sometimes considered part of the Kirtland Formation , which dates to the late Maastrichtian stage of the Cretaceous period). Some researchers have claimed to find isolated non-avian dinosaur remains in the younger Kimbeto Member. If this is the case, it would represent

112-538: The next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus . It is a kind of relative dating . A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in

128-503: The oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount to stratigraphic dating , which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. To illustrate the practical applications of superposition in scientific inquiry, sedimentary rock that has not been deformed by more than 90° will exhibit

144-498: The oldest layers on the bottom, thus enabling paleontologists and paleobotanists to identify the relative ages of any fossils found within the strata, with the remains of the most archaic lifeforms confined to the lowest. These findings can inform the community on the fossil record covering the relevant strata, to determine which species coexisted temporally and which species existed successively in perhaps an evolutionarily or phylogenetically relevant way. The law of superposition

160-1298: The only known instance of a non-avian dinosaur population persisting after the Cretaceous–Paleogene extinction event . However, most scientists consider these to have been stratigraphically misinterpreted or reworked from the older Naashoibito member. The following species are known to be present in the Naashoibito Member "Alamo Wash Fauna". Fish Myledaphus M. sp. ? Squatirhina ?S. sp. Amphibians ? Batrachosauroididae Indeterminate Testudines Compsemys C. sp. Hoplochelys H. sp. Plastomenus cf. P. sp. Adocidae Indeterminate Basilemys B. sp. Squamates Peneteius P. sp. Crocodylians Dinosaurs Dineobellator D. notohesperus Ojoraptorsaurus O. boerei Caenagnathidae Indeterminate Ornithomimidae Indeterminate Richardoestesia R. sp. Troodontidae Indeterminate Tyrannosaurus cf. T. sp. Alamosaurus A. sanjuanensis Glyptodontopelta G. mimus Ankylosauridae Indeterminate Noted to be similar to Euoplocephalus and Ankylosaurus Unconformity An unconformity

176-534: The overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions . In these cases, the hiatus in deposition represented by

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192-543: The rock record. A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, then the plane of juncture is a nonconformity. An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with

208-412: The silt back-fill of an underground drain would form some time after the ground immediately above it. Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall. Superposition in archaeology requires a degree of interpretation to correctly identify chronological sequences and in this sense superposition in archaeology

224-526: The unconformity may be geologically very short – hours, days or weeks. A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems . A buttress unconformity also known as onlap unconformity, occurs when younger bedding

240-721: Was first proposed in 1669 by the Danish scientist Nicolas Steno , and is present as one of his major theses in the groundbreaking seminal work Dissertationis prodromus (1669). In the English-language literature, the law was popularized by William "Strata" Smith , who used it to produce the first geologic map of Britain. It is the first of Smith's laws , which were formally published in Strata Identified by Fossils (1816–1819). Superposition in archaeology and especially in stratification use during excavation

256-517: Was shown by James Hutton , who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region or were subsequently eroded before

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