The Ermaying Formation is a geological formation of Anisian ( Middle Triassic ) age in north-central China. It is found across much of the Ordos Basin , at outcrops within the provinces of Shaanxi , Shanxi , and Inner Mongolia . It is composed of up to 600 m thick sequence of mudstone and sandstone , overlying the Heshanggou Formation and underlying the Tongchuan Formation . In the southern part of the Ordos Basin, the Zhifang Formation is equivalent to the Ermaying Formation.
19-477: The Ermaying Formation is divided into two members , each with a distinctive assemblage of tetrapod fossils. A 2013 study used SHRIMP U-Pb radiometric dating to assign an imprecise age of 245.9 ± 3.2 Ma for the upper member. A 2018 study assigned a more precise age of around 243.53 Ma based on three ash samples near the base of the upper member. This would indicate that the Upper Ermaying Formation
38-484: A formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of
57-662: Is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Marker horizon Marker horizons (also referred to as chronohorizons , key beds or marker beds ) are stratigraphic units of
76-550: Is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup
95-796: Is no older than the late Anisian stage. A few studies apply the name "Ermaying Formation" to a sedimentary unit in the Yanshan belt , a fold-thrust belt northeast of Beijing . In the Yanshan belt, reported exposures of the formation are dated to the Late Triassic, lying below the Early Jurassic Xingshikou Formation . The Yanshan belt exposures are also known as the Huzhangzi Formation , an alternative name proposed to reflect their chronological and geographic divergence from exposures in
114-543: Is the third "kannemeyeriid" genus known from the Upper Member, as it is distinct from both Parakannemeyeria and Sinokannemeyeria . Member (geology) A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic , lithologic or paleontologic features ( facies ) that characterize it. Units must be mappable and distinct from one another, but
133-517: The Chicxulub crater , produced an iridium anomaly that occurs in a thin, global layer of clay marking the Cretaceous–Paleogene boundary . Iridium layers are associated with bolide impacts and are not unique, but when occurring in conjunction with the extinction of specialised tropical planktic foraminifera and the appearance of the first Danian species, signal a reliable marker horizon for
152-587: The Cretaceous–Paleogene boundary. Fossil faunal and floral assemblages , both marine and terrestrial, make for distinctive marker horizons. Some marker units are distinctive by virtue of their magnetic qualities. The Water Tower Slates, forming part of the Hospital Hill Series in the Witwatersrand Basin , include a fine-grained ferruginous quartzite which is particularly magnetic. From
171-647: The Ordos Basin. The Ermaying Formation is notable for its diversity of well-preserved tetrapods. The upper member occupies most of a biozone historically known as the Sinokannemeyeria fauna . This fossil assemblage has more recently been termed the Sinokannemeyeria - Shansisuchus Assemblage Zone , including approximately coeval sediments of the Kelamayi Formation in Xinjiang . The biozone also extends up to
190-479: The contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank,
209-487: The early part of the Tongchuan Formation. Tetrapod burrows are known from the formation, occupying both large and small size classes. The larger burrow is reniform (kidney-shaped) in cross section, about 13 centimetres (5.1 in) in height and 30 centimetres (12 in) in width. It shallowly slopes down when seen from the side and smoothly undulates when seen from above. Scatches and grooves are readily visible on
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#1732855882764228-501: The form of peat beds, coal beds, shell beds, marine bands , black shales in cyclothems , and oil shales . A well-known example of a key bed is the global layer of iridium -rich impact ejecta that marks the Cretaceous–Paleogene boundary (K–T boundary). Palynology , the study of fossil pollens and spores, routinely works out the stratigraphy of rocks by comparing pollen and spore assemblages with those of well-known layers—a tool frequently used by petroleum exploration companies in
247-437: The formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group
266-483: The history of the earth. They usually consist of a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area. As a result, a key bed is useful for correlating sequences of sedimentary rocks over a large area. Typically, key beds were created as the result of either instantaneous events or (geologically speaking) very short episodes of
285-573: The inside. The burrow-maker was medium-sized animal, likely a juvenile dicynodont. The smaller burrows are low tapered chambers with incomplete or collapsed entry ramps. They may have been dug out by procolophonids or juvenile cynodonts. Apart from the taxa listed here, fossils of an unnamed genus of kannemeyeriiform dicynodont have been found in the Upper Ermaying Formation at the Sanjiao site in Shanxi Province. Not counting Shansiodon , this unnamed form
304-415: The main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for the unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to
323-431: The same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no doubt about their being of equivalent age ( isochronous ) and of common origin. Such clear markers facilitate the correlation of strata , and used in conjunction with fossil floral and faunal assemblages and paleomagnetism , permit the mapping of land masses and bodies of water throughout
342-464: The search for new fields. The fossilised teeth or elements of conodonts are an equally useful tool. The ejecta from volcanoes and bolide impacts create useful markers, as different volcanic eruptions and impacts produce beds with distinctive compositions. Marker horizons of tephra are used as a dating tool in archaeology , since the dates of eruptions are generally well-established. One particular bolide impact 66 million years ago, which formed
361-456: The widespread deposition of a specific types of sediment . As the result, key beds often can be used for both mapping and correlating sedimentary rocks and dating them. Volcanic ash beds ( tonsteins and bentonite beds) and impact spherule beds, and specific mega turbidites are types of key beds created by instantaneous events. The widespread accumulation of distinctive sediments over a geologically short period of time have created key beds in
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