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Chuska Sandstone

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The Oligocene Chuska Sandstone is a geologic formation that crops out in the Chuska Mountains of northeastern Arizona and northwestern New Mexico . The formation is a remnant of a great sand sea, or erg , that once covered an area of 140,000 square kilometres (54,000 sq mi) reaching from the present locations of the Chuska Mountains to near Albuquerque and to the southwest. This erg deposited a succession of sandstone beds exceeded in thickness only by the Navajo Sandstone on the Colorado Plateau.

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32-414: The Chuska Sandstone is up to 535 meters (1,755 ft) thick and is divided into two members . The Deza Member, to which the lowermost beds of the formation are assigned, is up to 81 meters (266 ft) thick and consists mostly of pale orange to yellow-gray sandstone (66%), claystone (16%) and sandy siltstone (16%). Sedimentary structures are present that indicate deposition by running water, and

64-482: A rock unit is a description of its physical characteristics visible at outcrop , in hand or core samples , or with low magnification microscopy. Physical characteristics include colour, texture, grain size , and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone , slate , basalt , or limestone . Lithology

96-409: A body of water or beneath ice. Unconsolidated surficial materials may also be given a lithology. This is defined by grain size and composition and is often attached to an interpretation of how the unit formed. Surficial lithologies can be given to lacustrine , coastal, fluvial , aeolian , glacial , and recent volcanic deposits, among others. Examples of surficial lithology classifications used by

128-411: A continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist, such as greenschist (metamorphosed basalt and other extrusive igneous rock) or quartzite (metamorphosed quartz sand). In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in

160-484: 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

192-408: A hand lens, the visible mineralogy is included as part of the description. In the case of sequences possibly including carbonates , calcite - cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate ) using dilute hydrochloric acid and looking for effervescence . The mineralogical composition of a rock is one of

224-461: A particular depositional environment and may provide information on paleocurrent directions. In metamorphic rocks associated with the deeper levels of fault zones , small scale structures such as asymmetric boudins and microfolds are used to determine the sense of displacement across the zone. In igneous rocks, small-scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻā basaltic flows, and pillows showing eruption within

256-569: A rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting , grading , shape and roundness of the clasts. Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation—before deformation porphyroclast —after deformation porphyroblast . Igneous textures include such properties as grain shape, which varies from crystals with ideal crystal shapes ( euhedral ) to irregular crystals (anhedral), whether

288-636: A sample. The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system . The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks

320-694: Is crossbedded and arkosic (it contains significant feldspar ). The formation overlies Mesozoic formations across an angular unconformity . The youngest of these is the Tohatchi Formation of the Mesaverde Group . The Chuska Sandstone is in turn overlain by lava flows of the Navajo Volcanic Field . Radiometric dating yields ages of 34.75 ± 0.20 million years for the Deza Member and 33.31 ± 0.25 million years for

352-639: Is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified . Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure. Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by volcanic processes ( tephra ) are classified as pyroclastic rock . Pyroclastic rocks are further classified by average fragment ( clast ) size and whether

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384-564: 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. Lithology The lithology of

416-563: 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

448-540: Is impractical, they may be classified chemically using the TAS classification . This is based on the total content of silica and alkali metal oxides and other chemical criteria. Sedimentary rocks are further classified by whether they are siliciclastic or carbonate . Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments. Carbonate rocks are classified with

480-557: Is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations , lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7 . The naming of a lithology is based on the rock type . The three major rock types are igneous , sedimentary , and metamorphic . Igneous rocks are formed directly from magma , which

512-477: Is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate , sandstone , or mudstone ). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are " pebble conglomerate" and "fine quartz arenite ". In rocks in which mineral grains are large enough to be identified using

544-475: The Dunham or Folk classification schemes according to the constituents of the carbonate rock. Metamorphic rock naming can be based on protolith , mineral composition, texture, or metamorphic facies . Naming based on texture and a pelite (e.g., shale , mudrock ) protolith can be used to define slate and phyllite . Texture-based names are schist and gneiss . These textures, from slate to gneiss, define

576-464: The QAPF classification , which is based on the relative content of quartz , alkali feldspar , plagioclase , and feldspathoid . Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or carbonatites . Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition

608-553: The Chuska, assigning beds previously included in the lowermost Chuska to his Deza Formation. Repenning, Lance, and Irwin reported in 1958 that the contact between the Deza Formation and Chuska Sandstone was gradational and difficult to map, and concluded that the Deza Formation should be abandoned and its beds included in the Chuska, a conclusion shared by later researchers. However, Spencer G. Lucas and Steven M. Cather assigned

640-527: The Deza Member was deposited in an environment with permanent water bodies. The formation has been interpreted as an erosional remnant of a large erg (sand sea). Other remnants include sandstone units of the Spears Group on the north flank of the Mogollon-Datil volcanic field and a sandstone unit found only in the deep subsurface in the northwest Albuquerque Basin . Conservative estimates of

672-534: The Deza beds to the Deza Member of the Chuska Sandstone. Member (stratigraphy) 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

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704-479: 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,

736-437: 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

768-440: The fragments are mostly individual mineral crystals , particles of volcanic glass , or rock fragments. Further classifications, such as by chemical composition , may also be applied. Igneous rocks that have visible mineral grains ( phaneritic rocks) are classified as intrusive , while those that are glassy or very fine-grained ( aphanitic ) are classified as extrusive rock . Intrusive igneous rocks are usually classified using

800-458: The lower Narbona Pass Member, and overlying volcanic rock has been dated at 25.05 ± 0.16 million years in age. The only fossils found in the Chuska Sandstone are shell fragments from emydid turtles found in the Deza Member. These cannot be more precisely identified, and so are of little value for dating the Deza Member. However, emydid turtles are almost exclusively aquatic, so the presence of these shell fragments provides supporting evidence that

832-415: 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

864-454: The main visible fabric is normally bedding , and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism ), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, forming cumulates . The texture of

896-487: The major ways in which it is classified. Igneous rocks are classified by their mineral content whenever practical, using the QAPF classification or special ultramafic or carbonatite classifications. Likewise metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in

928-423: The member fills shallow paleovalleys eroded in the underlying Mesozoic beds. The Deza Member is not always present, and when present, it grades into the overlying Narbona Pass Member without a sharp contact. The Narbona Pass Member makes up most of the Chuska Sandstone, and consists of as much as 535 meters (1,755 ft) of wind-deposited ( eolian ) sandstone. The sandstone is pinkish-gray to yellowish-gray and

960-547: The original extent of the erg suggest it once covered an area of 140,000 square kilometres (54,000 sq mi). The erg was almost entirely eroded away during the early Miocene . The formation was named by Herbert E. Gregory in 1917 for exposures in the Chuska Mountains . H.E. Wright restricted the formation to the caprock of the Chuska Mountains, and adjusted the definition of the lower boundary of

992-401: The rock shows highly nonuniform crystal sizes (is porphyritic ), or whether grains are aligned (which is described as trachytic texture). Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks this may include sole markings , ripple marks , mudcracks and cross-bedding . These are recorded as they are generally characteristic of

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1024-478: The rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive. Metamorphism of rock composed of mostly a single mineral, such as quartzite or marble , may increase grain size ( grain growth ), while metamorphism of sheared rock may decrease grain size (syntectonic recrystallization ). In clastic sedimentary rocks, grain size

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