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Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
Conodonts are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which are usually found in isolation and are now called conodont elements. Knowledge about soft tissues remains limited. They existed in the world's oceans for over 300 million years, from the Cambrian to the beginning of the Jurassic. Conodont elements are widely used as index fossils, fossils used to define and identify geological periods. The animals are also called Conodontophora to avoid ambiguity.
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.
Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.
The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. Most of the evidence for this event is in Nevada but the limits of its reach are unknown. A great volume of conglomeratic deposits of mainly Mississippian age in Nevada and adjacent areas testifies to the existence of an important tectonic event, and implies nearby areas of uplift and erosion, but the nature and cause of that event are uncertain and in dispute. Although it is known as an orogeny, some of the classic features of orogeny as commonly defined such as metamorphism, and granitic intrusives have not been linked to it. In spite of this, the event is universally designated as an orogeny and that practice is continued here. This article outlines what is known and unknown about the Antler orogeny and describes three current theories regarding its nature and origin.
In the geosciences, paleosol is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science.
A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic are characterized by aragonite seas.
Wills Creek Formation is a mapped Silurian bedrock unit in Pennsylvania, Maryland, Virginia, and West Virginia.
Paleontology or palaeontology is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. This includes the study of body fossils, tracks (ichnites), burrows, cast-off parts, fossilised feces (coprolites), palynomorphs and chemical residues. Because humans have encountered fossils for millennia, paleontology has a long history both before and after becoming formalized as a science. This article records significant discoveries and events related to paleontology that occurred or were published in the year 1926.
The Marcellus Formation or the Marcellus Shale is a Middle Devonian age unit of sedimentary rock found in eastern North America. Named for a distinctive outcrop near the village of Marcellus, New York, in the United States, it extends throughout much of the Appalachian Basin.
The Phosphoria Formation of the western United States is a geological formation of Early Permian age. It represents some 15 million years of sedimentation, reaches a thickness of 420 metres (1,380 ft) and covers an area of 350,000 square kilometres (140,000 sq mi).
The Foraminiferal Colouration Index (FCI) is a tool for assessing the thermal alteration of organic matter buried in sedimentary rock. It uses temperature-controlled colour changes in the organic cement of agglutinated foraminifera (microfossils) to estimate thermal alteration. The method is empirical and based on determination of colour by visual comparison of fossil specimens to the Geological Society of America Rock-Color Chart under a binocular microscope.
The Fayetteville Shale is a geologic formation of Mississippian age composed of tight shale within the Arkoma Basin of Arkansas and Oklahoma. It is named for the city of Fayetteville, Arkansas, and requires hydraulic fracturing to release the natural gas contained within.
The Shady Dolomite is a geologic formation composed of marine sedimentary rocks of early Cambrian age. It outcrops along the eastern margin of the Blue Ridge province in the southeastern United States and can be found in outcrops in the states of Alabama, Georgia, Tennessee, North Carolina, and Virginia. It can also be found in the subsurface of Kentucky, Ohio, and West Virginia. The Shady is predominantly composed of dolomite and limestone with lesser amounts of mudrock. It contains fossils of trilobites, archaeocyathids, algae, brachiopods, and echinoderms, along with the enigmatic fossil Salterella. The Shady Dolomite was first described by Arthur Keith in 1903 and was named for exposures in the Shady Valley of Johnson County in the state of Tennessee. Near Austinville, Virginia, the Shady hosts ore deposits that have been mined extensively for lead and zinc ore.
The Kettle Point Formation, also known as the Kettle Point (black) Shale, is a geologic formation that consists of thinly laminated, siliciclastic, organic-rich black shale with thin to thick interbeds of organic-poor mudstone. It is largely restricted to the subsurface of southwestern Ontario.
Ozarkodina is an extinct genus of conodonts in the family Spathognathodontidae.
The geology of Morocco formed beginning up to two billion years ago, in the Paleoproterozoic and potentially even earlier. It was affected by the Pan-African orogeny, although the later Hercynian orogeny produced fewer changes and left the Maseta Domain, a large area of remnant Paleozoic massifs. During the Paleozoic, extensive sedimentary deposits preserved marine fossils. Throughout the Mesozoic, the rifting apart of Pangaea to form the Atlantic Ocean created basins and fault blocks, which were blanketed in terrestrial and marine sediments—particularly as a major marine transgression flooded much of the region. In the Cenozoic, a microcontinent covered in sedimentary rocks from the Triassic and Cretaceous collided with northern Morocco, forming the Rif region. Morocco has extensive phosphate and salt reserves, as well as resources such as lead, zinc, copper and silver.
Orbitolina is an extinct genus of foraminifera belonging to the order Textulariida and family Orbitolinidae. Fossils of this genus are widely found in beds deposited in the Tethys Ocean ranging from Aptian ) to Cenomanian in age. It has been used as a shallow carbonate platform facies marker and as a Cretaceous index fossil.
References
- ↑ Epstein, Anita G.; Epstein, Jack Burton; Harris, Leonard Dorreen (1977). "Conodont color alteration - an index to organic metamorphism". USGS Professional Paper. Professional Paper. 995: 1–27. doi: 10.3133/pp995 .
- Epstein, A., Epstein, J., Harris, L. (1977). "Conodont Color Alteration - an Index to Organic Metamorphism." United States Geological Survey Professional Paper 995, 1-27.
- Königshof, P. (2003). "Conodont deformation patterns and textural alteration in Paleozoic conodonts: examples from Germany and France". Senckenbergiana Lethaea . 83 (1/2): 149–156. doi:10.1007/BF03043310. S2CID 134743916. (Image)
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