The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.
The geology of India is diverse. Different regions of India contain rocks belonging to different geologic periods, dating as far back as the Eoarchean Era. Some of the rocks are very deformed and altered. Other deposits include recently deposited alluvium that has yet to undergo diagenesis. Mineral deposits of great variety are found in the Indian subcontinent in huge quantity. Even India's fossil record is impressive in which stromatolites, invertebrates, vertebrates and plant fossils are included. India's geographical land area can be classified into the Deccan Traps, Gondwana and Vindhyan.
The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.4 million years, from 445.2 to 443.8 Ma. The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.
Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time.
The Geology of Bangladesh is affected by the country's location, as Bangladesh is mainly a riverine country. It is the eastern two-thirds of the Ganges and Brahmaputra river delta plain stretching to the north from the Bay of Bengal. There are two small areas of slightly higher land in the north-centre and north-west composed of old alluvium called the Madhupur Tract and the Barind Tract, and steep, folded, hill ranges of older (Tertiary) rocks along the eastern border.
The Decorah Shale is a fossiliferous shale that makes up the lowermost formation in the Galena Group. The Decorah lies above the Platteville Limestone and below the Cummingsville Formation in the sedimentary sequence that formed from the shallow sea that covered central North America during Ordovician Time. The Decorah consists of three members : Spechts Ferry, Guttenberg, and Ion. The Spechts Ferry member is organic-rich and suggests a large influx of terrigenous sediment during deposition. The Guttenberg is characterized by nodular calcareous beds and contains several K-bentonite deposits. The Ion Member, present in the southern Decorah in Iowa, is characterized by alternating beds of shale and limestone.
The Ordovician Kittatinny Formation or Kittatinny Limestone is a dolomitic limestone formation in New Jersey. The Kittatinny Limestones are located primarily in the Kittatinny Valley where it lies above the Ordovician Martinsburg Formation within the long valley running from Picatinny Arsenal in Rockaway Township, southwest toward Chester Township. It overlies the Cambrian Hardyston Quartzite.
The Cincinnati Arch is a broad structural uplift between the Illinois Basin to the west, the Michigan Basin to the northwest and the Appalachian Basin and Black Warrior Basin to the east and southeast. It existed as a positive topographic area during Late Ordovician through the Devonian Period which stretched from northern Alabama northeastward to the southeastern tip of Ontario. Fossils from the Ordovician Period are commonplace in the geologic formations which make up the Cincinnati Arch, and are commonly studied along man made roadcuts. The Nashville Dome of Tennessee and the Jessamine Dome or Lexington Dome of central Kentucky make up the central portion of the Arch. In the northern part, north of Cincinnati, Ohio, the Cincinnati Arch branches to form the Findlay and Kankakee arches. The Findlay plunges under Ontario and reappears as the Algonquin Arch further north.
The Jordan Formation is a siliciclastic sedimentary rock unit identified in Illinois, Michigan, Wisconsin, Minnesota, and Iowa. Named for distinctive outcrops in the Minnesota River Valley near the town of Jordan, it extends throughout the Iowa Shelf and eastward over the Wisconsin Arch and Lincoln anticline into the Michigan Basin.
The Lexington Limestone is a prominent geologic formation that constitutes a large part of the late Ordovician bedrock of the inner Bluegrass region in Kentucky. Named after the city of Lexington, the geologic formation has heavily influenced both the surface topography and economy of the region.
The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period. It is defined as the finely sandy argillaceous or clayey near-shore/marginal-marine shale that lies above the older, non-marine Dakota sand and mud, but below the younger, chalky open-marine shale of the Greenhorn. This definition was made in Colorado by G. K. Gilbert and has been adopted in other states that use Gilbert's division of the Benton's shales into Carlile, Greenhorn, and Graneros. These states include Kansas, Texas, Oklahoma, Nebraska, and New Mexico as well as corners of Minnesota and Iowa. North Dakota, South Dakota, Wyoming, and Montana have somewhat different usages — in particular, north and west of the Black Hills, the same rock and fossil layer is named Belle Fourche Shale.
The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle of the Western Interior Seaway.
The Vinini Formation is a marine, deep-water, sedimentary deposit of Ordovician to Early Silurian age in Nevada, U.S.A. It is notable for its highly varied, mainly siliceous composition, its mineral deposits, and controversies surrounding both its depositional environment and structural history. The formation was named by Merriam and Anderson for an occurrence along Vinini Creek in the Roberts Mountains of central Nevada and that name is now used extensively in the State.
The Folkeslunda Limestone is a thin limestone and mudstone geologic formation of Sweden. The formation crops out on the island of Öland to the east of Kalmar, where Folkeslunda is located. Other exposures of the formation are in Dalarna, Jämtland and Östergötland. The Folkeslunda Limestone was deposited in an open marine environment with an estimated water depth of 150 to 200 metres in a eustatically transgressive phase.
The Sinnipee Group is a geological group in Wisconsin. It consists primarily of sedimentary carbonate rocks. Primarily made of dolomite, it also has limestone as a secondary component and can even have shale imbedded with it. It was formed in the Ordovician period and has three rock members: Galena, Decorah, and Platteville formations.
The geology of Missouri includes deep Precambrian basement rocks formed within the last two billion years and overlain by thick sequences of marine sedimentary rocks, interspersed with igneous rocks by periods of volcanic activity. Missouri is a leading producer of lead from minerals formed in Paleozoic dolomite.
The geology of Thailand includes deep crystalline metamorphic basement rocks, overlain by extensive sandstone, limestone, turbidites and some volcanic rocks. The region experienced complicated tectonics during the Paleozoic, long-running shallow water conditions and then renewed uplift and erosion in the past several million years ago.
The Bertie Group or Bertie Limestone, also referred to as the Bertie Dolomite and the Bertie Formation, is an upper Silurian geologic group and Lagerstätte in southern Ontario, Canada, and western New York State, United States. Details of the type locality and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database. The formation comprises dolomites, limestones and shales and reaches a thickness of 495 feet (151 m) in the subsurface, while in outcrop the group can be 60 feet (18 m) thick.
![Douglas Lake Member Geologic formation in [[Tennessee]], United States](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Casterlorum_crispum_slab.jpg/313px-Casterlorum_crispum_slab.jpg)
The Douglas Lake Member is a geologic unit of member rank of the Lenoir Limestone that overlies the Mascot Dolomite and underlies typical nodular member of the Lenoir Limestone in Douglas Lake, Tennessee, region. It fills depressions that are part of a regional unconformity at the base of Middle Ordovician strata, locally the Lenoir Limestone, that separates them from the underlying Lower Ordovician strata, locally the Knox Group.
The Edinburg Formation is an Ordovician-age geological formation in Virginia. It is primarily composed of basinal limestone and shale, and is one of the younger units in the "Middle Ordovician" sequence of the Shenandoah Valley. However, fossils have shown that it actually was deposited in the early part of the Late Ordovician. There are two major facies encompassed by the Edinburg Formation. The more abundant Liberty Hall facies consist of evenly bedded black limestone and shale. In a few areas, the Liberty Hall facies intertongue with the Lantz Mill facies. The Lantz Mill facies are grainy or cobbly wackestone which weathers to a buff brown color. Fossils are diverse, including graptolites, brachiopods, and trilobites.
