PERMIAN PERIOD


Meaning of PERMIAN PERIOD in English

Table 4: Geologic time scale. To see more information about a period, select one from the chart. last period of the Paleozoic Era. It began about 286 million years ago and ended 245 million years ago, extending from the close of the Carboniferous to the outset of the Triassic (see Table). The Permian is often divided into the Early Permian Epoch (286 to 258 million years ago) and the Late Permian Epoch (258 to 245 million years ago). The changes in the general distribution and types of sediments and fossils during these 41 million years suggest that the Permian was a time of progressive climatic shifts that resulted in major environmental challenges to marine and terrestrial life. The rocks that formed during this period make up the Permian System. interval of geologic time from 286 to 245 million years ago. The Permian is the last of the six periods of the Paleozoic Era; it follows the Carboniferous and precedes the Triassic of the subsequent Mesozoic Era. The Permian is often divided into the Early Permian Epoch (286 to 258 million years ago) and the Late Permian Epoch (258 to 245 million years ago). The rocks that originated during the period make up the Permian System. The Permian was named in 1841 by the early British geologist Sir Roderick I. Murchison for the Perm region of the Russian Urals, whose rocks became a standard reference for the Permian System. The rocks of the system are characterized by an abundance of red land-laid sediments and evaporites (salts formed by a sequence of chemical precipitation in an acid environment). Massive movements of crustal plates continued from the preceding Carboniferous Period into the Permian. The northern hemispheric continent of Laurasia (composed essentially of present-day North America, Greenland, Europe, and Asia) became linked to the southern hemispheric landmass of Gondwana (composed of what is now South America, Africa, India, Australia, and Antarctica), forming one single supercontinent by the mid-Permian. This immense assembly of landmasses, known as Pangaea, stretched across all the climatic temperature zones virtually uninterrupted from pole to pole. The extensive glaciation that had begun in the Carboniferous continued into the Permian, mainly in the southern portion of Gondwana that was located around the south paleopole. Hot, dry conditions prevailed almost everywhere else on Pangaea, and, by the Late Permian, deserts had become widespread in several tropical and subtropical areas on the supercontinent. Laurasia and Gondwana remained separated at their eastern extent by the Tethys Sea, and the rest of the Earth's surface was covered by a vast ocean known as Panthalassa. Orogenies (mountain-building episodes) associated with the formation of Pangaea by the collision of these continents continued into the Permian, further forming the Appalachian mountain chain of North America and the Hercynian mountain chains of Europe and Asia. In eastern Europe the stable Russian Platform in the east flanked a deep-sea trough, the Ural Geosyncline, which was compressed, folded, and uplifted to form the Ural Mountains when the Angaran segment of western Siberia collided with eastern Laurasia. In western Europe continental and restricted basin deposits, such as red sandstone, conglomerate, shale, dolomitic limestone, and evaporites, predominated. Permian deposits are widespread in North America, particularly in Texas, New Mexico, Nevada, and Utah, where marine and continental deposits accumulated in great thicknesses. North American Permian deposits are principally sandstones, shales, limestones, and evaporites. Deposits of limestone, sandstone, shale, cherts, and basalt pillow lavas were also formed in the shallow and deepwater environments of the Tethys Sea. The Permian deposits on the landmasses that made up Gondwana exhibit similar characteristics; they contain tillites (consolidated sedimentary rocks formed by the lithification of glacial till) and sediments deposited by glacial meltwater, as well as sandstones, shales, and coal. This rock sequence indicates that during the Permian the world climate became milder through alternating warm and cool stages. Late Permian deposits in Pangaea are characterized by dune sands, evaporites, and red beds (e.g., shale and sandstone of reddish colour), indicating the onset of desert conditions at the close of the period. Economically important mineral deposits of Permian age consist of evaporite minerals (such as halite, sylvite, and gypsum), coal, petroleum, and copper ore. Significant evaporite deposits of rock salt and potash minerals are found in the southwestern United States, Russia, Poland, The Netherlands, Denmark, and England. Important Permian coals are found in northern Siberia, China, Korea, India, Australia, and southern Africa. Permian rocks also hold significant oil and natural-gas deposits in the southwestern United States (western Texas, Oklahoma, and New Mexico) and in Russia. The fossil record indicates that life evolved from the preceding Carboniferous into the Permian as a continuation of established faunas. The marine invertebrates, mainly brachiopods, ammonoids, bryozoans, bivalves, and foraminiferans, evolved into several lineages during the period. The rapid evolutionary rate of the ammonoids and fusulinid foraminiferans make them extremely useful for geologic dating in the Permian. Marine and freshwater fish lineages and the amphibians thrived in the Permian. Of the vertebrates, the reptiles evolved into three markedly distinct groups: the cotylosaurs, the pelycosaurs, and the therapsids (mammallike reptiles). Land plants evolved from the Carboniferous ferns and seed ferns to the conifers and adapted to drier and well-drained land conditions. An abrupt change occurred in Gondwana with the widespread development of the Glossopteris flora, which were tongue ferns adapted to the colder climates of the Southern Hemisphere. Toward the close of the Permian, however, many forms of life suffered mass extinction. In Gondwana the Glossopteris flora disappeared, but the coniferous flora of the Northern Hemisphere survived into the Triassic. Many animal groups did not survive the Permian. Trilobites and fusulinids became extinct, and the brachiopods, ammonoids, and bryozoans were severely reduced. Uncertainty still exists over the events that led to these extinctions at the close of the period. Additional reading Specific references include Charles A. Ross and June R.P. Ross, "Permian," in William A. Berggren, Treatise on Invertebrate Paleontology, part A, Introduction-Fossilization (Taphonomy), Biogeography, and Biostratigraphy (1979), pp. 291-350, and "Late Paleozoic Sea Levels and Depositional Sequences," in Charles A. Ross and Drew Haman (eds.), Timing and Depositional History of Eustatic Sequences: Constraints on Seismic Stratigraphy (1987), pp. 137-149; Charles A. Ross, "Paleozoic Evolution of Southern Margin of Permian Basin," Geological Society of America Bulletin, 97(5):536-554 (1986); L.L. Sloss (ed.), Sedimentary Cover, North American Craton, U.S. (1988); and Garry D. McKenzie (ed.), Gondwana Six: Stratigraphy, Sedimentology, and Paleontology (1987). June R.P. Ross Charles A. Ross Permian environment Paleogeography Figure 2: Paleogeography and paleoceanography of (top) the Early Permian and (bottom) the early 1/4 The Permian constitutes an important crossroads both in the history of the Earth's continents and in the evolution of terrestrial life. During roughly the first half of the period, Gondwana collided with and joined western Euramerica, to which the Angaran sector of Siberia was subsequently fused (see Figure 2). Thus, the assembly of what is often referred to as Greater Pangaea was completed by mid-Permian time, giving rise to a single mountainous continental landmass that extended across all the climatic temperature zones without interruption virtually from one pole to the other. This megacontinent was surrounded by the immense world ocean Panthalassa, which, with the Tethys Sea, was the site of a small number of microcontinents, island arcs, oceanic plateaus, and trenches. Paleoclimate Extensive glaciation persisted in the Early Permian, largely in what is now India, Australia, and Antarctica but also in Siberia near the north paleopole. Hot, dry conditions prevailed elsewhere on Pangaea, and deserts became widespread in various tropical and subtropical areas of the continent by the Late Permian. The gradual climatic warming that took place during the Permian at first encouraged evolutionary expansion among shallow-water marine faunas but later resulted in marked extinctions (see below Permian life). On the other hand, this warming trend, combined with climate diversity, provided an opportunity for broad adaptive radiation in terrestrial plants, insects, and reptiles, particularly among mammallike reptiles. Permian life Life during the Permian was very diverse, and the marine life of the period was perhaps more diverse than that of modern times. The previous period, the Carboniferous, had had two instances of significant marine faunal extinctions that had followed one another in relatively rapid succession. One was at the end of the Early Carboniferous (Mississippian subperiod in North America) and the other at the end of the Middle Carboniferous (the close of the Middle Pennsylvanian). Both may be attributed to global cooling during continental glaciations. The latest Carboniferous witnessed the establishment of new or highly modified marine lineages with relatively little ecological competition from the remnants of what had been highly successful earlier phylogenetic lineages. Most of these lineages are identified as new families or suborders among the foraminifers, ammonoids, brachiopods, bryozoans, bivalves, and some less-studied groups. In Wolfcampian, Leonardian, and Guadalupian times, these newly established lineages underwent rapid evolution and filled a remarkable number of specialized marine ecological niches. Some of this rapid diversification was probably the result of filling the ecological niches left vacant by the extinctions of the Carboniferous. Other factors undoubtedly included the gradual warming of oceans during the latest Carboniferous and the more rapid warming trends of the Early Permian. In paleotropical areas, a succession of carbonate bank, bioherm, and reef faunal associations evolved, which culminated in the late Guadalupian Capitan Reefs of the western United States and the more faunally diverse associations that formed even thicker fringing reefs in the Tethyan area. Because these two sets of tropical marine shallow-water faunal associations were separated by the large supercontinent Pangaea on the one hand and the deep oceanic basin of Panthalassa on the other, they tended to evolve independently of each other. Extinctions Near the end of the Guadalupian both of the these tropical faunal realms suffered major but incomplete extinctions, and the Djulfian saw a brief and relatively minor evolutionary re-expansion in some of the foraminifers and ammonoids. The trilobites were extinct by the end of the Leonardian. Only a few Permian bryozoan and brachiopod genera, and only one or two species of those genera, survived into the earliest Triassic. Although the magnitude of the extinctions among shallow-water marine organisms during the later parts of the Permian was great, the process took several million years and was accomplished in a series of steps followed by unsuccessful attempts by the surviving faunas to rediversify. Terrestrial life in the Permian was closely keyed to the evolution of terrestrial plants, which of course were the primary food source for terrestrial animals. The fossil plant record for the Permian consists predominantly of ferns, seed ferns, and lycophytes, which is attributable to their adaptation to marshes and swampy environments. A less abundant fossil record of early coniferophytes and even some protoangiosperms suggests a broad adaptation of these plant groups to progressively drier areas. As discussed earlier, evidence seems to point to gradually warming and drier climates, which would have encouraged plant adaptations to drier conditions. Another line of evidence suggesting broad plant diversification is found in the evolution of insects; these animals tend to be highly selective in choosing their plant hosts. Among the superclass Hexapoda of the phylum Arthropoda, at least 23 orders are known from the Permian, and of these orders 11 are extinct. By comparison, 250 million years later, there exist only 28 insect orders, and the new orders are mainly those that have adapted to living on the angiosperms or mammals that evolved after Permian time. Permian insects include a huge dragonfly-like creature that had a wingspan of 75 centimetres. Permian rocks Occurrence. Figure 2: Paleogeography and paleoceanography of (top) the Early Permian and (bottom) the early 1/4 Figure 2: Paleogeography and paleoceanography of (top) the Early Permian and (bottom) the early 1/4 Permian rocks are found on all present-day continents; however, some have been displaced considerable distances, sometimes thousands of kilometres from their original site of deposition (Figure 2) by subsequent tectonic transport during the Mesozoic and Cenozoic eras. For example, Permian glacial and glacial marine deposits typical of the cold high latitudes of the Southern Hemisphere during the Permian are now found in Antarctica, southern Africa, India, Thailand, and Tibet, while Permian glacial deposits of the Northern Hemisphere are found in northeastern Siberia. By contrast, some Permian tropical and subtropical carbonate deposits, typical of deposition in low latitudes, have been relocated to high latitudes, and the present location of certain tropical provincial faunas suggests that other deposits have been moved considerable distances longitudinally to form tectonic belts of accretionary material added during Mesozoic and Cenozoic times to the former late Paleozoic continental margins. The commonly conflicting occurrences of different faunal provinces and tropical deposits in juxtaposition with temperate and cold (glacial) deposits became more readily explainable once the rates of motion that result from seafloor spreading and plate tectonics became established as 0.5 to 1.5 centimetres per year. When viewed on a Permian paleogeographic reconstruction (Figure 2), these apparent depositional conflicts disappear and a climatically compatible depositional pattern emerges. The principal geographic features of the Permian world were the supercontinent Pangaea (which included all the then-existing major continents except North and South China) and a huge ocean basin called Panthalassa, with its branch, the Tethys (a large indentation in the tropical eastern side of Pangaea). Panthalassa and Tethys encompassed scattered fragments of continental crust (microcontinents) and basaltic volcanic island arcs that featured extensive fringing limestone reefs and platforms. They are generally viewed as being analogous to the present-day Indian and Pacific ocean basins in terms of geologic construction. Cathyasia, comprising both the continents of North and South China, lay within western Panthalassa. The existence of several other, smaller (or now disrupted) microcontinents also has been proposed. Pangaea contained extensive high mountain ranges along its orogenic suture between Euramerica (also known as Laurussia in Devonian and Carboniferous times) and the South American-Northwest African portion of Gondwana. These mountains influenced local climates and sedimentation during the early part of the Permian to a considerable extent. Later, during the middle Permian, the Angaran portion of western Siberia joined eastern Euramerica to form the Ural orogenic belt and mountains. Parts of Pangaea continued to be sheared and deformed by large linear zones of tear faults (steeply inclined faults along which movement has been largely horizontal and which include several failed rift structures) and by the vertical warping and faulting of the persistent North American transcontinental arch and similar tectonic activity in the northwestern segment of Europe. Figure 1: The marine series and stage names of the Permian, the correlation of regional reference 1/4 Because these orogenic and related tectonic events progressively changed sedimentation conditions during the Permian Period, it is possible to consider shifts in depositional patterns in several phases. The Asselian and Sakmarian represent intervals of gradual transition from Late Carboniferous depositional facies typified by large sea-level fluctuations and strongly developed cyclical sediments (Figure 1). Less rapid and pronounced extremes in sea-level changes and more uniform sedimentation characterized the Artinskian. The Kungurian and later Permian deposition reflect increasing exposure and aridity on the cratonic shelves and well-developed intracratonic basins with more extensive evaporites. Marine deposits were primarily along the shelf margins. The latest phase, seen in Djulfian sedimentation, is not widely distributed and probably was restricted to the cratonic shelf margins or even to the upper shelf slopes. Distribution and types In terms of geologic setting, the Permian sediments that were deposited as thick sedimentary wedges along the tectonically active margins of the major cratons are the least understood in detail. Because there was one megacontinent, Pangaea, most of these wedges were facing into the huge active ocean basin of Panthalassa-Tethys. All these Permian sediments have subsequently been thrusted, metamorphosed, and involved in major geologic deformation. In many cases, it is difficult to determine the original thickness of these wedges, and much of the fossil evidence is from clastic material that was derived from shallow shelf environments or eroded from older rocks and deposited as deepwater debris fans. Great deposits-perhaps originally one to three kilometres thick-are known in central Nevada, Idaho, and northward into Canada. Similar deposits have been found in the Middle East, China, Japan, and eastern Siberia. Interleaved with these thick clastic wedges are other thrust slices of ocean-floor deposits. These are thinner, about 0.5 kilometre thick or less, and are characterized by radiolarian-rich cherts, basaltic volcanic dikes, sills, and submarine lava flows, as well as silts and clays (commonly metamorphosed to slates) of the distal ends of turbidity flows (i.e., those of density currents). Such completely deformed deposits are the only remaining record of Permian (and older) ocean-floor deposits, because only Jurassic and younger oceanic sediments survive in the present-day ocean basins. Most of the Permian (and older) ocean-floor deposits and thick sedimentary wedges have been caught up in subduction zones along the plate boundaries and lost to the Earth's mantle. Associated with some oceanic basalts are thick accumulations of reef limestone that formed on seamounts and volcanic island arcs. These reefs were tropical and subtropical, as are modern oceanic limestone reefs. Because limestone is comparatively less dense than adjacent oceanic rocks, such as basalt or chert, many of the Permian reef limestones were not as easily subducted and are present in many modern mountain belts. Limestones of this kind are known from Tunisia, the Balkan Peninsula, Turkey, the Crimea (in Ukraine), the Middle East, northern India, Pakistan, Southeast Asia, New Zealand, China, Japan, eastern Siberia, Alaska, and the western Cordillera of Canada, the United States, and northwesternmost Mexico. These limestones commonly have complex structural relationships with adjacent rocks, usually of Triassic, Jurassic, or early Cretaceous age, suggesting that most such Permian limestones were incorporated into the mountain belts by those times. Cratonic shelf sedimentation in low paleolatitudes during the Permian was characterized by the gradual withdrawal of shorelines and the progressive increase in eolian sands, red beds, and evaporites. Many intracratonic basins, such as the Anadarko, Delaware, and Midland basins in the western United States, the Zechstein basin of northwestern Europe, and the Kazan Basin of eastern Europe, show similar general changes. In most cases, the inner parts of these basin systems became sites of red bed deposition during the Leonardian, followed within a short time by locally extensive evaporites. Sand sources along the Ancestral Rocky Mountains supplied eolian sand and silt in great quantities. The outer portions of the intracratonic basin systems, as in the Delaware and Zechstein basins, were involved in some transform faulting and extensional tectonics and locally developed considerable depositional relief. Although some of this relief was from rotated fault blocks, most of it resulted from the very rapid growth of limestone reefs on upthrown blocks (i.e., the sides of faults that appear to have moved upward) and the slower accumulation of clastic sediments on downthrown blocks. Striking examples of these reefs form the Guadalupe Mountains of West Texas and New Mexico. Such reefs also occur in the subsurface along the Central Basin Platform in West Texas, where they produce petroleum. Similar reefs are known from northern England, Germany, and the subsurface of the North Sea. Lower Permian limestone reefs also are known from the western and southern Urals of eastern Europe. At higher paleolatitudes, limestone is rare, and clastic rocks dominate the succession. Australia, Namibia, South Africa, peninsular India, southern Tibet, and southern Thailand all report Permo-Carboniferous tillite. These areas, as their paleogeographic reconstruction indicates, would have been in relatively high latitudes as parts of Gondwana during the Permian. Tillites are also known from the northern high paleolatitudes in northeastern Siberia. Some of the Gondwanan areas were tectonically very active during Permian time, as evidenced by extensive basaltic, andesitic, and other volcanic rocks in eastern Australia and by the development of intracratonic sedimentary marine basins, such as the Carnarvon Basin in Western Australia where nearly five kilometres of Permian sediments accumulated. Continental rocks were widespread on all the cratons during the Permian. The Dunkard Group is a limnic coal-bearing succession that was deposited from latest Carboniferous into Early Permian time along the western side of the then-newly formed Appalachian Mountains. In what is now Kansas and areas of the western United States, coal-bearing cyclothems that formed along the margins of shallow seas in continental interiors persisted well into the Early Permian. Coal-bearing Lower and Upper Permian beds, up to three kilometres thick, are widely distributed in Australia, peninsular India (the lower part of the Gondwana System), southern Africa (the lower part of the Karoo [also spelled Karroo] System), the Kuznetsk Basin of southwestern Siberia, and the Paran and Precordillera basins of southern Brazil and western Argentina in South America. Red beds were common in continental beds in tropical and subtropical paleolatitudes. During the Permian Period red beds became more abundant and widespread, suggesting that the climate became progressively warmer and tropical conditions became more widespread. This warming trend is also hinted at by a significant increase in the amount of dolomite in shelf sediments.

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