وبلاگ بزرگ مقالات زمین شناسی - پرمین

وبلاگ بزرگ مقالات زمین شناسی

بانک مقالات،کتابها،مطالب،فلشها و نرم افزارهای تخصصی زمین شناسی





The name applied to the last period of geologic time in the Paleozoic Era and to the corresponding system of rock formations that originated during that period. The Permian Period commenced approximately 290 million years ago and ceased about 250 million years ago. The system of rocks that originated during this interval of time is widely distributed on all the continents of the world.

The Permian System is presently divided into three series: the lower Cisuralian Series with type sections in the western Ural region of Russia; the middle Guadalupian Series with type sections in western Texas and southern New Mexico; and the upper Lopingian Series with type sections in southern China.

Permian rocks contain evidence for a paleogeography that was greatly different from present geography. The Permian Period was a time of variable and changing climates, and during much of this time latitudinal climatic belts were well developed. During the later half of Permian time, many long-established lineages of marine invertebrates became extinct and were not immediately replaced by new fossil-forming lineages. Rocks of Permian age contain many resources, including petroleum, coal, salts, and metallic ores.  See also: Living fossils


Eastern European Permian


The Permian System was proposed in 1841 by R. Murchison for a succession of marine, brackish, evaporitic, and nonmarine deposits exposed in the former province of Perm on the western flank of the Ural Mountains in Russia (Fig. 1). During the Permian Period, this region was near the boundary between the stable Russian Platform on the west and the tectonically active Ural Geosyncline to the east (Fig. 2). The ancestral Ural Mountains were gradually formed by the uplift and compressive collapse of the Ural Geosyncline during this time. The geosynclinal deposits are mostly clastic sediments that reach a thickness of several thousand meters. During the early part of the Permian, the edge of the Russian Platform was marked by the development of extensive carbonate banks and reefs. These carbonate deposits contrast markedly with the thick clastic beds of the geosynclinal facies and the thin shales and limestones of the platform facies.



Fig. 1  Correlation of major Permian rock units.




fig 1






Fig. 2  East-west section of the Permian System in its type area.




fig 2




Continental deposition


By the middle of Early Permian time, the marine connection from the Russian Platform southward into the Tethyan ocean was closed, apparently by uplifts along the southern and western margins of the platform. During middle Permian time, the Russian Platform was covered by a broad, shallow, brackish gulf which had limited water circulation with an ocean to the north. Brackish conditions fluctuated with evaporites, redbeds, and other continental deposits. During later Permian time, detrital material eroded from the uplifted ancestral Ural Mountains, filled the remnants of the deformed geosynclinal depression, and were progressively transported westward onto the Russian Platform. Continental deposition continued into Triassic time.




Both the lower and upper boundary of the Permian System have been subject to considerable discussion. As Murchison originally defined the system, the lower boundary was at the base of the evaporitic facies of the Kungurian beds and the upper boundary at the top of the continental facies of the Tatarian beds. Based on similarities in lithology, Murchison correlated the thick Artinskian clastics that lie beneath the Kungurian within the Ural Geosyncline with the lower Carboniferous of western Europe. He believed the limestones and shales that lie beneath the Kungurian beds on the Russian Platform were part of the upper Carboniferous. Later studies showed that these original correlations were not correct. A. P. Karpinsky in 1889 described Artinskian ammonoids that were closely related to faunas considered Permian in age in other parts of the world. The suggestion to lower the Permian boundary in its type region to include these Artinskian faunas gained general acceptance.


Cisuralian Series


The limestones of the Ufa Plateau continued to be classified as upper Carboniferous until after 1930. During the 1930s V. E. Ruzhentsev recognized prolific ammonoid faunal assemblages that were present in both the carbonates and shales of the Ufa Plateau and the Artinskian clastics. Based on these studies, Ruzhentsev subdivided the original Artinskian into a lower Asselian Stage, a middle Sakmarian Stage, and an upper (revised) Artinskian Stage (Fig. 1). In the 1990s, these stages were combined with the Kungurian Stage to form the lower Permian Cisuralian Series (Fig. 1).

The Asselian, Sakmarian, and Artinskian stages include normal marine faunas with abundant fusulines, conodonts, and ammonites, which are extremely useful in interregional correlation. Large, massive reef mounds and pinnacle reefs, 1800 ft (600 m) or more in thickness, are common on the edge of the Russian Platform (Fig. 2), and these pass eastward into thick shales and sandstones in the Ural Geosyncline and westward into thin dolomitic limestone, dolostone, and evaporites.

On the Russian Platform and on the margin of the Ural Geosyncline, the upper part of the Artinskian beds intertongues with Kungurian red shales and evaporites. The Kungurian extends stratigraphically higher and locally is the source of major salt, anhydrite, and potash salts, as near Solikamsk. Fossils are rare and not diverse, and show affinities at some localities to Artinskian species.


Middle Permian deposits


On the Russian Platform, the Ufimian beds lie at the base of the middle Permian and are less continuously distributed and are red shales and sandstones, variegated clays, and marly shales less than 450 ft (150 m) thick. They primarily contain a meager fresh-water fauna of bivalves and ostracodes. Eastward near the Ural Mountains, the Ufimian reaches 4500 ft (1500 m) and is coarser grained.

The Kazanian Stage unconformably overlies the Ufimian and consists of about 300 ft (100 m) of greenish-gray impure limestone that has brackish-water or restricted marine faunas that are mostly bryozoans and brachiopods. The Kazanian deposits were laid down in an elongate shallow basin that paralleled the western flank of the Ural Mountains. This basin was closed to the south, where it locally includes evaporites, and open to the north, where a few additional marine invertebrates, including rare ammonoids, are locally present.


Upper Permian deposits


The youngest stage of the Russian Permian is the Tatarian, which is formed of brightly colored, variegated sandstone, conglomerate, and other clastic beds that were deposited by rivers, streams, and lakes, and includes some dune sands. Plants and terrestrial animals, including insects and vertebrates, are well known from these sediments. The original Tatarian of Murchison had five main faunal zones; however, only the lower two are now considered Permian, and the name Tatarian is restricted to these two zones. The upper three zones are included in the Vetlushian Stage and are Triassic.  See also: Redbeds


Western Texas and southern New Mexico


One of the finest Permian sections known is in the Delaware Basin of western Texas and southeastern New Mexico (Fig. 3), where the Permian sequence reaches a thickness of more than 12,000 ft (4000 m). The rocks of the lower 6600 ft (2200 m) are of marine origin and are highly fossiliferous. Although a few fossils from this region were described in 1858, this succession of strata remained virtually unknown until 1909. Intensive study followed the discovery of oil in the region about 1920. In 1939 a committee of Permian specialists proposed to subdivide the American Permian section into four series, and this usage was widely accepted. It has become the standard section for North America and, to a considerable degree, for the world. It is divided into four stages in ascending order: Wolfcampian Stage, 1500 ft (500 m); Leonardian Stage, about 3000 ft (1000 m); Guadalupian Stage, about 3000 ft (1000 m); and Ochoan Stage, about 4500 ft (1500 m).



Fig. 3  Depositional and structural relationships in western Texas during Permian time.




fig 3




The Wolfcampian correlates closely with the Russian Asselian, Sakmarian, and part of the Artinskian stages (Fig. 1). The Leonardian correlates with the upper part of the Artinskian and the Kungurian, and has larger and more varied faunas. The Guadalupian cannot be correlated in detail with the middle part of the Russian section, because the latter has restricted faunas. The Ochoan is virtually unfossiliferous, except for a thin zone near the top (in the Rustler Dolostone) which contains productid brachiopods and a few other types of Paleozoic invertebrates.


Wolfcampian and Leonardian


During the Wolfcampian Epoch, most of the region was a broad, shallow marine intracratonic basin in which a rich and varied fauna thrived. A fore-deep basin bordered the northern flank of the active Marathon orogenic belt. By Leonardian time, three distinct basins (Fig. 3) were subsiding more rapidly than the surrounding area, which became a broad shelf occupied by wide, shallow lagoons. Light-colored, fossiliferous limestones (Victorio Peak Limestone) accumulated on the shelves, while black limestone and black shale accumulated in the basins. Evidently the threshold to the basins, which was somewhere in Mexico, was so shallow that water in the basins was density-stratified and the bottom was stagnant and foul, so that almost no benthic organisms could survive. The black Bone Spring Limestone is generally barren of fossils.




During Guadalupian time (middle Permian), the basins continued to deepen, and as the climate became markedly arid, surface water flowed radially out of the basins onto the adjacent platforms to replace the water lost by evaporation in the shallow lagoons. Narrow limy banks grew along the margins of the basin to form the great Capitan Reef. Figure 4 shows the complex relations within the Leonardian and Guadalupian deposits along the face of the Guadalupe Mountains. Probably no other great reef complex is so well exposed or has been so intensively studied as that of the Capitan Reef. Massive deposits of reef talus were derived from the growing front of the reef. These dip steeply into the basin, become finer down dip, and grade out into thin tongues of calcarenite. The back-reef deposits are calcareous for distances of 1.5–6 mi (2–10 km) and then grade rapidly into gypsiferous shales and anhydrite. Gray, beach, and offshore-bar sands intertongue from the landward margins of the lagoon. Farther back the sands pass into redbeds.



Fig. 4  Section across the Capitan Reef Complex. 1 m = 3.3 ft; 1 km = 0.6 mi.




fig 4





Late Permian


During Ochoan time, the basins became evaporitic under intensely arid conditions. Enormous deposits of anhydrite and, later, of halite were precipitated. Interbedded in the salt in the center of the Delaware Basin are several lenses of potash salts—sylvite, carnallite, and polyhalite.


Other areas


In central Texas, Oklahoma, and Kansas, the Wolfcampian equivalents are largely of marine origin. Marine conditions persisted well into Leonardian time in central Texas, but in Kansas a very large salt deposit (in the Wellington Shale) is followed by a thick redbed sequence. By Guadalupian time, most of the deposits in central Texas were nonmarine redbeds in which several thin marine dolostones intertongued from the west. In Oklahoma, all but the lower part of the Wolfcampian is in redbed facies because of the local influence of the Oklahoma mountains.

In other areas of the central and southern Rocky Mountain states, block faulting during Wolfcampian time resulted in a series of horsts, which were local sources of sediments, and adjacent grabens, which became local basins of deposition. Although most of these basins had initial deposits of marine origin, they were mainly filled by the end of Wolfcampian time. Subsequently, Leonardian and early Guadalupian sediments formed thin, blanketlike continental deposits over much of the region. Dunes, redbeds, and local evaporites are common.

The western margin of the North American craton during the Permian was located far to the east of the present west coast. The Permian margin extended generally northward from southern California through southern and eastern Nevada, southeastern Idaho, eastern British Columbia, and just into the eastern edge of Alaska. The southern part of this margin had a thick succession of Wolfcampian, Leonardian, and early Guadalupian limestones deposited on it. In northern Utah and Idaho, sandstone becomes more prevalent, and in the Guadalupian, phosphorite and chert become important deposits. Farther north, sandstone and chert continue to increase in abundance and form most of the Permian sediments on the old cratonic margin.

West of the Permian cratonic margin, Permian strata are found in four or five tectonically disturbed belts. Although these belts include rocks of the same age, each structural belt contains different fossil faunal assemblages that are quite distinct, and in each the lithologic succession and depositional history appears to be independent. These different assemblages of faunas and sediments were apparently deposited at considerable distances from one another, and their present-day geographic proximity indicates that each belt has been added by tectonic accretion to the western margin of the North American craton in post-Permian time—that is, during the Mesozoic or Cenozoic. Of all these structural belts, the best known is the Cache Creek (British Columbia) belt, which extends from southern British Columbia north-northwestward into southern Yukon. The Cache Creek belt is composed of oceanic ribbon cherts, dark shales and sandstones, basic igneous flows, and massive limestone reefs having a tropical fossil fauna and flora which show affinities to the Tethyan faunal realm rather than to the Midcontinent-Andean realm of North American and South American cratons.



Northwestern Europe


Permian beds are widespread in northwestern Europe and the North Sea and produce large amounts of gas and oil, which have had an important influence on the economies of northwestern European countries since the late 1960s. These Permian beds are divisible into two parts. The lower part (Rotliegend beds) is mainly red sandstone. The upper part (Thuringian Series) consists of conglomerate, chalcopyritic shale, dolomitic limestone, evaporites, and shale. The Rotliegend beds are subdivided into the Autunian Stage (or Lower Rotliegend) in the lower part and Saxonian Stage (or Upper Rotliegend) in the upper part. Saxonian strata are younger than the last major movements of the Hercynian orogeny, and an unconformity separates them from the overlying Thuringian.

The Thuringian contains a basal conglomerate, a thin copper-bearing shale (Kupferschiefer), dolomitic limestones (Zechstein), and seven important evaporitic units that near Stassfurt include potassium salts. The Thuringian sea extended east across Poland, and possibly connected with the Kazanian sea of the Russian Platform. The Magnesian Limestone of England is a thin western tongue of the Thuringian Series.

The Atlantic Ocean did not start to open until middle Mesozoic time, so that the Permian deposits of northwestern Europe are very similar to, and are thought to have been formerly continuous with, Permian beds found along the central east coast of Greenland, on Spitsbergen, and on the Canadian Arctic islands. They also connected, by way of the Barents shelf, to the northern entrance to the Ural Geosyncline and Russian Platform.


Tethyan regions


South of the Hercynian orogenic belt, the sediments and faunas and floras of the Permian change markedly in Mediterranean Europe, northern Africa, and southern and central Asia (Fig. 5). Limestone, dark shales and sandstones, cherts, and basaltic volcanics are common. The species and generic diversity is great in contrast to that of northwestern Europe and the Russian Platform.



Fig. 5  Paleogeography during the earliest part of the Permian Period.




fig 5




The marine Tethyan invertebrate fauna evolved rapidly during the middle part of the Permian and formed a distinctive biogeographic realm, called the Tethyan realm. This diverse fauna is characterized by the verbeekinid fusulinaceans (foraminiferal Protozoa) and colonial and solitary waagenophyllid corals (Rugosa). Later orogenic movements during the Mesozoic and Cenozoic have greatly complicated the interpretation of these Permian deposits; however, several linear belts of Permian rocks appear to be present. Each may have included shallow reef deposits and shallow- and deeper-water clastic and carbonate deposits, and some belts may have fringed a number of small fragments of cratonic crustal blocks. In post-Permian time, sea-floor spreading and crustal subduction apparently displaced these several depositional belts and cratonic fragments and lodged them as accreted terranes in orogenic belts against larger cratons, such as Europe, central and eastern Asia, and western North America.

The Permian Tethyan faunas were apparently tropical, and mostly shallow-water because of the abundance of calcareous algae, bryozoans, brachiopods, and echinoderms, and the reeflike nature of most of the limestones. The close association of these limestones with basic igneous rocks (including pillow lavas), ribbon cherts, and dark sandstones and shales suggests that much of the Tethyan region was made up of island arcs and oceanic carbonate plateaus similar to those of the present tropical Pacific Ocean.


Southern China


The lower part of the Permian in southern China includes the upper part of the Maping Stage and the Longlinian (Changshan) Stage which are widespread, but locally deeply eroded, beneath a middle lower Permian unconformity. Overlapping this unconformity are extensive limestones of the Chihsia Series which comprise the remainder of the lower Permian in the area. An extensive unconformity at the top of the Chihsia represents the mid-Permian sea-level event. Higher, widely distributed limestones of the Maukou Series are present. They show major lateral variations in lithologic facies.


Lopingian Series


The type area for the upper series of the Permian, the Lopingian Series, overlies the Maukou in continuous succession in much of southern China. This series is divided into the the Wuchiapingian Stage (below) and the Changhsingian Stage (above) and the top of the succession passes conformably into the Lower Triassic.



Gondwana continents


The Permian successions are remarkably similar in all of the parts of southern Africa, Australia, South America, Pakistan and India, and Antarctica that formed the large supercontinent of Gondwana during the late Paleozoic. In southern Africa these deposits form the lower part of the Karoo System and commence with a tillite (Dwyka Tillite), which is followed by dark shales (Ecca Series) overlain by sandstones and red shales (Beaufort Series). The Ecca includes the late Paleozoic coals of southern Africa, and the Beaufort has a distinctive and extensive mammallike reptilian fauna.

The Permian deposits of much of South America have close similarities to those of southern Africa. In the Paraná Basin the Guata Group with glacial sediments, coal, and marginal marine beds, and, in its upper part, Eurydesma (Bivalvia) and Glossopteris (plant) is of probable Early Permian age. Above, the Irati Formation has the same distinctive reptiles found in the Beaufort of southern Africa and also has an extensive plant fossil succession.

Antarctica also has a generally similar late Paleozoic succession to those found in southern Africa and southern South America, including coal and plant and reptile fossils.

Australia, Pakistan, and India have late Paleozoic successions preserved in a number of fault-bounded basins. The adjacent parts of the Gondwanan craton were apparently emergent. Many of these basins include tillites and glacial marine deposits in their Carboniferous and earliest Permian deposits. These are commonly followed by marine deposits having some early and middle Permian limestones. Several important New South Wales coalfields, such as the Newcastle Coal Measures, are of late Permian age and have nontropical Tatarian insect faunas. Coal formation continued into the Mesozoic in many of these basins.




During the Permian Period, several important changes took place in the paleogeography of the world. The joining of Gondwana to western Laurasia (Fig. 5), which had started during the Carboniferous, was completed during Wolfcampian time (earliest Permian). The addition of eastern Laurasia (Angara) to the eastern edge of western Laurasia finished during Artinskian time (middle to latest early Permian) and completed the assembly of the supercontinent Pangaea (Fig. 6). The climatic effects of these changes were dramatic. Instead of having a circumequatorial tropical ocean, such as during the middle Paleozoic, a large landmass with several high chains of mountains extended from the South Pole across the southern temperate, the tropical, and into the north temperate climatic belts. One very large world ocean, Panthalassa and its western tropical branch, the Tethys, occupied the remaining 75% of the Earth's surface, with a few much smaller cratonic blocks, island arcs, and atolls.  See also: Continental drift; Continents, evolution of



Fig. 6  Paleogeography near the middle of the Permian Period.




fig 6




Early Permian glacial deposits of the Gondwanan continents lie within an area about 40° from the south paleopole. Recently reported Permian glacial beds in eastern Angara would have been within 30 to 40° of the north paleopole. Middle and late Permian sediments, such as sand dunes, marine beds having a few invertebrate fossils with warmer-water affinities, and coals, suggest warmer conditions but not tropical or subtropical. Considerable evidence also suggests the world climate became progressively milder through a series of fluctuating warming and cooling steps during the later part of the early Permian and late Permian. Desert conditions became widespread in many parts of tropical and subtropical Pangaea with dune sands, evaporites, redbeds, and calcic soil zones.

Vast deposits of salt and anhydrite accumulated in Kansas, New Mexico, and the Permian Basin in western Texas. On the eastern part of the Colorado Plateau, extensive dune sands were deposited, such as in the Canyon DeChelly area. Elsewhere similar conditions are shown by the evaporites of the Kungurian on the Russian Platform and the dune sands and salt deposits of the Thuringian of northwestern Europe.  See also: Paleogeography; Saline evaporites




Most marine invertebrates of the Early Permian were continuations of well-established phylogenetic lines of middle and late Carboniferous ancestry. During early Permian time, these faunas, dominated by brachiopods, bryozoans, conodonts, corals, fusulinaceans, and ammonoids, gradually evolved into a number of specialized lineages. The tropical shallow-water faunas of southwestern North America evolved almost in isolation from those of the Tethyan region, because faunal exchanges had to cross either the cooler waters of a temperate shelf or deep waters of Panthalassa. Fluctuating climates permitted rare dispersals of some faunas between these two tropical faunal realms during the early part of early Permian time. The closure of the Uralian seaway during Artinskian time, however, extended that dispersal path north around Angara and into cold boreal waters through which the tropical species could not disperse. With the extension of this dispersal path, the faunas of the two tropical realms evolved independently, with only extremely rare dispersals between them.

The Siberian traps, an extensive outflow of very late Permian basalts and other basic igneous rocks (dated at about 250 million years ago), are considered by many geologists as contributing to climatic stress that resulted in major extinctions of many animal groups, particularly the shallow-water marine invertebtares. The end of the Permian is also associated with unusually sharp excursions in values of the carbon-12 isotope (12C) in organic material trapped in marine sediments, suggesting major disruption of the ocean chemistry system.




The warm to tropical shallow-water foraminiferal faunas during the Permian were dominated by the fusulinaceans. One group, the verbeekinids, which evolved in the Tethyan realm, formed the distinctive foraminiferal fauna of reefs and atolls of the Tethyan realm. Schwagerinids were less abundant and occurred in sandy sediments adjacent to the reefs. Many of the schubertellids appeared to be adapted to lagoonal environments. In the tropical Midcontinent-Andean realm along the western coast of Pangaea, the schwagerinids filled more of the shallow water niches than in the Tethyan realm, but they were not as diverse in number of new genera and species. Away from the paleotropics in both realms, species diversity decreases rapidly, nearly in proportion to the amount of limestone that was deposited in the succession. Near the end of Guadalupian time, fusulinaceans declined markedly. In the Lopingian specialized genera persisted in the Tethyan region until the end of the period.  See also: Foraminiferida




Among the brachiopods, the strophomenids became highly specialized and important framework builders in many bioherms and small reefs. Rhynchonellids and spiriferids are of interest because of their relict pattern of distribution after their evolutionary diversification during the Devonian. Locally they are abundant and important. Near the end of the Guadalupian, brachiopods also were greatly reduced, and about half of those that survived the Lopingian became extinct before the Triassic. Only a few genera and species in one-fifth of the families that were present in the early Permian survived into the Triassic.  See also: Brachiopoda




These have much the same history as other marine invertebrates during the Permian. Several families of cryptostomes, such as the polyporids, hyphasmoporids, and nikiforovellids, and several families of trepostomes, such as the eridotrypellids and araxoporids, became increasingly diverse during the middle part of the Permian. Their greatest geographical diversity was in the Tethys. During Guadalupian time, many genera within the bryozoan families became extinct and, by the end of that epoch, more than 10 families were extinct. Six more families became extinct during the Lopingian (latest Permian). At least five families range into the Triassic.  See also: Bryozoa




This important Permian fossil group also had a rapid middle Permian diversification, followed by a rapid reduction in genera late in Guadalupian time. Of about 55 Guadalupian genera, 15 survived into the earliest Lopingian (earliest late Permian), where they evolved into about 50 genera. Only 6 of these survived into the late Lopingian (late late Permian), however; they evolved into nearly 30 genera. Only one ammonoid genus survived the end of the Permian and ranged into the Early Triassic.




Terrestrial faunas included insects which showed great advances over those of the Carboniferous Coal Measures. Several modern orders emerged, among them the Mecoptera, Odonata, Hemiptera, Trichoptera, Hymenoptera, and Coleoptera. Extensive insect faunas are known from the lower Permian rocks of Kansas and Oklahoma, the Permian of Russia, and the upper Permian of Australia.  See also: Insecta


Land plants


During the Permian, plants, including lepidodendrons and cordaites, were well adapted to the moist conditions of low-lying coal swamps. Several lineages also adapted to the drier, well-drained conditions of mountains and alluvial plains, particularly conifers. In glaciated areas of Gondwana, the tongue ferns Glossopteris and Gangamopteris are common and were apparently adapted to cold climates.




Of the vertebrates, labyrinthodont amphibians were common and varied; however, reptiles showed the greatest evolutionary radiation and the most significant advances. Reptiles are found in abundance in the lower half of the system in Texas and throughout most of the upper part of the system in Russia and also are common in Gondwana sediments. Of the several Permian reptilian orders, the most significant was the Theriodonta, or mammallike reptiles, that evolved in the Triassic into mammals. These reptiles carried their bodies off the ground and walked or ran like mammals. Unlike most reptiles, their teeth were varied—incisors, canines, and jaw teeth as in the mammals—and all the elements of the lower jaw except the mandibles showed progressive reduction. Most of the known theriodonts are from South Africa and Russia.  See also: Paleozoic; Reptilia; Therapsida



Charles A. Ross

June R. P. Ross






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