CARBONIFEROUS PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. fifth interval of the Paleozoic Era, succeeding the Devonian and preceding the Permian. In terms of absolute time, the Carboniferous began approximately 360 million years ago and ended 286 million years ago (see Table). It is often divided into the Early Carboniferous Epoch (360 to 320 million years ago) and the Late Carboniferous Epoch (320 to 286 million years ago). Its duration of roughly 74 million years makes it the longest period of the Paleozoic and the second longest of the entire Phanerozoic time scale. The rocks that were formed or deposited during the period constitute the Carboniferous System. The name Carboniferous is derived from coal deposits typical of strata in the upper portion of the system throughout the world. interval of geologic time from 360 to 286 million years ago. It is often divided into the Early Carboniferous Epoch (360 to 320 million years ago) and the Late Carboniferous Epoch (320 to 286 million years ago). The fifth period of the Paleozoic Era, the Carboniferous Period follows the Devonian and precedes the Permian Period. The rocks that originated during Carboniferous time make up the Carboniferous System. The Carboniferous System was identified by William Daniel Conybeare and John Phillips in Great Britain in 1822. Its name refers to the presence of coals in many parts of the succession of strata between the Old Red Sandstone (Devonian) and the New Red Sandstone (Permian) formations in Britain. In North America the Carboniferous is often represented by the Mississippian and Pennsylvanian periods, which are named after the states in which the strata are widely developed and which correspond to the Early Carboniferous and Late Carboniferous time periods. In Europe the Carboniferous System is often subdivided into the Lower Carboniferous and Upper Carboniferous, which correspond to the time units Early and Late. The classification of the Carboniferous rock units and their corresponding time units had become so complex by the end of the first quarter of the 20th century that the International Geological Congress and later the Carboniferous Subcommission of the International Union of Geological Sciences set out to establish criteria by which to define an international time scale for the period. The boundaries of the system and their time equivalents have been generally agreed on, but minor subdivisions are still subject to debate. The distribution of the continental landmasses continued to change during the Carboniferous Period. All the landmasses drew closer together as tectonic plate movements gathered them toward the equatorial regions and the southern half of the globe. The enormous continent of Gondwana, made up of what is now Africa, South America, India, the Middle East, Australia, and Antarctica, occupied much of the Southern Hemisphere. Early in the period, Laurussia, consisting principally of present-day North America, Greenland, and northern Europe, advanced toward Gondwana's northern margin. By the end of the Carboniferous Period, most of Laurussia had collided with Gondwana, closing the Tethys seaway between them. The ensuing Appalachian-Hercynian orogeny fused the two continents together. Siberia and China (including Southeast Asia), which were individual continents at this time, remained at high latitudes in the Northern Hemisphere. In Gondwana a heavy and prolonged continental glaciation occurred at a south polar centre situated in the vicinity of southern Africa. Later, as the ice diminished, the sea level was restored and the climate ameliorated. Swamps became widespread in both northern and southern continental areas and forest vegetation thrived. The coal deposits of the later Carboniferous times were formed from the debris of these forests, and the widespread and repeated growth of the coal swamp forests was a most characteristic feature of the Carboniferous Period. This episode of coal formation was terminated by a rise in the landmasses and an increasingly arid climate. The end of these environments presaged the great crisis that was to affect much of the living world in the following Permian Period. The immense volume of carbon incorporated in plant debris during Carboniferous times came from the atmosphere. Some scientists have maintained that the removal of so much carbon dioxide from the atmosphere affected the climate, but this is disputed and not believed to have been a significant factor in the period. Coal, formed during the Carboniferous Period, is found in many areas of the world, including eastern North America, Europe, North Africa, and northern China and Korea. The process of coal formation lasted about 40 million years, and no other period was to see the transformation of so much plant material into coal. In addition to the great advances made by plants with their complex forest assemblages, the vertebrates were undergoing an evolutionary radiation. Amphibians became widespread and diverse, with some as large as 2 m (6 feet). Reptiles appeared for the first time and rapidly adapted to many habitats. The Carboniferous Period thus was marked by great changes to world geography. Life-forms made significant advances and adapted to a range of climates that included the periglacial, the equatorial rainy, and the hot arid. Continental climates characterized by seasonal extremes were probably as widespread as at any time in the history of the Earth. Additional reading Discussions of the Carboniferous period are included in William B.N. Berry, Growth of a Prehistoric Time Scale: Based on Organic Evolution, rev. ed (1987); Steven M. Stanley, Earth and Life Through Time, 2nd ed. (1989); Colin W. Stearn, Robert L. Carroll, and Thomas H. Clark, Geological Evolution of North America, 3rd ed. (1979). The results of ongoing research in the field are presented at congresses and published as International Congress on Carboniferous Stratigraphy and Geology, Compte rendu (irregular), including materials in French, English, and German. The Decade of North American Geology project of the Geological Society of America includes works on Carboniferous geology in many of its series of publications. Another series, undertaken by the International Union of Geological Sciences, summarizes the Carboniferous geology of all continents: Carlos Martinez Diaz (ed.), The Carboniferous of the World (1983 ). Walter L. Manger Carboniferous environment Paleogeography Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the The figure shows the assumed positions of the continents during the Upper Carboniferous. As will be noted, the land-sea distribution at the time did not correspond to present-day geography. The bulk of the Earth's landmass was concentrated in the Southern Hemisphere. Relative lithospheric plate movements had brought the continents close together on one side of the globe. Only five major continental masses existed. Laurussia, formed by the joining of Laurentia (principally North America and Greenland) and Baltica (most of northern Europe and Scandinavia) in the Devonian, constituted the single largest landmass in the Northern Hemisphere during the Carboniferous Period. In addition to Laurussia, Siberia, Kazakhstania, and China, which included Southeast Asia, were each separate continents. The enormous continent of Gondwana occupied the Southern Hemisphere. Gondwana combined what are now Africa, India, South America, Australia, the Middle East, and Antarctica. The Tethys seaway separated the landmasses of the Northern and Southern hemispheres near the equator. During the Lower (early) Carboniferous, Laurussia was apparently more fragmented than Gondwana. Siberia, Kazakhstania, and China occupied positions at high latitudes. Northern Europe and Scandinavia were joined to North America, with what is now northern Canada lying at a mid-latitude position and the United States, Poland, Ukraine, European Russia, Belarus, and adjacent areas near the equator. Gondwana lay entirely in the Southern Hemisphere, although little of the landmass was situated at the South Pole. By the Upper (late) Carboniferous, plate movements had brought most of Laurussia into contact with Gondwana, thereby closing the Tethys. The two became fused by the AppalachianHercynian orogeny, which began in the Upper Carboniferous and continued into the Permian. The United States and northern Europe retained their equatorial position. China and Siberia remained at high latitudes in the Northern Hemisphere. The distribution of land and sea followed fairly predictable limits. The continental interiors were terrestrial, and no major marine embayments apparently existed. Upland areas of the continental interiors underwent substantial erosion during the Carboniferous. Shallow seas occupied the shelf margins surrounding the continents. It must be remembered that areas that were marginal to the Carboniferous continents may very well have become continental interiors in the present geographic setting (as, for example, much of the United States). Deeper troughs (geosynclines) lay seaward of the continental masses, and their sedimentary record is now represented by mountains. Significant geologic events The Carboniferous marks a period of relatively stable crustal conditions between major mountain-building episodes in the Devonian and Permian. Nevertheless, the rocks of both the Lower and Upper Carboniferous show evidence of orogeny and isostatic adjustments in previously formed mountains. Among the major continental masses constituting present world geography, only Antarctica exhibits no recognizable trace of Carboniferous earth movements (tectonics). Significant mountain building resulted from the collision of Laurussia and Gondwana. The movement of these continents toward each other began in the Lower Carboniferous and continued into the Permian. Their collision resulted in the formation of three mountain belts and ranges essentially at the same time. The Hercynides (occasionally called the Variscan Belt) were formed across southern Europe (including Britain). The Appalachians were formed along the eastern coast of North America, and their counterpart, the Mauritanides, emerged in North Africa. A fourth belt, the Ouachita (Marathon) Mountains, were formed along the southern border of North America. Usually viewed as an extension of the Appalachians, the Ouachita orogeny predates most movements in the Appalachians and is linked to the collision of the southern margin of North America with the South American portion of Gondwana rather than with North Africa. Less significant tectonism occurred in the Cordilleran (Rocky Mountain) region during a pulse of the Antler orogeny that elevated the Ancestral Rockies and produced thick clastic wedges, such as the Fountain Arkose. Precordilleran movements occurred in South America, and similar events took place in northern Asia (the Tien Shan, Kunlun, and Timan mountains), Australia, and New Zealand. The Ancestral Urals also were active during the Carboniferous. Carboniferous life Invertebrates The Late Devonian experienced major extinctions within some marine invertebrate groups, and Carboniferous faunas reflect a different composition than what had prevailed in the middle Paleozoic. Most notably, reef-forming organisms, such as tabulate corals and stromatoporoids, were limited, and this dearth of framework builders resulted in poorly developed reefs during the Carboniferous. Yet, this period was still one of diverse marine invertebrates. More striking, however, was the remarkable variety of terrestrial animals and plants associated with the coal swamps for which the period is noted. Arguably the most famous Carboniferous fossil occurrence is at Mazon Creek, Ill., in the north-central United States, where ironstone concretions formed around a multitude of different invertebrate and plant specimens. The concretions contain many soft-bodied forms, attesting to the rapid formation and burial of these structures. Benthic marine communities in the Carboniferous included a variety of invertebrates. As discussed elsewhere, the crinoids grew in great profusion attached to the seafloor. These animals were solitary suspension-feeders. Their large numbers may have caused baffling of bottom currents, resulting in the deposition of carbonate mud around their bases that developed into mounds on the seafloor. The blastoids, a group of budlike stalked echinoderms related to the crinoids, are abundant in Carboniferous deposits as well. Areas favourable to the crinoids and blastoids were also occupied by the cryptostomous, or lacy, bryozoans. Their fanlike colonies were attached to the seafloor and may have aided in baffling fine sediment. Brachiopods, particularly the winged (or spiriferid) and the spiny (or productid) types, anchored their bivalved shells to the substratum by a fleshy stalk and pumped the water column with a ring of tentacles to recover food in much the same manner as the bryozoans. Brachiopods were particularly common during the Carboniferous, and all orders except the Atrypida and Pentamerida, which became extinct at the end of the Devonian, are found in rocks of the period. Both calcareous and agglutinate foraminifers are represented in Carboniferous deposits, especially limestones. An unusual group of these protozoans, called the fusulinids, appear in the rocks of the Upper Carboniferous and dominate the assemblages through the Permian, when they become extinct. The fusulinids secreted a tightly coiled, calcareous test that was chambered. They exhibited rapid evolutionary diversification, and the sequence of various morphologic features reflecting this diversification is used to subdivide and correlate the Upper Carboniferous on an intercontinental basis. In addition to marine invertebrates that inhabited the seafloor, the calcareous algae were well represented. Platelike red varieties were abundant enough to form mounds in the Upper Carboniferous. Benthic organisms on the decline during the Carboniferous were the trilobites, corals, and sponges. The ammonoid cephalopods, extinct relatives of the chambered Nautilus, were common in deep marine waters. They swam by means of jet propulsion and either caught prey or were scavengers. The ammonoids, like the fusulinids, exhibited rapid evolution through the Carboniferous and serve as useful index fossils for correlation of the interval. True nautiloids also were represented in Carboniferous marine environments. While they are not as diverse as the ammonoids, both straight and coiled forms are common as fossils, and some straight forms grew exceedingly large (more than three metres) for invertebrates. Plants Carboniferous terrestrial environments were dominated by plants, ranging from small, shrubby growths to tall trees reaching heights of more than 30 metres. The most important vascular plants were the lycopods, sphenopsids, cordaiteans, seed ferns, and true ferns. Lycopods are only represented in the modern world by the club mosses, but in the Carboniferous they included tall trees that had dense, spirally arranged leaves and spore-bearing organs on their leaves (cones might be present). Lepidodendron, with diamond-shaped leaf bases, and Sigillaria, with ribs and round leaf bases, were the dominant lycopod genera. They produced fossil logs that measure as much as one metre at their bases. Sphenopsids are trees and shrubs that have a distinctly jointed stem and leaves arranged in spirals from the joints. The horsetail rush (Equisetum) is the only living representative, while Calamites is the most common Carboniferous genus. Smaller than the lycopods, sphenopsids also occupied somewhat dry, more upland environments. The cordaiteans belong to the gymnosperms and are precursers of the conifers. They also favoured upland environments, grew tall, and had needles and cones like modern conifers, although the group itself has no living representatives. The genus Walchia probably formed forested areas much as modern pines do. Seed ferns, or pteridosperms, had fernlike foliage, but reproduced by seeds rather than by spores. They, too, are gymnosperms with no living representatives. The pteridosperms include such trees as Glossopteris, a genus that was characteristic of Permian floras from Gondwana; and such low shrubs (usually represented by fragments of their foliage) as Neuropteris, Pecopteris, and other forms from Mazon Creek (see above). The seed ferns and true ferns formed the under foliage associated with most Carboniferous coal swamps. Carboniferous rocks Occurrence and distributionLower Carboniferous The Carboniferous is traditionally broken down into lower and upper divisions that reflect major differences in depositional regimes and biota as well as in age and correlation. The Lower Carboniferous, or Mississippian, is characterized by shallow-water limestones deposited on broad shelves occupying most continental interiors, but particularly in the Northern Hemisphere. By contrast, geosynclinal facies, especially turbidites, formed in deeper troughs along continental margins. Shallow terrigenous clastic facies, such as sandstone and shale, are more poorly developed, and coals are rare. TypesLower Carboniferous Limestones of the Lower Carboniferous are for the most part composed of the disarticulated remains of crinoids. These echinoderms attached themselves to the seafloor by means of a long stalk. The stalk and the flowerlike crown of such animals are composed of plates consisting of single crystals of calcite (calcium carbonate). The crinoids grew in great profusion, forming meadows of thousands of individuals. Calcite is extremely stable in warm, shallow marine conditions, and so when individual crinoids died their plates would accumulate on the seafloor as sand-sized sediments, which were subsequently cemented together by calcium carbonate. The crinoid fragments were frequently reworked by currents, and the deposits exhibit both cross-bedding and ripple marks. Deposits of crinoidal limestone approaching 160 metres in thickness are not uncommon for certain intervals of Lower Carboniferous time, particularly in North America. Limestone of this kind is exploited as quarry stone. In addition to the crinoidal limestones, oolitic limestones and lime mudstones formed in shallow-water marine environments of the Lower Carboniferous. Ooliths are concentric spheres of calcium carbonate that were inorganically precipitated around a nucleus on warm, marine-shelf margins subject to high wave energy (such as the Bahama Shelf and northern Red Sea today). These deposits also exhibit cross-bedding and ripple marks, which testify to the high-energy conditions. Mixtures of ooliths and abraded fossil fragments, particularly foraminifers, are common in the Lower Carboniferous. Lime mudstones reflect quiet, shallow-water environments, such as are found in Florida Bay and on the west side of Andros Island in the Bahamas, which may have been exposed by tidal change. The carbonate mud is produced through the life cycle of green algae, but fossils are not particularly common in these lithologies. Deposits of these Lower Carboniferous limestones are frequently used as quarry stones as well. In the upper portion of the Lower Carboniferous, marine cycles are developed, probably reflecting the beginning of mountain building in the Appalachian region of eastern North America. Quartz sandstones typically appeared at the outset of each of these cycles as the seas transgressed across the continental interiors. Shales may succeed the sandstones, followed by limestone development, which suggests the clearing of the water and the establishment of carbonate production by animals and plants. Limestones of Lower Carboniferous age are typically associated with lenses and beds of chert (silicon dioxide). The origin of this chert is somewhat problematic, but it appears to be of either primary or secondary origin. Chert of both origins may occur within a single limestone unit but reflect different times of silicification. Primary cherts form penecontemporaneously with the deposition of the limestones in slightly deeper water settings. Secondary chert forms as a later replacement by groundwater usually involving shallower water deposits. Primary cherts are frequently dark-coloured (flint) and disrupt the bedding rather than follow it. They usually lack fossils. Secondary chert is light-coloured, follows the bedding, and is usually fossiliferous. In deeper-water carbonate regimes on the margins of the continents, limestones become finer-grained and the biotic component less readily recognizable; crinoids and bryozoans are only a minor component. These deposits are termed the Waulsortian facies, and mounds cored by carbonate mud formed on ramps that extended from the shelf areas into deeper water. Famous exposures of such mounds occur in the Franco-Belgian Basin near Namur in south-central Belgium; southwestern Ireland, particularly County Galway; the Craven basin in northern England; and the Sacramento Mountains of southern New Mexico in the United States. The Waulsortian mounds lack an obvious baffling or framework-building organism that would have formed these cores, although cryptostomous bryozoans have been observed in mounds in New Mexico and crinoid halos are associated with mounds in both Europe and North America. In contrast to the shallow-shelf areas that received carbonate sediments, the deeper intracontinental basins and geosynclines are characterized by terrigenous clastics. In Europe these clastics are termed the Culm facies, and they may also contain volcanic units. In North America the eastern continental margin received thick sequences of coarse clastics derived from the highlands to the east that were formed during the late Devonian. Turbidites of Lower Carboniferous age were deposited in both the OuachitaMarathon and Cordilleran geosynclines. Turbidites, as well as marine and terrestrial clastics of Lower Carboniferous age, have been reported from South America, Australia, and both North and South Africa. Coal-bearing deposits occur in portions of Scotland, Belgium, China, Russia (Siberia), and Kazakstan. Evaporite deposits and red beds were formed as part of the Lower Carboniferous record of the Maritime Provinces of Canada. Igneous activity, particularly volcanism and granitoid intrusions, characterizes the Lower Carboniferous record of Australia. The Clyde Plateau lavas of Scotland also are of Lower Carboniferous age.