Carboniferous Period

Interval of geologic time 354–290 million years ago, marked by great changes in world geography.

All the landmasses drew closer together as a result of tectonic plate movements. The supercontinent Gondwana occupied much of the Southern Hemisphere. By the end of the period, present-day North America, Greenland, and northern Europe were also part of Gondwana. Siberia and China (including Southeast Asia) remained individual continents located at high latitudes in the Northern Hemisphere. During this period, swamp forests became widespread, and enormous coal deposits formed. Plants made great advances in the complex forests, and vertebrates were undergoing extensive evolution. Amphibians became widespread and diverse, and reptiles appeared for the first time and rapidly adapted to many habitats.

* * *

▪ geochronology

Introduction

 fifth interval of the Paleozoic Era, succeeding the Devonian Period and preceding the Permian Period. In terms of absolute time, the Carboniferous Period began approximately 359.2 million years ago and ended 299 million years ago. Its duration of approximately 60 million years makes it the longest period of the Paleozoic Era and the second longest period of the Phanerozoic Eon. The rocks that were formed or deposited during the period constitute the Carboniferous System. The name Carboniferous refers to coal-bearing strata that characterize the upper portion of the series throughout the world.

 The Carboniferous Period is formally divided into two major subdivisions—the Mississippian (Mississippian Subperiod) (359.2 to 318.1 million years ago) and the Pennsylvanian (Pennsylvanian Subperiod) (318.1 to 299 million years ago) subperiods—their rocks recognized chronostratigraphically as subsystems by international agreement. In Europe, the Carboniferous Period is subdivided into the Dinantian and suceeding Silesian subsystems, but the boundary between those divisions is below the internationally accepted Mississippian-Pennsylvanian boundary.

The Carboniferous environment

paleogeography

 The Early Carboniferous (Mississippian (Mississippian Subperiod)) world is characterized by Laurussia, a series of small cratonic (craton) blocks that occupied the Northern Hemisphere, and Gondwana, an enormous landmass made up of present-day South America, Africa, Antarctica, Australia, and the Indian subcontinent in the Southern Hemisphere. Lithospheric plate movement brought the continents close together on one side of the globe. The orogenies (orogeny) (mountain-building events) taking place during the Devonian Period had formed the “ Old Red Sandstone” continent. The principal landmass of Laurussia was made up of present-day North America, western Europe through the Urals, and Balto-Scandinavia. Much of Laurussia lay near the paleoequator, whereas the cratons (craton) of Siberia, Kazakhstania, and most of China existed as separate continents occupying positions at high latitudes. During this time, the Tethys Sea separated the southern margin of the Old Red Sandstone continent completely from Gondwana.

      By Late Carboniferous (Pennsylvanian (Pennsylvanian Subperiod)) times, plate movements had brought most of Laurussia into contact with Gondwana and closed the Tethys. Laurussia and Gondwana became fused by the Appalachian (Appalachian orogenic belt)-Hercynian orogeny (Hercynian orogenic belt) (mountain-building event), which continued into the Permian Period. The position of the landmass that would become the eastern United States and northern Europe remained equatorial, while the China and Siberia cratons continued to reside 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 continental shelf margins surrounding the continents. Fringe areas of Carboniferous continents may very well have become the continental interiors of the present day. Deeper troughs (geosynclines (geosyncline)) lay seaward of the continental masses, and their sedimentary (sedimentary rock) record is now characterized by mountains.

Paleoclimate (paleoclimatology)

      During the Carboniferous Period, the climate of various landmasses was controlled by their latitudinal position. Since prevailing wind patterns were similar to those on Earth today, tropical conditions characterized the equatorial regions; the midlatitudes were dry, and higher latitudes were both cooler and moist. Although both western Europe and Balto-Scandinavia resided in latitudes low enough to produce evaporite (minerals in sedimentary rock deposits of soluble salts resulting from the evaporation of water) deposits in shallow continental settings, only North America occupied an equatorial setting during the Mississippian. Wetter areas on other continental blocks in higher latitudes of the Northern Hemisphere began to form coal swamps during this time.

 In contrast, the bulk of Gondwana was below 30° South latitude and experienced colder conditions that allowed the formation of continental glaciers (glacier). These glaciations were similar to those occurring in the Northern Hemisphere during the Pleistocene Epoch. Coeval (parallel) continental glaciations did not occur in the high latitudes of the Northern Hemisphere, probably because the landmasses were too small to sustain large ice fields.

      Paleogeography continued to control climates into Pennsylvanian times. As sea-level cyclicity became more pronounced, the equatorial settings changed from carbonate shelves to coal basins. Continental glaciation expanded in Gondwana, allowing glaciers to extend into lower latitudes that otherwise might have formed coal swamps. As mountains developed toward the close of the Pennsylvanian, rain-shadow effects became more influential, inhibiting the process of coal deposition in the basins of western Europe. Flora of the Carboniferous followed the same climatic gradients as glacial deposits. Fossil plants found in areas located in high latitudes during the Carboniferous exhibit seasonal growth rings, while those of the presumed equatorial coal swamps lack such rings, as do modern tropical trees.

Carboniferous life

Invertebrates (invertebrate)

      The Carboniferous was a time of diverse marine invertebrates. The Late Devonian Period experienced major extinctions within some marine invertebrate groups, and Carboniferous faunas reflect a different composition from what had prevailed earlier in the Paleozoic Era.

      Most notably, reef-forming organisms, such as tabulate corals (coral) and stromatoporoids (stromatolite) (large colonial marine organisms similar to hydrozoans), were limited. Consequently, Carboniferous reefs were poorly developed because of this lack of framework builders. Benthic, or sea-bottom, marine communities were dominated by the crinoids (crinoid), a group of stalked echinoderms (echinoderm) (invertebrates characterized by a hard, spiny covering or skin) that still lives today. These animals were solitary suspension feeders that grew in such great profusion that they affected bottom currents and water circulation. The calcareous (containing calcium carbonate) remains of these organisms are significant rock-forming materials.

  A related, but extinct, group of stalked echinoderms, the blastoids (blastoid), also characterize Carboniferous deposits. Areas favorable for crinoids and blastoids were occupied by other filter-feeding organisms. Colonies of stenolaemate bryozoans (moss animal) (moss animals) and articulate brachiopods (lamp shells) (lamp shells) are common associates of the crinoids. The bryozoans attached their undersurfaces to the seafloor and formed either fanlike, twiglike, or small knobby colonies of calcium carbonate in areas characterized by low rates of sedimentation. Articulate brachiopods formed a bivalved shell of calcium carbonate that either rested free on the seafloor or was attached by a fleshy stalk. The brachiopods and bryozoans both pumped the water column and removed food and oxygen by a tentacular lophophore (lophophorate) (a horseshoe-shaped feeding organ). Brachiopods are particularly common, and all orders except the Pentamerida are found in Carboniferous rocks. Both calcareous and agglutinate foraminifers (foraminiferan) (pseudopod-using unicellular organisms protected by a test or shell) are represented in Carboniferous deposits, particularly limestones (limestone).

      In the Pennsylvanian, an unusual group of these protozoans (protozoan) (single-celled eukaryotic organisms), the fusulinids (fusulinid) (single-celled amoeba-like organisms with complex shells), appeared and dominated assemblages through the Permian Period, when they became extinct. The fusulinids secreted a tightly coiled calcareous test that was chambered, but they lived free on the seafloor.

      Some benthic organisms that were common to early and middle Paleozoic times began to decline during the Carboniferous. These included the trilobites (trilobite) (which became extinct at the end of the Permian), rugose corals, and sponges (sponge). The pelagic, or water column, environment was inhabited by a profusion of cephalopods (cephalopod). These included both straight and coiled nautiloids (early relatives of the chambered nautilus), the ammonoids (ammonoid) (extinct members of the same class), and the first squids. Carboniferous cephalopods were either predators or scavengers, and they swam by jet propulsion. Some of the straight nautiloids grew exceedingly large (greater than 3 metres [10 feet]). The ammonoids exhibit rapid evolutionary development through the Carboniferous and, along with the calcareous foraminifers, provide the biostratigraphic data for age dating and correlating the boundaries and various subdivisions of the period. Graptolites (graptolite) (small colonial planktonic animals) extend into the Carboniferous, but they became extinct during the Mississippian.

      Insects (insect) had occupied terrestrial environments since the Devonian, but they diversified during the Carboniferous Period. No winged insects are known from Devonian or Mississippian times, but wings (wing) probably evolved during the Mississippian. By the Pennsylvanian subperiod, dragonflies (dragonfly) and mayflies were abundant and had reached large sizes. Fossils of more advanced insects capable of folding their wings, particularly cockroaches (cockroach), are well represented in rocks of the Pennsylvanian subperiod. Other Pennsylvanian insects include the ancestral forms of grasshoppers (grasshopper) and crickets (cricket) and the first terrestrial scorpions (scorpion).

Plants

 Carboniferous terrestrial environments were dominated by vascular land plants (tracheophyte) ranging from small, shrubby growths to trees exceeding heights of 100 feet (30 metres). The most important groups were the lycopods (lycophyte), sphenopsids (plant), cordaites (Cordaitales), seed ferns (seed fern), and true ferns (fern). Lysopods are represented in the modern world only by club mosses (club moss), but in the Carboniferous Period they included tall trees with dense, spirally arranged leaves. Reproduction involved either cones or spore-bearing organs on the leaves. Lepidodendron, with diamond-shaped leaf bases, and Sigillaria, with ribs and round leaf bases, were the dominant lycopod genera. They have produced fossil logs that exceed 1 metre (3.3 feet) at their bases. Sphenopsids are trees and shrubs with a distinctly jointed stem and leaves arranged in spirals from those joints. The horsetail rush (rush) (Equisetum) is the only living representative, but Carboniferous floras contained several members of the group. Calamitesis was the most common Carboniferous genus. Although small in comparison with lycopods, Calamitesis grew in profusion in drier, more upland environments.

      Cordaites (Cordaitales) are extinct members of the gymnosperms (gymnosperm) (nonflowering vascular plants), and they were the precursors to the conifers. They also favoured upland environments, where they grew tall and possessed tiny scalelike leaves and cones similar to modern conifers. Walchia is a typical genus that probably grew in forested areas in stands similar to where modern pines would grow.

      Seed ferns, or pteridosperms, are gymnosperms with fernlike foliage, but they reproduce by using seeds rather than spores. They have no living representatives. This group includes trees, such as the Permian genus Glossopteris, but they are represented in Carboniferous floras by taxa such as Neuropteris and Pecopteris, both of which were low scrubs. Both seed ferns and true ferns formed the underfoliage associated with most Carboniferous coal swamps.

Fishes (fish)

      The diversity of fish from the Devonian continued into the Carboniferous in both marine and freshwater environments. The arthrodires (arthrodire) (armoured, jawed fish) became extinct almost immediately in the Mississippian, while both the chondrichthians (chondrichthian) (cartilage skeleton) and osteichthians (bony fish) (bony skeleton) are represented throughout the Carboniferous. The common Carboniferous shark, Cladoselache, dominated marine settings, which also included the bradyodonts (a group of shell-crushing, pavement-toothed cartilaginous fish). A freshwater shark, Orthacanthus, is also known from Pennsylvanian freshwater deposits in both Europe and North America. Osteichthians also occupied freshwater environments. These included the crossopterygians (crossopterygian) (lobe-finned fishes), Dipnoi (lungfishes), and palaeoniscoids (small ray-finned fishes). In the Devonian, the crossopterygians and dipnoi were the dominant forms, but the palaeoniscoids dominated the Carboniferous assemblages. All these groups have living relatives.

Amphibians (amphibian) and early reptiles (reptile)

 The Carboniferous Period was the time of peak amphibian development and the emergence of the reptiles (reptile). Among the amphibians, the labyrinthodonts (labyrinthodont) are represented by members of order Embolomeri, such as Calligenethlon, Carbonerpeton, and Diplovertebron, and members of family Eryopoidae, such as Eryops, Arkserpeton, and Amphibamus. These forms had large skulls, small trunks, and stocky limbs. They were derived from the crossopterygians that emerged in the Devonian from freshwater settings. Embolomerians and Eryopodaens probably remained close to water, favouring the abundant coal swamps of the time. None of these forms have living representatives.

      Another group of unusual forms, the lepospondylians, resembled amphibians but are not included within the class in most recent classifications. This group contained a great variety of semiaquatic forms such as the snakelike Ophiderpeton, the “horned” Keraterpeton, and the microsaurs, such as Asaphestera. The lepospondylians became extinct during the Pennsylvanian subperiod.

      The development of the reptiles was characterized by the improvement of terrestrial reproductive systems during the Carboniferous, a feature not preservable in the record as such. The fossil record of early reptiles is poor, particularly in comparison with that of Permian and Mesozoic (Mesozoic Era) times. The earliest reptiles, the captorhinid Hylonomus, were recovered from Lower Pennsylvanian lycopod tree stumps in Nova Scotia. These small animals, which measure less than 0.3 metre (1 foot), were preserved after becoming entrapped in the cavities left by rotting trees.

Carboniferous rocks

Major subdivisions of the Carboniferous System

 The Carboniferous System is divided into two subsystems—the Mississippian and Pennsylvanian—and seven depositional stages that also correspond to time units (ages) of the same name. The Mississippian subsystem is made up of the Tournasian (359.2 to 345.3 million years ago), Viséan (Viséan Stage) (345.3 to 326.4 million years ago), and Serpukhovian (Serpukhovian Stage) (326.4 to 318.1 million years ago) stages, whereas the Bashkirian (Bashkirian Stage) (318.1 to 311.7 million years ago), Moscovian (Moscovian Stage) (311.7 to 306.5 million years ago), Kasimovian (Kasimovian Stage) (306.5 to 303.9 million years ago), and Gzhelian (Gzhelian Stage) (303.9 to 299 million years ago) stages make up the Pennsylvanian subsystem.

Distinctive features

      The Carboniferous Period was a time of highly variable depositional settings, characterized by both shallow marine and continental environments. In the Northern Hemisphere, the Lower Carboniferous (Mississippian time) is typified by shallow-water limestones (limestone), while the Upper Carboniferous (Pennsylvanian time) possesses cyclical sedimentary deposits (sedimentary rock) that reflect an alternation of marine and nonmarine conditions and a frequent occurrence of coal swamps. In contrast, the Southern Hemisphere experienced widespread continental glaciations during much of these same intervals.

      Plants and animals were diverse during the Carboniferous and had a decisive effect on the accumulation of sedimentary materials. Most Mississippian limestones are composed of the disarticulated remains of stalked echinoderms (echinoderm) (invertebrates characterized by a hard, spiny covering or skin) known as crinoids (crinoid). Bryozoans (moss animal) (moss animals) and brachiopods (lamp shells) (lamp shells) were also both common and diverse during this time. Coals and the associated rock strata of the Pennsylvanian subperiod contain abundant remains of unusual vascular plants (plant), such as the sphenopsids (Equisetopsida), lycopods (club moss) (or lycopsids), and seed ferns (seed fern). More coal was formed during Pennsylvanian times than at any other time in the entire geologic record. In the Southern Hemisphere a cold-climate flora, typified by seed ferns, dominated upland environments and became a source of coal deposits as well. Amphibians (amphibian), which appeared in the Devonian Period, were joined on land by a great variety of insects (insect). In addition, the first reptiles (reptile) appeared in the late portion of the Pennsylvanian.

      Pulses of orogenic activity (orogeny) (mountain building) occurred in the Cordilleran (Rocky Mountain) region of North America, in the Hercynides and Ural Mountains of Europe, and in Asia and Africa. The preservation of these physical and biological events is one of the most extensive in the entire Phanerozoic (Phanerozoic Eon) record, and Carboniferous strata are well exposed on all continents.

Significant geologic events

 The Carboniferous was a time of relative continental stability. Continental margins, and some continental interiors, such as that of North America, were covered by shallow, epicontinental seas that resulted in the development of the most extensive carbonates since those occurring in the lower latitudes of the Ordovician Period. No mountain building is associated with the Mississippian, but the isostasy following the orogenies that closed the Devonian Period certainly affected the continental blocks of Laurussia (a series of small cratonic (craton) blocks that occupied the Northern Hemisphere) and Gondwana (an enormous landmass made up of present-day South America, Africa, Antarctica, Australia, and the Indian subcontinent).

      Widespread continental ice sheets had developed in Gondwana to the point that their expansion and contraction began to drive the rise and fall of sea level, producing cyclic depositions that characterize the later part of the Carboniferous. A major drop in sea level produced the unconformity (an interruption in the deposition of sedimentary rock) at the Mississippian-Pennsylvanian boundary and an associated global extinction event, particularly among the crinoids and ammonoid cephalopods. The Pennsylvanian record reflects the continued Gondwanan glaciations, which produced the extensive coal cyclothems (cyclothem) (repeated sequences of distinctive sedimentary rock layers) throughout the Northern Hemisphere. These cyclothems produced the most extensive coal deposits in the entire geologic record. The Pennsylvanian concludes with the Ouachita (Ouachita orogeny)-Alleghenian (Alleghenian orogeny)-Herycnian orogeny, which developed mountains through the collision of the major landmasses of Laurussia and Gondwana.

      The movement of Gondwana toward the paleoequator closed the remaining salient of the Tethys Sea and formed the Ouachita Mountains (Arkansas, Oklahoma, Texas), southern Appalachians (southeastern United States), Hercynide Mountains (southern Europe), and Mauritanide Mountains (northern Africa). These events continued the creation of a supercontinent, Pangea, that would finally end in the Permian Period.

      Less-significant orogenic events occurred in the Cordilleran (Cordilleran Geosyncline) region of present-day North and South America with a pulse of the Antler orogeny that elevated the ancestral Rocky Mountains, precordilleran movements in western South America, and similar tectonism in northern Asia (as in the Tien Shan and Kunlun ranges), Australia, and New Zealand. The ancestral Ural Mountains also date from this time.

Economic significance of Carboniferous deposits

      Most of the great coal basins of the world are of Carboniferous age. These include the famous coal basins of central and eastern North America, England, western and eastern Europe, Russia, Ukraine, China, and Australia. Coal powered the Industrial Revolution, and mining has exploited coal in most of these regions for more than 200 years. In addition, major accumulations of natural gas and liquid petroleum are associated with the strata of the Carboniferous Period throughout the Northern Hemisphere. Furthermore, where Carboniferous strata (both limestones and sandstones (sandstone)) occur near the surface, they are utilized for high-quality building stone and aggregate for the manufacture of concrete and asphalt. Other commercially valuable materials derived from Carboniferous rocks include fertilizer and refractory clays (clay) and gypsum.

Occurrence and distribution of Carboniferous deposits

      The Mississippian is characterized by shallow-water limestones deposited on broad shelves occupying most continental interiors, particularly in the Northern Hemisphere. Turbidite facies, deep-water sandstones, and shales (shale) deposited as submarine fans by ocean floor currents formed in deeper troughs (geosynclines (geosyncline)) along continental margins. Terrigenous clastic facies (sedimentary rock exposures composed of fragments of older rocks), such as sandstone and shale, are more poorly developed during this time, and coals are rare. The Southern Hemisphere preserved a similar record of carbonates (carbonate rock) until, during the later portion of the Mississippian, cold-water conditions prevailed and terrigenous clastics predominated.

      The Pennsylvanian strata of the Northern Hemisphere are characterized by cyclothemic deposits reflecting the alternating advance and retreat of shallow seas into continental interiors. These widespread deposits included both terrigenous clastics and limestones. Nonmarine strata typically became coal beds, and Pennsylvanian cyclothems contain the major portion of world coal reserves. Oceanic troughs continued to receive clastic facies, particularly turbidites (sedimentary rock formed by a turbidity current), and pulses of mountain-building began to markedly affect the depositional sequences and their thicknesses. In the Southern Hemisphere, glacial deposits reflecting the Gondwanan continental glaciation were common, although shelf deposits of terrigenous clastics and even limestones were present in some areas. All Gondwanan strata reflect cold-water conditions.

Mississippian limestones

      Mississippian limestones are composed of the disarticulated remains of crinoids (crinoid). Upon their death, the plates of individual crinoids accumulated as sand-sized sediment on the seafloor to be cemented later by calcium carbonate. Crinoid fragments were frequently reworked by currents, and their associated deposits exhibit both cross-bedding and ripple marks. Deposits of crinoidal limestone approaching 150 metres (500 feet) are not uncommon in intervals of Mississippian age, particularly in North America, and they are exploited as quarry stone. In addition to the crinoidal limestones, oolitic (oölite) limestones and lime mudstones (mudstone) also formed in shallow-water marine environments of the Mississippian. Ooliths are concentric spheres of calcium carbonate inorganically precipitated around a nucleus. They were deposited on warm marine shelf margins receiving high wave energy similar to the present-day Bahama Shelf and northern Red Sea. These deposits also exhibit cross-bedding and ripple marks reflective of high-energy conditions. Mixtures of ooliths and abraded fossil fragments, particularly foraminifers (foraminiferan) (pseudopod-using unicellular organisms protected by a test or shell), are common in the Mississippian strata.

      Lime mudstones reflect quiet shallow-water environments, such as are found in Florida Bay and on the west side of Andros Island, Bahamas, that may have been exposed by tidal change. The carbonate mud was produced through the life cycle of green algae, but fossils are not particularly common in these lithologies. Deposits of these Mississippian limestones are frequently used as quarry stones as well. In the upper portion of the Mississippian, marine cycles are developed, probably reflecting the beginning of mountain-building in the Appalachian region of eastern North America. quartz sandstones typically began each of these cycles as the seas transgressed across the continental interiors. Shales may have succeeded the sandstones and were followed by limestone development reflecting the clearance of the water and the establishment of carbonate production by animals and plants.

      Limestones of Mississippian age are typically associated with lenses and beds of chert (chert and flint) (silicon dioxide). The origin of this chert is somewhat uncertain, but it appears to reflect either primary or secondary origin. Chert of both origins may occur within a single limestone unit but reflect different times of silicification. Primary cherts formed penecontemporaneously (with small folds and faults) with deposition of the limestones in slightly deeper water settings. Secondary chert formed as a later replacement by groundwater usually involving shallower water deposits. Penecontemporaneous cherts are frequently dark coloured (flint) and disrupt the bedding rather than follow it. They usually lack fossils. Later chert is light coloured, follows the bedding, and is usually fossiliferous.

      Deeper, intracontinental basins and deep ocean troughs (geosynclines) are characterized by Mississippian terrigenous clastics deposited as turbidites.

Pennsylvanian cyclothems (cyclothem), tillites (tillite), and turbidites

      Cyclothems occur on a worldwide basis throughout Pennsylvanian strata; however, they have been most widely studied in North America. The cyclothems display one of two types of development. In the eastern interior of North America, where they were first studied, one cyclothem may consist of as many as 10 separate beds reflecting a single advance and retreat by shallow seas. The lower portion of the cyclothem is predominantly nonmarine and consists of (in ascending order) a sandstone, shale, “freshwater” limestone, underclay (buried soil), and a coal bed. The upper portion of the cyclothem reflects marine conditions and exhibits alternating shale and limestone beds, both of which usually contain fossils. The nonmarine sequence probably represents deltaic conditions associated with the regression that allowed swamp conditions to develop on a delta plain. Marine transgression began with the shale beds overlying the coal. Rapid retreat ends each cycle, which is capped by an unconformity. Most cyclothems are incomplete; they do not exhibit the full sequence of beds.

      Cyclothems of the Appalachian Basin coal fields in Ohio, Pennsylvania, and West Virginia typically have good representation of the nonmarine portion of the sequence with thick coals (coal). These coals formed from the carbonization of plant debris, and it is generally held that one metre of coal equals the compaction of approximately five times as much plant material. Some coals exhibit remarkable thicknesses. The Mammoth coal bed of the Anthracite Belt in eastern Pennsylvanian has an average thickness of 10–12 metres (35–40 feet) throughout its extent. The Pittsburgh seam in western Pennsylvania averages 4 metres (13 feet) thick and is reported workable over 15,540 square km (6,000 square miles). More than 60 coal seams have been identified in Pennsylvania, although only about 10 have ever been exploited. Coeval cyclothems in the western midcontinent have better development of the marine portion of the sequence with fewer and thinner coals.

      In contrast to the coal cyclothems, predominantly marine intervals of Pennsylvanian age in the western midcontinent (Kansas, Iowa, and Missouri) exhibit cyclothems involving alternations of limestone and shale. These cyclothems also reflect transgression and regression by shallow seas, but the lower portion of the cycle is the transgressive event, followed by regression in the upper part. The cyclothem begins with a sandy shale containing marine fossils. It is succeeded by dark, carbonate mudstones that are, in turn, overlain by black shale. The black shale marks the maximum marine transgression. Above the black shale, marine carbonate mudstones and grainstones occur, followed by a return to sandy shale. One striking feature of both coal and marine cyclothems is the tremendous lateral persistency of beds within the sequence. Tracing of a single bed from outcrop to outcrop over a distance of hundreds of kilometres is not uncommon in the midcontinent.

      Depositional cycles similar to those of eastern North America can be recognized in Europe, but distribution of those sediments is confined to small isolated basins instead of a broad cratonic shelf. Nonmarine sequences predominate, and indeed some sequences exhibit no marine influence at all. Positioning of the basins is the result of folding and faulting reflective of the mountain-building of the Hercynian orogenic belt. The Middle and Upper Carboniferous record of western Russia and Ukraine is similar to that of North America.

      In the Southern Hemisphere, there is a marked cooling event beginning near the boundary separating the Mississippian and Pennsylvanian subperiods. Faunas and floras after that time are highly provincial, impoverished, and adapted to the cold climates that persisted into the Permian Period. Pennsylvanian glacial deposits of the Gondwana Realm are characterized by tillites (tillite) resting on polished and striated bedrock surfaces. Striated cobbles, glacio-fluvial deposits, and varved (deposited in still water) lacustrine (lake) sediments occur over large areas of present-day South America, Africa, India, Australia, and Antarctica. The extensive development of these unusual deposits has been used to support the theory of continental drift. The timing of these glacial episodes is still uncertain, and they may have actually begun during Mississippian times. Furthermore, many glacial advances and retreats occurred that were not necessarily simultaneous over the whole of Gondwana.

      Areas marginal to continental masses continued to receive turbidites, particularly in the Ouachita-Marathon region of Arkansas, Oklahoma, and Texas and the Cordilleran geosyncline in the western United States. Evaporites (evaporite) formed in restricted basins, such as those in Montana-North Dakota (Williston) and the Four Corners area in Utah-Colorado (Paradox Basin), that lay near the Pennsylvanian paleoequator. Igneous (igneous rock) and metamorphic rocks (metamorphic rock) of Pennsylvanian age reflect the Hercynian orogeny and its equivalents in North America, Europe, and North Africa.

Correlation of Carboniferous strata

Mississippian (Mississippian Subperiod) subsystem

      The type region for the Mississippian Subsystem lies in the central Mississippi Valley of the United States. Most of the formations representing the type sequence are found in Missouri, Iowa, and Illinois. The Kinderhookian Series includes the Hannibal Formation and the Chouteau Group. It is succeeded by the Osagean Series, which includes the Burlington Limestone and overlying Keokuk Limestone. The Meramecan and Chesterian Series overlie previous layers. Other well-known Mississippian units in North America include: the Pocono Group and Mauch Chunk Shale of the Appalachian region; Fort Payne Chert of Tennessee and Alabama; the Caney and Goddard shales of the Arbuckle region, Oklahoma; the Stanley Shale of the Ouachita Mountains of Arkansas and Oklahoma; the Madison Group and Big Snowy Groups of the northern Rocky Mountains; Redwall Limestone of the Grand Canyon region; and the Lisburne Group of the Brooks Range of northern Alaska.

      Mississippian units exposed at the famous Avon Gorge section at Bristol, Eng., include (in ascending order): Shirehampton beds, Lower Limestone Shale, Black Rock Limestone, Gully Oolite, Clifton Down Mudstone, Goblin Combe Oolite, Clifton Down Limestone, Hotwells Limestone, and the Upper Cromhall Sandstone. Other well-known Mississippian formations outside North America include: limestones at Waulsort and the Black Marble of Dinant, Belg.; Montagne-Noire of the French Massif; and limestones in Spain, the Ural Mountains, the Moscow Basin in Russia, and the Donets Basin of Ukraine.

Pennsylvanian subsystem (Pennsylvanian Subperiod)

      The type region for the Pennsylvanian Subsystem is located in central West Virginia. There the interval is represented by the following groups or formations (ascending order): Pocahontas, New River, Kanawha, Charleston Sandstone, Conemaugh, Monongahela, and basal Dunkard. Other well-known Pennsylvanian units in North America include: the Jackfork and Johns Valley shales of Oklahoma and Arkansas; the Atoka Formation of Arkansas and Oklahoma; the Supai Group of the Grand Canyon region; the Amsden and Tensleep formations of the northern Rocky Mountains; Fountain Arkose of the central Rocky Mountains; and the Haymond and Gaptank formations of the Marathon region, west Texas. Major coal fields in the United States include the Appalachian region (Pennsylvania, West Virginia, and Ohio), Illinois Basin, midcontinent region (Iowa, Missouri, and Kansas), Arkoma Basin (Arkansas and Oklahoma), and north-central Texas.

      The Pennsylvanian part of the type Carboniferous in Great Britain includes the Millstone Grit and the Coal Measures—names in use since the naming of the system. Local names are applied to specific intervals, and marine horizons, called bands, are named either for their characteristic fossil occurrence (i.e., Listeri Marine Band) or for a geographic locality (i.e., Sutton Marine Band). This process is followed in most areas outside North America. Major Pennsylvanian coal fields occur throughout Europe, especially in the central Pennines (Lancashire coal basin), Scottish border, southern Wales, Great Britain; Franco-Belgian Basin, northern France; Saar-Lorraine Basin, border of France and Germany; central French Massif (St. Etienne and Gard coal basins); Ruhr and Westphalian basins, Germany; Silesian Basin, Poland; Moscow Basin, Russia; and Donets Basin in Ukraine.

Walter L. Manger

Additional Reading

Discussions of the Carboniferous Period are included in Reed Wicander and James S. Monroe, Historical Geology, 5th ed. (2007); William B.N. Berry, Growth of a Prehistoric Time Scale: Based on Organic Evolution, rev. ed (1987); and Steven M. Stanley, Earth and Life Through Time, 2nd ed. (1989). 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 (IUGS), summarizes the Carboniferous geology of all continents: Carlos Martinez Diaz (ed.), The Carboniferous of the World (1983– ).Walter L. Manger

* * *