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Carboniferous
The fifth period
of the Paleozoic Era. The Carboniferous Period spanned from about 355 million
years to about 295 million years ago. The rocks that formed during this time
interval are known as the Carboniferous System; they include a wide variety of
sedimentary, igneous, and metamorphic rocks. Sedimentary rocks in the lower
portion of the Carboniferous are typically carbonates, such as limestones and
dolostones, and locally some evaporites. The upper portions of the system are
usually composed of cyclically repeated successions of sandstones, coals,
shales, and thin limestones. See
also: Sedimentary rocks
The economic
importance of the Carboniferous is evident in its name, which refers to coal,
the important energy source that fueled the industrialization of northwestern
The base of the
Carboniferous is placed at the first appearance of the conodont Siphonodella
sulcata, a fossil that marks a widely recognized biozone in most marine
sedimentary rocks. The reference locality for this base is an outcrop in
Subdivisions
The term
Carboniferous Order was originally applied by W. D. Conybeare and J. Phillips in
1822 to rocks in the
Fig. 1 Development of stratigraphic
nomenclature for the Carboniferous.
In 1891, the
U.S. Geological Survey introduced the terms Pennsylvanian Series for the Coal
Measures and Mississippian Series for the Subcarboniferous. In 1906 T. C.
Chamberlain and R. D. Salisbury raised the Mississippian and Pennsylvanian to
the rank of systems (Fig. 1), noting that they were separated by a major
unconformity and were quite different lithologically in both North American and
in Europe. This nomenclature was extensively adopted in
A third means of
subdividing the Carboniferous was developed during the 1920s and 1930s on the
Russian Platform, the western slopes of the Urals, and the greater
The
International Subcommission on Carboniferous Stratigraphy reached general
agreement in the 1970s and 1980s that the Carboniferous would be divided into
two parts: a Lower Carboniferous Mississippian Subsystem and an Upper
Carboniferous Pennsylvanian Subsystem.
The two
Carboniferous subsystems are subdivided into a number of series and stages (Fig.
2) that are variously identified in different parts of the world, based on
biostratigraphic evidence using evolutionary successions in fossils or
overlapping assemblage zones. Many fossil groups have been studied in order to
establish consistent worldwide zonations; they have included foraminifers,
corals, ammonoid cephalopods, brachiopods, bryozoans, sponges, bivalves,
crinoids, radiolarians, conodonts, and plants. To a large extent the
distribution of each of these fossil groups was determined by climate,
temperature, and other environmental and ecological conditions, evolutionary
adaptations, and paleobiogeographic opportunities for dispersal. As the
Carboniferous progressed, changes in each of these factors caused biotic
redistributions and dispersals and, at other times, restrictions in distribution
and strongly provincial biotic associations. These biogeographic differences
have made detailed correlations between some regions more difficult, and as a
result, there remains strong preferences for regional series and stage
names. See also: Index fossil;
Stratigraphy
Fig. 2 Curves outlining the fluctuations of sea
level during the Carboniferous. The Mississippian sea-level curve is typical of
times when glaciation was not a major factor in controlling sea levels, and the
Pennsylvanian sea-level curve is characteristic of extensive and protracted
glaciation in the Early Pennsylvanian followed by repeated glaciation and
nonglaciation events in the Middle and Upper Pennsylvanian. The time scale is
estimated based on a few reliable radiometric ages for the interval. 1 m = 3.3
ft. (After C. A. Ross and J. R. P. Ross, Late Paleozoic sea level and
depositional sequences and biostratigraphic zonation of late Paleozoic
depostional sequences, Cushman Found. Foraminiferal Res. Spec. Publ., no. 24,
1987)
Paleogeography and
lithology
Perhaps the
strongest of the many ecological factors that controlled biotic distributions
were the paleogeographic changes within the Carboniferous that were brought
about by the initial assembling of the supercontinent Pangaea and the associated
mountain-building activities, which greatly modified climate, ocean currents,
and seaways. In the Early Carboniferous (Fig. 3a), a nearly continuous
equatorial seaway permitted extensive tropical and subtropical carbonate
sedimentation on the shelves and platforms in North America, northern and
southern Europe, Kazakhstan, North and South China, and the northern shores of
the protocontinent Gondwana (such as northern Africa).
The gradual
collision of northern Gondwana against northern Europe–North America (also
called Euramerica or Laurussia) started the formation of the supercontinent of
Pangaea. By the middle of the Carboniferous, this process divided the tropical
equatorial seaway into two isolated segments (Fig. 3b). One, which represented
the beginning of the Paleotethys faunal region, lay to the east of Pangaea as a
large arcuate ocean with a number of small island arcs and oceanic plateaus.
This warm-water ocean with vast west-flowing equatorial currents resulted in
warm-water currents being directed both north and south of their normal
latitudes.
The other
segment of the former equatorial seaway lay to the west of Pangaea and faced a
relatively large, deep
Orogeny
The collision of
Gondwana with North America–northern Europe, in addition to changing the
patterns of equatorial ocean currents, created the long chain of mountains that
made up the Ouachita-Appalachian-Hercynian orogenic belt. This complex set of
mountains also is evident in the sedimentary patterns of clastic rocks, such as
conglomerates, river and shoreline sandstones, and coals and other nonmarine
clastic deposits, as well as in thick turbidite successions in several foredeep
marine basins that formed along the front of the advancing orogenic thrust
belts. Although this chain of mountains was mainly within the tropics and
subtropics, it became very elevated, perhaps similar to the present-day Himalaya
ranges; it disrupted terrestrial climates to a similar degree. See also: Orogeny
Glaciation
An additional
ramification of the formation of Pangaea was the beginning of very extensive
glaciation in the Southern Hemisphere polar and high-latitude regions of the
supercontinent. Glacial deposits are also known from smaller continental
fragments that were at high paleolatitudes in the Northern Hemisphere. The cause
for this great increase in ice accumulation is not entirely known. It may be
that the diversion of warm-water currents into more southerly (and northerly)
latitudes increased precipitation (as snow). Perhaps the mountain heights of the
newly formed orogenic belts disrupted general atmospheric circulation from the
tropics into high latitudes. Solar radiation possibly was reduced a few percent
to initiate these more extensive glacial conditions. Or other, still unknown
factors may be the primary causes. In any case, the Earth's climate cooled,
tropical carbonate-producing areas became restricted toward the Equator, and
eustatic sea-level fluctuations became prominent in the sedimentary record. See also: Glacial epoch
Sea-level changes
The sedimentary
record of sea-level changes is well documented from the beginning of the latest
Mississippian through the Pennsylvanian and into the Permian. These are cyclical
sediments in which the succession of depositional facies is regularly repeated
by each sea-level fluctuation. As a result, these depositional sequences have
great lateral extent and continuity. Because local depositional conditions
depended on many other, often unique circumstances, the sedimentary patterns
have considerable lateral variation (Fig. 4). For example, the cyclical
sediments near the Appalachian orogenic belt contain vast amounts of clastic
debris, such as sands, silts, and gravels, which were transported and deposited
in rivers, lakes, deltas, and coastal features. Many of these cycles are
nonmarine or only marginally marine, and many are viewed as being more strongly
influenced by changes in tectonic activity and climate. The suggestion is that
the amount of clastic material was supplied mainly by erosion, and sedimentation
responded to wet and dry climatic cycles and only indirectly to sea-level
fluctuations. In marine-dominated sequences, supratidal carbonates commonly
passed upward into thin, fluvial and eolian beds that were usually reworked as
debris into the succeeding basal marine transgressive beds. See also: Depositional systems and
environments
Farther west
(Fig. 4) at greater distances from the Appalachian orogenic belt, as in Ohio,
many of these clastic-rich cyclic sediments intertongue with marine sediments,
and the influence of sea-level fluctuations becomes increasingly evident. The
Illinois-Kentucky basin has about equal representation of marine deposits and
fluvial channels and delta deposits. Western Missouri and eastern Kansas were
far enough away from the large influx of clastic materials to have predominantly
marine sediments and minor amounts of nonmarine, mostly silt and fine sand
clastics. Carbonate sedimentation dominated farther from sources of clastic
sediments. Oceanic plateaus in the Paleotethys and Panthalassa ocean basins,
which lacked influxes of clastic materials, have cyclical sediments that are
entirely in various carbonate facies that reflect differences in water depth.
The fluctuations
in sea level during the Carboniferous reflect different rates of sea-level
change, different durations, and different magnitudes. Minor sea-level
fluctuations of a meter or two (3–6 ft) with cyclicities of 20,000–40,000 years
and intermediate fluctuations of 4–6 m (13–20 ft) with cyclicities of about
100,000 years may be preserved. More easily recognized are fluctuations of about
400,000 years–1 million years, which have amplitudes of 60–200 m (200–660 ft)
[Fig. 2]. Late in the Mississippian and early in the Pennsylvanian, for an
interval of approximately 15–20 million years, sea-level high-stands were
consistently low and infrequently reached higher than the continental shelf
margins (Fig. 4), a condition that was not repeated until quite late in the
Permian Period. See also:
Continental margin
The sea-level
fluctuations resulted in large areas of the cratonic shelves being frequently
exposed to long intervals of weathering, erosion, and diagenesis. The effects of
these exposure-related events suggest that within the Carboniferous climates
were not uniform and different types of paleosols (fossil soil profiles) formed
under a variety of pedogenic processes. These paleosols reflect changes in world
climate as well as the continued northward motion of continents across
latitudinal climatic belts as a result of plate tectonics and sea-floor
spreading. See also:
Paleoclimatology; Sedimentology; Weathering processes
Life
During the
Carboniferous, life evolved to exploit fully the numerous marine and nonmarine
aquatic environments and terrestrial and aerial habitats. Single-cell protozoan
foraminifers evolved new abilities to construct layered, calcareous walls and
internally complex tests. These single-celled organisms diversified into nearly
all shallow-water, carbonate-producing environments from the intertidal, lagoon,
shelf, shelf margin, and upper parts (within the photic zone) of the shelf
slope. Ammonoid cephalopods, from their first appearance in the later part of
the Devonian, diversified rapidly during the Early Carboniferous, and their
occurrences were used to establish one of the earliest biostratigraphic zonation
schemes for the Lower Carboniferous. Brachiopods, bryozoans, conodonts,
crinoids, and corals were also widespread and locally important parts of the
marine faunas. The Tournaisian and Visean shallow tropical seas abounded in
blastoid echinoderms; however, these creatures became nearly extinct and were
geographically restricted to a small area in the eastern Panthalassa Ocean after
the Early Pennsylvanian (Morrowan). Conodonts were widespread in deeper-water
deposits.
Insects have
remarkable evolutionary histories during the Carboniferous. They adapted to
flight and dispersed into many terrestrial and fresh-water habitats.
Carboniferous insects include many unusual orders, such as one with three pairs
of wings (although it is not clear that the foremost pair functioned directly in
propulsion). By the Late Pennsylvanian, many large cockroachlike orders were
present and also several huge dragonflylike groups, some of which reached
wingspans of 90 cm (3 ft). Carboniferous insects included representatives of
five Paleoptera orders (mayflies and dragonflylike orders) and at least six
orthopterid (cockroachlike) orders. Insects are commonly preserved in coal swamp
deposits and display the amazing diversity of life within these swamps. See also: Insecta
Vertebrates also
evolved rapidly. Although acanthodian fish declined from their Devonian peak,
sharklike fishes and primitive bony fishes adapted well to the expanded
environments and the new ecological food chains of the Carboniferous. Some
sharklike groups invaded fresh-water habitats, where they were associated with
coal swamp deposits. For the bony fish (ray-finned fish and air-breathing
choanate fishes with lobed fins), the Late Devonian and Carboniferous was a time
of considerable evolutionary diversity and ecological expansion, with many
lineages independently adapting to both fresh-water and marine conditions.
Lungfish are one of the fresh-water choanate lineages which evolved adaptations
for survival in temporarily dry lakes and rivers. Near the
Devonian-Carboniferous boundary, a lineage from the choanate fish had evolved
into the first amphibians. See
also: Vertebrata
Carboniferous
amphibians evolved rapidly in several directions. The earliest were the
labyrinthodont embolomeres, which had labyrinthodont teeth and were mainly
aquatic. Another significant labyrinthodont group was the rhachitomes, which
originated in the Early Carboniferous and became abundant, commonly reaching
about 1 m (3 ft) or more; they were widespread in terrestrial habitats during
the Late Carboniferous and Permian. Some of the Carboniferous amphibians
reverted to totally aquatic habitats, such as the lepospondyls, which lost their
bony vertebrae and limbs, had large flattened broad heads, and were snakelike in
appearance. Ancestors of the present-day anurans (frogs), urodeles
(salamanders), and caecilians (apodans) probably date from the later part of the
Carboniferous, but their record as fossils is meager. See also: Amphibia
Primitive
reptiles evolved from one of the embolomere amphibian lineages during the Late
Carboniferous. They formed the basal stock from which all other reptiles have
evolved including the earliest mammallike reptiles in the Late Carboniferous.
During the Late Carboniferous, early reptiles coexisted with several advanced
amphibian groups which shared at least some, but probably not all, of their
reptilelike characters. See also:
Reptilia
Terrestrial
plants also showed major diversification of habitats and the evolution of
important new lineages during the Carboniferous. Initially, Early Carboniferous
plants were predominantly a continuation of latest Devonian groups; however,
they were distinguished in part by their large sizes with many arborescent
lycopods and large articulates, and pteridosperms (seed ferns) and ferns became
increasingly abundant and varied. By the Late Carboniferous, extensive swamps
formed along the broad, nearly flat coastal areas; and these coal-forming
environments tended to move laterally across the coastal plain areas as the sea
level repeatedly rose. Other coal-forming marshes were common in the floodplains
and channel fills of the broad rivers of upper delta distributary systems.
During the Late Carboniferous, primitive conifers appeared and included
araucarias, which became common in some, probably drier ecological habitats. One
of the features of Late Carboniferous plant paleogeographic distributions is the
recognition of a southern, high-latitude Gondwanan floral province, the
Glossopteris province, and a northern high-latitude Anagaran floral province,
which were cool adapted. These provinces contrasted with an extensive equatorial
belt of much greater plant diversity.
See also: Paleobotany; Paleozoic
C. A. Ross
June R. P. Ross
Bibliography
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R. H. Dott, Jr., and D. R. Prothero, Evolution of the Earth, 5th ed., 1994
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H. L. Levin, The Earth Through Time, 4th ed., 1994
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A. L. Palmer (general ed.), Decade of North American Geology (DNAG) Project, The Geology of North America, Geological Society of America, 1986–1994
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R. C. Moore et al., The
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C. A. Ross and J. R. P. Ross, Late
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J. W. Skehan et al., The Mississippian and Pennsylvanian (Carboniferous) Systems in the United States, USGS Prof. Pap., no. 1110-A–DD, pp. A1–DD16, 1979
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R. H. Wagner, C. F. Winkler Prins, and L. F. Granados (eds.), The Carboniferous of the World (M. C. Diaz, general ed.), Instituto Geologico y Minero de Espana, Madrid, and Nationaal Natuurhistorisch Museum, Leiden, Parts I, II, III, IUGS Publ. 16, 20, 33, 1983, 1985, 1996
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H. R. Wanless and C. R. Wright, Paleoenvironmental Maps of Pennsylvanian Rocks, Illinois Basin and Northern Midcontinent Region, Geol. Soc. Amer. Publ., no. MC-23, 1978
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