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Silurian
The third oldest
period of the Paleozoic Era, spanning an interval from about 412 to 438 million
years before the present. The Silurian system includes all sedimentary rocks
deposited and all igneous and metamorphic rocks formed in the Silurian Period.
Both the base and top of the Silurian have been designated by international
agreement at the first appearances of certain graptolite species in rock
sequences at easily examined and well-studied outcrops. See also: Geologic time scale
Divisions
The type area of
the Silurian System is in adjoining parts of
Intervals with
durations shorter than those of the epochs have been recognized in
Paleogeography and
lithofacies
Plate positions
and plate motions during the Silurian significantly influenced the depositional
environments, climates, and life of the period. Integration of data from
remanent magnetism, distributions of reefs and other prominent successions of
carbonate rocks, positions of shorelines, and positions of glacial deposits may
be used to indicate certain features of Silurian paleogeographies and changes in
them through the Silurian (see illus.).
See also: Depositional systems and environments; Facies (geology);
Paleogeography; Paleomagnetism; Plate tectonics
The most
prominent feature of Silurian paleogeography was the immense Gondwana plate. It
included much of present-day South America, Africa, the Middle East, Antarctica,
During the
Silurian, many plates continued the relative northward motion that had commenced
during the mid-Ordovician. As a consequence of these motions, the Avalon plate
(South Wales, southern England, and nearby continental Europe; and the Avalon
Peninsula of Newfoundland and parts of maritime Canada) collided with the
eastern margin of Laurentia (much of modern North America, Greenland, Scotland,
and portions of northwestern Ireland and western Norway) in the latest
Ordovician. As that collision developed, a large mountainous area formed. It was
the source of siliclastics that comprise the Queenston deltaic deposits in
Appalachian rock sequences. It was also the source of siliclastic sediments that
accumulated in Avalon plate sites. Baltica (the Baltic states,
Plate positions
and plate motions as well as topographic features of the plates controlled
depositional environments and lithofacies. These, in turn, significantly
influenced organismal development and distributions.
Much of
high-latitude Gondwana was the scene of continental glaciation in the latest
Ordovician. Initially, the ice melted rapidly, leading to a rapid sea-level rise
that continued from the latest Ordovician into the early Silurian. Central areas
of Gondwana rose after the ice melted, presumably as a result of isostatic
rebound. That rebound was followed by tectonic doming. The consequence of a
rising central Gondwana is reflected in the steady northward and westward spread
of continental and nearshore marine deposits away from land areas and across
continental shelves during the Silurian. The Gondwanan land was the source for
much of the highly organic-rich mudstone that characterizes North African and
Middle Eastern Silurian strata. Isotopic ratios of strontium-87 to strontium-86
may be obtained from brachiopod shells and conodonts. In such analyses, a
relatively higher proportion of strontium-87 to strontium-86 suggests that an
old granitic source land is being eroded. Studies of the ratio of the
strontium-87 to strontium-86 for Silurian brachiopods and conodonts are
consistent with continued erosion of a relatively old granitic source during the
Silurian. As the southern European and modern Alpine plates moved northward to
enter the tropics, they became sites of, initially, cool-water carbonate
deposition. That was followed by reef formation and extensive bahamian-type
carbonate deposition. See also:
Chemostratigraphy; Glacial epoch; Isotope; Strontium
Three brief
glacial episodes have been recognized in the Llandovery stratigraphic record of
the Amazon Basin. Each of these episodes took place essentially at the time of
each of the boundaries between the stage divisions of the Llandovery Series. The
South American stratigraphic record also suggests that a brief glaciation took
place in the late Wenlock, at about the height of impact of the Baltic plate
against Avalon-Laurentia. The South American Silurian record of glaciation
suggests that the modern eastern part of South America lay close to or across
the South Pole at that time.
Plates that were
within the tropics were sites of extensive carbonate deposition. Reefs developed
along and near the margins of most plates that were within the tropics.
Silurian
Northern Hemisphere plates, other than a portion of Siberia, are not known north
of the Northern Hemisphere tropics. Presumably, nearly all of the Northern
Hemisphere north of the tropics was ocean throughout the Silurian.
Ocean
circulation
Absence of
plates bearing continental or shallow shelf marine environments north of about
45° north latitude indicates that ocean circulation in most of the Silurian
Northern Hemisphere was zonal. Ocean currents were relatively strong and flowed
from east to west north of 60° north. Major ocean currents between 30 and 60°
north flowed from west to east. Surface currents would have been deflected south
along the west side of Siberia, and upwelling conditions could have formed in
that area.
Ocean surface
currents in the tropics would have been influenced strongly by the prevailing
westerlies. Primary surface circulation from the Equator to 30° north and south
of it would have been from east to west. The large peninsulalike salient of
Gondwana would have deflected currents northerly along it in the Northern
Hemisphere and southerly along it in the Southern Hemisphere. North China's
eastern shores would have been sites of upwelling. The presence of richly
fossiliferous coralline limestones and volcanics there is consistent with
surface-water turbulence and upwelling conditions along the margins of volcanic
islands. Much the same conditions appear to have prevailed along the margins of
the Kazakh plate.
The large size
of the Gondwana plate and the presence of land over much of it would have led to
development of seasonal monsoon conditions. Monsoonal conditions would have led
to seasonal reversal of major surface circulation adjacent to the land, as is
seen in modern India. In the austral summer–Northern Hemisphere winter, winds
would have blown offshore from the warm land and resulted in an east to west
current flow near 30° south and a west to east surface current near the Equator.
In the Northern Hemisphere summer and austral winter, surface water flow west of
the lands on the Gondwana plate would have reversed. These seasonal reversals in
surface current directions could have created long-term, year-long upwelling in
a pattern similar to that observed in modern oceans off the Somali coast.
Consequently, intense upwelling across northern Africa could have been generated
during the Silurian in a pattern similar to that in the modern Arabian Ocean.
That upwelling generated vast quantities of organic matter which became
organic-rich shales that characterize the North African Silurian. In general,
surface circulation west of tropical Gondwana would have been east to west.
However, surface circulation between the several plates that drifted into the
tropics as well as those that moved closer to each other than they had been
previously would have created many small east-to-west flowing gyres in the
tropics. Modest-to-strong upwelling along many plate margins was likely as a
consequence. The occurrence of numerous long-studied reefs, such as those on the
island of Gotland off the Swedish coast, close to or along tropical plate
margins is consistent with plate margin upwelling.
Surface
circulation south of 30° south would have hit the western side of Laurentia and
flowed generally northward along it to about 30°, at which latitude the surface
currents probably turned to flow east to west between 30° south and the
Equator. See also: Coriolis
acceleration; Paleoceanography; Upwelling
Climate and
hydrology
Collision of the
Avalonian and Laurentian plates in the latest Ordovician coincides with
development of the Southern Hemisphere continental glaciation. Erosion of the
land area formed at the Avalon-Laurentian plate collision generated a large
volume of coarse to fine-grained siliclastic materials. Such extensive erosion
could have reduced the atmospheric concentration of carbon dioxide, creating
climate conditions cool enough to allow glaciation to develop. Although
atmospheric carbon dioxide concentration probably was significantly greater
during the Silurian than it is today, solar heat coming to the Earth was about
4–5% less than today. Thus, depression of the atmospheric carbon dioxide content
could have been enough to allow glaciation. Reduction in the extent and height
of the land being eroded could have resulted in increased atmospheric carbon
dioxide concentration, ending the conditions suited to generating glaciers by
the latter part of the Wenlock.
That part of
South America (modern eastern South America) near the South Pole for the early
part of the Silurian was not just cold, but also the site of as many as four
brief glacial episodes. These episodes took place during the time that the
Avalon and Baltica plates collided with each other and with the Laurentian
plate.
Rainfall would
have been significant on the mountainous lands formed along the
Avalon-Laurentian plate collision boundary because that land lay within the
prevailing winds. See also:
Paleoclimatology
Atmosphere
Analyses of
major rock suites have led to the suggestion that the Silurian global air
temperature was about 9°F (5°C) warmer than today. The postulated global air
temperature is consistent with an estimated carbon dioxide content of three to
four times that of the modern atmosphere. Oxygen content of the Silurian
atmosphere has been estimated to have been about two-thirds to three-fourths the
present content. A lesser oxygen concentration in the atmosphere would have
resulted in less oxygen available to be stirred into the oceans. A lesser oxygen
concentration in the surface oceans coupled with a warmer global temperature
meant that less oxygen was available to marine organisms in the Silurian seas
than is available to modern oceanic organisms. Silurian marine organisms,
especially those living at depths significantly below the surface, must have
survived on significantly less oxygen than do modern organisms. Furthermore, the
ocean oxygen minimum zone would have attained shallower depths than it does in
modern oceans. Oxygen content in ocean waters may have declined to near zero by
330 ft (100 m) in tropical waters. Atmospheric conditions would have had a
significant influence on the distributions of marine benthic dwellers. See also: Atmosphere, evolution of
Land
life
Both nonvascular
and vascular plants continued to develop in land environments following their
originations in the early mid-Ordovician. Many of these Silurian plants were
mosslike and bryophytelike. Plants with vascular tissues had developed in the
mid-Ordovician. These plants continued their spread in terrestrial environments
during the Silurian. Psilophytes assigned to the genus Cooksonia were relatively
widespread in Late Silurian terrestrial environments. The probable lycopod (club
moss) Baragwanathia apparently lived in nearshore settings in modern Australia
during the latter part of the Silurian.
Silurian land
life also included probable arthopods and annelid worms. Fecal pellets of
wormlike activity have been found as well as remains of centipede-, millepede-,
and spiderlike arthropods. See
also: Paleobotany; Paleoecology
Marine
life
Shallow marine
environments in the tropics were scenes of rich growths of algae, mat-forming
cyanobacteria, spongelike organisms, sponges, brachiopods, bryozoans, corals,
crinoids, and ostracodes. Nearshore marine siliclastic strata bear ostracodes,
small clams, and snails and trilobites. Certain nearshore strata bear the
remains of horseshoe-crab-like eurypterids. Some of them may have been
significant predators. See also:
Algae; Brachiopoda; Bryozoa; Crinoidea; Cyanobacteria; Ostracoda; Trilobita
Fish are
prominent in a number of Silurian nearshore and some offshore marine
environments. Jawless armored fish of several kinds occur in Silurian strata.
These fish include many species of thelodonts that had bodies covered with
minute bony scales, heterostracans, and galeaspids that had relatively heavily
armored head shields, and anaspids that possessed body armor consisting of
scales and small plates. Jawed fish were relatively rare in the Silurian. They
were primarily spiny sharks or acanthodians. As well, there are remains of true
sharklike fish and fish with interior bony skeletons (osteichthyes) in Late
Silurian rocks. See also: Anaspida;
Heterostraci; Osteichthyes; Thelodontida
The acanthodians
appear to have been relatively common in the latter part of the Silurian. The
oldest known placoderms were found in Silurian strata in South China. Fish began
to diversify in the latter part of the Silurian in many shelf sea environments.
The planktic colonial marine graptolites are the prominent organism found in
rocks that formed under anoxic or near-anoxic waters. Their remains are most
plentiful in rocks that accumulated on the outer parts of shelves and in basins
of the Silurian. See also: Anoxic
zones
The extinct
microfossil group, the conodonts, were relatively common in many carbonates
deposited in shelf seas. Small, slender shells of squidlike cephalopods occur in
many shelf-sea rock suites, including certain of the black, organic-rich
graptolite-bearing sequences. These cephalopods appear to have been nektic in
habit. See also: Cephalopoda;
Conodont
Biogeography
Both land plants
and marine animals were distributed in patterns reflective of latitudinal
temperatures during the Silurian. Those organisms living along the margins of
the Gondwana plate in the general position of 30 to 45° south latitude
constituted one floral and faunal realm, the Malvinokaffric, which persisted
throughout the Silurian. Organisms that typify it were adapted to cool climates.
Land plants and
marine organisms living during the early part of the Silurian (Llandovery into
Wenlock) in warm temperate to tropical conditions outside of the cool temperate
to polar Malvinokaffric Realm were essentially cosmopolitan. This distribution
reflected sparsity of tropical marine life in the tropics after the major Late
Ordovician into earliest Silurian extinctions among marine organisms. Both land
plants and marine organisms were increasingly provincial during the latter part
of the Silurian. Marine bottom-dwelling invertebrates living on shelves of
plates in the tropics were noticeably provincial by the close of the Silurian.
The northern part of the Siberian plate was characterized during the latter part
of the Silurian by a unique association of shelled marine invertebrates of which
the brachiopod Tuvaella is characteristic. Laurentia-Baltoscanian shallow marine
environments were inhabited by marine bottom-dwelling faunas, of which the
brachiopod Salopina has been selected to give its name. The Kazakh plate shelf
seas had a unique, endemic marine invertebrate fauna.
Nearshore marine
fish had distributions similar to those of the marine bottom-dwelling
invertebrates. Silurian fish from South China are an exception. They constituted
a distinct faunal province.
Among Late
Silurian land plants, the truly tracheophytic Baragwanathia known from eastern
Australia was distinct from coeval land plants known in other parts of the
tropics. Although sparse, tropical land plants appear to have had distributions
similar to those of the marine organisms. This evidence suggests that plate
positions and the ocean and wind circulation patterns they influenced were major
factors in the distribution of life during the latter part of the Silurian. See also: Biogeography
Economic
resources
Silurian dark,
organic-rich rocks form one of the six prominent suites of petroleum source
rocks known. The six suites have provided more than 90% of the world's known oil
and gas reserves. Silurian source rocks have generated approximately 9% of the
world's reserves. Most of the Silurian source beds for petroleum and gas are
those that lay under the upwelling waters along the northern and northwesterly
margins of the Gondwana plate. Other known prominent Silurian source rocks are
those that accumulated under upwelling waters on the eastern side of the
Baltoscanian-Avalonian plates and those that accumulated under upwelling waters
along the westerly side of the Laurentian plate.
Sedimentary iron
ores accumulated in tidal wetlands on the Avalonian, especially the West Avalon,
plates during the Llandovery. At that time, the Avalonian plates lay south of
the tropics.
Silurian
carbonate rocks have been quarried in many parts of the world for building stone
and for the raw material for cement. Quarries in the area of the type Wenlock,
at Wenlock Edge in
William B. N.
Berry
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M. G. Bassett,
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W. B. N. Berry and A. J. Boucot, Correlation of the African Silurian Rocks, GSA Spec. Publ. 147, 1973
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W. B. N. Berry and A. J. Boucot, Correlation of the North American Silurian Rocks, GSA Spec. Publ. 102, 1970
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J. Gray and W. Shear, Early life on land, Amer. Sci., 80:444–456, 1992
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J. C. Gutierrez-Marco and Rabano (eds.), Proceedings of the 6th International Graptolite Conference of the GWPG (IPA) and the 1998 Field Meeting of the International Subcommission on Silurian Stratigraphy (ICS-IUGS), Instituto Tecnologico Geominero de Espana, Temas Geologico-Mineros, 23, 1998
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C. H. Holland and M. G. Bassett (eds.), A Global Standard for the Silurian System, Nat. Mus. Wales Geol. Ser. 10, 1989
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E. Landing and M. Johnson (eds.), Silurian Cycles: Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic, and Tectonic Changes, New York State Mus. Bull. 491, 1998
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G. T. Moore et al., A paleoclimate simulation of the Wenlockian (Late Early Silurian) world using a general circulation model with implications for early land plant paleoecology, Palaeogeog. Palaeoclimatol. Palaeoecol., 110:115–144, 1994
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