SILURIAN PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. interval of geologic time from 438 to 408 million years ago (see Table). It is often divided into the Early Silurian Period (438 to 421 million years ago) and the Late Silurian Period (421 to 408 million years ago). As the third period of the Paleozoic Era, the Silurian Period follows the Ordovician Period and precedes the Devonian. The rocks that originated during the period make up the Silurian System. interval of geologic time from 438 to 408 million years ago. It is often divided into the Early Silurian Period (438 to 421 million years ago) and the Late Silurian Period (421 to 408 million years ago). As the third period of the Paleozoic Era, the Silurian Period follows the Ordovician Period and precedes the Devonian. The rocks that originated during the period make up the Silurian System. The Silurian System was named by Sir Roderick Impey Murchison in 1835 after the Silures, the ancient British tribe that occupied much of central and southeastern Wales, where rocks of the aforementioned age are well developed. Silurian rocks are found all over the world. The Silurian Period marks an important episode in Earth history at the end of early Paleozoic times, with the first appearance of land plants and jawed fish. Important, too, are the graptolites (especially the single-branched monograptids, whose fossils are useful for correlating Silurian rocks in widely separated areas) and many other marine invertebrate groups, such as the trilobites, brachiopods, conodonts, corals, stromatoporoids, crinoids, and mollusks. The upper boundary of the Silurian System is defined, by international agreement, at a locality called Klonk in the Czech Republic. The lower boundary has been fixed at a section of rocks in Dobb's Linn near Moffat in the Southern Uplands of Scotland. In stratigraphy, the Silurian is divided (from oldest to youngest) into the Llandovery, Wenlock, Ludlow, and Pridoli (the equivalent of the British Downtonian) epochs. Though these are regarded as the standard divisions, there are many alternatives in various parts of the world. In North America the Silurian System was formerly divided into the Alexandrian below the Niagaran and the Cayugan above. Silurian rocks are extensively exposed in the Appalachians and across the Midwest, especially in the vicinity of the Great Lakes. Large coral-stromatoporoid-algal reef developments in the middle time interval of the system have often formed oil reservoirs. These roughly correspond, on a much grander scale, to the reef developments on the Swedish island of Gotland and on a smaller scale, to the reef developments on Wenlock Edge in Shropshire in the Welsh borderland of England. Niagaran limestones are probably best known in the escarpment that forms Niagara Falls, between Lakes Erie and Ontario. These limestones are followed, notably in the Michigan Basin, by tremendous thicknesses of salt deposits, which are evidence of a widespread regression of the seas at the end of Silurian times. This regression also is seen in Europe in the form of transitional beds yielding fossils such as the large arthropods known as eurypterids. These salt deposits grade into continental facies known as Old Red Sandstone, which are of Devonian age and are found across northern Europe from Ireland to Russia, and in more distant places such as eastern Canada and Kashmir. During the Silurian Period the continents were distributed in a totally different way than they are today. Such areas as Arctic Canada, Scandinavia, and Australia were probably in the tropics; Japan and the Philippines may have been inside the Arctic Circle; South America and Africa were most likely in the region of the South Pole, with either present-day Brazil or western Africa as the locus of the pole. In any case, the land surface was buried by an ice sheet that may have been about as large as that covering Antarctica today. Volcanic activity was comparatively minor during the Silurian Period, but there were important earth movements, notably the Taconian orogeny (mountain-building event) of North America, which marks the boundary between the Ordovician and the Silurian, and the Caledonian orogeny in northwestern Europe, which probably reached its climax toward the end of Silurian time. The latter orogeny, the effects of which are particularly evident in Scandinavia and the northwestern part of the British Isles, probably resulted from the closing of an early Paleozoic precursor of the Atlantic Ocean. Additional reading C.H. Holland and M.G. Bassett (eds.), A Global Standard for the Silurian System (1989), provides working definitions for systemic, series, and stage boundaries at stratotypes in the United Kingdom and the Czech Republic, summaries of other reference localities, and reviews of the major index fossils. Anders Martinsson (ed.), The Silurian-Devonian Boundary (1977), applies the golden spike concept in defining a chronostratigraphic boundary. James A. Secord, Controversy in Victorian Geology: The Cambrian-Silurian Dispute (1986), presents a historical treatment of Murchison's development of the Silurian system and the conflict that arose therein. A.M. Ziegler et al., Silurian Continental Distributions, Paleogeography, Climatology, and Biogeography, Tectonophysics, 40(12):1351 (1977), offers an especially thorough treatment of Silurian weather systems that remains valid despite minor revisions in paleogeography. A thorough review of issues related to tectonics, paleogeography, sea-level events, reef development, and sedimentary patterns is found in Special Papers in Palaeontology, no. 44 (1990), the whole issue being devoted to the papers of the first international symposium on the Silurian System that took place in early 1989. Markes E. Johnson Silurian environment Paleogeography Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during Early The Silurian world consisted generally of a north polar ocean, a ring of at least six equatorial to middle-latitude continents, and one south polar supercontinent. A combination of paleomagnetic, paleoclimatic, and biogeographic data can be used to reconstruct the approximate orientations of the Silurian continents. The Earth's magnetic field leaves its signature on volcanic rocks and certain sedimentary rocks rich in such minerals as magnetite. As rocks capable of being magnetized were cooled or otherwise lithified, their component crystals or grains conformed to an alignment with the terrestrial magnetic field consistent with their latitudinal origin. Unless the rocks were reheated or reworked by erosion, they should retain this signature regardless of subsequent geographic displacement. The Earth's zonal climate also has an effect on global patterns of sedimentation (see above). Strata formed in arid regions, for example, differ from those formed in regions with high annual rainfall. Endemism provides an index sensitive to continental isolation. The geographic summary that follows is based on a global reconstruction specific to middle Silurian (Wenlock) time (seefigure). Much of North America, including Greenland, northwestern Ireland, Scotland, and the Chukotsk peninsula of northeastern Russia, belonged to the paleocontinent Laurentia. (The name is derived from Quebec's portion of the Canadian Precambrian shield.) With respect to the present-day Great Lakes and Hudson Bay, Laurentia was rotated clockwise during Wenlock time to fully fit within latitudes 30 N and S of the paleoequator. The present south shore of Hudson Bay was at the centre of Laurentia, with the Wenlock paleoequator crossing near Southampton Island. The microcontinent Barentsia (including Norway's island of Svalbard) probably was appended to Laurentia off eastern Greenland. Island arcs and highland areas, such as Taconica and Pearya (see above), rimmed the flooded continent. The narrow, northsouth Iapetus Ocean still separated Laurentia from Baltica during Wenlock time. (The continent's name is derived from the Baltic Sea and Baltic statesEstonia, Latvia, and Lithuaniaat the core of northern Europe.) The Uralian and VariscanHercynian sutures marked the eastern and southern margins of this paleocontinent, respectively. The northern tip of Scandinavia was situated just below the Wenlock paleoequator, but the islands of Novaya Zemlya extended well above it. The most prominent features were the Caledonian highlands of Norway, although a lowland may have existed in the vicinity of Finland. The microcontinent of Avaloniaits name derived from the Avalon Peninsula of eastern Newfoundlandprobably was appended to Baltica by the end of Ordovician time. It included what is now England, Wales, southeastern Ireland, the Belgian Ardennes, northern France, eastern Newfoundland, part of Nova Scotia, southern New Brunswick, and coastal New England. Separated from Baltica by the Pleionic Ocean, the paleocontinent of Siberia assumed an orientation rotated 180 from its present alignment (as recognized by the inverted position of Lake Baikal). A huge Siberian platform sea was almost completely surrounded by lowlands. Kazakhstania was a neighbouring continent in the same middle northern latitudes. North China (including Manchuria and Korea) and South China (the Yangtze platform) were two separate continents situated in a more equatorial position. In contrast to the foregoing continents, most of North and South China were elevated above sea level during Wenlock time. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during Early The vast supercontinent centred over the South Pole was Gondwana. In addition to Australia, Antarctica, India, Arabia, Africa, and South America, Silurian Gondwana also included smaller pieces of Florida, southern Europe, and the Cimmerian terranesnamely, Turkey, Iran, Afghanistan, Tibet, and the Malay Peninsulaon its outer fringes. The eastwest ocean separating the southern European sector of Gondwana from northern Europe (Baltica) is called the Rheic Ocean. Present-day Brazil or contiguous West Africa was the locus of the South Pole, buried by an ice cap probably comparable in size to Antarctica. During Wenlock time, India, Tibet, the Malay Peninsula, and Australia projected into subtropical or tropical latitudes (see figure). Significant geologic events Resumption of environmental conditions favourable to faunal recovery from the late Ordovician extinctions was the most significant geologic event of the Silurian Period. It is estimated that these extinctions claimed 12 percent of all marine invertebrate families. Sixty percent of late Ordovician brachiopod genera survived the start of the Silurian Period, but only 20 out of 70 tabulate and heliolitoid coral genera and 14 out of 38 trilobite families made the same transition. Dramatic unconformities between the Silurian and Ordovician systems indicate how extreme the glacially induced drawdown in late Ordovician sea level had been. The maximum global fall in sea level was as much as 170 metres and drained immense areas of former marine habitat. River valleys up to 45 metres in relief were eroded into Upper Ordovician marine shales stretching across Iowa, Wisconsin, and Illinois on the Laurentian platform. On Baltica, marine carbonates in Norway and Sweden were transformed into karst surfaces through subareal exposure; a network of extensive tidal channels was developed across a formerly much deeper shelf in Wales. Close to the edge of the Gondwanan ice sheet in Saudi Arabia, the Jabal Sarah paleovalley was deeply incised by glacial outwash streams eroding through Ordovician shales and sandstones. These features and many others like them elsewhere were eventually filled and buried with the return of marine sedimentation in early Silurian time. Basal Silurian strata virtually everywhere record a rapid rise in the level of the sea, which reflooded vast continental platforms. The few localities where marine sedimentation continued uninterrupted from late Ordovician to early Silurian time have been scrutinized for unusual trace elements. A major iridium spike of the sort widely found at the CretaceousTertiary boundary has not been detected at the basal Silurian stratotype in Scotland nor at the parastratotype on Anticosti Island in Quebec. The introduction of high iridium levels in the environment requires unusual volcanic activity or the impact of a large asteroid. Neither of these agents seems to be linked with the Late Ordovician extinctions or subsequent Silurian developments. The small Lac Couture crater (eight kilometres in diameter) in Quebec is the only known impact structure of Silurian age, but no known iridium layer is associated with it. Large-scale reduction of marine habitat space in combination with global cooling probably were the primary agents of the Late Ordovician extinctions. Some Silurian continents recovered more rapidly than others. The niche of the large-shelled pentamerid and stricklandiid brachiopods on the Laurentian platform, for example, was filled belatedly by immigrants from Siberia and Baltica. Smaller fluctuations in sea level between 30 and 50 metres in magnitude continued to occur on a global basis throughout the Silurian. In contrast to the Late Ordovician event, these fluctuations did not strongly affect the shelly bottom-dwelling invertebrates perched on continental platforms. Benthic faunas accommodated by living conditions at particular bathymetric levels simply tracked changing sea level by sifting upslope or downslope. The amount of available habitat space was not drastically altered as a result of these maneuvers. Data from three or more different paleocontinents indicate that at least four globally coordinated highstands (i.e., intervals during cycles of relative sea-level changes when the sea level lies above the continental shelf edge of a given area) took place during Llandovery time. The first event probably corresponds to the maximum rise in sea level achieved following recovery from the major drawdown in Late Ordovician sea level. This highstand occurred during the transition between the Rhuddanian and Aeronian ages near the boundary between the Coronograptus cyphus and C. gregarius graptolite biozones. A second highstand of mid-Aeronian age corresponds to the basal parts of the Stricklandia lens progressa lineage zone and the Monograptus sedgwickii graptolite biozone. The third matches an early Telychian event linked to the Stricklandia laevis and M. turriculatus zones, and the fourth, a late Telychian event, is correlated with the lower range of the Costistricklandia lirata and M. crenulata zones. Standard geochronology suggests that these cycles of rising and falling sea level had an average duration of about 2.5 million years during the Llandovery. Present data are not as complete for the rest of the Silurian, but a mid-Wenlock highstand in sea level is widely reported as coeval with the Monograptus riccartonensis to Cyrtograptus ellesae graptolite biozones. A mid-Ludlow lowstand in sea level also is commonly equated approximately with the Saetograptus leintwardinensis biozone, separating an early Ludlow highstand from at least one subsequent Ludlow highstand. Information on sea-level changes during Pridoli time is fragmentary and globally inconsistent. Late Silurian lowstands were sufficient to downgrade circulation patterns to a degree that stimulated widespread evaporite deposition in Laurentia, Baltica, Siberia, and the Australian sector of Gondwana. Some bathymetric changes clearly were local in effect, as brought about by submarine volcanism or by the tectonic elevation or subsidence of the seafloor. Those fluctuations recorded on different paleocontinents during the same interval of geologic time may have been coordinated by minor changes in the size of the surviving Gondwanan ice cap. South American tillites interpreted as Wenlock in age (see above) lend support to this model. Several small extinction and radiation events in the evolution of nektonic and pelagic organisms appear to be linked to Silurian fluctuations in sea level. Five graptolite radiations are recorded in the Silurian System. Four of these sudden increases in diversity occurred, respectively, in early Aeronian, early Telychian, early Sheinwoodian, and early Gorstian times during or immediately after highstands in sea level. The basal Wenlock (Sheinwoodian) radiation, for example, involves distinct new genera such as Cyrtograptus and as many as 20 new species. Among conodonts, a significant radiation is indicated by species within the Pterospathodus amorphognathoides biozone, which straddles the LlandoveryWenlock boundary and includes the late Telychian highstand. Extinction of key species followed by the origination of several new species during early Sheinwoodian time was one of the most drastic changes in the Silurian conodont succession. Acritarchs are microfossils that may represent the pelagically dispersed spore cases of benthic algae. Four major turnovers in Silurian acritarch species are recognized. Among those coinciding with highstands in sea level, the mid-Aeronian and early Gorstian turnovers are the most extensive. The various nektonic and pelagic organisms may have been affected by changes in water temperature related to minor episodes of glaciation. Silurian life Marine benthic invertebrates of the Silurian Period belonged to persistent assemblages or communities that commonly conformed to ecological zonation. One way in which zonation expresses itself is through bathymetric gradients. Paleoecologists studying in Wales, Norway, Estonia, Siberia, South China, and North America have used very similar models to explain the geographic distribution of Silurian communities. Some of these communities were adapted to life under conditions of stronger sunlight and more vigorous wave energy in shallow nearshore waters; others were restricted to darker, quieter environments in deeper offshore waters. Pentamerid communities The Pentamerus Community was an early Silurian community dominated by the large-shelled brachiopod Pentamerus oblongus. The community often included from 5 to 20 associated species, although enormous monospecific populations sometimes are found preserved in growth position. The Pentamerus Community and its slightly older or younger equivalents dominated by similar pentamerid species in the genera Virgiana, Borealis, Pentameroides, and Kirkidium all occupied a bathymetric zone of medium water depth. These pentamerid communities are known to have lived in sunlit waters because they are associated with robust, calcareous green algae. The waters were not too shallow, however, because pentamerid brachiopods lost their pedicle (the fleshy appendage that tethers the shell to the seafloor) as they matured, and thus unsecured populations were vulnerable to disruption by steady wave activity. The pentamerid communities thrived within a depth range of perhaps 30 to 60 metres. This was below the level of normal (fair weather) wave activity but still in reach of storm waves. At their lower depth limit, the pentamerid communities were out of reach of all but the most intense and infrequent storms. In regions such as Wales that are characterized by clastic deposition, an onshoreoffshore array of five brachiopod-dominated communities may be mapped in belts running parallel to the ancient shoreline. Listed in order from shallowest to deepest position, they are the Lingula, Eocoelia, Pentamerus, Stricklandia, and Clorinda communities. Below a relatively steep gradient, the centre of the Welsh basin was filled by graptolitic shales. Other areas, such as the Laurentian and Siberian platforms characterized by carbonate deposition, typically developed a continuum of stromatolite, coral-stromatoporoid, Pentamerus, and Stricklandia communities. Clorinda communities were rare in this setting. Stricklandia communities sometimes included smaller, less robust individuals of calcareous green algae, indicating a slightly deeper-water environment than that occupied by the Pentamerus Community. Coral-stromatoporoid communities, which sometimes formed reef mounds, preferred wave-agitated waters shallower than 30 metres. Much like the reef communities of today, they could not tolerate the more excessive rates of sedimentation typical of clastic settings. Bathymetric relief on carbonate platforms was very gentle; the full spectrum of available communities usually was expressed over a gradient hundreds of kilometres long. In contrast, the bathymetric gradient on the Welsh shelf was no more than a few tens of kilometres long. Like the Pentamerus Community, the other early Silurian communities have ecological equivalents that took their place in later Silurian time. Sea-level fluctuations (see above) are reconstructed by studying community replacement patterns through well-exposed stratigraphic sequences and then comparing the timing of trends on an interregional to intercontinental basis. Silurian rocks Types and distribution Excluding peat and coal, the same kinds of strata in the process of forming today were also deposited during Silurian time. Owing to the heightened state of sea level, coupled with the low relief of many continents, production of certain Silurian sediments was proportionately different than that observed in the present world, however. Chief among these are limestones, which form primarily from the carbonate detritus of coral skeletons, shells, and calcified algae. Unless such detritus is produced in great quantities or rapidly buried, it tends to dissolve in cold (temperate to polar) waters. In shallow warm (tropical to subtropical) waters, carbonates may collect more gradually to form continuous layers of limestone. The geographic locations of Laurentia, Baltica, and in part Siberia within 30 latitude on either side of the Silurian equator ensured the development of extensive platform carbonates. In North America, Silurian limestones or dolomites (altered from limestone by partial secondary substitution of magnesium for calcium) are found across an enormous territory stretching along one axis from northern Greenland to West Texas and along another axis from Quebec's Anticosti Island to the Great Basin of Utah and Nevada. Parts of Baltica where carbonate deposition was prevalent include Sweden's Gotland, Estonia, and the Ukrainian region of Podolia; carbonate deposition was also prevalent over much of Siberia. Platform carbonates of this kind rarely exceed 200300 metres in thickness. Important limestone units more restricted in Silurian time and space include the Wenlock Limestone (Shropshire, Eng.), the Ryterraker Formation (southern Norway), the Xiangshuyuan Formation and lateral equivalents (South China), and the Hume Limestone (New South Wales, Australia). Evaporites Evaporites, including salt (halite), anhydrite, and gypsum, are chemical precipitates that usually accumulate as layers through evaporation of marine waters isolated in shallow bays. This process is most effective under a warm, arid climate commonly found at latitudes of about 30 or less. Distributed through parts of Michigan, Ohio, and New York state, the Upper Silurian (LudlowPridoli) Salina Group is one of the world's most famous evaporite deposits. A maximum aggregate thickness of 600 metres occurs in Michigan, where one individual halite bed reaches a thickness of l65 metres. A 2-metre halite bed occurs in the Interlake Formation (Wenlock) of North Dakota. Gypsiferous beds occur in parts of the Upper Silurian Yangadin and Holuhan formations of Siberia, as well as in comparable formations in Latvia and Lithuania. Upper Silurian (Pridoli) evaporites are characteristic of three different basins in Western Australia. Minor amounts of halite and anhydrite occur in the Dirk Hartog Formation in the Carnarvon Basin; more extensive halite or anhydrite beds or those of both have been discovered in comparable formations from the Canning and Bonaparte Gulf basins.