TRIASSIC PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. first period of the Mesozoic Era. It began about 245 million years ago, at the close of the Permian Period, and ended approximately 208 million years ago, when it was succeeded by the Jurassic Period (see Table). It is divided into the Early Triassic Epoch (245 to 240 million years ago), the Middle Triassic Epoch (240 to 230 million years ago), and the Late Triassic Epoch (230 to 208 million years ago). The rocks that originated during this time interval make up the Triassic System, which consists mostly of rocks of sedimentary type. Because there are relatively few igneous rocks to provide reliable radiometric dates, the time span and absolute ages cited by different investigators for the Triassic Period tend to vary (e.g., some indicate that the period extended from about 240 million to 195 or 200 million years ago). Such dates are subject to revision as new and more accurate age determinations are made. Additional reading A. Logan and L.V. Hills (eds.), The Permian and Triassic Systems and Their Mutual Boundary (1973), includes papers of an international conference on the subject. Information on the fauna and flora of the period, as well as its paleogeography, is found in N.J. Silberling and E.T. Tozer, Biostratigraphic Classification of the Marine Triassic in North America (1968); E.T. Tozer, A Standard for Triassic Time (1967), Triassic Time and Ammonoids: Problems and Proposals, Canadian Journal of Earth Sciences, 8:9831031 (1971), and The Trias and Its Ammonoids: The Evolution of a Time Scale (1984); and A. Hallam (ed.), Patterns of Evolution as Illustrated by the Fossil Record (1977). Alan Logan Triassic environment Paleogeography Paleogeography and paleoceanography of Early Triassic time. The present-day coastlines and tectonic As has been noted, at the beginning of the Triassic the present continents of the world were grouped together into one large supercontinent, Pangaea, which covered about one-quarter of the Earth's surface. Pangaea stretched from pole to pole in a narrow belt of about 60 of longitude and consisted of a group of northern continents, Laurasia, and a group of southern continents, Gondwana. The rest of the globe was covered by Panthalassa, the enormous world ocean that stretched from pole to pole and extended to about twice the width of the present-day Pacific Ocean at the equator (seefigure). Scattered across Panthalassa, within 30 of the Triassic equator, were islands, seamounts, and volcanic archipelagoes, some associated with reef carbonates, which would later be driven into the lithospheric plates on either side of the Panthalassa spreading centre as displaced terranes. A deep embayment of Panthalassa projected westward between Gondwana and Laurasia along an eastwest axis approximately coincident with the present-day Mediterranean Sea. This ancient sea, the Tethys, was later to extend farther westward, as rifting between Laurasia and Gondwana began in Late Triassic time. This seaway would eventually link up with the eastern side of Panthalassa by Middle to Late Jurassic time when Gondwana proceeded to separate from Laurasia. The evidence for these paleogeographic reconstructions comes from many sources, of which paleomagnetic data and the matchups of continental margins, rock types, orogenic events, and distribution of fossil land vertebrates and plants prior to the breakup of Pangaea are the most important. In addition, the recalculated polar-wandering curves for Africa and North America converge between the Carboniferous and Triassic and then begin to diverge in the Late Triassic, indicating the exact time of the onset of separation of these two continents as the Tethys seaway began to open up. Thick sequences of clastic sediments accumulated in marginal geosynclines bordering the present-day circum-Pacific region and the northern and southern margins of the Tethys, while shelf seas occupied parts of the Tethyan, circum-Pacific, and circum-Arctic regions but were otherwise restricted in distribution. Much of the circum-Pacific and the northeastern part of Tethys were active plate margins, as indicated by tectonic and igneous activity, but its northwestern and southern margins were passive during the Triassic. Significant geologic events The Triassic Period is characterized by few geologic events of major significance, in contrast to the Jurassic and Cretaceous when Pangaea fragmented and the new Atlantic and Indian oceans opened up. The beginning of continental rifting in the Late Triassic, however, caused stretching of the crust in eastern North America along the Appalachian Mountain belt from the Carolinas to Nova Scotia, resulting in normal faulting in this region. Here grabens received thick clastic sequences, which were later intruded by dikes and sills. In similar fault-controlled basins between Africa and Laurasia evaporites formed in arid or semiarid environments as seawater from the Tethys periodically spilled into these newly formed troughs. Evaporites of Late Triassic and Early Jurassic age in Morocco and off eastern Canada were apparently the result of such tectonism. Mountain building was equally restricted during the Triassic, with relatively minor orogenic activity taking place along the Pacific coastal margin of North America and in China and Japan. The unmetamorphosed nature of the Triassic rocks of the Newark Group indicates that they were formed after the main phase of the Appalachian orogeny in the late Paleozoic. Triassic life Periodic large-scale mass extinctions have occurred throughout the history of life; indeed, it is on this basis that the geologic eras were first established. One of the most severe and widespread of such extinction events took place at the end of the Mesozoic; another was at the end of the Paleozoic, and it is the latter that profoundly affected life during the Triassic. A third episode of mass extinctions occurred at the end of the Triassic, drastically reducing some marine and terrestrial groups, such as the ammonoids, mammallike reptiles, and primitive amphibians. The Permo-Triassic extinction was perhaps the most drastic in the history of life on Earth, although it should be noted that many groups were showing evidence of a gradual decline long before the end of the Paleozoic. The trilobites, a group of arthropods long past their zenith, made their last appearance in the Permian, as did the closely related eurypterids. Rugose and tabulate corals became extinct at the end of the Paleozoic. Several superfamilies of Paleozoic brachiopods, such as the productaceans, chonetaceans, spiriferaceans, and richthofeniaceans, also disappeared at the end of the Permian. Fusulinid foraminiferans, useful as late Paleozoic index fossils, did not survive the crisis, nor did the cryptostomate and fenestrate bryozoans that inhabited many Carboniferous and Permian reefs. Gone also were the blastoids, a group of echinoderms that persisted in what is now Indonesia until the end of the Permian, although their decline had begun much earlier in other regions. Many possible causes have been advanced to account for these extinctions. Cataclysmic events, such as intense volcanic activity and the impact of a celestial body, or more gradual changes brought about by widespread marine regression, oceanic salinity and nutrient fluctuations, climatic cooling, and cosmic radiation, have been proposed to explain the Permo-Triassic crisis. Any theory, however, must take into account that not all groups were affected to the same extent. Whatever the cause, Early Triassic biotas were impoverished, although they progressively increased in diversity and abundance during Middle and Late Triassic times. While the fossils of many Early Triassic life-forms tend to be Paleozoic in aspect, those of the Middle and Late Triassic are decidedly Mesozoic in appearance and clearly the precursors of things to come. Invertebrates The difference between Permian and Triassic faunas is most noticeable among the marine invertebrates; the number of families was reduced by half, with an estimated 95 percent of all species disappearing at the PermianTriassic boundary. The ammonoids were common in the Permian but suffered drastic reduction at the end of that period. Only a few genera belonging to the prolecanitid group survived the crisis, but their descendants, the ceratitids, represented by such Early Triassic genera as Otoceras and Ophiceras, provided the rootstock for an explosive adaptive radiation in the Middle and Late Triassic. Ceratitids have varying external ornamentation, but all share the distinctive ceratitic internal suture line of rounded saddles and denticulate lobes. The group reached its acme in the Carnian, with more than 150 genera; it declined to less than 100 in the Norian and to less than 10 in the Rhaetian. In the Late Triassic bizarre heteromorphs with loosely coiled body chambers, such as the genus Choristoceras, or helically coiled whorls (e.g., Cochloceras) evolved. These aberrant forms, however, were short-lived. A small group of smooth-shelled forms with more complex suture lines, the phylloceratids, also arose in the Early Triassic. They are regarded as the earliest true ammonites and gave rise to all post-Triassic ammonites, even though Triassic ammonoids as a whole almost became extinct at the end of the period. Other marine invertebrate fossils found in Triassic rocks, albeit much reduced in diversity compared with the Permian, include gastropods, bivalves, brachiopods, bryozoans, corals, foraminiferans, and echinoderms. These groups are either poorly represented or absent in Lower Triassic rocks but increase in importance later in the period. Most are bottom-dwellers (benthos), but the bivalve genera Claraia, Posidonia, Daonella, Halobia, and Monotis, often used as Triassic guide fossils, were planktonic and may have achieved widespread distribution by being attached to floating seaweed. While the role of colonial scleractinian corals as reef-builders in Middle and Late Triassic structures has already been mentioned, cavities in Rhaetian reefs from Austria were colonized by a cryptofauna of echinoids, foraminiferans, spongiomorphs, and small arthropods. Many successful Paleozoic articulate brachiopod superfamilies became extinct at the end of the Permian, leaving the spiriferaceans, rhynchonellaceans, terebratulaceans, terebratellaceans, thecideaceans, and some other less important groups to continue into the Mesozoic. The brachiopods, however, never again achieved the dominance they held in the benthos of the Paleozoic. Fossil echinoderms are represented in the Triassic by crinoid columnals and the echinoid Miocidaris, a holdover from the Permian. The crinoids had begun to decline long before the end of the Permian, by which time they were almost entirely decimated, with both the flexible and camerate varieties dying out. The inadunates survived the crisis; they did not become extinct until the end of the Triassic and gave rise to the articulates that still exist today. Coccolithophores, an important group of living marine pelagic algae, made their first appearance during the Late Triassic, while dinoflagellates underwent rapid diversification during the Late Triassic and Early Jurassic. Triassic rocks Occurrence and distribution Major depositional troughsi.e., geosynclinesdeveloped around Panthalassa, the ancestral Pacific Ocean, during the Early and Middle Triassic. Great quantities of marine sediments, mainly sandstones, shales, and graywackes, collected in these troughs, as indicated by deposits now found in the western Pacific geosynclinal belt (New Zealand and Japan) and the eastern geosynclinal belt (Alaska, Arctic Canada, British Columbia, western United States, and the west coast of South America). As an example, more than 3,000 metres of Triassic sediments accumulated in the Sverdrup Basin of Arctic Canada. A deep, narrow arm of Panthalassa, the Tethys Sea, stretched along an eastwest belt separating what is now Africa from southern Europe, and it also received geosynclinal deposits. In the northern Tethyan geosyncline, these deposits now outcrop in the Alps, Turkey, Iran, Pakistan, and the Himalayas, mainly as limestones, with deep-sea sediments such as radiolarian cherts that formed in troughs in the deeper parts of the Tethys Sea. To the south was the southern Tethyan geosyncline, bordering Gondwana and stretching from northern India through the Middle East to northern Africa. Shallow shelf-sea embayments of limited distribution occurred landward of these geosynclines and are represented mainly by limestones in low latitudes, as around the margins of the Tethys Sea. Such tropical and subtropical shelf seas were warm and often supported small reefs, the forerunners of the more extensive coral reefs of today. Although the Permo-Triassic extinction of rugose and tabulate corals resulted in an absence of Lower Triassic corals, small reeflike mounds of early Middle Triassic age were succeeded later in Middle Triassic times by more extensive reef complexes with some Permian biotic elements retained. Such reefs have been described from the Tirolian Alps of Austria and the Dolomites of Italy. Late Triassic reef complexes, more modern in aspect and dominated for the first time by scleractinian corals, occur as thick sequences in the Dachstein and Steinplatte regions of Austria and Germany, as well as in Iran and the Himalayas. In the circum-Pacific region some shelf-sea deposits, generally of clastic facies (sandstones and shales), occur in Western Australia, Siberia, and the circum-Arctic region, including Arctic Canada, Alaska, eastern Greenland, and Spitsbergen. Continental sediments, dominated by red beds (e.g., sandstones and shales of red colour) and evaporites, accumulated on land throughout the Triassic Period. The Bunter and Keuper Marl of Germany and the New Red Sandstone of Britain are examples of such red beds north of Tethys, while to the south are similar deposits in India, Australia, South Africa, and Antarctica. Although deposits of this kind usually indicate accumulation in arid regions, such as inland desert basins, sediments of fluvial or lacustrine origin may also be represented by these red beds. Large basins containing Triassic continental sediments occur in South America (Colombia, Venezuela, Brazil, Uruguay, Paraguay, and Argentina) and in western North America, particularly in Utah, Wyoming, Arizona, and Colorado. In eastern North America great thicknesses of sedimentary rocks of continental origin were deposited during the Late Triassic in a series of fault basins, of which the Newark Basin is probably the best known. Here sequences of continental red clastics with dinosaur tracks and mud cracks, along with black shales containing fossils of freshwater organisms, indicate a depositional environment of rivers draining into freshwater lakes in a generally arid or semiarid region, which from paleomagnetic evidence appears to have been located about 20 N of the paleoequator. Triassic igneous rocks are not common, and reliable radiometric dates are available only from Upper Triassic rocks. Examples of extrusive basalt flows are known from Australia, South America, and eastern North America, while intrusive rocks include the well-known Palisades Sill of the Newark series. This 300-metre-thick diabase intrusion has yielded a potassiumargon age of 193 million years. Lava flows in the Hartford Basin of Connecticut have been used to estimate the age of the TriassicJurassic boundary as between 184 and 195 million years, based on potassiumargon and argon-40argon-39 geochronology. Types As a broad generalization, Triassic geosynclinal and shelf-sea deposits formed in low paleolatitudes are dominated by limestones, with minor amounts of clastics (e.g., sandstone, shale, and graywacke), while high-paleolatitude depositional basins and the circum-Pacific geosyncline are dominated by clastics, with minor amounts of limestone. Volcanism is usually associated with faulting and is represented by basalt flows and diabase intrusions.