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Triassic
Major
events
Triassic strata record profound paleontologic
changes that reflect major physical changes in Earth history. Two of the five
most catastrophic extinctions of the Phanerozoic Eon mark the beginning and end
of the Triassic. More than 50% of all Permian families died out at the beginning
of the period, including 85–90% of all marine species and 75% of land species;
and more than 50% of all marine genera became extinct at the end of the period.
The Triassic was also when many new families of plants and animals evolved,
including the earliest known mammals.
As a very brief interval of geologic time (about
40 million years), the Triassic Period uniquely embraces both the final
consolidation of Pangaea and the initial breakup of the landmass, which in the
Middle Jurassic led to the opening of the Central Atlantic Ocean and formation
of modern-day continental margins. The Triassic marks the beginning of a new
Fig. 1
Paleogeography of the Late Triassic Period: after the accretion of south
China and Cimmeria (Turkey, Iran, and Tibet) to Laurasia; during the Incipient
rifting of Pangaea in eastern North America and northwest Africa along the
Allegheny-Mauritanide-Variscan orogeny; and concurrent with oceanic subduction
and formation of deep-sea trenches and magmatic arc along the western plate
boundary of North America. (After R. K. Bambach, C. R. Scotese, and A. M.
Zlegler, Before Pangea: The geographics of the Paleozoic world, Amer. Sci.
68:26–38, 1980)
Fig
2.
(a) Map
of CAMP. (b) Late Triassic–Early Jurassic reconstruction of eastern North
America and northwest Africa, outlining rift basins, lithofacies, and tectonic
elements, during breakup but prior to sea-floor spreading of the Central
Atlantic in the Middle Jurassic. Arrows indicate the direction of intraplate
movement along continental fracture zones. (After W. Manspeizer, ed.,
Triassic-Jurassic Rifting: Continental Breakup and the Origin of the Atlantic
Ocean and Passive Margins, Elsevier,
1988)
Final consolidation of
Pangaea
The initial consolidation of Pangaea, which was
marked by the formation of the Allegheny-Mauritanide-Variscan mountain chain in
the middle Carboniferous (320 Ma), resulted from the collision of Gondwanaland
and the combined Laurasia-Baltica-Siberian-Kazakhstania landmass (Fig. 1). Major
plate accretion continued into the Middle-to-Late Triassic (230 ± 5 Ma), when
southern
Pangaean
supercontinent
The final phase of deformation produced a broadly
convex continental plate that extended from the north to the south paleo poles,
covered about 25% of the Earth's surface, and was surrounded by a global ocean
called Panthalassa. It had a central arch standing about 1 mi (1.6 km) high and
an average elevation of more than 4300 ft (1300 m) above the early Mesozoic sea
level (Fig. 1). Because of its size, location, and pronounced orographic peaks
that probably rivaled the
As the plate migrated north, transgressing about
10° of latitude between the Middle Triassic and Middle Jurassic, the plate was
subjected to increased aridity as it moved under the influence of the
subtropical high-pressure cell. Because of its large size, the landmass must
have been subjected to monsoon circulation. Winters along the future central
Atlantic probably were dominated by subtropical high-pressure cells bringing in
cool dry air from aloft, whereas summers were dominated by equatorial
low-pressure systems bringing in warm moist air from the Tethys seaway to the
east. As moist air was uplifted almost 1.2 mi (2 km) over the
Alleghenian-Variscan chain, it would have cooled adiabatically, yielding
rainwater that fed major rivers (for example, the Congo River) flowing thousands
of miles away from the axis of uplift across broad alluvial plains to the
coastal regions of Alaska, Patagonia, India, and Siberia. See also: Monsoon meteorology
With the onset of rifting in the Late Triassic
and subsequent topographic changes, small ephemeral streams flowed into the rift
valleys, creating huge lakes that may have been comparable in size to present
Crustal
extension
The most important tectonic event in the Mesozoic
Era was the rifting of the Pangaea craton, which began in the Late Triassic,
culminating in the Middle Jurassic with the formation of the
Atlantic rift
basins
Continental rift basins, passive continental
margins, and ocean basins form in response to divergent stresses that extend the
crust. Crustal extension, as it pertains to the
Fig. 3
Diagrammatic cross section of the Atlantic-type continental passive
margine of North America and North Africa, taken at the beginning of the Middle
Jurassic with the onset of sea-floor spreading that resulted from crustal
thinning and mantie upwelling Note the setting of the Late Triassio-Early
Jurassic continental and marine rift basins and their relation to the future
passive margins, the postrift unconformity, and the overtying Middle Jurassic
drift sequence. (After W. Manapelzer, ed., Triassic-Jurassic Rifting:
Continental Breakup and the Origin of the
Within the proto-Atlantic, off eastern North
America and
Figure 2, showing a predrift paleogeographic
reconstruction of the circum-Atlantic region, outlines the major Triassic basins
and lithofacies. Two major basin types are recognized (Fig. 3): Newark-type
detrital basins, which are exposed onshore as half-grabens and contain a thick
(approximately 2.5–5 mi or 4–8 km) sequence of fluvial-lacustrine strata and
border fanglomerates; and evaporite basins, which occur seaward of the string of
detrital basins and contain a thick evaporite facies with interbeds of red
mudstones and carbonates. As more than 3300 ft (1000 m) of salt was
concentrated, these basins must have acted as huge evaporating pans. The
Triassic-Jurassic systemic boundary, throughout the broad region of the
Atlantic, typically is marked by tholeiitic lava flows and intrusives that are
dated about 200 Ma (Early Jurassic), or only slightly older than the oldest
dated crust of the
Central Atlantic magmatic
province
The breakup of Pangaea was accompanied by the
most extensive outpouring of continental basaltic lava known, covering an area
estimated to be about 4 million mi2 (10 million km2). Basaltic remnants (flood
basalts and feeder dikes) of this igneous province, named the Central Atlantic
Magmatic Province (CAMP), are found on the rifted margins of four
circum-Atlantic continents, particularly eastern North America, South America,
western Africa, and southwestern Europe [Fig. 2(a)]. Almost all of CAMP rocks
are mafic tholeoiites that were intruded into or extruded onto clastic rocks in
Newark-type rift basins. The Palisades Sill, along the west shore of the lower
Hudson River in Northern New Jersey, is an example of this magmatic event. It is
thought that the immensely thick and widespread seaward-dipping basaltic wedges,
manifested by the East Coast magnetic anomaly, are linked to CAMP. See also: Geomagnetism
Recent multidisciplinary studies in stratigraphy,
palynology, geochronology, paleomagnetism, and petrography indicate CAMP formed
as a singular episodic event in Earth's history, occurring during a very brief
interval of geologic time, perhaps no longer than 4 million years. Importantly,
this event occurred about 200 Ma and was contemporaneous with widespread mass
extinction at the Triassic-Jurassic boundary. A causal relationship is
postulated by many scientists to climate change that was forced by emission of
huge quantities of volcanic gasses, estimated by researchers to be in the order
of from 1–5 × 1012 metric tons. Radical shifts in climate due to the ejection of
aerosols into the atmosphere and destruction of environments by lava flows, ash
falls, fires, and toxic pollution of soil and streams are suggested consequences
of this event. A similar explanation has been offered for the mass extinctions
that occurred at the end of the Paleozoic and Mesozoic Eras, with major volcanic
eruptions of the Siberian Traps and Deccan Traps, respectively.
Western North
America
Permian-to-Triassic consolidation of Pangaea in
western North America led to the Sonoma orogeny (mountain building), which
resulted from overthrusting and suturing of successive island-arc and
microcontinent terranes to the western edge of the North American Plate.
However, toward the end of the Triassic Period, as crustal extension was
occurring in the Central Atlantic region, the plate moved westward, overriding
the Pacific Plate along a reversed subduction zone. This created, for the
remainder of the Mesozoic Era, an Andean-type plate edge with a subducting sea
floor and associated deep-sea trench and magmatic arc. These effects can be
studied in the Cordilleran mountain belt, from Alaska to California, where great
thicknesses of volcanics and graywackes were derived from island arcs to the
west, and in Idaho and eastern Nevada, where thick Lower Triassic marine
limestones and sandstones were laid down adjacent to the rising Cordilleras on
the west and interfinger with continental redbeds derived from the stable
interior to the east. See also:
Cordilleran belt; Limestone; Orogeny; Redbeds; Sandstone
As the epicontinental seas regressed westward,
nonmarine fluvial, lacustrine, and windblown sands were deposited on the craton.
Today many of these red, purple, ash-gray, and chocolate-colored beds are some
of the most spectacular and colorful scenery in the American West. For example,
the Painted Desert of Arizona, known for its petrified logs of conifer trees,
was developed in the Chinle Formation, and the windblown sands of the Wingate
and Navajo formations are exposed in the walls of Zion National Park in southern
Utah. See also: Petrified forests
Life
The Triassic is bracketed by two major biotic
crises that terminated many groups of organisms. Triassic marine faunas can be
distinguished from their predecessors by the absence of groups that flourished
in the Permian, such as the fusulinid foraminiferans, the tabulate and rugose
corals, the trepostome and cryptostome bryozoans, the productid and other
brachiopod groups, the trilobites, and certain groups of echinoderms. Owing to a
very low stand of sea level, Early Triassic marine faunas are not common, and
show very small diversity except for ammonites. This is partly ecologic. The
reef community, for example, is not known from the Early Triassic deposits; yet
when it reappeared in mid-Triassic time, it contained sponges that were major
members of Permian reefs, and that must have survived in settings that have not
been found. See also: Brachiopoda;
Bryozoa; Echinodermata; Foraminiferida; Fusulinacea; Permian; Rugosa; Tabulata;
Trilobita
Triassic faunas are also distinguished from
earlier ones by newly evolved groups of plants and animals. In marine
communities, molluscan stocks proliferated vigorously. Bivalves diversified
greatly and took over most of the niches previously occupied by brachiopods;
ammonites proliferated rapidly from a few Permian survivors. The scleractinian
(modern) corals appeared, as did the shell-crushing placodont reptiles and the
ichthyosaurs. In continental faunas, various groups of reptiles appeared,
including crocodiles and crocodilelike forms, the mammallike reptiles, and the
first true mammals, as well as dinosaurs.
See also: Cephalopoda; Crocodylia; Dinosauria; Mammalia; Mollusca;
Placodontia; Scleractinia
The Jurassic faunas lack numerous stocks lost in
the Rhaeto-Liassic faunal crises. These include survivors of the Permian crises,
such as the orthoceratid cephalopods and the conodonts. However, stocks that had
flourished greatly in the Triassic also became extinct (phytosaurs, placodonts)
or nearly extinct: ammonites were reduced to one or two surviving lineages,
which then underwent no other great evolutionary surge in Jurassic time.
Furthermore, new groups such as the plesiosaurs and pterosaurs appeared.
Triassic land plants contain survivors of many Paleozoic stocks, but the
gymnosperms became dominant and cycads appeared. The Permo-Triassic and
Rhaeto-Liassic crises record a severe stressing of the biosphere, but the nature
and origin of these stresses have not been established. See also: Conodont; Cycadeoidales;
Extinction (biology); Index fossil; Paleobotany; Paleoecology; Paleontology;
Pinophyta; Pterosauria
Warren Manspeizer
Bibliography
-
A. Hallam, The end-Triassic bivalve extinction event, Paleogeo. Paleoclimatol. Paleoecol., vol. 35, pp. 1–44, 1981
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W. E. Hames et al. (eds.), The Central Atlantic Magmatic Province: Insights from Fragments of Pangea, American Geophysical Union, Washington, D.C., 2003
-
G. D. Klein (ed.), Pangea: Paleoclimate, Tectonics, and Sedimentation During Accretion, Zenith, and Breakup of a Supercontinent (Spec. Pap. No. 288), The Geological Society of
-
P. M. Letournea and P. E. Olsen (eds.), The Great Rift Valleys of Pangea in Eastern North America, vol. 1: Tectonics, Structure and Volcanism, Columbia University Press, New York, 2003
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W. Manspeizer (ed.), Triassic-Jurassic Rifting: Continental Breakup and the Origin of the
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D. R. Prothero and R. H. Dott, Evolution of the Earth, 7th ed.,
-
S. M. Stanley, Earth System History,
Additional
-
Paleomap Project
-
Permian-Triassic Extinction
-
Triassic Period
-
Triassic-Jurassic Working Group
-
Ecology of the Triassic
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