Cretaceous period (krĬtā´shəs), third and last period of the Mesozoic era of geologic time (see Geologic Timescale, table), lasting from approximately 144 to 65 million years ago. The Cretaceous was marked, in both North America and Europe, by extensive submergences of the continents. Changes both in the earth's surface and in its flora and fauna brought the Mesozoic to a close.
Historical Geology of the Period
The Lower Cretaceous Period
At the beginning of the Lower Cretaceous in North America, the Mexican Sea of the late Jurassic period spread over Texas, Oklahoma, New Mexico, and parts of Arizona, Kansas, and Colorado. Deposits from this inland sea, known as the Comanchean Sea, were chiefly limestone (up to 1,500 ft/457 m thick in Texas) but some continental sediments (i.e., sandstone, shale, and conglomerate) mark the reemergence of land, which brought the Lower Cretaceous to a close. The Comanchean Sea was probably separated by a land barrier from contemporaneous seas in the California areas, where 26,000 ft (7,925 m) of Shastan shales, with sandstone and thin limestone, were laid down. The sediments were derived by rapid erosion from the recently elevated Sierra Nevada and Klamath mts. In Montana, Alberta, and British Columbia the Kootenai deposits of sandstone and sandy shale, which contain workable deposits of good coal, were formed; along the Atlantic coast the unconsolidated sandy clay, gravel, and sand of the Potomac series were deposited.
The Lower Cretaceous opened in NW Europe with the deposition of a continental and freshwater formation, the Wealden sand and clay, best displayed in England. The sea, meanwhile, expanded from the Mediterranean, finally overlaying successive Wealden strata with limestone. There was at the same time an extensive sea in N Europe. At the close of the Lower Cretaceous, there was some recession of the seas; by the Upper Cretaceous, the great transgression of seas submerged lands that had been open since the Paleozoic.
The Upper Cretaceous Period
The Upper Cretaceous opened in W North America with the deposition of continental sands (now the Dakota sandstone), which, however, were covered by the ensuing rise of the Colorado Sea. The Colorado Sea was the greatest of the North American Mesozoic seas and extended all the way from Mexico up into the Arctic, covering most of central North America. The Colorado deposits were composed chiefly of shales, limestone, and some chalk in Kansas and South Dakota. Slight shifting of the sea was followed by the deposition of the Montana shale and sandstone and then by withdrawal of the sea. Near the end of the Upper Cretaceous, conditions in the west were similar to those of the Carboniferous period in other regions; swamps and bogs were formed that later became valuable deposits of coal.
At the close of the Cretaceous the Laramide revolution occurred—at least two different epochs of mountain building and one of relative quiet. In this disturbance the Rockies and the E Andes were first elevated, and there were extensive flows of lava. The Appalachians, which had been reduced almost to base level by erosion, were rejuvenated, and the seas retreated from all parts of the continent. The intermittent character of the Laramide disturbance makes difficult the demarcation of the Mesozoic and the succeeding Cenozoic era.
The striking feature of the European Upper Cretaceous are great chalk deposits from small carbonate-bearing marine algae and calcareous fauna, now exposed in the cliffs of the English Channel. In India the late Upper Cretaceous was marked by an overflow of lava in the Deccan plateau. The area covered by igneous rocks dating from this period now comprises over 200,000 sq mi (518,000 sq km) and was formerly much larger, having been reduced by erosion. Near Mumbai the formation is 10,000 ft (3,000 m) thick.
Movement of the Continents
During the Cretaceous period the massive continents of Gondwanaland and Laurasia continued to separate. South America and Africa had separated, with the consequent widening of the S Atlantic. The N Atlantic continued to expand, although it appears that Europe, Greenland, and North America were still connected. Madagascar had separated from Africa, while India was still drifting northward toward Asia. The Tethys Sea was disappearing as Africa moved north toward Eurasia. Antarctica and Australia had yet to separate.
Evolution of Plant and Animal Life
The Lower Cretaceous is characterized by a revolution in the plant life, with the sudden appearance of flowering plants (angiosperms) such as the ancestors of the beech, fig, magnolia, and sassafras. By the end of the Cretaceous such plants became dominant. Willow, elm, grape, laurel, birch, oak, and maple also made their appearance, along with grass and the sequoias of California. Closely associated with the angiosperms were insects, including a form of the dragonfly, and most were similar to today's insects. This prepared the way for the increase in mammals in the late Cenozoic. The marine invertebrates of the Cretaceous included nautiluses, barnacles, lobsters, crabs, sea urchins, ammonites, and foraminifers. Reptiles reached their zenith, including the dinosaurs Triceratops, Tyrannosaurus, Stegosaurus, Apatosaurus (Brontosaurus), and Iguanodon, and ranged from herbivores to carnivores. Flying reptiles such as the pterosaurs were highly developed, while in the sea there were ichthyosaurs, plesiosaurs, and mosasaurs. Other reptiles living in this period include crocodiles and giant turtles; snakes and lizards made their first appearance at this time. True mammals, which had already appeared in the Triassic period, were rare, as the Cretaceous reptiles dominated.
The climate of the Cretaceous was apparently fairly mild and uniform, but it is possible that toward the end of the period some variant zones of climate had appeared, making the overall climate cooler. Such changes, along with changes in both the earth's surface and its flora and fauna, brought the Mesozoic to a close.
By the end of the Cretaceous, about 75% of all species, including marine, freshwater, and terrestrial organisms, became extinct. The rather abrupt disappearance of Cretaceous life remains a mystery; similar mass extinctions have occurred at other periods in the earth's history. Theories for the extinctions have included one or a mixture of the following: drastic cooling of the globe, retreat of the seas, breakup of the continents (see continental drift), biological disease, reversals of the earth's magnetic field, or a change in atmospheric carbon dioxide and oxygen. However, the most widely accepted theory was introduced in 1980 by Luis and Walter Alvarez and colleagues at the Univ. of California. Alvarez proposed that the earth was struck by an asteroid or comet about 6 mi (10 km) in diameter around 65 million years ago. Such an impact (or collection of impacts) would spread dust into the atmosphere, suppressing photosynthesis and changing the food chain. Evidence for an impact includes an anomalous iridium layer, typical of meteorites, and some probable impact craters dated to the late Cretaceous. Other scientists have proposed that the cause of the final Cretaceous extinction was the huge volcanic eruptions that created the lava flows of the Deccan Traps in what is now India. One model has put these two theories together, hypothesizing that shock waves from the impact of a large asteroid moved through the earth, shaking the earth's crust and triggering or intensifying the volcanic events. Another suggests that the lava flows set off a chain of events that killed off much marine life a couple hundred thousand years before the impact extinguished the dinosaurs.
Cretaceous is the name given to a period of time in Earth's history (i.e., Cretaceous Period) from 145.6 to 65 million years ago. Also, all the rocks that formed during that time have the same proper name of Cretaceous (i.e., they are referred to as the Cretaceous System). Said differently, the Cretaceous System is the rock record of events that occurred—and organisms that lived—during a span of geological time that is called Cretaceous Period. Cretaceous was the third and final period of the Mesozoic Era .
Cretaceous is a name derived from the Latin word for chalk, creta. Chalk is a common type of sedimentary rock formed during this interval of Earth history. The term Cretaceous was first used in 1822 by d'Omalius d'Halloy (1707–1789), a Belgian geologist who was engaged in pioneering efforts at geological mapping of parts of France. He mapped the Terrain Cretac (Cretaceous System) within the Paris Basin and later in Belgium. D'Halloy's strata were easily correlated with chalks mapped earlier, but not formally named, by William Smith (1769–1839) on his revolutionary 1815 geological map. Other geologists of the day rapidly correlated the Cretaceous System with chalks they found in northern France, Holland, Denmark, northern Germany, Poland, and Sweden. Since their recognition and definition during the nineteenth century, Cretaceous strata have been mapped on all the world's continents.
During Cretaceous, the breakup of the supercontinent of Pangaea became nearly complete. The Atlantic Ocean opened sufficiently so that a substantial body of water existed between North America and Europe and South America and Africa were widely separated. While the Atlantic was opening, the Pacific Ocean continued to close rapidly and an episode of major tectonic change occurred as a result in western North and South America. Specifically, a major subduction zone developed along the western coast of the Americas and substantial tectonic uplift and volcanic activity occurred along this western margin.
During most of Cretaceous, global sea levels were at some of their highest elevations in the last 500 million years of Earth history. Much of the low-lying areas of the world's continents were covered by shallow seas , also known as epicontinental seas. During part of Late Cretaceous, sea level was so high (approximately 275 m above present level) that an epicontinental sea (i.e., Western Interior Seaway) connected the Gulf of Mexico with the Arctic Ocean through the center of North America. (The Rocky Mountains were not formed at this time, and thus this area was rather low-lying and flat).
Cretaceous was a time of elevated global temperatures and there were essentially no polar or high-altitude glaciers . This contributed to elevated sea levels as did the vast development of volcanic activity along Earth's mid-ocean ridges . Such volcanic activity and accompanying swelling of these undersea ridges displaced a considerable volume of seawater (strongly exacerbating sea-level rise). Also, there was rapid sea-floor spreading during Cretaceous. Further, effusive volcanism from mantle hot spots caused flood basalts to erupt on land, the most noteworthy of these produced the massive Deccan Traps of India.
Cretaceous life in the oceans was very plentiful, with numerous species of fish, sharks, rays, ammonites, turtles, mosasaurs, plesiosaurs, and other creatures plying ocean waters. Numerous massive reef systems developed, including one rimming the Gulf of Mexico during Early Cretaceous, which was dominated by a specialized type of large clams called rudists. Large oysters were common at this time. The main plankon, the golden-brown alga, produced massive amounts of calcareous platelets that settled to the sea floor along with abundant foraminifers thus forming an ooze that eventually became the famous Cretaceous chalk (as seen at the White Cliffs of Dover, England, and many other areas).
Cretaceous life on land was dominated by the reptiles, which dwelt for the most part in heavily vegetated terrains. Lush Cretaceous cycad-ginko-conifer forests and swamps are well preserved in vast tracts of coal and lignite deposits of this age. Late Cretaceous was the time of development of flowing plants and co-evolution of a very diverse insect population. Land animals such as snakes, lizards, crocodiles, dinosaurs, pterosaurs, birds, and small mammals were common. Among the dinosaurs, Cretaceous ecosystems saw the rise of many diverse types, which were indigenous to the rapidly separating continental land masses. The rising groups of dinosaurs during this time included pachycephalosarus, ceratopsians, iguanodonts, hadrosaurs, coelurosaurs, and carnosaurs (e.g., tyrannosaurids).
The end of Cretaceous came abruptly with global ecosystem collapse and mass extinction. The end-Cretaceous catastrophe resulted in rapid death of nearly 50% of all living species of organisms. Known since the nineteenth century as an inexplicable sudden mass death marker, the Cretaceous-Tertiary (K-T) boundary has attracted considerable interest from researchers in recent years because the boundary clay layer contains a substantial enrichment in certain chemical elements (e.g., iridium), which are much more common in asteroids and comets than on Earth. Discovery in the 1980s of a 180-km diameter impact crater in the state of Yucatan, Mexico, which has the same age as the K-T boundary, indicates a strong connection between the mass death and cosmic impact at this time in Earth history. Subsequent studies have shown that impact of a 10-km diameter asteroid at Yucatan could have resulted in such a global ecosystem collapse and accompanying mass death. The impact crater is known as Chicxulub, which is a Mayan word meaning "tail of the Devil."
See also Chronostratigraphy; K-T event; Impact crater; Mesozoic Era; Stratigraphy; Supercontinents
The Cretaceous period is the third of the three divisions of the Mesozoic era of the geologic time scale. The period lasted 79 million years, from 144 million to 65 million years ago. The Cretaceous is named for chalk beds found in England.
|Era||Period||Epoch||Million Years Before Present|
Cretaceous and surrounding time periods.
Laurasia and Gondwanaland, the northern and southern landmasses that resulted from the initial breakup of the supercontinent Pangea, continued to separate from each other during the Cretaceous period. These landmasses also began to fragment within themselves to form our modern continents. Throughout most of the Cretaceous, North America was divided by a vast inland sea that extended from the Gulf of Mexico to Canada.
Early in the Cretaceous, the climate was warm and semitropical, very much like at the end of the Jurassic period (144 million years ago). However, during the second forty million years the climate became colder at the polar regions and warmer at the equator, setting in motion ecological changes that affected the evolution of plants and animals.
Shallow oceans supported abundant marine life, including new forms of oysters, diatoms, and algae as well as fish and sharks, corals, echinoderms, ammonoids, and mollusks. The edges of these shallow seas provided important habitat for mammals, turtles, crocodiles, fish, lizards, and many invertebrates.
Dramatic changes occurred in plant life during the Cretaceous. Pollinating insects such as bees and butterflies allowed the emerging flowering plants—the angiosperms —an advantage over seed-bearing plants that relied on the wind or a chance encounter with an animal to disperse their seeds. Today, nearly 90 percent of plants on Earth are angiosperms, signifying a remarkable evolutionary success story. Forests of oak and willow, cypress, magnolia, palms, and sycamore slowly replaced the cycad forests—palm-like plants with a barrel shaped trunk and many long leaves growing from the top. These new plant communities provided new sources of food and habitat for many kinds of animals.
During the Cretaceous, the dinosaurs reached the height of their evolutionary success. Fossil bones found in Africa, the Gobi Desert, South America, China, Mongolia, and North America suggest that many new species emerged while earlier dinosaurs went extinct. The carnivorous animals such as Albertasaurus and Tyrannosaurus remained the top predators as they roamed and hunted their prey. Hadrosaurs, (duck-billed dinosaurs), Ankylosaurs (armored dinosaurs), and Ceratopsians (horned dinosaurs) replaced the giant Jurassic sauropods as the main herbivores. Triceratops fossils by the hundreds have been found, suggesting that these cows of the Cretaceous traveled in huge herds across the plains. Both herbivore and carnivore nests found in Montana, Mongolia, China, and South America suggest that many Cretaceous dinosaurs nested in colonies and possibly even cared for their young after they hatched.
Mammals became more abundant during the Cretaceous. One group, the multituberculates, were a successful group of early mammals. By the end of the Cretaceous, when the dinosaurs were becoming extinct, the mammals survived and became a very successful group of animals.
A worldwide mass extinction occurred at the end of the Cretaceous period. This extinction killed off nearly 50 percent of Earth's existing species, including all of the remaining terrestrial dinosaurs other than birds, all marine and flying reptiles, and the ammonoids and other invertebrate and microscopic marine organisms. Many groups, however, including most plant species, birds, lizards and snakes, crocodiles and turtles, fish and sharks, many invertebrates, and the mammals survived into modern times.
The causes of this mass extinction are still not completely understood and have led to many lively debates among scientists. One theory suggests that massive volcanic eruptions ejected enormous amounts of ash and harmful gases into the atmosphere, creating dark and cold conditions inhospitable to some plants and animals. One hypothesis suggests that a large asteroid hit the Earth, contributing to devastating climate changes. Still others, however, counter with the argument that the global climate was already in the process of cooling off, perhaps in part because of the drying up of North America's inland sea, and that this slow cooling may have led to more gradual changes in plant and animal life. Another proposition is that multiple factors, rather than one single catastrophic event, was responsible for the mass extinction.
see also Geological Time Scale; K/T Boundary.
Lane, Gary, A., and William Ausich. Life of the Past. 4th ed. Englewood, NJ: Prentice Hall, 1999.
Russell, D. A. An Odyssey in Time: The Dinosaurs of North America. Chicago: North-wood Press, 1989.