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Artwork of Cretaceous period scene, showing many dinosaur and other species, including pterosaurs, nautoloid mesosaur, pleisiosaur, hesperomis, belemnite, and other life-forms. (Publiphoto/Photo Researchers, Inc.)
of seafloor spreading were dramatically increased. The consequential displacement of seawater caused global sea levels to rise, so the Late Cretaceous was marked by high sea levels and the deposition of shal low water limestones in many epicontinental seas around the world. on the North American craton, the Zuni sequence was deposited across wide parts of the craton during this transgression. Increased magmatic activity in the Cretaceous may reflect more rapid mantle convection or melting, as marked by a number of igneous events worldwide. The south American Cordillera and the western united states saw unusual amounts of intrusive and volcanic activity. The giant flood basalt provinces of Paraná in south America and the Deccan of India were formed, and kimberlite pipes punctured the lithosphere of south Africa and Greenland. The dispersal of Pan-gaea was associated with the opening of the Atlantic ocean. Africa rotated counterclockwise away from south America, closing the Tethys ocean in the process of opening the Atlantic. The closure of Tethys was associated with the emplacement of many ophi-olites onto continents, including the giant oman (semail) ophiolite that was thrust to the south onto the Arabian continental margin.
Cretaceous sedimentary patterns suggest that the climate was warming through the period and was more varied and seasonal than in the earlier Meso-zoic. The famous Cretaceous chalks were formed by the accumulation of tests of calcareous marine algae known as coccoliths, which thrived in the warm shallow seas. The chalks are in many places interbedded with fossiliferous limestones with abundant brachio-pods and rudist coral fragments.
Life on the Cretaceous continents saw the development of the angiosperms, which became the planet's dominant flora by the middle of the period. Invertebrate and vertebrate animals were abundant and included many species of dinosaurs, giant flying pterosaurs, and giant marine reptiles. Dinosaurs occupied many different geological niches, and most continents have fossil dinosaurs, including herbivores, carnivores, and omnivores. Birds had appeared, both flying and swimming varieties. Mammals remained small, but their diversity increased. Many life-forms began a dramatic, progressive disappearance toward the end of the period. These marine and land extinctions seem to be a result of a combination of events including climate change and exhalations from the massive volcanism in the Indian Deccan and south American Paraná flood basalt provinces, coupled with an impact of a six-mile (10-km) wide meteorite that hit the Yucatán Peninsula of mexico. The extinctions were not all abrupt—many of the dinosaur and other genera had gone extinct, probably from climate stresses, before the meteorite hit the Yucatán Peninsula. When the impact occurred, a 1,000-mile (1,600km) wide fireball erupted into the upper atmosphere, and tsunami hundreds or thousands of feet (hundreds of meters) high washed across the Caribbean, southern North America, and much of the Atlantic. Huge earthquakes accompanied the explosion. The dust blown into the atmosphere immediately initiated a dark global winter, and as the dust settled months or years later, the extra carbon dioxide in the atmosphere warmed the Earth for many years, creating a greenhouse condition. Many forms of life could not tolerate these rapid changes and perished. The end Cretaceous extinction, commonly referred to as the K-T event, is one of the most significant mass extinction events known in the history of life.
See also Cenozoic; mass extinctions; Mesozoic.
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