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Topic%2016_Cretaceous - Cretaceous Period The Cretaceous...

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6-1 Cretaceous Period The Cretaceous Period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic Period (i.e. from 145 million years ago to the beginning of the Paleocene epoch of the Tertiary Period about 65 Ma.. The youngest and longest geological period of the Mesozoic, the Cretaceous comprises about 80 million years. The end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras. The Cretaceous (from Latin creta meaning 'chalk' ) as a separate period was named for the extensive beds of chalk found in the upper Cretaceous of continental Europe and the British Isles (including the White Cliffs of Dover). The great Idaho batholith (including Moscow Mountain) is Cretaceous in age. Most periods of the Mesozoic and Paleozoic eras are about 50 million years long except this one. Most granite near Moscow is of this period. Dating No great extinction or burst of diversity separated the Cretaceous from the Jurassic. However, the end of the period is most sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 65.5 Ma. This bolide collision is probably responsible for the major Cretaceous– Tertiary (K-T) extinction event. Paleogeography During the Cretaceous, the late Paleozoic - early Mesozoic supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin orogenies that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies. Though Gondwana was still intact in the beginning of the Cretaceous, it broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. Geography of the US in the Late Cretaceous Period http://en.wikipedia.org/wiki/Image:US_cretaceous_general.jpg
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To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged. This happens to sea level when oceanic rifting occurs. The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic. Mid-ocean ridge activity--or rather, the circulation of seawater through the enlarged ridges--enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton. These widespread
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Topic%2016_Cretaceous - Cretaceous Period The Cretaceous...

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