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Unformatted text preview: 6-1Cretaceous PeriodThe Cretaceous Period is one of the major divisionsof the geologic timescale, reaching from the end ofthe Jurassic Period (i.e. from 145 million years ago tothe beginning of the Paleocene epoch of the TertiaryPeriod about 65 Ma.. The youngest and longestgeological period of the Mesozoic, the Cretaceouscomprises about 80 million years. The end of theCretaceous defines the boundary between theMesozoic and Cenozoic eras.The Cretaceous (from Latin creta meaning 'chalk' ) asa separate period was named for the extensive beds ofchalk found in the upper Cretaceous of continentalEurope and the British Isles (including the WhiteCliffs of Dover). The great Idaho batholith (includingMoscow Mountain) is Cretaceous in age.Most periods of the Mesozoic and Paleozoic erasare about 50 million years long except this one.Most granite near Moscow is of this period.DatingNo great extinction or burst of diversity separated theCretaceous from the Jurassic. However, the end ofthe period is most sharply defined, being placed at aniridium-rich layer found worldwide that is believed tobe associated with the Chicxulub impact crater inYucatan and the Gulf of Mexico. This layer has beentightly dated at 65.5 Ma. This bolide collision isprobably responsible for the major CretaceousTertiary (K-T) extinction event.PaleogeographyDuring the Cretaceous, the late Paleozoic - earlyMesozoic supercontinent of Pangaea completed itsbreakup into present day continents, although theirpositions were substantially different at the time. Asthe Atlantic Ocean widened, the convergent-marginorogenies that had begun during the Jurassiccontinued in the North American Cordillera, as theNevadan orogeny was followed by the Sevier andLaramide orogenies.Though Gondwana was still intact in the beginning ofthe Cretaceous, it broke up as South America,Antarctica and Australia rifted away from Africa(though India and Madagascar remained attached toeach other); thus, the South Atlantic and IndianOceans were newly formed. Such active rifting liftedgreat undersea mountain chains along the welts,raising eustatic sea levels worldwide. Geography of the US in the Late Cretaceous Periodhttp://en.wikipedia.org/wiki/Image:US_cretaceous_general.jpgTo the north of Africa the Tethys Sea continued tonarrow. Broad shallow seas advanced across centralNorth America (the Western Interior Seaway) andEurope, then receded late in the period, leaving thickmarine deposits sandwiched between coal beds. Atthe peak of the Cretaceous transgression, one-third ofEarth's present land area was submerged.This happens to sea level when oceanic riftingoccurs.The Cretaceous is justly famous for its chalk; indeed,more chalk formed in the Cretaceous than in anyother period in the Phanerozoic. Mid-ocean ridgeactivity--or rather, the circulation of seawater throughthe enlarged ridges--enriched the oceans in calcium;this made the oceans more saturated, as well asincreased the bioavailability of the element forcalcareous nanoplankton. These widespreadcalcareous nanoplankton....
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This note was uploaded on 08/23/2009 for the course GEOL 102 taught by Professor Kennethsprenke during the Fall '09 term at Idaho.
- Fall '09