A cause for the Cretaceous-Tertiary mass extinction
By: Sarah Flaskerud
Several mass extinctions have occurred during geological time. One of these occurred at the Cretaceous-Tertiary boundary. There are many hypotheses that have been posited to explain the events that caused the K-T mass extinction. The hypothesis that is the focus of my seminar is that the mass extinction occurred as a result of an asteroid impact. A site that has been selected for the impact is on the Yucatan Peninsula. The Chicxulub crater, named after a city near the impact site, is claimed to be 120 miles (180 km) wide and 1 mile deep. Some evidence supporting the impact is the dent and rings that circle the crater which are found submerged underneath the water in the Gulf of Mexico. More supporting evidence is in the form of elevated iridium levels and shocked quarts. Shocked quarts forms in 3 settings: high-pressure laboratory experiments, nuclear test sites, and known giant meteor or asteroid impact sites. Tektites have also been found which are commonly formed during a meteorite or comet collision. Tsunami deposits are also reported from all around the Gulf of Mexico including Texas. The uppermost Cretaceous and lowermost Tertiary rocks of the Brazos River in Texas are continental margin deposits of shelf mudstone with no sandstone beds except at the K-T boundary. The boundary sandstone contains iridium. The impact hypothesis states that an asteroid hit Earth sending a cloud of dust and debris into the stratosphere, blocking out the sun, stalling photosynthesis and killing plants. This would have resulted in the collapse of the global food chain causing extinction in some, but not all species. The impact hypothesis is widely considered to be the cause for the Cretaceous-Tertiary mass extinction.
Examples of an unshocked quartz grain (left) and a shock-metamorphosed quartz grain (right). The planar features, cross-cutting the shock-metamorphosed grain are essentially microcrystalline faults. The shock metamorphosed grain was ejected from the Chicxulub crater and found in Haiti.
Image showing the 4 rings of the Chicxulub crater at the Yucatan Peninsula. Image from Earth in Space Vol. 8, No. 4, Dec. 1995.
Image showing altered tektites from the Chicxulub impact deposited at Dogie Creek, Wyoming. The scale bar shows millimeters. Image courtesy of Geologica Survey of Canada. Image from http://www.dsa.uqac.uquebec.ca/~mhiggins/MIAC/chicxulub.htm
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