Title: Manson didn't kill dinosaurs.   
Subject(s): METEORITE craters -- Iowa
Source: Astronomy, Feb94, Vol. 22 Issue 2, p16, 2p
Abstract: States that for the past several years, the buried impact
crater at Manson, Iowa, has been a minor suspect in the mystery of
dinosaur extinction 65 million years ago. Details of research
conducted by Glen Izett and his colleagues at the US Geological Survey
in Denver.
AN: 9401037822
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

Section: ASTRONEWS

MANSON DIDN'T KILL DINOSAURS

For the past several years, the buried impact crater at Manson, Iowa,
has been a minor suspect in the mystery of dinosaur extinction 65
million years ago. No longer. A team of researchers led by Glen Izett
of the U. S. Geological Survey in Denver has conducted new field work
at the 35-km-wide crater, which lies buried under about 300 feet of
glacial debris in central Iowa.

Their studies, using minerals that preserve their potassium-argon
radioactive dates better than those used in previous studies, show
that the Manson crater formed 73.8 million years ago. That's almost 9
million years before the colossal event that ended the Cretaceous
period in a mass extinction.

The new date leaves the Chicxulub structure in Mexico's Yucatcn as the
only known crater dated exactly at the Cretaceous-Tertiary (K-T)
boundary. However, Chicxulub's newly revised size (see previous news
story) appears to make it more than adequate to explain the K-T mass
extinction.

The new date for Manson may also help solve a different puzzle. A
South Dakota rock layer called the Crow Creek Member is full of sand
and broken shale bits, very unlike the quiet-water shales that lie
immediately above and below it. Crow Creek's date is about 74 million
years. The team speculates that the Manson impact may have triggered a
tsunami, a tidal wave, which stirred up the shallow ocean then
covering South Dakota and laid down a layer of crater ejecta and other
disturbed material.
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Source: Astronomy, Feb94, Vol. 22 Issue 2, p16, 2p.
Item Number: 9401037822