A buried Iowa crater finally comes of age.

Like and aging movie star, a large crater buried beneath the fields near Manson, Iowa, has deceived people with the appearance of youth. Scientists had thought that an asteroid or comet carved the crater 65 million years ago, concurrent with the mass extinction of the last living dinosaurs and many other species at the boundary between Earth's Cretaceous (K) and Tertiary (T) periods. But new work suggests that the Manson crater is older and played no role in the K-T die-offs.

Glen A. Izett from the U.S. Geological Survey (USGS) in Denver and his colleagues dated rocks collected last year when a team of researchers drilled 12 holes through the 100 meters of glacial deposits covering the Manson structure. Using the radioactive decay of potassium as a clock, they calculated that the impact occurred 73.8 million years ago.

That finding, reported in the Oct. 29 SCIENCE, surprised scientists with the drilling project. "We started our pretty confident the Manson was somehow involved in thje Cretaceous-Tertiary event. It turns out it isn't," says Eugene M. Shoemaker of the USGS in Flatstaff, Ariz.

A member of the group that misdated the Manson crater during the late 1980s, Izett says the earlier effort arrived at the wrong age because of the rock used in the analysis. Those researchers dated samples of a mineral called microcline, pulled up during drilling in the 1950s. Microline has relatively loose crystaline structure, which allows some of the argon produced by radioactive decay to escape. Thus, the rock appears younger than it is. In the most recent effort, the researchers dated sanidine, an ideal mineral because of its tight crystalline lattice.

The redating of the Iowa crater leaves only one other known suspect in the K-T drama: the Chicxulub crater in the Yucatan Peninsula. Geologists reported last year that this 80-kilometer-wide crater formed at the end of the Cretaceous and has the same age as clay layer found around the world (SN: 8/15/92, p.100). Scientists believe the clay to be fallout from an impact dust cloud that would have circled the Earth, blocking out light and knocking the climate out of kilter.

At 35 kilometers wide, Manson ranks as the second largest impact crater known in the United States. By redating the structure, Izett and his colleagues may help explain a curious layer of rock in South Dakota that has puzzled geologist for 50 years. Called the Crow Creek Member, the stratum consists of sand and broken pieces of shale quite unlike the smooth shale layers found above and below the Crow Creek. Geologists know the shale formed in the quiet bottom of the sea that covered the interior of North America at the time, but they have wondered what created the unusual Crow Creek layer.

Becasue the Crow Creek Member formed roughly 74 million years ago, Izett's tema suggests that the Manson impact created the layer, perhaps during a huge tsunami generated by the crash. In support of that theory, the group notes that the size of the sand grains and shale pieces grows bigger close to Manson. Izett's group also found that the Crow Creek layer contains silver of "shocked" quartz -- grains were lofted skyward by the Iowa impact and landed in South Dakota.