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Asteroid origin of the Everglades?

Asteroid origin of the Everglades?

A crowd of earth scientists, eager to hear Edward J. Petuch's saga of how the Everglades formed, packed the lecture room at last week's Geological Society of America meeting in Orlando, Fla. What enticed so many people was Petuch's idea that the evolution of the United States' largest tropical wetland began when an asteriod slammed into the region 36 million years ago, at the end of the Eocene epoch.

However, at least one crater specialist in the audience remained unconvinced. "There are seeds of something very interesting in his talk, but I don't think it points to an impact,' observes Gene Shoemaker of the U.S. Geological Survey in Flagstaff, Ariz.

The Everglades is a swampy and forested area surrounded by an ovalshaped system of ridges upon which most of southern Florida's cities sit. "The general consensus has been that the Everglades is a surficial [surface] feature, a little puddle or very shallow basin produced . . . in the Holocene [the epoch of the last 1 million years] as ground water etched down into the limestone and sand built up around the rim,' says Petuch, a paleontologist at Florida International University in Miami. But now, drawing on nine years of his own field work and the data of others, Petuch presents a much different picture.

He and Jack Meeder of the University of Miami in Coral Gables, Fla., recently discovered that a giant, oval-shaped coral reef, dating from the Pliocene epoch 6 million years ago, is buried beneath the rim surrounding the Everglades. "My curiosity was really stimulated,' says Petuch. "No other carbonate platforms [limestone beds] in the world have anything comparable to this.'

So Petuch started poring through the literature. He found that the layer of limestone formed 25 million years ago, during the Oligocene epoch, dipped under the southern tip of the Everglades; at its lowest point, other scientists had measured a magnetic field strength greater than 25 times that of the surrounding region. Then Petuch discovered that 250 to 300 meters of the next-deepest section, originally laid down about 40 million years ago, during the Eocene epoch, were missing over most of the southern part of the Everglades. Other scientists had noted that an extensive network of fractures in the rocks works its way up through the Eocene layers but then abruptly stops.

For Petuch, all of this adds up to an impact. Near the end of the Eocene, he postulates, an asteroid slammed into the limestones covered by about 180 meters of water, fracturing rocks and generating enormous tidal waves that may have swept away much of the nearby Eocene sediment. Perhaps a piece of the asteroid became lodged in the crater, or the impact induced molten rock to well up from the underlying mantle. Either of these events might explain the anomalous magnetic field readings obtained today. Petuch thinks the Everglades basin actually formed during the next epoch, the Oligocene, when the sea level dropped about 300 meters and the climate became much colder. Groundwater percolating down the limestone may have leached out the mineral gypsum, weakening the limestones and causing the collapse of a basin about one-third the size of the present Everglades.

Petuch proposes that as the world warmed at the end of the Oligocene, coral reefs began to form around the essentially circular collapsed basin. But as sea level rose, the reefs grew toward the north, where the ground was higher. Petuch thinks that by 4 million years ago, this process had caused the reef system to elongate into an oval that completely enclosed an area much larger than either the original crater or collapsed basin. Sediment and sand filled in the Everglades basin, built up the coral atoll and eventually connected the atoll to the mainland.

What really convinced Petuch of the impact idea, he says, were studies of the Eocene-Oligocene boundary layer in the Caribbean island of Barbados. Scientists had found that this layer contains high levels of iridium (an element abundant in extraterrestrial bodies but rare on earth) but none of the silicate spherical particles called tektites that are usually created when an asteroid collides with continental rocks. Petuch argues that had an asteriod bombarded the carbonate rocks under Florida, carbon dioxide and calcium oxide would have been produced. And since calcium oxide grains are water soluble, any trace of them at the Eocene-Oligocene layer would have long ago dissolved, leaving only the iridium dust.

While Shoemaker has no quarrel with Petuch's scenario from the growth of the coral reefs onward, he doesn't think there is evidence for an impact. He argues that the proposed asteriod would have deposited material outside the crater and would not have wiped away the Eocene layers. He also contends that chunks of asteriods are never found buried beneath craters and that the magnetic anomaly in Florida is not consistent with the way impacts are known to alter the magnetic fields of rocks.

Petuch says he welcomes other theories explaining the Florida magnetic anomaly, but he thinks the impact idea is the only plausible one now in the running. "This is the last place in the world you'd expect to see such an anomaly, because the nearest igneous rock [that would have a magnetic signal] is over 5 miles straight down through solid limestone,' he observes. Moreover, he argues that the bulk of impact research to date has focused on craters in continental crust and that the record of an impact may look considerably different in carbonate rocks. The Florida impact could represent "a whole class of craters to itself, completely different from any other one known,' he says.

If Petuch is proved wrong, he has, at the very least, raised some tantalizing questions about the geology of Florida--questions that he hopes will inspire more field work. And if he is right, he should expect standing-room-only crowds for some time to come.

Photo: At right, dark ridges surround the ovalshaped Everglades. The stippled area represents the missing Eocene layer. Petuch thinks an asteroid hit near the Everglades' southern tip, creating the fracture zone and leading to the sedimentary patterns shown in the cross section above. (Letters refer to geologic formations and geographic locations.)
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Author:Weisburd, S.
Publication:Science News
Date:Nov 9, 1985
Words:1032
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