Observer, Volume 16, Number 18, Thursday May 25 1995


Searching for spherules

by Bruce Simonson

In the course of a study of ancient sedimentary strata in Western Australia, I discovered a thin layer that contains an abundance of spherules that were originally molten (like BBs of red-hot lava). Deposits such as this form when a large comet or meteorite hits the earth at high velocity, along the lines of the collisions that took place on the planet Jupiter last summer. The spherules are formed by the melting of the impactor and/or target rocks and are dispersed over large areas known as strewn fields. The most famous strewn field formed 65 million years ago when a large body hit the earth at exactly the time the dinosaurs went extinct (coincidence? you be the judge). I have studied the spherule layer we discovered in Australia in some detail, as it is one of the oldest and best-preserved strewn fields on earth.

Single continent?

It's been known for decades that strata in the Transvaal Basin of South Africa are strikingly similar to the ones I've studied in Western Australia. Some researchers have suggested these two areas were originally part of a single continent that was torn apart by continental drift over a billion years ago. This interpretation, while plausible, has yet to be proven, and I plan to test it by searching for the spherule layer in the Transvaal Basin. Impact strewn fields are so big that, if Western Australia and the Transvaal Basin were part of a single continent at the time, the spherule layer could easily have blanketed the whole thing. In addition to the spherule layer, I will also search for two other distinctive layers formed by unusually large volcanic eruptions 2.6 billion years ago. These three layers make a very distinctive triumvirate that my co-workers (mostly Oberlin geology students) and I learned to identify rapidly and unequivocally during our five field seasons in Western Australia. With the new research and development grant I will travel to South Africa and spend about a month searching for these same layers in the Transvaal Basin. Dr. Nic Beukes, a geology professor at Rand Afrikaans University in South Africa and the world's leading authority on the geology of the Transvaal Basin, will give me expert guidance in the field to get the project started.

If I succeed in finding equivalents to one or more of the Australian marker beds in the Transvaal Basin, it would prove the theory that these two areas were part of a single continent at the dawn of earth history. It would also point the way to a new technique for correlating strata that are Precambrian in age, i.e., more than 600 million years old. Any new technique is welcome for rocks this old because, unlike younger rocks, Precambrian strata contain none of the shelly fossils that scientists generally use to determine if strata in widely separated locations are of the same age.

Bruce Simonson is professor of geology.


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