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Grooving on exhumed carbonate margins.

Some of the largest accumulations of oil are found in the kinds of rocks that make up carbonate platforms, which are underwater shelves composed primarily of sea-animal shells. Scientists like to study the sides, or margins, of such platforms to better understand how the platforms, and potential oil reserves, develop. Unfortunately, the margins of most modern platforms lie in deep, dark waters and can only be studied bit by bit in the light beam of a submersible.

Now, a discovery by two graduate students at the University of California at Berkeley will expedite the study of one kind of margin, in which sediments "bypass" or completely flow down the margin slope. David Bice and Keven Stewart recently found, in Italy's Sibillini Mountains, the first two examples of ancient bypass margins that are now elevated well above sea level.

The researchers believe the margins originally formed about 180 million years ago in the Jurassic period, when faults sliced through a carbonate platform as the crust near what is now Italy stretched out and thinned during the opening of an ocean basin. Heavy sediments from the top of the platform blocks then flowed down the sides, eroding the underwater carbonates and etching deep channels into the margin. Eventually, however, sediments filled in the grooves, obscuring this bypass channel pattern and contributing to the drowning of the platform blocks.

"Just by fortune, this [bypass] surface has been perfectly exhumed," says Bice. When the Sibillini Mountains formed, the crust was compressed and the carbonate blocks were uplifted. Over time, the sediments filling in the grooves were eroded to expose the bypass pattern. "Since it's so nicely exposed, it gives us a good chance to see what kind of processes shape these margins," adds Bice.
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Title Annotation:petroleum geology discovery in Italy
Author:Weisburd, Stefi
Publication:Science News
Date:Nov 16, 1985
Previous Article:Direct view: dynamics of development.
Next Article:Modern antibodies date ancient shells.

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