Geologist helps to revive 'layer cake' model of stratigraphy

November 05, 2001

An older model of the geological record is making a comeback-with new twists-partly through the research of University of Cincinnati geologist Carlton Brett.

In an invited address Monday morning, Nov. 5 at the annual meeting of the Geological Society of America, Professor Brett will explain why certain features of the once-derided "layer cake" model of stratigraphy actually make good sense in light of recent data.

"If you look at the Bahamas today, it's a patchwork...a mosaic of different environments," explained Brett. And that's exactly how most geologists in the 20th century believed the geologic record of marine environments should look. During the 19th century, however, geologists were most interested in outcrops and believed they could find widespread events buried in the fossil record.

"I see much the same thing," said Brett who uses a variety of techniques to trace major events from Ontario and New York into Ohio, Indiana and Kentucky. "We're developing the ability to recognize events at extremely high resolution. New techniques may allow us to track some events even though the subsurface-through rocks buried deeply below the ground surface."

Brett said it was the drive to find new pockets of oil and natural gas that helped revive the "layer cake" theory in a new guise. The more geologists looked, the more they found that certain surfaces in rocks could be found across wide geographic areas. Brett's personal interest is in reconstructing the Earth's paleogeography, examining major changes such as seismic events, sea level changes and climate change and linked biotic changes.

Ultimately, Brett believes it's possible to trace even some small-scale events over large areas. The seismic stratigraphy techniques used in oil exploration only work at a scale about 10-20 meters thick. Brett is looking at a scale of meters and even decimeter-level resolution.

As one example, he and other UC geologists recently began exploring trilobite mass mortality and mass molting events during the Ordovician. A late-summer flood along Sycamore Creek in northeastern Cincinnati, exposed a new section of rock, including the edge of a shale slab covered with the molted exoskeletons of Triarthrus, a small trilobite typically found in the New York-Pennsylvania area. Excavation by amateur paleontologist, Dan Cooper revealed the presence of more than 1500 well preserved molt parts on less than a square meter of the slab.

"It's the first time we've been able to document in the Ordovician that these animals were behaving in this organized way," said Brett, who believes the mass molting was linked to the organism's reproductive cycle. Here, apparently, a layer of mud from a massive storm buried the fragile trilobites molt parts shortly after the Ordovician "orgy" had gotten underway. In Greater Cincinnati, Triarthrus is not present at most levels; these beds also appear to record incursions of "exotic" species. The geologists believe they will be able to make use of this incursion to track the narrow bed containing the trilobite across the wide I-275 loop which surrounds the city and its suburbs. He has already found hints of this same layer as much as 20 miles away in Kentucky.

"Some beds of rock represent tens to thousands of years of geologic time," said Brett. "With this type of layer, you're seeing one day in the very bad day. That's dramatic. That's a time plane."

And for Brett, it's also important evidence that "the geologists in the 1800s weren't all wet about layer cake." Because very large events are preferentially represented in the geologic record it is perhaps not surprising that many layers can be traced over broad areas.

In addition to Brett's keynote talk, graduate student Patrick McLaughlin will present evidence Tuesday, Nov. 6 showing how sea level changes and tectonic events can be traced from New York through Pennsylvania into Ohio and Kentucky.

Another graduate student, Sean Cornell will present a poster Monday afternoon Nov. 5 explaining the use of gamma ray analysis to study both surface and subsurface rock layers, again with an emphasis on regional correlation.

University of Cincinnati

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