Geotimes - July 2003 - Jumbled Missouri geology Linked to Impact

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Impacts
Jumbled Missouri geology linked to impact

Combining a good bit of geologic sleuthing with a fortunate twist, Kevin Evans and colleagues at Southwest Missouri State University (SMSU) have uncovered what may be the fifth-largest impact structure in the United States.

Dubbed the Weaubleau-Osceola Impact site by the researchers, it has been a conundrum since geologists first studied it in the 1950s. Explanations for the region’s highly deformed, folded, faulted and brecciated rocks centered upon volcanic events and other terrestrial processes; most researchers did not consider impact hypotheses until the last few decades.

Already working on the hunch that a meteorite was responsible for the swath of complex geology in southwestern Missouri, Evans uncovered the structure’s larger-than-expected circular outline while saving a graphics file.

“I drove to Osceola to visit the site after I had found it through computer mapping. That’s when I saw the polymict breccia and other tantalizing features,” Evans says. “A week or so later, half of the department, geology students and faculty, were poring over the roadcuts and quarry exposures.” Although terrestrial processes can create polymict breccias, which have clasts from multiple source rocks, the breccias can indicate impact events.

The Weaubleau-Osceola impact structure features extensively folded limestone rocks. The site, which may be the fifth-largest impact structure in the United States, has attracted attention and support from the local community — from science students to “interested folks from all walks of life,” says researcher Kevin Evans at Southwest Missouri State University. Photo by Chris Barnhart, SMSU.

Using a software tool for merging digital elevation maps, Evans stitched together the graphics file from four 7.5-minute U.S. Geological Survey quadrangles, which he presented in March at a Geological Society of America North-Central Section meeting in Kansas City. Because the corners of the four maps dissect the impact structure, Evans could not see the telltale round shape until he looked at a thumbnail image of his work.

Although the shape of the 19-kilometer-diameter structure is intriguing, some of the most telling clues are from shocked minerals in the area. Riddled with multiple sets of parallel fractures and deformation features, these grains could have only resulted from the intense pressures of two types of events (outside of laboratory experiments): impacts and nuclear explosions. Charles Rovey, one of the SMSU team members, identified shocked quartz grains in the breccia that overlies the impact structure, suggesting that the jumbled rock may be fallout from the impact.

Ray Anderson, a geologist with the Iowa Geological Survey and an expert on Iowa’s Manson impact structure — the largest intact, on-shore meteorite crater in the United States — finds the SMSU data interesting but is hoping further study will bring more conclusive evidence.

“Like all impact structures, the identification won’t be definitive unless abundant shocked quartz is found,” Anderson says. “Scattered quartz in any deposit may contain planar deformation features as detritus from the weathering of other impact sites, so rocks must be found containing abundant [shocked] quartz grains.”

The proposed Weaubleau-Osceola impact structure is roughly outlined by the northwest, southwest, and northeast drainage patterns seen on this image of the region, created in part with MacDEM software. Image by Kevin Evans, SMSU.

The SMSU team has correlated the sandstone that overlies the breccia with formations that are roughly 310 million years old, while the deformed limestones below correlate to formations from approximately 340 million years ago. The limestone and breccia had a chance to weather before the deposition of the cleaner sandstone, so the researchers have linked the impact’s age more closely to 340 million years.

“One of the truly amazing aspects of the Weaubleau-Osceola impact site,” Evans says, “is that it was more or less hermetically sealed following the event, give or take a few million to tens of millions of years ... amazing for [a structure] more than 300 million years old.”

The structure’s age may correlate to two other Missouri structures, Decaturville and Crooked Creek, that some have proposed are remnants from a string of related impacts; however, age constraints are still rough. Says Anderson: “I hope that the SMSU researchers are able to continue their work on the structure and develop additional data that will advance our understanding of the string of features.”

Josh Chamot
Geotimes contributing writer

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