The Weaubleau-Osceola structure is thought to be a meteorite impact site in western Missouri near the towns of Osceola and Weaubleau. It is believed to have been caused by a 1200-ft (366 m) meteoroid between 310 and 340 million years ago during the late Mississippian period.
The structure consists of an area of severe structural deformity and extensive brecciation that was poorly understood and had been thought to be the result of either faulting or of a cryptoexplosive event. A 19-km-diameter (12 mi) circular structure was discovered by geologist Kevin R. Evans, Assistant Professor of Geology at Missouri State University, through examination of digital elevation data. Originally called the Weaubleau structure after Weaubleau Creek that cuts through it, now that its scope is better understood it is known as the Weaubleau-Osceola structure.
Because the site was soon sealed by Pennsylvanian period sediments, and only partially exposed to erosion relatively recently, its structure is well preserved, and its age can be determined with fair accuracy. It is one of a series of known or suspected impact sites along the 38th parallel in the states of Illinois, Missouri, and Kansas. These 38th parallel structures are thought to possibly be the result of a serial impact, similar to that of comet Shoemaker-Levy 9 on Jupiter, an extremely unlikely event on Earth. The argument for a serial strike would be greatly strengthened if the ages of the other 38th parallel structures could be constrained to the same period as the Weaubleau-Osceola structure.
The Weaubleau-Osceola structure is one of the fifty largest known impact craters on earth and the fourth largest in the United States. The three larger ones in the US either have been glaciated and buried (Manson crater), are under water (Chesapeake Bay crater), or have been subjected to orogeny (Beaverhead crater). Therefore the Weaubleau-Osceola structure is the largest exposed untectonized impact crater in the US.
Long thought to be a glacial remnant, these conglomerate rocks are found in the area of Osceola. They are nearly perfectly round, and are referred to locally simply as "round rocks" or "Missouri rock balls".
Current theory suggests that these rocks are chert concretions, created when the impact threw pieces of shale away from the center of the crater, and later silica-rich materials formed around the shale seeds.