AN UPLAND GRAVEL COMPLEX IN WEST PLAINS, MISSOURI

Ridge-top gravels, historically called “Lafayette,” are present at various sites across the Midwestern U.S. They commonly contain highly polished chert and cobble-to-boulder size quartzite with a matrix of red sand. Their age(s) is controversial. Such a deposit is preserved in a gravel pit on a drainage-divide summit in West Plains, Missouri. The highwall exposes up to 7m of rounded gravel and cobbles above a massive structureless clay. Most clasts are chert, but quartzite, silica-cemented sandstone, and chert breccia are also present. The gravel is clast supported, but very poorly sorted with a wide range in clast size and a sand matrix. The quartzite seems to include both metamorphic and sedimentary varieties, but serious petrologic study has not been undertaken. The largest quartzite clast found to date is 17cm in diameter. The contrast between the advanced state of chemical stability of the clasts and the textural immaturity is enigmatic. The quartzite’s provenance is another riddle, given that quartzite is not exposed for hundreds of kilometers from West Plains.

The highwall also exposes multiple clastic dikes sourced from the underlying clay. The dikes extend upward at least 3m into the gravel, indicating that the clay was liquified after gravel deposition, likely by seismicity associated with the nearby New Madrid fault zone.