Bonneterre Formation
The Bonneterre Formation is an Upper Cambrian geologic formation which outcrops in the St. [Francois Mountains] of the Missouri Ozarks. The Bonneterre is a major host rock for the lead ores of the Missouri Lead Belt.
Description
The formation is dominantly dolomite with areas or layers of pure limestone. A shaley or glauconitic zone occurs in the lower portion and the base contains sand and conglomerate or breccia where the formation overlaps the Lamotte and lies directly on the granite of the mountain core.Stratigraphy
Early geologists offered a variety of names for what is now known as the Bonneterre Formation. In 1894,Missouri state geologist Arthur Winslow proposed St. Francois limestone as a name for thick limestone beds, including everything between what are now known as the Lamotte Sandstone and the St. Peter Sandstone. He described the lower part of that formation separately as the St. Joseph limestone. Charles Rollin Keyes's Fredericktown limestone included everything between the Lamotte and the Potosi Dolomite when he first described it in 1896, but his later uses of the name were in a more restricted sense equivalent to the modern Bonneterre.
In 1901, Frank Lewis Nason was the first to apply the name Bonneterre to these rocks, identifying a type section near the city of Bonne Terre, Missouri.