The St. Francois Mountains igneous complex is a shallow batholith about 1,500 m.y. old which has intruded a roof formed by its own volcanic ejecta. It is similar to other shallow-roofed batholiths such as the Boulder batholith. However, it has been tilted and beveled by erosion so that the roof is available for study at a wide range of stratigraphic levels. The volcanic roof is composed mostly of ash-flow tuff sheets, implying that caldera-collapse eruptions were the major mechanism of formation of the roof. Only two calderas have been proposed in the St. Francois Mountains thus far, the Taum Sauk caldera and the Butler Hill caldera.
The southern ring fault of the Taum Sauk caldera is located in the Ketcherside Mountain area of southeastern Missouri. The fault is nearly vertical, strikes about N50°E, and is intruded by a small body of porphyritic granite with vertical flow foliation. Rocks in the down-dropped block are broken by high-angle faults as much as 3 km from the ring fault, but extracaldera units are little deformed. Associated with the ring fault are bedded tuffs and volcaniclastic sedimentary units, some of which are mineralized. These data support the hypothesis that the Taum Sauk caldera is a trap-door caldera with major faulting along the eastern and southern walls. There is little evidence of faulting along the western wall.
The Butler Hill caldera occurs in the eastern St. Francois Mountains and is centered approximately on the Butler Hill Granite. The caldera, one of the earliest features to develop, has been obscured by later structural modification, but the major collapse ash-flow tuff of the caldera, the Grassy Mountain Ignimbrite, is well exposed. Analysis of flow directions of the ignimbrite indicates a source for the unit in the eastern St. Francois Mountains, as predicted by the Butler Hill caldera model.