Calderas in the Precambrian terrane of the St. Francois Mountains, southeastern Missouri.

Precambrian igneous rocks in the St Francois Mountains of SE Missouri consist of silicic volcanic rocks, mostly ash flow tuff, which crop out mostly in the W and SW part of the region, and epizonal granitic plutons which are mostly exposed in the NE part of the area. These relations suggest that the exposures in the NE are of a somewhat deeper part of the crust and that rocks there are now exposed because of W-southwesterly tilting and beveling by erosion. In the eastern St Francois Mountains the occurrence of a thick rhyolitic ash flow tuff, Grassy Mountain ignimbrite, and its distribution relative to a large, high-silica pluton, Butler Hill granite, suggest that the ash flow is the principal eruptive material of a caldera, the Butler Hill caldera, and that Butler Hill granite is the frozen magma chamber that was originally beneath it. Two other calderas are also postulated.-after Authors