FAULTING AND POLYPHASE DEFORMATION NEAR MILLCREEK, SOUTHEASTERN MISSOURI

A recent road cut along U.S. highway 67 in southeastern Missouri exposes five stages of faulting near Millcreek, 4 km southwest of Fredericktown, Missouri. Stage-1 faulting produced a 40° ccw rotation of Mesoproterozoic volcanic and fluvial units in the hanging wall relative to the footwall along a fault striking N15°E and dipping 70°NW. This deformation was followed by intrusion of a microgranitic body and a large tridymite rhyolite dike. Stage-2 deformation involved a second rotational cycle along the same NE-trending fault that rotated the hanging wall segment of the tridymite dike 8° cw relative to the footwall segment. Stage-3 deformation occurred in Late Cambrian time along a left lateral strike-slip fault that strikes N5°E and dips 65° NW; this fault places Late Cambrian Bonneterre Formation (hanging wall) in contact with Mesoproterozoic rhyolite (footwall). These units are separated by a 2m wide megabreccia zone containing clasts of both wall rocks cemented by a coarsely crystalline mixture of dolomite and calcite. This fault has been traced over a distance of 2 km to the southwest. Stage-4 deformation is characterized by post-Bonneterre transpressional faulting that produced a splay of reverse faults (positive flower structure) deforming the overlying Bonneterre Formation in the southwestern portion of the road cut. Stage-5 deformation produced thrusting from the southwest along a detachment surface at the contact between the Lamotte and Bonneterre formations; this surface steps up into the Mesoproterozoic microgranite. Immediately above this thrust surface, boudinaged trains of sheared rhyolite appear in both faces of the road cut. The sole of this thrust sheet is marked by numerous northeast trending lineations varying in trend between N5°E and N43°E.