The Middle Proterozoic St. Francois igneous terrane of southeast Missouri is located on the northwest margin of the Reelfoot rift and was modified into a northeast-trending structural block during periods of Late Proterozoic-Early Cambrian extension and shoulder uplift. Late Cambrian facies patterns indicate that the St. Francois terrane was not a passive basement feature during early periods of thermal subsidence of the Reelfoot rift. Facies relations also show that basement response to syndepositional tectonics is the principal control on rift-margin geometry, and not sedimentary loading as assumed by models. Syndepositional tectonics that influenced southeast Missouri during Late Cambrian time are interpreted to be a result of in-plane stress related to changing regional stress fields.

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