In mid-Mohawkian time at the Turinian–Chatfieldian (Blackriverian–Trenton) transition, the extensive, shallow-, warm-water Blackriverian carbonate platform, in a low-latitude, subtropical setting across east-central Laurentia, underwent an abrupt change to temperate-water, sedimentary and faunal regimes. The change occurs across a regional unconformity, the formation of which is coincident with inception of a major Taconian tectophase and the related breakup of the Blackriverian Platform along old zones of structural weakness into smaller and higher platforms and shelves separated by a linear low area called the Sebree Trough. This breakup is interpreted to reflect far-field, foreland deformation, and temperate-water carbonates soon predominated across...