The Penokean Orogeny, a major event that occurred at 1850–1900 Ma in the Great Lakes region, involved deformation, metamorphism, and igneous activity. In Minnesota, the effects of the Penokean deformation are most clearly seen in the Thomson Formation. Here, two main Penokean deformational events are recognized. The first involved isoclinal recumbent folds (F1) with a well developed axial-planar foliation (S1). The second deformation involved open, upright and subhorizontal folds (F2) which folded bedding (S0) and the first foliation (S1), and refolded the early folds. A crenulation cleavage (S2) developed axial planar to the F2 folds. Post-Penokean structural features include joints, kink bands, and basalt dikes.Evidence for the first Penokean deformation is found in the southern region of Thomson Formation outcrop, whereas the features associated with the second Penokean deformation are found throughout its outcrop area. Thus, the only Penokean-age structural features seen at the type locality of the Thomson Formation are from the second Penokean deformation.

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