The high-grade sylvinite orebody (28% K2O) mined by the Potacan Mining Company (formerly the Denison Potacan Potash Company) near Sussex, New Brunswick, Canada, has undergone intense internal folding. The folds vary in size from centimetres to hundreds of metres, are mostly tight to isoclinal, recumbent and asymmetrically overfolded towards the southeast, and belong to a single varied style group suggesting one deformation event. Folds are noncylindrical to sheath-like locally and have highly variable cross-sectional morphologies. Slickensides are locally common and penetrative. Slickenline trends in all parts of the mine are northwest–southeast. All the evidence is consistent with a southeast-directed movement across the top of the evaporite body having caused extensive ductile drag folding within both the sylvinite and halite lithologies. The evaporites, in fact, have all the characteristics of a near-horizontal mylonite zone. Variscan deformation during the Late Carboniferous is generally cited as being the most recent tectonic activity immediately north of the Caledonia Highlands, and the evaporite deformation has, in the past, been attributed to these events. This viewpoint is here reexamined, since reverse faults that are locally associated with the Variscan orogeny do not adequately explain the evaporite deformation. With increasing evidence for significant Mesozoic extensional faulting and the need for low-angle detachments, it is more likely that the evaporite deformation is linked to these later events than to earlier Palaeozoic deformation.