Conventional fold belts have easily recognizable folds and thrusts, but pillow fold belts are harder to recognize because shortening is subtle and superposed. Pillow fold belts contain (1) preshortening halokinetic pillows and (2) synshortening features such as thrusts, contractional kink bands, box folds, and local, strong onlap and truncation.
The Atwater fold belt has long been thought to die out landward of the frontal high-relief folds. We propose instead that the frontal anticlines pass landward into a pillow fold belt. This belt continues to the present Louisiana coastline to support an ultradeep shelf play, like the Davy Jones discovery. Here mid-Miocene subcanopy shortening is juxtaposed with late Miocene supracanopy extension. Supported by physical modeling, we infer that a lower linked system detaches on autochthonous salt and an upper linked system detaches on Miocene salt canopies. The two stacked detachments explain why extension above the canopy was almost coeval with shortening below the canopy, which is otherwise inexplicable. Shortening below the canopy in the pillow fold belt created the anticlines for the subsalt play.