Structure of Winnfield Salt Dome, Winn Parish, Louisiana¹

Megascopic structural features were mapped of that part of the Winnfield, Louisiana, salt dome exposed in the Carey Salt Company’s mine, located 811 feet below the surface and about 400 feet below the flat top of the dome.

Layering of the salt is visible throughout the mine. White to dark gray beds of halite and anhydrite, 1 inch to 1 foot in thickness, are intricately folded. Both the bedding layers and fold axes dip and plunge steeply toward the center of the dome. The increased intensity of layering and the concordancy near the boundary of the dome show the close relation between the structural attitude of the plastically deformed original salt layers and the edge of the salt dome.

Pencil-shape aggregates of anhydrite, as long as 18 inches, parallel the fold axes; most of them plunge steeper than 70° inward toward the dome and support the interpretation of upward and outward movement of the salt during its diapiric emplacement in the surrounding rocks. The structural data also suggest the possibility that the dome overhangs the enclosing sedimentary rocks on the south and west.

Structurally the Winnfield dome is similar to the Jefferson Island, Louisiana, and Grand Saline, Texas, domes as mapped by Balk.

Special features in the Winnfield dome include the emanation of interstitial brine and CO₂ into the mine. “Blowouts” which result in cavities up to 100 feet in length have occurred in parts of the mine near the salt-dome boundary. Microscopic examination and chemical analyses of the salt indicate the “blowouts” probably are due to instability caused by high CO₂ pressures in numerous small gas-filled cavities close to a free-air surface.