ABSTRACT

Along the north rim of the Mississippi Salt Basin, the Louann salt was deposited as a basinward thickening wedge. Shallow piercement domes are found only in the deeper portion of the basin where the salt has attained a considerable thickness. Around the rim of the basin, between the shallow piercement dome area and the updip limits of the salt, more obscure structures are formed which take the shape of salt ridges, pillows and anticlines. Salt structures having similar characteristics are developed in areas of similar salt thickness. In the area studied along the northeast rim of the Mississippi Salt Basin, these salt structures can be classified as four common types.

The farthest updip structures are along the wedgeout of the salt. These are formed by flowage of the salt toward the wedgeout, which developed a salt ridge parallel to the wedgeout and immediately basinward from it. The suggested name for this type structure is "peripheral salt ridge." Reverse dip is exhibited from the crest of this ridge to the wedgeout. Associated with this type structure is a complex graben system in overlying sediments. This graben system is caused by stresses created by salt movement adjacent to a stable floor updip from the salt wedgeout. Flowage of salt in the peripheral salt ridge has been slow and continuous since deposition of first overburden.

Immediately downdip from these peripheral structures, the mother salt is still relatively thin and forms "low relief salt pillows." These exhibit low relief anticlinal closures aligned in ridge-like patterns. Salt movement in the pillows was rapid during early deposition of overburden but showed little structural movement after deposition of the Haynesville.

Basinward from the "low relief salt pillows," the mother salt attains greater thickness and structures developed have the appearance of "intermediate salt anticlines." These attain a fair size and show major growth during deposition of Jurassic rocks but little movement during Cretaceous and younger deposition. The differential salt uplift on these structures is usually sufficient to form a primary graben over its crest. This graben is sometimes spread apart at Smackover and Norphlet horizons to form a "drawbridge" pattern. On structures exhibiting the "drawbridge" pattern younger sediments are deposited directly on salt within the graben area.

Basinward from the intermediate salt anticlines, the salt attains greater thickness and results in formation of "high relief salt anticlines." These structures exhibit major differential growth from deposition of earliest overburden through Upper Cretaceous times. Commonly a primary graben was developed during the Jurassic which terminates upward in Lower Cretaceous rocks. Continued movement of the salt has caused development of a secondary graben within the primary graben which is most prominent in Upper and Lower Cretaceous sediments. A pronounced "drawbridge" effect at Smackover and Norphlet horizons is common on the high relief salt anticlines.

A study of the thin salt area around the north rim of the Mississippi Salt Basin indicates that salt flowage and structural deformation has taken place even in areas of very thin mother salt. As the salt thickens from its wedgeout basinward, the structures formed progressively farther downdip have more salt available and thereby develop a progressively thicker salt core. Salt flowage began very early after deposition of a small amount of overburden. Most salt structures exhibited a fast rate of growth in the early stages which continued as long as abundant salt was available from the mother salt bed. Faulting is confined to strata above the salt.