Recent exploration activity in the Smackover trend along the northeastern part of the Mississippi Salt basin has provided much new information for the interpretation of salt-formed structures. The study area is Clarke and Wayne Counties, Mississippi, and Choctaw County, Alabama. Improved seismic techniques and much new well control on the Louann Salt provide data to group structure types.

All structures studied are the result of salt flowage, and structural types are correlative primarily with amount of salt available to the structure from the mother salt bed.

Four principal categories are recognized in the area studied. These are, progressing from thinnest salt along the rim of the basin to thicker salt basinward:

1. Periphery salt ridges:
Around the periphery of the Mississippi Salt basin, the Louann Salt wedges out against an older Paleozoic shoreline. With subsidence of the basin after deposition of the salt, differential forces were created which caused flowage of the salt toward the wedgeout. This has produced a series of salt ridges along the updip limit of the salt. Stresses caused in overlying beds by this salt flowage resulted in a complex system of graben faulting in shallower beds. Such faults generally parallel the updip limit of the Louann Salt (the Pickens-Gilbertown-Pollard fault system). The salt thickness at the apex of peripheral salt structures ranges from approximately 500 to 2,000 ft. The Quitman field, Clarke County, Mississippi, and the Choctaw Ridge field, Choctaw County, Alabama, are examples of th s type.

2. Buried salt ridges:
Downdip from the peripheral salt-ridge structures, the mother salt layer is relatively thin and salt supply to structures very limited. Salt flowage apparently took place soon after deposition of a moderate sedimentary overburden. Available salt was depleted during early structural growth so that there is very little structural growth indicated after Jurassic time. These structures appear as elongated ridges usually aligned parallel with regional strike. The salt thickness at the apex of these structures ranges from 500 to 2,500 ft. The Nancy field, Clarke County, Mississippi, is an example of this type.

3. Intermediate salt structures:
Basinward from the buried salt ridges, the mother layer is thicker, and the salt-formed structures have the appearance of an elongated deep-seated dome with a greater uplift. These structures are found directly downdip from the salt ridges and produced the traps at such fields as Cypress Creek. These structures show pronounced growth in older Mesozoic rocks and may produce an initial graben. In some places sufficient salt uplift took place after the graben was initiated to spread the graben faults apart (generally less than 0.75 mi), so that younger Jurassic beds were deposited over the salt within the graben, and pre-Haynesville (Late Jurassic) beds may be found only on the upthrown sides of these original faults. Apparently there was not sufficient salt available to permit significant rowth of these structures during Late Mesozoic and younger times. As a result, only slight indications of structure are observed in beds above the Jurassic. The salt thickness at the apex of the intermediate structures ranges from 2,500 ft to 5,000 ft.

4. High-relief salt structures:
Farther basinward, the mother salt layer is sufficiently thick to furnish large amounts of salt. Consequently, the salt uplifts attain very large size and may have grown throughout Mesozoic time. The long duration of growth produced steeply dipping and very complex structures in the early Mesozoic beds; some of them were breached by a major salt extrusion. In these high-relief structures, the initial graben which was formed during early Mesozoic time commonly spread apart from 0.75 to 1.5 mi, but spreading ceased before late Mesozoic time. Younger grabens which affect late Mesozoic and Cenozoic beds are developed within the older graben. Potential Jurassic traps are present on the flanks of the large salt uplift, and generally are controlled by the original graben faults. The Pool Creek ield is an example of this type of structure. A salt thickness ranging from 5,000 to 15,000 ft or more may be expected on this type of structure.

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