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Faulting Associated with Deep-Seated Salt Domes in the Northeast Portion of the Mississippi Salt Basin
Dudley J. Hughes (1)
Faulting in the northeast portion of the Mississippi salt basin is principally of the local graben-type resulting from salt doming. On deep-seated salt dome structures, the faulting appears to exhibit certain common characteristics throughout the area which can be applied to great advantage in subsurface interpretations.
The strike of the faults associated with deep-seated salt domes through this area does not exhibit a consistent regional alignment, as the faults are localized over each dome. The general fault strike is usually parallel to the long axis of the deep-seated dome with which it is associated.
Faulting over deep-seated salt domes can usually be related to derivative gravity minimums which are expressions of the salt uplifts causing the faulting. The primary faults generally strike parallel to the long axis of the derivative gravity minimum, and faults on the outer margin of a graben structure are usually downthrown towards the long axis of the minimum. Generally, the relative intensity of the derivative gravity minimum becomes greater as the complexity of the faulting becomes greater.
On complexly faulted structures in this area, the outer faults of a graben system are considered to be the primary faults as they were usually the first faults initiated and they predominate with depth.
Fault dips over deep-seated domes in the northeast portion of the Mississippi salt basin average approximately 45° in the Upper Cretaceous and 60° in the Lower Cretaceous.
An increase in throw with depth is exhibited by faults over deep-seated domes at a rate determined principally by the rate of differential uplifting which the salt exhibited during deposition of the sediments through which the faults cut. The increase in throw with depth is principally a result of lengthening of stratigraphic section in the downthrown block relative to the same section in the upthrown block. This lengthening of section is caused by thickening of downthrown beds, and by preserved wedges below unconformities in the downthrown block which are absent in the upthrown block.
As the unconformable contact between the Upper and Lower Cretaceous is the only horizon at which a large increase in throw on faults is evident, it appears that this unconformity is the only one of significant hiatus between Wilcox to basal Hosston time.
The crest of structures at Lower Cretaceous horizons through this area are commonly located near one side of a graben system. The faults on this side, termed "axial faults", generally bisect the anticlinal crest so that closure is present on both their upthrown and downthrown sides. Lower Cretaceous production is most commonly found along the structural crest on both sides of the axial faults.
Faults with opposing dip on the opposite side of the graben, termed "flank faults", are farther removed from the structural crest and exhibit closure only on the upthrown side. Flank faults provide potential traps if upthrown reservoir beds remain against impervious strata in the downthrown segment during growth of the fault.
Lateral changes in the throw of faults takes place more rapidly on progessively older horizons. The lateral termination of a primary fault off the flanks of a deep-seated salt dome appears to take place on all horizons approximately the same distance from the apex of the dome.
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