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The major structure at Javelina and East McCook fields is a decollement surface on the Jackson Formation. Movement along this glide plane was contemporaneous with sedimentation and has resulted in extreme rollover (up to 50°) and great vertical and horizontal displacement of overlying Vicksburg sediments. Changing rollover trends observed in dip logs of lower Vicksburg strata, and structure of this glide plane surface demonstrate its listric nature both updip and along strike.
Structure within the Vicksburg Formation is dominated by 3 major listric, normal growth faults. The faults penetrate the entire Vicksburg section, which is over 8,000 ft (2,438 m) thick. These faults have significantly affected Vicksburg deposition, as the entire section expands more than 40% or 2,500 ft (762 m) across the field. Expansion ratios for individual units are higher. Structural displacement and complexity of the section increase with depth, with stratigraphic throws greater than 2,000 ft (610 m) in deeper Vicksburg beds. Fault drag is commonly observed in dip logs. Antithetic faulting is conspicuously absent.
Structural evolution of the Vicksburg was controlled by formation of a diapiric shale ridge updip early in Vicksburg deposition. Diapiric uplift resulted in evolution of a major growth fault on the basinward shoulder of the ridge. Once initiated, the fault maintained itself throughout Vicksburg deposition and resulted in decollement on, or within, Jackson shales. Development of this fault aided in formation of the other 2 growth faults. Formation of these faults was also related to movement along the glide plane, shale diapirism, and sediment-loading stresses.
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