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  • 1
    Publication Date: 2015-01-21
    Description: The northern Flinders Ranges and eastern Willouran Ranges, South Australia, expose Neoproterozoic salt diapirs, salt sheets, and associated growth strata that provide a natural laboratory for testing and refining models of allochthonous salt initiation and emplacement. The diapiric Callanna Group (~850–800 Ma) comprises a lithologically diverse assemblage of brecciated rocks that were originally interbedded with evaporites that are now absent. Using stereonet analysis to derive three-dimensional information from two-dimensional outcrops of stratal geometries flanking salt diapirs and beneath salt sheets, we evaluate 10 examples of the transition from steep diapirs to salt sheets, 3 of ramp-to-flat geometries, and 2 of flat-to-ramp transitions. Stratal geometries adjacent to feeder diapirs range from a minibasin-scale megaflap to halokinetic drape folds to high-angle truncations and appear to have no relationship to subsequent allochthonous salt development. In all cases, the transition from steep diapirs to salt sheets is abrupt and involved piston-like breakthrough of thin roof strata, which permitted salt to flow laterally. We suggest two models to explain the transition from steep diapirs to subhorizontal salt: (1) salt-top breakout, where salt rise occurs inboard of the salt flank, thereby preserving part of the roof strata beneath the sheet; and (2) salt-edge breakout, where rise occurs at the edge of the diapir with no roof preservation. Lateral emplacement of salt sheets is dependent on the interplay between the rate of salt supply to the front of the sheet and the sediment-accumulation rate. When the ratio of salt-supply rate to sediment-accumulation rate is high to moderate, thrust advance produces base-salt flats and truncation ramps, respectively. Halokinetic folds are absent because the thrust emerges at the base of the sea-floor scarp and mass-transport complexes are rare as a result of relatively low scarp relief. If the ratio is low, pinned inflation leads to drape folding of the top salt and cover into a fold ramp, with occasional slumping of the sheet and its roof and further breakout on thrust or reverse faults. In the shallow-water depositional environments of South Australia, lateral emplacement of salt sheets occurred through some combination of thrust advance, extrusive advance, and open-toed advance, with no evidence for subsalt thrust imbricates, shear zones, or continuous rubble zones. In deep-water environments, such as the northern Gulf of Mexico, thrust imbricates and rubble zones, which represent slumped carapace, are more common. The presence of slumped carapace is caused primarily by higher topographic relief related to thicker hemipelagic roofs, a lack of dissolution, and gravity-driven transport of overburden strata to the toes of large canopies.
    Print ISSN: 0149-1423
    Electronic ISSN: 0149-1423
    Topics: Geosciences
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  • 2
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    American Association of Petroleum Geologists (AAPG)
    Publication Date: 2016-11-16
    Description: Megaflaps are steep stratal panels that extend far up the sides of diapirs or their equivalent welds. They have multiple-kilometer fold widths and structural relief and are thus distinct from smaller-scale composite halokinetic sequences. Maximum dips range from near-vertical to completely overturned. Although overturned megaflaps are associated with flaring salt, there is no direct link between megaflap formation and the initiation of salt sheets. Strata within a megaflap are usually convergent, and the lower boundary is typically concordant with the top salt. The upper boundary ranges between a prominent onlap surface and a more diffuse zone of gradual rotation and thinning, and growth strata likewise display both onlap and stacked wedge geometries. We use quantitative cross-section restoration to elucidate the origin and development of megaflaps. Megaflaps typically represent the relatively thin roofs of early salt structures that include single-flap active diapirs, passive diapirs, salt pillows, and salt sheets. They develop during halokinetic drape folding as the minibasin sinks, during contractional squeezing of the diapir and its roof, or during some combination of the two. The kinematics are dominated by either limb rotation or kink-band migration, in which roof strata move through a fold hinge into a lengthening steep megaflap. Both restoration results and direct field evidence suggest that internal strain is minor, with little bed lengthening and thinning. Recognition and understanding of megaflaps are critical to successful petroleum exploration of three-way truncation traps against salt. Megaflaps also have implications for the lateral seal of stratigraphic traps and fluid pressures in minibasins.
    Print ISSN: 0149-1423
    Electronic ISSN: 0149-1423
    Topics: Geosciences
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