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Megaflaps and Composite Halokinetic Sequences: Different Scales of Drape Folding adjacent to Salt Diapirs

Abstract

Abstract

Strata in salt basins are typically folded to steep attitudes along the flanks of passive diapirs. Relatively narrow zones of stratal upturn, formed in response to local drape folding of diapir roofs, are termed composite halokinetic sequences (CHS). These stacked, unconformity-bound stratal packages are characteristic of the mature, vertical to flaring portions of passive diapirs and involve minibasin strata with a broad age distribution that are locally thinned adjacent to diapirs. The unconformities are local to the diapir, intersect the salt at cusps, and are commonly overlain by mass wasting deposits. The roof thickness determines the style of deformation: thin roofs generate tabular CHS, with up to 90 degrees of folding and angular truncation within <200 m of the salt; and thick roofs form tapered CHS, with the amount of upturn and truncation progressively decreasing over a distance of up to 1 km from the diapir and a maximum relief at any given stratigraphic level of about 1.5 km.

Strata adjacent to salt diapirs may also be folded to steep attitudes over wider zones with greater structural relief. In these megaflaps, the oldest minibasin strata become near vertical or even overturned adjacent to diapirs, with the zone of folding extending multiple kilometers away from the diapir and having relief of up to 5 or more kilometers. Megaflaps typically represent the relatively thin, conformable roofs of early salt structures and develop during drape folding as the minibasin sinks, contractional squeezing of the diapir and its roof, or a combination. Wedge and onlap growth geometries record 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.

Both megaflaps and composite halokinetic sequences have important implications for drilling three-way traps adjacent to diapirs. First, strata are generally steeper and older than expected or even imaged on seismic data. Second, the surface topography generated by drape folding exerts a profound influence on reservoir extent and facies. Third, the structural style impacts trap geometry, hydrocarbon migration/entrapment, seal, and column height.