ORIGIN OF “DRAG” FOLDS BORDERING SALT DIAPIRS

Sharply upturned strata commonly bordering salt diapirs have long been attributed to shear, or drag, from the rising salt. Finite-element models show that contact shear likely plays a minor role in the origin of these folds. Substantial drag-induced folds only form in exceedingly weak pre-existing country rock, such as shale overpressured to at least 90% of the total pressure. Even then, most folding develops by near-surface outward spread of the salt. Protrusions of overburden into a diapir, where the contact dips shallowly outward, are most susceptible to folding. Sedimentation during salt rise is much more likely to produce large and broad folds. New protrusions, in the form of onlapping wedges, may be continually produced during sedimentary downbuilding. Upward and outward rotation of these onlaps over rising salt can form folds even in strong layers. This process is really drape folding, so “drag folds” is a misnomer. “Onlap” or “flap” folding is more appropriate. Episodically deposited layers enhance the potential for onlap folding. For the same average aggradation rate, pulsed deposition of thick layers generates larger folds than do more continuously deposited thin layers. Variation in thickness and timing of deposition increases the likelihood of multiple wide onlaps that evolve into substantial folds. Older layers tend to form the broadest and tallest folds because those layers prograded farthest across the diapir crest. As the salt contact steepens, the folded zone narrows as younger layers onlap only a short distance onto the salt or onto older folds. Onlap folds can only form for a limited range of relative salt rise and deposition rates. Deposition much faster than the net rate of salt rise buries the diapir. Deposition too slow creates steep contacts or salt spilling outward onto the sediment surface, both of which result in little folding. Large thicknesses or stacked sequences of onlap folds may be due to natural variations in sedimentation rates and changes in salt rise rate as loads, thickness of source layers, and dissolution rates change.