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Loading-Driven Subsidence of Minibasins Into Salt


Salt evacuation and rise can be driven by differential loading. Even less-dense (siliciclastic) overburden can subside with adequate differences in pressure head due to lateral variations in overburden density and/or thickness. Although the base of the thickest load sinks into salt, the top remains a topographic high so that further sedimentation in the same area is unlikely and subsidence should cease. Yet there are numerous examples where subsidence of less-dense sediment has apparently continued in the absence of accepted driving mechanisms such as extension, contraction, or progradation. These include minibasins in parts of the northern Gulf of Mexico, the Paradox Basin, the Nordkapp Basin, and the Precaspian Basin.If true, this gives rise to two related conundrums. First, how do minibasins less dense than salt sink enough to accumulate more sediment and thereby keep subsiding? Second, what are the competing effects of a differential load driving salt in one direction (lateral variation in pressure head) when the top salt is dipping in the opposite direction (lateral variation in elevation head), as is the case on the flanks of minibasins? In other words, what is the direction of bulk salt flow in different scenarios where the density contrast, the top-salt dip, and the top-overburden dip are all factors?It may be that improved numerical models using lubrication squeeze flow or other mechanics-based analyses can solve this dilemma. Alternatively, the explanation may lie in taking regional, 3-D approaches rather than focusing on isolated minibasins in two dimensions. Or maybe we need a new mechanical paradigm, one based on fluid dynamics rather than fluid statics. In any case, this is an outstanding problem that warrants further investigation and new insights.