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The salt of the salt domes of some areas is known to be sedimentary. Geological observation of those domes and laboratory experimentation show that salt flows plastically under differential pressure. The German salt domes are known to have been formed by the plastic flowage of the Zechstein salt series. Any type of differential pressure should tend to produce plastic flowage of sedimentary salt whenever certain critical conditions such as those of pressure, temperature, and time have been exceeded. The plausible sources of pressure are two: (1) the static pressure of the overlying sediments; and (2) the dynamic pressure of tangential compression or thrust. Under (1), growth of the dome by upthrust can take place only if the available energy is sufficient both to overcome riction and to uplift the salt core and some sediments against gravity; growth of the dome by downbuilding can take place if the mother salt bed is sinking in earth space; the position of maximum uplift is below that of isostatic equilibrium of the salt core; and the form of the salt dome should evolve progressively through a characteristic series of forms. Under (2), the horizontal dynamic pressure will act indirectly upward through anticlines and downward through synclines in competent beds; and directly through horizontal squeezing of the salt in relatively upthrust cores. The static thrust of (1) will be active and may be more important than the dynamic thrust of (2); the position of maximum upthrust of the salt core may be far above its position of isostatic equilibrium; the form of the domes should be varied.
The Gulf Coast domes have been formed by the plastic flowage of sedimentary salt intrusively into the overlying sediments. The evidence for that origin of the domes comes from the structure which is revealed by oil-field drilling, from algal remains in the salt, and from the close similarity of the American salt domes to the German salt domes. The age of the salt is greater than most of the Lower Cretaceous. The motive force of the formation of the domes has been the static weight of the sediments. Growth of the domes has taken place throughout the Tertiary and has taken place on a few domes in the most recent past. There was no dynamic tangential compression in the Gulf Coastal Plain area during the Tertiary and Quaternary; therefore, the motive
force presumably must have been the static thrust of the sediments. Subsidence of the mother salt bed took place almost continuously through the Tertiary and into the Quaternary. The difference between the specific gravity of the salt and of the sediments is small; and the calculated force of upthrust is small, rather too small to overcome friction and to uplift the salt core and some sediments against gravity. Growth of the domes, therefore, must have been largely by downbuilding. Partially corroboratory evidence is given by the concomitant cessation of growth of the Clay Creek dome and cessation of the regional subsidence of the general surrounding area. But some actual upthrust has taken place on the Gulf Coast domes; and, as theoretically expectable, it tends to be greater on the omes of larger diameter. Growth has not continued into recent time on all the domes and has ceased finally at different times on different domes. The law holds crudely that the deeper the dome, the older the time of cessation of growth. The final cessation of growth in general may have been caused by exhaustion of the salt in the mother salt bed, attainment of isostatic equilibrium, frictional freezing of the salt core to the sediments, and, in the case of downbuilding, by cessation of the subsidence. The succession of retrograde movement of the salt core after the cessation of growth is suggested, inconclusively, by the Clay Creek dome. The presence of rim synclines has been suggested by the results of torsion-balance surveys, but has not been identified from geologic data. Rim syncline could be formed: (1) by solution of the flank of the salt, and (2) by the pushing-in of the deep flank of a flaring salt core in the growth of the dome. Overhang of the salt and cap is present in two types: (1) tilting of the vertical axis of the dome, and (2) mushrooming (Barbers Hill). Type (1) is produced perhaps by the seaward flowage of the deeper sediments. The explanations of type (2) are not satisfactory.
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