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The Origin and Development of Graben Structures Associated with Salt Domes
EDITOR'S NOTE: Dr. Currie's original paper on this subject was published in the Bulletin of the American Association of Petroleum Geologists, Vol. 40, No. 1, January, 1956. However, this abstract is from his manuscript prepared for his trip around the country as an A. A. P. G. Distinguished Lecturer.
The problem as presented by the above title is herein approached in part by the use of a model. In the interpretation of a geologic structure certain inferences are made as to direction of forces, the reaction of the rock materials involved to these forces and the progressive development of the pattern of strain, either as folding or faulting. In many cases the task of structural interpretation is not easy, for it is often difficult to visualize a static or, more so, a changing geometry in three dimensions. The use of models in geologic research encounters similar difficulties, and must of necessity represent somewhat idealized systems. However, models do allow investigation of a process from the standpoint of the structural principles involved.
In this particular case we are dealing with the question of deformation which accompanies relative vertical uplift of sediments with contemporaneous deposition of additional beds. In the formation of salt domes, the plastic flow of salt under differential compression is accepted as most important. Present research suggests that the differential lies within the range of 60 to 100 Kg/cm2 at surface temperatures and pressures. On the basis of these figures a flowage of salt could be expected when overburden pressure in a particular locality exceeds by some 800 to 1000 feet of sediments the overburden pressure in an adjacent area. Experimental work indicates that the strength of rock salt decreases with rising temperature by an estimated 20 to 40 percent per 100°C. Further, it may be estimated that the strength of salt increases with confining pressures, and from the time dependence of salt deformation it is also possible to calculate an equivalent viscosity. All of this indicates the importance of the low fundamental strength of rock salt in salt dome development.
Other concepts include the postulates that the lower density of salt relative to the overlying sediments would create the development of a rising salt mass surrounded by a peripheral sink, and that the salt in a dome may have remained at the same level relative to the surface of deposition, while the mother salt layer and the surrounding beds sank continuously around the dome. From this the additional idea develops that in a shallow piercement dome the top of the salt mass must remain close to the surface of deposition; if not the salt core may be buried finally under an increasing sedimentary load.
The factors suggested in the control of the initiation and termination of salt movement include variations in overburden thickness or density, normal faults above the mother salt layer or fault blocks below the salt layer. Some linear trends of domes suggest such fault control. However, it has been found in scale-model experiments that there exists a "critical overburden thickness" which would stop dome growth, and observation suggests that strata over a salt mass possess a strength which must be exceeded if salt movement is to continue.
The foregoing comments serve to outline the physical boundary conditions which exist for an investigation of graben development over and around a dome. Briefly, these are (1) salt movement is essentially viscous; (2) movement of points at the axis of the rim syncline will be in the vertical direction; (3) at the upper surface of the system deposition of the overburden is contemperaneous with salt dome movement; and (4) the choice of the horizontal reference plane is arbitrary. Within this framework the problem of graben faulting can be approached from the standpoint of plane deformation, or of deformation of the overburden in a plane normal to the axis of the anticlinal structure. Results from the study of such two dimensional deformation can then be applied to the more complicated case of three dimensional deformation. This is done by means of slides showing by structure maps and two and three dimensional cross-section arrangements, the fault pattern of different domes which bring out the graben nature of the structure in the sediments overlying the salt.
The only presented with this abstract shows the model by which the development of a graben structure contemporaneously with deposition is brought about. The cross-section is in one plane. A thin plastic strip below the stratified section, which lies between two transparent sheets, together with a curved metal plate capping a rising jackscrew acts as the salt mass. The sifter above the stratigraphic section travels rapidly back and forth depositing additional strata while the "dome" rises. The resulting graben faulting and stratigraphic attitudes and thickness variations are diagrammatic of the structure associated with a salt dome.
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