Centrifuge Modeling of the Effects of Aggradation and Progradation on Syndepositional Salt Structures

The manner in which sedimentary overburden accumulates significantly influences the growth of syndepositional salt structures. To determine the general principles of this influence for application to specific instances in nature, we carried out 16 experiments using scaled centrifuged models to simulate the effect of variable sedimentary loading on the geometry, kinematics, and dynamics of syndepositional salt structures. This topic has been almost entirely ignored in both physical and mathematical modeling of salt structures.The rheology of four modeling materials was determined. The equivalent shear viscosity of terrigenous clastic overburdens in the Mesozoic section of a Gulf Coast interior basin and in the Pleistocene section of the northern Gulf of Mexico was estimated by dynamic scaling to be 5 x 1020 Pa*s and 3 x 1020 Pa*s, respectively; the equivalent viscosity of Mesozoic Gulf Coast carbonates was estimated to be 2 x 1020 Pa*s.Thirteen models were centrifuged under 1200 g; 14 experiments were dynamically scaled so that the ratios of viscous, pressure, and gravity forces were equivalent to conditions thought to be found in nature. Four types of experiments were carried out: (1) illustration of edge effects; (2) comparison of diapiric growth by upbuilding versus downbuilding; (3) simulation of static differential loading; and (4) simulation of prograding differential loading. Edge effects represent the influence of the shape of the experimental container on the geometry and growth of source structures one to two diapir spacings inward from the container boundary. The most important edge effect was the rapid growth of a peripheral wall at the model perimeter. This effect was filtered out during model interpretation. Upbuilding occurs during postdepositional or interdepositional diapiric growth; the diapir crest rises while the base remains static. Downbuilding occurs during syndepositional diapiric growth while the overburden aggrades (accumulates vertically); the crest is static while the base sinks along with the basin floor. Both processes are extreme and ideal. R/A growth curves summarize objectively the incremental and finite growth history of natural and model domes. An overall trend of downbuilding growth can be produced by alternating episodes of burial and upbuilding during episodic sedimentation.The experiments showed that, compared with analogous upbuilding structures, downbuilding domes (1) rose more slowly, (2) formed more closely together, (3) formed double-stalked domes more readily, (4) formed exposed walls more commonly, and (5) developed peripheral hanging lobes more readily. A stiff, dense "carbonate" layer, such as the Upper Jurassic and Cretaceous units in Gulf Coast interior basins, had an important control on the form of domes rising through the layer.Several features characterized both upbuilding and downbuilding models. Where the uppermost cover was the same density as the source layer, mature domes were surmounted by a bulb. The bulb spread laterally at roughly the same depth as the deepest part of the uppermost cover.Where the uppermost cover was denser than the source layer, diapirs that pierced it extruded to form mounds surrounded by moats depressed by differential loading. Lateral flow was centrifugal over rising fingers of source and was convergent or parallel in sinking walls of cover. Upbuilding and downbuilding models were characterized by spoke circulation, in which shallow polygonal walls of cover enclosing rising source fingers overlay deep polygonal walls of source enclosing sinking cover fingers. A model containing a wedge-shaped source layer simulated the zonation of salt structures on the flanks of Gulf Coast interior basins. As proposed in analytical theory, domes grew fastest where the wedge was thickest. During static differential loading by a half-layer over tabular cover and buoyant source layer, a frontal wall was squeezed ahead of and parallel to the edge of the half-layer. Differential loading was equally effective on the source layer and on the overlying tabular cover, causing all diapirs to tilt toward the foundered edge of the half-layer. Diapirs next to the half-layer were more tilted and more asymmetrical than those beneath it. During simulated sedimentary progradation, buoyant source material was initially squeezed ahead of the depocenter, but flow reversed direction as source layers became completely buried. After being overridden by progradation, frontal bulges of source initiated diapiric walls parallel to the advancing linear depocenter. The walls were tilted in a direction opposite that of progradation, analogous to salt walls in the Gulf of Mexico. Dense cover also prograded across the edge of source layers beyond the confines of the basin. A recumbent, isoclinal wall of source material grew laterally for distances of at least five times the height of mature vertical domes. The recumbent wall, decorated with asymmetrical appendages and overhung by smaller recumbent walls, geometrically resembled the Sigsbee nappe complex on the continental slope of the northern Gulf of Mexico. The recumbent wall acted as a horizontal recycled source layer, allochthonously perched high in the stratigraphic pile, from which a second generation of vertical structures rose. Several effects, such as those due to source dip, cover wedge, and gravity spreading, favor growth of diapirs that tilt and are asymmetrical toward-basin centers under both static and dynamic loading conditions. First-order circulation of both source and cover in movement cells encompassing several diapirs strongly affects the geometry of second-order movement cells comprising individual diapirs.