The role of salt mobility in the development of supra-salt sedimentary depocentres and structural styles

Whilst the basin morphologies that develop in a salt-free setting are generally well understood and relatively predictable, the morphology and internal architecture of sedimentary depocentres developed in salt basins are much more complex and influenced by a greater number of controls. With the aim of understanding the relative role of salt mobility and other controls on depocentre development in salt basins, results obtained from strategic, targeted observations of three different sedimentary basins are presented. These include the Paradox Basin of Utah/Colorado, USA, where sequences which are the stratigraphic response to Pennsylvanian-Triassic mobility of a Pennsylvanian salt sequence are exposed and investigated at the outcrop scale. The other basins are the Sole Pit/Silver Pit Basin (Southern North Sea) and Shearwater area (Central North Sea), both of which are exclusively subsurface examples that have experienced Mesozoic and Tertiary mobility of Upper Permian salt sequences. These later study areas are investigated using high-resolution 3D seismic data which allow the large-scale structural and stratigraphic geometries to be investigated well beyond the outcrop scale. Observations reveal a wide variety of complex depocentre styles with varying controls on their development. Controls include tectonics, differential sedimentation, availability of salt to move and potential of the overburden to flex or be penetrated. Despite this complexity, depocentre morphologies are considered to be predictable based on the concept that salt moves as a response to the pressure state in the salt layer exerted upon it by its overburden. As salt flows down pressure gradients, subsidence resulting from salt mobility is predictably greatest where the pressure from the overburden is greatest. Subsidence creates accommodation space and consequent sediment accumulation increases the load providing an intricate feedback between salt mobility and sedimentation. Where depocentres are associated with extensional-style faulting, their morphology is influenced both by the interactions between salt mobility and sedimentation and by the relative uplift and subsidence associated with faulting. Faults in salt basins show significant variability from established concepts of fault development derived from salt-free settings and salt mobility can be seen to have a fundamental influence on how fault displacement is manifested. Observations show that footwall uplift can be of equal or greater importance in accommodating fault movement than hanging wall subsidence and salt mobility can locally both increase or decrease displacement along a fault. Fault and salt influenced depocentres do not necessarily thicken towards a fault plane and as salt must withdraw from adjacent areas into an uplifting footwall area, they may be significantly offset from the fault plane. Models of fault and depocentre development derived from salt free settings are therefore, not considered applicable to salt basins. The incompactable nature of salt means that upon burial, the contents of a salt-influenced depocentre reduce in volume through compaction significantly more than the associated salt structures do resulting in the development of anticlines in overlying sequences situated directly above the salt structures. Differential compaction around salt structures is considered to provide an alternative to structural inversion or late stage salt mobility which are the favoured interpretations of many workers for the development of folds overlying salt structures.