Salt tectonics

Salt tectonics refers to structural deformation involving bodies composed of evaporite minerals. The Dutch subsurface is characterized by a thick sequence of Zechstein salt deposited during the Late Permian. Its deformation in response to several tectonic phases had a pronounced influence on Meso-Cenozoic tectonostratigraphic and depositional evolution. Salt layers of the Zechstein Group are the most prominent and regionally significant intervals that were affected by salt tectonics in the Netherlands. Other salt layers in the middle to early late Permian Silverpit Formation and of Triassic age (e.g. Main Röt Evaporite Member) were affected by more local and less extensive salt tectonics than the Zechstein and are of secondary importance. The main salt tectonic structures observed in the Dutch sector are due to large volumes of Zechstein salt that were mobilized laterally or vertically, feeding large salt bodies and allowing basins to form in response to a combination of salt withdrawal, extensional- and contractional tectonic forces. These growth basins include mini basins, sub-basins, rim synclines, rollover anticlines and collapse grabens. Successive crustal-scale tectonic events induced the salt tectonics in the Netherlands, localizing salt movements, enhancing salt migration, and deforming pre-existing salt bodies (e.g. through diapir squeeze). In this chapter we discuss the main salt tectonic processes, the original Zechstein salt thickness and the five types of salt structures observed in the Dutch subsurface (salt pillows, diapirs, walls, rollers, and sheets), including their geometry, their geo graphic distribution, their kinematics and their effects on associated structures such as growth fault/raft systems, crestal faults, turtle structures, caprock and collapse grabens. The Meso-Cenozoic sediment pathways, sediment distribution/preservation and the overall basin architecture were also affected by accommodation changes resulting from salt withdrawal and migration. In the Netherlands, limited syn-depositional faulting during the Tubantian I phase (Geluk, 1999) caused local thickness variations within the Zechstein Group. The actual movement of Zechstein salt started as early as the Early Triassic but most of the significant salt tectonics occurred between the Middle Triassic and the Paleogene in response to a combination of gravitational forces (halokinetics) and tectonic events. Since the Paleogene, salt tectonic intensity has diminished but salt bodies and their crestal faults affect several processes in the shallow overburden up until the Quaternary. << Rhythmic millimetre-scale calcite (brown) and gypsum-bearing (white) laminites interprete.