Reconstructing 3D subsurface salt flow

Abstract

Archimedes' principle states that the upward buoyant force exerted on a solid immersed in a fluid is equal to the weight of the fluid that the solid displaces. In this 3D salt-reconstruction study we treat Zechstein evaporites in the Netherlands as a pseudo-fluid with a density of 2.2 g cm−3, overlain by a lighter and solid overburden. Three-dimensional sequential removal (backstripping) of a differential sediment load above the Zechstein evaporites is used to incrementally restore the top Zechstein surface. Assumption of a constant subsurface evaporite volume enables the stepwise reconstruction of base Zechstein and the approximation of 3D salt-thickness change and lateral salt redistribution over time. The salt restoration presented is sensitive to any overburden thickness change caused by tectonics, basin tilt, erosion or sedimentary process. Sequential analysis of lateral subsurface salt loss and gain through time based on Zechstein isopach difference maps provides new basin-scale insights into 3D subsurface salt flow and redistribution, supra-salt depocentre development, the rise and fall of salt structures, and external forces' impact on subsurface salt movement. The 3D reconstruction procedure is radically different from classic backstripping in limiting palinspastic restoration to the salt overburden, followed by volume-constant balancing of the salt substratum. The unloading approach can serve as a template for analysing other salt basins worldwide and provides a stepping stone to physically sound fluid-dynamic models of salt tectonic provinces.