Fault sealing in unconsolidated sediments through interporous accumulation of fines

Abstract Faults in unconsolidated sediments within the Roer Valley Rift System (RVRS), situated in the Netherlands, Belgium, and Germany, exhibit reduced permeability, which impacts near-fault groundwater flow. These faults act as semi-impermeable barriers, inducing groundwater-level steps of up to several meters. In a trench excavated across one of the faults of the Peel Boundary Fault Zone in Uden, the Netherlands, fault-sealing processes were studied to understand the hydrogeological behaviour of the fault zone. The post-sedimentation mobilisation, migration, and accumulation of interporous fines are identified as a novel fault-sealing process, in addition to the juxtaposition of sediments, reorientation of elongated grains, and the precipitation of iron (hydr)oxides. The mobilisation of fines, comprising clay, silt, very fine sand, and organic matter, within the unconsolidated, porous foot-wall sediments, can be initiated by fault activity, repeated freezing and thawing, as well as groundwater flow. Acting as a transport mechanism, groundwater flow facilitates the migration of fines towards the fault. The gradual accumulation of interporous fines results in a dark grey, fault-parallel zone along the fault core on the foot wall. The combined effect of the fault-sealing processes causes a mean groundwater-level step of 1.83 m. The accumulation of fines as a fault-sealing process in unconsolidated sediments has not been documented before, and may have been previously overlooked.