Soft-sediment deformation involves complex interactions between discrete fracturing and diffuse bulk strain, described in terms of volume change and shear strain in a critical state mechanics framework. This study reports on a mesoscale normal fault zone, intraformational in Oligocene argillaceous sediments from the Boom Formation (Belgium), containing several metre-scale normal fault strands. They form either discrete fault planes or decimetre-wide shear zones with internal fabric. The faults have been subjected to microtectonic and petrophysical analysis. Small but significant changes occur in the porous network of the argillaceous matrix approaching a fault or shear zone, indicating compactional strain in both footwall and hanging wall. Internal compaction associated with faulting is put forward as a ductile–brittle feedback mechanism in the kinematics of intraformational fracture systems. Small differential stress induced by compaction and minor regional tectonic forces (differential uplift and tilt) and subsequent gravitational forces (downslope shear stress) induce small shear bands in nearly critically stressed weak mud. Shear banding is accompanied by layer-parallel shortening and bulk volume loss. This provides an additional extension of endogenous origin, accommodated by further deformation. This ductile–brittle feedback mechanism eventually leads to commonly observed intraformational collapse structures called polygonal fault systems.