Layer-bound, polygonal fault systems (PFS) in marine, fine-grained sediments have been documented with seismic data over large areas in over 100 basins. The questions of how, when and how fast these muds broke without tectonic stress were still open. This study covered the missing observational length scales from 100 m to inns with a continuous range of outcrop observations and related lab results from two quarries in Ypresian marine unlithified swelling clays in the Belgian part of the North Sea Basin. The observed coeval dilatant hydrofractures and folded faults imply highly unstable and transitional tensile fracturing behaviour under shallow overpressure. The random fault plane orientations, radial striation vectors and conal micro-faults exclude far-field stress and slope instability as the driving force, and show that gravity was the major principal stress. A contributing driving force was overpressure in the oxidising fluviatile and lagoonal sands below, which further weakened its clayey caprock and supplied at least part of the water evacuating through it. This fracture water flow eroded and oxidised bits of wall clay and tell-tale microfossil mixtures along the way up. The sharp contrast of black gouge against barely stained wall clays implies that such dilatant fracture and fault growth was almost instantaneous. This resulted in winner-takes-all shear localisation along often razor-thin, slickensided fault mirrors. Faults that locked-up by different degrees of compaction folding and local rotational flow imply: 1) that this clayey mud was still an underconsolidated viscoplastic gel just prior to plastic faulting, capable of syneresis and therefore capable of supplying isotropic contractional stress; 2) not episodic growth but episodic nucleation of additional fractures and faults and punctuated dewatering along them, until the clayey gel matrix and the overpressure in it were fully deflated.