The present study is based on drained ring-shear experiments and fieldwork on Bornholm, Denmark. An extensional fault zone in Lower Cretaceous Robbedale Formation with a vertical separation of minimum 13 m, has a fault core and a damage zone that are clearly distinguishable. The fault core has an architecture that is dominated by lensoid rock bodies (extensional horses). Thus, the fault core is dominated by lenses of sand, separated by continuous clay membranes. Parts of massive, pure clay also occur, but are less frequent, whereas zones of mixted sand and clay and patchy clay characterise the distal part of the fault core. The damage zone is characterised by deformation bands mainly without clay membranes. Some of the deformation bands are connected to the fault core, while others are not.
The clay membrane probably developed as a result of combined drag and shearing and the intrusion of liquid or plastic clay into the fault core from above. Pockets with a lower pressure than in the source layer permitted downward flow of the clay. Indications of such internal pressure gradients suggest the fault zone was active simultaneously with clay intrusion, so that repeated shearing contributed in making room for clay and in transport of clay along the fault core.
Based on ring-shear experiments and field observations, the following conditions are suggested for the clay membrane development: The normal stress was low (<500kPa). The clay possessed low shear strength (<300-400kPa), and a high water content (>>30%). The strain rate was high. The sand was wet having a higher competency than clay causing the clay to act in a ductile fashion during faulting. Because of a continuous impermeable clay membrane, the fault zone is considered to be sealing.
Local disrupted and continuous clay membranes are observed for more small-scale extensional faults occurring in the Lower Jurassic Rønne Formation. Sand lenses occur frequently, and ramp-flat-ramp geometry is observed in some of the faults. A shear mechanism is suggested to explain the development of the clay membrane in these faults in a broader context. The faults are considered to be sealing only where clay membranes occur. Features similar to those observed in experiments are also seen in the field. These include sand lenses, semi-continuous to continuous clay membranes, and mixtures of sand and clay. Hence, it can be concluded that the ring-shear experiments are well suited for studying fault zones generated at shallow depths.