Fault zone deformation and displacement partitioning in mechanically layered carbonates: The Hidden Valley fault, central Texas

The Hidden Valley fault is exposed in Canyon Lake Gorge (central Texas) and cuts the Cretaceous Glen Rose Formation. This exposure provides an opportunity to explore the relationship between deformation mechanisms and fault displacement along 830 m (2723 ft) of a normal fault typical of those in carbonate reservoirs and aquifers around the world. The fault zone has five domains: gently deformed footwall damage zone, intensely deformed footwall damage zone, fault core, intensely deformed hanging-wall damage zone, and gently deformed hanging-wall damage zone. Footwall deformation is more intense and laterally extensive than hanging-wall deformation, and the intensely deformed hanging-wall damage zone is narrow and locally absent. The fault core contains thin clay-rich gouge or smear in most places but is locally represented by only a slickensided surface between limestone layers. The 55- to 63-m (180-207-ft) fault throw across a 43- to 98-m (141- to 322-ft)-wide fault zone is accommodated by slip along the fault core, layer tilting (synthetic dip development) in footwall and hanging-wall damage zones, and distributed faulting in footwall and hanging-wall damage zones. Total offset across the fault overestimates actual stratigraphic offset by 8 to 12 m (26-39 ft) or about 14 to 21%. In our interpretation, the Hidden Valley fault zone records both early extensional folding of the Glen Rose Formation and subsequent normal faulting that propagated downward from the overlying competent Edwards Group. The damage zone width is thus established before fault breakthrough.