Fault-controlled base-of-scarp deposits

The term base-of-scarp is proposed for those submarine deposits controlled by a fault and physically disconnected from their more proximal counterpart located on the footwall, although genetically linked to it. These systems differ from conventional fault-controlled deltas, such as shoal- and Gilbert-type, because they are entirely subaqueous and lack equilibrium morphology—a steady state in which the system grows in size without altering its shape. We present field examples of fault-controlled deposits from the Crati Basin and the Messina Strait (southern Italy) consisting of stratigraphic clastic wedges that thin towards and onlap onto the active margin with primary inclined bedding. Beds are composed of immature coarse-grained gravels and sand, lack structures representative of wave-action and reflect gravity-driven processes such as debris flow, debris fall and high-density turbidity currents. These deposits represent the unsteady-state phase in which the system grows reducing its slope angle leading to conditions under which the unsteady state may eventually turn into a Gilbert-type or shoal-water system. A diagram for fault-controlled base-of-scarp (B), Gilbert (G) and shoal-water (S) deposits is presented, including their steady- and unsteady states, and the conceptual conditions under which a base-of-scarp system might evolve into Gilbert-type or shoal-water systems and vice versa.