Volcaniclastic aggradation in a semiarid environment, northwestern Vulcano Island, Italy
Most studies of volcaniclastic facies and aggradation cycles have been conducted in regions with humid climates, and on regional scales. Here we document a volcaniclastic succession that formed in a semiarid climate characterized by rare, heavy rains onto a relatively barren volcanic landscape on Vulcano Island, Italy. The deposits, which we informally call the Cuesta succession, occur in a small valley between two rhyolite domes, and consist of a sequence of pyroclastic surge and fallout deposits interbedded with their reworked equivalents. A phase of eruptive activity characterized by sporadic hydrovolcanic explosions supplied ash to the valley and its flanks. Runoff events during the eruptive phase continually washed ash down to the valley floor in the form of hyperconcentrated flows. The inferred transport mechanism and depositional facies of these beds reflect the control of primary volcanic fragmentation processes on sedimentation; the abundant, medium-to-coarse ash supplied by eruption was incorporated sufficiently easily into the runoff to hyperconcentrate the flows, but not sufficiently fine grained to make the flows cohesive. These flows moved out onto the floor, merged, and transported ash down the valley axis, depositing the ash as a monotonous succession of massive to faintly laminated beds. The combination of primary deposits and the deposits washed off the valley flanks led to aggradation of the valley floor. As eruptions waned, ash was eroded off the flanks faster than it was replenished, leading to a stratigraphic upward increase in lithic clasts derived from the underlying lavas and a wider range of sedimentary facies as the ash load became more variable. After eruptions ceased and all remaining ash was removed from the flanks, aggradation gave way to degradation to form small canyons that expose the succession.