VOLCANICLASTIC AGGRADATION IN A SEMIARID ENVIRONMENT, NORTHWESTERN VULCANO ISLAND, ITALY

Most studies of volcaniclastic facies and aggradation cycles hale been conducted in regions with humid climates, and on regional scales. Her e we document a volcaniclastic succession that formed in a semiarid cl imate characterized by rare, heavy rains onto a relatively barren volc anic landscape on Vulcano Island, Italy, The deposits, which we inform ally call the Cuesta succession, occur in a small valley between two r hyolite domes, and consist of a sequence of pyroclastic surge and fall out deposits interbedded with their reworked equiv equivalents, ri pha se of eruptive activity characterized by sporadic hydrovolcanic explos ions supplied ash to the valley and its flanks. Runoff events during t he eruptive phase continually washed ash down to the valley floor in t he form of hyperconcentrated flows, The inferred transport mechanism a nd depositional facies of these beds reflect the control of primary vo lcanic fragmentation processes on sedimentation; the abundant, medium- to-coarse ash supplied by eruption was incorporated sufficiently easil y into the runoff to hyperconcentrate the no rvs, but not sufficiently fine grained to make the no,vs cohesive. These nons moved out onto th e floor, merged, and transported ash down the valley axis, depositing the ash as a monotonous succession of massive to faintly laminated bed s. The combination of primary deposits and the deposits washed off the valley flanks led to aggradation of the valley floor As eruptions wan ed, ash was eroded off the flanks faster than it was replenished, lend ing to a stratigraphic upward increase in lithic clasts derived from t he underlying lavas and a wider range of sedimentary facies as the ash load became more variable. After eruptions ceased and all remaining a sh was removed from the flanks, aggradation gave way to degradation to form small canyons that expose the succession.