Miocene submarine fire fountain deposits, ryugazaki headland, oshoro peninsula, hokkaido, japan: Implications for submarine fountain dynamics and fragmentation processes

Three in situ Miocene submarine basaltic andesite fire fountain deposits are preserved at Ryugazaki Headland, Oshoro Peninsula, Hokkaido, Japan. Emplacement occurred below storm wave base based on the absence of shallow water fractional reworking structures. The basal facies is a mass flow resedimented pillow clast and spatter clast breccia, indicating early effusive pillow lava and fire fountain eruptive activity upslope from the depositional site. The overlying fire fountain deposits consist of a porphyritic spatter clast breccia facies [oldest], an aphyric spatter micro-breccia facies, and a spheroidal clast breccia facies [youngest]. Only the youngest has a feeder dike exposed. The aphyric spatter micro-breccia facies indicates a high submarine fountain, fed by a high magma discharge, low viscosity magma source. Abundant blocky matrix clasts, indicate pervasive quench fragmentation in the high fire fountain. The fountains for the porphyritic spatter breccia and spheroidal clast breccia were lower, less water was able to enter the fountain, producing less quench fragmentation, and coarser deposits. In submarine fire fountains, the primary fragmentation process is shearing and extensional stretching of the magma fountain as it flares above the vent and decelerates. Quench fragmentation is an important second-order fragmentation process, especially at the margins of the fountain. The low vesicularity of Ryugazaki spatter [generally < 30%] indicates that explosive magmatic vesiculation was not an important process, and there is no direct evidence for phreatomagmatic influences. Spatter debris was deposited by fallout, with only minor evidence for re-sedimentation.