Eruption sequence of the Suwanoharu tuff ring in the Pliocene Oyano formation, western Kyushu, Japan

Volcanism in the Ariake sea region, west Kyushu, Japan, where is the northern extension of the Okinawa trough, have been active during the last 5 million years, from Pliocene to present Unzen volcano. The region has been subsided with development of the tectonic graben. Pliocene Oyano formation is predominated in shallow marine pyroclastic deposits including some base surge deposits. A series of stratified pyroclastic deposits, which were originated by phreatomagmatic eruption cycles is exposed on a sea cliff and is considered to be a remnant of tuff ring, which is called Suwanoharu tuff ring. Suwanoharu tuff ring is expected to preserve the complete eruption sequence. The sequence is basically alternation of massive white tuff layers and laminated gray coarse to lapilli tuff layers. We can identify seven eruption cycles bounded by the massive white air fall pyroclastic deposits (unit 1- 7). The gray pyroclastic deposits consist of massive dark gray medium tuff, crudely laminated medium to coarse-grained tuff, inversely graded lapilli tuff, cross-bedded lapilli tuff, and lapillistone and are considered to be a base surge deposit. Ascending to the eruption cycles, the thickness of massive white tuff layers is decreasing, but the gray layers are increasing. Representative samples were collected from 42 horizons of the tuff ring sequence and analyzed the bulk rock chemical compositions using XRF and the mineral assemblages using XRD. The bulk rock chemical compositions of the gray layers changes from andesite at the lower horizon to basalt at the upper horizon. On the other hand, massive white tuff layers have almost identical and are andesite. Because the compositions of basal layers are very similar to the silt of the basement rock, the beds in the unit 1 may have been deposited fragmented basement rocks by the first phreatomagmatic eruption. At the upper layers of the first eruption cycle (unit 1), chlorine contents are up to the maximum level (ca. 4%) and the level is decreasing exponentially according to the later eruption cycles. Mafic minerals, such as hornblende and hypersthene, are scarcely observed in massive white tuff layers, but relative abundance of kaolin is increasing with progressing the eruption cycle from unit 1 to unit 4. As rapid increasing of the chlorine contents observed at the basal part of the sequence, interaction between magma and seawater generated the early phreatomagmatic eruption. Because the basement rocks were indurated silt, the sequence lacks basal breccia, which is commonly observed in a tuff ring sequence. Increasing of the kaolin abundance implies the expansion of the vent area at the boundary between magma chamber and wall rocks. The widened vent may promote the relative abundance of the essential material in the pyroclastic deposits due to the direct connection of the magma chamber and surface. As a result, the eruption style changed from phreatomagmatic to strombolian type at the final eruption cycle.