Emplacement processes of channelized submarine lava flows composed of jumbled sheet flows and pillowed flows, inferred from the Middle Miocene Ogi Basalts, Japan.
Abstract:
Flows of the early phase I, contain lots of flattened pillows and horizontally connected pillows, branched from a subhorizontal massive sheet. The phase II flows are composed of sheets with jumbled surface, pillows, and flow-foot breccia (Fig. 1). The sheets are subdivided into inclined sheets and subhorizontal sheets, the latter of which compose the uppermost part of a flow, feeding the inclined sheets and pillows. The inclination is to the south and also to the east and west sides showing foreset bedding structures. Thickness of the pillow pile increases to the distal side, and the inclined sheets are formed where lava flowed over the pillow bank. The phase III flow is of pillowed flow, in which pillows show foreset bedding and intercalations with alternated horizontally elongated or flattened pillows. Undulated pillows and thin flat sheet are observed at the top, and sediment-filled pit-like relief is observed in north of the flow. These suggest that the phase III might be an inflated pillow flow.
The phase II and III flows probably represent jumbled sheet flow and its terminal, as an analogue was observed in channelized flows formed during the 1998 eruption of Axial Seamount, Juan de Fuca Ridge. Aggradational and progradational propagation is proposed for the emplacement of channelized flows in the Itsubo unit. Large bulbous pillows commonly occur as buds appeared from the inclined cylindrical pillows. Flow-top breccia of the sheets is formed by in-situ fragmentation of folded and wrinkled surface, which is caused by movement of molten lava in relation with dynamic interaction of lava and water. The phase I flow might represent propagation front of such a channelized flow.