Abstract
A change from pyroclastic to reworked deposits in the Lower Pliocene Ohta Tephra Bed is preliminary clarified, based on facies analysis. The Ohta Tephra Bed is a wide spread marker-tephra, distributed over a distance of more than 150km. Eight sedimentary facies are recognized in this tephra bed. The facies are interpreted as pyroclastic fall, pyroclastic flow, and reworked (hyperconcentrated flood flow, normal flood flow, channel fill, swamp or backswamp, and lacustrine) deposits. In underlying and overlain non-volcaniclastic beds, four sedimentary facies are recognized and indicate a fluvio-lacustrine depositional system. According to the successions of the volcaniclastic sedimentary facies, the deposition of the Ohta Tephra Bed can be divided into 5 stages. The depositional process is reconstructed using these 5 stages as follows: Stage I (first eruption stage); the north part of the study area was covered with air fall ash. Stage II (second eruption stage); air fall ash was distributed over all of the study area. Stage III (quiescent stage); development of a widespread lacustrine environment. Stage IV (pyroclastic flow stage); pyroclastic flow deposits reached at the north of the study area. Stage V (reworking stage); volcaniclastic sediments were deposited from a fluvio-lacustrine system including hyperconcentrated flood flow.
