DEPOSITIONAL PROCESSES OF PRIMARY TO REWORKED VOLCANICLASTICS ON AN ALLUVIAL PLAIN - AN EXAMPLE FROM THE LOWER PLIOCENE OHTA TEPHRA BED OF THE TOKAI GROUP, CENTRAL JAPAN

A change from primary to reworked volcaniclastic deposits in the Lower Pliocene Ohta tephra bed has been identified, using facies analysis. The Ohta tephra bed is a widespread marker-tephra, distributed over a distance of more than 130 km. The source volcano of this tephra is inf erred to lie 50 to 100 km north of the most proximal Ohta tephra bed. The Ohta tephra bed shows a change in volcaniclastic facies in distal areas. Eight sedimentary facies are recognized. These are interpreted as pyroclastic fall, pyroclastic flow, and reworked (mudflow, flood fl ow, channel fill, swamp or backswamp, lacustrine and ephemeral pond) d eposits. In underlying and overlying non-volcaniclastic beds, four sed imentary facies are recognized, indicating a fluvio (-lacustrine) depo sitional system on an alluvial plain. From the successions of volcanic lastic sedimentary facies, deposition of the Ohta tephra bed can be di vided into stages I to VI. Each depositional stage is intimately relat ed to volcanic activity. Stage I is the first eruption, during which o nly the northern part of the study area was covered with fallout ash. The second eruption stage of stage II involved fallout ash distributed over all the study area. These two fallout ash beds are derived from precursor eruptions. Stage III reflects a period of volcanic quiescenc e and development of widespread lacustrine conditions. The fallout ash may have destroyed vegetation, and prompted greater discharge to form the large lacustrine deposits of stage III. A pyroclastic flow stage in stage IV is associated with an explosive eruption, and resulted in pyroclastic flow deposits reaching the northern part of the study area . Stage V is characterized by mudflow deposits. Widespread mudflow dep osits also originated from high water and clastics discharge on the un vegetated surface. Mudflows extend more than 180 km from the source vo lcano. Abundant supply of volcaniclastic material and high water and e lastic discharge enchanced a rapid progradation of alluvial plains. Fi nal reworking occurred during stage VI, when volcaniclastic sediments were deposited from a fluvio-lacustrine system. The reworking system c omprises initial mudflows, followed by a normal fluvial system (normal stream flow). The mudflow deposits of stage V are traceable with roug hly constant thickness, but the volcaniclastic content of the deposits decreases in distal areas. The fluvial system deposits of stage VI ar e thicker and have less abundant volcaniclastics in distal areas. Fluv ial systems were braided streams promoted by volcanic eruption, while they were meandering streams during the pre-eruption period. Some erup tion-induced braided streams remained for a while following the cessat ion of volcanic activity.