Process of formation of large-ignimbrite plateaus

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Ignimbrite plateaus are distributed extensively in southern Kyushu, Japan. The plateaus are composed mainly of large Ito pyroclastic flow deposits, which is a product of big eruptions at Aira caldera about 22,000 y.B.P. The Ito-ignimbrite field was selected as a representative example for study of formation of large-ignimbrite plateaus.The top surface of the Ito-ignimbrite plateaus are classified into two: the original depositional surface of the ignimbrite and the fluvial surfaces with water-laid(reworked) deposits derived from Ito ignimbrite. The latter is divided into the upper surface with deposits of probable sheet flood origin and the fluvial terrace surfaces.The sheet flood deposits are widely distributed on the top of the Ito-ignimbrite plateaus, horizontally stratified, and usually less than 3 meters in thickness. fluvial terraces are distributed along main rivers in the Ito-ignimbrite field. Thickness of the terrace deposits is usually less than several meters.Tephra layers several meters in total thickness uniformly cover both the depositional and the fluvial surfaces. The age of the lowermost tephra on the immediate top of the reworked deposits is about 18,000 y.B.P. This indicates that the reworked deposits were formed between 18,000 y.B.P. and 22,000 y.B.P.It is generally considered that original depositional surface immediately after the emplacement of pyroclastic flows suffers most violent dissection because of lack of vegetation. Very high erodibility of the unwelded ignimbrite and the above-mentioned ages suggest the probability that the formation of the reworked deposits and the ignimbrite plateaus were accomplished rapidly in a very short period immediately after the emplacement of pyroclastic flows.