Sedimentary facies, processes and environments of the Akakura caldera lake, the South Kurikoma geothermal area, northeast Japan

Abstract

The Pleistocene Kan-nodai Formation was deposited in the collapse basin formed by felsic volcanism of the Akakura caldera, South Kurikoma geothermal area, northeast Japan. The middle and upper members of the Kan-nodai Formation represent postcaldera lacustrine slope apron and basin plain deposits of the Akakura caldera. The slope apron deposits consist mainly of interbedded sandstone and mudstone and massive tuff breccia deposited from turbidity currents and debris flows that resulted from slope failures of the unstable caldera wall. The beds are laterally continuous and show neither systematic vertical depositional patterns nor distinctive erosional features. Slump beds are common, indicating slope instability. The basin plain deposits overlie the slope apron deposits and are dominated by fine-grained turbidites and debris flow deposits, and suspension fallout deposits, indicating that coarse-sediment supply is not affected. Plant fossils are abundant. The slope apron and basin plain deposits include voluminous graded pumice lapilli tuff. A fining-upward succession from the slope apron to the basin plain deposits reflects retreat of the caldera wall due to slope failures, and rise of lake level with increasing water volume within the caldera basin. The lake deposits record volcanic activities of the felsic pyroclastic eruption in the area east of the lake and the postcaldera andesitic effusive eruption in the southeastern part of the caldera.