2011 Volume 33 Issue 1 Pages 15-27
The Pleistocene Kan-nodai Formation of three geothermal exploration wells (N63-MA-3, N63-MA-4 and T-3) represents caldera fill deposits of the Akakura caldera in the Kurikoma geothermal area and has a well-recorded sedimentary process of collapse basin after caldera-forming pyroclastic eruption. Based on grain size, composition and sedimentary structure of the Kan-nodai Formation of the drillcores, seven sedimentary facies (Gm, Gc, Sg, Sh, SM, Mb and Tf) are defined and are divided into three facies associations; A, B and C in ascending order. Facies association A is predominantly composed of massive, matrix-supported breccia (Gm), crudely graded, clast-supported breccia (Gc) and massive or crudely stratified, graded pebbly sandstone (Sg) together with horizontally stratified sandstone (Sh), whose depositional processes are chiefly debris flow, hyperconcentrated flow and high-energy tractional current. These sediments are interpreted to have been accumulated in an alluvial fan environment. Facies association B is mainly composed of interbedded sandstone and mudstone (SM) and felsic graded tuff (Tf) with massive, matrix-supported breccia (Gm), which is characterized by the depositional processes of sediment gravity flows such as turbidity currents and debris flows that result from the slope failures of the caldera wall. The most likely place in which these sediments are emplaced is inferred to be slope apron within a caldera lake. Facies association C consists of bedded mudstone (Mb) and felsic graded tuff (Tf). The bedded mudstone reveals that the depositional process is dominated by suspension fallout, suggesting the deposition on a basin plain in a caldera lake, in which there is no coarse-sediment supply. The Kan-nodai Formation consisting of postcaldera alluvial fan, slope apron and basin plain deposits forms a large-scale fining-upward sequence, which presumably reflects the retreat of the caldera walls due to erosional process. Such retreat of the caldera walls has resulted in a significant gap between the structural walls (about 5 km in diameter) and the present-day topographic walls (about 10 km in diameter).
Three geothermal exploration wells are located outside of the structural caldera wall. Based on thermal conductivity of the caldera fill and the basement rocks, geothermal gradient of the wells, and the subsurface geological structure, a promising geothermal reservoir is not expected in the western part of the caldera. However, the eastern part of the caldera in which the postcaldera subaqueous volcanic activity occurred during 0.8 to 1.4 Ma, as documented by the Mimizuku-yama Volcanics, might have to some extent anomalous temperature.
