The outline of Tertiary volcanostratigraphy in the Akita Basin is summarized. In particular, the volcanic facies in the Sunakobuchi Formation and the Manaitayama Volcanic Rock-Fukuyama Andesite which form oil and gas reservoirs is reviewed from a view of occurrence, depositional environment and eruption style.
The Sunakobuchi Formation is composed of basaltic volcanic rocks and its volcaniclastics. Those eruption styles change from magmatic eruption in terrestrial to shore environments at lower part, to submarine magmatic eruption mainly composed of pillow lava at upper part, through phreatomagmatic/phreatic explosion and subaqueous fountain eruption at middle part.
The Manaitayama Volcanic Rock comprised of subaqueous volcanic rocks and volcaniclastics is divided into M1 (olivine pyroxene basalt), M2 (pyroxene andesite), M3 (hornblende pyroxene andesite) and M4 (pyroxene hornblende dacite).
The Fukuyama Andesite consists of pyroxene andesite (FKV-1) correlated to M2 and hornblende pyroxene andesite (FKV-2) correlated to M3. The volcanic facies indicate that the andesites erupted under submarine environment and some of the subaqueous lava flows intruded into surrounding unconsolidated sediments.
The Manaitayama Volcanic Rock and the Fukuyama Andesite construct volcanic mounds on sea floor. Part of them is eroded at subaerial condition and reworked-volcaniclastic sediments develop around the mounds.