The zeolitic alteration of vitric tuffs in the Neogene oil-bearing formations in Akita oil field was studied. Zeolitic zonation at Yuri hilly land is shown in Fig. 1, and Table 2. Zeolitic columnar sections of underground rocks at lowland are shown in Figures 2 and 3. Zeolitization of underground vitric tuffs at the lowland area more moved upwards in geologic horizen as compared with that of the hilly land. Two different types of alteration facies, the upheaval type and the depression type, are distinguished among the underground tuffs, as showing in the following table: Marginal area of the Akita sedimentary basin, in which Yuri hilly land is situated, were widely and largely upheaved by the Dewa disturbance. After this disturbance, the physical conditions under which vitric tuffs had been altered into zeolite was diminished in these upheaved area. In the remained basin, where the depression was succeeded, the zeolitization upwardly somewhat increased. During the Tentokujian stage, the sedimentary basin was differentiated into raised belts and depressed zones. At the raised belt the favorable condition for zeolitization gradually decreased. While, in the depressed zone younger formations successively deposited and increased the zeolitic alteration of vitric tuffs until the end of the Sasaokan stage. In Akita oil field, pyroclastic rocks, of which acidic tuffs are most extensive, are commonly intercalated in oil-bearing formations, and they often formed oil and natural gas reservoirs.
To make efficient use of isopach maps and lithofacies maps that are indispensable in oil exploration, it is required that such maps were prepared on the basis of stratigraphic units distinguished by significant time plane. The Cretaceous sediments which are widely distributed in the axial zone of Hokkaido are divided, in descending order, into the Hakobuchi group, the Upper Yezo group, the Middle Yezo group, the Middle Yezo group, the Lower Yezo group, and the Sorachi group. However, since this stratigraphic division is based largely on lithology, the boundaries of the respective groups may sometimes intersect the time plane obliquely. If such is the case, then the maps that were prepared from the stratigrephic division would be meaningless. Therefore, the present writer examined the propriety of the past methods of defining the base of the Middle Yezo group. His aim was to discern the base of the Middle Yezo group as a time plane and clarify the scope and significance of the Intra-Yezo disturbance. The writer has reached the following conclusion : The Intrayezo disturbance took place in a limited period of time during late Albian. It formed the uplifted zone that stretches north-south centering on the Ashibetsu mountains, and also upraised the west marginal area of the Hidaka mountains. This disturbance probably represents a crustal movement in the middle period of the history of development of a sedimentary basin.