The western offshore area of the Oki Islands is evaluated as one of promising area for oil and gas exploration, because of the thick accumulation of sedimentary rock based on the results of seismic surveys. To establish the chronostratigraphic framework with high resolution in the area, the biostratigraphic study of fossil diatoms was carried out for the samples collected from the Miocene Kumi and Tsuma Formations mainly exposed in the national park in Dogo, Oki Islands, Shimane Prefecture.
The Crucidenticula kanayae Zone (16.6 - 17.0 Ma) was recognized from the samples about 10 m above the lowermost lacustrine Kumi Formation in the Kumi River, which indicates that the geologic evidence of the first marine invasion related to the Japan Sea spreading was recorded in the northern part of Dogo. The Zone including the biohorizon D33, the last occurrence of C. kanayae (16.7 Ma), was found in the Nogoshi section exposed in the northwestern part of the island. The stratigraphic change of diatom assemblage suggests a gentle transgressive trend through the C. kanayae Zone. The specimens of C. kanayae yielded from the Kumi Formation are identified as C. kanayae var. kanayae showing a wide range of morphological variation. The shallow marine sandstone is expected to be a good reservoir rock distributed in the lowstand systems tract correlated with the layer just above the C. kanayae zone.
The Crucidenticula nicobarica, Denticulopsis praedimorpha and Denticulopsis dimorpha Zones were recognized from the Kumi Formation in an about 10 m thick section in the Nagu Peninsula area of the southwestern part of the island. This suggests a condensed sequence developed from late Middle to early Late Miocene in the area. The sequence which is correlative with the black mudstone, the Utagi Mudstone Member, seems to have the highest source rock potential in the area. The lower Neodenticula kamtschatica Zone was recognized from the Igo Mudstone Member along the Nakanoura shore of the northern part of the Island. The Tsuma Formation showing lateral lithofacies changes in Dogo indicates low possibility of the cap rock existence for the unconformity play of the uppermost Kumi Formation.
