Cretaceous to Paleocene depositional history of North-Pacific subduction zone: reconstruction from the Nemuro Group, eastern Hokkaido, northern Japan

Abstract

The Campanian–Paleocene Nemuro Group comparing the oldest strata in the Kuril Arc, is distributed in the east of Hokkaido Island, northern Japan. Strata of the group in this region are sedimentologically classified into eight depositional facies, most of which are interpreted as sediment gravity flow deposits. These depositional facies comprise four facies associations. The distal and proximal basin plain facies associations are composed mainly of hemipelagic mudstones and sandstones that are interpreted as gravity flow deposits, and were deposited in a topographically flat environment inferred by good lateral continuity of lithofacies. The channel-levee complex and submarine slope facies associations, in contrast, are composed of hemipelagic mudstone, turbidites and debris flow deposits. These four facies associations stack in ascending order, and represent a regressive succession. Palaeocurrent data indicate that the deposits of the Nemuro Group were transported from the northwest. Hence, the group in the study area is concluded to record slope progradation away from the northern source area. Hiertherto, it has been known that the sea regressed from the Kuril Arc during the Eocene. This was attributed to ridge subduction beneath the Kuril Arc. My study has revealed that regression began as early as Maastrichtian. It may have been induced by volcanic activity in the Kuril Arc or by a significant eustatic fall in sea level.