Temporal variations in major depositional processes of prograding deltaic systems in the Tertiary Taishu Group, Tsushima Islands, southwestern Japan

Abstract

The Tertiary Taishu Group in the Tsushima Islands, southwestern Japan is characterized by deltaic systems. The group is subdivided into the Lower, Middle, and Upper Formations. The Lower Formation (Early Eocene) represents a tide-influenced, fluvial-dominated deltaic system, and the Upper Formation (Early Miocene) represents a wave-dominated deltaic system. During the deposition of the Upper Formation, the Japan Sea is interpreted to have started opening and a shallow marine environment appears to have expanded around the Tsushima Islands area. This paleogeographic change is interpreted to have resulted in changes in major depositional processes for the delta formation.