2019 Volume 125 Issue 1 Pages 87-105
Through the comparison of lithofacies fossils, marker tephras, and radiocarbon ages of different subsurface strata, this review aims to characterize the geologic features of the major coastal plains of Shikoku, southwestern Japan, thus promoting a better understanding of the coastal plain formation process. In the Tokushima Plain, the subsurface strata are divided into the alluvial Tokushima and the Pleistocene Kitajima formations. The Tokushima Formation is 30-50 m thick and comprises, from bottom to top, gravel, sand, mud, and sand beds, with marine molluscan fossils in the middle mud beds. This formation is latest Pleistocene to Holocene in age and unconformably overlies the Kitajima Formation. Although a similar sequence is identified under most of the coastal plains in Shikoku, the presence of gravel and sand in the lower parts has been confirmed only in the Takamatsu Lowland. The thickness of the alluvial deposits varies from plain to plain, with this variation probably being due to the long-term subsidence of the basin and the differential erosion of former incised valleys. The Kitajima Formation is characterized by the repletion of gravel and mud beds, with each attaining a thickness of several decameters. Marine fossils are found in the mud beds, suggesting that they were formed during high sea-level periods, namely, interglacial intervals. The Kitajima Formation can be correlated with the strata underlying the alluvial sediments of the coastal plains of Shikoku by means of widespread tephra marker beds. Based on the correlation results of this study, a similarity of lithofacies among the upper Pleistocene and Holocene deposits is identified. In many cases, the bottom part of the alluvial deposits comprises gravel covered with thin marine muds, deltaic muds, and/or sands. Sandy gravels cover the plains, often as fan deposits. The thickness of the alluvial deposits tends to be greater in areas of subsidence compared with that in areas of uplift.
