Stratigraphic position for the Kikai-Akahoya tephra in the Holocene Numa Formation, southernmost part of the Boso Peninsula, Chiba, Japan: An approach from the viewpoint of biogenic sediment mixing process

Abstract

Stratigraphic position of the Kikai-Akahoya (K-Ah) tephra in the Holocene Numa Formation distributed in the southernmost part of the Boso Peninsula, Chiba, Japan was mainly examined from the viewpoint of bioturbation. The K-Ah tephra in muddy sediments of the Numa is not recognizable as a visible geologic record due to complete dispersion of volcanic glasses induced by the intense biogenic sediment mixing after the deposition. A muddy sediment interval, which is cropped out along the Tomoe-gawa River, Tateyama City was selected for this study.
Petrographic and petrologic data on volcanic glasses in the muddy sediments show that the volcanic glasses of the K-Ah origin were found in mud interval above the Tsunami deposit T 3.1. Sedimentologic, paleontologic, and paleogeographic data indicate that biogenic sediment mixing process seems to represent the most important factor for controlling the diffusion mechanism and the final distribution pattern of volcanic glasses in the sediments. Stratigraphic distribution pattern of volcanic glasses of the K-Ah origin and mixing process strongly suggest that the initial stratigraphic position of the K-Ah in the Numa presents the restricted interval about 15 cm above the T 3.1 bed.