Lonestones as indicators of tsunami deposits in deep-sea sedimentary rocks of the Miocene Morozaki Group, central Japan

Abstract

Lonestones are isolated larger clasts enclosed in muddy fine-grained deposits, and are usually interpreted as iceberg-rafted dropstones. This interpretation implies the existence of glaciers (continental ice sheets) and, consequently, a cool climate. However, alternative interpretations are possible, as lonestones may also be deposited by non-glacial processes. Therefore, clarification of the depositional processes associated with lonestones is fundamental for studies based on lonestone-bearing deposits. A field survey of lonestone-bearing deposits from Early Miocene deep-sea sedimentary rocks found around the Chita Peninsula of central Japan suggests that tsunami-induced flows on the sea bottom may also form lonestones. These lonestones are associated with sandy to gravelly deposits, and were deposited by high-energy episodic currents. The main features of these deposits are the multiple stacking of normally graded units, and the laterally discontinuous distribution of coarse-grained clastic material (sands and gravels). Such features are consistent with deposition by tsunamis and suggest that lonestone-bearing depositional successions must be carefully interpreted, especially where lonestones are used as glacial indicators, as some lonestones were probably put in place by ancient tsunami events.