Origin of mudstone clasts in turbidites of the Miocene Ushikiri Formation, Shimane Peninsula, Southwest Japan

Abstract

Some turbidites of the Miocene Ushikiri Formation in the Shimane Peninsula, Southwest Japan include many mudstone clasts within their normally graded and/or massive lower parts. The mudstone clasts are at most 1–2 m long and 10–20 cm thick. They are undulated, folded or pulled apart, and are often concentrated at certain levels within the lower parts. The turbidites with mudstone clasts usually have sharp, planar soles with no distinct channels, but occasionally they penetrate and tear up the underlying mudstone layers. In addition, most, if not all mudstone clasts lithologically resemble the underlying mudstone. These observations on outcrops suggest that the most of mudstone clasts were torn off by turbidity currents from underlying hydroplastic muds that possess a breaking strength, and are entrained into the currents and deformed into various attitudes during transport. The force which played such an important role in the process probably came from large shear stress developed within the turbidity currents.