Tako brecciastone-bearing mudstone layer in the southern coastal region of the Kii Peninsula, southwestern Japan — its age and the formation process —

Abstract

Peculiar mudstones beds containing sandstone breccia or blocks of various size named the Tako brecciastone-bearing mudstone layer, called “Sarashikubi beds”, occur in the coastal area of southern Kii Peninsula, southwest Japan. The formation of the Tako brecciastone-bearing mudstone layer has been ascribed to a mud volcano or a submarine debris flow. The beds are subdivided into three units: “the Ozarashi”, “the Tomiyama conglomerates”, and “the Kozarashi” in stratigraphic ascending order. We collected samples four types of sandstone breccia, one gravelly sandstone block from the Ozarashi, and one sandstone from the Tomiyama conglomerates. Modal composition analyses and detrital zircon U–Pb ages dating were conducted by LA–ICP–MS. Sandstone breccias from “the Ozarashi” and sandstone from the Tomiyama conglomerates are feldspathic wacke to arenite, and their composition is consistent with that of the Muro Accretionary Sequence (AS) that underlies the Tako brecciastone-bearing mudstone layer. In contrast, the gravelly sandstone block is quartz-rich lithic arenite. The youngest cluster of detrital zircon U–Pb ages from the sandstone breccias, spans the range of 32–62 Ma. Based on the compositional data and U–Pb ages, the breccia clasts are inferred to have been derived from the Muro AS. The sample of gravelly sandstone, which is thought to be foreshore deposit, yielded a detrital zircon U–Pb age of 27 Ma (Late Oligocene), younger than the Muro AS (Early Oligocene). This result ruled out the possibility that the Tako brecciastone-bearing mudstone layer was formed via mud volcano. Characteristic features of bedded formation are observed in “the Ozarashi”. It is inferred that “the Ozarashi” was transported by a submarine debris flow. It is likely that the Tako brecciastone-bearing mudstone layer comprises rock-fall deposits and shallow marine sediments. These deposits are presumed to have originated from the activity of thrust faults cutting the Muro Accretionary Sequence in a forearc basin.