Paleo-mud-volcanoes of the Lower Miocene Tanabe Group on the Southern Kii Peninsula, Southwest Japan

Abstract

The Lower Miocene Tanabe Group, exposed on the southern Kii Peninsula, is a thick pile of fore-arc basin sediments, which clino-unconformably covers the Paleogene Shimanto accretionary complex. Many mud diapirs and mud dykes intrude into the Tanabe Group. Several thick sequences of bedded breccia are found at Tanoi and Fukuro in Shirahama-cho, Wakayama Prefecture. In this paper, bedded breccias with shallow-marine sediments are described. The facies analysis shows that the bedded breccias are mud-volcanic deposits, and that two submarine mud volcanoes were involved in the southern Tanabe Group. The Tanoi sequences, which reach a thickness of 490 m, are mainly composed of sand-matrix, bedded breccia associated with mud-matrix, and bedded breccia. The bedded breccias range from 5 to 150 cm in thickness. They contain angular to sub-rounded clasts consisting of sandstone and mudstone from granule to cobble in size. The bedded breccia is matrix-supported with scattered clasts, which develop with inverse grading. It is considered to be a subaqueous debris flow deposit. The Fukuro sequences are mainly composed of mud-matrix, bedded breccias associated with clast-bearing sandstone of 1 ⊖ 15 cm thickness, which have turbidite-like sedimentary structures. The bedded breccias range from 5 to 150 cm in thickness. They contain angular to sub-rounded clasts consisting of mudstone and sandstone from granule to cobble in size. The bedded breccias are matrix-supported with scattered clasts, which develop with inverse grading. It is considered that the cause is a subaqueous debris flow deposit. The upper part of the clast-bearing sandstone is likely to have been reworked later by storm waves and tidal currents. The paleocurrent deduced from the sole marks of mud-matrix, bedded breccia flowed from northeast and east. The shallow-marine sediments develop wave ripple, planar cross-stratification, trough-type cross-stratification, chevron structure, off-shooting foreset, and hummocky cross-stratification, which indicate that the Fukuro mud-volcanic products were deposited at a depth near the lower limit of the storm wave base from the lower shoreface to the shelf. During the Early Miocene, submarine mud volcanism took place at Tanoi and Fukuro in the southern Tanabe Group. It is believed that the Tanoi mud volcano caused the Tanoi mud-volcanic deposits to erupt from the Tanoi mud diapir, and that the Fukuro mud volcano caused the Fukuro mud-volcanic deposits to erupt from the Migusagawa-Hirukawadani mud diapir.