Chaotic beds in the Paleogene Muroto Formation at Muroto Peninsula, Kochi Prefecture, SW Japan

Abstract

The deformation features of the Paleogene Muroto Formation at Cape Gyodo, Muroto Peninsula, SW Japan, are described in terms of a lithofacies map and stratigraphic columns. Nine chaotic beds account for about 40% of the total stratigraphic thickness of 220 m in the study area and have the following characteristics : (1) a certain deformation pattern, although some parts are missing (2) soft-sediment deformation that indicates layer-parallel extension and shortening, (3) simultaneous sand intrusions with the soft-sediment deformation, (4) development of their chaotic nature before the formation of map-scale tectonic faults, and (5) migration in the paleo-downslope direction, as inferred from turbidite deposits. Based on these characteristics, the chaotic beds are inferred to be mass-transport deposits. The lithofacies and deformation structures indicate that the chaotic beds were formed by sliding and/or slumping.