Prelithification shear structures of mélange unit in Shiofuki-iwa area, the upper Cretaceous Shimanto Belt, Wakayama Prefecture, Japan

Analyzing of sedimentary facies and structure together with regional field mapping were studied in the mélange unit, upper Cretaceous Shimanto Belt, Wakayama Pref., SW Japan. The field survey was concentrated in the coast of Shiofuki-iwa, Miyama Complex, Hidakagawa Belt where soft deformations penetrate throughout the mélange unit. Brittle to ductile faults associated as later deformations are excluded. The mélange unit is subdivided into the Y-shear zone (YSZ) and P-foliation zone (PFZ). The YSZ is composed of thick lenticular sandstone (trends subparallel to the direction of the mélange), intense shear band (thin layer with concentrated prelithification shear deformation) and mudstone dominant mélange. The PFZ is composed of mudstone dominant mélange (associated with foliations which slightly oblique to the direction of the YSZ) and lenticular sandstone (slightly oblique to the direction of the YSZ). The intense shear band separates the YSZ and PFZ. Preserved radiolarian fossils free from deformation in the intense shear band is the evidence of the prelithification shear deformation in the mélange. A remarkable-fact is that the structures of mélange unit are formed under sinistral sense of shear. The mélange unit in the Shiofuki-iwa is ascribed to be formed by sinistral sense of shear during sediments were prelithified.