Mesozoic and Paleozoic Systems in the central area of the Kii Mountains, Southwest Japan (Part VI) ―Mesozoic of the Tsujido area in Nara Prefecture―

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  • 紀伊山地中央部の中・古生界(その6) ―辻堂地域―
  • キイ サンチ チュウオウブ ノ チュウコセイカイ ソノ 6 ツジドウ チイキ

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Abstract

Six units of sedimentary complexes are discriminated in the Tsujido area, west of Omine Range, namely the Daifugendake, Wasabidani, Takahara, Obadanigawa, Ui and Akataki Formations in structurally desending order. The first three units belong to the Chichibu Terrane, while the rest belongs to the Shimanto Terrane. All the units are stacked up to form a low-angle piled nappe structure showing generally downward-younging polarity from the Daifugendake Formation to the Akataki Formation except for the Ui Formation. The sole thrusts of the Akataki and Obadanigawa Formations may be regarded as out-of-sequence thrusts. The nappe structure is modified by later high-angled faulting such as the E-W trending Shimotakogawa Fault and the N-S trending Kitozumi Fault which were newly recognized here. The Daifugendake Formation (Upper Jurassic) is composed of chert-clastic sequences. The Wasabidani Formation (Upper Jurassic melange) is characterized by containing huge blocks of limestone, greenstones and chert. The Takahara Formation (lower to middle of Lower Cretaceous) is a melange containing blocks of chert, limestone and greenstones. The Obadanigawa Formation (Albian〜Cenomanian) consists mainly of sandstone-rich clastic sequences. The Ui Formation is newly introduced here, and is characterized by repetition of sandstone-rich clastic sequences (mainly Cenomanian) and melange facies (Turonian〜Santonian). The Akataki Formation (Coniacian〜Campanian?) is composed of a melange with blocks of red chert, red mudstone and greenstones. It has been well-known that the Chichibu Terrane is missing in the central part of the Kii Mountains. Our research has revealed that the Obadanigawa Formation which occupies the uppermost part of the Shimanto Terrane structurally is also missing in the same area.

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