下部白亜系関門層群砂岩の岩石学的性質と起源  [in Japanese] Petrological nature and origin of sandstones of the Lower Cretaceous Kanmon Group  [in Japanese]

関門層群下部の脇野亜層群の砂岩組成や鉱物成熟度などを検討して慶尚層群下部の新洞亜層群との供給源の比較を行った。その結果, 脇野と新洞亜層群の砂岩は岩片の組成変化により区分できる。脇野亜層群の砂岩は石英と岩片の含量比が高く, 片岩, 安山岩, 花崗岩類などの岩片で特徴づけられる。新洞亜層群の砂岩は石英と長石の含量比が高く, 片磨岩やほかの変成度が高い変成岩類, 花崗岩類などの岩片が特徴である。両亜層群砂岩の斜長石とカリ長石の含量比は同じであるが, 鉱物成熟度からみると, 新洞亜層群の砂岩が脇野亜層群よりも成熟した砂岩である。新洞亜層群砂岩の供給源としては先カンブリア紀の基盤の嶺南地塊が考えられる。脇野亜層群砂岩の供給源は基盤の三郡変成岩類と大陸起源の花崗岩類が考えられる。基盤の時代, 砂岩組成, 供給源や堆積環境などから, 関門盆地と慶尚盆地は別々の堆積盆の可能性もある。

The Lower Cretaceous Wakino Subgroup, the lower part of the Kanmon Group, is exposed extensively in the western area of Chugoku and North Kyushu, Southwest Japan. It rests uncomformably on Permian metamorphic rocks and Upper Jurassic brackish-water deposits and displays a wide range of depositional facies from fluvio-alluvial to marginal lacustrine and lacustrine environments. This paper presents the results of a petrological study of sandstones from the Wakino Subgroup in terms of texture and mineralogical composition. The results are compared with that of the Lower Cretaceous Sindong Subgroup, the lower part of the Kyeongsang Supergroup, South Korea. The sandstones of the Wakino Subgroup are characterized by a predominance of lithic components which mainly consist of schist and subordinates late, andesite and other igneous rocks. On the other hand, sandstones of the Sindong Subgroup are highly feldspathic in nature, including clasts of gneiss and other high-grade metamorphic and granitic rocks. The mineralogical maturity of the Sindong sandstones is much higher than that of the Wakino sandstones. It is, however, noted that the plagioclase to alkali-feldspar ratio is similar between both subgroups and that clasts of orthoquartzite and granitic rocks are included in both subgroups. The provenance of the Sindong Subgroup was Pre-Cambrian continental basement rocks. However, two different provenances might be inferred for the Wakino Subgroup; one was related to older plutonic rocks, probably continental granitic rocks, and the other was directly underlying Sangun metamorphic rocks. The wide variety of both vertical and lateral sedimentary facies and sandstone compositions may suggest that the Wakino Basin was developed as an on-land strike-slip basin on the basement of the Permian accretionary complex. These results indicate that the Kanmon and Kyeongsang Basins may have originated separately.