丹波帯の三畳系-下部ジュラ系の砂岩組成とその意義  [in Japanese] Triassic-lower Jurassic sandstone compositions from the Tamba Belt, Southwest Japan, and their geologic significance  [in Japanese]

西南日本内帯の丹波帯に分布する丹波層群のうち, その上部に分布するユニットTIIdの砂岩およびその上位のポンポン山層と高城山層の砂岩について, 堆積岩石学的検討を行なった。ユニットTIIdの砂岩は, ユニットTIIcの砂岩より相対的に石英や斜長石が多く岩片量が少ない。高城山層やポンポン山層の砂岩は, 成熟度・淘汰ともに良く, 石英やカリ長石が多く岩片量が少ないアルコース質砂岩である。これらの砂岩は, 地帯を越えて舞鶴帯の三畳紀新世の砂岩に類似しており, しかも丹波層群の堆積環境より浅い堆積場のものである。丹波帯の砂岩が, 三畳紀新世のアルコース質でより浅海を示す砂岩からジュラ紀古世の石質で深海相へ変化していったことは, この時期に丹波帯の後背地および堆積場で, 主要な構造運動の活発化があったことを示している。

We describe in detail petrographical features and the detrital mode of sandstones from the TIId Unit (late Triassic), the Takashiroyama and Pompomyama Formations in the Tamba Belt in the Inner Zone of Southwest Japan. The geologic age of the Takashiroyama and Pompomyama Formations is regarded as the sameas, or slightly older than, that of the TIId Unit. Sandstone of the TIId Unit is richer in quartz and plagioclase, and poorer in rock fragments than that of the TIIc Unit (early Jurassic). In contrast, sandstones from the Takashiroyama and Pompomyama Formations are rich in potash feldspar and quartz, but poor in rock fragments, with good sorting and high maturity. We have termed them feldspathic sandstones. These sandstones are very similar in composition to those of the late Triassic Nabae Group in the Maizuru Belt beyond the Ultra-Tamba Zone, and both of them display features of shallower facies compared with those of the TIId Unit. Petrographical and sedimentological data reveal that these sandstones changed from feldspathic and shallower facies in the late Triassic time to lithic and deeper facies in the early Jurassic time. This change may correspond to activation of tectonic movement in the source area and depositional basin of the Tamba Belt at that time.