西南日本外帯中新統熊野層群の砕屑岩組成と堆積相  [in Japanese] Detrital composition and sedimentary facies of the Miocene Kumano Group in the Outer Zone, Southwest Japan  [in Japanese]

西南日本外帯に分布する中新統熊野層群の砕屑岩(砂岩・礫岩)の組成及びその層位的変化を調べた。砂岩組成は堆積環境の変化と対応して顕著な層位的変化を示す。下位層準で淘汰不良で未成熟な石質ワッケが, 上位層準で単結晶石英に富む石英長石質アレナイトが卓越する。砂岩中の岩片組成・礫組成及び古流向解析によって, 四万十・秩父・三波川・領家の各地帯からの砕屑物の供給が想定され, 特に基盤の四万十累層群からの供給が卓越していたと考えられる。熊野層群の砂岩組成は, Dickinson & Suczek (1979)やIngersoll & Suczek (1979)らによって提案されたプレートテクトニクスに基づく供給源組成三角図において, 予想される供給源フィールドに必ずしもプロットされない。これは, 砕屑物(岩)組成の堆積環境に対する依存性に起因していると考えられる。

Detrital compositions of clastic rocks from the Kumano Group, which is one of the post-Shimanto Miocene strata in the Outer Zone of Southwest Japan, are investigated. The characteristic composition of distinct suites of sandstone can serve as a guide to the interpretation and relation among detrital composition, depositional environment and provenance. Detrital compositions of sandstone are clearly affected by the sedimentation process, and become more quartzofeldspathic stratigraphically upwards. Petrographic analysis permits subdivision of sandstones into seven petrofacies. Sandstones deposited during a transgressive phase, of petrofacies 1 (basal shallow marine sandstones), petrofacies 2 (sandstones deposited under shelf or bathal conditions) and petrofacies 3 (turbidites) are mainly composed of ill-sorted immature lithic wacke. On the contrary, sandstones deposited during a regressive phase, of petrofacies 4 (nearshore storm sandstones and associated thick sandstones), petrofacies 5 (shoreface to littoral sandstones) and petrofacies 6 (shallow marine or marginal marine sandstones) are characterized by quartzose or feldspathic arenites which are rich in monocrystalline quartz grains. The modal analysis indicates that 335 sandstones from the Kumano Group have mean QmFRt percentages of 57-24-19 (standard deviation 12-9-15), QFR 68-24-8 (9-9-9), QpLvLs 62-19-19 (19-12-15), and LmLvLs 4-48-48 (9-24-26). The compositions of lithic grains in sandstones and conglomerate clasts, together with paleocurrent data support an idea that clastics were derived mainly from the basement Shimanto Supergroup, and partly from the Chichibu, Sambagawa and Ryoke terrains which consist of non-metamorphosed or metamorphosed accretionary complex rocks. Detrital compositions of sandstones from the Kumano Group do not necessarily plot in a predictable provenance field in the triangular plate tectonic provenance diagram proposed by Dickinson & Suczek (1979) and Ingersoll & Suczek (1979). This is presumed to result from the dependence of detrital compositionon the sedimentary environment.