コンデンス・セクションとして形成された非熱帯性石灰岩 : 岡山県南西部,中新統浪形層石灰岩の堆積環境

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Although the latest Early to early Middle Miocene sediments in the Southwest Japan arc are mostly of terrigenous and volcanic origin, small limestone masses with common lithological and paleontological properties occur within an intra-arc basin of the arc. This paper elucidates the formative process of the limestone bed intercalated in the Namigata Formation in Okayama Prefecture, at the northern margin of the intra-arc basin, and infers the regional paleoenvironment of the intra-arc basin. The Namigata Formation, ca. 120m thick, consists of well-sorted sandstone with conglomerate and limestone, and is a sediment on a transgressive rocky coast bordering the northern margin of the intra-arc basin. The basal unconformable surface of the formation shows that the rocky coast had a staircase geomorphology consisting of three-stepped wave cut terraces and an elevated peneplain behind. The limestone layer (ca. 15m thick) intercalated at the middle horizon of the formation comprises biosparudite (grainstone) which is a mixture of skeletal fragments of molluscs, bryozoans, cirripeds, echinodermata, foraminifera, brachiopoda and algae, and of terrigenous sands with various contents (42% on an average), without any other calcareous grains and lime-mud. These sedimentological features indicate that the limestone is classified as "non-tropical shelf carbonates", particularly "bryomol". The predominance of sessile benthos and suspension feeders, the remarkable fragmentation of the skeletals (except for non-worn benthic foraminifera tests), and the abundance of intergranular pore space show high water energies in both the carbonate factory and accumulation site for the "bryomol" limestone. The staircase rocky coast buried beneath the Namigata Formation is a product of intermittent rises of relative sea level. Benthic foraminifera in the "bryomol" bed show that it accumulated in the inner to middle sublittoral zone and the sea level at that time rose up to the altitude of the elevated peneplain behind the rocky coast. Hence we concluded that the "bryomol' bed is a condensed section formed under the following environmental conditions; 1) the marked reduction of terrigenous supply due to trapping on the drown elevated peneplain, and 2) the slow but exclusive supply of skeletals from the carbonate factory on the highest wave cut terrace lowered below "surf base". Extensively occurring in the intra-arc basin of the southwest Japan arc are limestone masses similar to the "bryomol" bed in the Namigata Formation and molluscan faunas preferring rapid water circulation. This suggests that high water energy is characteristic of the regional maritime environment of the intra-arc basin and that the intra-arc basin was a coastal open sea.
地学団体研究会の論文
1995-01-25

コンデンス・セクションとして形成された非熱帯性石灰岩 : 岡山県南西部,中新統浪形層石灰岩の堆積環境

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