Taphonomy of fish fossils from the Miocene Tottori Group, Southwest Japan : Part 2: Sedimentary facies and the formative process of a fish-fossil stratigraphic unit

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  • 鳥取層群産魚類化石のタフォノミー : その2:堆積相と魚類化石層の形成プロセス
  • トットリソウグンサンギョ ルイカセキ ノ タフォノミー(ソノ 2)タイセキソウ ト サカナ ルイカセキソウ ノ ケイセイ プロセス

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Abstract

Mid-Miocene transgressive sediments at Miyanoshita, Tottori City, yield abundant fossils of shallow marine fishes. As a part of investigation on the taphonomy of the fish fossils, this paper aims to analyze the sedimentary facies of the lower and middle Miocene Tottori Group, and to clarify the formative process of the fossiliferous unit. The major conclusions are as follows: 1) Facies analysis of the Tottori Group depicts the diachronous change of sedimentary environments: alluvial fan to meandering river environment for the Entsuji Conglomerate, Sandstone and Mudstone Member, lacustrine delta for the Shozan Sandstone and Mudstone Member, meandering river and flood plain for the Fuganji Mudstone and Sandstone Member, and outer shelf to deeper marine for the Tochimoto Shale Member. 2) The fish-fossil stratigraphic unit at the base of the Tochimoto Member comprises alternation of very fine-grained sandstone and siltstone in millimeter to sub-millimeter thickness. It contains abundant remains of shallow marine fish fossils, but no body- and trace-fossils of benthos. These facts indicate that the fossiliferous unit has accumulated in a coastal low-energy environment, i.e., in a lagoon, and that the hypersaline bottom water of the lagoon basin has been steadily anoxic due to strong density stratification, which is characteristic of deep and choked lagoons. 3) Although shoreface erosion frequently removes coastal lagoon sediments in transgressive stages, the fish-fossil unit at Miyanoshita was preserved through a rapid increase in accommodation. The increase was due to tectonic subsidence (0.45 mm/y on the average), eustatic rising (TB2.3) during the mid-Miocene climatic optimum, and sediment starvation resulted from extensive submergence of hinterland. 4) The fish-fossil unit appears to have formed through the following processes: (a) at a stormy high tide, sea water with shallow-marine fishes invaded into the coastal lagoon, (b) the fishes soon became feeble and dead through exhaustion of dissolved oxygen in the bottom seawater, and (c) very-fine sand grains and silt particles suspended in river floodwater settled down to envelope the fish remains at the lagoon bottom.

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