Depositional environment of the Cretaceous Shimanto bedded cherts from the Fukura area, Kochi Prefecture, inferred from major element, rare earth element and normal paraffin compositions.

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  • 高知県福良地域の四万十帯白亜紀層状チャートの堆積環境　　ノルマルパラフィン組成，主要元素組成，希土類元素組成からの推定
  • コウチケン フクラ チイキ ノ シマントタイ ハクアキ ソウジョウ チャート ノ タイセキ カンキョウ ノルマルパラフィン ソセイ シュヨウ ゲンソ ソセイ キドルイ ゲンソ ソセイ カラ ノ スイテイ

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The depositional environment of Cretaceous bedded cherts and intercalated shales from the Fukura area, Kochi Prefecture, in the Shimanto Terrane is discussed on the basis of analyses of n-paraffins (C12∼C38), major elements and rare earth elements (REEs). Radiolarian biostratigraphy confirms that the bedded cherts had deposited during Barremian.N-paraffins in most shale partings are low in concentration and are composed mainly of short-chained (C14∼C18). The low abundance of long-chained n-paraffins with a strong odd-carbon number predominance suggests that the bedded cherts deposited in an environment with little contribution of terrigenous organic matter.Some samples, in spite of their sampling horizon, shows lower Al2O3/TiO2 ratios, suggesting that the bedded cherts deposited around a field of mafic volcanism with a large contribution of basaltic debris.Samples from the lower horizon have higher tot.Fe2O3/TiO2 and MnO/TiO2 ratios than those of average shales and marine sediments. In addition, these are characterized by a negative Ce anomaly. This suggests a large contribution of Fe-Mn precipitates from hydrothermal emanations, originating from mafic volcanism. Samples from the upper horizons have lower tot.Fe2O3/TiO2 and MnO/TiO2 ratios than those from the lower horizons and are close to those of average shales and marine sediments. Moreover, these samples show no or slight negative Ce anomaly. The upper horizon bedded cherts, therefore, deposited in an environment with little contribution of Fe-Mn precipitate from hydrothermal emanations. Accordingly, the hydrothermal effect observed in the bedded cherts under consideration is inferred to have weakened drastically in a short period of time (about 5 m. y.).

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