Depositional process of the Pliocene Anno Formation, the uppermost part of the Awa Group, Boso Peninsula, central Japan-A case study on a turbidite depositional system of a forearc-basin filling-

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  • Ishihara Yoshiro
    Department of Earth System Science, Fukuoka University
  • Tokuhashi Shuichi
    Fuel Resource Geology Research Group, Research Institute for Geo-Resources and Environment, GSJ / AIST (National Institute of Advanced Industrial Science and Technology)

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  • 房総半島安房層群最上部安野層の堆積様式－前弧堆積盆を埋積するタービダイト・システムの一例－
  • ボウソウ ハントウ アワソウグン サイジョウブ アンノソウ ノ タイセキ ヨウシキ ゼンコ タイセキボン オ マイセキ スル タービダイト システム ノ イチレイ

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The forearc basin fill turbidite succession of the Pliocene Anno Formation is well exposed in Boso Peninsula, central Japan. As the turbidite succession of the Anno Formation is distributed along a couplet of anti- and syncline axes trending WNW to ESE, three-dimensional analysis of depositional system of the formation can be carried out by using many tuff beds as key markers. Lateral distribution and successive change of turbidite facies and palaeocurrent directions of turbidites were investigated, based on the detailed lithologic map of the study area.<br> The six sedimentary facies, 1) channel fills of pebbly sandstones and conglomerates originated from sedimentary gravity flows, 2) natural levee deposits of rhythmic alternation of thin turbidites and hemipelagic mudstones, 3) depo-center deposits in depositional lobes of thick-bedded turbidites without distinctive erosions, 4) marginal deposits 1 of alternation of thin turbidites and hemipelagic mudstones, 5) marginal deposits 2 of alternation of tuff beds and hemipelagic mudstones, and 6) extensive slump deposits composed of various depositional forms, are defined and laterally correlated by using key tuff markers. Most palaeocurrent directions of turbidites, measured from current ripple marks of Bouma Tc division, sole marks and fabric of mud clasts, show southward spreading from the central part of the peninsula, though some of them show northward in the southern part and variable directions in the western part. The southward palaeocurrent directions indicate a main feeder channel or a canyon distributed in the northern part, while the northward palaeocurrent directions may indicate the reflection from the southern local topographic highs. The variable dispersive palaeocurrent directions may be caused by winnowing processes. <br> Occurrence of two successive depositional systems, channel-levee system in the lower part and channel-lobe system in the upper part, forming a submarine fan, is suggested by the facies distributions and palaeocurrent directions. The upward retrogradation can be detected through the formation of the forearc basin fillinig.<br>

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