Mud diapirs and associated intrusion structures in the Miocene Tanabe Group

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  • 中新統田辺層群にみられる泥ダイアピル類の貫入構造
  • チュウ シントウタナベソウグン ニ ミラレル ドロ ダイアピルルイ ノ カンニュウコウゾウ

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We examined the material properties and intrusive structures of several mud diapirs in the Miocene Tanabe Group with the aim of determining the style and process of fluid intrusion. Tank experiments were performed to examine the factors controlling the intrusive. Four types of intrusive structures (cylinder, dome, sill, and associated dikes) were observed in ascending stratigraphic order in the Shirahama Formation, overlying the muddy Asso Formation of the Tanabe Group: (1) cylinder type: the Ichieminami mud diapir, about 20 m in diameter, intruded near-vertically into bedded sand and siltstones; (2) dome type: the dome-shaped Ichiezaki mud diapir is about 150 m in diameter and contains blocks and sand grains of the host sediments, incorporated by stoping; dikes intruding radially into the host rocks are clayey in the early stage and sandy in the later stage; (3) sill type: the Migusa mud diapir represents a lenticular laccolith of at least 200 m in length, consisting mainly of pebbly mudstone containing blocks of surrounding strata; and (4) associated dikes: these diapirs are commonly accompanied by mudstone dikes. The tank experiments produced a series of lenticular, intrusive mud-slurry bodies on the top of the vertical conduit during dome-like upheaval, transforming into a mud chamber that grew in size with stoping of the roof sediments. When the chamber collapsed, a conduit of upward-escaping fluids, and sill and dike structures, formed above the chamber. This style of intrusion is comparable with that observed in the Tanabe Group. Consequently, we interpret that mud diapirs of the Tanabe Group show a variety of intrusive structures resulting from a single event in which high-pressure fluid with a minor mud component was injected through a narrow conduit to the level where successive, large mud chambers began to form. The nature of intrusive structures is determined by the permeability contrast between layers and by the degree of solidification of the host sediments.

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