A Case study of analysis of faults in Neogene siliceous rocks

Ishii, Eiichi   ; Fukushima, Tatsuo

Because some fractures might be water conducting features in rocks, it is necessary to consider sufficiently the fractures, for development of a geological model. In this study, we have tried to understand geological characteristics and formative processes of faults in Neogene siliceous rocks, based on results of outcrop observation, boring-core observation, and borehole-wall observation, in order to develop an investigation technique for water conducting features. In addition, we have tried to estimate permeability of faults and to develop a geological conceptual model including water conducting features. The results of this study suggest the follow: Concerning faults without filling minerals and hydro-thermal alteration in Neogene siliceous rocks including fold structures, (1) in order to classify the faults, an attention of relation between fault direction and fold axis/bedding direction, of displacement sense, and of displacement is effective. (2) in order to understand the formative process of the faults, an observation of crosscutting relations of faults, an analysis of relationship between faulting and folding, and an inference of stress conditions of faulting by fault striation analysis are effective. (3) in order to estimate hydro-geological property, an observation of redox front along faults near the surface, and an analysis of fault density, of formative process of faults, and of relation between fault direction and stress axis are effective.