Lenticular Pores and Groundwater Flow in Sandstone of the Torinosu Formation, Chichibu Belt

― An Example in Tosa City, Kochi Prefecture

Abstract

Lenticular pores were found in a boring core drilled to 150m depth in sandstone of the Chichibu Belt, Tosa City, Kochi Prefecture. The pores were formed by the dissolution of calcite veins. The pores occur in intervals with high porosity and low P-wave velocity, and they contribute to porosity and water permeability, as their occurrence coincides with the zone of groundwater flow. The results of controlled source audio-frequency magnetotelluric surveys and surface geology surveys indicate the occurrence of NNW-SSE trending faults and fissures near the well. We analyzed the water quality and stable oxygen and hydrogen isotopes of water in wells and of floodwaters near the boring site. We infer that the pores were formed by the dissolution of vein calcite by calcite-undersaturated groundwater that flowed downward along cracks throughout the drainage basin at an elevation of ≧ 300 m, south of the well.