| Abstract/Description or Keywords |
To identify differences in fracture distribution and character with respect to lithology and different generations of geologic structures, and to apply this fracture distribution and characterization to the development of a conceptual model for fractured controlled groundwater. A total of over 8000 fracture measurements were made at 157 stations on 8 islands using scanline, grid mapping and random data collection techniques. Results indicate that fracturing related to the Late Cretaceous to Neogene tectonic history is heterogeneously distributed and can be separated into groups. Four primary groups were identified: deformation bands, bedding-perpendicular fractures, faults and fracture zones, and non-bedding-perpendicular fractures. Relationships between structures, lithology and fracture spacing are used to define hydrostructural domains, areas of differing bulk permeability. Four domains are defined and supported by geochemical, geophysical and well yield data. The four domains are: discrete fault and fracture, fracture zone, bedding-perpendicular fracture, and fault zone. The bedding-perpendicular fracture domain includes two subdomains, the mudstone-dominant and sandstone-dominant domains. The fault zone and mudstone-dominant domain are the highest relative permeability. A methodology is proposed for delineating domains using a combination of lineament analysis and geologic mapping. The net effect has implications for flow system controls, the amount and location of recharge, and potential for saltwater intrusion along shorelines. Regional structural history is determined to have a direct and significant effect on groundwater resources via the distribution of brittle fractures |
