The influence of dolerite dykes on saline seeps in southwestern Australia

Abstract

Geophysical surveys, soil and geological mapping and hydrologic studies have been used to show an association between dolerite dykes and the occurrence of saline seeps on two catchments in southwestern Australia. The role of the dykes in the location of the saline seeps is explained and the potential contribution of routine geophysical surveys to salinity studies is highlighted. Magnetic surveys and soil and geological mapping identified dolerite dykes crossing the main drainage line of each catchment. Soil resistivity and conductivity surveys showed that these dykes are associated with saline soils. Seismic refraction surveys over these dykes indicate deeper weathering profiles. Groundwater pressures and hydraulic conductivities measured in bores across one of the weathered dykes showed that the clay saprolite formed above the dolerite is less permeable than the surrounding weathered granite. The lower permeability is probably due to the finer texture of the saprolite and/or a lower frequency of preferred flow pathways. The clay formed above the dolerite acts as a linear hydraulic barrier to lateral groundwater flow and results in the discharge of saline groundwaters into surface soils.