Geophysical studies over the north Horowhenua sedimentary basin

Abstract

Geophysical surveys have been carried out to investigate the buried Torlesse basement in North Horowhenua, between the Manawatu and Ohau Rivers. Extensive gravity and resistivity surveys have resulted in new information concerning the geometry of obscured basement rocks. The depth to basement, interpreted from gravity data, is less than 1.5 km. A North - Northeast orientated, high angle fault (informally named Levin Fault) has been located West of Lake Horowhenua. The vertical displacement of the basement rocks along this fault is estimated to be greater than 500 metres. This makes it a more significant feature than previously recognised. The Levin Fault almost certainly extends well beyond the survey region. Interpretation of resistivity data collected over the upfaulted Poroutawhao High shows a fractured and shattered greywacke zone between Quaternary sands and the basement. At several sites this fractured rock contains brackish water, possibly as a result of deeper saline groundwater rising along the fault zone. It appears there is little potential for locating a reliable groundwater source in Poroutawhao. The present gravity survey and borehole information confirms that the area between Lake Horowhenua and State Highway 57 has the best groundwater resource potential.