Geophysical characterisation of the Foulden Maar, Waipiata Volcanic Field, southern New Zealand

The Foulden Maar, located in Central Otago, New Zealand, is a monogenetic volcanic crater filled with lacustrine sediments. It is part of the latest Oligocene - Early Miocene Waipiata Volcanic Field. The geological structure of the Foulden Maar has been characterised using a combination of geophysical techniques. These include seismic reflection surveys, a gravity survey and magnetic surveys. A new technique for interpretion of maar sediments in reflection profiles has also been incorporated, with generation of a synthetic seismogram from a 184 m core. Seismic surveying has revealed a series of high amplitude reflections between 0.15 and 0.3 s that indicate the presence of debris flow deposits below a bowl shaped accumulation of laminites. Lower reflectivity below 0.3 s is probably caused by the stratified part of the diatreme breccia. Two normal fault structures have also been identified along with a series of antiformal reflections at ~1 s depth which are probably related to a sill structure. The synthetic seismogram had six significant amplitude horizons within it and these were tied with the borehole log to give a detailed re-interpretation of one seismic profile. Gravity surveying revealed a -6.1 mGal oblate residual gravity minimum with a diameter of ~1100 m and data from this survey has been modelled in 2D and 3D. Magnetic surveying has enabled the identification of volcanic features around and within the maar structure with short and long wavelength trends observed in data from ground-based and airborne surveys. A series of short wavelength anomalies have high intensities with a maximum of 1730 nT, and are primarily caused by basaltic dykes and flows around the edge of the structure. Two longer wavelength trends have a much lower magnitude with anomalies between 80 and 250 nT, and these are caused by root structures buried at a depth of >1500 m. Overall, the Foulden Maar probably has a structure that is similar to the generalised model proposed by Lorenz (2003) for a hard rock environment.