Geophysical investigation of the Nelson Haven sedimentary basin, Nelson Boulder bank and the Flaxmore fault in the Nelson region

Abstract

Five geophysical techniques (seismic refraction, reflection, gravity, resistivity and magnetotelluric [MT]) were conducted landward of the Nelson Boulder Bank region. The Boulder Bank is a 13 kilometre long gravel barrier composed of a ridge of well-sorted, small sized gravels that lie on a wide boulder platform. The seismic velocity of the ridge gravels is 360m/s, while that on the surface of the boulder platform is approximately 2200m/s. A refractor with a velocity of 2400-3000m/s beneath Nelson Haven correlates to the erosional surface at the top of the Pliocene, Port Hills Gravel. Nelson Haven sediments above this refractor are presumably quaternary in age and have accumulated in a tectonically formed basin with an axis parallel to the Boulder Bank. Gravity data modelled to fit the seismic data show these sediments thicken from 150 metres in the southwest to at least 270 metres in the northeast part of this study area. The steep slope of the seismic refractor and gravity profiles close to the Boulder Bank suggests that an offshore fault may be associated with the northwestern side of the valley.