SURVEY OF WAITAKI VALLEY SEDIMENTS

Press, Volume C, Issue 29635, 4 October 1961, Page 17

SURVEY OF WAITAKI VALLEY SEDIMENTS

The tibicknesss of sediments in parts of the upper Waitaki valley where power schemes »re planned wiU be probed during the next few weeks by members of the Geophysical Survey, in association with the Ministry of Works. The party, led by Messrs R. A. Garrick and C. E. Ingham, of Wellington, will set off explosive charges and measure the speed at which the shock wave travels through the ground. The work was suggested by Mr L. E. Oborn, of the Geological Survey, Christchurch. Mr Oborn has been engaged for some time in geological investigations connected with the proposed new power schemes, but was unable from the surface evidence to get an accurate indication of the depth bf the sediments. The geophysicists will arrive in the South Island with their equipment next week from Taupo, where they have been exploring the depth of the sedimentary basin. The main features to be explored for the Ministry of Works in the Waitaki are the Mary Range, in connexion with the tunnel for the Maryburn scheme, the site of the proposed Ohau Bridge dam, the power-house site for the same scheme at Aviemore,

and the Pukaki-Ohau lakes region. The Ostler fault will also be examined.

Mr Oborn explained yesterday that in many places there was a veneer of recent

glacial outwash gravel on top of earlier sediments. During earlier interracial periods, pre-existing gravels had been excavated by erosion, and there had also been a considerable , amount of glacial sculpture.

In some districts, the depth of the pareiit rocks could be assessed by magnetic surveys, but where neighbouring strata were non-magnetic or had a similar amount of magnetism the method was of little use, and this was the position in the area to be investigated. Gravity surveys were not, in general, accurate enough for the kind of work required, but he expected that the seismic method might provide the answer because of the marked difference in the speed the explosion shock-waves travelled in the gravels as compared with the parent rock.