Tekapo-Pukaki canal, Maryburn crossing : S100, report no. 2, November 1972

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Mansergh, G.D. 1972 Tekapo-Pukaki canal, Maryburn crossing : S100, report no. 2, November 1972. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 143 21 p.

Abstract: A previous report (Mansergh 1972) gave the geology of the site and described the presence of a lacustrine stiff clay and silt called the Maryburn Unit, lying beneath Recent Alluvium, the interface lying at about RL 2050'. It also suggested that a trench at least 70' deep had been cut into the Maryburn Unit surface between canal footage 59500 and 59800. This report discusses the results of further investigations by drilling and resistivity survey. Three drill holes, BH183, BH184 and BH185 were put down over the possible site of the channel by percussion and rotary methods, and sediment similar to that of the Maryburn Unit was encountered between RL 2045' and RL 2070'. Variations within the sediment from clay silt to fine sand, particularly over the lengths of hole drilled by percussion methods, made correlation of the sediment with the Maryburn Unit doubtful. Resistivity values can be assigned to various geological units recognised with moderate confidence but the resistivity value assigned to the clay silt and fine sand sediment lies just outside the limits proposed by Macdonadl (1972) for the Maryburn Unit Reinforcing the doubts about its correlation with this unit. Resistivity also suggests the presence of a material interrupted as a saturated clay silt between RL 1978' and RL 2015' below BH 183. Investigations have thus failed to prove the existance of a channel but indicate that one may exist about footage 59650. If a channel does exist it has been back-filled with fine to medium sand and loose blocks of clay-silt derived from the Maryburn Unit; if it does not exist a sandy facies of the Maryburn is present instead. No compressible materials have been found over the site but a plastic clay may possibly exist between RL 1978' and RL 2015' at canal footage 59630. (auth)