Gisborne port study: preliminary geological report

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Mansergh, G.D. 1979 Gisborne port study: preliminary geological report. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 325 16 p. 1 map.

Abstract: A preliminary study of the geology of two proposed sites for a new harbour for Gisborne revealed the presence of a sequence of Tertiary sedimentary rocks overlain unconformably by Pleistocene alluvium and by Holocene sediments. The Tertiary beds have been ruptured by the extrusion under high hydrostatic pressures of underlying bentonitic beds, and a bentonitic extrusion breccia has been deposited at the surface in a zone of over one kilometre wide. The diapirism is centred along an axial fault zone which passes through Site B (Fig 1) and has tilted the Tertiary beds at both sites. Coastal erosion during Quaternary time has cut back into the Tertiary beds and into the extrusion breccia, and oversteepened the hills behind the sites. Landslides in the area are therefore common. Each geological unit, and the engineering geology of each site is discussed in general terms. It is suggested that the rock at Site A should provide an adequate foundation and should not present any major difficulties in working but that Site B should be abandoned. The geological risk of landslide, earthquake and tsunami are discussed. Four sources of armour rock and rock underlay are suggested, and it is recommended that, because of the paucity of good quality rock in the Gisborne distrcit, a policy of stockpiling of suitable rock be initiated.