Late quaternary deposits of the Christchurch metropolitan area

Abstract

Deep wells sunk for artesian water beneath Christchurch penetrate three formations. The youngest, the Christchurch Formation, consists of marine, estuarine, and fluviatile deposits of which at least the upper part was formed during the post-glacial rise of sea-level. It extends down to more than 200 ft below present sea-level, and radiocarbon dates record part of the rise of sealevel, from — 73 ft 9400 years ago (S83/501) to — 12 ft 6100 years ago (S84/523). Other dates, together with topographic and soil evidence, are interpreted as indicating the 'Subsequent period of development by the Waimakariri River of a fan of gravel over the Christchurch area, probably completed about 2,000 years ago when sea-level was a little higher than at present. The aggradation surface of this fan is termed the Yaldhurst Surface. The Riccarton Gravel, on whose eroded surface the Chri˜tchurch Formation lies, is thought to be the sub-surface representative of the gravel that forms an earlier fan, whose 'surface—the Halkett Surface—also has characteristic soils. This fan is believed to have developed as the result of aggradation by outwash streams during the last but one maj or advance of “Last” Glaciation age. Glacial outwash gravels of a still earlier“Last”Glaciation advance form an older fan whose surface—the Darfield Surface—is also recognized from topographic and soil evidence. No sub-surface representative of these .gravels is recognized beneath Christchurch. The Bromley Formation, estuarine and marine gravel, sand, silt, and peat, extends down to several hundred feet below the Riccarton Gravel beneath most of Christchurch; it is thought to be of “Last” Interglacial age. In the western part of the Christchurch area, thick gravel beneath the Riccarton Gravel may perhaps be of “Penultimate” Glaciation age. © Taylor & Francis Group, LLC.