The regional boundaries of the Canterbury Plains (Figure 2.1) extend from Timaru (~100 km south to Rakaia River in Figure 2.1) to Waipara River (30 km North of the Waimakariri River marked in Figure 2.1) and east to Banks Peninsula, covering a total area of 7500 km2 (Browne and Naish, 2003). The geologic basement of the Plains is hard sandstone (greywacke) and mudstone (argillite) of the Torlesse Composite Terrane of Triassic age (Browne et al., 2012). The Plains are formed by a series of coalescing alluvial fans built by the Waimakariri River and other Canterbury rivers coming from the Southern Alps and their foothills (Brown et al., 1988). The building of the Canterbury Plains is related to the events of the late Pleistocene and to the uplift of the Southern Alps during the Kaikoura Orogeny that started at the beginning of the Miocene Period (Soons and Selby, 1985 ).
44 The Waimakariri River was the major player in the study area. Postglacial surfaces are found near the lower Waimakariri River as several very young inset fan terraces and abandoned river channels. Some of the currently abandoned channels can be traced through the wider area of Christchurch and were probably active within the last 500 years (Basher et al., 1988). Older channels document avulsions of the Waimakariri River to a channel position south of Banks Peninsula, reaching the sea in the region of the shores of the present Lake Ellesmere.
The Quaternary geologic units on the Canterbury Plains have been described in detail (e.g. Brown and Weeber, 1992; Brown et al., 1988; Gage, 1958; Suggate, 1958; Suggate, 1990, and Figure 2.2 ). The late Pleistocene units most relevant to the zone affected by CES liquefaction are Burnham, Springston and Christchurch Formations, from oldest to youngest, and are summarized below. Figure 2.2 shows the landforms constructed from the late Quaternary formations described.
45 Figure 2. 2 Main landforms on the Quaternary Formation of the Canterbury Plains in the area affected by liquefaction (from Barrell, 2015).
The Burnham Formation, and its lateral equivalent the Riccarton Gravel, (ca 18-25 ka Brown et al., 1988; Oborn and Suggate, 1959; Suggate, 1963; Suggate, 1965, Figure 2.2 and 2.3 ) represents the last phase of fluvio-glacial aggradation and coastal extension of the Canterbury Plains at the time of the last glacial maximum (LGM) when sea level was about 130 m below present (Brown et al., 1988). According to Browne and Naish’s (2003) seismic study in the Canterbury Bight, the Burnham Formation comprises coarse-grained, fluvio-deltaic sediment that forms a regressive systems tract (RST) up to 40 m thick grading to the LGM low sea level stand. On land, the Burnham Formation’s Surface is recognised as the Darfield Surface with soils of Lismore age (see below).
46 Figure 2. 3 Cross-section through the northern portion of the Canterbury Plains near Christchurch to the edge of the continental shelf showing the stratigraphy of alternating lowland fluvial gravels and highstand sand, silt, clay, and peat (modified after Brown and Weeber, 1992). Numbers refer to inferred oxygen isotope stages based on radiocarbon dating. Burnham formation is the latest fluvio glacial outwash gravel deposited during the last glacial maximum; Springston and Christchurch Formations are the main Canterbury Plains Holocene formations.
Holocene Formations are the Springston and the Christchurch Formations (Figure 2.2 and 2.3) (Brown et al., 1988; Suggate, 1963; Suggate, 1965). The Christchurch Formation began accumulating at the end of the Otira Glaciation about 14,000 years ago (Brown et al., 1988). It is characterized by beach, estuarine, lagoonal deposits, dune and coastal swamp deposits.
The Springston Formation (Figure 2.3) is an aggradational deposit resulting from the Postglacial sea level rise (Suggate, 1963), and it is mainly fluvial sediment, but includes swamp deposits as well. The Springston Formation is inset within channels incised into Burnham Formation in the upper plains, but overlies the Burnham Formation (and its equivalent, the Riccarton Gravel) in the lower plains, and it is interfingered with the Christchurch Formation near the Rakaia River mouth (Figure 4 and Brown et al., 1988). The Springston Formation has a maximum thickness on the west side of Christchurch City of up to 20 m, and its sediment is underlain by several terraces southwards of the Waimakariri River. These terraces, the sediments beneath which are identified as members of the Springston formation, mark different periods of deposition, and are mainly coarse grained deposits. The Springston Formation members are identified on the basis of soil characteristics, for example presence or absence of loess and dune sand, degree of weathering of gravel and matrix, and also by radiocarbon dating.
47 The members listed in order of decreasing age are: Bleak House Member, Riverview Member, Courtenay Member, Halkett Member and the Yaldhurst member (Brown et al., 1988).
The Halkett member represents younger stages of activity of the Waimakariri and it is capped by numerous sand dunes (Brown et al., 1988). The Yaldhurst member represents the most recent flood and overflow sediment originating from the Waimakariri River when it flooded into the Avon, Heathcote and Halswell Rivers. The Yaldhurst Surface is subdivided (Barrell, 2015) into Yaldhurst 1 Surface, characterized by Waimakariri soil age and Yaldhurst 2 and 3 Surfaces, characterized by Selwyn soil age (Cox and Mead, 1963)2.
As a consequence of the Canterbury Plains geologic setting, to the west the Plains are characterized by a succession of fan and alluvial gravel forming aquifers. In contrast, to the east a succession of interglacial fluvial, marine and estuarine sediment of silty, clay and peat forms aquitards (Cox et al., 2012) separating a sequence of glacial outwash gravels forming confined and semiconfined aquifers, the youngest of which is the Burnham Formation (Figure 2.4).
In general, the aquifers occur in well sorted and permeable alluvial gravel sediment. Any surface water course is associated with a shallow unconfined aquifer with a water table in hydraulic connection, then intermediate aquifers at depths between 30-80 m and deeper aquifers at more of 80 m depth (Cox et al., 2012). Consequently, to the west the water table lies several meters or more below the surface and away from modern river floodplains. The soils are usually well drained and gravelly. To the east and closer to the coast the water table rises and coupled with fine textured sediment results in poorly drained soils (Brackley et al., 2012).
2 In this paper Cox and Mead stated that it is possible to recognise four main soil age group on the Springston
Formation: Templeton (3-6 k years ago), Waimakariri (700-2400 years ago) and Selwyn (<300). The fourth group is the Lismore (14-20 k years ago) that forms on the Burnham Formation. This thinking has been revised and updated by Basher et al 1988, Webb, 2008, Brackley et al., 2012.
48 Figure 2. 4 Outline of aquifers and aquitards on the Canterbury Plains and; A, map of the Canterbury Plains with the hydrogeological system in plan view; model of the aquifers and aquitards across the succession of gravel and alluvial sediment through the Plains. From Cox et al., (2012).