The basement beneath the AVF consists of indurated marine sedimentary units, including greywacke, chert, quartzite and crystalline limestone, deposited between Late Paleozoic to Early Mesozoic (e.g. Kermode, 1992). Such sedimentary rocks were deposited in coastal to marine environments, associated with a continental margin and subduction zone in the Mesozoic (Isaac et al., 1994). They are subdivided into two NNW-trending terranes: the Murihiku Supergroup and the Waipapa Group (Figs. 2.1 and 2.2), based on their componentry, as well as textural and structural characteristics. The basement of the AVF is predominantly Waipapa Group units, outcropping on the western flanks of Hunua Ranges and on Motutapu Island at the eastern margin of the volcanic field (Fig. 2.1). Below the AVF, there is also thought to be a 6–8 km wide band of variably serpentinised ultramafic ophiolitic rocks, termed the Dun Mountain– Maitai Terrane (Coombs et al., 1976; Woollaston, 1996; Eccles et al., 2005; Cassidy and Locke, 2010). This narrow zone is interpreted to be the source of the regionally
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dominant NNW-trending gravity and magnetic anomaly, known as the Junction Magnetic Anomaly (e.g. Woollaston, 1996; Cassidy and Locke, 2010). Tectonically, the basement is characterised by a series of uplifted and subsided blocks formed during extension periods between Late Miocene to Pleistocene, forming dominantly N, NE and NNW trending sets of normal faults (Spörli, 1978; Boedihardi, 1990; Wise et al., 2003; Kenny et al., 2012). This age range, however, is poorly constrained (Wise et al., 2003; Kenny et al., 2012).
Figure 2.1 Simplified geologic and tectonic map of the broader Auckland region, based on Kermode (1992) and Edbrooke (2001). The black rectangle shows the area of the AVF with the locations mentioned in the text (1 – Takapuna Beach, 2 – Cheltenham Beach, 3 – Rangitoto, 4 – Motutapu Island, 5 – Browns Island, 6 – One Tree Hill, 7 – Pukaki maar, 8 – Crater Hill.). The roman numerals show the area of the three domains within the AVF mentioned in the text (I – North Shore, II – Central Auckland, III – Manukau Lowlands). The coordinates are in metres (New Zealand Map Grid).
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After the depositional unit of Murihiku and Waipapa Groups, there is erosion discordance in the stratigraphic column, corresponding to a hypothesised uplift and erosional period between Cretaceous and Early Eocene (Kermode, 1992; Raza et al., 1999). In the Late Eocene, terrestrial deposition and erosion was gradually overwhelmed by marine transgression, forming basin-wide deposition of sediments of the Te Kuiti Group (e.g. Kear and Schofield, 1959). The resultant sequence of the Te Kuiti Group comprises, from the base, Waikato Coal Measures, Mangakotuku and Glen Massey Formations (Fig. 2.2), with the latter reflecting the full marine incursion, from the Late Eocene to Oligocene (Nelson, 1978; Nelson and Hume, 1987; Edbrooke et al., 1998). The Waikato Coal Measures have been penetrated by drill cores in the AVF at a depth of >500 m below sea level (Edbrooke et al., 1998), and are capped by marine silt-, mud- and sandstones of Mangakotuku and Glen Massey Formations (Kermode, 1992).
The majority of the pre-eruptive terrain in the AVF is dominated by deposits of a volcanogenic flysch and marine sandstones, known collectively as the Waitemata Group (Figs. 2.2 and 2.3). This sequence was formed about 20 Ma ago (Spörli and Browne, 1982; Hayward and Brook, 1984; Spörli, 1989; Shane et al., 2010). These deposits
accumulated within a former elongated Waitemata Basin (130×60 km) located between
two active southeast-trending volcanic arcs from Late Oligocene to Late Miocene (Ballance, 1976; Spörli and Browne, 1982; Kermode, 1992). However, the stratigraphy, dating and geochemistry of volcanogenic clasts from the Waitemata sequences have shown a lack of subduction signatures and more of an ocean island basalt chemical affinity (Shane et al., 2010). The up to 2 km-thick deposits include variously consolidated silt, mud and sand, along with consolidated varieties of sandstone, limestone and breccias/conglomerates from volcanic and non-volcanic origins (Ballance, 1964; Carter, 1971; Ballance, 1974; 1976; Hayward and Brook, 1984; Ricketts et al., 1989; Hayward, 1993; Shane et al., 2010).
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Figure 2.2 Simplified stratigraphic column with characteristics of the main geologic formations occurring in Auckland. Note that the depth indicated in the sedimentary column varies from region to region. The values used here are based on Edbrooke et al. (1998). Sedimentary characteristics are after Kermode (1992) and Edbrooke et al. (1998). The Dun Mountain–Maitai Terrane has not been penetrated by the drill core described in Edbrooke et al. (1998), but it is inferred to be located underneath Auckland based on magnetic anomalies (e.g. Eccles et al., 2005).
The silt and mud beds of the Waitemata Formation were deposited in a marine setting, along with intercalated coarser deposits from turbidites bearing sediments from terrestrial and volcanic rocks from the proximal two active volcanic arcs (Ricketts et al., 1989; Spörli, 1989; Kermode, 1992; Davidson and Black, 1994). Due to the turbidite origin, syn-depositional slumping and sliding is common (Gregory, 1969; Spörli, 1989; Spörli and Rowland, 2007); however, their discrimination from the post-Miocene faulting is often complicated (e.g. Gregory, 1969). In proximal position to the volcanoes, the formation of the Waitakere Group was contemporaneous with the Waitemata Group (Fig. 2.2), and contains volcanic-rich sediments from the nearby volcanoes (Allen, 2004; Shane et al., 2010). These form lenses within the thick Waitemata sequences, such as the Parnell Grit Member, found within a few tuff ring deposits in the AVF, including Orakei Basin (Németh et al., 2012), or Maungataketake (Conybeer, 1995; Agustín-Flores et al., 2014).
Deposition within the Waitemata Basin ceased about 8 Ma ago, due to slow continuous uplift. Following this, erosion and tectonic dissection began, associated with block faulting (Barter, 1976; Anderson, 1977; Berry, 1986; Wise et al., 2003; Irwin,
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2009; Kenny et al., 2012). A block of the Waitemata Group subsided to form the Manukau Lowlands (Figs 2.1) and is characterised by multiple sets of fault orientations (Hayward, 1975; Berry, 1986). These include NE–SW and WNW–ESE striking normal and reverse faults with some minor bedding-parallel thrusts, and N–S striking extensional normal faults (Berry, 1986). These faults are not expressed in the younger, post-Waitemata sedimentary units (Berry, 1986).
Most of the subsided Waitemata blocks were transgressed again, forming an estuarine environment (e.g. Kermode, 1992). During this time, the deposition of the Kaawa Formation took place (Fig. 2.2), followed by the Tauranga Group, which includes terrestrial fluvial sediments (e.g. Kermode, 1992; Edbrooke, 2001).
Figure 2.3 Outcropping Waitemata units with an extensive multidirectional fracture system at Takapuna Beach (A) and Cheltenham Beach (B) in the North Shore. For the location map, see Fig. 2.1.
Most recent sedimentary units, formed in the Pleistocene to Holocene, have been grouped into the Tauranga and Kaihu Groups (Fig. 2.2). Their deposits accumulated mostly on top of the low-lying, subsided basement blocks, such as those forming the Manukau Lowlands (Kermode, 1992; Edbrooke, 2001). These deposits interfinger with the basaltic volcanic products of the AVF (Kermode, 1992; Edbrooke, 2001). The sediments of Kaihu Group are exposed along the coastline, and include present day beach deposits (Kermode, 1992). The sedimentary units of the Tauranga Group
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comprise silt, mud, sand and gravel with intercalated organic materials (e.g. peat, organic-rich clay) and pumiceous volcanics from the Taupo Volcanic Zone (Kermode, 1992). They indicate mostly fluvio-lacustrine depositional settings, and rare shallow marine conditions (Nelson, 1978; Nelson and Hume, 1987).