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Volcaniclastic Rocks of the Orton-Bradley Formation, Banks Peninsula, New Zealand. General Geology
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Chapter 3: Terminology and Stratigraphy Chapter 5: Physical Volcanology Chapter 6: Interpretations and Lithofacies Analysis Chapter 7: Discussion and Geological History Acknowledgements and References
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Chapter 2
General Geology 2.1 Regional Setting New Zealand is situated on an active plate margin, with the Taupo Volcanic Zone in the North Island being currently the site of voluminous volcanic activity related to the subduction of the Pacific plate under the Indo-Australian plate. Cenozoic volcanism in the South Island is of quite a different character and is most likely the result of intraplate continental volcanism (Weaver and Smith 1989). It is predominantly mafic, in contrast to the Taupo Volcanic Zone, which is composed mainly of rhyolite with only subordinate amounts of more mafic rocks being erupted on to the surface. The composition of the Banks Peninsula volcanics correlates well with the hotspot or plume type of intraplate volcanism, usually when a plate moves over a fixed hotspot or plume within the mantle, a linear chain of progressively younger volcanoes forms, such as occurs at Hawaii. This does not appear to have occurred in this case, though here is some evidence of migration of volcanism from Lyttelton to Mt. Herbert to Akaroa, but with the eruption of the Diamond Harbour lavas this trend was reversed (Sewell 1985, Weaver and Smith 1989). Banks Peninsula consists of several large coalescing, composite volcanoes, composed predominantly of alkali rocks. Originally an island, the volcanoes were joined to the mainland by the progradation of the Canterbury Plains by deposition of loess and alluvium derived from the erosion of the Southern Alps. Banks Peninsula shows little evidence of deformation in the region. The volcanoes were formed on a pre-existing basement high, formed by the horst and graben structures in the Torlesse rocks, underlying the Canterbury Plains (Sewell et al. 1988). This results in Torlesse lithologies being present at 300m above sea level at the south end of the Lyttelton Volcano, whereas 30km to the west the Torlesse is not encountered until a depth of 1068m below sea level in the Leeston borehole (S.D. Weaver, pers. comm. 1989).
Table 2.1 Stratigraphy of Banks Peninsula (after Sewell and Weaver 1990, Sewell et al 1988) 2.2 Stratigraphy of the Study Area The following stratigraphy provides an outline of the units in close proximity to the Mt. Bradley Volcaniclastic Member (Fig. 3.3), formed during the period of predominantly alkaline, intraplate volcanism that formed most of Banks Peninsula in the upper Miocene period. More complete accounts (see table 2.1 for complete stratigraphy of Banks Peninsula) of the geology in this and other areas of Banks Peninsula can be found in Sewell 1985, Weaver and Sewell 1986, Sewell 1988, Sewell et al. 1988 and Sewell and Weaver 1990. 2.2.1 Charteris Bay Sandstone (Carlson et al. 1980) The Charteris Bay Sandstone consists of massive, indurated (quartz cemented), medium to fine sandstone. It is well sorted and a light grey to yellow brown in colour. 2.2.2 Allandale Rhyolite (Sewell et al. 1988) The Allandale Rhyolite represents the earliest Miocene volcanics and is exposed around the head of Lyttelton Harbour and around Gebbies Pass. The rhyolite consists predominantly of a creamy white to light brown, flow banded, porphyritic rhyolite. The outcrops are the eroded remains of rhyolite domes; breccias are found around the bases of the domes and rare tuffs are interbedded with the lava flows. The domes are radially, columnar jointed and contain large, black smoky quartz crystals. These rocks have a Rb-Sr radiometric age of 10.8±0.1 Ma (Barley et al 1988). 2.2.3 Lyttelton Volcanic Group (after Speight 1924) Rocks from Lyttelton Volcanic Group make up approximately one third of the volume of all the Miocene volcanics on Banks Peninsula. Lyttelton stratovolcano consists predominantly of dark grey to black hawaiite lava flows, mainly of the aa type, but of varying thickness, with minor flows of basalt, mugearite and trachyte interbedded with tuffs, lahars, volcanic breccias and agglomerates. The hawaiites are mainly porphyritic and contain phenocrysts of plagioclase, clinopyroxene and olivine. Lyttelton volcano was active from 11.0 to 9.7 Ma (Stipp and McDougall 1968; Barley et al 1988; Sewell et al 1988). There are numerous radial dikes of various composition, ranging from basalt to trachyte, centred on two eruptive centres of Lyttelton Volcano. The older one is located in the Head of the Bay and the younger one is located to the south of Quail Island, in Charteris Bay, dikes were emplaced from these centres throughout this period of volcanism (Shelley 1987, 1988). Towards the end of this phase of volcanism, the eruptive style changed from being dominantly Hawaiian to being mainly Strombolian, with a number of parasitic scoria cones formed on the flanks of the volcano. Several endogenous domes also formed, fed by trachyte dikes. 2.2.4 Mt. Herbert Volcanic Group (Sewell 1988) The Mt. Herbert Volcanic Group consists of a complex of mildly alkali basaltic plugs, lava flows, pyroclastic and volcaniclastic deposits situated between the two major stratovolcanoes of Lyttelton and Akaroa. These rocks make up the third largest unit of Miocene volcanics on Banks Peninsula and cover a significant proportion of the central area of the peninsula. 2.2.4.1 Orton Bradley Formation (Sewell 1988) The Orton Bradley Formation forms the bulk of the Mt. Herbert Group and since it also contains the Mt. Bradley Volcaniclastic Member (Sewell et al, 1988), is the most important in terms of this thesis. The Orton Bradley Formation consists predominantly of fine grained, black, aphyric hawaiite lava flows, with some rare flows of pyroxene-olivine-plagioclase-phyric hawaiite. Most flows are knobbly-jointed with columnar jointing visible in some locations. There are also subordinate dark grey to black, porphyritic basalt flows, with large phenocrysts of pyroxene, olivine and plagioclase. The earlier lava flows were probably sourced by the vent plug at the northern end of Orton Bradley Park, as well as possibly other vents within the Lyttelton crater. Later (after the deposition of the Mt. Bradley Volcaniclastic Member) flows came from vents to the south, in the vicinity of Mt. Herbert. K-Ar ages from Stipp and McDougall (1968) range from 9.5 to 8.6 Ma. The set of Orton Bradley Formation lava flows below the Mt. Bradley Volcaniclastic Member is 100m thick, with the lower flows being generally composed of porphyritic basalts. The upper lava flows, those immediately below the Mt. Bradley Volcaniclastic Member, are all composed of aphyric hawaiite, overlying flows of predominantly porphyritic basalt. The section of Orton Bradley Formation above the Mt. Bradley Volcaniclastic Member is 220m thick and has a sequence of surtseyan and strombolian deposits, interbedded with the lava flows. These deposits are confined to an area surrounding two tuff cones located on the flanks of Mt. Bradley and have been defined by this thesis as the Tablelands Volcaniclastic Member. The Mt. Bradley Volcaniclastic Member and the Tablelands Volcaniclastic Member are separated by a series of lava flows, predominantly porphyritic basalt. 2.2.4.2 Herbert Peak Hawaiite (Sewell 1988) The Herbert Peak Hawaiite flows form caps, up to 300m thick, on the tops of both Mt. Bradley and Mt. Herbert. They are roughly flat lying and are composed predominantly of grey aphyric hawaiites. The Herbert Peak Hawaiite is thought to have been erupted from a feeder dike near the summit of Mt. Herbert, during a Hawaiian style eruption. These rocks have K-Ar dates ranging from 8.5 to 8.0 Ma (Stipp and McDougall 1968). 2.2.5 Akaroa Volcanic Group (after Oborn & Suggate 1959) The Akaroa Volcanic Group represents the largest volume of rocks on Banks Peninsula and consists of seven different formations, ranging from plutonic gabbros and syenites to hawaiites and trachytes. K-Ar ages range from 9.1 to 8.0 Ma (Stipp and McDougall 1968; Evans 1970) 2.2.5.1 Mt. Sinclair Formation (Sewell & Weaver 1990) The Mt. Sinclair Formation outcrops in the vicinity of Mt. Sinclair. There are also outcrops near Mt. Herbert, that unconformably overlie the Orton Bradley Formation and interfinger with the Herbert Peak Hawaiite. The Mt. Sinclair Formation consists of shallow dipping lava flows of medium grained blue-black hawaiite. This has not been radiometrically dated, but Sewell and Weaver (1990) suggested an age range from 8.6 to 8.3 Ma. 2.2.6 "Church-Type" Lavas (ungrouped) This set of units consists of rocks that do not clearly fall into any of the other established groups. They are generally small volume deposits with distinct geochemical and mineralogical affinities. 2.2.6.1 Chateau Intrusives (Sewell 1988) This unit consists of a columnar jointed, light grey, aphyric hawaiite dome on the eastern side of Charteris Bay, with related dikes and a sill. The is sill is up to 130m thick and intrudes between the Lower Orton Bradley Formation and the Mt. Bradley Volcaniclastic Member on the western flank of Mt. Herbert. Stipp and McDougall (1968) obtained an age of 7.99±0.19 Ma for a sample from this unit. 2.2.7 Diamond Harbour Volcanic Group (Liggett & Gregg 1965) This group represents the youngest phase of volcanism on Banks Peninsula and the reverse of the eastward trend of volcanism on the peninsula. 2.2.7.1 Stoddart Basalt (after Liggett & Gregg 1965) The Stoddart Basalt consists of a series of basanite, olivine basalt and olivine hawaiite flows sourced from a number of monogenetic cones scattered around the flanks of Lyttelton Volcano as well as from within the crater. The thick basalt flow at Diamond Harbour forms a prominent 5km long dip slope, dipping into Lyttelton Harbour. The ages of these rocks range from 7.0 to 5.8 Ma (Stipp and McDougall 1968; Sewell 1988). 2.2.8 Christchurch Formation (Suggate 1958) The Christchurch Formation consists of gravel, sand, clay and silt deposited under marine and estuarine conditions during and after the Otira Glaciation (approximately 14000yrs ago, Suggate 1968). |