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Age of the Auckland Volcanic Field: a Review of Existing Data

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Age of the Auckland Volcanic Field: a Review of Existing Data New Zealand Journal of Geology and Geophysics ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzg20 Age of the Auckland Volcanic Field: a review of existing data JM Lindsay , GS Leonard , ER Smid & BW Hayward To cite this article: JM Lindsay , GS Leonard , ER Smid & BW Hayward (2011) Age of the Auckland Volcanic Field: a review of existing data, New Zealand Journal of Geology and Geophysics, 54:4, 379-401, DOI: 10.1080/00288306.2011.595805 To link to this article: http://dx.doi.org/10.1080/00288306.2011.595805 Published online: 07 Nov 2011. Submit your article to this journal Article views: 1375 View related articles Citing articles: 25 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tnzg20 Download by: [109.56.190.155] Date: 28 February 2017, At: 14:34 New Zealand Journal of Geology and Geophysics Vol. 54, No. 4, December 2011, 379Á401 Age of the Auckland Volcanic Field: a review of existing data JM Lindsaya,c*, GS Leonardb, ER Smidc and BW Haywardd aSchool of Environment, The University of Auckland, Auckland, New Zealand; bGNS Science, Lower Hutt, New Zealand; cInstitute of Earth Science and Engineering, The University of Auckland, Auckland, New Zealand; dGeomarine Research, St Johns, Auckland, New Zealand (Received 10 March 2011; final version received 1 June 2011) Determining magnitudeÁfrequency relationships, a critical first step in assessing volcanic hazard, has been hampered in the Auckland Volcanic Field (AVF) by the difficulty in dating past eruptions from the field’s c. 50 centres. We assessed 186 age determinations from 27 centres for reliability and consistency. Results indicate that only three centres (Rangitoto 0.6 ka; Mt Wellington 10 ka; Three Kings 28.5 ka) are reliably and accurately dated. Eight are reasonably reliably dated within a small age range: Crater Hill, Kohuora, Mt Richmond, Puketutu, Taylors Hill and Wiri Mountain (all 32Á34 ka); Ash Hill (32 ka); and Purchas Hill (11 ka). Tephrochronology of lake sediments and relative ages from stratigraphic relationships provide age constraints for a further 9 and 11 centres, respectively. Although recent ArÁAr studies show promise, ages of AVF centres generally remain poorly understood; this has implications for any statistical treatment of the distribution of volcanism in the AVF. Keywords: Auckland Volcanic Field; geochronology; ArÁAr; radiocarbon; KÁAr; tephrochronology; magnetic declination; magnitudeÁfrequency; volcanic hazard; New Zealand Introduction programme is needed to fill the gaps and provide reliable Determining eruption magnitudeÁfrequency relationships in data to improve the robustness of hazard models. This paper volcanic regions is a critical first step in assessing volcanic reviews existing age determinations for AVF eruptions as a hazard (e.g. Condit & Connor 1996; Conway et al. 1998; baseline to new age-dating approaches. Connor et al. 2006; Molloy et al. 2009). Such analysis has been hampered in the Auckland Volcanic Field (AVF) due Methods to a lack of reliable age data (e.g. Magill et al. 2005). Carbonised material is rare and many centres are older than We have reviewed all published literature that reports ages of 40,000 years thus limiting the use of 14C dating. Early volcanic rocks in the AVF, and have tracked down as many attempts to date Auckland lavas using the KÁAr technique unpublished age data as possible. Where the same age was revealed excess Ar, making the lavas appear older than they reported in several different publications, we have attempted to go back to the original source. In doing this, we have are (e.g. McDougall et al. 1969). The tephra record obtained identified several instances of typographical errors in papers from drilling the palaeolake sediments of Auckland’s maars subsequent to the original, which in some cases have been is providing excellent interpolated ages for basaltic AVF propagated in literature to the present day. For 14C ages we tephras (e.g. Molloy et al. 2009). However, only a few also queried and cross-checked with the Fossil Record tephras retrieved from these cores have been reliably Database (FRED; http://www.fred.org.nz) and extracted correlated to individual centres (e.g. Rangitoto tephra in the Fossil Record Number. Where dated samples were also Pupuke core, Crater Hill tephra in Pukaki core and Mt recorded in the PETLAB database (New Zealand’s main Wellington tephra in Panmure core) and correlation remains rock catalogue and geoanalytical database; http://pet.gns. challenging (Shane & Zawalna-Geer in press). Recent ArÁ cri.nz), we extracted the PETLAB collection number and Ar dating of AVF basalts has yielded promising results (e.g. cross-checked with FRED and published literature. In Cassata et al. 2008), and future dating using this technique almost all cases, the ages lodged in PETLAB are different has the potential to fill many of the gaps in the chronology from those in the original literature. In the case of KÁAr of the field. Past attempts to model probabilistic hazard in ages, this likely relates to an adjustment made to all the ages Auckland have drawn attention to the lack of good age derived in the 1960s and early 1970s based on the control, and have relied heavily on relative age order models recommended Steiger & Ja¨ ger (1977) decay constant (Nick rather than absolute analytical ages (e.g. Magill et al. 2005; Mortimer, GNS Science, pers. comm. 2009). The reason for Bebbington & Cronin 2011). A comprehensive dating the differences in 14C ages probably relates to recalculations *Corresponding author. Email: [email protected] ISSN 0028-8306 print/ISSN 1175-8791 online # 2011 The Royal Society of New Zealand http://dx.doi.org/10.1080/00288306.2011.595805 http://www.tandfonline.com 380 J.M. Lindsay et al. of pre-1977 ages carried out in the 1990s (Lindsay & Leonard Dating techniques used in the Auckland Volcanic Field 2009). The timing of past eruptions has been explored by previous 14 Note that most C ages presented in the literature for workers using five main approaches: (1) radiocarbon dating the AVF are conventional (uncalibrated) ages. Here we of organic material preserved beneath or within erupted present both conventional and calibrated ages. We cali- material; (2) direct dating of the erupted rock; (3) inferring a brated all ages using CalPal-Online (http://www.calpal.de) minimum age based on the presence of known-age tephras except for the young Rangitoto ages, for which we used and sedimentation rates in sediment cores from explosion OxCal 4.1 (curve ShCal04), considered to be more accurate craters (tephrochronology); (4) age correlation of palaeo- for young ages determined in the Southern Hemisphere. It magnetic signature to known-age volcanic rocks; and (5) should also be pointed out that the half-life value used for relative ages based on stratigraphic relationships (Table 1). calculating conventional 14C ages has been revised over See Lindsay & Leonard (2009) for a summary of the time. Our CalPal-Online calibrations were carried out on the different dating techniques that have been applied in the original published or worksheet ages, and are therefore AVF, a brief discussion of the advantages and disadvantages based on the half-life applied at the time of original analysis. of each and the specific problems that have been encoun- The term ‘age’ is used to refer to a time before present. The terms ‘date’ and ‘dated’ refer to a scientific result and are tered with these techniques with respect to the AVF. abbreviations of ‘age date determination’. The alternative meaning of ‘date’ as in a specific year (e.g. the year 1946) is not used here. The age of individual centres of the AVF Here we review 186 known radiometric and proxy age determinations available for volcanic rocks from 26 of the Geological framework volcanic centres of the AVF, discussed individually below in The AVF is a small-volume, intraplate basaltic field (Fig. 1) alphabetical order. The complete database of dates is located within New Zealand’s largest city, Auckland (popu- presented in the appendix. We also review minimum and lation 1.3 million). Activity in the field has occurred from relative age determinations (Tables 2 and 3, respectively). about 50 scattered eruption centres in the form of maars, Where possible, we present a best-estimate age (rounded to scoria cones and tuff rings over the past c. 250,000 years. three significant figures) for that centre based on these age The most recent eruption about 600 years ago produced the determinations and what is known about relative or mini- largest volcano in the field: Rangitoto. mum ages based on stratigraphy; best-estimate ages are Eruptions have typically progressed from explosive to summarised in Table 4. Unreliable ages are not included in effusive over the lifetime of a volcano. Where the magma the text but are included in the appendix. initially interacted with sufficient groundwater or surface water, a maar volcano formed with a wide low tuff ring around the vent. Examples preserved in the AVF include Ash Hill Panmure Basin, Onepoto, Te Hopua and Orakei Basin. Dry A single radiocarbon date is available for Ash Hill explosive volcanism produced scoria and/or finer tephras (31.890.2 cal kyr BP; calibrated kiloyears before present, and dry effusive activity, typical in the late stages of some present being 1950); this was collected and analysed in 2007 eruptions, led to the development of lava flows. While tuff (Hayward 2008a). This age appears reliable and is the basis rings and scoria cones form in and around the vent, lava for the current best-estimate age of c. 32 kyr for this centre, flows and tephra falls may extend for some distance from essentially the same as neighbouring Wiri Mountain.
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