Geocene Auckland GeoClub Magazine Number 24, December 2020 Editor: Jill Kenny CONTENTS Instructions on use of hyperlinks last page 19 NEW 14C RESULT CONFIRMS 28,000 YEAR OLD Elaine R. Smid, 2 – 5 MAUNGAWHAU / MT EDEN ERUPTION AGE Bruce W. Hayward, Thomas Stolberger, Roderick Wallace, Ewart Barnsley FORTY-SEVEN YEARS OF EROSION AND WEATHERING Bruce W. Hayward 6 – 7 OF LION ROCK BOMB, PIHA MAORI BAY MICROMINERALS Tim Saunderson 8 – 14 PROXY EVIDENCE FROM TAMAKI DRIVE FOR THE Bruce W. Hayward 15 – 16 LOCATION OF SUBMERGED STREAM VALLEYS BENEATH HOBSON BAY, AUCKLAND CITY THE FIRST EXPLANATION IS NOT ALWAYS THE BEST Bruce W. Hayward 17 – 18 Corresponding authors’ contact information 19 Geocene is a periodic publication of Auckland Geology Club, a section of the Geoscience Society of New Zealand’s Auckland Branch. Contributions about the geology of New Zealand (particularly northern New Zealand) from members are welcome. Articles are lightly edited but not refereed. Please contact Jill Kenny [email protected] 1 NEW 14C RESULT CONFIRMS 28,000 YEAR OLD MAUNGAWHAU / MT EDEN ERUPTION AGE Elaine R Smid1, Bruce W Hayward2, Thomas Stolberger1, Roderick Wallace1, and Ewart Barnsley3 1 The University of Auckland 2 Geomarine Research 3 City Rail Link In February 2019, City Rail Link (CRL) reported that their The large obstruction (>1 m diameter in a 2 m drill hole) micro-tunnel boring machine “Jeffie” became entangled caused Jeffie to veer off course. CRL removed the wood in a large tree at 15 m depth, approximately 50 m north fragments and pulled them to the surface (Fig. 3). Select of the rail line between Mt Eden Rd and Shaddock St samples of wood were chosen by DEtermining VOlcanic in Eden Terrace, Auckland (Fig. 1). The tree was found Risk in Auckland (DEVORA) researchers for radiocarbon 1–1.5 m below lava flows from Maungawhau/Mt Eden, analysis during a visit to CRL offices in April 2019. The as extrapolated from nearby boreholes by CRL (Fig. 2). tree fragments were in variable states of preservation, Fig. 1. The CRL tree was found at 15 m depth, approximately 50 m north of the rail line between Mt Eden Rd and Shaddock St in Eden Terrace, Auckland. Image from Google Earth Pro. Location data from CRL. The buildings around where the tree was excavated are all now demolished as part of the tunnel portal work. 100 m Fig. 2a. Cross-section excerpt of the area where the tree was found by CRL (turquoise star); Fig. 2b. Legend for stratigraphy shown in cross-section. Cross-section and legend courtesy of CRL. It is not necessary to be able to decipher the borehole information, but rather to get an idea of where the tree was found and its proximity to the basalt lava flow. Simplified - grey is fill, dark pink indicates basalt, yellow represents Tauranga Group sediments, and shades of orange are all Waitemata Group sediments. 2 Fig.3a. Tree in soft mud or tuff entangled in the City Rail Link micro-tunnel boring machine near Maungawhau. Figs b & c. Fragments of tree brought to the surface. Fig. d. Tree samples chosen for radiocarbon analysis. Photos a–c courtesy of CRL: https://www.facebook.com/ cityraillink/posts/1160953177398200 Photo d by Elaine Smid. from well- to poorly-preserved, with apparent light charring or carbonisation evident on portions of some fragments. Tree identification, growth environment and likely demise Samples were provided to RW, who determined that dense wood from two species was present: a) well- preserved tōtara (Podocarpus totara) (Fig. 4), and b) wood of a broadleaf species (angiosperm), too decayed to identify. The latter wood was dense and therefore could not have been from a whau tree, as previously postulated, as whau has a density less than that of cork (Kirk, 1889). Fragments examined by RW were not charred by a heat source. It is not possible to tell if the wood was from sub- surface roots or from above ground portions of trees. From borehole information and geologists’ accounts during drilling, the trees were likely growing in a forested valley that was later filled in by Maungawhau’s lava flows. The trees were found approximately 1.2 km from Maungawhau’s main vent (Fig. 5). There is no indication if the trees were standing in place, had already fallen Fig. 4. Image of well-preserved tōtara wood taken from over, or were knocked over when the volcano started the City Rail Link tunnel in February 2019. Image erupting. taken by Dr Rod Wallace. 3 400 m Fig. 5. Locations of radiocarbon samples used to date Maungawhau’s eruption. These include: 1. a tōtara log buried in tuff found during construction of buildings within the Mt Eden Corrections Facility; and 2. the CRL tree (see text for details). The trees were discovered ~1.1 and ~1.2 km from Maungawhau’s main vent, respectively, and are roughly 0.5 km apart. Image from Google Earth Pro. The wood was found in what looks to be soft mud or the agreement of the resulting age of this wood with other tuff (Fig. 3a). The presence of tuff from Maungawhau has ages for Maungawhau, the large size of the obstruction been inferred in Auckland maar cores (Hopkins et al., 2017) encountered by Jeffie, as well as the thickness (~10 m) and reported within the site description of a previously- of the lava flows above the layer where the trees were dated tōtara log, found during construction of two Mt found make it unlikely that the wood was emplaced post- Eden Corrections Facility buildings (Fig. 5; Table 1; East eruption. and George, 2003). The mud pictured may not be in situ, however, as Jeffie is a slurry microtunnelling boring Tree age and implications for Maungawhau eruption machine and copious amounts of drilling mud was used. age The GNS National Isotope Centre’s Rafter Radiocarbon The uncertainty about what material surrounded the trees Laboratory (sample ID CRL-1; NZA 69409) returned a leads to several potential scenarios for the trees’ demise. result of 23,916 +/- 172 yrs BP (conventional age), with a The lack of charring on the wood indicates that it was not calibrated 2-sigma age range of 27,666 to 28,274 cal yrs impacted by the eruption products, either base-surge- BP. This new result places the eruption age within error of derived tuff or lava flows, and was likely buried and/or two previously obtained ages: 1. A tōtara log found within insulated by sediment. This sediment cover may have tuff and 2. ash layers within five maar cores, sourced been tuff,in situ soft mud in a pond-like environment that to Maungawhau via geochemical correlation, with an the trees fell into up to several hundred years prior to the average estimated age calculated from sedimentation eruption, bulldozed in front of an advancing lava flow, rates (Table 1). The former was discovered 1.1 km north or soil cover if the wood found was root material. The of Maungawhau’s main vent and 0.5 km from where the apparent lack of black, organic-rich mud at the tunnelling CRL tree was found (Fig. 5). site indicates that the sediment was not associated with a swamp or peat bog. Ages obtained using other techniques (e.g. thermo- luminescence; K-Ar) were deemed unreliable (McDougall Another possibility is that the wood post-dates the et al. 1969; Lindsay et al., 2011). Overall this find and Maungawhau eruption, and grew on top of the lava flow, radiocarbon result confirm the age of the Maungawhau / with roots reaching 1–1.5 m below the flows. However, Mt Eden eruption at ~28,000 years BP. 4 Table 1. Known reliable ages for Maungawhau samples. Sample ID Description Location a Age Error (years) Source NZA-69409; Wood in soft 36°52’3.73”S, 27,970 b 304 This study CRL-1 mud or tuff 174°45’36.03”E; 15 m depth; 50 m N of Rail Line between Shaddock St & Mt Eden Rd, Eden Terrace; 1.2 km NNW of Maungawhau WK-7136 Tōtara log 36°52’3.58”S, 28,386 b 345 East & George, buried in tuff 174°45’56.90”E; under the 2003; Administration Building at Lindsay et al. 2011 Mt Eden Corrections Facility; 1.1 km N of Maungawhau AVF12 Geochemically Ōrākei Basin; Hopua; 28,030 c 260 Hopkins et al. 2017 correlated Onepoto; Pupuke; Pūkaki ash layers within maar cores a Coordinates in WGS 84 Web Mercator. b Radiometric age in calibrated years before present. c Estimated average age in years before present, calculated from sedimentation rates in five cores. References East, G.R.W., George, A.K. 2003. The construction Kirk, T. 1889. The forest flora of New Zealand. of the Auckland Central Remand Prison on the Mt G. Didsbury, government printer. Eden basalt flow. In Geotechnics on the Volcanic Lindsay, J.M., Leonard, G.S., Smid, E.R., & Hayward, Edge: Tauranga, March 2003, New Zealand B.W. 2011. Age of the Auckland Volcanic Field: a Geotechnical Society Symposium (p. 387). review of existing data. New Zealand Journal of Institution of Professional Engineers New Zealand. Geology and Geophysics 54(4): 379–401. Hopkins, J.L., Wilson, C.J., Millet, M.A., Leonard, G.S., McDougall, I., Polach, H.A., Stipp, J.J. 1969. Excess Timm, C., McGee, L.E., Smith, I.E.M. & Smith, E.G. radiogenic argon in young subaerial basalts from the 2017. Multi-criteria correlation of tephra deposits Auckland volcanic field, New Zealand. Geochimica to source centres applied in the Auckland Volcanic et Cosmochimica Acta 33(12): 1485–1520.