GEOLOGICAL SOCIETY OF NEW ZEALAND ANNUAL CONFERENCE 2ND-5TH DECEMBER, WHANGAREI NORTHLAND 2002 FIELD TRIP GUIDES Edited by Vicki Smith & Hugh Grenfell (with thanks to Bruce Hayward, Ashwaq Sabaa and Jessica Hayward for editorial assistance) Bibliographic Reference Smith V. & Grenfell H.R. Editors (2002): Fieldtrip Guides, Geological Society of New Zealand Annual Conference “Northland 2002”, Geological Society of NZ Miscellaneous Publication 112B, 116 pp. Recommended referencing of field trip guides (an example): Spörli K.B. & Hayward B.W. (2002): Geological overview of Northland. Field trip guides, GSNZ Annual Conference “Northland 2002”, Geological Society of NZ Miscellaneous Publication 112B, p.3-10 2002 ISBN 0-908678-90-8 Conference Organising Committee: Bruce Hayward (Convenor), Hugh Grenfell (Chair Programme Committee), Bernhard Spörli (Chair Fieldtrip Committee), Peter Andrews (Chair Social Events Committee), Ian Smith (Chair Finance Committee), Philippa Black, Matt Boyd, Sian France, Jack Grant-Mackie, Murray Gregory, Dan Hikuroa, Jane Olsen, Stuart Rodgers, Ashwaq Sabaa, Grace Schaefer, Nik Smith, Vicki Smith, Nikki Tonkin, Virginia Toy, Helen Williams. Contents Contents Geological overview of Northland 3 Bernhard Spörli and Bruce Hayward Field trip 1 Northland Allochthon emplacement, south Whangarei Harbour 11 Bruce Hayward, Bernhard Spörli and Mike Isaac Field trip 2 Kaipara allochthon – autochthon 19 Jack Grant-Mackie and Murray Gregory Part one of field trips 3, 4 & 5 Introduction to Whangarei geology 25 Bruce Hayward, Mike Isaac, Keith Miller and Bernhard Spörli Field trip 3 Part 2 Whangarei Heads geology 33 Petra Bach, Philippa Black, Bruce Hayward, Mike Isaac and Ian Smith Field trip 4 Part 2 Basement/Tertiary cover/allochthon interactions 39 (Ocean Beach) Bernhard Spörli Field trip 5 Part 2 Bushwalk on a Miocene volcano 47 Fred Brook Field trip 6 Geothermal Northland 49 Pat Browne, Stuart Simmons, Kathy Campbell, Wendy Hampton and Dion Pastars Field trip 7 A taste of Northland geology 59 Bruce Hayward and Ian Smith Field trip 8 Northland from the bottom up: P/Tr boundary to allochthon 79 Bernhard Spörli and Yoshiaki Aita Field trip 9 Geological gems of the Far North 91 Philippa Black and Murray Gregory References for all field trips 111 1 Introduction GEOLOGICAL OVERVIEW OF NORTHLAND Bernhard B. Spörli and Bruce W. Hayward Northland is surrounded offshore (from W to E) by the Norfolk Rise, Norfolk Basin, Three Kings Rise, and the South Fiji Basin. The continental extensional “ribbon” of the Norfolk Rise links it to New Caledonia, with which it shares quite a few geological traits. The northeastern shelf area occupied by large expanses of allochthonous material (Herzer et al 2002) and containing a ?sliver/metamorphic core complex? of enigmatic Tertary biotite schist (Mortimer et al. 2002) is very wide and still not well known. It and the bounding (dextral?)Vening Meinesz Fracture zone played an especially important role in the Cenozoic development of Northland In considering the geological history of the Northland region the following events are important: 1. Accretion of basement “greywacke terranes” (Waipapa terrane, Torlesse terrane) onto the Gondwanaland margin ( Paleozoic - Mesozoic) 2. Cretaceous rifting during the separation of New Zealand from Gondwanaland (100- 80 million years) 3. Possible further rifting in the early Tertiary (Challenger Rift) 4. Emplacement of the Northland Allochthon as an effect of renewed subduction under Northland 5. Establishment of a NW – trending calcalkaline volcanic arc, and the intravolcanic Waitemata Basin over the subduction zone (early Miocene) 6. Migration or jump of the arc and subduction zone, via the Coromandel arc,into its present location (Taupo Volcanic Zone) and into a NE trend to leave Northland in a behind–arc tectonic position during establishment of the present day Alpine fault regime (middle Miocene to present) Basement is only exposed in uplifted blocks along the east coast and in some inland exposures. There it almost entirely consists of what was until recently called the Waipapa terrane (Spörli 1978). However, Black (1994) suggested a new subdivision into units relating to the tectono-stratigraphic terranes of the South Island. A westernmost unit (Omahuta – Puketi area) is correlated with the Caples terrane. A middle terrane (Bay of Islands – northern Waipapa area) is compared with the Rakaia sub-terrane of the Torlesse terrane, whereas the easternmost (Helena Bay-Hunua unit) is linked with the Pahau subterrane of the Torlesse. Basic structure of these units is one of intense thrust slicing in an accretionary prism affected by terrane collision (Aita and Spörli 1992). A different “basement”, the Mount Camel terrane, is exposed at the northern end of Northland. It is younger than any “Waipapa terrane” and has experienced a different tectonic history (Toy et al. 2002), which is more similar to that of the Northland Allochthon (see below). A concealed boundary (?fault?) between it and the Waipapa terrane in the south trends WSW from Whangaroa Harbour to Ahipara (Fig I.1). What underlies the Mount Camel Terrane is one of the big unsolved problems of Northland. In the main part of Northland the “Waipapa” basement extends westward, but is covered by up to 2km of younger deposits, including the Northland Allochthon (see description further on). The western boundary of the Waipapa basement is formed by the Permian/Triassic Dun Mountain /Maitai terrane which can be traced in the subsurface from Auckland to Ahipara as the Magnetic Junction Anomaly (JMA, Fig. I.1). By definition any concealed basement rocks to the west of the JMA must belong to the highly fossiliferous Murihiku terrane which rises to the surface at Port Waikato as the Kawhia synclinorium, and together with the Dun Mountain Belt can be traced all the way to the South Island. Waipapa terrane rocks are unconformably overlain by Te Kuiti Group, ranging from coal measures to glauconitic sandstones and limestones (Whangarei and Onemama Lst). The unconformity documents Cretaceous uplift (rebound) of the Waipapa accretionary prism after metamorphism ranging up to pumpellyite–actinolite facies and the onlapping sediments document an upward deepening (Isaac et al 1994). The >50 000 km3 deep-water Northland Allochthon is the most widespread tectonic unit in Northland. It consists of Cretaceous to Oligocene sedimentary rocks tectonically overlain by the Cretaceous/Early Tertiary Tangihua ocean floor volcanics (Fig. I.2, I.3), interpreted as having been formed in a supra-subduction tectonic environment (Malpas et al. 1992). 3 Introduction Omahuta –Puketi unit Mt.Camel terrane Bay of crustal fault Islands unit JMA Whangarei N Helena Bay –Hunua unit Hauraki Rift Coromandel Peninsula Auckland Port zeolite greywacke Waikato prehnite-pumpellyite greywacke pumpellyite-actinolite greywacke Murihiku Manaia Hill.unit 100 km L.Taupo Fig. I.1: Map of the northern half of the North Island, showing the distribution of major tectonic units and of basement metamorphism. Thick lines (except “JMA”) are boundaries of terranes or their subunits (names are in bold script). Manaia unit is not mentioned in text. Modified after Woldemichael and Black (2002). 4 Introduction Fig. I.2: Lithostratigraphic summary of the main Permian-early Miocene autochthonous and allochthonous rocks of Northland. Mangakahia lithofacies are: 1. clastic flysch (Punakitere Sandstone); 2. siliceous mudstone (Whangai facies, Ngataturi Siltstone); 3. muddy limestone; 4. non-calcareous mudstone. Motatau lithofacies are: 5. calcareous mudstone; 6. sandstone; 7. fine-grained limestone. G = glauconitic (from Hayward et al. 1989). 5 Introduction Fig. I.3: Distribution of major lithological units within the Northland Allochthon outcrop (from Hayward et al. 1989) Autochthonous sedimentary rocks record rapid latest Oligocene (Ld-eLw) subsidence starting in northern Northland and migrating southwards through to Auckland over the suceeding 2 to 3 million (earliest Miocene, Po) years. Subsidence was followed by emplacement of the Allochthon from the north-east. After the main phase of nappe-like emplacement in the north, the Allochthon was uplifted, folded and subjected to further mobilisation, thrusting and terrestrial erosion. During this later phase, early Miocene marine sediments (Akarana Supergroup) accumulated in depressions on the surface of the slowly moving Allochthon at Parengarenga and Hokianga, and over its margins to the west and south. South of the southern toe of the Allochthon near Wellsford, subsidence formed the Waitemata Basin which began to fill with turbidites derived from erosion of the uplifted Allochthon. During infilling of the Waitemata Basin, the toe of the Allochthon was remobilised southwards into the basin (Hayward 1993). The Auckland region and Waitemata Basin were everted at the end of the early Miocene (Pl). The only marine Tertiary strata overlying Akarana Supergroup rocks on land are calcarenites of the mid Miocene Waikuku Limestone near North Cape, which indicate a brief period of shallow water conditions in the Far North. 6 Introduction Fig. I.4: Distribution of early Miocene arc-type volcanic centres in northern New Zealand (after Hayward 1993, Herzer 1995). Location of main early Miocene marine sedimentary basins on top of and to the south of the Northland Allochthon is also shown. Subduction must have begun in the late Eocene and Oligocene with the first evidence (tuffs) of the outbreak of calc-alkaline volcanism documented