Tane 35: 25 - 38 (1995)

GEOLOGICAL FEATURES OF LITTLE BARRIER ISLAND,

by Jan Lindsay1 and Phil Moore2 'Department of Geology, University of , Private Bag 92019, Auckland 2Peninsula Research, P O Box 120, Waihi

SUMMARY

Little Barrier Island is the emergent part of a large dacite stratovolcano situated in the Hauraki Gulf, 80km NE of Auckland. It is the largest known dacite volcano in , having an estimated volume of 10km3 (cf. Edgecumbe (1.6km3) and Tauhara (1.2km3)), and is composed of three main units: The Waimaomao Rhyodacite (3Ma), the Haowhenua Dacite (1.5Ma) and the Haowhenua Breccia (< 1.5Ma). Little Barrier Island is heavily forested and has very few inland outcrops. There are, however, excellent coastal exposures of the three volcanic units that form this stratovolcano. The Waimaomao Rhyodacite is restricted in outcrop to the north-eastern-most corner of the island and in places exhibits spectacular flow banding (e.g. at Horseshoe Bay). Numerous exposures of the Haowhenua Dacite can be seen along the coastline, where they are usually overlain by units of the Haowhenua Breccia (e.g. at The Pinnacles, Te Hue Point, Te Ananuiarau Bay). The occasional more resistant inland outcrop is also of Haowhenua Dacite (e.g. Bald Rock and the Citadel). Exposures in the coastal cliffs and valleys along the entire southern coastline consist of Haowhenua Breccia units. Breccia also overlies Haowhenua lavas in cliff exposures along the rest of the coastline. In most exposures the Haowhenua Breccia is extensively weathered, and has a characteristic "marble cake" appearance. The dominant feature of the north-eastern corner of Little Barrier Island is Pohutukawa Flat, a large rock slide consisting of large blocks of Haowhenua Dacite. It has a headscarp that rises 420m above sea level at its highest point. The slide may have resulted from dome collapse or cliff instability accentuated by the great height of the cliffs coupled with the presence of intersecting joint sets.

'Current address: Wairakei Research Centre, IGNS, Private Bag 2000, Taupo

25 Scale 0 1000m

Fig. 1. Geological map of Little Barrier Island.

INTRODUCTION

Little Barrier Island (Hauturu) lies on the northern edge of the Hauraki Gulf, 80km north-east of Auckland and 18km west of (36°12'S,175°7'E). The island is roughly circular in shape, with a maximum

26 Fig. 2. A typical exposure of semi-weathered Haowhenua Breccia exposed at Kiriraukawa Gorge. This photograph also shows the typical nature of stream mouths along the eastern and northern coastlines, where they commonly terminate as waterfalls and steep-sided gorges. diameter of 7.5km from north to south, and 5.8km from east to west (Fig. 1). Steep ravines radiate from a group of central peaks, which rise to a height of 722m at the summit (Mt. Hauturu). Water depth around Little Barrier Island averages 50m, so the total height of the summit above the sea floor is about 770m. On the northern, western and southern sides of the island ring plain surfaces form terraces that slope gently down from 400m to coastal cliffs generally 20-100m high. On the eastern side of the island, coastal erosion has removed these slopes, producing high sea cliffs (up to 200m). The island is fringed by a boulder beach, continuous except where rocky headlands meet the sea (e.g. the Pinnacles, Orau Cove and the Queen). Rapid cliff retreat post 6,500BP has produced spectacular hanging valleys (most at least 10m high), good examples of which are visible at the Hauruia, Te Wairere and Kiriraukawa stream mouths (see Fig. 2). As Little Barrier Island is a nature reserve, visitors to the island need a permit from the Department of Conservation and those visitors wanting to conduct scientific research on the island require a special permit. This paper summarises a detailed geological and geochemical examination of the island by Lindsay (1995) and observations by Moore in May 1990.

27 GEOLOGY

The geology of Little Barrier Island has received little attention in the literature. Excluding the occasional brief reference to the island, there are only three published accounts (Hamilton 1937, Hopgood & Barron 1954, Kear 1961) and each of these interpreted the island as a late Pleistocene andesite stratovolcano. A more recent study (Lindsay 1995) has showed that the island is dacitic in composition and mid Pliocene to early Pleistocene in age; It is, therefore, anomalously young given its location near the northern end of the Coromandel Volcanic Zone (CVZ). Its geochemistry suggests however that it is not part of the CVZ (where volcanic activity ceased 4 million years ago), but rather is related to a group of small dacite domes in Northland (Lindsay 1995). Little Barrier Volcano is the largest known dacite volcano in New Zealand with an estimated volume of 10km3, considerably greater than that of Edgecumbe (1.6km3; Duncan 1970) or Tauhara (1.2km3; Graham & Worthington 1988). The island has a complex stratigraphy, a common feature of andesite and dacite stratovolcanoes. Its lower slopes are made up of lava flows, debris flow deposits and minor pyroclastic flow deposits (Fig. 1). The lavas of Little Barrier Island have been subdivided into two formations, the Waimaomao Rhyodacite and Haowhenua Dacite. This subdivision combines the Haowhenua and Hauturu "andesites" of Kear (1961) into the Haowhenua Dacite. Samples of Waimaomao Rhyodacite from Lot's Wife have been dated by the uranium-lead radiometric method, yielding an age of 3 million years (Lindsay et al. in prep.). Potassium- Argon radiometric dating of several Haowhenua Dacite samples has shown this unit to be 1.5 million years old (Isaac et al. 1994, Takagi 1995). The reason for this unusually long (1.5 million year) time gap in the evolution of the volcano is unknown. The lavas of Little Barrier are overlain by a complex series of breccia deposits, known collectively as the Haowhenua Breccia. This is interpreted as the product of numerous small debris flow-type events occurring after volcanic activity had ceased, and resulting from gravitational collapse and other erosional processes. These breccias consistently lack the characteristics of deposits produced by pyroclastic events e.g. normal grading; gas escape pipes; vapour phase crystallisation; welding and clast homogeneity. Subdivision of Haowhenua Breccia is difficult. The only common sequence is one that relates the breccia to the underlying Haowhenua Dacite. This sequence is exposed at several places along the western and northern coast, and consists of grey Haowhenua lava overlain by 15-25 metres of oxidized, red brecciated lava, which in turn is overlain by less-weathered Haowhenua Breccia units.

28 Haowhenua Dacite The Haowhenua Dacite is a medium-dark grey lava, often with pronounced jointing, that forms the central part of the island and is exposed in many places along the coastline around the northern half of the island (Fig. 1). Tops of the predominantly grey lava flows are characteristically oxidised, resulting in a purple coating. In many localities there is evidence of flow fragmentation in the Haowhenua Dacite, which forms as the viscous upper surface of a lava flow cools and breaks up into blocks while the more fluid lava inside continues to flow. The deposits resulting from this flow fragmentation are known as autobreccia, and, on Little Barrier, are present as deposits up to 25m thick on top of some Haowhenua lava flows.

Waimaomao Rhyodacite The Waimaomao Rhyodacite was recognised as a separate rock unit by Lindsay (1995). It is a pinkish lava flow unit that is restricted to the north-eastern corner of Little Barrier Island (Fig. 1), and in places exhibits spectacular flow

banding. These lavas have a higher Si02 content than the Haowhenua lavas, which is reflected in the name "rhyodacite" (a rock type intermediate between a dacite and rhyolite).

Haowhenua Breccia Exposures in the coastal cliffs and valleys along the entire southern coastline from the Shag Colony in the south-west corner to just north of the Whekau stream mouth consist entirely of breccia units (Fig. 1). Breccia is also present overlying Haowhenua lavas in cliff exposures along the western and northern coasts, and in the area between Whekau and Nehupo steams on the east coast. The Haowhenua Breccia extends some distance inland but, as only a few river valleys are accessible, it is difficult to determine where the breccia outcrops end and the lava begins. A good indication of this transition is the presence of waterfalls in some of the major stream valleys (e.g. The Awaroa and Te Wairere streams). Where weathering is not pervasive, the breccia is generally well-bedded, with individual depositional units easily distinguishable (Fig. 2). Such beds are typically 0.5-2 metres thick, poorly sorted and grading is reverse or absent. Clasts are subangular to subrounded and range in size from a few centimetres to over 3 metres in diameter. One very large clast (c. 10m) is present in the coastal cliffs north of Nehupo Gorge, but the largest clasts in any one unit are commonly 1-3 metres in diameter. There is no preferred clast orientation. The matrix is bimodal; generally fine grained but including small clasts up to 2 centimetres in diameter. In fresh outcrops of breccia it is possible to identify individual clast lithologies. The four most common clast-types are: grey Haowhenua Dacite;

29 Fig. 3. The Queen (and opposite headland) from the north-west. purple (oxidised) Haowhenua Dacite; red Haowhenua autobreccia and a black dense dacite not seen in the form of lava flows on the island. Notably absent in all breccia units are clasts of the pink Waimaomao Rhyodacite. The fact that clasts in the breccia appear to be derived entirely from the Haowhenua Dacites is reflected in the name "Haowhenua" Breccia.

GEOLOGICAL FEATURES

The following is a brief description of some of the more significant geological features of Little Barrier Island (see Fig. 1).

The Queen There is a good exposure of thick, flow-banded Waimaomao lava at The Queen (Fig. 3), a small (20m high) island named after Queen Victoria because of its resemblance to a woman wearing a hoop petticoat. The adjacent headland is made up of a thick, strongly jointed and folded lava flow.

Waimaomao Bay Waimaomao Bay is the type locality of the 3 million year-old Waimaomao Rhyodacite. The lava is a pinkish colour and locally flow banded. In the centre of the bay there is an impressive natural arch in the flow banded lava.

30 Horseshoe Bay Horseshoe Bay is a small, horseshoe-shaped cove in the Waimaomao Rhyodacite. The cliff at the back of the cove is approximately 120m high, and shows good flow banding. Inside the cove there is a cave that has formed by erosion along prominent joint sets, and there is an excellent exposure of very thick, folded, flow-banded lava adjacent to the cave.

Haowhenua Point (The Pinnacles) This is the type locality of the 1.5 million year-old Haowhenua Dacite (Fig. 4). A grey, contorted Haowhenua lava flow at the base of the cliff is oxidised to a purple colour at the top. A layer of autobreccia 15m thick overlies this purple oxidation layer, and is itself oxidised to a deep red. This high level of oxidation may have resulted from the streaming out of gases from the cooling lava. At the top of this autobrecciated unit (which is made up of blocks < 2m

Fig. 4. Field sketch of a cliff just south of Haowhenua Point showing a common sequence exposed around the coast: purple (oxidised) Haowhenua Dacite overlain by autobrecciated lava 15m thick, which in turn is overlain by a series of Haowhenua Breccia units, each l-2m thick.

31 diameter derived from the underlying lava) there is a patchy yellow zone due to alteration of plagioclase to clay (probably montmorillonite). This zone of autobreccia is mantled by a layer of ash, which is in turn overlain by well-bedded Haowhenua Breccia deposits (Fig. 4). All these units dip gently to the south.

Lion Rock and Ngorengore Point Lion Rock consists of grey Haowhenua Dacite, oxidised on its upper surface to a purple colour. It is part of the c. 20m thick lava flow forming the adjacent headland (Ngorengore Point), notable for its interesting arcuate jointing/banding pattern (Fig. 5). An autobrecciated layer, which overlies the lava flow on the headland, also forms the very top (i.e. the "ears") of Lion Rock.

Te Hue Point Te Hue Point forms the northwestern corner of Little Barrier Island, and is a good example of the highly-jointed "platy" nature of many outcrops of Haowhenua Dacite. It forms a very sharp contact with the adjacent red autobreccia, and the jointing in the lava swings sub-parallel to the contact.

Fig. 5. Ngorengore Point and Lion Rock from the north, showing the arcuate jointing pattern in a grey/white Haowhenua Dacite lava flow as it reaches the coast.

32 Fig. 6. The western end of Te Ananuiarau Bay where three stacked Haowhenua Dacite lava flows reach the coast.

Te Ananuiarau Bay The western end of Te Ananuiarau Bay forms a point that is made up of three separate Haowhenua Dacite lava flows, each one separated by a zone of coarse, red autobreccia (Fig. 6).

Orau Cove The western margin of Orau cove appears to be formed by a dike in coarse red breccia, and aerial photographs reveal a N-S trending lineament reaching the coast at this point. The cliffs in the centre of the cove consist of faulted, well- bedded grey Haowhenua Breccia, which merges on both sides into coarse, red autobreccia associated with the bounding lava flows/dikes. The deposits at the top of the cliff, above the eastern side of the bay (c. 50m asl), may be old alluvial gravel.

Bald Rock (Mt. Hauruia) Bald Rock is the only large, inland outcrop on Little Barrier Island. It consists of fine-grained, grey, jointed, relatively resistant Haowhenua Dacite. It forms a NNW-trending lineament with the Citadel which, along with its resistant nature, suggests that it may be a plug or dike (Fig. 7).

33 Fig. 7. Bald Rock (Mt. Hauruia) from the north-west, showing the jointed nature of Haowhenua Dacite. The rock drops off another 70m in the foreground to the valley floor below (rock is approximately 250m from left to right).

South Coast In most coastal exposures (including the area below the Shag Colony) the Haowhenua Breccia has undergone extensive weathering. In these areas bedding is partly to completely obscured by pervasive chemical weathering, and the breccia has a characteristic "marble cake" appearance (Fig. 8). Both clasts and matrix appear to be equally weathered, suggesting that weathering of the clasts has occurred in situ rather than prior to deposition (by geothermal activity for example). The weathered breccia gives an impression of being strongly heterolithologic, a result of alteration of the four main clast lithologies (evident in fresher outcrops) to pink, red, yellow and grey weathering products. Because of the intensity of weathering it is difficult to determine the original clast compositions. In a few places, however, weathering of clasts has not been so pervasive, leaving an unweathered core with a highly weathered rim. Good examples of this can be found below the Shag Colony. Such clasts are almost always of the fine-grained black dacite lava, because of their lower permeability. The unweathered cores of these clasts commonly project out from the cliff. En masse weathering of breccia units occurs in stages, further adding to the

34 Fig. 8. Typical "marble cake" appearance of intensely weathered Haowhenua Breccia at Awaroa Point. Different colours result from multi-stage weathering of at least four different clast types. The dark, more resistant clast towards the top of the photograph is a dense almost black dacite (not seen outcropping as lava on the island).

35 Fig. 9. Pohutukawa Flat from the north-east, showing bush-clad Hingaia Rock Slide Debris. The cliffs forming the headscarp are 420m high. It is approximately 800m from one side of the photograph to the other. array of colours in these outcrops. Initially, weathering results in pinkish hues (due to oxidation). Continued weathering leads to the progressive development of yellow and orange clays. Most cliffs of this breccia have zones of pronounced honeycomb weathering, particularly around the south-west coast to the north of East Cape, where cliff erosion is not as intensive as along the rest of the coast. In areas of honeycomb weathering bedding is completely obscured. Fretting and cracking of beach boulders due to salt weathering is common along the entire coastline.

The Hingaia Rock Slide The dominant feature of the north-eastern corner of the island is Pohutukawa Flat, the product of a large rock slide (Fig. 9). The rock slide deposit consists of large blocks (up to 17m in diameter) of Haowhenua Dacite, and has a headscarp that rises 420 metres above sea level at its highest point. The slide material was mapped by Kear (1961) as the Hingaia Fall Debris, but the name Hingaia Rock-Slide Debris reflects a more realistic mode of origin as a rock slide rather than rock fall. The thickness of the debris is unknown and it is difficult to assess the volume accurately. Kear (1961) estimated the volume of the debris

36 above sea level as 25,000,000 cu. yd. (about 19,000,000m3) by assuming an average thickness of 200 feet and an area of 120 acres. Despite its name, the surface of the landslide is far from flat; large blocks of lava form ridges and depressions and the surface is generally very irregular. The age of the rock slide is unknown. Kear (1961) pointed out that the slide material seems unmodified by higher sea levels following the Flandrian Transgression (when sea levels were approximately two metres above the present level), suggesting that the slide occurred less than 2,270 years ago. It is however questionable whether evidence of the Flandrian Transgression would be preserved in such an environment. The rock slide does seem to relate to the present day sea level (as it does not appear to be swamped) and its age can therefore be estimated at less than 10,000 years, based on the time of sea-level rise at the end of the last major glacial period. The cause of the Hingaia rock slide is also unknown. It may have resulted from dome collapse, or more likely, cliff instability due to the great height of the cliffs, coupled with the presence of several joint sets weakening the rock mass.

Te Maraeroa and Te Titoki Point The only area of flat land on Little Barrier is Te Maraeroa on the south-west side of the island. This area is bordered by extensive boulder banks on both sides formed at the meeting point of two prevailing winds, and the area behind the boulder banks is filled by material from several streams that drain into it. Records and photographs taken by past visitors and caretakers show that the shape of the spit has changed considerably during the last hundred years, at times having a relatively symmetrical shape as it does today, at other times having a pronounced hook at the point (see fig. 11, Kear 1961). The age of the spit is unknown, but Hamilton (1961) suggests it formed during the Flandrian Transgression, and became exposed during the slight drop in sea level following the transgression.

ACKNOWLEDGEMENTS

We are grateful to the Department of Conservation for allowing us to conduct field work on Little Barrier Island, and to Tim Worthington and Bruce Hayward for critically reviewing the manuscript.

REFERENCES

Duncan, A.R. 1970: The Petrology and Petrochemistry of Andesite and Dacite volcanoes in Eastern Bay of Plenty, New Zealand. Unpublished PhD thesis, Victoria University of Wellington. Graham, I.J. & Worthington, T.J. 1988: Petrogenesis of Tauhara Dacite (Taupo Volcanic Zone, New Zealand) - evidence for magma mixing between high-alumina andesite and rhyolite. Journal of Volcanology and Geothermal Research 35: 279-294. Hamilton, W.M. 1937: The Little Barrier Island, Hauturu. New Zealand Department of Scientific and

37 Industrial Research Bulletin 54 (also in: NZ Journal of Science and Technology 17: 465-495). Hamilton. W.M. (ed) 1961: Little Barrier Island (Hauturu). New Zealand Department of Scientific and Industrial Research Bulletin 137. 198pp. Hopgood, A.M. & Barron, R.H. 1954: Notes on the Geology of Little Barrier Island. Tane 6: 7-19. Isaac, M.J., Herzer, R.H., Brook, F.J. & Hayward, B.W. 1994: Cretaceous and Cenozoic sedimentary basins of Northland, New Zealand. Institute of Geological and Nuclear Sciences monograph 8. Kear, D. 1961: Geology. In: Hamilton, W.M. (ed.): Little Barrier Island (Hauturu). New Zealand Department of Scientific and Industrial Research Bulletin 137. 198 pp. Lindsay, J.M. 1995: Little Barrier Volcano: Geology and Geochemistry. Unpublished MSc thesis, University of Auckland. Takagi, M. 1995: Miocene-Pliocene arc volcanism of the Hauraki region in , New Zealand. Unpublished MSc thesis, Hiruzen Research Unit, Okayama University of Science.

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