Tane (1971) 17:9-32 9

GEOLOGY OF WHALE ISLAND (MOTUHORA)

by W.R.H. Ramsay* and B.W. Hayward*

SUMMARY

The geology of Whale Island basically a Quaternary volcanic complex is described. Three informal formational units are erected: Camp Bay Formation (pre-volcanic marine sediments), Whale Volcanics (andesitic breccias and lavas and an informal member, Pa Hill Intrusive), and Motuhora Formation (post volcanic marine tuffaceous sands). Stratigraphic columns, penological descript­ions, a geological map and one new silicate analysis of the volcanic rock are presented.

INTRODUCTION

Whale Island lies in the Bay of Plenty situated some 11 kilometres NNW of Whakatane. The authors carried out seven days of field work on the island during the Auckland University Field Club scientific camp in August, 1970.

PREVIOUS WORK

MacPherson (1944) gave the first detailed geological description of Whale Island and presented a geological sketch map. He believed the island to be either the southern remnant of a volcanic cone, or the southern and south-west portions of a crater rim.

Healy (1967), however, claimed the island to be the remains of three volcanic cones down-faulted along the northern side.

Duncan (1969) in an unpublished thesis, included a map of the island together with a discussion on the petrochemistry of the igneous rocks. He considers Whale Island to have been inactive for 36,000 years (Wilson, 1971).

PHYSIOGRAPHY

This steeply sloping island, 2.4 kilometres east to west, and nearly 1 kilometre wide, rises to a maximum height of 345 metres. Three prominent valleys trend­ing NE cut the island and contrast with the much younger and shallower consequent radial drainage system in the initial stages of development on the main hill. Except at the mouths of the three valley systems, steep cliffs have developed which, on the northern side of the island, rise sheer for over 300 metres. Rapid erosion of the sediments on the northern and western cliffs has produced typical badlands topography, but where the cliffs are composed of more resistant lava, erosion is slower and promontories jut out into the sea. Blown, unconsolidated sand fills the valley behind McEwans Bay, and has also, blocked the mouth of the western-most valley (Camp Bay), impounding drainage and causing a swamp to develop. Boulder beaches are present along the SW margin of this latter valley, along the base of the northern and western cliffs and on the western side of McEwans Bay.

* Department of Geology, University of Auckland.

FIG. 1 View of Whale Island from the south west. The main hill consists of undifferentiated volcanic breccia and lava (Whale Volcanics); the prominent valley system to the left leads into Camp Bay and the promontory on the far left is Pa Hill consisting largely of the Pa Hill Intrusive. The sloping ground leading up to Pa Hill consists of volcanic breccias unconformably overlying Camp Bay Formation sediments and conformably overlain by marine tuffaceous sands (Motuhora Formation). (Photo: Olaf Petersen).

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STRATIGRAPHY

For the purposes of description of the rocks on the island, three informal formations have been erected: The Camp Bay Formation, unconformably over­lain by the Whale Volcanics conformable on which is the Motuhora Formation.

I. Camp Bay Formation. This formation is confined to the western portion of the island, and takes its

name from the nearby bay, here referred to as Camp Bay. Two cliff sections are exposed, one to the north of Camp Bay (northern section), and the other in the extreme west (western section). The latter section is the thicker of the two and is uninterrupted, and here designated the type section for the Camp Bay Format­ion. Bedded sediments belonging to this formation are also exposed in stream valleys draining southwards from Pa Hill and as remnant patches on top of Pa Hill .

(i) Lithology and Thickness The sediments are typically well bedded to massive tuffaceous sandstones

containing well-rounded greywacke, pumiceous and obsidian clasts of granule to pebble grades. The total thickness for the type section (western section) is 155 metres, and the minimum total thickness for the Camp Bay Formation exposed in the northern section is 105 metres. Descriptions of individual members com­prising these two sections commencing from the base, are as follows:

A. Western Section (a) Five metres of red stained, pumiceous, medium grained sandstone

in sheared contact with the Pa Hill Intrusive. This shearing has obliterated any sedimentary structures.

(b) Seventeen metres of laminated grey, pumiceous, medium grained sandstone with lenses of greywacke conglomerate and granule-size pumice. The basal 8 metres adjacent to the sheared member (a) is penetrated by clastic dykes of dark grey, fine sandstone with some greywacke granules and pebbles as illustrated in Fig. 3.

FIG. 3 Field sketch showing clastic dykes in unit (b), Western Section, Camp Bay Formation

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WESTERN SEC TI ON

P

4 - 4

NORTHERN SEC TI ON

b

. o .

• ° ' T i

x x x

& 2

A A

V

6 &

KEY

siltstone

sandstone

gwk. conglomerata

pumiceous pebbles

brecc ia

lava

(intrusive )

lava (flow )

fossiIs

carbonacaous material

t)' M69 I 520

a' O 10 20 30 40

METRES

FIG. 4 Stratigraphic columns - Whale Island.

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(c) Twenty-seven metres of well-bedded grey pumiceous, medium grained sandstone with fossil wood present. Greywacke and pumice-like pebbles, prevalent elsewhere through the column, are absent.

(d) Nine metres of varying lithologies consisting of a basal 6 metres of pumiceous tuffs with a few irregular lenses of greywacke conglom­erate and occasional carbonaceous bands covered by 1 metre of massive white pumiceous tuff, 1 metre of ripple-bedded, fine greyish sandstone, and finally 1 metre of cross-bedded very coarse sandstone with horizons of greywacke conglomerate,

(e) Three metres of finely interbedded fine sandstones, very coarse sandstones and conglomerates; the conglomerates contain well-rounded greywacke pebbles with minor pumice and rare obsidian.

(0 Ten metres of laminated sandstones interbedded with lenses and bands of pumiceous conglomerate. The pumice is typically rounded with boulders up to 0.5 metres in diameter, and minor obsidian and greywacke pebbles are also present. The basal 1 metre of this member contains carbonaceous matter.

(g) Fifteen metres of poorly bedded, well-sorted, fine carbonaceous, tuffaceous sandstone.

(h) Seven metres of conglomerate consisting of subequal proportions of well-rounded pumice, greywacke and sub-rounded obsidian pebbles contained in a sand-sized matrix.

(i) Seven metres of massive to poorly bedded, well sorted fine sand­stone with thin interbedded bands of pumice, greywacke and obsidian conglomerate.

(j) Eight metres of massive conglomerate with minor cross-bedding. The conglomerate comprises 50% sub-angular to rounded obsidian (sand to cobble size), 40% well-rounded pumice (sand to cobble size), 10% well-rounded greywacke pebbles.

(k) Ten metres of well-sorted buff grey, fine sandstone. Massive bed­ding with worm borings, abundant carbonaceous material and some rhyolitic tuff present.

(1) At least 35 metres of poorly exposed tuffaceous sandstones with a few greywacke conglomerate bands.

B. Northern Section (&) Seven metres of sheared rock containing angular igneous blocks,

up to 0.3 metre diameter, derived from the adjacent Pa Hill Intrusive, together with lenses of tuffaceous sedimentary rock, some containing greywacke pebbles.

G E O L O G I C A L M A P - W H A L E IS.

FIG. 2 Geological map of Whale Island.

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(b) Five metres of sheared dark grey, well-sorted medium sandstone. Relict humiliations are rucked up and contorted. Highly fossilifer­ous. (See Appendix.)

(c) Four metres of massive buff sandstone with interbedded grey­wacke conglomerates composed of well-rounded pebbles set in a sand sized matrix with minor carbonaceous material.

The above three units are inferred to predate the remaining units.

(d) Nowhere seen in sedimentary contact with unit (<f) being separated by an inferred fault; it consists of 20 metres well-bedded to massive conglomerates containing well-rounded greywacke pebbles up to 6 centimetres diameter, set in a buff-coloured sand-sized matrix.

(e) At least 40 metres of massive sandstones and light grey siltstones with bedding poorly defined.

(f) Fifteen metres of well-bedded alternating siltstones and medium to coarse sandstones with a few lenses and bands of greywacke conglomerate up to 0.75 metre thick, containing rare, red jasper pebbles. The uppermost 2 metres of this unit is very rich in poorly preserved marine bivalve casts and carbonaceous matter (see Appendix).

(g) Eight metres of poorly exposed massive sandstone with some greywacke conglomerate horizons.

This sequence is unconformably overlain by undifferentiated pyroclastics and minor flows (Whale Volcanics) and the unconformity can possibly be correlated with that in the western section beneath unit (m). The pre-volcanic sediments exposed on the southern flanks of Pa Hill consist of greywacke conglomerates, pumiceous tuffs and sands with carbonaceous material scattered through. Those sediments exposed on top of Pa Hill are well-bedded, fine tuffaceous sandstones and siltstones. One bed contains greyish siltstone inclusions, whilst elsewhere carbonaceous laminae and occasional greywacke conglomerates are present.

(ti) Sedimentary Structures Typically, sedimentary structures are lacking in the more massively bedded

conglomerates, although in fine tuffaceous sands lamina bedding is common. Determination of facing in the sedimentary sequences was obtained from minor occurrences of crossbedding (units (f), (d) and (j) ), and ripple bedding (units (d) and (1)). Three beds from the base of unit (1) were rotated back to a hori­zontal attitude using a Wulff stereo net, and bearings of the crests of 20 current ripples recorded in these beds are portrayed in Fig. 5. From this diagram it is deduced that the dominant paleo-current direction during the deposition of the basal portion of unit (I) was from the NW.

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(Ui) Correlation and Age (see Appendix)

FIG. 5 Rose diagram showing crestal bearings of twenty current ripples from three beds at the base of unit (1), Western Section, Camp Bay Formation.

2. Whale Volcanics These rocks constitute by far the greatest bulk of the island from which they

derive their name. Lava, tuffs, lapilli tuffs and volcanic breccia comprise this formation, and from the cliff section to the north of the central hill, these rocks attain a thickness in excess of 300 metres. Here the deposits dip to the south and east, suggesting that this main hill is the remnant of a former more extensive volcanic cone. Around the SW coastline of this cone, lava dominates with blocky aa lava exposed to the west of Brimstone Bay, and more massive flowbanded lava to the east. Further to the east along the coastline towards McEwans Bay, this massive lava passes up into poorly sorted volcanic breccia which in turn grades upwards to lensing breccia horizons contained in tuffs and lapilli tuffs. The headland to the east of McEwans Bay is dominantly massive lava unconformably overlain by pockets of volcanic breccia which in turn grade upwards to well-bedded marine tuffs (Motuhora Formation).

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The informal member Pa Hill Intrusive comprises dark massive lava, here con­sidered to have intruded the Camp Bay Formation.

(i) Lithology and Thickness

A. Western Section (m) Zero - one metre of lensing poorly bedded tuffaceous sandstone

with rare greywacke pebbles and angular volcanic pebbles and blocks (up to 65 cm. diam.).

(n) One - two metres bedded tuffaceous sand containing fern impres­sions (Pteridium sp.), logs and other plant material as well as angular volcanic blocks up to 5 centimetres diameter.

(0) Eighteen metres tuffaceous sediments containing numerous lenses of volcanic breccia. These lenses consist of either slightly graded, unsorted volcanic breccia with minor interstitial tuffaceous matrix (less than 35%) or very poorly sorted breccia containing angular blocks up to 2 metres in diameter with interstitial matrix more than 50%. The tuffaceous matrix in each case is identical to the surrounding tuff which encloses the breccia lenses. Some carbon­aceous matter, rare rounded greywacke pebbles and pumiceous material are all present in these tuffaceous sediments.

B. Northern Section (K) Ten metres of fine tuffaceous sands containing numerous pieces

of carbonised fossil wood occurring in a gouge-like feature which has cut unconformably into the underlying Camp Bay Formation. This is tentatively correlated with units (m) and (n) in the Western Section.

(1) Sixty metres minimum of poorly sorted, poorly bedded volcanic breccia, possibly agglomerate and thin interbedded lenses of lava.

(ii) Formational Boundaries In all cases observed, the Whale Volcanics were found to unconformably

overlie the Camp Bay Formation, and to conformably pass upwards into the Motuhora Formation. Pa Hill Intrusive is believed to have intruded the Camp Bay Formation because:

(a) Sheared contacts with the Camp Bay Formation up to several metres wide in both the Northern and Western Sections.

(b) Dips in the sediments of the Camp Bay Formation steepen on approach­ing the contact, especially the Western Section where the dips assume a near vertical attitude.

(c) Isolated remnants of Camp Bay Formation on top of Pa Hill . In places these sediments have well exposed contacts with the underlying lava and show minor evidence of baking and a 50 centimetre wide brick red coloured, brecciated zone.

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(iti) Correlation The writers follow Healy et al (1964) and correlate the lava and pyroclastic

deposits of Whale Island with Edgecumbe Andesite on the grounds of similar rock types (hypersthene andesites - see petrography).

(iv) Age Whale Volcanics post-date the Camp Bay Formation of probable Putikian

age - see Appendix. Recently Dr A.R. Duncan has been quoted (S.H. Wilson, 971) 1971) that Whale Island has been inactive for 36,000 years.

3. Post-Volcanic Deposits

(i) Motuhora Formation This consists of laminated tuffaceous sandstones containing minor rounded

greywacke granules and pebbles, together with some angular volcanic fragments. Originally referred to by Macpherson as "bedded rhyolitic t u f f these deposits occur in isolated pockets scattered over the island. In the Western Section such deposits comprise the uppermost 3 metres of the section (unit P) whilst exposures of similar lithology were found forming terraces on part of the eastern lava promontory. Such terraces approach sea level on the southern margin, but are raised to nearly 30 metres above sea level on the northern portion of the promontory, as well as at the head of the valley to MacEwans Bay. A minor deposit was also observed exposed as a terrace 4 metres above sea level on the eastern margin of Camp Bay. In all cases observed, the marine tuffaceous sands conformably overlie up to 0.5 metres of bedded volcanic breccia which, in the case of the eastern lava promontory, sits in hollows in the irregular lava surface.

(ii) Recent Deposits These consist of consolidated and unconsolidated sand dunes together

with boulder beaches. None of these appear to be Flandrian, thus suggesting that the boulder beaches are of recent age. (See fig. 2).

PETROGRAPHY

1. Obsidian and Pumice of Camp Bay Formation In Camp Bay Formation, both dark obsidian and light coloured pumice are

relatively common. The obsidian is the more ubiquitous and varies from sand to granule size, although some pebble grades were observed. Typically it has a course granular texture with numerous white feldspar phenocrysts. The glass from samples collected from unit (j) in the Western Section was found to have a refractive index n . m j n 1.495.

Pumiceous samples collected from the same horizon varied in size up to pebbles and, owing to their composition, are more subject to rounding. Typically light coloured, some exhibit bands of dark to light grey glass. Refractive index for the glass is n. • 1.505.

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2. Whale Volcanics Five representative specimens of volcanic rock were selected for thin-sectioning

and detailed examination. Specimens 19040 and 19041 are from volcanic blocks, specimens 19042 and 19044 are from massive lava and specimen 19043 is from Pa Hill Intrusive. Specimen numbers are those of the penological collections of the Department of Geology, Auckland University.

In hand specimen both the volcanic fragmental blocks and lava are similar and are of a medium grey to reddish grey colour with numerous creamish to colour­less plagioclase phenocrysts; largest dimension typically less than 4 millimetres. However, one colourless phenocryst observed had a diameter of 12 millimetres. Black phenocrysts hornbelende occurs in the Pa Hill Intrusive.

In thin section, the rocks are glomeroporphyritic with phenocrysts of plagioclase and ferromagnesian minerals seated in a hyalopilitic or more rarely pilotaxitic groundmass. Modal analyses of the five sections presented in Table 1, show groundmass to vary from 49.3% to 73.6%. Plagioclase feldspar, orthopyroxene, clinopyroxene, magnetite and apatite are present in all sections. Rock type in each case is considered to be a hypersthene andesite, although 19043 may be regarded as a hypersthene-hornblende andesite.

TABLE 1: Modal Analyses of Whale Island Thin Sections.

Section Number 19040 19041 19042 19043 19044

% % % % %

Groundmass* 49.6 49.3 73.6 50.8 62.7

Plagioclase 36.1 31.3 16.9 31.5 21.8

Orthopyroxene 6.0 11.1 4.5 4.9 4.6

Clinopyroxene 1.1 1.9 0.6 3.2 0.1

Hornblende - - - 0.7 -

Opaques 6.8 6.4 3.0 8.9 8.3

Glass 0.4 - 1.4 - 2.5

1. Crystals whose maximum dimension was less than 0.5 millimetres regarded as groundmass.

2. 1,000 points per section.

Plagioclase

Plagioclase feldspar occurs as microlites and idiomorphic to xenomorphic phenocrysts with a maximum observed dimension of 3 millimetres. Glomeroph-enocrysts, either monomineralic containing plagioclase or bimineralic with plagioclase and pyroxene, typically orthopyroxene, are also present. Phenocrystic plagioclase composition derived from 31 determinations using the "Revised

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.Turner Method" (Slemnums, 1962) proved to be much more variable than reported by Hutton (Macpherson, 1944) with the anorthite percentage ranging from AJI37 to Anyj . Typically there is a hiatus between An^Q-An^Q although exceptions do occur, e.g. 19043 where a zoned plagioclase has a core Ang^ and a rim Aiigj;. Twinning is on several laws:

Al bite 17

Carlsbad 4

Albite-Carlsbad 9

Other 1

with zoning prevalent, being both normal and oscillatory 2V"Z measured from 8 crystals varied from 86°-105°.

Inclusions are common with brown glass, magnetite and more rarely pyroxene present, often zonally arranged. Microlite composition determined from albite twins in sections normal to (010), Michel Levy Method, showed the average composition to be andesine, An-^Q-An^Q.

Orthopyroxene Orthopyroxene dominates over clinopyroxene in Whale Island Volcanics,

sometimes to the virtual exclusion of the latter pyroxene. Phenocrysts range in size up to 1 millimetre in length, and display faint to distinct pleochroism

x = pinkish brown z = pale green. 2 V X derived from 30 determinations indicates a more varied range than originally reported by Hutton (Macpherson, 1944). 2 V X varies from 74° to 46° with those with small angles falling completely off the curve given by Deer et al (1963). Average 2 V y is 61°, with the smallest 2 V X recorded in 19042,48°, 46°, 53°, and in 19043 53°, 52°, 48°, 50°. Zoning is present in some of the phenocrysts, the results of which are presented in Table 2. From 2 V X and refractive index a= 1.71 derived from the rock sample from which 19040 was sectioned shows the composition to fall in the hypersthene field as plotted from the chart of Deer et al (1963).

TABLE 2: Variations in 2 V X in zoned

Hypersthene phenocrysts, Section 19043.

Core Inner Rim Outer rim

69 59 71 70 - 63 62 - 54

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Clinopyroxene Clinopyroxene is either sub-equal in abundance with orthopyroxene e.g.

19043, or rare e.g. 19024 and 19044. Colourless to pale green under polarized light, it lacks pleochroism. 2 V Z measured on five crystals varied from 47° to 55° and this resembles 2 V Z 52°, 56° and 58° reported by Hutton (Macpherson, 1944). Z C i s 45°.

Amphibole Present in 19043 only, from Pa Hill Intrusive, it exists as idiomorphic to

hypidiomorphic phenocrysts having a maximum dimension of 4 millimetres with an average of 0.5-1.0 millimetres. Pleochroic dark yellow green to dull golden brown it has a 2 V X varying from 74-87° and from the chart of Deer et al (1963), the composition is 58-85 Mg: (Mg + Fe + + + Fe + + t + Mn).

TABLE 3: Chemical Analysis and CIPW Norm of Rock from Whale Volcanics, 19041. Analyst: T.H. Wilson, Department of Geology, University of Auckland.

S i 0 2 62.6 Q 16.68

TiC>2 0.5 C -

A 1 2 0 3 17.1 Or 7.23

Fe2(>3 0.8 Ab 34.58

FeO 4.7 An 24.74

MnO 0.1 | [Wo 0.93

MgO 2.0 D i . En 0.38

CaO 5.6 VPs 0.56

Na 2 0 4.1 Hyj

En 4.60

K 2 0 1.2 Hyj

Fs 6.73

P 2 0 5 0.1 Mt 1.16 S - Ap 0.34

C 0 2 - I 1 0.91 + H 2 0 0.8

Total 99.6

Magnetite Scattered throughout the groundmass and present both as micropheno-

crysts and as a resorption product in rims surrounding amphibole in 19043.

FIG. 6 Zones of hydrotheimal alteration. Whale Island.

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Glass Common constitutent of matrix and in 19043 forms large patches inter-

grown with phenocrysts and glomerophenocrysts. Also in 19042 orthopyroxene phenocrysts are in places completely mantled with a rim of reddish brown glass.

Apatite Rare small needle-like crystals were observed in phenocrysts feldspar.

HYDROTHERMAL ACTIVITY

Numerous products of hydrothermal activity, both past and present, occur on the island in the form of sulphur and gypsum accumulations, altered andesite, fumaroles and hot springs. Such areas observed are indicated in fig. 6. Most of the present day activity is concentrated in a NNE trending zone running up Sulphur Valley and exposed again in the northern cliff face. Sulphur Valley itself is a narrow valley separated into two areas of sinter deposits by a Leptospermum and Metrosideros covered flat. The more southerly of the two areas is the larger and more active, having many sinter covered rises and depressions. Fumarolic activity is common in both portions of the valley, and is typically characterised by the deposition of yellow sulphur crystals. The temperatures of twenty-five fumaroles in the upper sinter area ranged from 57° to 99.8°C. The temperatures often fumaroles in the upper portion of the lower area varied from 90 to 100.8°C with an average of 96°C. The temperatures of ten fumaroles in the lowest part of the valley ranged from 77 to 100.7°C averaging 90°. Following a day of rain, three small hot springs were observed, just above high storm wave mark at the base of Sulphur Valley. After a period of dry weather, however, only the largest survived, the temperature of which was 88°C (March 1971), and the pH was 1.6 (August, 1970) owing to the presence of free H2SO4. A map of Sulphur Valley showing the position of fumaroles is given in fig. 7. Activity extends from Sulphur Valley into Brimstone Bay where, at low tide, hot water and steam are emitted from the beach sands, together with minor steam from the rocks around the eastern margin of the bay.

STRUCTURE

Structurally Whale Island displays evidence of tectonism in the form of igneous intrusion, extrusion, tilting and faulting.

1. Bedding Bedding in the western section of the Camp Bay Formation displays strikes

between 112° and 154° with dips to the SW steepening from 30° to 64° on proceeding northwards towards Pa Hill Intrusive. Sedimentary structures, including cross bedding and ripple marks, indicate that the strata young in a southerly direction away from Pa Hill. In the northern section, however, the strata have strikes varying from 26° through to 88° with dips to the NW increasing from 24° to near vertical adjacent to the Pa Hill Intrusive. Facing derived from sedimentary structures shows that the beds young towards the

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FIG. 7 Location of fumeroles and thermal springs, Sulphur Valley, Whale Island.

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intrusive, rather than away, as in the previous case. To the extreme west of the northern section against Pa Hill Instrusive, strata 16 metres thick and separated from the remainder of the sequence by a gully in which no exposures occur, strike 69° and dip 39° to the SE. Poor evidence for facing derived from minor graded bedding suggests that the beds young to the SE.

Bedding in the unconformably overlying, undifferentiated breccias and lavas is irregular and often indistinctly defined. However, a measurement obtained on the northern flank of the island gives a strike of 38° and a dip of 32° to the east.

The terrace deposits of the Motuhora Formation indicate the presence of late tilting of the island with sediments at the head of the valley leading into McEwans Bay having a dip in the bedding of 6° to the south. Similar deposits to the west of Camp Bay dip 30° SE. These bedding attitudes are believed to be of tectonic origin rather than primary dip because the beds at the head of the above former valley dip towards the underlying Whale Volcanics which half surround and contain the pockets of Motuhora Formation.

2. Faulting Macpherson (1944) mapped in three major faults striking approximately

NNE and corresponding with the three present valley systems. The present authors infer the presence of two of these faults only: one in Sulphur Valley and the other up the valley behind Camp Bay. The fault in Sulphur Valley is con­sidered to be associated with the present fumarolic activity. A prominent joint or fracture exposed in the northern cliff face and dipping 50° SE, displays evidence of hydrothermal alteration along its length, and is believed to approxi­mate the attitude of the fault. The second fault is believed to extend up the most westerly valley and out through the northern section along a gully separat­ing the exposures of oppositely dipping sediments.

In the northern section, a sequence of near vertically bedded sediments -unit (f) - is cut by at least five parallel faults, which strike between 280° and 290° and dip steeply to the south at angles between 81° and 88°. A total lateral displacement of the steeply dipping sediments was found to be approximately 2.5 metres, with the sense of movement being north side east. Two possible explanations are:

(i) Faulting with a dominant dip slip to oblique slip component occurred before tilting of the Camp Bay Formation. However, so extreme is the present dip of the sediments that it can reasonably be expected that renewed movement along the fault planes would be initiated if these faults pre-dated tilting. Evidence of renewed movement along the fault planes is entirely lacking.

(ii) Faulting with a dominant strike slip component occurred after tilting of the Camp Bay Formation. This possibility is thought to be more likely.

Evidence of minor faulting is also present in the western section where five normal faults strike between 90° to 120° and dip steeply both to the north and south.

FIG. 8 Cross-sections showing geology of Whale Island. (True vertical scale).

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GEOLOGICAL HISTORY

During the mid-Pleistocene or later, the Camp Bay Formation comprising well-bedded to massive tuffaceous sands, greywacke conglomerates and pumiceous and obsidian rich horizons, was deposited in a shallow harbour or sheltered estuary. Several source areas contributed to the sediments present:

(1) Well-rounded greywacke granules and pebbles derived from a nearby eroding landmass on which Permo-Cretaceous basement was exposed; two such likely areas being Otamarakau, NW of Whakatane, or more probably from the region south of Whakatane e.g. Raungaehe Range.

(2) One or more volcanic sources supplied obsidian, pumice and tuffaceous sands.

The dominant current direction for at least part of the period of sediment­ation was from the NW across the present trend of the White Island Trench. Uplift and tilting of this formation was followed by erosion. Igneous activity of the Whale Volcanics was initially subdued, associated with plant material derived from land close at hand. Lava intrusion (Pa Hill Intrusive) and extrusion was in turn followed by an eruptive phase and subaerial deposition of pyroclastic deposits.

Depostion of Motuhora Formation followed upon cessation of volcanic activity on Whale Island, and the origin of these tuffaceous marine sands appears to be the Central Volcanic Region. Tilting to the south and south-east of some 10° then occurred, followed by deposition of the present boulder beaches and wind blown sand.

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APPENDIX

SHALLOW-WATER PLEISTOCENE FOSSILS FROM WHALE ISLAND

by R.F. Whitten*

SUMMARY

Poorly preserved shallow-water fossil faunas from Whale Island indicate a probable Putikian (mid-Pleistocene) age for the Camp Bay Formation.

INTRODUCTION

Two small collections of fossils were made by W.R.H. Ramsay and B.W. Hayward from the Camp Bay Formation on the northern coast of Whale Island.

The sample numbers refer to Fossil Record forms held at the New Zealand Geological Survey Office, Rotorua. N69f520 was obtained from a well-sorted medium sandstone, (b'), 5 metres SE of the contact of the sediments with the Pa Hill Intrusive on the northern coast. N69f521 was obtained from a carbon­aceous siltstone near the base of (f), 50 metres SE of f520. f521 lies at a mini­mum height of 70 metres above f520. The exact distance is not known because of faulting.

LIST OF WHALE ISLAND FOSSILS

N69f520 Foraminifera

Lenticulina subgibba Parr, 1950 Robulus sp. Fissurina sp. Bulimina marginata d'Orbigny, 1826 Virgulopsis turris (Heron-Allen and Earland, 1922) Discorbinella bertheloti (d'Orbigny, 1839) Cribroelphidium charlottensis (Vella, 1957) Cribroelphidium simplex (Cushman, 1933) Notorotalia zelandica Finlay, 1939 Notorotalia aff.finlayi Vella, 1957 Globorotalia inflata (d'Orbigny, 1839) Globigerina sp. Globigerinoides ruber (d'Orbigny, 1839) Cibicides aff. deliquatus Finlay, 1940 Nonionella magnalingua Finlay, 1940 Nonionellina flemingi (Vella, 1957) Zeaflorilus pani (Cushman, 1936) Oridorsalis tenera (Brady, 1884)

*Department of Geology, University of Auckland.

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Ostracoda 8 spp.not determined

Bryozoa

Fenestrulina malusii (Audouin, 1826)

Mollusca ?Pecten aff. novaezelandiae aotea Fleming, 1953 'Maurea sp. Dentalium zelandicum Sowerby, 1860

N69f521 Mollusca

Chione (Austrovenus) stutchburyi (Gray, 1828) Cyclomactra ova ta (Gray, 1843)

Annelida Polydora sp.

AGE

The specimen from f520 identified as ?Pecten aff. novaezelandiae aotea is a poorly preserved fragment showing several subangular ribs with intercostal lam­ellae. Some of the ribs show a weak secondary groove. These features suggest an affinity wi\h Pecten n.aotea which is known from the upper Putikian (Upper Castlecliffian) Landguard Sand, Wanganui and the Te Piki Shellbed, Cape Runaway (Fleming, 1953;1957). The tentative nature of the identification, however, does not allow a definite correlation with these beds.

The appearance of the genus Pecten in New Zealand marks the beginning of the Putikian Substage of the Castlecliffian Stage (Fleming, 1953). As the specimen does not resemble any known species of Pecten from the Hawera Series a probable Putikian age is given for the fauna. All of the other species identified from this locality are known from rocks older than the Putikian and are of little significance in the age determination.

The fauna from f521 which lies 70+ metres stratigraphically above f520 is probably Putikian also although there is no faunal evidence to rule out a younger age.

Other Putikian fossiliferous deposits in the Bay of Plenty are known from Te Piki (Fleming, 1957), Ohope, (Fleming, 1955), and Matata (Meaty and Ewart, 1965).

PALEOECOLOGY

N69f520 The dominant foraminifera in this sample are Notorotalia zelandica,

N.aff.finlayi, Cribroelphidium charlottensis, C. simplex, Zeaflorilus parti and Cibicides aff.deliquatus. The depth distributions of all these species fall within

31

the Elphidium Biofacies of Vella (1962a) which represents depths of 20-200 feet. Zeaflorilus pani is rare in the Elphidium Biofacies and where it is found in abundance it indicates the Zeaflorilus Biofacies with a depth range of 0-20 feet (Vella, 1962b). Notorotalia finlayi, Cribroelphidium charlottensis and C. simplex are also known from the Zeaflorilus Biofacies. Thus the fauna probably lived at a depth of about 20 feet.

Fenestrulina malusii, a membraniporiform bryozoan, is found encrusting molluscan fragments. The membraniporiform bryozoa are adapted to life in littoral and sublittoral zones and although they are known from deeper waters, they are numerically unimportant at greater depths (Stach, 1936). The presence of F. malusii would seem to support a fairly shallow water environment for the fauna.

Chione stutchburyi and Cyclomactra ovata are associated in esturine and enclosed harbour conditions. Chione stutchburyi has a maximum depth of 2 fathoms (Powell, 1939) and both species are known from the littoral zone. Cyclomata ovata extends into deeper water than Chione stutchburyi in the Waitemata Harbour (Powell, 1937).

Most of the specimens of Chione stutchburyi in the sample have conjoined valves and appear to have been preserved in their life position. Their abundance would seem to suggest that they lived in the mid to upper part of their depth range, possibly in less than 6 feet of water. This indicates a relative fall in sea level of 15-20 feet between the time of deposition of (b') and (f).

Casts closely resembling the tubes of the spionid worm Polydora poly-bran ehia are present on the external casts of two specimens of Chione stutchburyi. The thin hairpin-shaped borings of this worm are not uncommon on recent shells of C. stutchburyi (Morton and Miller, 1968).

The authors wish to thank Mr T.H. Wilson for the silicate analysis, Dr G. W. Gibson for checking the foraminifera, Dr P.M. Black and Mr J.A. Grant-Mackie for critically reading the manuscript, and Mr G.R.V. Anderson for additional information on fumarole temperatures obtained during a visit in March 1971.

N69f521

ACKNOWLEDGEMENTS

REFERENCES

DEER, W. et al 1963 "Rock-Forming Minerals" Vol. 2, Longmans.

DUNCAN, A.R. 1970 Unpublished PhD Thesis, Victoria University.

FLEMING, C.A. 1953 The Geology of the Wanganui Subdivision. Bull. N.Z. Geol. Survey. 52-

1955 Castlecliffian fossils from Ohope Beach, Whakatane (N69). N.Z. Jl. Sci. Technol. B36(5): 511-22 -

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1957 The Genus Pecten in New Zealand. Paleont. Bull. N.Z. Geol. Surv. 26-

H E A L Y , J., et al 1964 Sheet 5 Rotorua "Geological Map of New Zealand 1:250,000" D.S.I.R. Wellington.

HEALY, J . , 1965 EWART, A.

Coastal section Matata to Otamarakau. In Thompson, B.N., Kermode, L.O. and Ewart, A. (Eds). "Volcan-ology: Central Volcanic Regions". Inf.Ser.Dep.Scient. ind.Res.50.132-139.

H E A L Y , J. 1967 Geological History of the Whakatane District. / Whakatane and District Historical Society 15,1: 9-27.

MacPHERSON, E.O. 1944

Notes on the geology of the Whakatane District and Whale IslandTV.Z. /. Sci. Tech, 26B: 66-76.

MORTON, J.E., 1968 MILLER, M.C.

"Ihe New Zealand Sea Shore". Collins, London.

POWELL, A.W.B. 1937 Animal communities of the sea-bottom in Auckland and Manukau Harbours. Trans.R.Soc.N.Z.(56:354-401.

1939 Notes on the importance of recent animal ecology as a basis of paleoccologyJh-oc.6th.Pacif.Sci. Congr: 607-17.

SLEMMONS, D.B. 1962 Determination of Volcanic and Plutonic Plagioclases Using a three- or Four-Axis Universal Stage. Geol. Soc. Amer. Special Paper 69.

STACH, L.W. 1936 Correlation of zoarial form with habitat. Jour.Geol.44: 60-65.

V E L L A , P. 1962a Biostratigraphy and paleoecology of Mauriceville District, New Zealand.Trans.R.Soc.N.Z.(Geol). I (12): 183-99.

1962b Late Tertiary nonionid foraminifera from Wairarapa, New Zeaiand.Trans.R.Soc.N.Z.(Geol).I(20/: 285-96.

WILSON, S.H. 1971 Geochemistry of Volcanic Rocks. N.Z. Geochemical Group Newsletter No. 2.