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GEOLOGY OF RED MERCURY ISLAND (WHAKAU)
9
by B.W. Hayward* and P.R. Moore*
SUMMARY
The geology of Red Mercury Island is presented in the form of a map,
twenty-four columns, three cliff sections and with a brief description.
One informal formational unit, the Whakau Volcanics, is erected to cover the
complex sequence of interdigitating basalt flows, breccias, scoria and possible
rhyolitic, tuff beds. These are intruded by several dykes and one large intrusive
body (Te Kokowai Intrusion), which are also considered here as part of the
Whakau Volcanics.
INTRODUCTION
Red Mercury Island, the second largest of the Mercury Group of islands, lies
off the east coast of the Coromandel Peninsula, some 30 kilometres north-east of
Whitianga. Geological mapping of the island was carried out by the authors
during the six day Auckland University Field Club scientific camp there in
August, 1971.
WHAKAU PT. M E T R E S
Fig. 1. Geological map of Red Mercury Island.
*Department of Geology, University of Auckland.

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PREVIOUS WORK
Little previous work has been done on the island.
Thompson (1966) and Schofield (1967) placed the rocks of Red Mercury
Island within the andesitic Beesons Island Volcanics. In the course of mapping
all the islands in the Mercury Group in 1968, Drs. D.N.B. Skinner and R.N.
Brothers made a short survey of Red Mercury. The results of this programme are
expected to be published shortly in Skinner's N.Z. Geological Survey Bulletin
on the geology of the Northern Coromandel Peninsula (Dr. D.N.B. Skinner,
pers. comm.).
PHYSIOGRAPHY
Red Mercury Island, 2.6 kilometres across at its widest part, forms a slightly
dissected raised tabular block sloping gently southwards. Except for Te Roroi
Bay and the southern end of Rolypoly Bay, the entire island is bounded by steep
cliffs, in places up to 100 metres high. The highest point, Te Rengarenga;
(154 metres A.S.L.), occurs above the northern cliffs and from here most of the
island's streams flow southwards, subparallel with the overall slope of the
topography.
Boulder beaches occur around much of the island and an extensive boulder
flat, approximately two to three metres above sea level, is preserved at the
southern end of Rolypoly Bay.
Almost all information was collected from cliff sections, as rock exposure
inland was poor and limited to occasional unvegetated patches exposing highly
weathered flows and breccias.
Fig. 2. Western stack (approx 30m high), Ngatoka Puta
Bay. Sketch shows south-easterly striking dykes cutting
massive breccia and thin flows; a wall of columnar >— V"jointed
basalt forms the western face. The margin ^-O^g
of the Te Kokowai Intrusion is exposed in the £%
lower portions of the eastern face.

11
STRUCTURE
The flows, breccias and tuff beds that constitute the bulk of Red Mercury
Island generally have a dip of from 2° to 15° in a southerly or westerly direction.
Several dyke complexes and the Te Kokowai Intrusion, penecontemporaneous
with the emplacement of the uppermost flows, intrude the lower sequence of
the Whakau Volcanics. These dyke complexes, situated on either side of Te
Rengarenga headland and in Ngatoka Puta Bay (fig. 1), generally consist of one
or more major dykes that often give rise to several secondary offshoots. No dyke
exceeds three metres in thickness and most are between one and two metres
thick; the majority are vertical or near vertical and often somewhat sinuous.
A textbook example of a dyke, two metres thick, vertical and striking 115°,
is located in Ngatoka Puta Bay. Here it extends from the cliff out through the
shore platform as a natural jetty and into the nearby stack where it forms a
striking wall of horizontal columns (fig.2).
SECTION A-A
Ngatoka Puta Bay
dykes
numerous trim flows i9S»*
and scoria bands
SECTION B-B'
thick bush
...„ i £ • -
Te Kokowai Intrusion '
Te Kokowai Intrusion
thick bush
-X) metres
Fig. 4. Cliff sections from Ngatoka Puta Bay to Von Luckner's Cove.
V o n
Luckner's
Cove
weathered flows
and scoria 1
Numerous dykes were seen to extend into the overlying scoria and flows from
the Te Kokowai Intrusion (fig.4) and to feed sills or flows above. The Te
Kokowai Intrusion forms the lower twenty to fifty metres of the north eastern
portion of Te Kokowai. At the contact in the eastern end of Von Luckner's
Cove, the Intrusion appears to have baked and disrupted the flows and breccias,
while in Ngatoka Puta Bay it appears to have squeezed in between flows and
incorporated large blocks of country rock (fig.3).

12
Faults were rarely observed on the island but three subvertical fractures
exist in Ngatoka Puta Bay, striking approximately NNW and with one to six
metres downthrow to the west.
One cliff section of particular interest is that on the east coast of Te Renga-
renga in section C — C (fig.5). Here a large mass of basaltic lava (c'), 200 metres
across and 30 metres high, conformably overlies a tuff bed, breccias and flow c.
This mass (c') is subhorizontal with a comparatively flat upper surface and very
steep (50 — 60°) irregular sides. This upper surface is overlain by a tuff bed
that is semi-continuous down the sides, where it has accumulated in irregular
hollows. On either side of c' the overlying tuff bed is directly on top of, and
conformable with, the tuff bed that underlies c'.
Because there is no evidence of erosion or intrusion, it can only be assumed
that c' was emplaced in its present form as a huge ejected volcanic plug, or more
likely as an extremely viscous lava flow, c' is of considerable importance when
considering the sequence of events during the emplacement of the lower Whakau
Volcanics sequence, as the character of the flows and breccias to the east is
is markedly different from that to the west (see Stratigraphy).
SECTION c-c Te Rengarenga
Fig. 5. Detailed cliff section around Te Rengarenga.
STRATIGRAPHY
Red Mercury Island is composed of a complex sequence of interstratified
basalt flows, breccias, scoria and tuff beds, intruded by dykes and the Te
Kokowai Intrusion, that together comprise what is here termed the Whakau
Volcanics. The authors consider that this informal formational unit cannot be
correlated with the andesitic Beesons Island Volcanics or any other recognised
unit.
The island was circumnavigated by boat and on foot and the entire cliff
exposure mapped. Twenty-four stratigraphic columns have been constructed

13
to illustrate the sequence (figs.6,7,8). Correlation of flows around the northern
cliffs, from column 1 through to 18, is possible due to continuous exposure.
Because of the boulder terrace at Rolypoly Bay and the Te Kokowai Intrusion
however, these flows cannot be correlated accurately across to the southern
coastal cliffs of Te Atiaroa and Whakau Point (columns 19 to 24).
Description of Columns
(a) Von Luckner's Cove to Te Rengarenga (figs.5,7).
In Von Luckner's Cove (column 18) numerous irregular interlensing flows,
breccias, scoria bands and tuff beds dip at some 10° west, away from an inferred
9
f
'V"-"-
Bay - bos art flows
"* - ^
A
7
-i A
-
9
i
? —
?
tuff beds
/
flay
t
'I
f
7
7
1
( 1
* * • I*
~ - * -
t y t
19994
e /
** * "a
*«* * a
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d c
'?,

14
centre of volcanicity off Ngatoka Puta Bay. These flows flatten off to subhorizontal
to the north west and extend as far as column 14 where they abut
against c' (see Structure and fig.5). This body, c', effectively divides the northern
cliff section into two parts. To the south-east the flows are thin, irregular and
often discontinuous, with thick rubbly breccia and tuff beds between; the flows
themselves often grade into breccia bands. Because of the complex nature of
this sequence it was impossible to accurately sketch and correlate all these flows,
but a sample section immediately south of c' was sketched to illustrate their
structure (fig.5).
Correlation from one side of c' to the other is facilitated by tuff beds and the
overlying flow g. A tuff bed that immediately underlies c' is overlain on either
side by further tuff that is seen to pass over the top of c'. This particular bed can
be traced from column 18 in Von Luckner's Cove through to column 11 in
"North Bay" (fig.7). Two other tuff beds can be traced from the sequence in
Von Luckner's Cove over the top of c' and into the sequence in "North Bay".
One of these, between flows e and f, can be traced right around Koterai-o-maru
to column 1 in Rolypoly Bay, while the other, beneath flow g, can be traced from
column 17 in Von Luckner's Cove to column 10 in "North Bay".
Flow g extends from Von Luckner's Cove (column 18) to "North Bay"
(column 8), and thickens westward from 6 metres to a maximum of some 50
metres beneath Te Rengarenga (column 15). Thickness decreases to 20 metres
in column 8 and the flow has been removed by erosion further west on Koteraio-maru.
(bjTe Rengarenga to Rolypoly Bay (figs. 6,7).
To the west of c' the flows are fewer in number, more distinct and thicker,
correlation being aided by this less complex structure. A maximum of seven
flows is shown in column 10. Flows a and b are only exposed in the lower
eastern portion of the "North Bay" cliff section (columns 10 and 11), while
flow c, underlying c' in columns 12 and 13, and varying in thickness from 10
to 20 metres, can be traced continuously, except for a short gap in "North
Bay", from Te Rengarenga to Koterai-o-maru (column 4).
Flows d and e, like flows u and w in Te Atiaroa Cliffs, lens out to the west in
the cliffs approaching Koterai-o-maru, and in doing so they grade into thick
breccia and scoria layers that also decrease in thickness and lens out to the west.
Flow f can be traced from c' through to column 1 in Rolypoly Bay and has a
fairly constant thickness of some 20 to 30 metres.
All these flows are separated by breccia or scoriaceous bands and/or tuff beds.
(cj Ngatoka Puta Bay to Whakau Point (figs. 4,8).
Numerous thin westerly-dipping flows and interbedded scoria bands are
exposed in the cliffs of Ngatoka Puta Bay. Twenty-three separate flows (0.3 to
4 metres thick) were counted in one sixty metre high cliff exposure, and these
flow flatten from approximately 14° to 3° westerly dip within a distance of
200 metres to the west.
The exact relationship of the Ngatoka Puta Bay flows to those in the Te
Atiaroa Cliffs is not clear due to thick vegetative cover between the two areas.

The authors infer, however, that as the flows flatten to the west they thicken up
or lens out to form the sequence of flows shown in columns 19 to 24.
Seven different flows (t to z) were mapped in the Te Atiaroa Cliffs. Two of
these flows u and w, lens out to the west (fig.8), and together with the
decreasing attitude of the flow, provide evidence for a centre of volcanism to
the east, off Ngatoka Puta Bay. Flow v can be traced continuously, except for a
short gap across Te Roroi Bay, from column 19 to 24 and provides a good
reference plane for the remainder of the flows.
15
Red and yellow tuffs beds arc sparse in the Te Atiaroa Cliffs and discontinuous,
occuring only as elongate pods interbedded with the breccias between flows.
(d) Te Kokowai (fig.4).
The Te Kokowai Intrusion underlies a sequence that contains abundant
scoria bands, in places up to 20 metres or more thick. Flows are sparse in the
immediate 40 metres above the instrusion but are more numerous forming the
cap on Te Kokowai itself.
Bare patches of reddish clay, derived from the weathering of the red scoria
and baked tuff beds, on the western side of Te Kokowai were once quarried by
the Maoris. It is from this activity that Te Kokowai was named (kokowai =
red ochre).
Whakau Point Te Atiaroa Cliffs
Fig. 8. Stratigraphic columns, Whakau Pt. and Te Atiaroa Cliffs. For locations see Fig. I.
Holocene Deposits
An extensive boulder terrace, two to three metres high, occupies the southern
end of Rolypoly Bay and is probably a remnant of the Flandrian Transgression.
This is slightly higher than usual Flandrian terraces but may be explained in that
storm wave and talus built boulder terraces are often higher than normal
coastal terraces. Other than the boulder beaches that' surround the island,
there are no other Holocene deposits of note.

16
Genesis of Whakau Volcanics Clastic Deposits
(a) Breccias
The breccias that lie between the flows may be heterogenetic, but the majority
appear to have a direct origin in the lava flows, to which they are petrographi-
cally similar. Moreover, the lava flows were often observed to lens out and
break up into breccias.
Flows often have a layer of breccia directly above and below, with red
scoria or tuff beds between. The authors envisage that the viscous lava broke up
into angular blocks around the margins as it flowed, rafting along and over­
riding the breccias, while the slower cooling central portion of the flow remained
intact as a coherent mass.
(bj Scoria
In the course of mapping, distinction between scoria and breccia was often
found difficult especially where these were weathered. Consequently, the
symbols used on the stratigraphic columns may not give an accurate picture of
the distribution and relationships of the scoria bands.
Although some of the scoria interbedded with flows in the Te Rengarenga -
Koterai-o-maru section may have been directly deposited subaerially, it seems
likely that a proportion was later rafted along by the flows. This hypothesis may
also explain such features as the scoria and breccia pods or lenses in the lava
flows of Whakau Point.
At Te Kokowai a sequence of scoria bands and thin flows overlying the Te
Kokowai Intrusion appears to be the remnant of a once more extensive scoria
cone, presumably built around a centre off Ngatoka Puta Bay.
(c) Tuff Beds
Laminated tuff beds, varying in thickness from 1 cm up to 3 metres, show
few of the features typical of aqueous deposition and were probably all subaerially
deposited. The even distribution of ash over such steep sided features
as c' (see structure) makes the idea of aqueous deposition unacceptable.
In places these tuff beds are separated into two or even three bands by
lensing lava flows and associated breccias, indicating that they were deposited
from a series of ash showers over a period of time.
In numerous places the heat of the overlying flows has baked the tuff beds,
and in Rolypoly Bay the fine-grained tuffs under-lying flow f (column 2) have
been baked to an extremely hard orange-red brick.
PETROGRAPHY
Specimen numbers are those of the petrological collections of the Department
of Geology, University of Auckland.
Whakau Volcanics
(a) Dykes, Flows and Breccias
Nine representative specimens of dykes and flows were selected for thin

17
sectioning and examination. Specimens 19985 to 19988 are from four major
dyke localities (fig. 1) and specimens 19993 to 19997 are from five different
flows exposed at various points around the island. Modal analyses from thin
sections of the four dykes and three of the flows are presented in Table 1.
TABLE 1: Modal analyses of Whakau Volcanics' thin sections.
Sect inn Number
Flows: % % * * % * * % * % * *
19993 (flow h) 47.4 44.3 1.1 1.3 52.6 26.6 21.4 2.9 1.7 70.9 22.7
19994 mow c) 37.9 33.9 3.4 06 62.1 32.3 22.6 5.2 2 0 66.2 23.2
19995 (now f) 14.5 10 0 4.5 85.5 51.1 18.1 9 3 7 0 62.1 18 1
Dykes:
19985 19 8 116 6.8 1.4 80.2 53.2 13.6 1.3 0.8 113 64.8 14.0
19986 16.0 109 4.0 11 84.0 53 4 22.6 3.3 4.7 64.3 23.7
19987 35.5 30.9 5.0 0.6 64.5 29.0 25.1 3 8 3.7 1.9 59 9 25.7
19988 7.6 2.9 4.7 92.4 57 8 25.8 3 1 5.7 60.7 25.8
19984 5.2 1.3 3.9 94.8 21.1 0.8 0 7 4.6 67.5
I 1,000 points per section,
micro
t 2. Crystals whose maximum dimension was less than 0.3 mm were counted as groundmass.
Pheno " Phenocryst; Plag. • Plagioclase; 01. = Olivine; Cpx. = Clinopyroxene; Gmass = Groundmass; Micro • Microliter; Opaq = Opaques; Alk. fetds. -
Alkali feldspars; Indel gmass = Indeterminate groundmass (less than 0 01 mm maximum dimension).
In hand specimens the breccia blocks, flows and dykes are similar and vary
from light purplish to grey in colour. They contain numerous small creamywhite
plagioclase feldspar and orange-brown altered olivine phenocrysts, with
largest dimension of 2mm. Samples vary from vesicular to non-vesicular.
In thin section the rocks are porphyritic, with plagioclase feldspar, olivine
and rare clinopyroxene phenocrysts. Rare glomerophenocrysts of clinopyroxene,
plagioclase and olivine occur in several sections. The groundmass texture in the
flows is generally intergranular, but in the two thicker flows sectioned the
texture verges on pilotaxitic (19995) and subophitic (19996). In the dykes the
groundmass texture is more variable, ranging from the strongly pilotaxitic
(19988) to the subophitic (19987) and the intergrannular to weakly pilotaxitic
(19985 and 19986).
All sections, except for 19988, contain between 10 and 45% euhedral to
subhedral stubby, twinned and often zoned plagioclase feldspar phenocrysts.
These phenocrysts are often poikilitic, containing granules of devitrified glass
and opaques.
Two to seven percent of the bulk composition of all sections are composed of
subhedral olivine phenocrysts, ranging from 0.2 to 2.0mm diameter. These
olivine phenocrysts are always rimmed or even completely replaced by goldenbrown
deuteric iddingsite.
Sparse subhedral, weakly pleochroic light green-brown clinopyroxene phenocrysts
occur in most sections.
A noncrystalline groundmass of slender plagioclase feldspar microlites.

18
anhedral clinopyroxene crystals, opaques and alkali feldspar constitutes between
52 and 93% of the bulk composition of all sections. Small anhedral clinopyroxene
crystals and opaques, of less than 0.3mm diameter, fill the angular interstices
between the plagioclase microlites. The often indistinguishable colourless alkali
feldspar occurs as a late crystallising phase in cavities between the plagioclase
and clinopyroxene crystals.
Coarseness and groundmass constituents is highly variable, with microlites
generally less than 0.3mm in length, although 19987, with subophitic texture,
contains some up to 0.6mm long.
basalt.
Rock type for all flows and dykes examined is considered to be an olivine
(b) Tuff Beds
Three samples from the tuff beds were examined. Colours vary from
whitish through red-brown to dark brown, though all appear to be of similar
composition. The whitish sample, however, consists of what seems to be highly
weathered pumiceous material, with common euhedral to anhedral opaques
and rare quartz grains. Some relatively unweathered darker scoriaceous fragments
are also present.
The other two samples consist largely of whitish and grey to red-brown rock
fragments, often very glassy and vesicular, and common feldspar phenocrysts
and abundant colourless to white glassy shards. The remaining fragments
consist of both black and red-brown glass in varying amounts, minor black ores,
and rare quartz grains. The red-brown glass shows a "grain" or sheen similar to
that found in some of the obsidians of Mayor Island (pers. obs.).
(c) Te Kokowai Intrusion
One fresh specimen (19984) of the Te Kokowai Intrusion was sectioned and
examined. The modal analysis is presented in Table 1. In hand specimens the
sample is indistinguishable from the flows and dykes. It is light grey, vesicular,
and with golden-brown phenocrysts of altered olivine, of maximum dimension
1.5mm, scattered throughout.
In thin section the rock is porphyritic although the phenocrysts, mostly
iddingsite pseudomorphs after olivine, comprise only 5% of the bulk composition.
Stubby subhedral poikilitic plagioclase phenocrysts are rare.
The groundmass has an intergranular to intersertal texture of which 70%
is indeterminable fine grained material of less than 0.01mm diameter. Slender
plagioclase microlites, 0.05 to 0.2mm long, are the coarsest and most common
groundmass mineral indentifiable, with very fine grained crystals of clinopyroxene
rarely discernible. Opaques are in minor proportions, and small
patches of alkali feldspar occur as late crystallising groundmass cavity fillings.
GEOLOGICAL HISTORY
During an undetermined period in Pliocene and/or lower Pleistocene, the
Whakau Volcanics were erupted onto a terrestrial surface. The nearest centre of
activity appears to have been off Ngatoka Puta Bay where a considerable

quantity of scoria, together with interbedded thin flows, formed a small cone.
Further from this centre, viscous basaltic aa lava flowed over and rafted along
breccias and scoria forming a complex interdigitating sequence.
During the course of eruption of the basalt flows, laminated tuffs were
deposited within the sequence, the ash possibly coming from a distant rhyolitic
eruptive centre on Great Mercury Island or the Coromandel Peninsula.
Intrusion of dykes and the Te Kokowai Intrusion, occurred during the later
stages of activity and the observed faults were probably associated with the
emplacement of the Te Kokowai Intrusion.
The south sloping surface of Red Mercury and the apparent overall southerly
dip of flows suggests post-volcanic tilting occurred. No direct evidence for uplift
or for major offshore faulting was found, but these processes may have also
played a major role in the history of Red Mercury.
Since the cessation of volcanism, erosion has been most marked around the
coast with fluviatile erosion of only minor importance in shaping the island's
present physiography.
The higher sea level of the post-glacial Flandrian transgression was responsible
for the formation of a 2-3 metre boulder terrace at Rolypoly Bay.
19
ACKNOWLEDGEMENTS
The authors wish to thank Dr R.N. Brothers and Mr J.A. Grant-Mackie for
critically reading the manuscript.
REFERENCES
Schofield, J.C. 1967 Sheet 3, Auckland. "Geological Map of N.Z." 1:250,000.
Thompson, B.N. 1966 Geology of the Coromandel Region. N.Z.G.S. Report 14.
Skinner, D.N.B. (in press) Geological map of N.Z. 1:63,660. Part sheets N35, 36, 39
and sheet N40. Northern Coromandel.
(in prep) Aspects of the Cenozoic geology of northern Coromandel.
Bull. N.Z.G.S.