149
G E O L O G Y OF T H E K A W E R U A COASTLINE, N O R T H A U C K L A N D
by B.W. Hayward*
SUMMARY
An outline of the geology of an 8 k m . long coastal section around Kawerua
on the west coast of Northland, including the lower 2 k m . of the Waimamaku
River, is presented.
Fossiliferous, shallow water Otaua Formation siltstones, sandstones and conglomerates of Awamoan age are conformably overlain by the Miocene Waipoua
Basalt. Conformably overlying these interbedded basalt flows, breccias and sediments, are interlensing conglomerates and sandstones with minor siltstones and
lignites, called the Pukorukoru Beds (informal formational unit). A l l these three
units are here included within the Waitemata Group.
Horizontally bedded Pleistocene conglomerates, minor lignites and coastal
sands (Kaihu Group) overlie an unconformity cut in the Pukorukoru Beds and
Waipoua Basalt.
Holocene beach sands, boulder beaches, low sandy terraces and moving sand
dunes are prominent along the coast.
INTRODUCTION
This paper is intended to provide a general outline o f the geology of the coastline around the Auckland University Field Club Scientific Station ait Kawerua,
North Auckland. It is not intended as a comprehensive cover of the geology but
rather as a starting point for more detailed work in the future.
Kawerua is situated on the west coast of North Auckland, 50 k m . north of
Dargaville and 17 k m . south of Hokianga South Head. The 8 k m . coastal section
between Wairara and Pukorukoru Streams, including the lower 2 k m . of the
Waimamaku River, was mapped during a two day visit in February 1972 (figs. 1
and 2).
Rock specimen numbers referred to in this paper are those of the Auckland
University Geology Department, whilst fossil record numbers are the sheet numbers of the New Zealand Geological Society. Both fossil and rock specimens are
lodged in the collections of the Auckland University Geology Department.
PREVIOUS WORK
The area described was first mapped by Harrington
ever, to have done no work on the coastal section south
and although he mapped the Waimamaku River area
does not record the basaltic flows that outcrop near its
* Department of Geology, University of Auckland.
(1944). He appears, howof the Waimamaku River,
in considerable detail he
mouth.
F I G . l Geological map of the lower Waimamaku Valley and coastal strip north of Kawerua.
Inset shows location of figures 1 and 2. (See fig. 2 for legend).
151
FIG.2 Geological map of coastal strip around Kawerua. Northland. Numbers indicate
stratigraphic columns for fig. 8.
152
Laws (1947, 1948) published two papers on molluscan faunules from Miocene
sediments of the Waimamaku Valley but no collection came from within the area
mapped.
Thompson (1961) in the 1:250,000 map of New Zealand, recorded Waipoua
Basalt near the Waimamaku River mouth and along the rocky Kawerua coastline,
but on that scale could give few detailed observations.
Woodward (1970) includes the area to the north of the Wairau River in his
magnetic and gravity anomaly maps of the Hokianga region, and Schofield (1970)
has analysed two beach sand samples from near the mouth o f the Waimamaku
River in the course o f his survey of Northland beach sands.
PHYSIOGRAPHY
Twelve kilometres of uninterrupted exposed sandy beach extends south from
Waiotane Stream to Maunganui Bluff and another three kilometre stretch exists
between the Waimamaku and Wairau Rivers. Behind these beaches there is a
narrow strip of low, moving sand dunes, whilst at the mouths of the larger streams
low, partially fixed, sandy terraces occur.
In several localities (figs. 1 and 2) within the moving dunes, wind deflation of
the loosely consolidated sand has formed small basinal areas or regs covered with
sandblasted pebbles (ventifacts).
Between Waiotane Stream and Kawerua, and between the Waimamaku River
and Pukorukoru Stream, patches of rock extend seawards as reefs from the sandy
beach behind. Between Kawerua and the Wairau River, however, basalt flows
form an almost continuous row of reefs broken in several places by stretches of
sandy beach. R o c k y shore platforms and boulder beaches predominate here, at
the foot of 15—20 m . cliffs (fig. 3). Actively eroding coastal cliffs are almost
solely restricted to this section of shoreline.
Behind the sandy beaches and moving dunes,
stabilised coastal sands rise
steeply to heights of 1 0 - 4 5 metres. Inland from the coast, the entire southern
area is covered with these fixed Pleistocene sands, which are eroding in places to
expose limonite cemented and leached bluffs, although in most places these sands
give the country a subdued topography. Indistinct fossil coastal terraces are
identifiable and indicate past sea levels. The larger streams have cut down through
these Pleistocene sands and exposed the harder underlying basalt. Small swamps
are common and river terraces abound near the mouths of streams.
The Waimamaku River is the largest river in the area mapped, and towards its
mouth a long and winding tidal estuary has formed. Flood plains extend across
a wide valley floor for at least 4 k m . upstream (fig. 1). The 1.5 k m . stretch im- «
mediately upstream from the mouth has been cut through hard lithologies of the
Waipoua Basalt and Otaua Formation thus forming a wide "gorge" leading to the
sea. Good exposures of lava flows and the upper beds of the Otaua Formation
occur in these bluffs. The interbedded conglomerates and coarse sandstones of
the latter form a long continuous ridge on the south side of the Waimamaku
River. This ridge is bluffed on the north eastern side and has a steep dip slope on
the south west — a textbook example of structural control of erosion.
FIG.3 Coastal cliff section immediately north of the Ohae Stream mouth showing Kaihu
Group sands overlying the unconformity cut in Waipoua Basalt. Reefs of basalt lava Hows
extend seawards (right foreground) with short stretches of sandy beach behind.
FIG.4 Granule and pebble conglomerate filled fossil stream channel preserved in the unconformity cut in the Waipoua Basalt (W.B.). Pleistocene pebble and cobble beach conglomerate
above is overlain by coastal sands.
154
STRUCTURE
In the lower Waimamaku Valley, Otaua Formation sediments of the western
limb of the Waimamaku anticline (Thompson, 1961) have a south westerly dip.
There appears to be a general flattening of the beds towards the coast from 4 3 °
to 3 7 ° (fig. 5). Still further west Waipoua Basalt appears to overlie the Otaua
Formation conformably and dip south west at an even shallower angle (c. 2 0 ° ) .
On the coast, further south near the mouth of Ohae Stream, Waipoua Basalt
flows and interbedded sediments have a low westerly dip of about 5 ° .
Pleistocene
sands
,
W A I M A M A K U
TASMAN
SEA
^
beach
E S T U A R Y
O T A U A
W A I M A M A K U
R
F O R M A T I O N
KILOMETRES
FTG.5 Sketched N . E . - S . W . geological cross-section of the lower Waimamaku Valley area.
(True vertical scale).
Although no contact is exposed, the Pukorukoru Beds that outcrop on the
coast to the north of Waimamaku River estuary are inferred to conformably
overlie the Waipoua Basalt. These sediments are disturbed by lensing, folding and
faulting, and the problems in determining whether or not bedding attitude is
primary makes structural interpretation difficult.
A n unconformity above these sediments in the north, and above the Waipoua
Basalt to the south, is overlain by Pleistocene conglomerates, lignites and coastal
sands. In the south this unconformity and the overlying sediments have a slight
westerly dip and cover an extensive coastal strip. To the north around the Waimamaku estuary the unconformity rises steeply inland and sands overstep a 200
m. high fossil coastline.
STRATIGRAPHY
For the purpose of description the rocks in the area mapped will be considered
in five units: three Waitemata Group units - Otaua Formation, Waipoua Basalt,
155
and Pukorukoru Beds (informal formational unit); Pleistocene sands - Kaihu
Group; and Holocene deposits (fig.6).
K A I H U G R O U P - coastal sands (30 metres +)
basal conglomerate
W A I T E M A T A G R O U P - P U K O R U K O R U B E D S (25 metres +)
conglomerates, sandstones, siltstones and lignites
- W A I P O U A B A S A L T (200 metres)
basalt flows, breccias and baked sediments
I
unexposed
- O T A U A F O R M A T I O N (280 metres +)
conglomerates and sandstones
unexposed
r
'
|
alternating sandstones and siltstones
I
FIG.6 Complete stratigraphic column for lower Waimamaku Valley (scale 1:666).
1.
Waitemata Group; Otaua Formation.
(i)
Name.
Hay (1961) erected the Otaua Group for rocks in the Punakitere and
Tutumoe Survey Districts similar to those of the Waitemata Group of Dargaville
- Rodney and north east Parengarenga, but provided no evidence to justify the
distinction. Thompson (1961) followed this lead and included all Pareora and
lower Southland sediments in the region within the Otaua Formation. Lithologic, paleontologic and tectonic similarities all indicate that sediments o f the socalled Otaua Group and Waitemata Group accumulated in the same depositional
basin at the same time and have experienced similar post — depositional
histories, so that there exists no valid reason for retaining a separate status for
the Otaua Group ( J . A . Grant-Mackie, pers. comm.) and it is here submerged in
the Waitemata Group, which can be used to cover all similar Pareora and lower
Southland rocks of Northland and their southwards extension. The unit Otaua
Formation is retained however to cover the Waitemata rocks that underlie the
Waipoua Basalt in this area.
156
(ii)
Distribution.
Otaua Formation rocks werc mapped only in the lower Waimamaku Valley,
between 1.5 and 2 k m . from its mouth. Two well exposed sections on the southern side of the river were studied in detail (fig. 7).
0
\5
J
10
! a
conglomerate
\io
25
METRES
200 metres
unexposed
f
6 2 3
f 6
24
FIG.7 Stratigraphic column of the Otaua Formation sediments of the lower Waimamaku
Valley, together with a sketch of the slumped unit and location of N18/f,623 and N 18/f.624.
(Hi)
Lithologies and thickness.
A . Eastern Section (Grid reference N18/950127)
Here the lowest exposed beds o f the Waitemata Group consist of 17 of
alternating graded medium sandstones and siltstones with each bed 1 - 5 cm.
thick. Interbedded within this sequence are several 1 - 50 cm. thick pebble conglomerates containing rounded argillite and argillaceous limestone pebbles. Conformably overlying these lower beds is a further 12 m. of alternating, sparsely
fossiliferous, sandstones ana siltstones without conglomerate bands. Siltstones
average 0.2 - 2 cm. thickness and the graded medium to fine sandstones are 1 - 20
cm. thick. Eight metres of laminated, shelly, medium to coarse sandstone conformably overlies this sequence. Fossils (N18/f.623) were collected from this
bed (fig. 7).
157
To the west, a westerly dipping penecontemporaneously emplaced slump
truncates the shelly sandstone (fig. 7). Immediately above the slump sole, angular
granules of sandstones, mudstone and rounded granules of argillite are caught up
in a matrix of shell fragments and coarse to fine sand. Numerous whole shells
were found above this slump sole (N18/f.624). The uppermost 5 metres exposed
in this section grades upwards away from the sole into a massive light grey-brown
siltstone and mudstone.
B. Western Section (Grid reference N18/945 123)
This section occurs 400 m. south west of the eastern section and is inferred
to be some 200 m. stratigraphically above. The sediments exposed are alternating
sandstone and conglomerate beds. The sandstones consist of well sorted, laminated, medium to coarse sands with rare rounded pebbles up to 10 cm. in diameter,
and occasional interbedded granule conglomerate bands. The conglomerates are
massive to poorly laminated sandy, granule, pebble and cobble gravels (terminology from Folk, Andrews and Lewis, 1970). The gravel clasts are polyvolcanic
in origin but the majority are basaltic. Wedges and lenses of coarse sands occur in
in some of conglomerate beds, and large-scale cross bedding is common in the
granule conglomerates and sandstones.
The basal 14 metres of this section exposes eighteen alternating sandstone
and conglomerate beds in approximately equal quantity (fig. 7). Beds range from
15 cm. to 3 m. in thickness. The upper 35 metres of the section is predominantly
conglomerates consisting of a slightly graded massive pebble and cobble conglomerate, 25 m . thick, overlain by a sequence of interbedded pebble and fine granule
conglomerates.
On the southern side of the Waimamaku River, a total thickness of some
280 m . o f Otaua sediments is inferred to exist between the basal exposure (in the
eastern section) and the base of the Waipoua Basalt (above the western section),
(fig. 6).
(ivj
Paleontology and age.
Two fossil collections were made from the eastern section (fig. 1). These
were:
N18/f.623 from laminated shelly medium to coarse sandstone; Grid reference
N18/950127; Macrofauna and microfauna.
N18/f.624 from gravelly coarse to fine sandstone; Grid reference N18/950127;
Macrofauna only identified.
Preliminary Fossil List
f.623
Foraminifera
Quinqueloculina
sp.
Nodosaria sp.
Robulus 2 spp.
f.624
158
Lagena 2 spp.
Spirillina sp.
Elphidium sp.
Notorotalia targetensis Hornibrook, 1961
Pararotalia mackayi (Karrer, 1864)
Miogypsina intermedia
Droogcr, 1952
Globorotalia (Turborotalia) zelandica incognita
Walters, 1965
Globigerina sp.
Globigerinoides trilobus trilobus (Reuss, 1850)
Globoquadrina dehiscens (Chapman, Parr, Collins, 1936)
Amphistegina aucklandica
Karrer, 1864
Cibicides temperata Vclla, 1957
Cibicides sp.
Globocassidulina
sp.
Melonis simplex (Karrer, 1864)
Coelenterata
Oculina
sp.
Notocyathus
(Paradeltocyathus)
orientalis
(Duncan, 1876)
Bryozoa
Brachiopoda
Terebratuloid
Bivalvia
Nucula sp.
Saccella duplicarina (Laws, 1939)
Saccella sp.
Jupiteria cf. provoluta (Dell, 1950)
Ledella pakurangiensis
Laws, 1941
Ctenoides aff. naufragus Marwick, 1928
Lima colorata colorata
Hutton, 1873
Nemocardium
sp.
Scaphopoda
Dentalium mantelli Zittel, 1864
Laevidentalium pareorense (Pilsbry and Sharp, 1897)
Gastropoda
n. gen. aff. Zeminolia
Zeacolpus sp.
Frignatica fSulconacca)
Frignatica (Sulconacca)
Mima sp.
Falsicolus sp.
? Marginella sp.
Turrinae
Awateria sp.
? Puha sp.
Vaginella torpedo
Ostracoda
Decapoda
(crab chela)
vaughani
n. sp.
Marshall, 1918
Marwick, 1924
159
Echinoidea
(spines)
Teleostei
(otolith)
This list is not comprehensive and contains only those specimens collected
during the short mapping visit.
The presence o f the foraminifera Notorotalia targetensis (Pa), Miogypsina
intermedia (Po-Pa), Globorotalia (Turborotalia)
zelandica incognita (Pa) and
Globigerinoides trilobus trilobus (Pa-Wn) indicates a lower Miocene, Awamoan
age (Pa). The macrofauna is also a typically Waitematan assemblage.
Although of limited diversity, this fauna contains elements that are present at
Pakurangi Point on the Kaipara Harbour (Jones, 1970) and in similar sediments
from the Hokianga Harbour and upper reaches of the Waimamaku River (Laws,
1947:1948).
(v)
Paleogeography
The lower Otaua Formation sediments in Waimamaku Valley are inferred
to have been deposited in shallow water on the margin of a lower Miocene basin,
which possibly extended from Mercer in the south through the entire Northland
Peninsula. Evidence for shallow water comes from the interbedded conglomerates
and the fauna. The most abundant foraminiferan in N 1 8 / L 6 2 3 is Amphistegina
aucklandica which is generally regarded as typical of a warm shallow sea environment. A n inner shelf environment is also supported by the common occurrence
of Miogypsina, Elphidium, Notorotalia and Miliolidae and the poor planktonic
microfauna. Modern representatives of the macrofauna in f.623 and f.624 also
confirm a shallow marine environment. It seems likely that the upper Otaua
beds of alternating gravels and sands are a shallow water, possibly deltaic deposit.
2.
Waitemata Group; Waipoua Basalt.
(ij
Distribution
and contacts.
This unit is exposed almost continuously along the coast from Waiotane
Stream to the Wairau River, and sporadically further north to the Waimamaku
River mouth.
In the lower 1.5 k m . of the Waimamaku Valley, Waipoua Basalt appears to
conformably overlie the Otaua Formation (fig. 5), although the actual contact is
nowhere exposed.
Along the coast the basaltic flows form large reefs which extend out into
the Tasman Sea, protecting cliffs of Pleistocene sediment.
(iij
Lithologies
and thickness.
The Waipoua Basalt consists of basalt flows and breccias with bedded sedi-
160
merits often occurring between the flows, and interbedded with the breccias. The
sediments vary from chocolate - brown mudstones, tuffs, lapillituffs and carbonaceous siltstones to sandstones, muddy granule conglomerates and cobble conglomerates. They are generally considerably baked by the heat of the overlying
flow and altered by later chemical weathering. This weathering has been most
intense immediately beneath the Quaternary deposits, making it difficult in many
places to distinguish between flow and breccia.
At least three flows, separated by thin bands of breccia and red to orange
baked sediment, are exposed in the bluffs on either side of the wide Waimamaku
gorge where the Waipoua Basalt is approximately 180 metres thick (fig. 5). A t no
other place in the area mapped can an estimate of total thickness be made, as the
lower contact is exposed only in the Waimamaku Valley.
5
6
t >
11
s
l D n c
s a n d s t o n e
b r e c c i a
lava
n
f l d w e,
F I G . 8 Stratigraphic columns illustrating the coastline geology around Kawerua (see figs. 1
and 2 for column locations).
(Hi)
Description of stratigraphic columns (fig. 8).
Nineteen stratigraphic columns have been erected along the coastline section
to illustrate the geology. Location of each column is marked on Figs. 1 and 2.
Rocks of the Waipoua Basalt occur in columns 2 to 18 inclusive, and vary
in thickness in any one column from 3 to 12 metres.
Columns 2 and 3 are separated by considerable unexposed stretches, both
from each other and from the main Waipoua Basalt cliff exposures (columns 4 to
18). N o correlation of flows can be made between columns 2 and 3 or the
remainder.
The basalt flow exposed at the mouth of the Waimamaku River and underlying the Pukorukoru Beds (Column 2), is the uppermost flow of those exposed
inland in the Waimamaku "gorge".
In column 3, immediately south of the Wairau River mouth, two flows are
exposed separated by 0.2 to 1 metre of baked sediment.
Continuous exposure from column 4 through to column 17, except for a
161
short distance immediately south of the Ohae Stream mouth (between columns
) and 10), allows correlation of flows, breccias and sediments over a distance of
1 k m . (fig. 2). Only three flows can be seen in outcrop over this distance.
(
The basal flow is exposed from column 4 in the north to column 7, where
it disappears beneath the shore platform. Possibly this same flow reappears above
sea level to the south, at the base of the cliff of column 17 and in the reef and
small high-water exposure of column 18.
The middle flow is separated from the basal flow by lensing breccias, conglomerates and sediments (fig. 8). These vary from 1 to 10 metres in total thickness. The usual situation is illustrated in column 6 where a 1.5 metre thick bed of
silty sandstone has been deposited over a basaltic breccia that overlies the basal
flow. This sandstone has penetrated the upper portions of the breccia to form a
sandy matrix. Basaltic lava then appears to have flowed over the top, leaving an
irregular band of volcanic breccia between the solidified flow and baked sediment.
The middle flow, almost continuously exposed between columns 5 and 15,
varies in thickness from 2.5 to 5 metres, and is overlain by 1 to 1.5 metres of
chocolate-brown siltstone. This siltstone has been baked, presumably by the overlying uppermost flow which is only exposed over a distance o f some 50 metres
(column 8). Elsewhere this flow has been removed by the erosion that cut the
almost horizontal unconformity, and which forms the upper contact of the
Waipoua Basalt in columns 3 to 18.
(ivj
Petrography.
Five representative specimens of Waipoua flows (19919 to 19923 inclusive)
were sectioned for study (see figs. 1 and 2 for localities).
In hand specimen, fresh samples are medium to dark grey, slightly to nonvesicular, with numerous creamish to colourless plagioclase feldspar and rarer
altered ferromagnesian phenocrysts.
In thin section the rocks are porphyritic, with phenocrysts seated in a
finely intergranular or more rarely coarsely intergranular to subophitic (19919)
groundmass. The sample with coarse groundmass conies from one of the thick
flows exposed in the Waimamaku "gorge", whilst the remainder are from thinner
flows exposed to the south. Modal analyses of the five sections show phenocrysts
to comprise between 28 and 48% o f the total bulk composition (Table 1).
Subhedral to euhedral plagioclase feldspar is the dominant phenocrystic
mineral present. Slight embayment of the plagioclase phenocrysts is common in
all sections but in 19922 many are rounded and anhedral. Most are zoned and
twinned and many angular broken crystal fragments are scattered throughout the
groundmass. These plagioclase phenocrysts, 0.4 to 8 mm. maximum dimension,
are generally poikilitic containing small inclusions of clinopyroxene, opaques and
altered glass. Chlorite appears to have preferentially replaced the glass and penetrated the phenocrysts along fracture planes.
Clinopyroxene phenocrysts, 0.5 to 2.5 m m . maximum dimensions, are
present in small quantities in all sections (see Table 1). These weakly pleochroic,
light brown to green phenocrysts are subhedral to euhedral in all sections, except
19922, where they are rounded and anhedral.
162
T A B L E 1 : Modal analyses of Waipoua Basalt thin sections.
Section Number
19919
19920
19921
19922
19923
%
%
%
%
Phenocryst total (>0.3 mm.)
31.4
41.4
42.9
47.9
28.1
Plagioclase phenocrysts
29.8
40.0
40.5
46.4
27.7
Clinopyroxene phenocrysts
0.8
1.4
1.5
1.5
0.4
Olivine
0.8
-
0.9
-
-
Groundmass total ( < 0 . 3 mm.)
68.6
58.6
57.1
52.1
71.9
Plagioclase microlites
43.7
29.3
37.7
28.9
42.8
Clinopyroxene
18.9
15.4
1 1.7
14.3
20.3
Opaques
2.4
4.6
2.0
2.6
3.7
Alkali feldspar
1.0
0.6
1.5
0.8
1.4
Chlorite
1.9
6.7
3.1
2.4
3.1
Indeterminate groundmass
0.7
2.0
1 1
VI
0.6
1,000 points per section.
Olivine is rare and occurs as small subhedral altered phenocrysts in sections
19919,19920 and 19921.
Plagioclase microlites comprise 50 to 66% of the total groundmass constituents and vary in average length from 0.06 mm. (19922) to 0.2 m m . (19919 and
19920). Small stubby clinopyroxene crystals and subhedral opaques occupy the
angular interstices between the plagioclase microlites. Alkali feldspar occurs in
patches as a late crystallising phase within the groundmass. Patches of dark green
chlorite replace portions of the groundmass and part or all o f the various phenocrysts.
Rock type for all five samples of Waipoua Basalt examined is considered to
be pyroxene basalt of the alkali basalt magma group.
(v)
Correlation and age.
The inferred conformability of the Waipoua Basalt on Otaua sediments in
the lower Waimamaku Valley would indicate an age not much younger than
Awamoan. Stipp and Thompson (1971) gave a K / A r date of 15.1 i 0.49 m. yis.
(i.e. lower Miocene) for a sample of Waipoua Basalt from a quarry 25 k m . south
east of Kawerua. This age agrees with the field evidence in this area.
Stipp and Thompson also noted that the "Waipoua Basalt was erupted almost contemporaneously with the Manukau Breccia". Other similarities with
rocks of the Waitakere Ranges, 150 k m . to the south, may be seen in the character of the overlying Pukorukoru Beds in this area (see later) and the similar
164
The Pukorukoru lithologies are indicative of a shallow water environment of
deposition, possibly freshwater. Together with their structural character they are
very similar to parts of the Manukau Breccia.
4.
Pleistocene sands (Kaihu Group).
A l l of the Pleistocene coastal sands, basal conglomerates and lignites that
overlie the unconformity, which truncates the Waipoua Basalt and Pukorukoru
Beds, are considered to be within the Kaihu Group. More detailed work should
allow correlation with one of the formations of this Group.
(ij
Distribution.
These sands occur along the entire length of this coastal strip and extend
at least 5 k m . inland in places and to over 300 metres above sea level.
(ii)
Lithologies and
paleogeography.
Where the basal contact is exposed in the coastal cliffs ( 1 - 5 metres A . S . L . )
these deposits directly overlie an extensive unconformity which is almost horizontal parallel to the coastline but has a very low westerly dip (fig. 3). In the lower
Waimamaku Valley the only inland exposures of the contact were observed
(fig. 1), the contact rising steeply to the east to a height of 200 metres in less than
1.5 k m . The exact nature of the unconformity inland further to the south is
unknown but topographic relief and sand exposure indicates that the
have been to the north at that time. Further study may reveal higher
fluctuations. Vague topographic terrace levels can be recognised in the inland
Pleistocene sand deposits but no work has been attempted on them.
In several places (columns 9, 11 and 12) lignite and silty carbonaceous
sandstones occur in small depressions or elongate wedges directly overlying the
unconformity. A pebble and cobble conglomerate layer, varying from 20 cm. to
1.5 metres in thickness, generally overlies the unconformity or these latter lenses,
and can be traced through most of the columns (fig. 8). In column 7, two layers
of conglomerate have been formed by the deposition of a small lens of sand
between; in columns 5, 10 and 11 sands were deposited over the unconformity
before the conglomerate advanced over them.
Between columns 16 and 17, a 2 - 3 metre wide fossil stream channel has
been cut into the unconformity and filled by a granule and pebble conglomerate
before being overlain by the extensive conglomerate layer (fig. 4).
These conglomerates, exposed only in the coastal cliff sections at Pukorukoru Stream and between the Wairau River and Kawerua, are overlain by coastal
sands of maximum observed thickness c. 25 metres, but probably considerably
more inland to the south. The sands are usually horizontally laminated, often
with a slight westerly dip, although in several places small scale cross bedding was
seen. In column 12, one exposure contains numerous narrow westerly dipping
sand bands, several of which wedge out towards the sea i n the manner of present
day beach sands (pers. obs.).
s
sub
[63
relationship of the Waipoua Basalt to the Waitemata basin sediments (Halliday,
Ramsay and Wilson, 1970).
The sequence observed i n the lower Waimamaku Valley, from alternating
sandstones and conglomerates to massive conglomerates overlain by basaltic
flows, breccias and sediments, is virtually identical to that described by Hay
(1960) from the Kaihu Valley, 25 k m . to the south east.
3.
Waitemata Group; Pukorukoru Beds.
This informal formational unit within the Waitemata Group is used here to
include the interlensing conglomerates and sandstones, together with minor siltstones and lignites that conformably overlie the Waipoua Basalt (fig. 5).
(i)
Distribution.
These beds are exposed along the coast north of the Waimamaku River
mouth in a series of intertidal reefs and also on the southern bank of the Pukorukoru Stream mouth.
(iij
Lithologies and thickness.
The southern exposures (column 2) consist of at least 5 metres thickness
of irregularly bedded volcaniclastic granule conglomerates, massive poorly sorted
pebble conglomerates, chocolate-brown siltstones and laminated medium to coarse
tuffaceous sandstones containing carbonaceous laminae.
The heterolithologic conglomerates contain rounded and subrounded pebbles and cobbles, 0.5 to 10 cm. maximum dimension, of basalt and rarer sandstone
and scoriae, in a coarse sandy tuffaceous matrix. One thin section (19927) o f a
basaltic pebble was cut for examination. This was found to be a highly altered
pyroxene basalt containing large patches of green chlorite and finely divided
opaques. It contains fewer and smaller phenocrysts than the five samples of
Waipoua Basalt studied but otherwise is very similar to them.
Both conglomerates and sandstones contain patches o f secondary cementing calcite and zeolites, and in places large pumice pebbles occur within the
sandstones.
A n indurated light grey medium sandstone, exposed in a reef 600 metres
north of the Waimamaku River mouth, contains broken plant stems and other
carbonaceous material.
Further north, at the mouth of the Pukorukoru Stream, a 12 metre thick
sequence is exposed (column 1 in fig. 8) and appears to overlie the interbedded
conglomerates and sandstones of the intertidal reefs. It consists of 6 metres of
alternating light grey mudstones and lignite bands, up to 50 cm. thick, overlain
by a further 6 metres of medium sandstones and thin granule conglomerate bands.
fHi)
Correlation and
paleogeography.
Although no contact is exposed, these beds are inferred to conformably
overlie the Waipoua Basalt and be only slightly younger in age. Their induration,
lithologies and the overlying unconformity, all indicate that they are part o f the
Otaua Formation - Waipoua Basalt, sedimentary - volcanic - sedimentary cycle
that appears to have occurred in this area during the Miocene (Waitemata Group).
FIG.9 Cumulative grain-size graphs of Pleistocene coastal sands and Holocene beach sands.
166
Three grain size analyses of these sands ( S I , S2, S3) were carried out — see
Fig. 2 for locations. The results are plotted as cumulative curves in Fig. 9. S2 and
S3 are almost identical while SI is slightly coarser, but all are well sorted (CTj0.37
to 0.44) medium sands. Calculation of parameters (Folk, 1968) shows the curves
to be near symmetrical to fine skewed and platykurtic to leptokurtic. The curves
are very similar to both modern beach and dune sands (Schofield, 1970).
It is possible that the basal conglomerates are fossil beach deposits overlying
an extensive shore platform and covered by sands that were deposited as the sea
advanced over them. The sands appear to be from a coastal environment, deposited in shallow water, beaches and dunes. Whether conglomerates overlie the unconformity at higher levels further inland is yet to be determined.
Near the surface the sands form a hard white leached crust up to 3 metres
thick, beneath which an iron enriched, limonite stained band exists, often forming an irregular hardpan.
5.
Holocene deposits.
These are briefly described because of their potential for future extensive
sedimentological studies.
(i)
Sandy beaches.
Long uninterrupted stretches of sandy beach occur south of Waiotane
Stream and in the north between the Waimamaku and Wairau Rivers, and as less
continuous stretches between and behind reefs along the remainder of the coastline.
Grain size analyses of two samples (S4, S5) were made (see figs. 1 and 2 for
locations) and the results plotted as cumulative curves in Fig. 9. The shape of the
curve is almost identical to those of the Pleistocene sands. Both samples are very
well sorted (aj0.27 to 0.34) although S4 is a coarse sand and S5 a medium sand.
Calculation of parameters (Folk, 1968) shows the curves to be near symmetrical
to coarse skewed and leptokurtic to platykurtic.
(ii)
Moulder beaches.
These are restricted to areas where reefs protect the coastline, and the
beach is backed by eroding cliffs exposing Waipoua Basalt at the base.
(Hi)
Fluviatile
deposits.
Horizontally bedded fluviatile conglomerates and finer sediments are exposed in the banks of the Waimamaku River, between 1 and 3 k m . from its mouth,
where they form extensive flood plains.
On several other smaller streams, flat swampy areas have been mapped as
fluviatile deposits (figs. 1 and 2).
(iv)
Low sand terraces.
Low sandy terraces, partially covered by lupins and other vegetation, occur
above high water at the mouths of many rivers and streams. Some may occasionally be covered by water at high tide in severe storms, whilst others such as those
at the mouths of the Wairau River and Ohae Stream may be remnant terraces
167
from the Flandrian transgression.
(v)
Deflation areas - regs.
These occur behind sandy beaches, within the moving dunes in several
localities (figs. 1 and 2), where wind is removing all the finer sand grains and
leaving the basinal regs covered with ventifacts.
(vi)
Moving sand dunes.
Nowhere in the mapped area do these extend more than 40 or 50 metres
inland. They invariably occur behind the sandy beaches where the Pleistocene
sands are not steeply cliffed. The dunes are low and never reach a height greater
than 2 metres and most are lower than 1 metre high.
(viij
Estuarine deposits.
These are of too limited extent to be placed on the map, but certainly exist
in the lower stretches of the tidal Waimamaku estuary where anaerobic muds are
accumulating.
GEOLOGICAL HISTORY
During the lower Miocene, alternating sandstones and siltstones were
deposited on the warm shallow margins of an elongate marine basin, in which
sediments of the Waitemata Group throughout Northland were accumulating. The
sedimentary sequences indicate a shallowing of the sea in this area, with consequent deposition of deltaic conglomerates and sands.
The regression continued so that basaltic lavas of the Waipoua Basalt flowed
over a probably low lying land surface, and between these flows tuffs, lapillituffs
and possible freshwater sediments accumulated. The Pukorukoru Beds were
deposited over the Waipoua Basalt in the north, apparently in a shallow, possibly
freshwater environment, although available evidence does not exclude the possibility that the conglomerates and sands are of shallow marine origin.
This Miocene cycle was followed by tectonic folding that produced the
Waimamaku anticline and general uplift relative to sea level (i.e. the local effect
of the Kaikoura Orogeny).
A period of erosion resulted in deep incision into the underlying rocks
and formed the extensive unconformity that overlies the Waipoua Basalt and
Pukorukoru Beds. At one time in the Pleistocene, this unconformity in the south
appears to have been an extensive shore platform over which a transgressing sea
first deposited beach conglomerates and later beach and other coastal sands.
Recent fluviatile and marine erosion has modified the topography considerably but remnant relief from Pleistocene times can still be recognised.
ACKNOWLEDGEMENTS
The author would like to thank the following people: Mr P . R . Moore for
valuable assistance in the field; Messrs R . F . Whitten and L . L . Wakefield for
168
assistance in identifying the macrofauna and for checking the microfaunal identifications; Messrs J . A . Grant-Mackie and P.R. Moore for critically reading the
manuscript.
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