119 TANE (1967) 13: 119 - 142

AN OUTLINE OF THE GEOLOGY OF THE ST HELIERS

B AY-GLENDOWIE AREA, ,

by B. G. Jones *

LOCATION

The area is situated on the eastern side of Auckland citj (Fig. 1, & la); dominantly in the residential suburb of Glendowie. It consists of the coastal section between St. Heliers Bay (N42/374608) and Glendowie Basin (N42/396594), and is bounded inland by St. Heliers Bay Road to the west, and Riddell Road to the south. The Tamaki River Estuary and Waitemata Harbour form the eastern and northern boundaries respectively.

Map coverage of the area consists of the standard N. Z. M. S. I, provisional map series, sheet N42, and all grid references refer to this sheet. The area is also covered by 10 chain Parish and Lands and Survey maps.

Thin section and rock numbers refer to Auckland University Geology Department collections.

PHYSIOGRAPHY

The area has a moderate undulating relief, generally not exceed• ing 35 metres (120 ft. ) in height - a well established Pleistocene terrace level in the Auckland area. The highest point in the area (65. 5 metres) is located at N42/387613 - on the eastern edge of the tuff ring surround• ing Glover Park (St. Heliers Bay Volcano). Another small Pleistocene volcanic cone and lava flow (Taylors Hill) occurs as a prominent feature on the southern boundary of this area. Inland, the relief does not appear to be structurally controlled. Churchill Park Valley is an area of lower relief partially filled with Pleistocene clays. A 1-2 metre high Flandrian terrace can be seen in St. Heliers and Karaka Bays and in the Glendowie Basin.

The coastal cliffs exposed to the north and east are retrograding rapidly under subaerial and wave attack, and numerous landslips testify to the rapidity of this erosion. The main effect of the rapid erosion is to produce a wide (20-40 metres), wave-cut shore platform which enables many of the structural features of the area to be interpreted in three dimensions. The shore platform generally shows no structural control, * Department of Geology, University of Auckland Present address: Dept. of Geology, Australian National University, Canberr 120

Fio 1». LOCALITY MAP 121 with the exception of two areas where a coarse volcanic grit ("Parnell Grit") forms reefs extending out into the harbour at Achilles and West Tamaki Points.

PREVIOUS GEOLOGICAL OBSERVERS

Hochstetter (1864) was probably the first geologist to observe the area although he did not give any specific details. The first brief reports on the area were made by Hector (1886) and Park (1886). Park also gave a rather inaccurate cross-section of the area between Achilles and West Tamaki Points. Later reports by Mulgan (1901) and Fox (1901) gave reference to the area, with special attention being paid to the "Parnell Grit" and "Orakei Greensands". Fox gave a more accurate interpretation of Park's cross-section (Plate XXXVIII, Fig. 5). Since 1901 little attention has been paid to the stratigraphy of the Glendowie area, except for two papers by Searle (1959 and 1964) on the volcanic and associated accidental rocks of the St. Heliers Bay Volcano and Taylors Hill. A brief, but fairly detailed, stratigraphic survey of the area was carried out in 1964 as a mapping exercise by students of the Geology Department, University of Auckland. This paper is largely based on the 1964 surveys of Hopkins and Pharo, together with supplementary field and laboratory work by the present writer.

STRUCTURE

A notable feature of the Waitemata Group is the horizontal or gently undulating attitude of the beds, with localized areas of intense deformation (Mulgan, 1901).

The structure of the St. Heliers Bay-Glendowie area is relatively simple - a general low, south to south-easterly dip of the Waitemata beds. However, tectonic faulting and folding have complicated this picture and have made correlation of the undifferentiated Waitemata beds extremely difficult. A localized area of intense deformation due to slumping is situated 100-500 metres west of West Tamaki Point.

In general, the structural features of the region can be grouped into two classes — penecontemporaneous deformational features and post- depositional tectonic features.

(i) Penecontemporaneous Deformation: Penecontemporaneous deformation caused by clumping of uncon• solidated sediments is frequently far more intense in localized areas than that produced by post-depositional tectonic movement. 122

Within the bulk of the Waitemata sequence exposed in the St. Heliers Bay-Glendowie area penecontemporaneous deformation is only developed on a very small scale; as minor contorted siltstone horizons below graded sandstones. These siltstones usually show overturning to the south or south-west, thus indicating that the sandstones probably arrived in the area of deposition as turbidity currents from northerly or north-easterly sources. However, the latter may merely indicate that the turbidity currents were deflected down the axis of the depositional trough and hence they may not give a true direction of original provenance. Small scale normal and reverse faults are quite common throughout the sequence and are due to differential rates of compaction. They generally show displacements of 1-20 cm. but they may occasionally reach 1-2 metres. They are frequently non-continuous either in length and/or vertical eleva• tion. Such faults generally show very few drag or shear effects.

Large scale slumping occurs west of West Tamaki Point and is believed to be partially associated with the emplacement of the "Parnell Grit" horizon. The beds underlying the "Parnell Grit" at West Tamaki Point and near Glover Park show contorted bedding and small scale thrust faulting, all indicating derivation of the grit from a western source.

However, the intense deformation and large scale slumping occurring 100-500 metres west of West Tamaki Point cannot be entirely attributed to the emplacement of the "Parnell Grit" and is more likely to be due to penecontemporaneous, large-scale, slump movement of uncon• solidated sediments associated with tectonic fault movements. The area in question is bounded to the east and west by normal faults with throws exceeding 30 metres. Movement on the eastern fault (dip 60° to the east) occurred after the deposition of the "Orakei Greensand" and prior to the deposition of the "Parnell Grit" - the latter bed not having been displaced by the fault. The western fault is largely obscured but shows a large downthrow to the east. The area between these faults consists of a series of tightly folded northwest-southeast trending anticlines and synclines which plunge at 15-20° to the south-east. There is considerable lensing and contortion of siltstone horizons, while sandstone beds are tightly folded and frequently fractured. Decollement is a common feature within these contorted, slumped masses. Numerous small, and not so small, normal and reverse faults further complicate the picture in this area and generally separate the major fold features. Both the faulting and direction of overturning of slump features indicate a direction of slumping and sliding from south-west to north-east. Hence, the slumping shown in the Glendowie area shows similar trends to that in the Musick Point- area (chappell, 19R3). 123

(ii) Post-depositional Deformation: One large, low-amplitude anticline between and Glendowie Basin (see Section D-D') can be attributed to post-depositional tectonism. The axis of this fold trends northeast-southwest and it was probably produced during regional faulting since its northern and southern extension is delimited by normal faults. This is in contrast with the north-south trending folds on Musick Point where folding is assumed to have preceeded faulting, and probably also uplift (Chappell, 1963).

The majority of large faults in the St. Heliers Bay-Glendowie area have a north to north-east strike. Where visible, the majority of the faults are normal, and are either approximately vertical or steeply dipping (60-80°) to the east or south-east. These faults generally have a throw exceeding 20 metres and the largest one is believed to have a throw of 60 metres (N42/387614) and can be traced inland to the west of Glover Park. Splinter faults were frequently observed. The fault planes are often marked by a shear zone (e. g. N42/381616) and the fault movement is generally reflected in drag effects in the adjacent beds.

The tectonic faulting shows a similar pattern to that described in the Musick Point-Bucklands Beach area (Chappell, 1963). It is assumed that faulting accompanied the uplift of the Waitemata sediments during the Kaikoura Orogeny.

STRATIGRAPHY

The major stratigraphic unit exposed in the St. Heliers Bay- Glendowie area is the Waitemata sequence of alternating siltstones and graded sandstones, together with the interstratified "Parnell Grit" horizon(s). There has been very little post-Waitemata sedimentation with the exception of Pleistocene clays and silts, and Recent sands. During the Pleistocene sporadic, short-lived volcanicity produced tuff rings at Glover Park and Taylors Hill.

WAITEMATA GROUP

1. Stratigraphic Classification:

The history of stratigraphic classification of the lower-mid Miocene rocks in the Auckland area has been complicated by lack of formal definition of the beds. The name Waitemata beds was first used by Hochstetter (1864) and this informal name, or Waitemata Series or Sequence, has been used extensively by later writers referring to the alternating sequence of sandstones and siltstones outcropping in the Auckland area. Brothers (1954) introduced the terms Waitemata Formation 124

FIG Z CROSS - SECTIONS LOCALITieS SHOWN ON Fig 1

H43/S»tt04 KANAKA weST TAMAKI *Af POINT 125 and waitemata Group (first proposed by Brown, 1942) without formally defining them, and these terms have been used by most subsequent writers even though they are informal.

It is far beyond the scope of this present work to define formally the Waitemata Formation, and hence the beds in the St. Heliers Bay- Glendowie area have not been assigned to a formation, but rather they have been included within the generalized Waitemata Group of sediments.

2. Stratigraphic Columns:

The alternating sequence of turbidite sandstones and siltstones of the Waitemata Group is well exposed in the cliff sections around the St. Heliers Bay-Glendowie area. (Fig. 2) However, as in most Auckland areas, the exposure is not continuous and correlation of the undifferentiat• ed sandstones and siltstones across breaks in coastal exposure is extremely difficult and often only tentative.

Unweathered sandstones are porous, uncemented, blue-grey subgreywackes which occur in well bedded horizons varying in thickness from 5 cm. to 3 cm., - the average thickness of these turbidite sandstones being 45-50 cm. They generally weather to form a yellow or yellow-brown, friable bed which appears to be massive rather than graded. The inter- bedded siltstones are generally light grey to white and finely laminated. Occasional calcareous concretionary horizons occur within the thicker turbidite sandstones — usually at the horizon between two graded beds.

Two composite stratigraphic columns have been established in the St. Heliers Bay-Glendowie area. They are separated by faults and a large area of slumped sediments between Achilles and West Tamaki Points. One composite column (Nos. 1-4 on accompanying stratigraphic columns) extends from St. Heliers Bay to east of Achilles Point. The basal 7 m. exposed near St. Heliers Bay consists dominantly of V&-2 m. thick, graded turbidite sandstones with thin (2-40 cm. ) sequences of alternating, laminated, fine sandstones and siltstones. Above these sandstones is an 8 m. sequence dominantly consisting of alternating fine sandstones and siltstones with occasional interbedded turbidite sandstones up to Vi m. thick. The overlying 55 m. consists dominantly of '^-2 m. thick, graded, turbidite sandstones generally separated by thin sequences of alternating, laminated, fine sandstones and siltstones. This column of Waitemata sediments is overlain disconformably by the 6 m. thick "Parnell Grit". Waitemata sediments again occur above the "Parnell Grit" but they are too weathered to be identified. FIG 3 STRATIGRAPHIC COLUMNS

COLUMN NUMBERS REFE» TO LOCALITIES MARKED OHFIS1. 127

The second composite column extends from West Tamaki Point to Glendowie Basin and reasonably accurate correlation across the unexposed areas and small faults has been possible. This composite column includes Nos. 5-15 on the accompanying stratigraphic columns. The basal portion of this composite column (Nos. 12-15) is exposed just north of Glendowie Basin, and it consists of 5 m. of graded sandstones with slightly less abundant alternating fine sandstone and siltstone horizons. This is overlain by a prominent 3-5 m.. coarse, graded sandstone whose basal portion has numerous incorporated siltstone blocks up to 5-30 cm. in size. This sandstone is succeeded by a 32 m., regular sequence dominantly consisting of ri-1 m. thick, turbidite sandstones separated by thin horizons of alternating fine sandstones and siltstones. This is overlain, in turn, by a 15 m. sequence of subequal quantities of turbidite sandstones and alternating laminated, fine sandstones and silt• stones. The next 15 m. consists dominantly of graded sandstones with only minor proportions of fine sandstone and siltstone - the latter becom• ing prominant towards the top of the 15 m. sequence. The overlying 40 m. is predominantly laminated, fine sandstones and siltstones with occasional graded sandstone beds 1-2 m. thick. Graded sandstones predominate in the succeeding 14 m. which immediately underlie the 6 m. thick "Parnell Grit". Included near the base of this 14 m. sequence is the massive, 2-3 m. thick, "Orakei Greensand" horizon.

The accompanying stratigraphic columns (Pig. 3) have been drawn up to show correlation of the "Parnell Grit" horizon. However, at West Tamaki Point, this horizon appears to rest unconformably on the underlying Waitemata sediments, i. e. the thickness of strata above the "Orakei Greensand" horizon varies from 20 m. at the fault 100 m. west of West Tamaki Point to 2 m. at the Point itself. If this unconformity below the "Parnell Grit" is widespread in the St. Heliers Bay-Glendowie area then the "Parnell Grit" cannot be used as a horizon to correlate the under• lying Waitemata sediments east and west of the area of large scale slump• ing. The presence of an unconformity is also shown when correlation between columns No. 4 and No. 6 is attempted. In the western column graded sandstones are dominant below the "Parnell Grit", while in the eastern column alternating laminated, fine sandstones and siltstones predominate. This difference could possibly represent a fades variation, but it is more reasonable to assume that faulting and slumping of the beds 100-500 m. west of West Tamaki Point, and erosion of the unconsolidated sediments, took place after the deposition of the "Orakei Greensand" and prior to the deposition of the "Parnell Grit".

If this assumption is upheld and correlation of the beds immed• iately below the "Parnell Grit" is rejected, then quite good correlation 128 can be obtained between the dominantly turbidite sequences shown in columns Nos. 1-4 and columns Nos. 9, 11 and 12. The evidence for this proposed correlation is not conclusive since there are no marker horizons within this part of the Waitemata sequence. If, however, this correlation is correct then the total thickness of Waitemata sediments exposed in the coastal sections in the St. Heliers Bay-Glendowie area would amount to 152 m.

From this proposed stratigraphic column several generalizations can be made. The frequency of arrival of turbidity currents was much greater in the lower part of the column than in the upper, and turbidite sandstones occupy approximately 60% of the column while the interturbidite fine sandstones and siltstones occupy 40%.

Takapuna is the only other area within the Waitemata sediments where detailed stratigraphic columns have been drawn and Ballance (1964) noted that the establishment of stratigraphic correlations within the Waitemata Group on the basis of such columns is likely to be unsuccess• ful due to the irregular extent of individual turbidite sandstones. A very tentative correlation based on the general abundance of turbidite sand• stones compared with interturbidite fine sandstones and siltstones would place the upper portion of the Takapuna sequence on a similar level to the basal portion of the St. Heliers Bay-Glendowie sequence.

3. Turbidite Features:

Most of the characteristic sedimentary structures shown in the graded Waitemata sandstones of the St. Heliers Bay-Glendowie area can be explained simply as features of high-density turbidity current deposition.

(i) Basal Contact: The basal contact of the turbidites is abrupt and usually gently undulating. Erosional sole structures include rare groove casts, indicating a north to north-east trending current direction, and slightly more numerous, irregular, erosion hollows extending into the underlying siltstone. Occasional minor drag-folds and rare streaked-out ripple marks are visible below a turbidite sandstone south-west of Ladies Bay. These suggest that the depositional current flowed from the north• east to the south-west. Inclusions of siltstone pellets torn up from the underlying siltstone and incorporated within the turbidite sandstone are quite common.

(ii) Graded Bedding: Vertical grading is a typical feature developed in the basal portion of the turbidite sandstones. There is a concentration of larger particles at and near the basal contact, and the upward diminution 129 of grain size is fairly regular up to the base of the overlying siltstone. It was suggested by Ballance (1964) that the interbedded siltstones were not deposited from the tail of the turbidity current, but rather they were deposited from bottom traction currents operating in the intervals between turbidity currents.

(iii) Repeated Grading: Thick sandstone units are often found to contain two or three graded sequences within them. The lack of intervening silt- stones could be due to erosion by the succeeding turbidity current, or it could be due to two or three turbidity currents following each other closely enough to preclude deposition of finer, interturbidity material.

(iv) Laminations and Convolute Laminations: These were occasionally seen directly above the graded portion of the turbidite and the convolute laminations are usually associated with ripple-drift bedding.

(v) Ripple-drift Bedding: Cross-laminated, ripple-drift bedding is charact• eristic of the upper 2-10 cm. of each turbidite and it usually grades very rapidly up into the overlying, interturbidite siltstone. Ripple-drift bedding generally indicated that the depositional currents flowed from the north• east quadrant.

(vi) Gaps in the Rhythmic Sequence: In many of the turbidite sandstones one or more members of the typical sequence may be absent. Convolute laminae are frequently absent, while all other members, including graded bedding, are occasionally missing. The lack of the basal, coarse, graded sandstone is simply explained by its deposition prior to the arrival of the turbidity current in the St. Heliers Bay-Glendowie area. Hence many of the laminated fine sandstones may represent reworked, tractive current deposits from turbidity currents whose main flow did not reach this area of deposition.

4. Petrography

A limited number of thin sections were studied from the various sediments of the Waitemata Group. The three major classes of rocks present in the St. Heliers Bay-Glendowie area - graded sandstones, laminated fine sandstones, and siltstones - showed a marked similarity throughout the sequence. The only bed which does not conform to one of these general classes is a massive, fossiliferous, 2-3 metre thick bed occuring 2-20 metres below the "Parnell Grit" at West Tamaki Point. This latter bed has been correlated with the "Orakei Greensand" (Fox, 1901). 130

(J) Graded Sandstones: The graded sandstones consist predominantly of sand-sized, fine-grained, sedimentary rock fragments, together with detrital quartz and feldspar grains. The argillaceous fragments (25-50%) are generally well-rounded, up to 1-2 mm. in diameter, and typically have an almost opaque appearance. The most abundant fragments are muddy siltstones lithologically extremely similar to the interbedded Waitemata siltstones. The next most abundant argillaceous fragments are partially recrystallized, and are similar in constitution and metamorphic grade to the argillites of the Waipapa Group (undifferentiated Permian-Jurassic) which forms the immediate basement of the Waitemata Group in the Auckland area. A few fragments of shale containing aligned laths of chlorite were also recorded. Other rock types, represented to a lesser extent in the graded sandstones, include fragments of andesite, calcite cemented sandstone, dolerite, and rarer schistose grains of Chlorite and Albite-epidote-amphibolite grades.

Quartz and feldspar are the most important detrital minerals and may make up 10-25% of the rock, quartz usually being predominant. Their typical grain size is within the 0. 1-0. 5 mm. range. The quartz grains are generally subangular to subrounded, clear, unstrained and contain relatively few inclusions — usually gaseous, although rare needles of apatite and ?rutile were recorded. A few quartz grains with undulose, strained extinction were also noted. The feldspar consists dominantly of fresh, twinned, plagioclase laths, although occasional zoned crystals with sericitized cores were noted. The plagioclase composition (Slemmon's method) was dominantly andesine-labradorite, more acid varieties being relatively rare.

Regularly occuring accessory minerals include pale-green augite and rarer, colourless hypersthene, opaque minerals including some pyrite, glauconitic grains, dark brown flakes of pleochroic biotite, and very rare zircon, apatite and epidote.

Occasional abraded foraminifera were present.

Detrital matrix of these graded sandstones is dominantly argillaceous — there being a complete gradation of grain sizes down to fine silt and clay. Occasional patches of calcite, and rare zeolites, occur as secondary matrix constituents. In similar graded sandstones at Takapuna (Ballance, 1964) primary detrital matrix and primary pore spaces totalled nearly 30% of the rock, and in the typical uncemented and unindurated sandstones the effective porosity was as high as 40-45%.

The heavy mineral suite from similar graded sandstones, below 131 the "Parnell Grit" half a mile to the east in the Musick Point-Bucklands Beach area, included abundant zircon, hypersthene and ore minerals; moderate apatite, epidote and biotite; and sparse to rare sphene, hornblende, garnet, titan-augite and clinozoisite (Chappell, 1963).

The proportion of rock fragments and matrix varies inversely — rock fragments predominating at the base while matrix becomes more abundant higher in the graded beds. The unsorted nature, and the proportion of quartz and feldspar (20%) to rock fragments (50%), and matrix (30%) indicate that these graded beds could be termed unconsol• idated, lithic subgreywackes (Pettijohn, 1957, p. 291).

(ii) Laminated Pine Sandstones: These rocks are essentially similar in mineralogical composition to the graded sandstones. However, all grains are less than 0. 1 mm. in diameter. Rock fragments are less abundant (10-25%) and are dominantly of fine-grained, well-rounded argillite and siltstone. Detrital quartz and feldspar grains are in subequal proportions (10-15%) and are typically fresh, clear and subangular to subrounded. Accessory minerals are slightly more abundant than in the graded sand• stones — probably because of the flood of rock fragments in the latter. They include rounded and abraded glauconitic material, opaque minerals including spheroidal pyrite; minor augite, zircon and biotite; and rare apatite and epidote. Unabraded foraminifera are relatively common. The matrix is generally more calcareous and less argillaceous than in the graded sandstones (e. g. section 12309).

(iii) Siltstones: The siltstones are extremely fine grained, and often very muddy, equivalents of the laminated, fine sandstones. Identifiable detrital grains are rare, including argillite. quartz, feldspar and opaque minerals, all set in an indeterminable argillaceous matrix.

(iv) "Orakei Greensand" horizon: This massive, 2-3 metre thick bed is similar in general lithology to the graded sandstones except in the relative proportions of the various constituents. In this horizon rock fragments are again the most abundant consitituent (40-55%) — argillaceous fragments still predominating, while andesite fragments are more abundant than usual. The remaining detrital mineral and matrix constituents are the same as those occuring in the graded sandstones, although glauconitic material is slightly more abundant in this horizon. The major distinctive feature of this horizon is the abundance of biogenic fragments. Large and small foraminifera, including Amphistegina, and bryozoan fragments are common and their tests are usually filled with indeterminable argillaceous material. Comminuted shell material is also 132 abundant. Rare ostracods, coral fragments, and an echinoderm radiole were also recorded (e.g. section 12308).

5. Fossil Content.

The graded sandstones, laminated fine sandstones, and siltstones of the Waitemata Group contain no recognisable macrofossils, although trace fossils are quite common in the laminated fine sandstones and siltstones. Four types of trace fossils have been recognised and their identification is based on descriptions given by Ballance (1964) and Gregory (1966).

Type A: Long, sinuous, non-branching trails 3-5 cm. wide and consisting of a succession of fairly regular, arched, transverse ridges together with a longitudinal median ridge.

Type B: Cylindrical burrows (0.5-1.5 cm. diameter) orientated sub-parallel to the stratification and filled with a segmented series of siltstone pellets separated by darker or coarser convex laminae.

Type D: Irregularly branching, randomly orientated, cylindrical burrows (0.5-2.0 cm. diameter) filled with light grey siltstone. They are similar to the very common trace fossil type Chondrites.

Type H: Cylindrical burrows up to 1 cm. in diameter which are orientated approximately normal to the bedding. Probably domicile burrows of the Domichnia type.

A more detailed study of the trace fossils present would probably enable other forms to be recognised.

Two slightly more fossiliferous horizons occur within the Waitemata Group in the St. Heliers Bay-Glendowie area - the "Orakei Greensand" horizon and the "Parnell Grit". The "Orakei Greensand" horizon contains abundant comminuted shell material and possibly rare macrofossils — none were collected. It also contains abundant foraminifera, including Amphistegina, fragmental bryozoan tests, and rare ostracod, coral and echinoderm fragments. The "Parnell Grit" contains minor quantities of comminuted shell material, including rare "Pecten" (Fox, 1901), and rare foraminiferal and bryozoan tests — usually abraded to some extent. Further detailed palaeontological study of these horizons would probably provide a small collection of identifiable material.

Carbonised plant debris is abundant throughout the sequence, especially in the laminated siltstone horizons. The largest fragments are up to 1 m. long and 10-20 cm. in diameter, but the vast majority of it is in the form of finely comminuted sand- and silt-sized particles. 133

None of the plant debris is identifiable.

6. Provenance:

The petrography of the Waitemata sediments suggests that they have probably been derived from two or more distinct sources — a greywacke and metamorphic landmass and an andesitic volcanic chain. The bulk of rock fragments are either intra-formational or greywacke derived, with only rare andesitic fragments. This suggests that the greywacke landmass was the major source of detrital rock fragments. However, in the "Parnell Grit" the position is reversed and the abundance of andesitic fragments suggests a volcanic source. Small quantities of other rock types, e.g. dolerite, slate, and schistose rocks of Chlorite and Albite-epidote-amphibolite grade, may be of second cycle origin or may represent more distant provenance areas. The quantity of intermediate plagioclase suggests an andesitic volcanic source. Accessory minerals such as augite, hypersthene and opaque minerals could have come from either an andesitic or greywacke source, whereas pyrite, biotite, zircon, apatite, epidote, garnet, hornblende and clinozoisite all suggest a greywacke or schistose provenance area.

Hence, the typical sediments of the Waitemata Group in this area were probably derived from a greywacke and schistose landmass to the east or north-east (e.g. Coromandel Range and the islands of the ), with the small quantities of volcanic material being derived from either an eastern (e.g. Coromandel Range) or western (e.g. Waitakere Range) chain of andesitic volcanicity.

"PARNELL GRIT"

The andesitic grits, outcropping on the shore-platform at Achilles and West Tamaki Points and in the cliffs at Glover Park and West Tamaki Point, are all lithologically extremely similar, both to one another and to the 4 m. thick, type "Parnell Grit" at Judges Bay, Auckland. Hence, although definite correlation of these grit horizons cannot be attempted, they will all be termed "Parnell Grit".

Mulgan (1901) described the type "Parnell Grit" as:- "The band consists of fine volcanic material, the fragments ranging from minute specks to particles somewhat larger than a pea." He went on to say that "... at St. Heliers Point, and again at Tamaki Point, a similar band appears, almost identical in texture and mineral contents with that out• cropping at Parnell. (In point of fact, the only difference is that the St. Heliers Bay beds are slightly coarser at the base.)" 134

The "Parnell Grit" in the Glendowie area is also extremely similar, lithologically, to a 10 m. thick grit band described from Army Bay, Whangaparaoa Peninsula by Gregory (1966).

In the St. Heliers Eay-Glendowie area the "Parnell Grit" horizons form the uppermost exposed strata in their respective strati• graphic columns. However, in all cases, they are overlain by a small thickness of indeterminable Waitemata sandstones and siltstones. In this area the "Parnell Grit" is about 6 m. thick compared with a thickness of 10 m. for the East Tamaki Grit ("Parnell Grit") of the Musick Point- Bucklands Beach area (Chappell, 1963). The latter is 40 m. below the exposed top of the Waitemata sequence.

1. Basal Contact:

The "Parnell Grit" appears to lie on an uneven eroded surface of underlying Waitemata sediments. This erosion break may represent an unconformity or disconformity prior to the deposition of the "Parnell Grit", or it may represent penecontemporareous erosion during the emplace• ment of the "Parnell Grit", or possibly it may represent a combination of both these factors. The latter alternative is the one favoured by the writer as he considers there was an erosion break and faulting prior to the deposition of the "Parnell Grit".

Small scale features associated with the basal contact of the "Parnell Grit" horizon are:-

(i) The incorporation of abundant blocks of undeformed siltstone and sandstone, up to 1 m. in diameter, within the basal portion of the grit horizon. This suggests that the underlying Waitemata sediments must have been partially consolidated prior to the deposition of the grit.

(ii) North-south trending exaggerated ripple marks up to 25 cm. deep are developed in the underlying sediments.

(iii) Eastward slumping and overturning of the underlying siltstones. This is especially well shown at West Tamaki Point.

(iv) Wedging and eastward thinning of the underlying siltstones.

(v) Low angle overthrusts to the east, developed in the underlying sand• stone horizons.

All these features of the basal contact of the "Parnell Grit" point to a 135 direction of derivation from the west.

2. Lithology:

The 6 m. thick "Parnell Grit" occasionally appears to be roughly graded, although when weathered the sense of grading is lost and the grit appears to be massive andunbedded. Grading, when visible, is usually confined to the basal portion of the grit — coarse andesitic fragments up to 1-2 cm. and larger included blocks of the underlying silt• stone and sandstone rapidly grade up into the more typical, finer grit where fragments are largely confined to the 0.5-1.0 cm. range.

The "Parnell Grit" is bluish-grey when fresh but it weathers to a dull, dark greyish or dark, greenish-brown colour. Red flecks are a characteristic feature of the grit. The fragments within the grit are dominantly andesitic but numerous sedimentary, and smaller quantities of metamorphic fragments, are also present.

The "Parnell Grit" beds are generally jointed on a large scale, and frequently contain veins of calcite and/or zeolite.

Concentric and spheroidal weathering is quite a common feature of the "Parnell Grit" and this criterion can be used to distinguish inland, weathered, "Parnell Grit" horizons.

3. Petrography:

The "Parnell Grit" contains abundant andesite fragments - up to 50% of the rock. The andesite fragments-are generally subrounded and reach a size of 3-7 mm. - occasionally larger. A porphyritic augite andesite with a hyalopilitic texture predominates, although fragments with a pilotaxitic texture are also quite common. The andesite fragments contain phenocrysts of pale-green, non-pleochroic augite and zoned and/or twinned andesine-Labradorite. The matrix of the andesite fragments is frequently very glassy and contains abundant plagioclase laths and ore minerals, and minor intergranular augite anhedra. Many of the vesicles are lined with chloritic minerals and they are occasionally filled with calcite or zeolite. The andesite fragments are, therefore, extremely similar petrographically to the porphyritic pyroxene andesites of the Manukau Breccia, described from the Waitakere Ranges by Searle (1932) and Brothers (1948).

Sedimentary and metamorphic rock fragments are the next most abundant constituent of the "Parnell Grit" (up to 25-30%). Rounded fragments of Waitemata siltstones are dominant and often make up nearly 136

15% of the rock. They may reach a size of 5 mm. (e.g. section 12307). Smaller rounded fragments of dolerite, greywacke, argillite (e.g. section 12307), and schistose rocks including albite-epidote amphibolites are scattered throughout the grits (e.g. sections 12305-12307).

Detrital mineral fragments are also quite common throughout the grits. They consist of shattered, twinned and/or zoned plagioclase laths, greenish augite euhedra up to 3 mm. long (e.g. section 12305), and opaque minerals. Minor detrital mineral constituents include subrounded quartz grains, and rare epidote, zircon, biotite, hypersthene and olivine — the latter almost entirely replaced by chloritic minerals (e.g. section 12305) .

Rounded and sub-angular grains of glauconitic minerals are occasionally present (e.g. section 12307). Rare foraminiferal and fragmented bryozoan tests are also occasionally present (e.g. section 12306) . Zeolites are occasionally developed in the matrix and are usually associated with altered andesite fragments (e.g. section 12305).

The matrix generally consists of indeterminable argillaceous material and glass shards. However, in section 12306 the matrix has been almost entirely replaced by calcite. In this section the calcite can also be seen replacing quartz, feldspar and andesite fragments.

The red specks, seen in hand specimens of the grit, probably represent blebs of lepidocrocite formed by the oxidation of ore minerals.

4. Fossil Content:

The outcrop of "Parnell Grit" off Achilles Point contains scattered fossils including "... Pecten zittelli, Pecten burnettif?), and Polyzoa too much weathered for identification," (Fox, 1901).

5. Origin:

There are two conflicting views on the direction of derivation of the "Parnell Grit". Several older writers suggest that it may have 3ome from an eastern source in the Coromandel Ranges - e.g. Hector (1886), "The 'Parnell Grit' as far as I have seen, contains no fragments of the volcanic rocks of the district, but is greensand, with well-rolled pebbles of cherty slate, quartzite, and other Paleozoic rocks, and occasional fragments of old trachyte and basic rocks of Cape Colville." Fox (1901) also stated that the "Parnell Grit" could possibly have come from an eastern source although he favoured derivation from a western andesitic volcanic chain. This latter view is the one proposed 137 by the majority of later writers, including Mulgan (1901), Searle (1932), Kuenen (1950), Brothers (1954), Chappel (1963) and Gregory (1966). Fox (1901) suggested that the coarseness of the "Parnell Grit" horizon decreases away from the Waitakere Ranges, e.g. the grit at St. Heliers Bay is about twice as far, and only half as coarse as a "correlative" 10 m. thick grit at White Bluff. Kuenen (1950) put forward the concept that the "Parnell Grit" horizons were emplaced by deposition from submarine lahar flows which had slurried off the slopes of a western andesitic volcanic chain (now represented by the Waitakere Ranges) and had flowed into the basin of Waitemata sedimentation. This suggested origin of the "Parnell Grit" horizons was accepted by Brothers (1954), Chappel (1963) and Gregory (1966), and it has also been accepted by the present writer since it explains all the features associated with the "Parnell Grit" beds in the St. Heliers Bay-Glendowie area.

PLEISTOCENE AND RECENT SEDIMENTS

The only post-Waitemata sediments in the area are superficial deposits of Pleistocene, white marine clays and silts, especially prominent in the Churchill Park Valley. However, inland exposures are almost nil and consequently the extent or thickness of the Pleistocene silts could not be determined with any degree of accuracy, except where the deposits could be seen outcropping in bays around the coast.

Holocene marine sands and silts occur within the St. Heliers Bay shopping area, and also form the Flandrian terraces at Karaka Bay and Glendowie Basin. Muds of recent age occur as wide flats extending out into the Tamaki River Estuary.

PLEISTOCENE VOLCANICITY

Two areas of Pleistocene volcanism occur in the Glendowie area. The earliest outbreak was the St. Heliers Bay Volcano (N42/886614) which is assumed to nave erupted more than 50,000 years ago (Searle, 1964). Searle states that the eruption was largely abortive and ceased after a phreato-magmatic phase that ejected large quantities of tuff and occasional accidental blocks of basement rock and basalt, but which produced neither a scoria cone nor lava flows. The tuff ring is now on the present coastline and it forms the highest point (65.5 metres) in the Glendowie area.

The tuff is composed of fragments predominantly derived from the mid-Tertiary grits ("Parnell Grit"), sandstones and siltstones of the 138

Waitemata Group, together with lesser quantities of juvenile basaltic ejectamenta. The latter is typically a vesicular, and rather glassy, olivine basalt that contains numerous small xenolithic inclusions (Searle, 1959).

Accidental blocks of pre-Tertiary age which are incorporated within the tuff include minor (15%) argillites, greywacke-sandstones and volcanic wackes - dominantly epidotized and prehnitized sediments of Chlorite 1 - Chlorite 2 grade (typical of the basement rocks of the Auckland and North Auckland areas). However, the majority of such blocks (85%) were metamorphic rocks that could be assigned to the Albite-epidote- amphibolite facies (Searle, 1959).

The second volcanic centre in the Glendowie area is Taylors Hill (N42/387591). This volcanic centre is believed to be 20,000-50,000 years old and is a more composite cone than the St. Heliers Bay volcano. The original eruption was dominantly explosive with the development of a tuff ring and crater. Later a complex scoria cone was built up and the tuff ring was partially destroyed by the accompanying explosions. One small lava flow rafted away part of the scoria cone but petered out before it was able to escape from the explosion crater. The main lava flow was to the north through a breach in the crater rim (Searle, 1964). The Taylors Hill debris completely blocked a small stream flowing into Glendowie Basin, thus forming a swamp upstream and causing the stream to adopt a new course to the south.

The tuff is very similar in appearance and composition to that of the St. Heliers Bay Volcano and it also contains a few accidental blocks of albite-epidote-amphibolite schists. These two volcanic centres are the only ones in the Auckland area which contain accidental blocks of base• ment schistose material.

PETROGRAPHY

The juvenile basaltic ejectamenta from both Taylors Hill and St. Heliers Bay Volcano is extremely similar petrographically — both being phenocrystic olivine basalts which often contain xenolithic inclus• ions of basement rocks (e.g. section 5269).

Phenocrysts are dominantly of olivine and augite, with relatively minor quantities of zoned and/or twinned plagioclase. Phenocrysts range in size up to 2 mm. with the average size being 0.75-1 mm. Olivine is often very abundant (e.g. 15% in section 5263), and is typically 139 fractured with the fractures lined with secondary chloritic minerals. Occasional olivine phenocrysts show resorption rims with the surrounding glass, and when present the resorbed area shows a high concentration of opaque ore minerals (e.g. section 5203). The olivine has a magnesium-

rich composition (2VX » 78-82°).

Augite is also an abundant phenocrystic mineral (10-20%). It typically occurs as pale-green, non-pleochroic, euhedral phenocrysts up to 1.5 mm. long, occasionally twinned and very rarely zoned. The augite has a 2VZ range of 47-52 °.

Occasional glomeroporphyroblasts of olivine, augite and ore minerals were observed (e.g. section 5191).

Plagioclase phenocrysts are rare and may be twinned and/or zoned — usually only the former. However, plagioclase laths are abundant in the ground mass. The composition of the plagioclase is dominantly labradorite (Michel-Le'vy extinction angle range 27-38°, average » 33°, » 58% An; Slemmon's method gives range 47-68% An, average 59% An).

The groundmass consists of a felsitic mozaic of plagioclase leths, minor blebs of augite and olivine, together with abundant glass and ore minerals. The groundmass of the Taylors Hill lava flow is much less glassy than that of ejected blocks incorporated within the tuffs. It contains more abundant plagioclase laths which frequently show signs of flow band• ing (e.g. section 5263).

Vesicles in some of the basalts arelined with chloritic minerals - probably an alteration product of the glass, although they could be a primary hydrothermal feature (e.g. section 5202). Occasional sections (e.g. 5204) show the development of calcite in the vesicles.

GEOLOGICAL HISTORY

The basement rocks of the St. Heliers Bay-Glendowie area consist of a lowerMesozoic greywacke — argillite suite of Chlorite 1 and Chlorite 2 metamorphic grade, and older, possibly pre-Mesozoic, schistose rocks of albite-epidote-amphibolite grade. These rocks occur as accidental blocks in the tuff of Taylors Hill and St. Heliers Bay Volcano, and the greywacke and argillite fragments are lithologically identical to rocks in the Hunua Ranges, the outlying islands in the Hauraki Gulf, and eastern Northland.

No sediments of Cretaceous to lower Tertiary age are known from 140 the area and the oldest Tertiary sediments exposed belong to the lower-mid Miocene Waitemata Group. The Waitemata sediments are assumed to have been deposited in a moderately deep north-south trend• ing trough which was bounded to the east by a greywacke landmass. and to the west by an andesitic volcanic chain now represented by the Waitakere Ranges. The Waitemata basin of deposition is assumed to have extended south to join the Waikato basin, and north to join the Kaipara basin.

Deposition in the Waitemata basin was fairly rapid and very few fossils are seen. However at some horizons, especially in the laminated siltstones, abundant trace fossils can be recognised. The typical Waitemata sediments are graded sandstones and alternating, laminated, fine sandstones and siltstones. The latter are assumed to be the normal sediments deposited in the area, while the massive and/or graded, coarser sandstones are thought to have been deposited from turbidity currents which flowed into the area from the north or north-east. The presence of abundant plant material, either in definite bands in the sandstones and siltstones, or finely disseminated throughout the siltstones, indicates that a landmass was not too distant.

During the deposition of these intercalated sands and silts, andesitic volcanicity in the west (or possibly east; Fox, 1901) produced abundant debris. When oversaturated with water, these accumulations of andesitic debris flowed to, and entered the basin of Waitemata sedimenta• tion as lahar flows. The submarine emplacement of these thick grit bands caused widespread deformation and slumping in the underlying, unconsol• idated sediments; the attitude of which indicates derivation of the grits from a western source.

Uplift during the Kaikoura Orogeny was accompanied by normal faulting and probably produced the anticlinal warp in the eastern section of the area. Erosion from later Miocene to the end of Tertiary time was followed by resubmergence and deposition of Pleistocene silts.

During one Pleistocene interglacial period of higher sea levels, possibly the last, the area was extensively peneplaned at a height of 35 metres (120 ft.) above present sea level. Subsequent regression has exposed this 35 metre terrace.

At some time during the later Pleistocene — about 50,000 years ago — the St. Heliers Bay volcano explosively erupted forming a tuff ring. Volcanicity from Taylors Hill (20,000-50,000 years ago) produced 141 a large tuff ring over the Pleistocene silts, followed by later breaching of the crater and development of flow basalt - a small tongue of which encroaches into the area being described.

The Flandrian transgression, following the last glaciation, resulted in deposition of marine sands and muds, especially well seen in St. Heliers Bay, as well as Karaka Bay and Glendowie Basin. Post• glacial regression of the sea exposed the extensive 1-2 metre high Flandrian terrace.

The present topography is one of undulating relief with some steep hills and valleys. Recent seas have cut extensive shore platforms and the high coastal cliffs.

ACKNOWLEDGMENTS

To Mr J. Hopkins and Mr C.H. Pharo for providing stratigraphic columns and preliminary information on the area studied and to Mr C.H. Pharo for draughting the map and critically reading the manuscript.

REFERENCES

Ballance, P.F., 1964: The Sedimentology of the Waitemata Group in the Takapuna Section, Auckland. N.Z. Journ. Geol. Geophys., Vol. 7, pp. 466-499.

Brothers, R.N., 1948: The Geology of the North-west Portion of the Waitemata County, Auckland. Unpublished. M.Sc. Thesis, University of Auckland.

Brothers, R.N., 1954: The Relationship of Waitemata Formation and The Manukau Breccia, Auckland, New Zealand. Trans. Roy. Soc. N.Z. Vol. 81. pp. 521-538.

Brown, D.A., 1942: Geology of the West Coast of the Firth of Thames. Trans. Roy. Soc. N.Z. Vol. 72. pp.69-84

Chappell, J., 1963: The Geology of the Musick Point-Bucklands Beach Area. Tane, Vol. 9, pp. 33-39.

Fox, C.E., 1901: The Volcanic Beds of the Waitemata Series. Trans. Roy. Soc. N.Z., Vol. 34, pp. 452-493. 142

Gregory, M.R., 1966: Rocks of the Waitemata Group, Whangaparaoa Peninsula, Northland. Unpublished M.Sc. Thesis, University of Auckland.

Hector, J., 1886: Progress Report by Director. N.Z. Geol. Surv. Rep. 17, 1885-86, p. xxxix.

Kuenen, P.H., 1950: Slump Structures in the Waitemata Beds around Auckland. Trans. Roy. Soc. N.Z., Vol. 78, pp. 467-475.

Mulgan, E.K., 1901: On the Volcanic Grits and Ash-Beds in the Waitemata Series. Trans. Roy. Soc. N.Z., Vol. 34, pp. 414-435.

Park, J., 1886: Waitemata, Eden and Manukau Counties. N.Z. Geol. Surv. Rep. 17, 1885-86, p. 150.

Pettijohn, F.J., 1957: Sedimentary Rocks. 2nd edition. Harper, New York.

Searle, E.J., 1932: The Geology of the Southern Waitakere and Western Auckland Area. Unpublished. M.Sc. Thesis, University of Auckland.

1959: Schistose Rocks from St. Heliers Bay, Auckland. N.Z. Journ. Geol. Geophys., Vol. 2, pp. 368-379.

1964: City of Volcanoes, A Geology of Auckland. Paul's Book Arcade.