Tane 11971) 17:181-195 181

PLIOCENE AND OUATERNARY SEDIMENTS FROM WEYMOUTH,

A U C K L A N D .

by P.R. Moore* and R.J. McKelvey* with appendix by D.C. Mildenhallf

•SUMMARY

Pleistocene pumiceous sediments of the Manukau Lowlands cover the remnants of a once more extensive Pliocene sequence. At Weymouth, previously unrecorded Upper Pliocene shallow-water, fossiliferous marine sands are found preserved in erosion hollows in Lower Miocene Waitemata sandstones. Pollen from Pliocene plant beds record a dominantly beech forest (with Nothofagus matauraensis as the main species), which existed in a probable near-coastal envir­onment, and warm to temperate conditions. A Waitotaran age for the beds is suggested from the pollen species present.

A record of fluctuating sea levels during the Ice Ages is preserved in the Quarternary sediments of the region, with a prominant terrace level at 9-18 m. (30-60 ft.) above MSL.

INTRODUCTION

The settlement of Weymouth is located on the south-eastern shores of the Manukau Harbour, approximately 15 miles from central Auckland city. Palmers Beach (grid. ref. N47/376378), where the present study was confined, lies one mile to the north of Weymouth.

The earliest description of the region was given by Hochstetter (1864), who described lignites and Quaternary sands and pumice slits outcropping along the low-lying shores of the Manukau Harbour. His "Lignite Formation" may also have included Pliocene plant beds, which are described in this paper.

A Pliocene fauna has been recorded from drillholes in the South Manukau area by Schofield (1958). Firth (1930), and Day (1948), have described the geology of the Manukau Lowlands.

GEOMORPHOLOGY

The southern and eastern shores of the Manukau Harbour are marked by a prominent terrace level - the "40-60ft." terrace. In the Weymouth area, the seaward edge of this terrace is approximately 9-11 m. above MSL, rising to 18m. inland. Preserved in many of the bays around the harbour is a further terrace level, generally about 2.3m. (7-8 ft.) above MSL, representing highest sea level during the post-glacial Flandrian transgression.

* Department of Geology, University of Auckland. f New Zealand Geological Survey, D.S.I.R., Lower Hutt.

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FIG. 1 Geological map of Palmers Beach, Weymouth.

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At Palmers Beach, an abandoned cliff line between the two terrace levels exposes a former valley carved in Waitemata sediments, and infilled by Pliocene sands (fig. 2). This "infilled valley" apparently extends inland for some distance, and locally, is a valuable water reservoir. A boreholeofindicated on Fig. 1) situated on the 9-18m. terrace, failed to penetrate Waitemata sediments at a depth of approximately 13m.

STRUCTURE

Waitemata sediments appear to dip generally to the west at 2° to 5° . Locally, however, dips and strikes vary considerably, especially in the vicinity of the "Parnell Grit", although in some places the grit beds are entirely concord­ant with Waitemata sandstones. Small-scale folding of Waitemata sandstones is commonly associated with the "Parnell Grits".

Overlying Waitemata rocks, often with a very marked unconformity (fig. 4), Pliocene sediments appear to lie roughly horizontal, although further to the south (fig. 6), plant beds dip at 6° to 7° to the north-west. This suggests a possible localized renewal of earth movements in post-Pliocene, pre-? Upper Pleistocene times.

Pleistocene sediments of the 9-18m. terrace truncate both Waitemata and Pliocene beds at between 6m. and 7m. above MSL.

STRATIGRAPHY Waitemata Group.

Lower Miocene sediments of the Waitemata Group can be divided into two distinct units:

(a) Massive, or thick bedded sandstones - the Waitemata sandstones

(b) "Parnell Grit".

(a) Waitemata Sandstones Typical sandstones are grey, poorly indurated, non-calcareous, and medium

to coarse grained. Grains generally consist of angular to sub-rounded quartz, feld­spar, and rock fragments in a fine-grained matrix, and without detailed petro-graphic analysis, the rock can be roughly classed as a litharenite (Folk, Andrews, and Lewis, 1970). In places these beds grade upwards into fine sandstones or siltstones, but siltstone bands are uncommon, and often very thin.

Sandstone beds range from a few cm. to 2-3m. thick, with many showing fine laminations or cross-bedding. Thicker beds tend to be well graded.

(b) "Parnell Grit" The "Parnell Grits" have been more fully described by Jones (1967), and

in the Weymouth area by Firth (1930). Correlation of these beds with the type Parnell Grit at Parnell Point (Brothers, 1959), is uncertain, and the term is used here only for convenience.

Typical grits contain numerous sub-rounded or angular fragments of

184

M S L

sands tone , "Parne l l Gr it" and andes i te bou lde rs

P LE I STOCENE P L I O C E N E

shell

shell (D

18S* Pi

o 2 feet

M I O C E N E

Sands , pumice"silts"

Sandstone

P lant beds

Wa i t ema t a sandstone

FIG. 2 Stratigraphic columns. See Fig. 1 for locations.

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andesite, up to 1 cm. in size, and sub-rounded siltstone inclusions, in a sandy matrix. The sand matrix, of fine to medium grain size, often contains fresh augite and feldspar crystals. Sandstone breccias, with blocks up to 3-5 m. diameter, are common on the southern shore platform, and rounded andesite boulders, weathered from the grits, were measured up to nearly 2 m. in diameter.

Waitemata sandstones and "Parnell Grits" were not examined for micro-fauna, but a Lower Miocene age for these rocks is assumed from the scarce fauna collected from other localities in the Auckland area (Grant-Mackie, 1965).

Pliocene sediments.

These can be divided into two members:

(a) A grey carbonaceous sandstone, here referred to as the "plant beds", and

(b) A yellow-brown sandstone.

The main exposure of these beds occurs in the central part of the beach, with a small pocket preserved to the north (fig. 4). To the south, sandstone beds appear to be quite extensive, although obscured by recent muds.

fa) "Plant beds". These are well compacted, light grey, non-calcareous siltstones or very

fine sandstones, often with abundant lenses and infilled burrows of poorly sorted medium sand. The beds contain abundant plant material, with well preserved leaves, fruits and seeds (fig. 3), and occasional remains of trees. The leaves and fruits are as yet unidentified, but most of the fruits appear proteaceous and not unlike the native Knightia or Persoonia (D.C. Mildenhall, pers, comm.) Type 'a' (fig. 3) resembles the introduced species of Hakea.

Occasional casts of the bivalves Modiolaria and Venericardia were also noted.

Pollen and spores from the "plant beds" (see Appendix) seem to indicate a Waitotaran or at least uppermost Pliocene age. Dating of similar beds described by Day (1948) from Kidd's and Clark's beaches on the southern shores of the Manukau Harbour may reveal a similar age.

(b) Sandstone beds. These are moderately sorted, grey to yellowish medium or fine sandstones.

Green sandstone exposed by digging under the 2.3 m. terrace rapidly turns yellow-brown on exposure to air. Beds show considerable lateral and vertical variation, and are strongly cross-bedded. Occasional small sub-angular granules of jasper and rounded argillite were also noted.

Thin and irregular shell beds contain a poorly preserved fauna, with Glycymeris being the most common species present. Tire following macrofauna was identified by Mr J.A. Grant-Mackie:

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T y p e d

FIG. 3 Pliocene fruits and seeds. (Natural size.)

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?Crassostrea ingens Ww ?Eucrassatella sp. Mactra c.f. discors ?Wo-R Glycymeris sp. Chione sp. ?Venericardia aff. purpurata Ww-R Amphidesma (Paphies) c.f. australe Amphidesma c.f. subtriangulatum Maoricrypta sp. Sigapatella c.f. novaezelandiae Naticidae ?Tawera duobrunnea Ww ^Balanus sp. ?Notopaphia sp. ?Bankia sp. Den tahum solidum ?Fellaster sp. c.f. (Potamididae or Terebridae) Perna or Aulacomya (internal cast) Modiolaria c.f. impacta (external cast) Ww-R Ostrea c.f. sinuata (internal cast)

FIG. 4 Pliocene sandstone, and unconformity with Waitemata sediments, north end, Palmers Beach. Sketched from photo. P - Pliocene sandstone. W- Waitemata sandstone.

2

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CUMULATIVE WEIGHT PERCENT (0

_»r\> OI 00 ID (0 <D <P -* m o o o o o oi to <p I 1—I I 1 ¥ 1—T 1 IT-

CUMULATIVE WEIGHT PERCENT 0 - . t o oi go to to to <p -g -» m o o o o o oi ID <b 1 i 1 — i — i 1 - 1 — i — i 1 r

FIG. 5 Cumulative grain-size graphs of Pliocene sandstone.

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The fauna would seem to indicate a Waitotaran (Ww) age, and suggests correlation with the Kaawa beds of Port Waikato (Laws 1936, 1940). Pliocene beds from Otahuhu (Marwick 1948, Laws 1950) are apparently older, and are dated as Lower Pliocene.

Grain size analyses of four sand samples were made, and the results plotted as cumulative curves on probability paper (fig. 5). Calculation of parameters (Folk, 1968) showed the curves to be fine skewed, and mesokurtic to leptokurtic. According to Visher (1969), the form of the curves would seem to indicate a deltaic or river channel depositional environment. The strong cross-bedding, marine shell beds, and location of the standstone beds in erosion hollows, suggest a tidal river mouth as the most likely environment. A near-shore environment is also suggested by the size of boulders marking the unconformity with Waitemata sediments. Numerous rounded boulders of Waitemata sandstone and "Parnell Grit" up to 1 m. diameter, and rounded andesite boulders up to 0.4 m. diameter were recorded on this unconformity.

Confined to the main outcrop of sandstone in the centre of the beach, and largely beneath the 2.3 m terrace, is a small deposit of opalised wood. Petrified logs up to 10 m. in length were recorded, and although none were immediately identifiable, a number of different varieties were found. Many of the logs have been reworked, so that they are commonly found in the pebble layer above (Holocene). A few specimens show evidence of marine boring molluscs, c.f. Bankia. Opalised seeds, identical to those found in the "plant beds" were also discovered.

Associated with the wood are large lenticular masses of blue to yellow common opal, also commonly found in the overlying pebble bed.

Groundwater at this locality has a peculiar greasy feel, and occasional gelatin­ous masses of this "greasy" material have been found in both Pliocene sandstones and overlying Holocene sands.

Chemical analysis of this gel (Mr T.H. Wilson), showed it to be a clay mineral, and values of 0.2% silica and 3.8% alumina indicate montmorillonite as being the chief mineral present. X-ray diffraction of an unorientated powder sample also showed montmorillonite to be the major constituent.

The high percentage of silica in the gel would appear to be connected with opalisation of the wood. Further detailed work on this deposit may also reveal a connection between opalisation and the montmorillonite gel.

The exact time of opalisation is unknown, but is almost certainly post-Waitotaran, and definitely pre-Holocene. However, if opalisation had occurred before advance of the Pleistocene seas, then the high silica values may have provided a source of attraction for the siliceous sponges whose remains are now found in overlying Pleistocene calcareous silts.

Silicification resulting from breakdown of immediately overlying pumiceous silts seems unlikely because of the extremely localised nature of the petrified wood deposit, and silica was probably carried by groundwater from a more distance source. No evidence of hydrothermal activity was found in the immed­iate vicinity.

The present distribution of Pliocene sediments in this area appears to be

190

confined to erosion hollows, probably former stream valleys, in Waitemata strata. However, these beds were likely to have been far more extensive before erosion and formation of the 9-18 m. terrace, as shown by drillholes in the South Manukau region (Schofield, 1958).

Unsilicified logs up to 10 m. in length, and lying in Pliocene sandstones to the south, appear to be in situ. Similar logs on the northern side of the southern shore platform however, appear to lie in "Parnell Grits", and may thus be much older.

Pleistocene sediments.

Pumiceous silts and sands underlying the 9-18 m. terrace are typical of Pleistocene sediments of the Manukau Lowlands.

Exposed in the scarp behind the beach, and approximately 7 m. above MSL, is a 0.3-0.6 m. band of well-weathered pebbly grit, marking the unconformity with Waitemata sediments (fig. 2). The majority of the grit consists of soft, sub-angular to rounded granules of siltstone and sandstone in a matrix of silty, medium sand. In places, the band contains numerous rounded or angular

DETAIL A

East

mln. 1 ft.

West

min. 1 ft

1 ft.

Bored erosion sur-face

Loose,grey pebbly sand.

Iron-stained pebbly sand.

> HOLOCENE

PLEISTOCENE

Carbonaceous sandstone with bivalve casts, and fossil fruits.

Carbonaceous pebbly muddy sandstone. Pebbles of Waitemata sandstone and argillite. Waitemata sandstone.

> 8 m z m

FIG. 6 Detail of Pliocene beds and unconformity.

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siltstone and sandstone inclusions up to 8 cm. in diameter, and occasional well weathered andesite pebbles, 15-20 cm. in diameter.

This layer grades upwards into soft brown fine to medium sands, which in turn are overlain by pumiceous sandy silts.

Overlying the Pliocene sands at the base of the scarp, and probably representing slumped material, is a 1 m. thick band of calcareous silty sand. The fine sand fraction contained abundant sub-angular quartz grains, with shell fragments, magnetite, and minor volcanic glass. Opaline sponge spicules, up to 1 mm. in length were fairly common, and protriene, plagiotriene, oxeas, and dichotriene forms were identified. Spicules belong to the order Choristida, and sponges of this group are presently found off the New Zealand coast from inter-tidal to 1400 m. depth. (Assoc. Prof. P.R. Bergquist pers. comm.).

This bed is obviously the source of identical spicules found in overlying Holocene sands of the 2.3 m. terrace. Underlying Pliocene sands were not found to contain spicules.

Holocene (Recent) sediments.

Sediments underlying the 23 m. terrace are loose grey-brown sands, and basal pebble band. (See Table 1).

The sands, medium to coarse grained and somewhat silty, contain abundant sub-angular to sub-rounded quartz grains, with volcanic and sedimentary rock fragments, and minor feldspar, shell fragments, and sponge spicules. Quartz grains are fresh, with abundant vacuoles, and suggest a volcanic origin.

"Hard" rock types, such as jasper, chert and quartz were generally sub-angular, while greywacke and argillite pebbles tended to be sub-rounded to rounded. This factor, together with the poor sorting of the pebble fraction, and

TABLE 1: Analysis of basal pebble bed.

Rock type Percentage occurrence

red-brown jasper 34%

chert 15%

quartz 13%

greywacke and argillite 13%

volcanic f 10%

opalised wood 7%

other * 8%

* "Other" included such types as chalcedony, silicified wood, common opal, and Waitemata sandstone.

t Volcanic types were largely andesites, with minor andesitic breccia and pumice.

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lensoid distribution, seems to indicate a fluviatile deposit, and the Waikato River would appear to be the most likely transporting agent.

Similar pebble bands have been recorded in the South Manukau (Day, 1948), and Mangatangi - Mangatawhiri areas (Lyons, 1932).

GEOLOGICAL HISTORY

Deposition of Waitemata sandstones continued through much of the Lower Miocene. Material was deposited rapidly, possibly by turbidity currents (Ballance, 1964), or bottom traction currents, and added abundant sediment to a moderately deep basin, which covered much of the Auckland area. Volcanic activity during this period gave rise to submarine debris flows or lahars which disrupted the unconsolidated sandstone beds. Firth (1930) suggests a source not far to the south for the "Parnell Grits" in the Weymouth area.

Uplift and erosion followed. Towards the end of the Pliocene, transgression resulted in the deposition of shallow-water marine sands, covering the former topography. This may Well have been the last warm period before the onset of glaciation. Sea level fluctuated during glacial and inter-glacial periods, resulting in removal of much of the Pliocene cover.

During the Pleistocene, pumiceous material was carried, either by wind, or by the Waikato River or sea-currents, from the Central Volcanic region lying to the south, and was widely distributed over the Auckalnd region. A period of standstill, possibly during the Main Monastirian (Brothers, 1954), resulted in the carving of a 9-18 m. terrace in these sediments. Following the last glaciation fluviatile Holocene sediments were deposited in the Manukau basin, and a "2-3 m." terrace formed at the point of maximum rise in sea-level during the post-glacial Flandrian transgression.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. P.F. Ballance for his advice, and helpful comments on the script; Mr J.A. Grant-Mackie for identification of the fauna; Assoc. Prof. P.R. Bergquist for help with identification of sponge spicules; Mr T. H. Wilson for permission to publish the chemical analysis; and Mr Ed. Gibbons for his interest and assistance during the study. A special thanks to Mr D.C. Mildenhall for his valuable contribution to this paper.

REFERENCES

B A L L A N C E , P.F. 1964 Ihe Sedimentology of the Waitemata Group in the Takapuna Section, Auckland. N.Z. Jour. Geol. & Geophys. 7 (3); 466-499.

BROTHFRS, R.N. 1954 The relative Pleistocene chronology of the South Kaipara district, N.Z. Trans. Roy. Soc. N.Z. 82: 677-694

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1959

DAY, J. 1948

FIRTH, C.W. 1930

FOLK, R.L. 1968

FOLK, R.L., 1970 ANDREWS, P.B., LEWIS, D.W.

GRANT-MACKIE, J.A. 1965

HOCHSTETTER, F. von 1864

JONES, B.G. 1967

LAWS,C.R. 1936

1940

1950

LYONS, R.R. 1932

MARWICK. J. 1948

SCHOFIELD, J.C. 1958

VISHER.G.S. 1969

"Parnell Grit" in Fleming, C.A. (Ed.) Lexique Stratigraphique International Vol. VI, Fascicule 4.

Geology of the Lower Waikato-Manukau area, Franklin County, Auckland. M.Sc. Thesis, University of Auckland.

The geology of the NW portion of Manukau County, Auckland. Trans. Roy. Soc. N.Z. 61: 85-137. Also M.Sc. Thesis, University of Auckland.

Petrology of Sedimentary Rocks (Hemphill). 170 pp.

Detrital sedimentary rock classification and nomenclature for use in N.Z. N.Z. Jour. Geol. & Geophys. 13 (4); 937-68.

"Ancient Life of the Auckland area" in Kermode, L.O. (Ed.) Science in Auckland 11th N.Z. Science Congress.

Geologie von Neu-Seeland. Translated by C.A. Fleming, 1959. 320 pp.

An outline of the geology of the St. Heliers Bay-Glendowia area, Auckland, N.Z. Tane 13; 119-142.

The Waitotaran faunule at Kaawa Creek, parts 1 and 2. Trans. Roy. Soc. N.Z. 66 (1): 38-59; 66 (2): 99-124.

The Waitotaran faunule at Kaawa Creek, part 3. Ibid. 69(4): 427-447.

Additional Lower Pliocene mollusca from Otahuhu, Auckland. N.Z. Geol. Surv. Pal. Bull. 17.

Notes on the geology of the Mangatangi-Mangatawhiri district, Auckland, N.Z. N.Z. Jour. Sci. & Tech. 13; 268-277.

Lower Pliocene mollusca from Otahuhu, Auckland. N.Z. Geol. Surv. Pal. Bull. 16.

Pliocene shell beds south of the Manukau Harbour. N.Z. Jour. Geol. & Geophys. 1(2); 247-255.

Grain size distributions and depositional processes. Jour. Sed. Pet. 39, 3; 1074-1106.

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APPENDIX

POLLEN AND SPORES FROM PALMERS BEACH, A U C K L A N D

by D.C. Mildcnhall

One sample from the "plant beds" was examined for pollen and spores. The resulting count was made on the basis of 100 grains of the dominant tree pollen type (Nothofagus matauraensis). Other species found after Ihe count were Compositae (tubuliflorae), Gramineae, Cyperaceae, and Proteacidites c.f. P. minimus Coupe r.

TABLE 2: Pollen and Spore Count. Reference: Sheet Fossil No. N47/619 - Slide L5803

POLLF.N & SPORF. T Y P E C O U N T

SPORES Lycopodium billardicri group 1

Cyathea spp. 10

Phymatodes diversifolium (Willd.) Pic.-Serm 4

Dicksonia squarrosa (Forst. f.) Swartz 2

unidentified monolcte spores 2

P O L L E N

Dacrydium cupressinum Lamb. 2

Podocarpus sp. 1

Nothofagus fusca group 17

N. matauraensis Couper* 100

N. longispina Couper* (Waitotaran index fossil) 1

N. spinosus Couper* 2

N. Menziesii (Hook, f.) Oerst 2

Triorites harrisii Couper* 1

Cranwellia sp. indct. 1

Metrosideros sp. 1

Eugenia sp. 1

cf. Avicennia sp. 2

Ascarina sp. 3

Araliaceao 1

total count 152

extinct pollen types

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The flora was poor in species but the presence of the extinct Nothofagus "brassi" group of beeches (N. matauraensis, N. longispina and A7, spinosus) as the dominant tree species indicates that the sediments are no younger than Waitotaran. The "brassi" group are never dominant in post-Waitotaran samples. Eugenia first appears in the Waitotaran and its presence seems to suggest a Waitotaran age for the sample.

The beeches indicate temperature conditions as warm or slightly warmer than the present day at the same latitude. The uncertain presence of Avicennia would indicate a coastal or near coastal depositional environment.