The Coastal Geology of the Ihumateo [Ihumatao] Area, Auckland The... · 2013. 11. 3. · Journal of Geology and Geophysics 2(5): 870-888. Searle, E.J 1959b. : Pleistocen and Recen

T A N E 23, 1977

T H E C O A S T A L G E O L O G Y O F T H E I H U M A T A O A R E A , A U C K L A N D

by B . D . Ricketts Department of Geology, University of Auckland, Private Bag, Auckland

(Present address: Geology Department, Carleton University, Ottawa, Canada K15 5BC)

S U M M A R Y

The Whau Formation (Pleistocene) in the Ihumatao area, Manakau Harbour, contains sedimentary structures indicating a shallow water depositional environ-ment that may have been under the influence of a weak tidal regime. Paleocurrents were predominantly towards the west or southwest. These deposits are unconformably overlain by a Kauri-bearing peat, which in turn has been covered by tuffs derived from the Ihumatao eruption centre (Late Pleistocene). The Okahu Formation (Holocene) in this area is represented by fossiliferous beach sands deposited on a shore platform eroded into the tuffs.

I N T R O D U C T I O N

Pleistocene and Holocene deposits are exposed in the cliffs and shore platform between Ihumatao and Auckland International Airport (Fig. 1). Volcanoes and lava flows of this area have been described by Searle (1959a) and wil l not be considered further. The nature of the sediments has been summarised by Fir th (1928) while formations belonging to the Tauranga Group have been mapped by Kermode (1966).

Deposits o f two of these formations are exposed along this section and they are described below.

P L E I S T O C E N E DEPOSITS

Whau Formation: Deposits o f the Whau Formation in this area underlie a 14m to 20m terrace. A typical exposure is illustrated in Fig . 2a. The strata can be divided into two types:

(i) The lowest 2*m thick is characterised by the presence of lenticular and wavy bedding. Lenses of sand and sandy mud are generally less than 2 to 3cm thick the tops of which often show ripple development. Both tabular and trough cross-bedding are present but scour and fil l structures are uncommon. Small mud intraclasts are also found.

The intervening muds and sandy muds are finely laminated, commonly with very small scale cross-bedding 2 to 10mm thick.

Paleocurrent orientations have been calculated from apparent cross-bedding dip directions and are illustrated in Fig. 3. Although only 26 measurements were

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Manukau

Harbour

2 Km

Scale

Auckland International Airport

K e y : v Basalt Flows

A Scoria

t Tuff

Ash

Whau Formation

Fossil Forest

Fig. 1. Coastal exposure adjacent to Ihumatao showing localities of cliff sections illus-trated in Fig. 2 [map after Searle (1959a) and Kermode (1966)].

made there appears to be a relatively distinct cluster of values for coarse grained lenses wi th flow towards the west to south-west indicated. Cross-bedding in the laminated muds however indicates a much wider spread in current directions.

Fine carbonaceous material is scattered throughout these sediments and often occurs in local concentrations in the laminated muds. Trace fossils are also common and three main types are recognised; short simple burrows and trails up to 10cm in length and a distinctly larger group of burrows up to 25cm in length and 2 to 2.5cm across. The last type exhibit internal concave-up surfaces and possibly represent 'escape' routes.

In this unit there is a marked lateral change from west to east with the ratio of laminated muds to intervening sand lenses increasing and the number of large burrows decreasing. The change from a relatively high energy to a low energy environment is indicated.

(ii) The upper part o f this sequence (Fig. 2a) is extremely bioturbated with

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Tuff 5m

Section C

•y- a Unconformity

Okahu Peat

1m

Intensely Bioturbated Unit

Whau Fm

Lenticular Bedding

' - \ 1 _ \ —

75m

s " 4 '

" * " -MHW

2m

1.5%

Section B Section A

Fig. 2. Typical cliff sections exposed along the portion of the coastline studied.

primary bedding almost completely destroyed. Pumice granules are scattered throughout and carbonaceous material is common. The nature of this unit remains virtually unchanged along the section studied although towards the east horizontal bedding becomes more distinct.

One local variation in this sequence occurs where the lower lenticular bedding unit is replaced by well sorted, medium grey-green sands. Occasional sets of trough cross-bedding up to 10cm thick can be seen and current directions similar to those above are indicated. Mud intraclasts are also present. Overlying these sands are three horizons, each grading upwards from sub-horizontally bedded sandy muds with moderate bioturbation, to extremely bioturbated sediments.

Interpretation: Lenticular bedding is commonly developed in marine environ-ments as a result of tidal flow, usually with bimodal current directions. In these sediments no distinctly bimodal distribution is seen although waxing and waning of currents is apparent. Similar features have been described from non-tidal environments for example mudflat or lacustrine environments associated with deltas (Coleman & Gagliano 1965; DeRaaf & Boersma 1971). A similar situation where tidal influence is weak is interpreted for those deposits.

Searle (1959b) has suggested that Whau sediments were deposited during the marine transgression that lead to the formation of the 35 to 40 metre terrace that is well developed in the Auckland area. The sediments are thus Terangian in age.

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60 50 40 3< D 20 1 0 % ^ Number Percent Plotted at 20 Intervals

N

Lenticular Sands

N i

^> Muds & w \ Sandy Muds Fig. 3. Paleocurrent directions from the lower lenticular bedded unit of the Whau Formation.

The Ihumatao Fossil Forest A peat which contains remains of a fossil Kaur i forest unconformably overlies

Whau sediments. It is well exposed in the shore platform immediately south of Ihumatao. Radiocarbon dating of this bed gives an age of 29 000 ± 1 500 years BP (Ferguson & Rafter 1959), similar to that obtained for timber-packed beds at Tamaki Estuary (Searle 1959a).

Tuffs (Late Pleistocene) Tuffs derived from Ihumatao eruption centre are well exposed along this

section of the Manukau coastline. The Kaur i forest appears to have been buried by these air-fall deposits and hence the age of 29 000 ± 1 500 years BP pin-points the time of eruption (Searle 1959a).

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Bedding in the tuffs is moderately well defined and in general has a shallow dip away from the eruption centre. However at one locality dips of up to 20 were recorded (Fig. 1). Bed thickness varies from about 5 to 100 millimetres. The dominant clast lithology is olivine basalt ranging in size from sand to cobbles, set in a silty yellow-brown matrix. A variety of 'country rock' fragments are also present for example grits, sandstone and limestone.

The tuffs are crudely graded usually becoming finer towards the top of the sequence. There is also a lateral decrease in overall clast size with increasing distance from Ihumatao. Drape structures are not uncommon, especially where tree stumps protrude into the base of the tuffs. Also near the base leaf impressions can be found, for example cf. Dacrydium cupressinum.

Minor deformation is present in the form of small conjugate faults. These only occur in close proximity to the overlying lava flows and are inferred to be a result of this localised overburden.

H O L O C E N E DEPOSITS

Okahu Formation: These deposits are found approximately 400m south of the beach access road and form part of a terrace that is 2 metres above the mean high water level. The contact with the underlying tuffs is erosional and at this locality it has a shallow dip to the southwest (Fig. 2b). Bedding dips 2° to 3° in a similar direction.

The base of this deposit consists of dark brown moderately cemented sandy grits with thin crudely graded lenses of rounded cobbles. The long axes of the clasts are often parallel to bedding. As with the tuffs, basalt is the dominant lithology while 'country rock' is uncommon. Iron stained quartz grains plus rock fragments predominate in the finer fraction. Limonitised bivalve casts are also scattered throughout. Immediately overlying the pebble horizon are medium to coarse grained sandstones with abundant limonitised bivalve casts. A marked lateral change in fossil preservation occurs in this horizon where sandstone completely leached of skeletal carbonate gives way to coarse shelly, moderately cemented sandstones. Most of these shells however exhibit carbonate leaching to some degree. The molluscs, mostly bivalves, are tightly packed and are generally parallel to bedding. Chione stutchburyi is abundant. Other forms include:

Amalda sp. indet. Cominella glandiformis Cellana sp. Melagraphia aethiops Melarapha oliveri Macomona liliana Paphies (Paphies) australis Xymene plebejus Zeacumantus lutulentus Zearcopagia disculus

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These deposits are thought to represent a beach environment not unlike that seen around parts of Manukau Harbour at the present time, where pebble horizons occur in the low to mid-tide zone and shell accumulations at the high tide level. Deposition probably occured as a result of the Post-Glacial rise in sea level.

R E F E R E N C E S

Coleman, J . M . & Gagliano, S .M. 1965: Sedimentary structures: Mississippi River Deltaic Plain. In: G . V . Middleton, (Ed.) Primary Sedimentary Structures and their Hydrodynamic Interpretation. Society of Economic Paleontologists and Mineralogists Special Publication 12: 133-148.

DeRaaf, J . F . M . & Boersma, J .R. 1971: Tidal deposits and their sedimentary structures (seen examples from Western Europe). Geologic en mijnbouw 50(3): 479-504.

Fergusson, G . J . & Rafter, T . A . 1959: New Zealand C 4 Age Measurements-4. New Zealand Journal of Geology and Geophysics 2: 208-241.

Fir th , C.W. 1928: The Geology o f the Northwest Portion of Manukau County. Unpublished Thesis, Geology Dept University of Auckland.

Kermode, L . O . 1966: Geological Map of New Zealand, 1:25,000, Sheet N42/8 , Mangere (1st Ed.). Department of Scientific and Industrial Research, Wellington.

Searle, E . J . 1959a: The Volcanoes of Ihumatao and Mangere, Auckland. New Zealand Journal of Geology and Geophysics 2(5): 870-888.

Searle, E . J . 1959b: Pleistocene and Recent Studies of Waitemata Harbour. Part 2: North Shore and Shoal Bay. New Zealand Journal of Geology and Geophysics 2(1): 95-107.

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