T A N E 23, 1977

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

by P .F . Ballance Department of Geology, University of Auckland, Private Bag, Auckland

S U M M A R Y

Motuora Island, Hauraki Gulf, is composed of sedimentary rocks belonging to the Pakiri Formation o f the Waitemata Group (Lower Miocene age, approxi­mately 20 mil l ion years). The strata are virtually horizontal, and include three beds or groups of beds: a) a lower complex Parnell Grit/slump sheet, 20 metres thick; b) a middle homogeneous sandstone, 8m thick; and c) an upper group of interbedded sandstones and mudstones (flysch) forming the cliffs at the south end of the island. The Parnell Grit/slump sheet is exposed around the entire island, and is the most extensive occurrence known of such a bed. Shore platforms carved in Parnell Grit are highly irregular and contain many crevices and small pools. Platforms carved in the thick sandstones are much less irregular, consisting of a flat bench at high tide level, surrounded by a raised rim up to 30cm high holding in extensive shallow pools, and dropping off to low tide level in a steep rock face.

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

Motuora is the southernmost of the cluster o f small islands lying between Mahurangi and Kawau Island in the Hauraki G u l f (Fig. 1). It is part o f the Hauraki Maritime Park, and carries a rich intertidal biota. A n opportunity to examine the geology of the island occurred on 14 July, 1976, through the courtesy of Mr Ben Thorpe, Chief Ranger to the Hauraki Maritime Parks Board, and Dr Robert Wassons, then of the Auckland University Geography Depart­ment. This description is given primarily for biologists visiting the shore platform, although the geology is o f considerable interest to geologists because it includes the most extensive known exposure of a Parnell Grit/slump sheet, as described below.

T H E S T R A T A

Motuora Island is composed entirely of sedimentary strata belonging to the Pakiri Formation of the Waitemata Group (Ballance 1977). These rocks are of Lower Miocene age, approximately 20 mil l ion years old. Similar strata form most of the western coastline of the Hauraki Gulf, from Omana Beach to Pakiri Beach, and are called Waitemata sandstones and mudstones by Morton and Miller (1968, fig. 10). The strata on Motuora have only a very gentle dip to the south-west. Three beds or groups of beds are readily identified, which can be

77

Fig. 1. Clockwise from top left: locality map; section along the island showing the three beds (a), (b) and (c); detail of the contact between (a) and (b) to show one of the mounds of Parnell Grit (see also Fig. 3); cliff and shore platform in the thick sandstone, bed (b); ditto in Parnell Grit, bed (a). Black blocks are lava.

followed around the entire island,

(a) Lowest; Parnell Grit/slump sheet This bed is more than 20m thick - the base is below low tide level — and has a complex internal structure. The background is a very poorly sorted and very coarse grained volcanic breccia-conglomerate, in which pebbles and boulders of dark coloured andesite lava are contained in a matrix of gritty sand. The boulders of lava range up to 60cm in diameter, although the largest common diameter is 20cm, and there is a complete gradation down to sand grain size. They vary in their density of packing: in places the pebbles and boulders are packed closely together, and elsewhere they are widely dispersed in the gritty matrix (see Figs 2 and 3). This background sediment is typical Parnell Gri t , such as can be seen in many localities around the Hauraki Gulf. However, on Motuora Island it contains, in addition, very many included blocks and rafts of light-coloured sandstone and mudstone, ranging in size from a few centimetres up to huge ones 90m x 5m x 15m. Many of the rafts have tongues of Parnell Grit matrix injected into them, sometimes for several metres. The presence of these included rafts is the reason for calling the bed a Parnell Grit/slump sheet; this particular occurrence of Parnell Grit is unique, as far as is known, in its prolific content of slump blocks and rafts. Occasional fossils occur in the matrix.

(b) Middle; a thick and homogeneous bed of sandstone, about 8m thick. This sandstone is coarse-grained, containing occasional grains up to 5mm in diameter and large quantities of fossil bryozoan remains. The contact on the underlying Parnell Grit is irregular, and covers mounds of Grit that are up to 5m high and 25m long (Fig. 3).

(c) Highest; interbedded sandstones and mudstones ('flysch'). These are arranged in packets of thin-bedded flysch up to 13m thick, in which individual beds of sandstone and mudstone average about 30cm in thickness, separated by thicker beds of sandstone ranging from 1.5 to 4m in thickness. This unit of flysch is confined to the cliffs at the south-west end of the island, and a maximum thickness of about 33m is present. It does not form any part of the intertidal platform, which is composed entirely of the two underlying beds.

O R I G I N O F T H E S T R A T A

The Waitemata Group, to which the Motuora strata belong, outcrops extensively around Auckland and southern Northland. The strata are interpreted as having been deposited 20 mill ion years ago in a deep marine basin, probably more than 2km deep, which was confined between an eastern ridge, located where Coromandel Peninsula and Great Barrier Island are now, and a western chain of island-arc volcanoes, the Waitakere Volcanic Arc , where the Waitakere Ranges and Kaipara Harbour are now (Ballance 1974). The sandstones were

79

Fig. 2. Shore platform in Parnell Grit /slump sheet Hammer is resting against a boulder of lava. A t the left hand rear is a concentration of lava pebbles. R = raft of sandstone -mudstone. Note the irregular surface and many small pools.

Fig . 3. The contact between Parnell Grit/slump sheet (bed (a) and the thick sandstone (bed (b) , showing one of the mounds on the top surface of the Parnell Gr i t . The shore platform, and the cliff wall under the overhang, are in Parnell Gr i t ; dark coloured blocks are lava, light coloured blocks - the majority - are sandstone/mudstone. Cl i f f at left, and roof of overhang, are in the thick sandstone, showing a prominent white vein of calcite. Pack and map case are for scale.

SO

carried down into the basin from the north-west by a type of bottom-hugging, sediment-carrying density current known as a turbidity current. Each sandstone bed is the deposit, virtually instantaneous, of one turbidity current. The intervening mudstone beds represent the normal slow accumulation of marine muds by settlement from sea water.

The Parnell Grit originated as a volcanic mud flow on the flanks of the Waitakere Volcanic A r c , and flowed eastwards into the deep basin. In doing so it passed over an area of unstable sediments, causing them to slump and slide, so that the resulting deposit, now seen on Motuora Island, is a mixed volcanic mud flow (Parnell Grit) and slump sheet. The irregular upper surface of this sheet was subsequently buried by the overlying thick sandstone (Fig. 3), and the sea bed restored to a plane surface.

The Waitemata marine basin was everted during important earth movements, the Kaikoura Orgeny, approximately 15 mill ion years ago. Since then the strata have been subjected to erosion.

N A T U R E O F T H E S H O R E P L A T F O R M

The shore platform of the entire island is carved in either the Parnell Grit or the overlying thick sandstone (beds (a) and (b)). There is a very slight tilt of the strata to the south-west, as a result of which the lower of the two beds, the Parnell Grit/slump sheet, forms the shore platform of the whole central and north-eastern part o f the island, and the overlying sandstone occurs there only in the cliff. In contrast, at the south-western end of the island the beds are at a slightly lower elevation, and the shore platform is carved partly in Parnell Grit and partly in the thick sandstone. The distinction is important, because the two beds react quite differently to intertidal erosive forces. Being very varied in its interna] composition, the Parnell Grit/slump sheet erodes to form an irregular platform, with many small crevices and small pools, which is somewhat rounded in overall form and which presents no large vertical faces (Figs. 1, 2 and 3).

The overlying sandstone, by contrast, is very homogeneous, and characteris­tically erodes to form an almost flat bench at high tide level (Fig. 4). Extensive shallow pools on the bench are surrounded by sandstone rims up to 30cm high, while the bench as a whole falls away in steep faces two or more metres high to an irregular, boulder-covered low tide region. The main crevices in the sandstone platforms occur along vertical joint cracks.

The erosive mechanisms acting on intertidal platforms in Waitemata Group rocks were discussed by Healy (1968).

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

The descriptions given here for Motuora Island can be applied to many shores in the Hauraki Gul f where the Waitemata Group outcrops. Note, however, that most o f the cliffs and platforms are carved in the flysch facies of the Group (bed (c)), which does not occur in the platform on Motuora Island. Morton and

81

Fig. 4. Cl i f f and shore platform in a thick sandstone, similar to bed (b). This particular platform is on the nearby Mahurangi Peninsula.

Miller's (1968) diagram (their fig. 10) of a typical Waitemata cl iff and platform applies in such cases. Note also that thick sandstones like bed (b) on Motuora only occur to the north of Waiwera.

A C K N O W L E D G E M E N T S

The paper was reviewed by Dr G.W. Gibson. 1 am grateful to the Hauraki Maritime Parks Board for the opportunity to visit the Island.

R E F E R E N C E S

Ballance, P .F . 1974: A n inter-arc flysch basin in northern New Zealand: Waitemata Group (Upper Oligocene to Lower Miocene). Journal of Geology 82: 439-71.

Ballance, P.P. 1977: Stratigraphy and bibliography of the Waitemata Group of Auckland. N.Z, Journal of Geology and Geophysics 19 (6): 897-932.

Healy, T . R . 1968: Bioerosion on shore platforms developed in the Waitemata Formation, Auckland. Earth Science Journal 2: 26-37.

Morton, J .E . & Miller, M . C . 1968: "The New Zealand Sea Shore." Collins, London -Auckland, 638pp.

82