Geology and fossils of Chatham Island, New Zealand

Paul D Taylor

The Chatham Islands are a far-flung outpost of New Zealand (Fig. 1). This isolated archipelago sits in the Pacific Ocean, some 850km east of Christchurch, and lies very close to the International Date Line, making the Chatham Islands one of the first places in the world to see the dawning of each new day. Few New Zealanders have ever visited the Chatham Islands – most know of them only from the country’s national weather forecast.

Just over 700 people inhabit the Chathams. The great majority live on the main Chatham Island, with about 30 on the smaller Pitt Island. The islands were first visited by Europeans in 1791, who arrived aboard HMS Chatham, captained by Lieutenant William Robert Broughton. At that time, they were inhabited by a peace-loving people called the Moriori, who had probably colonised them from the New Zealand mainland about 500 years ago. European visitors, including whalers, were later joined by Māori from the North Island of New Zealand, setting in motion a series of tragic events that culminated in the extinction of the native Moriori. The last full-blood Moriori – Tommy Solomon – died in 1933.

I’ve had the good fortune to visit the Chatham Islands on three occasions, each time to study and collect Cretaceous and Cenozoic bryozoans under the expert guidance of Hamish Campbell, the leading authority on the geology of the Chathams. This brief article focuses on the rocks and fossils I saw during my most recent visit to Chatham Island in February 2023.

Geologically, the Chatham Islands are important because of their position near the end of the Chatham Rise, an undersea ridge extending eastwards from the mainland of New Zealand. For islands of such a small size – Chatham Island covers an area of only 794 km² – the Chathams have a surprisingly diverse geology. This reflects their complex geological history, with metamorphic rocks forming the basement, which is covered by volcanic lavas and tuffs and fossiliferous sedimentary rocks.

Unlike the New Zealand mainland, the Chatham Islands do not suffer from earthquakes and there is no contemporary volcanic activity. The lack of earthquakes is because they are distant from the tectonic plate boundary that passes through the mainland of New Zealand, being situated almost 1,000km from the edge of the Pacific Plate.

Chatham Island has a very unusual and distinctive shape defined by its geology. The massive embayment of Petre Bay, taking a bit out of the west side of the island, is a conspicuous feature, as is the huge brackish lagoon called Te Whanga. The southernmost one-third of the island consists mainly of hard igneous and pyroclastic rocks, whereas the northern two-thirds is dominated by softer sedimentary rocks, along with some metamorphic rocks and localised igneous centres.

Holocene sands and gravels confine the Te Whanga lagoon on its northern and eastern sides, whereas Cenozoic limestones form a thin strip of land on the western side of Te Whanga and extend westwards to form a peninsula along the northern edge of Petre Bay. Overall, the stratigraphy is quite complex and some of the beds cannot be traced very far laterally, making it difficult to draw a simple geological column (see Campbell et al., 1993, fig. 1.3).

The oldest rock exposed rock is the Chatham Schist, which outcrops in the north of Chatham Island and is the basement rock for the islands. This is the equivalent of the Haast Schist on the South Island of New Zealand and, like the Haast Schist, was formed by metamorphism of Permian and Triassic sedimentary rocks.

Radiometric dating suggests that metamorphism of the Chatham Schist occurred during phases in the Late Triassic and Middle Jurassic. Green-coloured Chatham Schist outcrops on the foreshore at Kaingaroa near the north-eastern corner of Chatham Island (Fig. 2). Not surprisingly, fossils are entirely lacking through the almost 5,000 metres thickness of the Chatham Schist.

A large shield volcano developed in the area of the Chatham Island during the late Cretaceous, erupting about 80 to 85 million years ago to produce the basalt lava flows and ashes that constitute rocks known as the Southern Volcanics. The shield volcano seems to have formed at the time of the detachment of the New Zealand continent (Zealandia) from the rest of the supercontinent of Gondwana.

A feature of some of the basalt lava flows is columnar jointing, resulting from shrinkage during cooling. This is seen particularly well on the coast at Ohira Bay, a tourist destination reminiscent of the Giant’s Causeway, but on a smaller scale (Fig. 3). The basalt was mostly erupted onto the land surface, not into the sea. At Ohira Bay, it fills a valley in the Chatham Schist.

Resting on the Chatham Schist in the north of Chatham Island are sediments belonging to the Late Cretaceous–Early Paleocene Tioriori Group. These consist of phosphatic sandstone (Takatika Grit), glauconitic sand (Tutuiri Greensand) and poorly cemented limestone (Tumaio Limestone). The Takatika Grit is notable in yielding the reworked bones of a theropod dinosaur, which are exhibited at the small museum in Waitangi (Fig. 4). Other fossils found in this rock include birds, shark teeth, sponges and molluscs.

Although normally covered by sand dunes and only sporadically exposed (it was visible in 1997 and 1999, but not in 2023), the Tumaio Limestone has particular importance in being one of the first deposits in the whole of New Zealand from which invertebrate fossils were described in the scientific literature. In 1840, the German naturalist, Ernst Dieffenbach (1811–1855), collected fossil oysters from the Tumaio Limestone, which were described three years later by John Edward Gray of the British Museum. No fewer than 77 species of bryozoans have been recognised in the Tumaio Limestone, one of which is shown in Fig. 5.

Predominantly of Early Eocene age, the Red Bluff Tuff not only has a memorable name but can be dramatic when seen at outcrop, forming cliffs in shades of orange and brown (Fig. 6).

The Red Bluff Tuff is a cemented volcanic ash that was erupted from a series of small submarine volcanoes. Black volcanic bombs (Fig. 7) can be observed weathering out from the softer matrix of tuff, which was devitrified when the hot ash composed of basaltic glass became quenched after eruption in the cold seawater.

At some localities, the tuff contains abundant fossils of marine invertebrates: large sponges abound in some beds (Fig. 8), bryozoans and bivalve molluscs are common, and rare nautiloids and crinoids (Fig. 9) also occur.

Overlying the Red Bluff Tuff is the Eocene Te Whanga Limestone Formation, subdivided into two members: Matanganui and Te One. Calcareous green algae and larger foraminifera are present in some parts of the Matanganui Limestone, indicating deposition in relatively warm waters, and abundant echinoid spines also typify this member.

An unconformity, developed at the top of the Matanganui Limestone, separates it from the overlying Te One Limestone (Fig. 10).

Crossbedding in the Te One Limestone (Fig. 11) points to deposition from mobile, subaqueous dunes in a shallow water environment and under cooler conditions than the Matanganui Limestone.

The Te One Limestone is an almost white, poorly cemented, skeletal sand, packed with nodular and broken branches of ramose bryozoan colonies (Fig. 12). This profuse bryozoan fauna has yet to be described and the number of species present is unknown, but a diversity exceeding one hundred species would not be surprising.

The northern part of Chatham Island is peppered with hills representing the eroded cores of small volcanoes belonging to the Northern Volcanics formation (Fig. 13).

These volcanoes probably erupted at various times between the Eocene and the Miocene, although some may be as young as Pliocene in age. An exposure of basaltic pillow lava, erupted underwater at ‘Spatter Rock’ on the northwest coast of Chatham Island, is believed to be about five million years old and is full of phenocrysts of hornblende (Fig. 14).

West of Spatter Rock is the poetically named Momoe-a-toa, the type locality of the Pliocene Momoe-a-toa Tuff. Resting on basalt, the Momoe-a-toa Tuff is richly fossiliferous, a chaotic jumble of shells of marine animals. Pectinid bivalves – scallop shells – dominate (Fig. 15), but there are also brachiopods, bryozoans, serpulid worm tubes, occasional gastropods, and a variety of hydrozoan cnidarian known as a stylasterid (Fig. 16), which can easily be mistaken for a bryozoan at first sight.

Quarrying at Waitaha, where hard basalt for the airport runway was being extracted at the time of my visit, has entailed removal of an overburden of Titirangi Sand. This basal Pleistocene sediment is full of mollusc shells, especially burrowing bivalves. The presence of countersunk borings (Fig. 17) in a significant proportion of the bivalves indicate that they were preyed upon by naticid gastropods.

The concave inner surfaces of many of the bivalve shells after death supported a cryptic fauna of encrusting bryozoans and small serpulid worms, adding to the palaeontological interest of this locality.

The Chatham Ridge today marks an oceanic convergence between warm waters to the north and colder waters to the south. These conditions promote high productivity and, as a result, the fisheries around the Chatham Islands are rich. Judging by the abundance of marine fossils in the Cenozoic rocks of the Chatham Islands, similar conditions may well have existed for a considerable time.

Submarine volcanoes were probably important in introducing inorganic nutrients into the sea and in supplying initial hard substrates onto which bryozoans and other suspension feeders could attach. Once these communities had become established, their own skeletal remains would have provided further substrates – a positive feedback leading to the accumulation of large quantities of biogenic carbonate and the formation of limestones like the Tumaio and Te One limestones.

Tourism to the Chatham Islands is on the increase, with several companies now offering group tours focusing on the fascinating history and culture, dramatic scenery and fauna and flora, with several species endemic to the islands. However, independent geological fieldwork there is difficult because most of the localities, including those along the coast, are in private ownership and permission must be sought from numerous people to visit them. But there is something magical about the remote and unique Chatham Islands, which will always be enticing for geologists and other visitors.

Further reading

Campbell, H.J., Andrews, P.B., Beu, A.G., Maxwell, P.A., Edwards, A.R., Laird, M.G., Hornibrook, N., Mildenhall, D.C., Watters, W.A., Buckeridge, J.S., Lee, D.E., Strong, C.P., Wilson, G.J. & Hayward, B.W. 1993. Cretaceous–Cenozoic geology and biostratigraphy of the Chatham Islands, New Zealand. Institute of Geological & Nuclear Sciences Ltd, Monograph 2, 269 pp.

James, N. P., Jones, B., Nelson, C. S., Campbell, H. J. &Titjen, J. 2011. Cenozoic temperate and sub-tropical carbonate sedimentation on an oceanic volcano – Chatham Islands, New Zealand. Sedimentology, 58, 1007–1029.

Stilwell, J. D. & Mays, C. 2020. Lost World of Rekohu: Ancient ‘Zealandian’ Animals and Plants of the Remote Chatham Island. Cambridge Scholars Publishing, Newcastle upon Tyne.