Geology of New Zealand Field Trip Guidebook - ResearchGate

preserved as coal and the floodplain sediments entombed dinosaurs and a variety of marine
reptiles (McSaveney and Nathan 2007).
At the same time, a rift began to develop along the eastern margin of Gondwana, well
inland of the newly-formed coastal mountains. Rift-associated volcanoes spewed mafic
rocks onto the surface. These are today visible in Mount Peel, the Malvern Hills and Mount
Somers in Canterbury. By about 85 Ma, the ocean had flooded the rift (Fig. 3) and New
Zealand was isolated from Australia. Seafloor spreading continued for about 30 Ma and
mysteriously stopped, leaving the modern Tasman Sea (McSaveney and Nathan 2007).
A marine section at Woodside Creek in Marlborough preserves a thick clay layer with
high levels of the element iridium, marking the moment in time when a large meteor struck
Earth (Alvarez et al. 1980) and precipitating the extinction of the non-avian dinosaurs and
heaps of other animals and plants on land and in the sea. The ecological havoc wrecked by
this event is also witnessed in New Zealand in the form of a superabundance of fern spores
just above (after) the impact layer (Vajda et al. 2001, Hollis 2003). A similar layer is
observed in North America and is widely interpreted to indicate a disturbance flora rich in
ferns that colonized the landscape after the impact catastrophe (Tschudy et al. 1984, 1986).
Although the Woodside Creek sediments were marine, their pollen content shows that
land was nearby. Terrestrial rocks in Canterbury preserve the leaves of some of the plants
responsible for this pollen. Since leaves are so essential to the functioning of plants, they are
commonly highly adapted to the particular environments in which they are found. As a
result, fossil leaves can be used to reconstruct climates of the past. Work by Elizabeth
Kennedy showed that the diversity of angiosperms (flowering plants) declined significantly
following the Cretaceous-Tertiary extinction, but climate didn’t change too much (Kennedy
2003). New Zealand remained cool to mild (mean annual temperature 6-12°C, today’s MAT
ranges from 10-16°C depending on location) and temperate with abundant rainfall throughout
this period of change.
While a significant portion of the Zealandia was above sea level during the early
phases of rifting from Gondwana, as rifting slowed and eventually ceased, the continent
cooled and sank into the mantle below, submerging large regions. The Late Cretaceous and
Paleocene terrestrial rocks of Canterbury show clearly that this part of present-day New
Zealand was still above sea level, but a detailed look at the sediments shows that by
Oligocene time, less than one third of present-day New Zealand was above the waves. What
land remained was isolated into a serious of relatively small islands (McSaveney and Nathan
2007). Many of these islands were capped with low-lying coastal swamps, from which most
of New Zealand’s coal comes. Between the islands, carbonates (limestone) accumulated in
shallow seas (McSaveney and Nathan 2007).
This sinking may partially explain the paucity of animal diversity on New Zealand.
The only native mammals, for example, are bats, that could disperse to the island relatively
recently. Instead, New Zealand’s terrestrial fauna is rich in birds, both flying and flightless.
Some members of the ratite (emu) family, such as the moas, were likely flightless when
Zealandia rifted from Gondwana. Others likely lost flight after immigration. Nonetheless,
the extreme fragmentation of New Zealand’s land mass may explain some of its diversity
patterns. A good terrestrial fossil record is needed to flesh out this story. However, such a
record—if it exists—has yet to come to light.
By the end of the Oligocene, the modern configuration of plate boundaries had been
established (Fig. 3). To the north, the Pacific Plate was sinking beneath continental rocks of
the Australian Plate. The Pacific Plate was rotating relative to the Australian plate producing
shear stress that initiated the formation of the Alpine Fault. Despite the dominant shear
motion, significant compression still occurred, lifting much more land above sea level and
exposing more of modern New Zealand. Uplift continues today and appears to be
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