Precambrian and Paleozoic Events
The term Precambrian means, broadly, all the vast span of time from the formation of the earth's crust, thousands of millions of years ago, until the beginning of the Cambrian period of the Paleozoic era. It is in the Cambrian rocks of the world that the first well preserved fossils are found. Nothing is known with certainty of the events in this part of the Pacific before the Cambrian. Although the New Zealand islands are small, the expanse of undersea ridges and plateau from which they rise is huge; it includes such features as the Campbell Plateau, an under-sea plateau some 500 miles across immediately to the south of New Zealand. Seismologists deduce from earthquake-wave studies that the material of the earth's crust in and around New Zealand is “sial”; this term sial is a general one given to rocks, such as granite, of high silica and aluminium content, of which the earth's crust in continental areas, but not beneath the deep ocean floors, is composed. Beyond the New Zealand region no sial exists nearer than Australia; the Pacific and Tasman oceans are floored by thin sediments that overlie dark, heavy basaltic rocks — “the sima”. A fundamental problem of New Zealand geology is to know how, and at what time, this extensive, isolated mass of sial was formed — is it a fragment of a “scum” of lighter elements that thousands of millions of years ago floated to the surface of a molten earth to form the continental crust; is it a piece of continental crust that has “drifted” a thousand miles from its parent, Australia; or has some other process brought about this concentration of sialic material within the basin of the sima-floored Pacific?
There is no firm evidence of events even of later Precambian times in New Zealand; no Precambrian rocks are known for sure, although some granites and metamorphic rocks in Westland and Fiordland may be of this age. Some geologists think that the thick sequence of complexly folded, marine sedimentary rocks of the Waiuta and Greenland Groups of Westland and south-west Nelson may possibly be of late Precambrian age. They are predominantly greenish grey, lightly metamorphosed unfossiliferous greywacke and argillites, that probably accumulated in an extensive geosyncline. They have been studied closely because they contain the rich gold-bearing quartz reefs of the Reefton area. Little or nothing has been deduced, however, about the size and shape of the land areas from which these ancient sediments were derived.