The New Zealand Geosyncline
The Lower Paleozoic rocks just discussed are confined to the western South Island; nothing is known of the history of the rest of New Zealand during that time. Rocks of later Paleozoic age are, however, more widespread, and the events of these times better understood. In the late Paleozoic was formed the extensive New Zealand Geosyncline, in which accumulated an enormous thickness of volcanic and sedimentary rocks that now form much of the “basement” (the foundation of old hard rocks) of this country. On the geological maps the rocks of the geosyncline are seen now to make up much of the South Island: most of the expanse of “old sedimentary rocks” extending from East Nelson and Marlborough through Canterbury, Otago, and Southland consists of them; the Haast Schists — the metamorphic rocks are shown in Marlborough, along the western side of the Southern Alps, and in a belt across Otago — have also been formed by metamorphism of these sediments. All the “old sedimentary rocks” of the North Island map were deposited in the geosyncline; they extend as well beneath the cover of younger sedimentary and volcanic rocks that now occupies much of the North Island.
Although the oldest fossils so far discovered in the rocks of the New Zealand Geosyncline are Permian, it is likely that it was in existence in the Carboniferous. Stratigraphically beneath the proven Permian strata are tens of thousands of feet thickness of basic volcanic lavas, agglomerates, breccias, and tuffs. These old volcanic rocks came probably from a chain of volcanoes that lay west of the geosyncline early in its history; two belts of them are shown on the South Island map — in Nelson (Brook Street Volcanics), and in western Southland (Eglinton Volcanics). In Nelson and Southland serpentines, dunites, gabbros, dolerites, and other basic and ultramafic rocks are intimately associated with the Paleozoic volcanic rocks, probably being intruded at that time.