On the margins of Gondwana, the foundations of Zealandia are laid in a deep geosyncline, fed by sediments from a volcanic island arc
The geological history of the Nelson area starts with New Zealand's oldest rocks, in the Cobb Valley. Here, beds of siltstones and sandstones are found, containing lenses of chert and conglomerate, interbedded with basic volcanic rocks (mainly andesite and basalt). Trilobite and brachiopod fossils contained in the sedimentary beds link these rocks to the mid-Cambrian. Uranium-lead isotope analysis of zircon crystals from the rocks suggest a date of about 505 million years before present. At this time, New Zealand was part of the recently-formed super-continent, Gondwana, which included most of the land masses that now lie in the Southern Hemisphere, along with the Arabian peninsula and India.
The Cobb Valley rocks formed as part of the Takaka Terrane - a geological domain that now extends from Golden Bay south as far as the western edges of Mount Arthur. The rocks appear to have formed by accumulation of sediments in a gradually deepening marine basin (a geosyncline), adjacent to a volcanic island arc. Sediments eroded from the island arc were washed into the geosyncline by turbidity currents, while at intervals lavas were erupted onto the sea-floor from submarine volcanoes. The detailed history of the rocks is difficult to decipher, however, because subsequent faulting and erosion has broken the sequence up into several discontinuous slices. The exact number and geographic limits of these slices, and how they relate to each other, are all still a matter of debate. Likewise the stratigraphic naming of the rocks is disputable. Early mapping placed all of the rocks in the Haupuri Group. More recent mapping has identified two more-or-less contemporaneous groups: the Haupiri, containing all the sedimentary rocks, and the Devil River Volcanics Group which contains all the volcanic strata.
Access The best place to see these Cambrian rocks is at Trilobite Rock, near the Cobb Reservoir. Note, that the Cobb Valley is part of the Kahurangi National Park, and collection of fossils or other material from this area is strictly forbidden.