The Great Dying at the end of the Permian is estiumated to have seen the extinction of about 90 per cent of life on Earth. Afterwards, the world was relatively lifeless for several million years, until the depleted species that had survived could build up their populations and repopulate the empty areas of the world, and new species could evolve and spread. By the tihis had happened, life on Earth had changed substantially. Throughout most of the Palaeozoic, the vast majority of life was marine. The following Mesozoic Era saw the widespread emergence of terrestrial life, notably in the form of reptiles. The Mesozoic, as a consequence, has often been called the Age of Reptiles. By the end of the era, the first dinosaurs had appeared.
The Triassic Period comprises the first part of the Mesozoic Era, spanning the time from ca. 252 to 201 million years before present. In New Zealand, there is only limited evidence of land-based creatures during this period, but after an almost barren phase of some 13 million years, marine life returned in something like abundance. Fossils from this period comprise a wide range of relatively deep-water (benthic) creatures, including brachiopods, nautioids and ammonites, as well as shallow-water forms such as bivalves, gastropods, corals, bryozoa and anthozoa. Rarely, sharks' teeth and bone material from marine vertebrates can also be found. Amongst macro-fossils, land-based life is mainly represented by plant debris, such as wood fragments, leaves or - more rarely - nuts or fruits - which had been washed offshore and deposited at sea.
As this fossil assemblage indicates, the Triassic rocks of New Zealand must have formed in a marine environment not all that far from land. The same story is shown by the rock types. Though both igneous and volcanic rocks of Triassic age are relatively sparse, there is plenty of evidence for nearby volcanic activity in the materials that make up the sedimentary rocks of this period. These form a succession of alternating sandstones and siltstones, interrupted at times by thinner beds of conglomerate, that is collectively referred to as greywacke. The general interpretation is thus that New Zealand at that time lay at the margins of the vast continent of Gondwana, which was beginning to break up as a deepening geosyncline formed to the south. Associated with these tectonbic activities, a volcanic arc formed, spewing out the debris which provided the main source for the sediment.
In the Top of the South, the products of this activity are now preserved in a long but relatively narrow swathe of rocks, stretching from D'Urville Island in the north to the Wairau Valley in the south. To the east, the rocks overlie (and are faulted against) older Permian strata. To the west, they are bounded by the Waimea-Flaxmore Fault, which is one of the major branches of the Alpine Fault that runs through the Southern Alps. Interestingly, 450 km to the south, an almost identical sequence of Triassic rocks occurs in Southland. The two areas were once conjoined, but have been moved apart by repeated slippage of the Alpine Fault over the last 100 million years.
This swathe of Triassic rocks comprises a number of different stages and associated formations that represent most of the period. The Early Triassic is represented by the Greville Formation, best seen in the Lee Valley.This contains a meagre fossil fauna, mainly comprising small ammonoids. Above this lie the Wauia Formation and Stephens Subgroup, both of Middle Triassic age, and both of which are also more-or-less unfossliferous. In the overlying strata, however, fossils at last burst onto the scene. Through the Late Triassic we get a diverse assemblage of fossils that changes with the passage from one stage to another: from the the Kaihikuan to the Oretian to the Otamitan to the Warepan and eventually the Otapirian. The boundary between each of these stages is not always clear. Sometimes it's marked by a fault. On other occasions, as between the Oretian and Otamitan, and more distinctly between the Otamitan and Warepan, it's indicated by a distinct conglomerate bed, possibly showing a period of uplift caused by tectonic activity. At other times the transition is subtle. The fun, therefore, has been not just to find the fossils and identify them, but to match them to their correct geological stages and develop detailed maps of their distribution.
The Triassic of the Nelson area has been a special interest of mine for many years, and over recent years I've been actively researching the geology with a group of colleagues from the Nelson Rock and Mineral Club. As a result, the collection includes a large number of specimens of Triassic fossils. A report on this work is available on the Nelson Rock and Mineral Club website, which includes maps of the geology and the fossils. This provides a useful accompaniment to the fossils described here.
Further Reading: Ballance, P. 2009 New Zealand geology: an illustrated guide. GSNZ Miscellaneous Publication 148, Auckland: Geoscience Society of New Zealand, 397 pp. Campbell H.G. 2019 Biostratigraphic age review of New Zealand’s Permian–Triassic central terranes. Chapter 3 in: Robertson, A. H. F. (editor) Paleozoic–Mesozoic geology of South Island, New Zealand: subduction-related processes adjacent to SE Gondwana. Geological Society, London, Memoirs49, pp. 31–41.