Little is known about the geological history of New Zealand beyond the early Palaeozoic era (more than 541 My ago). There are no precambrian rock outcrops in continental New Zealand, but luckily the oldest rocks outcrop in the Nelson area, in the Buller and Takaka terranes (fragments of continental crust accreted to another region of continent, and bounded by major faults).

Mount Owen, at which Arthur marble outcrops, is situated at the southern tip of the Takaka terrane. The slice is bounded to the west by the major Anatoki thrust fault, which separates it from the Buller terrane.

The Takaka terrane is comprised of different entities of which the Arthur group is of interest. After the Devil’s River volcanics were emplaced in an island arc setting of the coast of Gondwana (a giant mass made of modern Africa, India, Australia, Southern America and Antarctica) in the Cambrian, massive limestones were deposited on the shallow fringes of the arc from the middle Ordovician to the beginning of the Silurian. In the deeper parts of the basin between the Takaka terrane and mainland Gondwana, argillites (or clay-rich sequences of rocks) were deposited.

Afterwards, Buller and Takaka were brought together, and both moved towards mainland Gondwana, which during the early Jurrassic became Pangea. At the break up of Pangea in the mid Jurassic, the Zealandia continent stayed with Australia and Antarctica until the three eventually broke up. Australia moved north at a rapid pace, while the microcontinent Zealandia, moved east.

Now, only 7% of the Zealandia continent sits above sea level, in the form of the North and South Islands plus minor archipelagos. The continent sits astride a major plate boundary, the Alpine fault, which separates the Indo-Australian plate from the Pacific plate. It is active today and responsible for the surrection of the Southern Alps. The rapid uplift recorded on the west of the major fault means that old rocks, deep in the earth’ crust will eventually crop at the surface.

The limestones of Mount Owen have thus seen a long and varied geological history. They have been deformed by the terrane accretion and thrust faulting, and the igneous intrusions nearby. The excessive heat and pressure have permitted the recrystallisation of calcite, the development of fabrics that metamorphosed the limestone into marble, and argillite deposits nearby into schists. At least three distinct, successive episodes of folding can be recognised for the study of microscopic fabrics, the latest of which has resulted in the surrection of the Southern Alps and Marino mountains, to which Mount Owen appertains.

Now this is well and good, but not very useful to the Homo Speleo Vulgarus. What will the marble look like on the field? It is dark and massive. The relic bedding is sometimes seen as a light/dark alternation, but the subsequent metamorphic fabrics are superimposed, often at an angle. The distinctive feature is the karstic landscape. On mount Owen, east of the Nuggety creek source is a conformable contact with the Wangapeka formation, another upper Ordovician metamorphic rock, which is composed of argillites, dark shales, and rare lenses of dark marble. No caves there!

Tanguy Racine

References

New Zealand tectono-stratigraphy and implication from conglomeratic rocks for the configuration of the SW margin of Gondwana A.M. Wandres and J.D. Bradshaw, Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand.