Geological background of karst in New Zealand
University of Auckland
Email: p.williams@auckland.ac.nz
Abstract
Zealandia is a piece of Gondwana, about one-third the size of Australia, but most of its continental crust is underwater as submarine plateaux. The emergent part, New Zealand, covers about the same area as Victoria plus Tasmania, and carbonate rocks occupy about 3% of its area (compared to 4.7% in Australia). So karst is scarce in NZ, making its conservation all the more important. Before the present Australia-Pacific plate boundary was established at the end of the Cretaceous Period, about 80 Ma ago, Zealandia had been eroded almost to sea level. By 60 Ma the Tasman Sea had reached its full extent, the oceanic crust was cooling and the subsiding sea floor drew down the land, such that by 35 Ma only small low islands protruded above sea level. The sinking continental crust first accumulated estuarine sediments, then sheets of shell fragments, silty sandstones and mudstones. Deep burial compressed the shell layers into limestones. Sediment accumulation ended in the late Miocene-Pliocene once the present plate boundary was established and resulted in convergence and uplift. Uplift started about 15 Ma and accelerated around 5 Ma. This resulted in the emergence of tilted blocks of country bounded by faults. Erosion commenced immediately uplift occurred, so the marine sediments were dissected as they were uplifted. This removed most of the Tertiary sediments from the highest country leaving fragments round the edges where uplift was less intense. Consequently, there are remnant patches of Oligo-, Mio-, and Plio-cene limestones in many parts of the country, predominantly located on the flanks of ranges but sometimes found to 1500 m. These carbonates range from a few 10s to 700m in thickness. In some places, exhumation of the eroded basement has revealed more ancient carbonates, including marble bands in Fiordland and especially NW Nelson where they attain up to 1000 m in thickness and occur to 1875m above sea level.
Most karst development has taken place in New Zealand since plate convergence generated uplift, mainly over the last 5 Ma and especially in the last 1 Ma. By the early Pleistocene the Southern Alps began to form and gravel accumulations started to develop the Canterbury Plains. During uplift the Cenozoic marine sediments were simultaneously eroded leaving the remnant patches we see today. In NW Nelson the Ordovician marbles were re-exposed and our greatest caves (to 1026 m deep; to 67.2 km long) started to develop, a process that still continues. In western North Island, near Waitomo, the Oligocene-Miocene limestones were exposed in places by the early Pleistocene, but the evolving karst was overwhelmed about 1.25 Ma ago by a vast thickness of ignimbrite from the central volcanic zone. This has now been largely stripped off and karstification has resumed. Most caves in the Waitomo region are probably less than 250,000 years old. Meanwhile in eastern North Island marine shelly limestones of Plio-Pleistocene age were uplifted only about 1 Ma ago and these thin young limestones have been karstified only in the last 10,000 years or so.