From highlands to lowlands
By 100 million years ago, in the middle Cretaceous period, a region of new mountains stretched for several thousand kilometres along the Australian and Antarctic margin of Gondwana. These mountains were made of uplifted Western Arc and Murihiku rocks, and the same grey Torlesse rocks that make up today’s Southern Alps.
Erosion, however, immediately began to take its toll. By the middle Cretaceous period, the mountains had largely worn down to extensive lowlands. Lush vegetation mantled the river flood plains and swamps of the low-lying region – this greenery was eventually converted into the coal beds now found in Otago, Southland, Westland and Nelson. Along the coast and off the eastern shores of the new landmass, layers of sediment eroded from the mountains built up. Preserved within their layers were the remains of many creatures of this period, including dinosaurs and marine reptiles such as mososaurs and plesiosaurs.
At the same time that erosion was wearing down the land, patterns of circulation below the earth’s crust were shifting. Hot rock began to well up beneath Gondwana and move outward, pulling the land apart. A rift developed in Gondwana’s crust, well inland of the coastal mountains. Along this rift, molten rock rose to the surface, producing the volcanic rocks now found in the Awatere and Clarence valleys, and the Mt Peel, Malvern Hills and Mt Somers areas of Canterbury.
By 85 million years ago, the sea had flooded into the rift. A large section of Gondwana – including inland areas of older rocks and the newer coastal region made of Western Arc, Murihiku and Torlesse rocks – moved off into the Pacific Ocean. New Zealand was now on its own – a drifting continent about half the size of Australia. The new region of ocean separating New Zealand from Gondwana became the Tasman Sea. From dating the rocks that make up the floor of the Tasman Sea, it is known that it took about 30 million years for it to reach its present width.
Age of the dinosaurs ends
At the end of Cretaceous period, the impact of huge meteors and large-scale volcanic activity resulted in the extinction of about half the plant and animal species, including the dinosaurs.
New Zealand rocks deposited at this time contain clues to these global catastrophes. A thin layer of clay at locations such as Woodside Creek in Marlborough contains high levels of iridium, an element that is abundant in meteorites but rare in normal rocks. The widespread destruction of forests is indicated by abundant soot in the clay layer, and changes in fossil pollen.
As the New Zealand continent moved away from the spreading centre, its crust began to cool and become denser. The low-lying land began to gradually sink into the ocean during the early Tertiary period. By the Oligocene period, about 35 million years ago, less than one-third of the area of modern New Zealand remained above sea level, as numerous islands.
During this time, on the land still above the sea the bulk of New Zealand’s coal deposits accumulated, including coal now mined at Greymouth, Buller and Waikato. Offshore from the islands a blanket of new sediments was laid down on top of the older rocks of ancestral Gondwana. Sandstones and mudstones were deposited close to shore. On shallow sea floors far enough from land to be clear of sediment, the calcareous remains of marine organisms built up, forming large areas of limestones. These limestones are now used for lime for agriculture, cement and Ōamaru’s famous building stone. They have formed well-known scenic features such as the Pancake Rocks at Punakaiki on the West Coast.
Today, parts of the North Island are still covered with the layers of sedimentary rock that formed during this time. In the South Island, however, most of this cover has been eroded away, and in a few areas only patches have survived.