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Sedimentary cover rocks

Sedimentary and Volcanic Cover Map

Cover rocks in New Zealand refer to those less than about 100 million years in age. They formed on the Zealandia continent that had separated from the Australian part of Gondwana with the opening of the Tasman Sea. Most of the cover rocks are sedimentary; sandstone, mudstone, conglomerate and limestone. These rocks accumulated in sedimentary basins, depressions in the landscape caused by the tectonic conditions at the time.

The older cover rocks, typified by the limestone and mudstone rocks around western Waikato, Greymouth, Oamaru and western Southland, are finer grained. This reflects the prevailing tectonic stability and the absence of an active plate boundary between 100 and 23 million years ago. Since 23 million years ago the modern plate boundary has developed resulting in mountain uplift, including growth of the Southern Alps. These new mountains eroded quickly as well and resulted in a lot of sandstone and conglomerate deposits forming in localised sedimentary basins in the West Coast, Nelson, Auckland and many parts of the East Coast. Farther from those eroding mountains the preserved sedimentary rocks are finer grained “papa” sandstone, mudstone and limestone such as those found in the King Country-Wanganui area and around Hawke’s Bay.

The cover rocks of North Island and East Cape have been disrupted by extraordinary tectonic deformation where some large packets of rock (thrust slices) have been pushed more than 100 km southwards over the top of younger rocks. Included in these packets are pieces of ocean floor basalt that now form some of the higher parts of Northland Peninsula.

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