THE GEOLOGY OF NEW ZEALAND
Pliocene (map symbol P)
Distribution Pliocene sediments are most extensive in the south and east of the North Island. They also have a limited extent on both coasts of the South Island and in Otago and Southland. While Pliocene sediments are rare in the north of the North Island, volcanic rocks are present in Northland, Coromandel and south Canterbury.
Content The Wanganui, Hawkes Bay, Wairarapa and Marlborough areas were the major sites of Pliocene deposition. The sediments around Wanganui are chiefly mudstone and sandstone with common shellbeds and rare limestone and conglomerate bands, the latter being more common near the Ruahine Range. In Hawkes Bay and Wairarapa the Pliocene sediments are characterised by the presence of coquina limestone (a loosely cemented shelly deposit composed mainly of barnacle plates). The coquina bands have a large lens-like form enclosed by thick muddy sandstone or sandy mudstone; there can be a transitional change from limestone into calcareous sandstone. Tuff bands are common throughout the Pliocene sequence. North of Gisborne, limestone becomes rare and calcareous sandstone more common. Minor marine deposits of sandstone with shell beds followed by non-marine mudstone are present on the coast south of Auckland, while non-marine mudstone and pumice breccia occur near Tauranga. In the South Island, gravel deposits were shed from rising mountain ranges in Marlborough. Marine mudstone and concretionary sandy mudstone are present near Ward while in north Canterbury near Parnassus, mudstone with conglomerate, and minor limestone are present. Marine sandstone is also known from the Waiau River in Southland and muddy sandstone and calcareous mudstone are present near Greymouth and further south at Jackson Bay. Non-marine deposits of conglomerate and freshwater sand, with or without lignite layers, are found throughout the South Island, in particular in Central Otago, Southland, inland Canterbury, northern Westland, and in Nelson.
Volcanism continued from the Miocene into the Pliocene in Northland and Coromandel, with andesite and volcanic breccia being present at Whangaroa, basalt at Ti Point, and widespread rhyolite, volcanic breccia, and minor andesite and dacite in eastern and southern parts of the Coromandel Peninsula. The oldest parts of Pirongia and Karioi volcanoes on the west coast of the North Island are Pliocene in age as are the earliest ignimbrites in the Taupo Volcanic Zone. At Timaru and Geraldine several basaltic flows were erupted onto the land surface.
Paleogeographic conditions The southern and eastern areas of the North Island were under water, and rapidly subsiding troughs were filled with sediment derived from rising land to the west and north. Generally, sedimentation kept pace with subsidence and the deposits were mainly in shallow water, with some exceptions. In the east, conditions favoured the growth of barnacle banks on the edges of deeper water channels.
The central part of the South Island was above sea level, and large gravel plains built out from mountain fronts in Nelson, Canterbury, Otago and Southland. Meanwhile the eastern part of the Canterbury - Marlborough region was undergoing warping and faulting, allowing local invasion by the sea.
Deformation Movements of the Kaikoura Orogeny continued throughout the Pliocene, but most Pliocene rocks are not highly deformed and generally they have been gently folded or tilted. In some areas of active faulting, the beds have been dragged up so that they are nearly vertical.
|<< Back Next >>|