The Permian
It was in the Permian, in many ways, that Zealandia really got going, with widespread sedimentation in a geosyncline, and the accumulation of interbedded volcanic rocks. Rock formation clearly occurred in a number of different areas, with material later being brought together by the action of plate tectonics, faulting and folding. In the Nelson area, three new terranes were eventually added (probably during the Cretaceous): the Brook Street, Dun Mountain-Matai and Caples terranes. Today, their rocks are responsible for much of the Richmond Hills, as well as smaller outcrops elsewhere.
The Brook Street Terrane is limited in area, and as its name suggests is centred on the Brook valley in eastern Nelson. It consists primarily of volcanic rocks - tuffs and some basalts - interbedded with breccia, sandstone, siltstone and mudstone. Most of the sedimentary rocks contain minerals clearly derived from nearby volcanic outcrops, and the sequence seems to have formed in an island arc, fed both by lava flows and ash deposits from active volcanoes, and reworking of volcanic rocks.
The Dun Mountain-Matai Terrane is far more extensive. As well as Nelson, it occurs in Otago and Southland, always in close proximity to the Alpine Fault. Interestingly, in the Nelson area it lies to the west of the fault; elsewhere to the east. Its distribution thus shows that it has been translocated almost 500 km by lateral movements along the faultline. In the Nelson area, it forms the Red Hills and crops out in a long slice from Croisilles Harbour (and beyond) to the Wairau River. Rock formation on the terrane continued for a long time, perhaps starting in the very late Carboniferous and extending throughout the Permian into the Early and Middle Triassic. The sequence as a whole shows a progression from volcanics at the base to primarily sedimentary rocks towards the top. During the Permian, the main formations included:
The Caples Terrane is similarly extensive, and found through much of the Marlborough Sounds and Otago, as well as western North Island. In the Nelson area, it makes up the bulk of the Richmond Range. It comprises a series of sandstones, containing interbedded siltstones and minor conglomerate. With time, the deposits became more schistose, until, at the end of the Permian, they were replaced by a distinct quartzite band containing schistose sandstone and siltstone (the Wakamarina Formation). The metamorphism responsible for these schistose characteristics probably occurred in the Early Jurassic. Fossils are rare, and details of the stratigraphy and history of the terrane remain unclear.
For more information
Campbell, H.J. 2000 The marine Permian of New Zealand. Developments in Palaeontology and Stratigraphy 18, 111-125.
Mortimer, N. 2004 New Zealand's geological foundations. Gondwana Research 7, 261-272.
The Brook Street Terrane is limited in area, and as its name suggests is centred on the Brook valley in eastern Nelson. It consists primarily of volcanic rocks - tuffs and some basalts - interbedded with breccia, sandstone, siltstone and mudstone. Most of the sedimentary rocks contain minerals clearly derived from nearby volcanic outcrops, and the sequence seems to have formed in an island arc, fed both by lava flows and ash deposits from active volcanoes, and reworking of volcanic rocks.
The Dun Mountain-Matai Terrane is far more extensive. As well as Nelson, it occurs in Otago and Southland, always in close proximity to the Alpine Fault. Interestingly, in the Nelson area it lies to the west of the fault; elsewhere to the east. Its distribution thus shows that it has been translocated almost 500 km by lateral movements along the faultline. In the Nelson area, it forms the Red Hills and crops out in a long slice from Croisilles Harbour (and beyond) to the Wairau River. Rock formation on the terrane continued for a long time, perhaps starting in the very late Carboniferous and extending throughout the Permian into the Early and Middle Triassic. The sequence as a whole shows a progression from volcanics at the base to primarily sedimentary rocks towards the top. During the Permian, the main formations included:
- the Dun Mountain Ultramafics Group, which consists of a range of relatively unusual rocks, including harzburgite, dunite, and wehrlite, as well as gabbro. High concentrations of nickel and magnesium in these rocks seems to limit plant development, with the consequence that the Group is often readily distinguishable as a broad, sparsely vegetated area of rather bare red-brown soils. To the north, these rocks have been heavily serpentinised. They also contain volcanic dikes made of rodingite (named after the Roding River) and lenses of chromite and areas of copper mineralisation, which have been worked in some places.
- the Livingstone Volcanics Group, consisting of gabbro - coarse-grained at the base, but grading up into microgabbro - and overlying basalts. Uranium-lead dating of zircons suggests formation in the Early Permian.
- the Matia Group. This is a widely distributed group of rocks. Deposition started in the Late Permian, with accumulation of breccia, siltstone and sandstone, but then gave way to fine-grained grey limestone and calcareous sandstone (the Wooded Peak Formation), up to 1 km in thickness. This, in turn, is overlain uncomformably by another kilometre or so of well-bedded sandstone and siltstone, limestone and interbedded mudstones. Andesite is common in these rocks, suggesting that volcanic islands were not far away. The Matai Group is not confined to the Permian, but continues long into the Triassic.
The Caples Terrane is similarly extensive, and found through much of the Marlborough Sounds and Otago, as well as western North Island. In the Nelson area, it makes up the bulk of the Richmond Range. It comprises a series of sandstones, containing interbedded siltstones and minor conglomerate. With time, the deposits became more schistose, until, at the end of the Permian, they were replaced by a distinct quartzite band containing schistose sandstone and siltstone (the Wakamarina Formation). The metamorphism responsible for these schistose characteristics probably occurred in the Early Jurassic. Fossils are rare, and details of the stratigraphy and history of the terrane remain unclear.
For more information
Campbell, H.J. 2000 The marine Permian of New Zealand. Developments in Palaeontology and Stratigraphy 18, 111-125.
Mortimer, N. 2004 New Zealand's geological foundations. Gondwana Research 7, 261-272.