The Ordovician
In many ways, the Ordovician in Zealandia was a case of more of the same. Zelandia remained a marginal part of Gondwana, , and sedimentation continued within and on the flanks of a developing geosyncline. Ordovician rocks, however, are far more extensive than Cambrian, and formed on two different terranes - the Buller Terrane in the west, and the Takaka Terrane in the east. Small differences in rock chemistry suggest that they formed in two different parts of the syncline, and received sediments from different areas of Gondwana.
The Ordovician rocks of the Buller Terrane consist primarily of quartzose sandstones and mudstones; volcanic rocks are almost absent. The rocks appear to be deep-water turbidites, laid down by sediment-laden waters flowing down the edge of the continental shelf, into the geosyncline. There is a suggestion that sedimentation moved gradually eastwards (relative to modern New Zealand) during the Ordovician, for in the west of the terrane the Greenland Group rocks are restricted to the early Devonian (and late Cambrian), and date from ca 495 million years before present. In the west, a more continuous sequence is seen, extending through much of the Ordovician. These have been divided into a number of different formations, of which the most notable is perhaps the Aorangi Mine Formation, which provided the source of the Aorangi gold. The rocks found in these two zones are broadly similar. However, small chemical differences are seen, which suggest that some volcanic debris was entering the geosyncline in the south, but this declined northwards.
The Takaka Terrane, further to the east, is a somewhat different beast. It contains a much wider variety of rocks, and is structurally much more complex. It also shows the marked influence of volcanic activity. Again, the terrane has been divided into two zones - a western and eastern, with slightly different depositional histories. In the eastern zone (west of Mount Arthur), the Ordovician is heralded by the formation of beds of shale and sandstone (the Owen Formation), suggestive of erosion from a nearby land mass. This is followed by a long period of carbonate formation (the Mount Authur Marble) in quieter and shallow waters at the margin of the continent. In the late Ordovician, shales and sandstones return (the Wangapeka Formation), perhaps as sediment supplied from land increases again. The western zone lacks the basal shales, but is otherwise similar, with the Summit Limestone being replaced in the Late Ordovician by the sandstones and shales of the Baldy Formation. In both parts of the Takaka Terrane, evidence for volcanic activity gradually declines throughout the Ordovician, suggesting that the volcanic arc was becoming less active, and eventually volcanic activity ceased.
Since they were laid down, Ordovician rocks have not remained totally passive. The Buller and Takaka terrains were brought together, for example, some time in the Devonian. The Karamea Batholith was intruded in the late Devonian, disrupting the Ordovician rocks and metamorphosing some of the the limestones to marble and the greywackes to gneiss. Tectonic activity also caused extensive faulting during the Silurian and Devonian, and again in the Cretaceous. As ever, in New Zealand, what we see today, therefore, is a small part of the Ordovician, much rearranged and altered by later geological activity.
Access
Rocks belonging to the Ordovician can be seen across large parts of of the Buller and Takaka region. Mount Arthur provides spectacular exposures for anyone willing to do the walking involved. Takaka Hill provides easier access to the Ordovician limestones of the Mount Arthur Group, and gives examples of karst scenery. Canaan Down - accessible via the long, gravel Canaan Road - is an especially interesting area, with access to a number of large cave systems (e.g. Harwood Hole), and to exposures that yield a range of minerals, including scheelite and hornfels.
For more information see:
Roser, B.P., Cooper, R.A., Nathan, S. and Tulloch, A.J. 1996 Reconnaissance sandstone geochemistry, provenance, and tectonic setting of the lower Paleozoic terranes of the West Coast and Nelson. New Zealand, New Zealand Journal of Geology and Geophysics 39, 1-16.
The Ordovician rocks of the Buller Terrane consist primarily of quartzose sandstones and mudstones; volcanic rocks are almost absent. The rocks appear to be deep-water turbidites, laid down by sediment-laden waters flowing down the edge of the continental shelf, into the geosyncline. There is a suggestion that sedimentation moved gradually eastwards (relative to modern New Zealand) during the Ordovician, for in the west of the terrane the Greenland Group rocks are restricted to the early Devonian (and late Cambrian), and date from ca 495 million years before present. In the west, a more continuous sequence is seen, extending through much of the Ordovician. These have been divided into a number of different formations, of which the most notable is perhaps the Aorangi Mine Formation, which provided the source of the Aorangi gold. The rocks found in these two zones are broadly similar. However, small chemical differences are seen, which suggest that some volcanic debris was entering the geosyncline in the south, but this declined northwards.
The Takaka Terrane, further to the east, is a somewhat different beast. It contains a much wider variety of rocks, and is structurally much more complex. It also shows the marked influence of volcanic activity. Again, the terrane has been divided into two zones - a western and eastern, with slightly different depositional histories. In the eastern zone (west of Mount Arthur), the Ordovician is heralded by the formation of beds of shale and sandstone (the Owen Formation), suggestive of erosion from a nearby land mass. This is followed by a long period of carbonate formation (the Mount Authur Marble) in quieter and shallow waters at the margin of the continent. In the late Ordovician, shales and sandstones return (the Wangapeka Formation), perhaps as sediment supplied from land increases again. The western zone lacks the basal shales, but is otherwise similar, with the Summit Limestone being replaced in the Late Ordovician by the sandstones and shales of the Baldy Formation. In both parts of the Takaka Terrane, evidence for volcanic activity gradually declines throughout the Ordovician, suggesting that the volcanic arc was becoming less active, and eventually volcanic activity ceased.
Since they were laid down, Ordovician rocks have not remained totally passive. The Buller and Takaka terrains were brought together, for example, some time in the Devonian. The Karamea Batholith was intruded in the late Devonian, disrupting the Ordovician rocks and metamorphosing some of the the limestones to marble and the greywackes to gneiss. Tectonic activity also caused extensive faulting during the Silurian and Devonian, and again in the Cretaceous. As ever, in New Zealand, what we see today, therefore, is a small part of the Ordovician, much rearranged and altered by later geological activity.
Access
Rocks belonging to the Ordovician can be seen across large parts of of the Buller and Takaka region. Mount Arthur provides spectacular exposures for anyone willing to do the walking involved. Takaka Hill provides easier access to the Ordovician limestones of the Mount Arthur Group, and gives examples of karst scenery. Canaan Down - accessible via the long, gravel Canaan Road - is an especially interesting area, with access to a number of large cave systems (e.g. Harwood Hole), and to exposures that yield a range of minerals, including scheelite and hornfels.
For more information see:
Roser, B.P., Cooper, R.A., Nathan, S. and Tulloch, A.J. 1996 Reconnaissance sandstone geochemistry, provenance, and tectonic setting of the lower Paleozoic terranes of the West Coast and Nelson. New Zealand, New Zealand Journal of Geology and Geophysics 39, 1-16.