An introduction and timeline geological events in the Nelson area
The current land area that we know as New Zealand is only a small part of a wider area of continental crust that is sometimes called Zealandia. This is some twenty times the area of the current land mass, and stretches in a broad lozenge shape from ca. 1000 km north-west of the current coastline to the same distance south-eastwards.
In studying geology, we also have to remember that many of the rocks that now make up any land mass were formed under water, so what is land now was ocean then. For most countries of the world, however, the process was relatively straightforward. Land rose and sank, or sea-levels changed, such that the march of geological periods saw the gradual build-up of a succession of rocks - albeit disrupted and distorted by folding and faulting. For Zealandia, however, the story is considerably more complex, for much of it has been built from left-over parts - by the bolting together of areas of ready-made continental crust that had been created elsewhere. These 'terranes', as they are known, were in many cases formed hundreds or thousands of kilometres away, and transported to their present relative position by plate movements, folding and faulting. We say 'relative'position, because while all this was happening, Zealandia itself was being pushed around the surface of the planet by plate tectonics. At different times, therefore, Zealandia found itself a neighbour or bed-fellow to many different continents, including Australia, North America, Europe and Southern Africa. To add to this complexity, several periods of igneous intrusion have occurred - some as the wandering Zealandia crossed magma hotspots in the mantle - leading to the formation of large batholiths which penetrated the surrounding rocks. Heating associated with these intrusions altered the nearby rocks - a process known as metamorphosis.
The geology of New Zealand that we see today, therefore, is just one of many configurations that the country has had over its history. Here in the Nelson area, we have evidence for much of this history, and the rocks that crop out around the region give us glimpses into many of the events that have helped to shape Zealandia.
The timeline below summarises the main events and indicates where yo go to see examples of the geological features for each period. To read more about any of the periods, click on the name.
Million Years BP
Where to see
Sea-level rise; climatic variation; human occupation and exploitation of the land.
Alluvium in river valleys Dune sands and gravel beaches on coast Hillwash, scree and landslide deposits in hill-lands
Tectonic uplift causing falling sea-levels and emergence of land; development of shallower marine (shelf) and estuarine deposits
Lower Blue Bottom Group: mudstones and sandstones Jenkins Group (Nelson city): siltstones, sandstones and breccia/conglomerates (continuous from Eocene-Early Miocene)
High sea-level causing complete(?) submergence of region; shallow-water marine sedimentation, with deposition of calcareous mudstones and limestones
Nile Group, comprising
Platform facies (limestones and muddy limestones formed on stable shelf environment);
Basinal facies (muddy limestone, calcareous mudstone and non-calcareous mudstones and sandstones, formed in rapidly subsiding basins)
Subsidence begins in Middle-Late Eocene, leading to progressive drowning of land area. In Early-Mid Eocene stable conditions lead to extensive weathering and development of estuarine and shallow-water sediments in coastal areas.
Sandstones, siltstone and conglomerate with thin coal seams (Jenkins Group in Nelson city; Maruia Formation in Murchison area); Mudstones and beach sands in Golden Bay (Kaiata Formation) Brunner Coal Measures: coal seams, shales, non-marine sandstones and conglomerates (Mid Eocene)
Extensive land areas, dominated by fluvial erosion and deposition, on braided floodplains; gradual sea level rise.
Farewell Formation: sandstones and pebbly conglomerates
New Zealand separates from Gondwana in the mid-late Cretaceous (>95 My), a. Associated tectonic activity causes faulting (e.g. Waimea and Flaxmore faults) and the formation of sedimentary basins with variable deposition. The Early Cretaceous is period of continued volcanic intrusion and tectonic activity, as the Median Tectonic batholith develops near the margins of Gondwana, and continued subduction brings together the terranes developed during the Permian-Jurassic. Extensive metamorphism of older rocks,
Pakawau Group: shallow marine sandstones and siltstones (North Cape Formation), overlying terrestrial coal measures (Rakopi Formation). Separation Point granites (biotite granite, biotite-hornblende granodiorite) and Wakapuaka Phyllonite (berated and schistose sedimentaries and volcanics).
The Median Tectonic Batholith develops along an old subduction zone at the margins of Gondwana, causing extensive intrusion of granites and granodiorites in New Zealand and occasional extrusive activity. The Rangitata Orogeny causes extensive folding and faulting. On and at the margins of the raised land areas, sandstones and breccias form.
Breccia, sandstone and siltstone with rare plant remains: Marybank and Botanical Hill Formations. Cable Granodiorite: hornblende granodiorite, quartz diary, tonality and syenite. Palisade Andesite: porphyritic hornblende and quartz-biotite andesites. Rotoroa Complex: biotite granite, granodiorite and amphibolite hornblende-biotite diorite.
The Murihiku terrane builds up in a marine shelf environment, at the edge of a gradually subsiding geosyncline, along a line of subduction.
Richmond Group: weakly metamorphosed and poorly bedded sandstones, siltstones and mudstones, commonly tuffaceous, with rare conglomerates; frequent shell beds. Folded and faulted.
In the Lower and Mid-Permian, volcanic activity (perhaps as part of an island arc to the SW) provides material that accumulates in a deep geosyncline. Later, volcanic activity subsides and limestones develop.
Powerful tectonic activity faults and folds pre-existing rocks, overturning Cambrian sequences in places. Extensive metamorphism. Sedimentation continues in a shallow marine environment, close to shore.
Pink granites and biotite diorites of Karamea Suite (Buller Terrane) and Riwaka Complex (Takaka Terrane). Reefton and Baton Formations: mudstone and fine-grained sandstone containing shellbeds.
Sedimentation in a volcanic island arc, associated with a deep geosyncline. Deposition occurs in at least two areas, which are later brought together as the Buller and Takaka terranes.
Anatoki and Mount Patriarch Formations: Fine quartz-mica siltstone, sandstone and conglomerate, passing into calcareous siltstone and carbonaceous limestone and shale. Haupiri Group: calcareous siltstone and fine sandstone, limestone and conglomerate. Contains trilobites and brachiopods. Interbedded volcanics, including basalt and andesite, with dacite and rhyolite. Gabbro and ultramafic rocks intruded in places, and melange associated with areas of faulting.