By mid Cretaceous, the Rangitata orogeny had run its course. The process once again of sedimentation, this time on land and on the sea floor, and volcanism began.
The process of erosion and redeposition on flood plains below this now-mountainous land of greenery gave way to the perfect conditions for basins filled with said greenery. Over time with burial and lithification came coal! Some of the coal deposits around New Zealand contain well-preserved fossil impressions of ferns and other plants from the mid-Cretaceous.
West of New Zealand was where the rifting began after the orogeny, splitting New Zealand once and for all from Gondwana. This MOR created what we now know as the Tasman Sea. The characteristic faults that run off of an MOR due to the large amount of stress it causes on the adjacent plates triggered the next set of volcanism in New Zealand as magma rose through these faults and fractures. The intermediate to felsic composition (primarily andesitic and rhyolitic) of the eruptions produced ash that buried the surrounding forests of the South Island, in the Canterbury region. These created some beautifully preserved petrified forests seen today. The rocks found in the North Island are more mafic in composition, mainly basalts. This could potentially be due to the positioning of New Zealand at that time, as the South Island volcanics show a more back-arc style setting with the North Island displaying a sea floor setting along the northeast, perhaps due to the extended faults from the MOR that upwelled deeper, more mafic magma to surface. This is proven by the presence of pillow basalts. Offshore of this area, once again there was sedimentation, incredibly similar to the Torlesse rocks during the first period.
This process continued throughout the Cretaceous, coupled by the mass extinction, the KT event, with huge losses of animals of this period.
Under the sea…again…
With the continued expansion of the Tasman Sea and New Zealand drifting further away from the large landmass of Gondwana, its buoyancy rapidly decreased and sinking began to occur. This was due to its continental crust not being nearly as thick as the well established supercontinent. All the current rocks, including the coal, volcanics, and older basement rocks were being recovered by marine sediments.
This type of sequence can be seen around the Greymouth area on west coast of the South Island. As you drive south along the coast you pass through the Cretaceous coal beds, followed by the overlying sandstones, mudstones, and limestones. The transition of the sandstones and mudstones to limestone created large carbonate concretions, sometimes containing fossils. These are beautifully displayed in the slowly eroding Moeraki Boulders along the east coast in the shallow seas, seen along the beaches near Moeraki, where they are slowly eroding out of their sedimentary host rock to display themselves, broken and also in tact, along the oceans edge.
Te Kuiti Group
There was one area that didn’t suffer the effects of Rangitata, along the eastern side of the North Island. So whilst the west was going through the same process described above as the South Island, the eastern side became your typical offshore depositional environment of silts, sands and eroding organics. These sediments were dubbed the Te Kuiti Group.
I drove through the Te Kuiti Group for quite some time along the west coast of the southerly part of the North Island. Passing through the characteristic stratigraphy of sandstones and siltstones to the overlaying limestone beds.
Some of the Te Kuiti siltstones along a river, with keys for scale
Where things start to get a bit more carbonaceous, siltstone interbedded with carbonates
The overlying limestone above the road where I was seeing the siltstones. see below the siltstone shown up close above in the riverbed below.