Excellence exemplar for AS 91192 - Describe the geological processes in a New Zealand locality.
Note that this exemplar contains a description of five different rock types, more than the two required in the standard. However, this full description then meets the requirements for discussing comprehensively as set out in the Assessment Schedule, as it places each rock type (unit) into the context of the ongoing series of processes that formed the present day Whangarei area. I would not expect that a description of only two units could achieve this and thus meet the 'Excellence' standard unless at least an overview of the geological history was given.
There MUST be some discussion of landscape-forming process to achieve; I have done this in a separate section as well as within the text of each unit to ensure adequate coverage.
Illustrations are in preparation.

Geology of the Whangarei area

The major rock units of the Whangarei area were formed over the last 200 million or so years and include sedimentary and volcanic rocks. Some were formed when New Zealand was still attached to Gondwana and others were formed after we split away 80 million years ago. The major rock units are listed here from oldest to youngest.

1. Greywacke
The oldest rock in the Whangarei area is greywacke. This rock occurs all over the eastern side of the Northland Peninsula in any places where there has been enough uplift for the overlying, younger rocks to have been eroded away. Examples can be seen on the coast east of Whangarei from Tutukaka northwards. It underlies the whole area and is referred to as the 'basement' rock for that reason.
Greywacke exposed on the coast near Tutukaka

Greywacke is a sedimentary rock formed by underwater currents of muddy water. It is a mixture of sand and mud that has been buried deep enough to become partly metamorphic. It is hard, grey in colour and fractured by many joints and faults. Some of these are filled with quartz or other minerals.
The sediment for the greywacke in Northland is formed from weathering and erosion of the Gondwana continent before the opening up of the Tasman Sea. At the time of deposition of these sediments, that continent lay perhaps about 100 km to the west (it is now part of Australia). The eroded material was transported by rivers and deposited near the river mouths. The sediments deposited there were unstable, and from time to time sandbanks collapsed forming underwater turbidity currents which flowed across the continental slope. They were deposited in an oceanic trench that existed at the time where the Pacific Plate was subducted beneath the Gondwana Plate. Evidence for this comes from occasional pillow lavas and deep sea sediments (chert) found in the greywacke, which was ‘scraped off’ the oceanic floor as the Pacific Plate was subducted at trench. These ocean floor rocks are found mixed up with the greywacke sediment throughout Auckland and Northland.
The greywacke contains some fossils and is dated as Triassic and Jurassic in age (200 – 140 million years).

2. Onerahi Formation
The Onerahi Formation is made of grey, muddy sedimentary rock that is often very extensively folded and fractured. It is named after the cliffs below the airport at Onerahi. It would be classified mostly as a mudstone, although it is somewhat variable in nature and highly disturbed.
Onerahi formation, Whangarei Harbour

The sediments that formed this rock were derived from material eroded from the Zealandia continent during and after it was separated from Gondwana by the action of plate tectonics (a new mid-ocean ridge). At this time, Zealandia (the future New Zealand) was quite a lot bigger than today but was fairly low-lying and the rocks were deeply weathered, forming much clay through chemical weathering. This clay was eroded and carried out to sea by muddy, slow-flowing rivers and was deposited quite a long way from land and about 100-200 km to the east of present Northland. After deposition it was compacted into rock, but not enough to make it very hard. This happened from Cretaceous to Eocene times, about 100 to 40 million years ago. From the time 80-60 million years ago the Tasman Sea was opening up, but that did not have much effect on the processes forming these sedimentary rocks.
About 22 million years ago a new plate boundary developed roughly parallel to the modern Northland Peninsula. The area where the Onerahi sediments were deposited was uplifted by the Pacific Plate (though probably remaining underwater), and the present Northland area was downwarped. The Onerahi sediments peeled off the higher area and slid to the west in a series of giant landslides, becoming very mixed up but not disintegrating into debris flows. They were deposited on top of younger sedimentary rocks (coal and limestone) that overlay the greywacke there. Therefore, in much of Northland, the usual rule of superposition (younger on top) is not always true and the Whangarei Limestone is younger than the Onerahi Formation that often overlies it.

3. Whangarei Limestone
waiomio_limestone.jpgThe Whangarei Limestone is a rock made mostly of calcium carbonate, derived from the skeletons and shells of dead marine organisms. It is a hard, flaggy limestone which occasionally forms caves and karst landscapes in parts of Northland.
After the erosion of Zealandia to form the Onerahi sediments, the continent was very low lying and boggy. When the sea-floor spreading that separated New Zealand and Australia stopped, the mantle gradually cooled down and contracted. This had the effect of lowering the whole landscape until it was invaded by the sea and only a few small islands were left. There was no plate boundary activity near Whangarei from 120 – 22 million years ago.
The reason the limestone formed is that there was little or no land to contribute sand or mud. The only sedimentary input was from biotic activity. The shallow submerged continent was extremely rich in fish, shellfish and other organisms. When these died, the fleshy parts decayed away, leaving shells and similar material made of calcium carbonate on the sea floor. When this was buried, it recrystallised into limestone because of the pressure.
The limestone occurs in several locations around Whangarei and forms the caves on the outskirts of the city. It is quarried for cement making near Marsden Point.
Some coal measures underly the limestone and are known in locations around Kamo and Hikurangi. These were formed on the swampy landscape before it sank into the sea.

4. Whangarei Andesite
Whangarei Heads and various other prominences in Northland such as the Hen and Chickens are the eroded remains of plate boundary volcanoes that were active 25-10 million years ago.
About 50 million years ago a new subduction zone started to form over a thousand kilome
Whangarei Heads - eroded volcanic remains

tres north of New Zealand. Over time, it spread south and the Pacific Plate started to subduct below the Indo-Australian plate beneat
The volcanic rock is andesite, an intermediate-silica rock formed by melting of the mantle triggered by water release from the subducting Pacific Plate. The present Whangarei Heads consist of the volcanic pipes and plumbing which fed the volcano, together with volcanic debris flows, lava flows and related rocks. They are heavily eroded, and the more resistant ‘plumbing’ forms the prominent rock outcrops at the top of the heads. The Pacific Plate began subducting beneath Northland about 25 million years ago. Two belts of andesite volcanoes were formed: one in the west, forming a line down the west coast from Ahipara to the Waitakeres and continuing later down to the present Taranaki volcanoes; the other in the east from Mt Camel at Hohoura down to Whangarei Heads and continuing down to Great Barrier and the Coromandels as time went on.
The activity near Whangarei had ceased by about 15 million years ago as the focus of volcanism moved south to Great Barrier and Coromandel, with the exception of some activity around Mangawhai about 10 million years ago and the ‘afterthought’ volcano of Little Barrier, which was active 3-1.5 million years ago.
Mineralisation associated with this volcanism is the source of the present interest in gold prospecting in the area between Whangarei and Kawakawa (Pukipuki gold prospect).

5. Whangarei Volcanic field
As well as the plate boundary volcanoes of the Miocene, Whangarei has some much younger basalt volcanoes about 1 to 3 million years old. These volcanoes are ‘hot spot’ volcanoes which are found well away from the plate boundary. It is thought ther are formed by dry melting within the mantle about 100 km down. The exact reason for this melting is unknown, but is thought to relate to the rotation of the convergent plate boundary from its former position parallel to the Northland Peninsula to its present orientation from Ruapehu to the Kermadec Islands and Tonga.
This rotation is thought to have caused tensional forces in the mantle which triggered melting and small intraplate basalt volcanism in several fields: Franklin to Auckland, Ti Point to Whangarei and Kaikohe. Each field shows a general tendency for the volcanic activity to move north over time and this may relate to ‘tearing’ of the crust by tectonic forces.
Basalt is a low-silica, dark volcanic rock which is highly fluid and easily allows gas to escape. As a result, basalt lava can flow considerable distances, often down valleys. Basalt flows have blocked river valleys in several places and are the cause of some of the prominent waterfalls in Northland. Whangarei Falls are an example of this.
Whangarei falls - over the edge of a lava flow

As well as lava, the basalt magma around Whangarei has formed a number of prominent scoria cones. This happens when the frothy lava cools and solidifies before it can flow away. The froth is light weight and forms steep sided cones, sometimes with a central crater. The lava flows pass under the lighter scoria.
The Whangarei activity was not particularly explosive, perhaps indicating a climate drier than today at the time, since explosive features (maars and tuff) tend to form where basalt meets groundwater. However, the age of the field also means that these features would not be well preserved as they are easily eroded away.
A slightly warm soda spring associated with this volcanism occurs at Kamo.

6. Erosion and landscape forming processes
Much of the erosion of the Whangarei area took place when sea-level was relatively low. The river valleys drained to points considerably beyond the present coastline. When the sea rose to its present level about 6000 years ago, the valleys were flooded. In the places exposed to wave erosion, shore platforms were eroded back forming cliffs. Some hillocks in the valleys were eroded to become ‘castle’ islands on an eroded platform, such as Limestone Island. Whangerei Harbour is a drowned river valley system.
On the greywacke coast, wave action more easily removed the weathered rock around faults and joints, leaving behind the harder rock. This led to a rugged coastline with many small bays, stacks, reefs and similar features on a generally restricted shore platform. Tutukaka is an example.
In the limestone, karst landscape erosion has produced some typical karst features such as flaggy limestone bluffs and caves. This occurs through chemical weathering, when the acidity of dissolved carbon dioxide reacts with the limestone rock to dissolve it away. The Abbey Caves, just outside the city, are an example.
Scoria cones are a prominent landscape feature e.g. Hurupaki. Whangarei falls occur where the Otanagerei Stream reaches the end of a lava flow and has eroded away the softer rock at the edge of the flow.