The Waitemata Group is an

Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene Epoch made up of two stages: the Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 Ma to 15.97 ± 0.05 Ma (million years ago). It was prece ...

geologic group that is exposed in and around the

Auckland Region Auckland () is one of the sixteen regions of New Zealand, which takes its name from the eponymous urban area. The region encompasses the Auckland Metropolitan Area, smaller towns, rural areas, and the islands of the Hauraki Gulf. Containing ...

of New Zealand, between the Whangarei Harbour in the North and the

Raglan Harbour Raglan is a small beachside town located 48 km west of Hamilton, New Zealand on State Highway 23. It is known for its surfing, and volcanic black sand beaches. History The Ngāti Māhanga iwi occupied the area around Raglan in the lat ...

in the South. The Group is predominantly composed of deep water sandstone and mudstone (

flysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ep ...

). The sandstone dominated units form the cliffs around the

Waitemata Harbour Waitemata or Waitematā may refer to: * Waitematā Harbour, the primary harbour of Auckland, New Zealand * Waitematā (local board area), a local government area in Auckland, New Zealand ** Waitematā Local Board, a local board of Auckland Council, ...

and rare more resistant conglomerates underlie some of Auckland's prominent ridges.

Sub-units and deposition

The Waitemata Group was deposited within fault controlled basins. These were bounded to the North and South by up faulted

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

basement sedimentary rocks and volcanic rocks to the East and West. The sedimentary source for the Group's sandstone is a mix of these basement sediments of the Waipapa Terrane and the mostly intermediate volcanic rocks. The maximum water depth of the Waitemata Group basin was 2,000 m. The basal strata (Kawau Subgroup) are from the early Miocene ( Otaian) and range in thickness from 10–45 m. These basal lithologies are varied and different from the overlying flyish basin. They overly the Te Kuiti Group sediments and the Mesozoic basement. This subgroup includes the Papakura Limestone, Tipakuri Sandstone Formation and the Cape Rodney Formation (

greywacke Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...

conglomerate and breccia). The Warkworth Subgroup is up to 1000m of inter-bedded sandstone and mudstone formed from

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

s. These accumulated in a bathyal submarine fan. The Meremere Subgroup is finer grained and represents a bathyal submarine fan and basin floor

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

.Edbrooke 2001, ''Geology of the Auckland area'' P.24-25 The Waitemata Group is overlain by the volcanic and volcaniclastic Waitakere Group.Edbrooke 2001, ''Geology of the Auckland area'' P.29

Paleontology

The Group was once thought to extend from the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

to the Early Miocene, however it is now confined exclusively to the Early Miocene.Edbrooke 2001, ''Geology of the Auckland area'' P.23 In the shallow water Kawau Subgroup at least 84 taxa have been identified, half of which were

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...

, however,

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

s and

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s were also found. The environment was inferred to be a rocky shore. Nereites facies trace fossils are common throughout the deeper water sequence.

Deformation

The Waitemata Group formed during the emplacement of the Northland Allochthon and is sometimes inter-bedded with it. The Allochthon continued to move South during deposition and some areas of the Waitemata Group are therefore extensively deformed.

Geotechnical properties

The Waitemata Group forms steep rapidly eroding cliffs and it is recommended that building should be avoided near them.Edbrooke 2001, ''Geology of the Auckland area'' P.63 Landslides are commonly caused by bedding plan failure in weathered Waitemata Group sedimentary rock. This is particularly the case when bedding dips towards the prominent coastal cliffs formed by the group's sandstones.Edbrooke 2001, ''Geology of the Auckland area'' P.49

References

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Bibliography

* Edbrooke, S.W. (2001) Geology of the Auckland area. Lower Hutt: Institute of Geological & Nuclear Sciences Limited. ''Institute of Geological & Nuclear Sciences 1:250,000 geological map p. 3''74

External links

List of Waitemata Group sub-units
Geologic groups of Oceania Geologic formations of New Zealand Miocene Oceania Sandstone formations Geography of the Auckland Region