The Waitemata Group is an

Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...

geologic group In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into superg ...

that is exposed in and around the

Auckland Region Auckland () is one of the 16 regions of New Zealand, which takes its name from the eponymous urban areas of New Zealand, urban area. The region encompasses the Auckland, Auckland metropolitan area, smaller towns, rural areas, and the islands o ...

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

Raglan Harbour Raglan ( or ) is a small beachside town located 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 late ...

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 railway station, the public transport hub in the central business district of Auckland, formerly Britomart Station * Waitematā ...

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 is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

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 to 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 ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

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 Ocean current, 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 ...

s. These accumulated in a bathyal

submarine fan Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater ve ...

. 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 distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

.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 (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

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 a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

, however,

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

s and

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s were also found. The environment was inferred to be a rocky shore. ''

Nereites ''Nereites'' is an ichnogenus of trace fossil. Modern tracemakers of incipient ''Nereites'' include worm-like organisms,Seilacher, A., 2007. Trace Fossil Analysis. Springer, Berlin, Heidelberg (226 pp.) horseshoe crabsMartin, A.J., Rindsberg, A. ...

'' trace fossils are common throughout the deeper water sequence.

Deformation

The Waitemata Group formed during the emplacement of the

Northland Allochthon New Zealand's Northland Region is built upon a Basement (geology), basement consisting mainly of greywacke rocks, which are exposed on the eastern side of the peninsula. In-place Eocene coal measures crop out at Kamo, near Whangārei, and Oligoc ...

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 groups Geography of the Auckland Region