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Waitemata Group
Waitemata Group
Stratigraphic range: Early Miocene
	Waitemata Group sandstones form the cliffs of Takapuna Beach
Type	Geological group
Underlies	Waitakere Group
Overlies	Waipapa Terrane and Te Kuiti Group
Area	130 km × 60 km (81 mi × 37 mi)
Thickness	up to 2,000 m (6,600 ft)
Lithology
Primary	Sandstone, siltstone, mudstone
Other	Conglomerate, limestone, volcanoclastic sediments
Location
Country	New Zealand
Type section
Named for	Waitemata Harbour

The Waitemata Group is an Early Miocene geologic group that is exposed in and around the Auckland Region of New Zealand, between the Whangarei Harbour in the North and the Raglan Harbour in the South.^[1] The Group is predominantly composed of deep water sandstone and mudstone (flysch). The sandstone dominated units form the cliffs around the Waitemata Harbour 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 basement sedimentary rocks and volcanic rocks to the East and West.^[2] 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.^[2]

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.^[3] 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 conglomerate and breccia).^[3]

The Warkworth Subgroup is up to 1000m of inter-bedded sandstone and mudstone formed from turbidity currents. These accumulated in a bathyal submarine fan. The Meremere Subgroup is finer grained and represents a bathyal submarine fan and basin floor facies.^[4] The Waitemata Group is overlain by the volcanic and volcaniclastic Waitakere Group.^[5]

Paleontology

The Group was once thought to extend from the Oligocene to the Early Miocene, however it is now confined exclusively to the Early Miocene.^[1]^[6] In the shallow water Kawau Subgroup at least 84 taxa have been identified, half of which were molluscs, however, corals and brachiopods were also found. The environment was inferred to be a rocky shore.^[7] Nereites trace fossils are common throughout the deeper water sequence.^[2]

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.^[8]

Geotechnical properties

The Waitemata Group forms steep rapidly eroding cliffs and it is recommended that building should be avoided near them.^[9] 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.^[10]

References

^ ^a ^b Ballance, P.F. (2011). "Stratigraphy and bibliography of the Waitemata Group of Auckland, New Zealand". New Zealand Journal of Geology and Geophysics. 19 (6): 897–932. doi:10.1080/00288306.1976.10420746. ISSN 0028-8306.
^ ^a ^b ^c Ballance, P. F. (1 July 1974). "An Inter-Arc Flysch Basin in Northern New Zealand: Waitemata Group (Upper Oligocene to Lower Miocene)". The Journal of Geology. 82 (4): 439–471. Bibcode:1974JG.....82..439B. doi:10.1086/627993. ISSN 0022-1376. S2CID 129503147.
^ ^a ^b Hayward, Bruce W.; Brook, Fred J. (1984). "Lithostratigraphy of the basal Waitemata Group, Kawau Subgroup (new), Auckland, New Zealand". New Zealand Journal of Geology and Geophysics. 27 (2): 101–123. Bibcode:1984NZJGG..27..101H. doi:10.1080/00288306.1984.10422521. ISSN 0028-8306.
^ Edbrooke 2001, Geology of the Auckland area P.24-25
^ Edbrooke 2001, Geology of the Auckland area P.29
^ Edbrooke 2001, Geology of the Auckland area P.23
^ Eagle, Michael K.; Hayward, Bruce W.; Carter, Glenn (1994). "Early Miocene Rocky Shore and Coarse Sediment Fossil Communities, Kawau Island, Auckland". Records of the Auckland Institute and Museum. 31: 187–204. ISSN 0067-0464. JSTOR 42906442. Wikidata Q58677467.
^ Spörli, K. B.; Rowland, J. V. (2007). "Superposed deformation in turbidites and syn-sedimentary slides of the tectonically active Miocene Waitemata Basin, northern New Zealand". Basin Research. 19 (2): 199–216. Bibcode:2007BasR...19..199S. doi:10.1111/j.1365-2117.2007.00320.x. ISSN 0950-091X. S2CID 140541189.
^ Edbrooke 2001, Geology of the Auckland area P.63
^ Edbrooke 2001, Geology of the Auckland area P.49

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. 374