Ōrākei Basin
Ōrākei Basin is a tidal basin and one of the extinct volcanoes in the Auckland volcanic field in the North Island of New Zealand. It has an explosion crater around wide, with a surrounding tuff ring. The present basin is slightly larger than the original maar crater. Sediments in the basin provided the first high-resolution palaeo-environmental reconstruction for northern New Zealand of the last 130,000 years. The basin supports recreational water sports activities for the local population.
Geography
Ōrākei Basin is between the suburbs of Remuera and Meadowbank, adjacent to the south shore of the Waitematā Harbour, close to the harbour entrance from the Hauraki Gulf. The western side of the basin has a road that connects the inland suburb of Remuera with the coastal suburbs and the northern side has been formed into a railway embankment which the basin drains into the sea through sluice gates at its north-east corner. The Ōrākei Creek drains some of the Remuera watershed into the north-east of the basin and the Pourewa Stream presently directly drains to the sea on the northern aspect of the basin.Geology
Basement is the mudstones and sandstones of the Miocene Waitemata Group East Coast Bays Formation. After eruption of the maar about 132,305 years ago, it became a freshwater lake that had an overflow stream in the vicinity of present Ōrākei Road bridge. About of the underlying rock was removed upon the explosive formation of the maar. There was for much the period since protection of the lake by a tuff ring crater wall. The stream inflows to the lake were from the north-east fairly close to its outlet to the north-west, so a central lake sediment core in 2007 confirmed external sedimentation was a minor component of the infill and there was a finely laminated sediment sequence. There was a high sedimentation rate averaging.The eruption tephra volume has been estimated to be with volcanic material making up about 30% of the tuff ring, so that volume of magma ejected was about About of basalt has been shown to remain underneath the crater by gravity and magnetic studies. The reason that the lake is larger than the original maar is believed to be because the inner tephra ring deposits may have slumped into the crater.
As sea level rose after the end of the last Ice Age, the lake, which by then had shallowed to a swamp, was breached by the sea about, and has been a tidal lagoon ever since. This has resulted in the deposition of marine mud over the top of the fresh water deposits. The tuff ring is generally stable except for the north-east side mainly on the northern bank of the Pourewa Stream. The tephra deposits may be over Plio-Pleistocene alluvium here, with a steep slope, and indeed the nearby slopes which are not connected to the overlying volcanics at all have been historically unstable.
Chronology
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PlotArea = right:50 top:10 left:50 bottom:10
DateFormat = yyyy
TimeAxis = orientation:vertical order:reverse
Period = from:-135000 till:2050
AlignBars = early
ScaleMajor = unit:year increment:10000 start:-135000
Colors =
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width:15 color:white
bar:test from:-135000 till:-131230 # nothing
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width:15 color:red
bar:test from:-131230 till:-129480 # Eruption
PlotData =
width:15 color:brightblue
bar:test from:-129480 till:-128000 # Phase 1 Freshwater oxic and warm
PlotData =
width:15 color:skyblue
bar:test from:-128000 till:-97000 # Phase 1 Freshwater oxic and cool and dry
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width:15 color:drabgreen
bar:test from:-97000 till:-73000 # Phase 2 Freshwater warming, autochthonous deposition and sub-oxic bottom, high autochthonous biological productivity
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width:15 color:powderblue2
bar:test from:-73000 till:-60000 # Phase 3 Freshwater autochthonous to allochthonous cooler and drier conditions
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width:15 color:drabgreen
bar:test from:-60000 till:-31000 # Phase 4 Freshwater like phase 5
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bar:test from:-31000 till:-17900 # Phase 5 cold and shallowing with sediment
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width:15 color:yellowgreen
bar:test from:-17900 till:-7050 # Bog
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bar:test from:-7050 till:2050 # Salt water and much more muddy
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at:-134999 shift: text:Ōrākei Basin main tephra deposits,
at:-132500 shift: text:lake type and climate
at:-131500 shift: text:CE
at:-129500 shift: text:
at:-130355 shift: textcolor:red text:Ōrākei Eruption
at:-128000 shift: textcolor:brightblue text:Freshwater, oxic, warm
at:-117050 shift: textcolor:red text:Ōwairaka / Mount Albert
bar:test at:-117050 mark:
at:-110000 shift: textcolor:skyblue text:Freshwater, oxic, dry cooling
at:-104220 shift: textcolor:red text:Grafton Volcano
bar:test at:-104220 mark:
at:-85000 shift: textcolor:teal text:Freshwater, anoxic bottom, warming
at:-69000 shift: textcolor:powderblue text:Freshwater, oxic, cool and dry
at:-65050 shift: textcolor:red text:Maungakiekie / One Tree Hill #
bar:test at:-65050 mark:
at:-57050 shift: textcolor:red text:Māngere Mountain #
bar:test at:-57050 mark:
at:-50000 shift: textcolor:teal text:Freshwater, anoxic bottom, warming
at:-43150 shift: textcolor:red text:Ōkataina, Rotoehu
bar:test at:-43150 mark:
at:-36450 shift: textcolor:red text:Taupō, Tahuna
bar:test at:-36450 mark:
at:-34150 shift: textcolor:red text:Ōkataina, Maketu
bar:test at:-34150 mark:
at:-33050 shift: textcolor:red text:Takarunga / Mount Victoria
bar:test at:-33050 mark:# Mt Victoria
at:-29500 shift: textcolor:drabgreen text:Freshwater, suboxic, sediment, cold
at:-26671 shift: textcolor:red text:Taupō, Okaia
bar:test at:-26671 mark:
bar:test at:-26050 mark: # Mount Eden tephra
at:-23410 shift: textcolor:red text:Taupō, Oruanui
bar:test at:-23410 mark:
at:-20345 shift: textcolor:red text:Ōkataina, Okareka
bar:test at:-20345 mark:
at:-15546 shift: textcolor:red text:Ōkataina, Rerewhakaaitu
bar:test at:-15546 mark:
at:-13685 shift: textcolor:red text:Ōkataina, Rotoroa
bar:test at:-13685 mark:
at:-10000 shift: textcolor:kelleygreen text:Freshwater, oxic, warming
at:-7050 shift: text:Sea level rise
bar:test at:-7050 mark:
at:-3000 shift: textcolor:blue text:Salt water, oxic, marine mud, warming
at:1920 shift: text:Basin sluice gates installed
bar:test at:1920 mark:
An accurate chronology exists over the last glacial cycle between about 9500 and 130,000 BP as a result of two cores taken of lake sediment in 2016. The stratigraphy has been validated against multiple dating standards, and the Laschamp event. New ages, consistent with other determinations, in all but the case of the Okareka tephra, were obtained for 14 basaltic, 18 andesitic and eight rhyolitic tephra horizons. Tephra studies, including compositional analysis, have defined the major recent Taupō Volcanic Zone eruptions where ash reached Auckland. A particularly thick deposit of is from the Rotoehu tephra from the Rotoiti eruption of the Ōkataina Caldera. This gives a slightly earlier time on analysis of 45,100 ± 3,300 years ago than the usual consensus now of 47,400 ± 3,000 years ago.