title: Sedimentology of Lake Taupo, Central North Island, New Zealand

authors: Lister, Guy Stafford

subjects: Lake Taupo

abstract: Lake Taupo, with an area of 616 km², is the largest of several lakes of volcano-tectonic origin in the Central Volcanic Region, North Island, New Zealand. In the past the lake has been affected by catastrophic volcanic events, the last being the Taupo Pumice eruptions of about 131 A.O. Since then the catchment has restabilised and sedimentation in the lake has diminished.
A number of depositional environments in Lake Taupo are characterised on the basis of surficial sediment texture and depositional processes. These environments include: shelf (sands, gravels, sand-gravel mixtures); slope (silty sands, sandy silts); rise (sandy silts, silts); basin (silts); sub-lacustrine talus slope and sub-lacustrine hill (gravel-sand-mud mixtures).
Close to existing or pre-existing river mouths, and on sub-lacustrine talus slopes, gravels typically include one or more types of catchment-derived rock fragments (andesite, rhyolite, ignimbrite, greywacke). In other parts of the lake, gravels are almost entirely pumice fragments derived from pumice tephras and breccias within the catchment. Shelf and slope sands consist of rock fragments, volcanic glass, quartz and plagioclase feldspar. Slope and rise sediments may also contain sand-sized benthic and aufwuch diatom tests (e.g. Synedra sp., Nitzschia sp., SurireZZa sp.). Basinal sediments include a minor,very fine-fine sand fraction of andesitic Ngauruhoe Ash. Silt in basinal sediments comprises planktic diatom tests (Melosira sp. » Cyclotella sp. ≃Epithemia sp.) and terrigenous grains (volcanic glass, rock fragments, quartz, plagioclase feldspar). Subordinate clay-sized material is dominated by diatomaceous and authigenic silica, detrital illite and kaolinite, and montmorillonite of probable hydrothermal origin.
The distribution and provenance of sediment components in the lake indicate that terrigenous material is derived mainly from southern catchment regions (greywacke and andesite lithologies) and to a lesser extent from the northeastern catchment (rhyolitic volcanics). Gravels and sands are deposited in shallow water where they may be re-worked by coastal processes; suspended silts and clays are transported offshore by surface or inter-flow currents, and deposited in basinal regions of the lake.
An extensive sub-bottom surface of sedimentary pumice, deposited in Lake Taupo during and soon after the Taupo Pumice eruptions, has been identified from sediment cores and seismic profiles of the lake bottom. Mud-layer thicknesses overlying this pumice surface indicate average basinal sedimentation rates of less than 30 mm/yr for central and northern lake regions, to more than 140 mm/yr in southern 1ake regions.
Areal distribution patterns of sediment texture and composition, together with depositional rates, form the basis for a proposed model of sedimentation in Lake Taupo.

text: Sedimentology of Lake Taupo, Central North Island, New Zealand Lake Taupo, with an area of 616 km², is the largest of several lakes of volcano-tectonic origin in the Central Volcanic Region, North Island, New Zealand. In the past the lake has been affected by catastrophic volcanic events, the last being the Taupo Pumice eruptions of about 131 A.O. Since then the catchment has restabilised and sedimentation in the lake has diminished.
A number of depositional environments in Lake Taupo are characterised on the basis of surficial sediment texture and depositional processes. These environments include: shelf (sands, gravels, sand-gravel mixtures); slope (silty sands, sandy silts); rise (sandy silts, silts); basin (silts); sub-lacustrine talus slope and sub-lacustrine hill (gravel-sand-mud mixtures).
Close to existing or pre-existing river mouths, and on sub-lacustrine talus slopes, gravels typically include one or more types of catchment-derived rock fragments (andesite, rhyolite, ignimbrite, greywacke). In other parts of the lake, gravels are almost entirely pumice fragments derived from pumice tephras and breccias within the catchment. Shelf and slope sands consist of rock fragments, volcanic glass, quartz and plagioclase feldspar. Slope and rise sediments may also contain sand-sized benthic and aufwuch diatom tests (e.g. Synedra sp., Nitzschia sp., SurireZZa sp.). Basinal sediments include a minor,very fine-fine sand fraction of andesitic Ngauruhoe Ash. Silt in basinal sediments comprises planktic diatom tests (Melosira sp. » Cyclotella sp. ≃Epithemia sp.) and terrigenous grains (volcanic glass, rock fragments, quartz, plagioclase feldspar). Subordinate clay-sized material is dominated by diatomaceous and authigenic silica, detrital illite and kaolinite, and montmorillonite of probable hydrothermal origin.
The distribution and provenance of sediment components in the lake indicate that terrigenous material is derived mainly from southern catchment regions (greywacke and andesite lithologies) and to a lesser extent from the northeastern catchment (rhyolitic volcanics). Gravels and sands are deposited in shallow water where they may be re-worked by coastal processes; suspended silts and clays are transported offshore by surface or inter-flow currents, and deposited in basinal regions of the lake.
An extensive sub-bottom surface of sedimentary pumice, deposited in Lake Taupo during and soon after the Taupo Pumice eruptions, has been identified from sediment cores and seismic profiles of the lake bottom. Mud-layer thicknesses overlying this pumice surface indicate average basinal sedimentation rates of less than 30 mm/yr for central and northern lake regions, to more than 140 mm/yr in southern 1ake regions.
Areal distribution patterns of sediment texture and composition, together with depositional rates, form the basis for a proposed model of sedimentation in Lake Taupo.

year: 1978

title: Products and Processes of Cone-Building Eruptions at North Crater, Tongariro

authors: Griffin, Anna Marie

subjects: Tongariro Volcano

abstract: North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre
and represents one of at least eleven vents which have been active on Tongariro since
the last glacial maximum. The most recent cone-forming activity at North Crater is
thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped
andesite cone. This project focused mainly on the cone-building eruptions at
North Crater, including stratigraphic correlations with distal tephra, interpreting
eruptive processes, and establishing the sequence of events during cone construction.
Detailed field work identified key stratigraphic sections and facies in the proximal,
medial and distal environments. These sections allowed stratigraphic correlations to
be made between proximal cone-building facies and distal sheet-forming facies at
North Crater, and established a complete North Crater eruption stratigraphy. In the
proximal environment, welded and non to poorly welded facies formed from fallout
of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the
medial and distal environment, the lithofacies consist of fallout from a convecting
plume and minor pyroclastic flow. Convective fall and non to poorly welded
pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting
explosive eruptions involving a gas-rich magma. A change to welded deposits
produced from lava-fountaining occurs later in the cone-building sequence and
suggest a change to lower explosively and eruption of gas-poor magma.
Grain size, componentry data, density, petrography and SEM analysis were carried
out on representative samples to characterise the different facies, and reveal
information about eruption processes. The non to poorly welded deposits are typically
made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and
lapilli. The welded facies are relatively dense and clast outlines are often difficult to
distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt
orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and
indicate crustal assimilation. Mingled clasts of light and dark glass were found to
have microlites present in the dark glass, but were absent in the light glass. Electron
microprobe analyses found that the dark and light glass components in a single clast
had similar compositions, showing that the contrasting physical appearance of the
glass is not due to a different chemical composition.
Forty three whole rock XRF analyses showed that the magmas ranged from basaltic
andesite to andesite, and Harker variation plots display linear trends typical of magma
mixing. Magma mixing as the most important magmatic process is supported by
disequilibrium of phenocryst compositions and phenocryst textures. Magma
viscosity, bulk density and temperature was determined using MAGMA (Kware), and
indicate that they fall into the range of typical andesites.
Eruptive activity involved vigorous lava-fountaining, minor convecting eruption
plumes and dominant collapsing eruption plumes. This activity has produced welded
and non-welded pyroclastic flow and fall deposits to form the large cone seen today.
There are significant volcanic hazards associated with this style of activity at North
Crater, characterised by lava-fountaining, eruption plume fallout, and widespread
pyroclastic flows and lahars extending beyond the ring plain. These could all be
potentially devastating to the central North Island of New Zealand.

text: Products and Processes of Cone-Building Eruptions at North Crater, Tongariro North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre
and represents one of at least eleven vents which have been active on Tongariro since
the last glacial maximum. The most recent cone-forming activity at North Crater is
thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped
andesite cone. This project focused mainly on the cone-building eruptions at
North Crater, including stratigraphic correlations with distal tephra, interpreting
eruptive processes, and establishing the sequence of events during cone construction.
Detailed field work identified key stratigraphic sections and facies in the proximal,
medial and distal environments. These sections allowed stratigraphic correlations to
be made between proximal cone-building facies and distal sheet-forming facies at
North Crater, and established a complete North Crater eruption stratigraphy. In the
proximal environment, welded and non to poorly welded facies formed from fallout
of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the
medial and distal environment, the lithofacies consist of fallout from a convecting
plume and minor pyroclastic flow. Convective fall and non to poorly welded
pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting
explosive eruptions involving a gas-rich magma. A change to welded deposits
produced from lava-fountaining occurs later in the cone-building sequence and
suggest a change to lower explosively and eruption of gas-poor magma.
Grain size, componentry data, density, petrography and SEM analysis were carried
out on representative samples to characterise the different facies, and reveal
information about eruption processes. The non to poorly welded deposits are typically
made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and
lapilli. The welded facies are relatively dense and clast outlines are often difficult to
distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt
orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and
indicate crustal assimilation. Mingled clasts of light and dark glass were found to
have microlites present in the dark glass, but were absent in the light glass. Electron
microprobe analyses found that the dark and light glass components in a single clast
had similar compositions, showing that the contrasting physical appearance of the
glass is not due to a different chemical composition.
Forty three whole rock XRF analyses showed that the magmas ranged from basaltic
andesite to andesite, and Harker variation plots display linear trends typical of magma
mixing. Magma mixing as the most important magmatic process is supported by
disequilibrium of phenocryst compositions and phenocryst textures. Magma
viscosity, bulk density and temperature was determined using MAGMA (Kware), and
indicate that they fall into the range of typical andesites.
Eruptive activity involved vigorous lava-fountaining, minor convecting eruption
plumes and dominant collapsing eruption plumes. This activity has produced welded
and non-welded pyroclastic flow and fall deposits to form the large cone seen today.
There are significant volcanic hazards associated with this style of activity at North
Crater, characterised by lava-fountaining, eruption plume fallout, and widespread
pyroclastic flows and lahars extending beyond the ring plain. These could all be
potentially devastating to the central North Island of New Zealand.

year: 2007

title: Te Awa Tupua (Whanganui river claims settlement) Bill – the endless quest for justice

authors: Te Aho, Linda

abstract: Linda Te Aho continues her review of the new legal framework for Te Awa Tupua (the Whanganui River system) as part of the settlement of historical claims by Whanganui iwi about the Whanganui River.

text: Te Awa Tupua (Whanganui river claims settlement) Bill – the endless quest for justice Linda Te Aho continues her review of the new legal framework for Te Awa Tupua (the Whanganui River system) as part of the settlement of historical claims by Whanganui iwi about the Whanganui River.

year: 2016-08