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Stratigraphy and volcanology of a submarine apron from an offshore stratovolcano, Waitakere Group, Muriwai, New Zealand


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Matthews , E 2003 , 'Stratigraphy and volcanology of a submarine apron from an offshore stratovolcano, Waitakere Group, Muriwai, New Zealand', Honours thesis, University of Tasmania.

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A mid to late Miocene volcanic sequence is well exposed along the west coast of the Northland
Peninsula, New Zealand. The Waitakere Group are uplifted submarine apron deposits from
offshore arc stratovolcanoes, and outcrop from Kaipara in the north, to Manukau Harbour in the
south. The submarine coherent and clastic succession at Muriwai (the Muriwai volcanic
succession) is well preserved and records the medial depositional environment of a
volcaniclastic apron from one or more submarine to emergent stratovolcanoes. Several major
Miocene volcanic centres have been identified offshore from Northland. The present location
of Muriwai is closest to the Manukau (WSW) and Kaipara (NE) centres. The stratigraphy is
dominated by resedimented volcaniclastics, thinly bedded fme sandstone, coherent lava and
pyroclastic fallout.
Six lithofacies groups were identified in the Muriwai volcanic succession, and each represents
various transport and deposition process. Basal thinly bedded fine sandstone represents slow
background sedimentation, prior to the onset of extensive eruptive activity. Scoriaceous and
pumiceous facies, volcanic conglomerate facies and coarse granule sandstone facies record
resedimentation of more proximal volcaniclastic deposits. Thin pumice and crystal rich facies
record water-settled fallout from explosive eruptions. Coherent facies probably represent
satellite vent eruptions on the volcano flanks. In general, the facies document rapid submarine
deposition in the closing stages of the eruptive history of the source volcano( es ).
Transport and deposition of the pumice and scoria, conglomerates and granule sands was
dominated by water-supported sediment gravity flows. Rounded to subangular volcanic clasts
are a significant component of these deposits, which suggests they were sourced from existing
deposits that became remobilised on the upper flanks of a volcanic edifice. Uncommon
allochthonous pebbles suggest this depositional environment also incorporated a component of
non-volcanic sediments, derived from the Northland Allochthon to the north.
Pumice and scoria clasts record significant explosive eruptions, whereas coherent bodies and
lava clasts record effusive eruptive activity. Bulk rock geochemistry defines the volcanic
succession as a medium-K, calc-alkaline suite, a characteristic product of arc volcanism. Lavas
and volcanic clasts have a compositional range from basalt to andesite. Representative sampling suggests each clast type corresponds to a distinct bulk rock composition. Lavas are
basalt to basaltic andesite, pumice are basaltic andesite, and scoria and hyaloclastite are
andesitic. The chemical variations suggest multiple eruptive events and varying degrees of
fractionation. Many pumice clasts are compositionally banded, suggesting magma mingling
and complexities in the magma chambers of the source stratovolcanoes.
Stratigraphic relationships, facies types and comparison with other arc stratovolcanoes suggests
that the bathymetry surrounding the source volcanoes once consisted of a series of radial
erosional submarine canyons, separated by depositional highs. Towards the final stages of
volcanism, mass wasting and edifice degradation disrupted background sedimentation in the
marine basin. Resedimentation processes lead to the extension of a volcaniclastic apron, which
in-filled submarine canyons at medial distances from the source volcano. The facies within the
Muriwai volcanic succession form a thick canyon filling sequence and represents part of a
clastic apron that formed in the basin adjacent to one or several volcanic centres.

Item Type: Thesis - Honours
Authors/Creators:Matthews , E
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