Stratigraphy and volcanology of a submarine apron
from an offshore stratovolcano, Waitakere Group,
Muriwai, New Zealand
Emma Mathews (BSc)
A research thesis submitted in partial fulfilment of the requirements of the
Degree of Bachelor of Science with Honours
University of Tasmania
Centre for Ore Deposit Research
(CODES)
School of Earth Sciences
Abstract
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
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
Acknowledgements
The production of this thesis would not have been achieved without the support of many, and I
am grateful for their advice, assistance and encouragement.
Thankyou to my supervisor, Dr. Sharon Allen, for the regular advice and direction given
throughout this year, it has been great working with you. Also, for patience and understanding
for those first ·.days in the field. Thanks also to my secondary supervisors Dr. Catherine Reid and
Dr. Cathryn Gillins, for the proof readings and support.
Much thanks to Dr. Bruce Hayward, whose expertise on the rocks at Muriwai proved invaluable.
Thankyou for the guidance and information provided while in the field, and for the many
insightful articles that you have produced.
Thankyou very much to Assoc. Prof. Jocelyn McPhie, for advice, and for initially suggesting this
project to me, which has been a terrific opportunity.
This research was funded by an ARC fellowship to S. Allen with additional support for analyses
from the School of Earth Sciences. Greatest thanks.
Thanks to Fernando Della Pasqua, Greg Ebsworth and Wally Hermann for advice. I appreciate
the work of Simon Stephens, for preserving my crumbly rocks in thin sections, Katie McGoldrick
and Phil Robinson, for analyses, and June Pongratz and Alistair Chilcot for resolving computer
issues. In addition, I would like to acknowledge Russell and Don for the use of their tracks to the
southern parts of the field area.
Thankyou my darling David, for the support, encouragement, and understanding that you have
given me all the way through. I appreciate you urging me to pursue my passions, and especially
for the cuddles. You are wonderful.
Lastly, to the honours group, what a year it has been. Now we have made it! All the very best
for the future.
Contents
Abstract i Acknowledgements iii
Contents iv List of Figures vii List of Tables viii
Chapter 1: Introduction
1
pages
1.1 Preface ... 1
1.2Aims and significance ... 2
1.3Thesis organisation ... 5
1.4Project Location ... 5
1 .5 Methods ... 6
1.6 Previous work ... 6
Chapter 2: Regional Geology and Setting
7
2.1 Introduction ... 72.2 Tectonic Framework ... 7
2.3 Regional Geology ... 1 0 2.4 Age ... 15
2.5 Volcanic History of Northland from the Miocene ... 16
2.6 Depositional Environment ... 18
Chapter 3: Stratigraphy
19
3.1 Introduction ... 193.2 Stratigraphic Framework ... 22
3.2.1 Site 1 (Otakamiro Point) ... 24
3.2.2 Site 2 (Maori Bay) ... 26
3.2.3 Site 3 (Collins Bay) ... 29
3.2.4 Site 4 (Pillow Lava Bay) ... 32
3.2.5 Site 5 (Powell Bay) ... 36
3.2.6 Site 6 (Bartrum Bay) ... 38
3.3 Unit Correlation Between Sites ... 41
Chapter 4: Lithofacies of the Muriwai volcanic succession 44
4.1 Introduction ... 44
4.2 Pumiceous and scoriaceous lithofacies group ... 46
4.3 Thin pumice and crystal rich lithofacies group ... 55
4.4 Volcanic conglomerate lithofacies group ... 57
4.5 Thickly bedded granule sandstone lithofacies group ... 62
4.6 Thinly bedded fine sandstone lithofacies group ... 67
4. 7 Bioturbated pebbly sandstone facies ... 71
4.8 Polymictic breccia facies ... 7 4 4.9 Coherent lithofacies group ... 76
4.1 0 Discussion ... 80
Chapter 5: The physical properties of coherent and clastic
volcanic components
82
5.1 Introduction ... 825.2 Lava ... 82
5.3 Hyaloclastite ... 84
5.4 Pumice ... 84
5.5 Scoria ... 85
5.6 Free Crystals ... 85
5. 7 Comparison of vesicular clasts ... 85
5.8 Diagenesis ... 86
5.9 Discussion ... 86
Chapter 6: Discussion
88
6.1 Introduction ... 886.2 The Muriwai volcanic succession ... 88
6.3 Regional Setting ... 89
6.4 Depositional Environment ... 90
6.5 Source ... 91
6.6 Eruption Processes ... 92
6.7 Petrogenesis ... 92
6.8 Transport and deposition ... 93
6.9 Facies architecture ... 95
