New Zealand Journal of Geology and Geophysics abstracts
Te Pouhawaiki Volcano and pre-volcanic topography in central Auckland:
volcanological and hydrogeological implications
DEV. K. AFFLECK
JOHN CASSIDY*
CORINNE A. LOCKE
Department of Geology
The University of Auckland
Private Bag 92019
Auckland, New Zealand
email: j.cassidy@auckland.ac.nz
Abstract Te Pouhawaiki Volcano in the Auckland Volcanic Field
was identified on the basis of a small scoria cone, but whether this cone
marked the location of a significant eruption centre has been unknown. Volcanic
stratigraphy in the central Auckland isthmus is complex, with older deposits
(possibly entire volcanic centres) obscured by younger deposits. The
distribution of lava flows in the central Auckland isthmus was strongly
influenced by the pre-volcanic topography, and is a major control on
present-day groundwater flow regimes.
Detailed gravity data from the central Auckland isthmus are used here to model
the thicknesses of volcanic deposits and hence determine the pre-volcanic
topography. The site of the former Te Pouhawaiki scoria cone is shown to
correlate with a distinct positive gravity anomaly (c. 6 μN.kg-1)
interpreted in terms of a lava-filled depression in the Waitemata surface,
surrounded by a tuff ring. This inferred explosive eruption centre is similar
in both size and eruption style to a number of others in the Auckland Volcanic
Field and suggests that the Te Pouhawaiki scoria cone may have been the surface
manifestation of a substantial eruption centre which also produced
phreatomagmatic deposits and lavas.
The gravity model also defines the location and geometry of the
paleotopographic divide between the ancestral Waitemata and Manukau River
systems, showing it to be a complex ridge system. These buried ridges peak at
c. 10-20 m depth (60-70 m a.s.l.) with a saddle in an eastern limb of the ridge
which may have allowed lava from One Tree Hill Volcano to flow north of this
divide. The configuration of the pre-volcanic Waitemata surface indicates that
the present-day groundwater flow regime is likely to be complex and divergent
away from the ridge system, controlled in some areas by narrow paleovalleys.
Within the ridge complex, an area in which groundwater flow is likely to be
convergent has been defined which correlates with the location of occasional
surface flooding.
Keywords Auckland Volcanic Field; Te Pouhawaiki; gravity;
paleotopography; hydrogeology
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44:
313-321
0028-8306/01/4402-0313 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (898K); (scanned from paper original: notes about this process)
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