METRES - Bookshelf Collection - The University of Auckland

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Faults were rarely observed on the island but three subvertical fractures
exist in Ngatoka Puta Bay, striking approximately NNW and with one to six
metres downthrow to the west.
One cliff section of particular interest is that on the east coast of Te Renga-
renga in section C — C (fig.5). Here a large mass of basaltic lava (c'), 200 metres
across and 30 metres high, conformably overlies a tuff bed, breccias and flow c.
This mass (c') is subhorizontal with a comparatively flat upper surface and very
steep (50 — 60°) irregular sides. This upper surface is overlain by a tuff bed
that is semi-continuous down the sides, where it has accumulated in irregular
hollows. On either side of c' the overlying tuff bed is directly on top of, and
conformable with, the tuff bed that underlies c'.
Because there is no evidence of erosion or intrusion, it can only be assumed
that c' was emplaced in its present form as a huge ejected volcanic plug, or more
likely as an extremely viscous lava flow, c' is of considerable importance when
considering the sequence of events during the emplacement of the lower Whakau
Volcanics sequence, as the character of the flows and breccias to the east is
is markedly different from that to the west (see Stratigraphy).
SECTION c-c Te Rengarenga
Fig. 5. Detailed cliff section around Te Rengarenga.
STRATIGRAPHY
Red Mercury Island is composed of a complex sequence of interstratified
basalt flows, breccias, scoria and tuff beds, intruded by dykes and the Te
Kokowai Intrusion, that together comprise what is here termed the Whakau
Volcanics. The authors consider that this informal formational unit cannot be
correlated with the andesitic Beesons Island Volcanics or any other recognised
unit.
The island was circumnavigated by boat and on foot and the entire cliff
exposure mapped. Twenty-four stratigraphic columns have been constructed