Maunganui Bluff, New Zealand.
Geology
The Waipoua Basalt Formation is the youngest (15 - 11 million
years ago) of the three Miocene Waitakere Group volcanoes. It covers
some 500 km2 between Kaihu and the Waimamaku Valley. From the coast the
basalt extends inland up to 20 km to form the south-westward tilted Tutamoe
Plateau, rising up 700m high. Erosion of the softer rocks under the basalt
has formed bluffs ringed with talus slopes of basalt boulders. Along the
Kawerua coast the basalt surface dips gently out to sea, but further south
lava flows built up to 450m in the Maunganui Bluff.
Waipoua Basalt comprises lava flows, pyroclastic deposits, volcanic
breccia, dikes and breccia dikes. Because the basalt is deeply weathered
and covered in thick bush, it is almost impossible to map units cross
country.
In the Kaihu - Mangatu Stream area, Anne Wright described the Whatoro
Breccia, a layer of bits of basalt in brown tuffs, with secondary limonite
and zeolite mineralisation. This layer, 5 - 17m thick, is usually covered
with a Waipoua Basalt lava flow.
Lava flows are the bulk of the Formation. Along the coast they
are typically 5 - 15 m thick, and have been traced for up to 4 km in
the north. Aa lava is in most of the flows along the coast; scoriaceous
rubble half a metre to several metres thick is overlain by massive lava,
which is capped by another scoriaceous layer. The scoriaceous basalt is
usually oxidised to a red or mauve colour, and in the Maunganui Bluff
area, where dikes have intruded, a white zeolite infills vesicles and
joints. Away from the coast this rubble is less commonly seen.
Anne Wright did not find evidence of flow direction or any eruptive
vents. However dikes in the southern section penetrate to the top of the
sequence and may have fed the flows, which were probably terrestrial.
There are tephra beds, especially
at Maunganui Bluff, up to 2 m thick. There are mafic fragments and broken
crystals in a matrix that is pale brown when not baked red by overlying
flows. Near a dike the whole deposit is permeated by a white zeolite.
Dike swarms penetrate the pile of lava in a zone from Maunganui
bluff to Toronui Stream. The dikes vary from 0.1 to 10 m in width. Most
have dark glassy edges and a central vesiculated zone. Some show evidence
of multiple intrusion, and most have baked contacts.
Anne Wright described the basalt as plagioclase - olivine - augite,
with phenocrysts of these minerals up to 30% of the rock. She only
found two secondary minerals, a dark green mineral lining vesicles, which
she believed to be an iron-rich saponite; and chabazite, which she believed
was restricted to the areas intruded by dikes.
Since then collectors have found microminerals in lavas, scorias
and dikes, mainly at Maunganui Bluff and in Aranga Quarry. The Waima
and Kaihu Rivers have been less visited and Sue Courtney examined minerals
in coastal rocks at Kawerua.
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maintained by Rod Martin
