METRES - Bookshelf Collection - The University of Auckland

16
Genesis of Whakau Volcanics Clastic Deposits
(a) Breccias
The breccias that lie between the flows may be heterogenetic, but the majority
appear to have a direct origin in the lava flows, to which they are petrographi-
cally similar. Moreover, the lava flows were often observed to lens out and
break up into breccias.
Flows often have a layer of breccia directly above and below, with red
scoria or tuff beds between. The authors envisage that the viscous lava broke up
into angular blocks around the margins as it flowed, rafting along and over­
riding the breccias, while the slower cooling central portion of the flow remained
intact as a coherent mass.
(bj Scoria
In the course of mapping, distinction between scoria and breccia was often
found difficult especially where these were weathered. Consequently, the
symbols used on the stratigraphic columns may not give an accurate picture of
the distribution and relationships of the scoria bands.
Although some of the scoria interbedded with flows in the Te Rengarenga -
Koterai-o-maru section may have been directly deposited subaerially, it seems
likely that a proportion was later rafted along by the flows. This hypothesis may
also explain such features as the scoria and breccia pods or lenses in the lava
flows of Whakau Point.
At Te Kokowai a sequence of scoria bands and thin flows overlying the Te
Kokowai Intrusion appears to be the remnant of a once more extensive scoria
cone, presumably built around a centre off Ngatoka Puta Bay.
(c) Tuff Beds
Laminated tuff beds, varying in thickness from 1 cm up to 3 metres, show
few of the features typical of aqueous deposition and were probably all subaerially
deposited. The even distribution of ash over such steep sided features
as c' (see structure) makes the idea of aqueous deposition unacceptable.
In places these tuff beds are separated into two or even three bands by
lensing lava flows and associated breccias, indicating that they were deposited
from a series of ash showers over a period of time.
In numerous places the heat of the overlying flows has baked the tuff beds,
and in Rolypoly Bay the fine-grained tuffs under-lying flow f (column 2) have
been baked to an extremely hard orange-red brick.
PETROGRAPHY
Specimen numbers are those of the petrological collections of the Department
of Geology, University of Auckland.
Whakau Volcanics
(a) Dykes, Flows and Breccias
Nine representative specimens of dykes and flows were selected for thin