Abstract:
A petrographic study has been made of most of the type area
of the Tangihua Group in the Northern Wairoa district .
Two formations have been recognized:
The Horokaka Formation a thick sequence of basalt and
camptonite pillow lavas and breccias which is dated as Otaian an
microfossil evidence; and the Houto Intrusives, a later suite of
ultrabasics, gabbros, teschenites and diorites .
The juxtaposition of this Miocene igneous complex with
Cretaceous and lower Tertiary rocks is explained by a process of
isostatic readjustment with the volcanics sinking into the mechanically weak country rock .
The Tangihua Group was erupted into m derately deep water
under stable non-orogenic conditions in the Otaian and was uplifted
during the Kaikoura Orogeny, very soon after emplacement . This
orogeny ushered in the Manukau Breccia volcanism which represents
a different rock association characteristic of orogenic belts.
