50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

investigations in the 60’s and 70’s. The active
central vents shows a combination of
prheatomagmatic to magmatic explosive and
effusive events that are seemingly strongly
controlled by the hydrogeology and architecture of
the island. Marum and Benbow are well-developed
tuff cones with deep and complex craters, where pit
crater formation as well as phreatomagmatic
explosive eruption induced crater subsidence play a
major role in their formation. Flank eruptions
forming monogenetic fields commonly started with
central eruptions such as in 1913, suggesting a
strong coupling mechanism of central and flank
eruptions. Magma rise in the centre of the island,
followed by intense degassing and drainage of
degassed magma along lateral dykes is inferred by
us to be the major means by which the central 12km
wide caldera slowly formed. This caldera
formed over a long-term stable shallow magma
storage system. Hence we suggest, that the theory
of the “giant tuff cone” is not necessary to explain
the volcaniclastic facies associations of Ambrym,
and the formation of its caldera. This volcano
should hence never be used as a comparative
example for large-scale explosive
phreatomagmatism – as has been done on several
occasions since the mid 90’s. This conclusion also
highlights that the volcanic hazard of Ambrym is
rather dominated by the potential interaction of
magma and water along the NW and SE rift edge
and the style of eruptions in the central vent rather
than potential major caldera forming events that
may disrupt the island and impact upon the region.
ORAL
EARLY PALEOGENE BIOSTRATIGRAPHIC
RECORD FROM THE NERITIC MID-
WAIPARA RIVER SECTION, NORTH
CANTERBURY
E. M. Crouch, H.E.G. Morgans, B.D. Field,
C.M.Jones,J.I.Raine,C.P.Strong&
G.J. Wilson
GNS Science, PO Box 30368, Lower Hutt
(e.crouch*gns.cri.nz)
Detailed studies of early Paleogene South Pacific
sedimentary records are limited, but are critical in
providing proxy data for validating general
circulation models and understanding natural
processes that led to major perturbations in carbon
cycling and global climate. Some of the most
complete South Pacific early Paleogene records are
from distal pelagic successions in the Clarence
Valley, Marlborough, but corresponding wellexposed
records from proximal settings are
currently limited. The mid-Waipara River Section,
North Canterbury, has a well-exposed and near
continuous early Paleogene shelf succession,
providing an opportunity to document biological
and sedimentological response to climatic changes
in a proximal setting during the Paleocene to
Middle Eocene (~65–46 Ma).
From the mid-Waipara River Section, a detailed
suite of 265 samples has been collected from
Paleocene, Paleocene/Eocene boundary, and Early
to Middle Eocene successions of the Conway and
Loburn Formations, Waipara Greensand and
Ashley Mudstone. All samples are incorporated in a
detailed tape and compass survey map, and the
collections are documented in six lithologic
columns that represent a geographic and
stratigraphic progression from the westernmost
(oldest) to the easternmost (youngest) location
(Morgans et al., 2005). Integrated studies of
sedimentology, foraminifera, dinoflagellate cysts,
calcareous nannofossils, spores/pollen, and carbon
isotopes are being completed.
Biostratigraphic results suggest the top of the
Conway Formation youngs in an eastward direction
from uppermost Cretaceous in the Waipara South
Branch section to lower Paleocene in the mid-
Waipara River section. A near-complete ~120 m
thick Paleocene record is sampled, with diverse
calcareous and organic-walled microfossil
assemblages in the Early and early Late Paleocene.
Calcareous faunas are absent throughout most of
the Waipara Greensand (Late Paleocene) and age
control is primarily from dinoflagellate cysts. Initial
bulk organic � 13 C isotope results suggest the late
Paleocene carbon isotope maximum (~59.5–56 Ma)
is present in the upper Waipara Greensand
(Stormont Member), along with a geochemical
signature that is characteristic of the Waipawa
(Black Shale) Formation. The Paleocene/Eocene
boundary, or initial Eocene thermal maximum (~55
Ma), is tentatively recognised in the uppermost
Waipara Greensand, however further palynological
and � 13 C isotope analyses are needed. While
continuous in-situ earliest Eocene (Waipawan)
sediments are not presently exposed, a detailed
Early to Middle Eocene (Mangaorapan–
Heretaungan) Ashley Mudstone record indicates
most of the Early Eocene climatic optimum (~53–
50 Ma) is represented.
Morgans, H.E.G., Jones, C.M., Crouch, E.M., et al.,
2005: Upper Cretaceous to Eocene stratigraphy and
sample collections, mid-Waipara River Section, North
Canterbury. Institute of Geological and Nuclear
Sciences science report 2003/08. 101 p.
ORAL
50 th Kaikoura05 -22- Kaikoura 2005