New Zealand Journal of Geology and Geophysics abstracts
Physical volcanology of Miocene basaltic pyroclastic deposits at
Pigeon Bay: remnants of flank scoria cones of Akaroa volcano,
Banks Peninsula, New Zealand
D. M. JOHNSTON
J. W. COLE
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
B. F. HOUGHTON
Institute of Geological & Nuclear Sciences
Wairakei Research Centre
Private Bag 2000
Taupo, New Zealand
Abstract Coastal exposures of four lithified pyroclastic
units at Pigeon Bay on Banks Peninsula in Canterbury, New Zealand, record flank
explosive activity at the Miocene Akaroa volcano. These eroded units are the
surface expression of lateral dike injections of hawaiite magma from a central
conduit beneath the Akaroa lava shield. The form of the deposits was severely
modified by erosion before burial within the volcanic pile and during
exhumation. Four lithofacies are proposed, based on the size and shape of
clasts and inferred degree of welding: (1) ash-rich deposits; (2) non-flattened
scoria deposits; (3) mixed scoria deposits; and (4) densely welded scoria
deposits. Spatial relationships between lithofacies, steep primary dips, and
rapid changes in thickness and grainsize of the beds enable us to infer that
the deposits are the eroded remnants of Hawaiian and Strombolian cones. Within
the deposits, variations in clast shape and size reflect subtle variations in
the processes of fragmentation, in turn controlled by the size and rise rate of
bursting gas bubbles. Different degrees of welding reflect local variation in
accumulation rates and clast temperature on deposition (in turn partially
dependent on residence time in the vent, distance from vent to point of
deposition, and clast size).
Keywords Banks Peninsula; Akaroa volcano; hawaiite; scoria
cone; Strombolian; welded tuff
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 109-115
0028-8306/97/4001--0109 $2.50/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (3347K); (scanned from paper original: notes about this process)
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