New Zealand Journal of Geology and Geophysics abstracts
Quaternary lahar stratigraphy of the western Ruapehu ring plain,
New Zealand
JEROME A. LECOINTRE
VINCENT E. NEALL
ALAN S. PALMER
Institute of Natural Resources
Massey University
Private Bag 11222
Palmerston North, New Zealand
Abstract One of the most widespread hazards associated with
explosive activity at Mt Ruapehu is the production of potentially destructive
lahars. Volcanic activity at this andesite stratovolcano has produced numerous
lahars throughout Quaternary time, with the resulting deposits being preserved
principally on the lower slopes of the composite cone and its ring plain. This
paper describes the stratigraphy of volcaniclastic sequences on the western
Ruapehu ring plain, in order to improve the record of known events. The
lithologic record is principally one of lahar deposits interbedded with datable
andesitic tephras, loesses, paleosols, and rhyolitic marker beds, allowing a
chronological reconstruction of volcanic events in the region during the last
120 000 yr. The main laharic surfaces include the extensive Porewan-aged
(>65 000 yr B.P.) uplifted Erua plateau (Waimarino Formation,
new) and voluminous Ratan-Ohakean (10 000-65 000 yr B.P.)
sequences of stacked diamictons east of the Waimarino Fault (Horopito
Formation, new, and Te Heuheu Formation). A marked change in
eruptive style occurred around 10 000 yr B.P. when pumiceous pyroclastic
flows were emplaced to the east and the west of the Mt Ruapehu cone (Taurewa
Eruptive Episode). At about this time, river entrenchment into the Ohakean
surface began. Holocene lahar deposits are preserved only in the Mangaturuturu
and Mangawhero catchments where they are confined to riparian strips. In the
Mangawhero catchment a mid-Holocene debris flow unit (Turoa diamicton)
contains numerous jigsaw-jointed bombs and vitric fragments, indicative of a
coeval vulcanian eruption. In historical time, only lahars in 1975 left
recognisable deposits in the Mangaturuturu catchment. Thus, the potential for
long-term preservation of modern, small-size sandy diamictons is strictly
limited on andesite stratovolcanoes such as Ruapehu.
Keywords Ruapehu; lahar; ring plain; Quaternary; volcanic
hazard; stratigraphy; new stratigraphic names; Waimarino Formation; Horopito
Formation
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 225-245
0028-8306/98/4103-0225 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (7210K); (scanned from paper original: notes about this process)
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