New Zealand Journal of Geology and Geophysics abstracts
Methods of identifying late Quaternary rhyolitic tephras on the ring plains of
Ruapehu and Tongariro volcanoes, New Zealand
SHANE J. CRONIN
V. E. NEALL
A. S. PALMER
R. B. STEWART
Department of Soil Science
Massey University
Private Bag 11 222
Palmerston North, New Zealand
Abstract On the ring plains of Ruapehu and Tongariro
volcanoes, distal rhyolitic marker tephras provide a valuable stratigraphic
framework. However, identification of many of these tephras has been imprecise.
Here we provide a quantitative approach for identifying tephras within the
ring-plain sequences. We extend from simple canonical discriminant function
models of glass chemistry to show how these, in conjunction with other
geological information, can be used in a practical field-based study. In two
stratigraphically distinguishable groups (10-22 ka and 22-65 ka), we
established discriminant models for possible tephra correlatives from standard
glass analyses. Testing analyses from unknown tephras against the models
classified 34 of the 41 samples with probabilities >0.75 to tephras that
were consistent with mineralogical and stratigraphic evidence. Unknowns with
lower probabilities of classification had several possible correlatives. Some
of these were improved when the tephras classified with >0.75 probability,
and which were consistent with stratigraphic and other evidence, were added to
the discriminant models. The classifications were improved because of an
increased number of samples for each tephra and also because the added analyses
were produced by the same EMP operator under the same instrument conditions.
Classifications of other unknowns were improved by considering them as mixed
tephras. In addition to more rigorously correlating several tephras previously
identified in this area, we have identified four tephras in the area for the
first time--the Okaia, Omataroa, and Hauparu Tephras and the Rotoehu Ash. These
occur as microscopic accumulations of rhyolitic glass shards within weathered
andesitic tephra deposits.
Keywords Ruapehu volcano; Tongariro volcano; ring plain;
stratigraphy; rhyolitic tephra correlation; discriminant function analysis;
Okaia Tephra; Omataroa Tephra; Hauparu Tephra; Rotoehu Ash
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 175-184
0028-8306/97/4002-0175 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (937K); (scanned from paper original: notes about this process)
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