New Zealand Journal of Geology and Geophysics abstracts
Transformation, internal stratification, and depositional processes within a
channelised, multi-peaked lahar flow
SHANE J. CRONIN
J. A. LECOINTRE
A. S. PALMER
V. E. NEALL
Soil and Earth Sciences
Institute of Natural Resources
Massey University
Private Bag 11 222
Palmerston North, New Zealand
email: S.J.Cronin@massey.ac.nz
Abstract On 25 September 1995, phreatomagmatic explosions
through Crater Lake at Ruapehu volcano, New Zealand, generated a closely spaced
sequence of lahars. From direct observations of the flows and timely
description of their deposits, we infer these debris flows transformed to
hyperconcentrated streamflows not by dilution with incorporated water they
overran, as previously proposed. Rather, the described debris flows diluted by
selective deposition of their coarse clasts as they thinned and decelerated
while spreading >700 m laterally over the Whangaehu fan. Deposits
recording this transformation are veneering (<100 mm thick) layers of
muddy sandy gravel interspersed with many boulders and cobbles. Downstream of
their transformation to hyperconcentrated streamflows, ephemeral near-channel
deposits indicate the flows were vertically stratified. A new depositional
model for these hyperconcentrated streamflows includes a basal, coarse,
sediment-concentrated "channel flow" that emplaced transitory near-channel
sediment wedges. The near-channel sediment was bouldery, massive, and poorly
sorted, like debris-flow deposits elsewhere in the Whangaehu catchment. The
upper and marginal parts of the lahars (the surface layer) were diluted, finer
hyperconcentrated flows that left voluminous overbank deposits. The overbank
sediment is poorly sorted gravelly sand, with some degree of horizontal
bedding, like other hyperconcentrated flow deposits elsewhere in the catchment.
The rapid erosion of channel-flow deposits within days to months of the events
indicates that geologic records will only preserve lateral-flow deposits of
such lahars. Hence, long after an event, interpretation of hyperconcentrated
streamflow mechanisms from geologic deposits can be misleading without the
near-channel record.
Keywords lahars; volcanic hazards; Ruapehu volcano;
hyperconcentrated streamflows; debris flows; volcaniclastic sediments
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
117-128
0028-8306/00/4301-0117 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (3476K); (scanned from paper original: notes about this process)
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