『Abstract
The Haast and Clutha rivers drain opposing flanks of New Zealand's
Southern Alps. Major element analysis of grain size fractions
(2-1 mm, 1 mm-355μm, 366-63μm, and <63μm) from bedload sediments
collected throughout the reach of each river suggest that weathering
is strongly partitioned between the chemical weathering of carbonates
and the physical weathering of silicates. Sand size fractions
from both rivers are depleted in CaO(〜0.2-2.1 wt%) relative to
source schists (〜3 wt% CaO), while silt fraction CaO concentrations
range from 2-5 wt%. The depletion of CaO in the sediments is interpreted
to be due at least in part to removal of carbonate during chemical
weathering of the schist protolith in the soil zone. The observed
covariance of CaO and P2O5
concentrations in all river sediment suggests that most CaO is
bound in a combination of phosphate-bearing minerals such as apatite
along with other heavy mineral phases with similar hydrodynamic
properties (e.g. epidote). Chemical index of alteration (CIA)
values for grain size fractions from both rivers are similar (Haast:54-63,
Clutha:4961) and do not systematically vary with grain size or
sample location. Al2O3-CaO*+Na2O-K2O (A-CN-K) relationships
suggest that CIA values are controlled by albite-muscovite mixing
rather than feldspar weathering. Both A-CN-K relationships and
modal mineralogical calculations from Clutha river samples indicate
progressive downstream attrition of muscovite from coarser to
finer grain size fractions. In contrast, Haast river sediments
display less variable normative muscovite concentrations and no
downstream enrichment/depletion trends.
The Haast and Clutha watersheds have drastically different sediment
yields, but the similarity of sediments from both rivers indicates
that there is minimal climatic control on the weathering intensity
of fluvial sediments. Rather, bedload geochemistry is controlled
primarily by mechanical breakdown of lithic fragments and subsequent
preferential attrition of muscovite>albite>quartz. The geochemical
signature of mechanical attrition and hydrodynamic winnowing is
more developed in Clutha river samples because of longer sediment
residence time within its fluvial system. These findings suggest
that high standing island (HSI) fluvial sedimentary evolution
is characterized by the dominance of physical weathering processes
and the absence of silicate chemical weathering signatures.』
1. Introduction
2. Geological setting
3. Methods
4. Results
5. Discussion
5.1. Sediment maturity and tectonic discrimination
5.2. Behavior of silicates and non-silicates during weathering
and transport
5.3. A-CNK-FM and S/10-CM-NK diagrams
5.4. Normative mineralogy
6. Conclusions
Acknowledgements
References