『Abstract
The Chubut is a medium-size (42,000 km2) river basin
that drains the arid-to semiarid Patagonian seaboard and pours
its waters into the southwestern Atlantic Ocean (ca. lat 43゜20'S,
long 65゜04'W). The materials eroded from the continent and deposited
in the sea are scarcely affected by chemical weathering (the chemical
index of alteration of riverbed sediment is 〜55) and bear a typical
chemical and mineralogical signature characteristic of volcanic
arcs. Clearly, flowing toward a passive margin, the river carries
the mineralogical and chemical signature of an active margin.
Physically weathered andesites and basalts occupy only about 25％
of the drainage area, and therefore most exported material must
be supplied by outcropping sedimentary beds of variable age. The
Chubut River headwaters are placed i a tectonically active region,
soil formation is incipient (“weathering-limited regime”), and
the rate of denudation (24.6 t km-2 yr-1)
is much lower than the rates exhibited by similar rivers in other
parts of the world. The depleted dissolved ad particulate load
is determined by scarce atmospheric precipitations (i.e., the
drainage basin is in the Andean rain shadow) and by the protective
effect of Cenozoic lava flows that often shield sedimentary formations
from denudation. Although the index of chemical variability suggests
that materials exported are products of the first denudational
cycle, the geological history supports the view that most materials
may have passed two or even three times through the exogenous
cycle without acquiring a chemical or mineralogical signature
indicative of repeated weathering. This is probably also true
for other basins in temperate Andean climates.』
Introduction
Material and methods
Study area
Physiographic, climatic, and hydrologic characteristics
Geological setting
Source signatures
The mineralogy of sediments
The chemical composition of sediments
The characteristics of dissolved constituents
Chemical weathering
The chemical index of alteration
The alpha indices
Discussion
Denudation regime
The issue of recycled sediments
Conclusions and implications
Acknowledgments
References cited