Material sources, chemical weathering, and physical denudation in the Chubut River basin (Patagonia, Argentina): Implications for Andean rivers

The Chubut is a medium-size (42,000 km(2)) river basin that drains the arid-to-semiarid Patagonian seaboard and pours its waters into the southwestern Atlantic Ocean (ca. lat 43 degrees 20'S, long 65 degrees 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 sediments is similar to 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 in 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 and 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.