Material Sources, Chemical Weathering, and Physical Denudation in the Chubut River Basin (Patagonia, Argentina):Implications for Andean Rivers

ANDREA I. PASQUINI, PEDRO J. DEPETRIS, DIEGO M. GAIERO, AND JEAN-LUC PROBST

JOURNAL OF GEOLOGY

The University of Chicago

Lugar: Chicago; Año: 2005 vol. 113 p. 451 - 469

0022-1376

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 SW Atlantic Ocean (ca. 43º20’S, 65º04’W). The materials eroded from the continent and deposited in the sea are scarcely affected by chemical weathering (CIAs of river bed sediments ~55) and bear a typical chemical and mineralogical signature characteristic of volcanic arcs. Clearly, flowing towards a passive margin, it carries the mineralogical and chemical signature of an active one. 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-2y-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 (ICV) 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.