Sediment provenance signatures of the largest river in the Andes (Marañón River, Peru): Implications for signal propagation in the Amazon drainage system

Abstract

The transcontinental Amazon River system carries vast amounts of sediment from the Andes Mountains and South American craton to the Atlantic Ocean. However, diagnostic provenance signatures for Andean headwater rivers and Amazonian lowland tributaries remain poorly defined. The Marañón River, the longest Andean river with the largest upland drainage area, is proposed here to be the principal tributary and contributor of orogenic sediment to the modern Amazon River. The intermontane Marañón River flows axially (longitudinally) northward for ∼550 km from 10° to 5°S in the high-relief hinterland region between the flanking 3000–6000 m high Western Cordillera and Eastern Cordillera in the Andes of Peru. Detrital zircon U-Pb ages from 18 samples (1922 analyses) of modern river sands collected at 235–3120 m altitude reveal the provenance signatures for the major sediment sources within the Marañón River drainage system. Detrital zircon U-Pb age distributions from the mainstem river and selected tributary watersheds (∼280–6600 km²) exhibit a large proportion of Precambrian to early Paleozoic grains, including prominent 1600–1400 Ma (Rondonia-San Ignacio), 1300–900 Ma (Sunsás), and 700–450 Ma (Brasiliano-Famatinian) age groups, with subordinate Permian-Triassic (300–200 Ma) and Cenozoic (66–0 Ma) ages. The detrital zircon age distributions can be linked to extensive outcrops of late Neoproterozoic-early Paleozoic metamorphic basement, Mesozoic sedimentary rocks, and Cenozoic sedimentary and volcanic deposits in the Andean fold-thrust belt. Limited variations in detrital signatures along the mainstem Marañón River match the uninterrupted exposures of chiefly pre-Cenozoic units along the length of the Western and Eastern Cordilleras. However, a greater downstream proportion of Cenozoic ages corresponds to expanded Cenozoic deposits in tributary catchments of northernmost Peru. For one tributary (Crisnejas River), we present zircon U-Pb ages from 9 bedrock source samples (866 analyses) to assess the degree to which river sand age distributions reflect the bedrock units exposed in the corresponding drainage area. A remarkable agreement between observed and predicted age distributions from sediment provenance unmixing models suggests that modern river sands faithfully preserve the relative proportions of contributing rock units. Further comparisons of U-Pb age signatures for the intermontane Marañón River with other Andean tributaries and downstream lowland segments of the Amazon River (7603 analyses of 67 samples) reveal striking similarities between Marañón and Amazon results, with many shared characteristic age components, suggesting that Amazon River provenance may be determined principally by Andean headwater tributaries such as the Marañón River.