212-2 Detrital Zircon Provenance Signatures from the Andean Marañón River and Implications for the Amazon Drainage System

Session: Reconstructing Earth Surface Processes in Orogenic Systems

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Abstract:

The Marañón River, the longest Andean river with the largest drainage area, flows axially in the high-relief hinterland between the Western and Eastern Cordilleras in the fold-thrust belt of Peru. The Marañón River is proposed to be the principal contributor of Andean sediment to the Amazon basin and is instrumental in understanding orogenesis, exhumation, and sediment routing in South America. Detrital zircon U-Pb ages from 18 samples of modern river sands reveal provenance signatures for major sediment sources within the Marañón drainage system. Results from the mainstem river and selected tributary watersheds exhibit 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) and Cenozoic (60–0 Ma) ages. The detrital zircon age distributions can be linked to Neoproterozoic-lower Paleozoic basement metamorphic rocks, Mesozoic sedimentary rocks, and Cenozoic sedimentary and volcanic deposits.  Limited variations in detrital signatures along the mainstem Marañón River match the uninterrupted exposure of pre-Cenozoic units along the length of the Western and Eastern Cordilleras. However, a greater downstream proportion of Cenozoic ages corresponds to extensive Cenozoic deposits in tributary catchments of northern Peru. For one tributary (Crisnejas River), we present zircon U-Pb ages from 9 bedrock samples 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 sedimentary and metamorphic rock units.  Further comparisons of U-Pb age signatures for the Marañón River with other Andean tributaries and downstream lowland segments of the Amazon River (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.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9404

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