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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Understanding Zircon Transport in the Modern Amazon River

Russell W. Mapes, Drew S. Coleman, and Afonso C. R. Nogueira
Geological Sciences, Univ of North Carolina at Chapel Hill, CB#3315, Mitchell Hall, Chapel Hill, NC 27599-3315, [email protected]
Department of Geological Sciences, Univ of North Carolina, CB#3315, Chapel Hill, NC 27599
Departamento de Geociências, Universidade Federal do Amazonas, Av. Gal. Rodrigo O.J. Ramos 3000, Manaus, AM 69.077-000

As detrital zircon provenance analysis increases in popularity, the techniques used during sample collection, preparation, analysis, and data interpretation are increasing in sophistication. An outstanding question in detrital zircon studies is how zircon age spectra represent the range of zircon sources. To address this, we have begun a study to document detrital age spectra along the length of the Amazon River. The Amazon is ideally suited for this study because: (1) it is of continental scale, (2) it has a traceable Andean zircon population in its headwaters, and (3) zircon budget modeling suggests Andean zircons should represent a statistically recognizable portion of the total zircon age spectra. To investigate zircon age population attenuation downstream, sand was collected at eleven localities along the Solimões and Amazon Rivers between the Madeira and Japurá rivers. Samples were collected from the lee face of recently deposited bedforms on emergent bars during low water. Sample spacing ranged from 50-100 km with focus on locations immediately up and downstream from major confluences. Samples were gathered from dunes and sandwaves that ranged in height from ~10 cm to 1 m, and reflected downstream transport. Grain size ranged from fine to medium sand and all samples contained abundant mafic minerals. Samples from tributaries including the the Purus and Negro Rivers were collected upstream of their confluences with the Solimões to explore age spectra mixing of partially Andean derived sediments with those with shield (Negro) and recycled sedimentary (Purus) sources. The Purus sample was collected from the leading edge of a vegetated mid-channel bar and was clean, coarse-grained quartz-sand. The Negro sample was fine-grained quartz-sand with abundant clay floccules.

Preliminary results from three existing samples suggest that age spectra are dominated by local sources. Results from the newly collected samples will test this observation and be used to understand how age-population spectra change along the course of a single, large-scale drainage system, and how mixing of zircon from distinct sources influences age spectra. Further work aims to quantify how far and how fast Andean zircons are traveling in the modern Amazon River. These results will be applied toward a better understanding of ancient fluvial deposits.

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).