Abstract

Modal sand compositions in nine rivers draining the proximal part of the Amazonian retroarc foreland basin in Peru and Bolivia provide support for the Recycled Orogen provenance model of Dickinson and Suczek. Average compositions of 52 sand samples from the Rio Madre de Dios and its tributaries are: %QFL = 74, 5, 21; %QmFLt = 60, 5, 35; %QpLvmLsm = 44, 10, 46; %LmLvLs = 69, 18, 13; and %QmPK = 91, 4, 5. These compositions accurately reflect the predominance of sedimentary and metasedimentary rocks in the Cordillera Oriental of the Andean orogen, the ultimate source terrane for the sands of the modern foreland basin. Rivers of the Madre de Dios basin can be classified in one of three ways: (1) rivers that head in the Subandean fold-thrust belt; (2) rivers that are actively incising a subtle intraforeland uplift overlain by Quaternary and upper Tertiary foreland-basin fill; and (3) trunk rivers that collect both type 1 and type 2 rivers. The rivers that drain the fold-thrust belt carry sands that are slightly more lithic than those carried by rivers that drain the intraforeland uplift. The intraforeland-derived sands are relatively enriched in volcanic rock fragments (%LmLvLs = 70, 22, 8), whereas sands derived from the fold-thrust belt are relatively enriched in sedimentary rock fragments (%LmLvLs = 66, 9, 24). The overriding control on sand composition is the relative area of bedrock type exposed in the fold-thrust belt. Sands reworked from Tertiary and Quaternary foreland-basin fill are relatively enriched in quartz, possibly owing to intense weathering of sediments in the soil profile. The preservation of predictable and systematic mixing patterns in sand compositions from different rivers suggests that sandstone petrologic data may be augmented by paleocurrent, isopach, and textural data to facilitate detailed reconstruction of drainage patterns in ancient, nonmarine foreland basins.

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