Abstract
Immature lithic arenite (55:7:38) of the Guaviare River in the Andean foothills of southern Colombia passes rapidly into sublithic arenite (87:3:10) as it travels eastward and downstream for 800 km across a low tropical savannah of the Colombian Llanos immediately east of the Andes. The sands of its long tributary, the Inirida River, are exceptionally pure (99:0:1) because they are derived from weathered molassic sands that underlie the Llanos. Small, isolated Precambrian crystalline monadnocks that pierce this Andean molasse have little influence on sand composition. This area has a seasonal rainfall of 2 to over 5 m and an average temperature of 26 to 27°C. The petrology of these streams has great significance, because it shows how effectively a seasonal tropical climate can alter the provenance and plate tectonic interpretation of ancient sandstones of foreland basins.
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