South America and a Few Grains of Sand: Part 1: Beach Sands

Abstract

To study correlations between petrography and provenance of modern sands, and thus contribute to improved interpretation of ancient sandstones, this study reports on the petrography of 218 modern South American beach sands obtained from a shoreline of 25,300 km. Judged by quartz-feldspar-rock fragment ratios, there are five major associations: Pacific (24:16:60), Argentina (28:14:58), Brazil (91:4:5), Caribbean East (59:8:23), and Caribbean West (54:17:29). Study of potash feldspar, volcanic rock fragments, and the $$SiO_{2}/Al_{2}O_{3}$$ ratio supplement Q: F: Rf ratios. Tectonics are the dominant control on the mineralogy. Both the Pacific and Brazilian associations, which form 75% of South America's coastline, identify correctly tectonic setting, that is, active margin composition for the Pacific and passive margin for the Brazilian. However, the Argentine association would yield a serious error, because sand with active-margin composition overprints a passive margin. Variation of total quartz and feldspar, plus volcanic rock fragments, rather than variation of total feldspar, largely define these provinces. The roles of climate, relief, and continental geography also emerge as important modifying factors that always deserve consideration when plate tectonics are inferred from the sandstones of ancient basins.