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Sedimentologists have investigated grain-size distributions of sediments from various environments in an attempt to gain uniformitarian insight for reconstruction of ancient environments. However, modern river and beach sediments occurring seaward from active volcanic highlands have been little studied even though ancient analogues may be common locally.
The Pacific coastal plain in Central America is widest in Guatemala where it is abruptly terminated 10-30 mi inland by the steep slopes of a row of active Quaternary volcanoes. The slopes are locally bare of vegetation, and the strongly seasonal, torrential rainfall provides abundant bed load to low-sinuosity streams that discharge onto the coastal plain and flow seaward in a roughly parallel pattern.
Guatemala river and beach-face sands are mostly moderately to moderately well sorted (river average ^sgrI = 0.87^phgr units; beach average ^sgrI = 0.54^phgr units), near symmetrical, mesokurtic, medium-grained (river average Mz = 1.44^phgr; beach average Mz = 1.45^phgr) volcanic arenites, composed predominantly of unweathered grains of angular volcanic rock fragments, plagioclase, and ferromagnesian and opaque minerals. River sands can be distinguished from beach sands by plotting mean size versus sorting, by coarsest percentile (C) versus median percentile (M), and by inspection of cumulative probability curves. Curves for river sands commonly show subpopulations inferred to reflect sliding and/or suspension populations. Saltation populations, in both river and each sands, are usually polymodal because of mixing of grain populations differing in specific gravity and shape.
Published bivariant plots and other techniques are assessed in terms of the Guatemalan sands.
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