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Shallow cores and surface samples from a beach, point bar, and channel mouth near Pascagoula, Mississippi, were used to study clay distribution in sand-rich sediments. Air photos, permeability measurements, grain-size data, photomicrographs, and X-ray diffractograms provided the data base.
Data from clay-silt-sand separations show that, in general, the clay/silt ratio increases as the sand content increases. The clay/silt ratio increases from 0.12 to 5.92 in beach samples as the "percent sand" increases from 52.6 to 99.9, from 0.25 to 9.05 in point-bar samples as the "percent sand" increases from 48.5 to 98.8, and from 0.08 to 1.19 in channel-mouth samples as the "percent sand" increases from 59.9 to 97.8.
The highest clay/silt ratios are in subaerially exposed sediments with generally high vertical permeabilities. Clay/silt ratios in the berm crest of the beach increase to about 5 as permeability increases to about 5 darcys. Clay/silt ratios in the beach portion of the point bar increase to about 9 as the permeability increases to about 1.4 darcys.
Scanning electron microscope (SEM) photomicrographs show that the clay can occur as coatings on much larger grains and as composite grains (aggregates of clay, silt, and sand). Only one possible example of authigenic growth in the clay fraction was observed with the SEM.
It is concluded that clay may be deposited in sand-rich environments through the settling of large clay-coated grains, as composite clasts, and as floccules. In-situ percolation of clay suspensions and biogenic activity may add to the clay content of sediments.
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