Clays in the fine fraction of nearshore marine sediments from the southeastern United States are mineralogically different from clays from the rivers which drain into nearshore areas. River clays average 60 percent kaolinite with vermiculite next in abundance, and illite, chlorite, and montmorillonite present in minor amounts. The main source of river clay must be the weathered feldspathic metamorphic rocks of the Piedmont. Small rivers which rise in the Coastal Plain carry montmorillonite and kaolinite, but their quantitative contribution to nearshore areas is very small.
Nearshore marine clays of Florida, Georgia, and South Carolina average 50 percent montmorillonite, whereas those of North Carolina contain both illite and montmorillonite in abundance. A significant part of the nearshore clays must, therefore, come from non-river sources.
Shelf clays are montmorillonitic except off North Carolina where they are illitic as well. Hence, some nearshore clays may be derived either from the shelf or perhaps from erosion of coastal Pleistocene outcrops.
Some illitic clay may be transported southward to North Carolina from shore and shelf areas to the north, but this mechanism appears not to operate south of North Carolina.
River vermiculite may absorb potassium in the marine environment and collapse to a 10-Å illite spacing.