The clay minerals in many estuaries of the eastern United States differ from those in the sediments of the rivers that feed these estuaries. Illite and chlorite are characteristic minerals of the estuaries as far south as Chesapeake Bay, yet chlorite is minor and illite is reduced in concentration in the rivers south of the Hudson. Although the relative kaolinite content in river sediments increases toward the south, the kaolinite content of the estuaries reaches a peak in Albemarle Sound, North Carolina, and decreases south of this area, as montmorillonite reworked from sedimentary formations underlying the continental shelf becomes a significant component. Dioctahedral vermiculite is common in river sediments along the entire coast, yet it is important in estuarine sediments only in Pamlico Sound and the Neuse River estuary. North Carolina. Diagenetic alteration of dioctahedral vermiculite as a source of chlorite is unlikely because chlorite is absent in many estuaries that receive abundant dioctahedral vermiculite from river sediments.
Two major clay mineral facies are observed in the estuaries and on the continental margin: a northern facies characterized by illite, chlorite, and traces of feldspar and hornblende, and a southern facies characterized by montmorillonite and kaolinite.
The similarity in composition between Chesapeake Bay and the estuaries to the north may be the result of reworking and redeposition of sediments contributed during Pleistocene time, inasmuch as the Chesapeake is the southernmost estuary fed by rivers that drained glaciated areas. Shoreline erosion and reworking of still older sediments on the shelf probably also contribute clay to the estuarine sediments. Bottom drift directions suggest that clay material is moved southward along the shelf and into the mouths of the estuaries; there tidal mixing homogenizes the clay mineral composition over large areas.