Abstract: The basal Cretaceous beds of the area are subdivided into three units: the Cape Fear formation, Middendorf formation and Bladen member of the Black Creek formation. Stratigraphie position and lithology indicate that the Cape Fear is the oldest and the Middendorf is in part the landward facies of the Bladen. The Middendorf sediments are fluviatile and the Cape Fear and Bladen sediments are probably marine.
The Cape Fear and Bladen contain abundant montmorillonite, some kaolinite and lesser amounts of illite. In the Middendorf kaolinite predominates, although a few samples have small amounts of montmorillonite and illite.
Preliminary data indicate that in the Cretaceous-Tertiary sediments of the Atlantic Coastal Plain, montmorillonite occurs in abundance in the marine beds and kaolinite in the nonmarine sediments. The basal Cretaceous beds of the area studied fit into this scheme of clay mineral distribution.
The origin of the clay minerals of the Cretaceous formations studied is approached from sedimentary and empirical standpoints. The montmorillonite-marine and kaolinite-nonmarine associations strongly suggest that the montmorillonite has formed by marine diegenesis, whereas the sedimentary evidence suggests that the montmorillonite of the Cape Fear formation is detrital and the kaolinite of the Middendorf formation has formed by prolonged postdepositional weathering of a more varied clay mineral assemblage. The sediments of the Bladen unit in themselves offer little evidence as to the origin of their clay minerals. However, the predominance of kaolinite in the contemporary deltaic Bladen suggest that kaolinite was the only clay mineral supplied to the depositional area, and that in the marine part of the sedimentation area part of this kaolinite was changed to montmorillonite by marine diagnetic processes.