Stratigraphic studies in northern Oaxaca brought to attention the importance of the clay minerals in the interpretation of the depositional history of Tertiary deposits. A complex series of interfingering continental sediments, of lacustrine, fluvial, and volcanic origin, fill adjacent intermontane basins of late Eocene-early Oligocene age. A study of the clay fraction of the former revealed the dominant presence of smectites and illites associated with attapulgite and other minor components.

The genetic significance of the clays and their relation to depositional and postdepositional processes are discussed. Water-table changes are believed to have affected the Eh and pH conditions under which these deposits were oxidized.

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