The clay-mineral composition of 86 samples from the Northern Pacific coast of Mexico was identified and the relative abundance of each clay mineral was obtained. The known minus 2µ fraction was analyzed in oriented aggregates using X-ray diffraction. Prior to X-ray, the samples were freed of carbonates, organic

End_Page 637------------------------------

matter, iron oxides, and colloidal silica and alumina.

Montmorillonite, illite, and chlorite were found to be the most abundant clay minerals. Kaolinite and vermiculite were present in some samples, usually in minor amounts.

The distribution patterns of clay minerals in the lower Gulf of California are determined by the source areas and strong diagenetic effects ensuing on contact with sea water.

The clay-mineral distribution shows a sharp contrast in clay-mineral assemblages between the marine and non-marine sediments. The montmorillonite, illite, and chlorite content of the normal marine samples is very uniform. In the warmer hypersaline environment of the swamps, formed in the depressions between cheniers, the clay-mineral assemblages have greater proportions of chlorite, indicating that intense chemical conditions behind cheniers are particularly effective in modifying the composition and structure of the clays entering the sea from rivers.

The clay mineralogy of the terrestrial samples is controlled by the source material and the weathering conditions of the area.

In both terrestrial and marine environments, anomalous clay-mineral assemblages reflect small-scale or local geographic conditions (provenance).

End_of_Article - Last_Page 638------------