| Abstract: | This study of the clay mineralogy of the surface sediments in the southwestern Gulf of Mexico and its adjacent river outlets is based primarily on X-ray diffractograms of the less than 2u size fraction of 184 samples. Montmorillonite, mica (illite), kaolinite and chlorite, as well as metahalloysite, tabular halloysite and attapulgite occur in the study area. The clay mineral suites found at the mouth of ten river systems in Mexico fall into four natural geographical groups. The controlling influence for this grouping is the lithologies of the parent rocks found in the drainage basins. The most distinctive province is one containing large quantities of volcanics. The clay suites in the rivers do not reflect climatic differences between their drainage basins. Very similar suites are present in the Rio Grande River, which drains an arid to semiarid region, and the Rio Grijalva River, which drains a tropical rain forest. Within the southwestern Gulf of Mexico the clay mineral suite is remarkably homogeneous in the northern and western portions because of mixing by the current system. Montmorillonite is the predominant clay mineral and varies between 40% and 45%. Mica is the next most abundant, averaging about 35%. Kaolinite and chlorite average about 12% and 8%, respectively. In the Bay of Campeche and adjacent shelves the areal distribution of the clay minerals is more variable in which case they reflect their source rivers. A cyclonic current system in this region tends to shift the clay minerals counterclockwise away from their sources. Halloysite as measured by a "halloysite index" is a minor but distinctive mineral in this area. No evidence of significant diagenesis or differential transport is found in the clay minerals in the southwestern Gulf. The results of this study places serious restrictions on the ability of clay minerals in ancient sediments to provide information concerning their source or depositional environment. |
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