Early Diagenesis of Clay Minerals, Rio Ameca Basin, Mexico

Abstract

ABSTRACT Chemical analyses of Recent sediments from Banderas Bay, Mexico, show that the non-exchangeable Mg content of the clay fraction is higher, and the Fe content is lower, in sediments from strongly reducing environments than in similar sediments from less reducing environments. Clay mineralogy (52% montmorillonite, 28% kaolinite, 20% illite) does not vary significantly among the different environments. The total cation-exchange capacity of the clays does not differ in the different environments although exchangeable Na is markedly lower in clays from the sulfide-rich sediments. A possible explanation for these effects is that in a strongly reducing environment, Fe leaves the structure to form a sulfide, and Mg enters the same sites from the surrounding water, so that gros clay mineralogy is unchanged. Mg is depleted in the interstitial water of the anoxic sediments and Mg must be diffusing into the sediment from the overlying sea-water to account for the increased Mg content of the clay. Analysis of clay minerals separated by continuous particle electrophoresis indicates that the montmorillonite contains approximately 1.2% K2O, and that the non-exchangeable Mg content of the in sediment from 1 m below the surface is 10% higher than in surface sediment. Alkali-soluble Al2O3 and SiO2 in the clay fraction of the sediments (amorphous material) decreases with depth in the cores, but there is no evidence that this represents a reaction which removes cations from sea water. Apart from initial ion exchange and glauconite formation, there is no evidence for uptake of K or Na by land-derived clay minerals.