Clay Minerals in Sediments from the Central Part of the Guaymas Basin, the Gulf of California, Hole 478

Abstract

The clay minerals in the Upper Pleistocene sediments from DSDP Hole 478 (a drilled depth of 464 m) located in the central area of the Guaymas Basin, Gulf of California, are studied. The composition of the clay minerals and their quantitative proportions in natural mixtures are determined by modeling of X-ray diffraction patterns for oriented specimens of finely dispersed particles. The terrigenous clay minerals are mainly dioctahedral (mixed-layer smectite–illite is dominant, the amount of illite is smaller) with admixtures of chlorite and kaolinite. They are distributed throughout the entire sedimentary section, except for thermally altered sediments from the contact zones with the top of the sills, where terrigenous dioctahedral clay minerals disappear, except for illite, and are substituted by trioctahedral mixed-layer chlorite–smectite and corrensite–chlorite; trioctahedral smectite also appears. Terrigenous illite is missing in the sediments at the base of the sedimentary cover exposed by the hole, on the top of the sill complex with a thickness of no less than 125 m.