Abstract <p>Clay minerals from Quaternary sediments intruded by basaltic sills of varying thickness were studied using data from Hole DSDP 485/485A drilled to a depth of 331 m in the southern Gulf of California. Modeling X-ray diffraction patterns from oriented specimens (fraction &lt;0.001 mm) saturated in ethylene glycol allowed for the determination of the phase composition of clay minerals and their structural parameters, and a quantitative assessment of their content. Terrigenous clay minerals are predominantly dioctahedral (mixed-layer smectite‒illite predominates and smectite, illite and kaolinite are rare), chlorite is scarce. Finely dispersed trioctahedral chlorite‒corrensite mixed-layer minerals with chlorite (Chl) to corrensite (Cor) ratio W_Chl : W_Cor = 0.80 : 0.20–0.90 : 0.10 formed from primary terrigenous clay minerals, mainly mixed-layer smectite‒illite in sediments above the roofs of the basaltic sills with a thickness of 14.20, 30.95, 16.92, and 16.50 m. No chlorite‒corrensite was found in sediments beneath the sills with a thickness of 14.20, 16.92 , and 3.57 m. Trioctahedral smectite formed under the 3.57 m-thick sill at the contact with its base. Biotite formed under the sill 30.95 m thick, at the contact of sediments with its base. Biotite formation was accompanied by the dissolution of all the primary terrigenous clay minerals. Authigenic biotite was not previously been encountered in the Gulf of California in sediment/sill contact zones in deep-sea holes. Mixed-layer chlorite‒corrensite, smectite, and biotite in sediments from Hole 485/485A were formed as a result of a contact metasomatic process.</p>