Abstract: The gray montmorillenite-rich beds （clay rocks）, discovered in the Upper Shaximiao formation of Middle Jurassic, Wanzhou City, were studied mineralogically and chemically. The clay minerals in the clay rocks are typically montmorillonite, illite and chlorite. Based on the comparison with post-Archean Australian shale （PAAS）, north American average shale （ NASC）, background value of earth crust clay and other typical rocks, the studied clay rocks are characterized by low abundances of V, Ti, and Fe group dements, and high Al2O3/TiO2 and Ti/Th ratios. Some beds have clear negative Eu anomalies. The sources of clay rocks are interpreted as mixtures of volcanic ash and continental debris. The variation of compositions in different beds shows different mixing ratios at different times when the Sichuan basin was formed. It is suggested that the volcanic ash might come from the middle Jurassic volcanic arc in the Tethys which was about 600 km to the west.