Abstract. Clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of Cardigan Bay Basin (Llanbedr–Mochras Farm borehole, northwestern Wales) Paris (Montcornet northern France) to highlight prevailing environmental climatic conditions. In both basins, located at similar palaeolatitudes 30–35? N, clay assemblages comprise chlorite, illite, illite–smecti...