Abstract. High resolution clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of the Cardigan Bay Basin (Llanbedr [Mochras Farm] borehole, northwest Wales) and the Paris Basin (Montcornet borehole, northern France) to highlight the prevailing environmental and climatic conditions. In both basins, located at similar palaeolatitudes of 30–35° N, the clay mineral assemblages comprise chlorite, illite, illite-smectite mixed-layers (R1 I-S), smectite and kaolinite in various proportions. Because the influence of burial diagenesis and authigenesis is negligible in both boreholes, the clay minerals are interpreted to be derived from the erosion of the Caledonian and Variscan massifs, including their basement and pedogenic cover. In the Cardigan Bay Basin, the variations in the proportions of smectite and kaolinite are inversely related to each other over the entire upper Sinemurian succession. As in the Pliensbachian, the stratigraphical distribution reveals an alternation of kaolinite-rich intervals reflecting strong hydrolysing conditions, and smectite-rich intervals indicating a semi-arid climate. Kaolinite is particularly abundant in the upper part of the obtusum Zone and in the oxynotum Zone, suggesting more intense hydrolysing conditions likely coeval with warm conditions responsible for an acceleration of the hydrological cycle. This interval is also marked by a negative excursion of δ13Ccarb and δ18Ocarb, which may confirm a warmer palaeoclimate, although these excursions may be exaggerated or overprinted by the carbonate diagenesis. In the north of the Paris Basin, the stratigraphical succession is much thinner compared to the Cardigan Bay Basin site, and the oxynotum Zone is either absent or highly condensed. The clay assemblages are dominantly composed of illite and kaolinite without significant stratigraphical trends, but a smectite-rich interval identified in the obtusum Zone is interpreted as a consequence of the emersion of the London-Brabant Massif following a lowering of sea-level. A long-term decrease of δ13Corg from the late oxynotum/early raricostatum zones is also recorded in the two sites and may precede or partly include the negative carbon-isotope excursion of the Sinemurian/Pliensbachian Boundary Event, recognised in most basins worldwide, and interpreted as a late pulse of the Central Atlantic Magmatic Province volcanism.
Million-year-scale alternation of warm-humid and semi-arid periods as a mid-latitude climate mode in the Early Jurassic (Late Sinemurian, Laurasian Seaway)
