Early Jurassic climate is characterized by alternating cold and warm periods highlighted by
studies based notably on oxygen isotopes measured on belemnite guards and other marine
invertebrate shells. These climatic changes include changes in the hydrological cycle and,
consequently weathering and runoff conditions. In order to clarify ...
Early Jurassic climate is characterized by alternating cold and warm periods highlighted by
studies based notably on oxygen isotopes measured on belemnite guards and other marine
invertebrate shells. These climatic changes include changes in the hydrological cycle and,
consequently weathering and runoff conditions. In order to clarify the erosion and weathering
conditions during the Pliensbachian, this study determined the mineralogical composition of
the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras
2
Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various
proportions of chlorite, illite, illite/smectite mixed-layers (R1 I-S), smectite and kaolinite, with
possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that
the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are
considered mainly detrital, and their fluctuations likely reflect environmental changes. The
variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is
particularly abundant at the base of the jamesoni Zone, in part coinciding with the d13C negative
excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the
davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones
and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite-rich
intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle,
while the smectite-rich intervals indicate a more arid climate. The spinatum Zone is
characterized by a distinct clay assemblage with abundant primary minerals, R1 I-S, kaolinite
reworked from previously deposited sediments or from Palaeozoic rocks, and probably
berthierine originating from contemporaneous ironstone-generating environments of shallower
waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition
from a dominant chemical weathering to a deeper physical erosion of the continent, probably
related to a significant sea-level fall consistent with a glacio-eustatic origin.
