Abstract
AbstractEarly 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 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 δ13C 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.
Highlights
The Early Jurassic is characterized by significant environmental changes, the most prominent of these being the Toarcian Oceanic Anoxic Event (T-OAE)
The main clay minerals identified are: (i) R0 type illite-smectite mixed-layer (17 Å based on the glycolated run condition) referred to smectite for the following sections; (ii) R1 type illite-smectite mixed-layer; (iii) chlorite (14.2 Å, 7.1 Å, 4.7 Å and 3.54 Å peaks); (iv) illite (10 Å, 5 Å, 3.33 Å peaks) and (v) kaolinite (7.18 Å and 3.58 Å peaks)
The analysis of Pliensbachian clay mineral assemblages from the Llanbedr (Mochras Farm) borehole reveals that the clay minerals did not undergo significant transformations during their burial
Summary
The Early Jurassic is characterized by significant environmental changes, the most prominent of these being the Toarcian Oceanic Anoxic Event (T-OAE). Highlighted are several cold intervals, the most prominent of these occurring in the latest Pliensbachian, time equivalent to the spinatum Zone (Bailey et al, 2003; Rosales et al, 2004; Dera et al, 2009; Suan et al, 2010; Harazim et al, 2013; Gómez et al, 2016a; Bougeault et al, 2017; Arabas et al, 2017). The current study attempts, through the identification of detrital clay mineral assemblages of Pliensbachian sediments from the Llanbedr (Mochras Farm) borehole, hereafter referred to as ‘Mochras’, to estimate the intensity of physical and chemical weathering as well as the intensity of hydrolysis and runoff
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