Origin and climatic implication of a fibrous clays occurrence in the late Lutetian-early Bartonian (Eocene) of the Paris Basin

Fibrous clay minerals associated to Mg-rich environments have been well recognized and sporadically studied in some Cenozoic units of the Paris Basin from decades (e.g. Cavelier, 1963; Trauth et al., 1968). However, their occurrence in the Basin has been little discussed in relation to the palaeoclimatic and tectonic contexts. Eocene has been recognized as the last very warm interval and records major climatic events known as hyperthermals (Westerhold et al., 2020 and references herein). Therefore, fibrous clay formation may have been controlled by climatic factors. Recent investigations based on detailed sedimentological and bio, chemo and sequence stratigraphy studies allow to refine the palaeogeographic evolution of the basin and to better constrain 3D geometric reconstructions (RGF program led by BRGM).
This study focuses on two cores (Le Tillet and Ussy-sur-Marne) and two outcrops (Saint-Maximin and Isles-les-Meldeuses) distributed along an E-W transect. Both the timing and spatialization of a fibrous clays episode are investigated. All these sections present an equivalent episode within the ‘Marnes et Caillasses’ Fm. (late Lutetian-early Bartonian), often associated with syn-sedimentary dolomite. Due to its stratigraphic position, this episode most likely encompasses the Late Lutetian Thermal Maximum (LLTM), previously recognized in the deep ocean and associated with extremely high insolation values resulting in arid conditions (Westerhold et al., 2018). Using a new dataset based on XRD, electron microscopy (TEM and SEM imaging) and petrography, we aim to discuss the formation processes of these fibrous clays in the Paris Basin. Elemental geochemistry was also carried out using Laser Induced Breakdown Spectroscopy (LIBS) on bulk sediments and compared to XRF measurements on the same sample sets.
Our results seem to provide evidence of a transformation from Mg-rich smectite to palygorskite under arid environmental conditions as proposed by some authors (e.g. Trauth, 1977 and Chen et al., 2004). An arid climate combined to a basin confinement in relation with a major Pyrenean tectonic event (Briais, 2015) might have been the key factors that favored fibrous clays sedimentation in the Paris Basin.