Thesis defense of France Polette

Palynological continental assemblages from French Lower Cretaceous deposits (Tithonian-Cenomanian) : palaeoenvironments, paleoclimats, stratigraphy, and taxonomy

Abstract

In order to improve our knowledge regarding the evolution of Lower Cretaceous plant paleoecosystems, being a fundamental period of their evolutionary history, here is presented the first bibliographical, paleoclimatic, palaeoenvironmental, stratigraphic, and taxonomic synthesis of continental palynological assemblages from French Lower Cretaceous deposits. This study is based on the observation of palynofloras recovered from 20 French sites (mostly Charentes, north of the Paris Basin, and subalpins massifs), and four sites from England, the age of deposits, mostly continental, ranging from Tithonian to Cenomanien.
446 palynological species were identified following the observation of 120 samples, mainly corresponding to spores and pollen grains, but algae and dinocysts were also recovered. A Morpho-statistical analysis of Lower Cretaceous verrucate spores was carried out in order to determine precisely the diagnostic features of the morpho-species, and improve their stratigraphic and palaeoenvironmental resolution in the long run.
The palaeoenvironmental and paleoclimatic evolution inferred from palynological observations suggests the existence of arid to semi-arid climatic conditions between late Tithonien and mid-Berriasian (sebkhas-type environments), followed by an abrupt change marked by a pronounced humid phase during late Berriasien and earliest Valanginian, and by a progressive decrease of the humidity from the mid Valanginian to early Aptian (floodplains and humid forests-types environments). Albian and Cenomanian ages are characterized by generally humid conditions, thus less so than the humid phase associated with the late Berriasian (mangrove-type environments). Several aridification phases are noted between late Berriasian and late Albian : at the end of early Valanginian, between late Barremian and early Aptian (thus not very pronounced in northern France), and to a lesser extent during the mid Albian in northern France.
Lower Cretaceous continental palynomorphs are rarely considered as strong briostratigraphic markers. Thus, two complementary dating methods have been developed herein, in order to facilitate the dating process of continental deposits. Their validity was tested using Lower Cretaceous continental assemblages from Purbeck and Wealden strata of England, and they were used to determine precisely the age of deposition of Angeac (Charente), and Pays de Bray (Normandie) strata.