Megaspores from mid Cretaceous deposits in western France and their biostratigraphic and palaeoenvironmental significance
Résumé
Six localities in western France contain deposits of mid Cretaceous age that have yielded small assemblages of megaspores. These are Archingeay-Les Nouillers, Puy-Puy, Les Renardières, La Buzinie and Le Brouillard quarries, and Fouras tidal flat. With one exception, all are within the Charente and Charente-Maritime regions; Le Brouillard is further north in Anjou Province. The productive horizons are in sub-units of previously defined stratigraphic units A, B and C, and range in age from latest Albian or earliest Cenomanian to earliest mid Cenomanian. Owing to a scarcity of records of mid Cretaceous megaspores that can be relied upon for comparison with those recovered in this study, a cautious approach to identification has been adopted. Most forms are only compared to previously described species or referred to in open nomenclature. The most productive sample was from the lower part of sub-unit A1 at Puy-Puy. The assemblage recovered is dominated by spores referable to isoetalean Minerisporites and Paxillitriletes, with a few baculate, rugulate and other lycopsid spores and marsilealean Molaspora rugosa in association. The occurrence of the last of these in particular might indicate that this lowermost unit of the mid Cretaceous succession is earliest Cenomanian rather than latest Albian in age, but the evidence is equivocal. The composition of the assemblages from sub-unit B2 (Fouras tidal flat) and higher levels (La Buzinie, Le Brouillard) is consistent with a Cenomanian age determination and, in common with those from Puy-Puy, Archingeay-Les Nouillers and Les Renardières, indicates a bias towards parent plants that lived in and adjacent to water bodies. Among the forms recorded are species of Ariadnaesporites, Dijkstraisporites and Tenellisporites. The mixed character of the assemblages and the presence of damaged specimens and fragments in all of them lend support to previous interpretations of lacustrine to paralic environments of deposition for the sedimentary units concerned.