The petrographic and geochemical studies of the organic material from a sediment core recovered from the central part of a marshland basin situated on a narrow plateau surrounded by the Amazonian rainforest has allowed characterization and evaluation of the relative percentages of the different organic fractions. The petrographical approach allowed confirmation and a better understanding of the geochemical results. In order to prove the potential of organic palaeoenvironment markers, the results of this study were compared with those of the palynology obtained from the same samples. This comparison is consistent and provides a new framework of interpretation of palaeoclimatic changes. In the organic levels, emplaced during the Holocene forest regression, the maximum fluxes of TOC coincide with the presence of carbonized wood debris, proving the existence of frequent forest fires. There is an extended English abstract. -English summary