In the humid tropics, deep chemical post-depositional weathering of arkosic materials may promote detectable distortions in the grain-size data. Under such conditions the usual statistical methods for environmental reconstruction based on grain-size parameters fail to give satisfactory results. Mineralogical data provided by the Macacu Formation a Neo-Cenozoic terrigenous non-marine deposit outcropping at the northern edge of Guanabara bay (Brazil) suggest a series of, quantitatively undetermined, post-depositional changes. These changes may be responsible for the differential behaviour of the quartz and feldspar mean content curves. Grain-size frequency distribution curves show a general trend toward bimodality, with a relative lack of silt-sized particles. By the analysis of coarse, well-stratified arkosic materials, one can notice the development of a secondary, argillaceous mode. This bimodality makes the grain-size data not characteristic of processes involved in the sedimentation and provides a gradual particle distribution convergence between such water-laid materials and the Upper Pleistocene-Holocene colluvial mantles. Post-depositional weathering is considered as a possible cause of such distortions of the grain-size data. The present analysis also provides some evidence to question the actual significance of primary structures as an indicator of changing processes of deposition. The great similarities of textural data obtained from the stratified and the structureless sections of single beds suggest that, under particular conditions, lack of structure may be due to very local mechanical deformations in the sedimentary body.