Late Cenozoic sedimentary sequences in Acre state, southwestern Amazonia: Fluvial or tidal? Deductions from the IGCP 449 fieldtrip

The IGCP 449 fieldtrip in June 2003 drew attention to the Late Cenozoic fluvial sequences of western Amazonia. In Acre state in western Brazil, underlain by relatively mobile crust, rivers have incised up to 70 minto the stacked latest Miocene (?)/Early Pliocene (?) sediments of the Solimُes Group, creating staircases of fluvial terraces and indicating regional uplift on this time scale. In contrast, in western Rondonia state, the Madeira River flows through the Early Proterozoic western part of the Amazon Craton, where Late Cenozoic vertical crustal motions seem minimal. The evidence in Acre suggests that the Solimُes Group was deposited by an ancestral river system associated with the incipient development of the modern eastward Amazon drainage.