SEDIMENTOLOGY AND GEOMORPHOLOGY OF THE RIO MADRE DE DIOS FROM DIAMANTE TO PUERTO MALDONADO, AMAZON BASIN, PERU

Quaternary fluvial sediments underlying three laterally extensive and sedimentologically complex terrace tracts along the Rio Madre de Dios and its tributaries in the Peruvian Amazon have preserved a record of aggradation and downcutting related to local complex-response mechanisms, as well as regional climatic and tectonic change. The terraces range in height from ~45 m to ~5 m above modern river level and represent three distinct phases of deposition and erosion.

The oldest and highest terrace (T1) is underlain by sediments of the Tertiary Ipururo Formation, fluvial deposits of the Quaternary Madre de Dios Formation, and younger fluvial strata. The sediments that comprise the T2 and T3 fill sequences record meandering fluvial environments similar to that in the modern river system, including channel, point bar, levee, crevasse splay, floodplain, and floodplain lake deposits.

Fossil log-jams are present in the channel and point bar deposits of all three terraced sequences and individual logs and wood fragments are preserved in several of the floodplain lake deposits. Radiocarbon dates from the fossil wood (in progress) will allow us to determine the depositional history of the sedimentary sequences, the timing of aggradation and downcutting, and the relationship between fluvial history and climate and tectonic change in this portion of the Amazon Basin.