An environmental reconstruction of the palaeo-Amazon River system (Middle–Late Miocene, NW Amazonia)

Abstract

New sedimentological and palynological data from the Tertiary sediments in the Upper Amazon River area suggest that these sediments are fluvio-lacustrine deposits of Middle to Late Miocene age. They were generated as a result of the uplift of the Eastern Cordillera (Andes) and constitute possibly the oldest relics of the Amazon River system. The palaeoenvironment in which these sediments were deposited is characterized by extensive wetlands environments formed by swamps, shallow lakes, crevasse splay channels and crevasse-delta lakes where the channel environment is poorly represented. The palaeovegetation was dominated by palms (e.g. Mauritia and Grimsdalea), riverine taxa (e.g. Bombacaceae, Amanoa and Alchornea), ferns and fern allies (e.g. Polypodiaceae and Selaginellaceae), floating meadows (Gramineae) and aquatic taxa ( Ceratopteris, Botryococcus and Azolla). The relative abundance of Gramineae and the occurrence of Andean-type pollen taxa is related to the Andean origin of the fluvial system. The varzeas of the present Upper Amazon River flood-basin are probably the best analogue for the Middle to Late Miocene environment. Intervals rich in marine palynomorphs, mangrove pollen, brackish tolerant molluscs and ostracods, and ichnofossils of the Thalassinoides-Teichichnus association suggest that the palaeoenvironment was characterized by brackish conditions and marine influence. These marine incursions are possibly related to the Langhian and the Serravallian global sea-level rise. Although in the Middle Miocene a global cooling is known to have occurred, no indicators of a cooler climate have been observed in the Miocene palynoflora of the Upper Amazon River area. Finally, four new sporomorph species are described belonging to the form-genera Psilatriletes, Clavainaperturites and Psilaperiporites.