Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: results of a palynostratigraphic study

Abstract

New palynological and sedimentological data permit the age of Neogene sediments in northwestern Amazonia to be established more precisely, and indicate that major environmental changes occurred in the area during this time. Based on a study of borehole samples the age of the Solimões Formation (Solimões Basin, northwestern Brazil) is determined as Miocene and five pollen zones are distinguished which are correlated with existing zonations for northern South America. In addition, 30 new sporomorphs are described which belong to the following 12 genera: Psilamonocolpites, Retimonocolpites, Retitricolpites, Retibrevitricolpites, Psilatriporites (nov. gen.), Bombacacidites, Psilatricolporites, Retitricolporites, Rugutricolporites, Psilastephanoporites (nov. gen.), Psilastephanocolporoties and Heterocolpites. A correlation is made between the Brazilian wells and some of the studied outcrops in Colombian and Peruvian Amazonia based on palynological marker species. The presence of coastal elements such as mangroves (Zonocostites), indicates that during the Miocene this area was influenced by marginal marine conditions caused by several marine incursions. These incursions may be related to global sea level fluctuations. Miocene marine phases also are known from sites elsewhere in northern South America which at present, like northwestern Amazonia, are entirely ruled by continental conditions. Sediment composition shows that during the Early Miocene the Guyana Shield was the major source area of sediment input in the basins of northwestern Amazonia. In the interval from Early to Middle Miocene the Andes became the major sediment source area. The change in provenance is related to the uplift of the Eastern Cordillera. This event caused a major change in palaeoenvironment and palaeogeography in northwestern Amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial-lacustrine system with an estuarine character.