Isotope studies of fossil shells give insight in the Miocene paleogeography of western Amazonia

Abstract

The Cenozoic landscapes of South America have been governed by the interaction of its large scale geological units. On the one hand these are the tectonically quiescent cratonic units ofthe Guyana and Brazilian Shields, that formed the core of the continent from the moment it separated from Africa in the Cretaceous. On the other hand, there is the Andean mountain chain that has been developing since that time, shaping and reshaping South American landforms and ecosystems. One of the more significant Andean mountain building phases took place in the Early and Middle Miocene, which probably shut down most Pacific intluences on South America, and established the wet Atlantic climate system which still dominates the area today. As a result, the Amazon Basin in the Middle and Late Miocene knew widespread wetland environments, the fossiliferous sediments of which are now known as the Pebas Formation. Good chronostratigraphy of the Pebas Fm long remained problematic. Since the 1990's it has become clear from the work ofNutall (1990) and Hoorn et al. (1993, 1994a, 1994b) that the Pebas Fm is of late Early to early Late Miocene age.