The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sediments of Middle to Late Miocene age. These sediments, here informally called Apaporis sand unit, are in nonconformable contact with the Precambrian basement, and were deposited in a low-sinuosity fluvial system with an anastomosing character that originated in the Guyana Shield. The predominantly sandy unit has organic-rich clay intervals that contain a palynological assemblage dominated by the mangrove Zonocostites ramonae (Rhizophora) and the palm Mauritiidites franciscoi (Mauritia). Abundant Zonocostites (25–85%), together with the occurrence of marine palynomorphs (dinoflagellates and foraminiferal inner-wall linings), indicate the presence of well-developed coastal mangrove forests and marine incursions. Occasional decrease of Zonocostites in favor of Mauritiidites suggests that the coastline fluctuated and the mangroves were replaced by palm vegetation. The Middle to Late Miocene age of these sediments is based on presence of the palynological marker species Grimsdalea magnaclavata and absence of the older biostratigraphic marker Crassoretitriletes vanraadshoovenii and the younger Asteraceae. This makes the unit equivalent in age to the upper Pebas/Solimões Formation. The marine ingression and coastal conditions in the heart of Amazonia possibly are related to a combination of global sea-level rise (Serravallian?) and subsidence in the periphery of the Guyana Shield. Modern analogues of the Miocene Apaporis fluvial/ coastal interface are present-day fluvial and coastal environments in Surinam and the Guyanas.