The Cenozoic stratigraphic sequence in the foothills of the Eastern Cordillera of Colombia is mostly fluvial in nature and very thick (∼8000 m), but it contains very few mollusk-bearing horizons. Recent fieldwork discovered a well-preserved molluscan assemblage that occurs near the top of the Carbonera Formation (lower Miocene) in the central foothills of the Eastern Cordillera. This level, named the Huesser horizon, is laterally extensive and can be followed for tens of kilometers. The horizon is 10 m thick and was divided into eight levels, five of them highly fossiliferous. Most of the levels are dominated by the freshwater gastropod Sheppardiconcha, with lower abundances of the bivalves Anodondites and Mytilopsis. The top level is dominated by specimens from the bivalve family Arcidae. The taxonomic composition of the assemblage is similar to that of the Magdalena and Amazonas Basins during the early-to-middle Miocene. Paleoecologic, taphonomic, and palynological analyses indicate that the Huesser accumulated in a freshwater lake system, capped by a marine incursion. The development of a large lake and the subsequent marine event could be related to increasing subsidence coincident with eustatic sea-level rise that has been identified for the basin during the early Miocene.