Strontium isotopic ratios and Sr contents of molluscs from the Miocene Pebas Formation in Colombian Amazonia indicate that the bulk of the Pebas Formation was deposited in a freshwater environment, predominantly draining the Andes, with minor contribution from the shield regions. The molluscan oxygen and carbon isotopic composition does not provide a palaeosalinity proxy, but points to a lacustrine palaeoenvironment. The faunal character confirms such an interpretation. Sr isotopic ratios further suggest that a marine incursion that reached the area was composed of a mixture of shield-derived freshwater and seawater, with a maximum salinity of about 5 psu. Faunistic components of this incursion-level support a shield-seawater mixing. The incursion most likely reached western Amazonia from the north, probably along the western Guyana shield.
Vonhof, H. B., Wesselingh, F. P., & Ganssen, G. M. (1998). Reconstruction of the Miocene western Amazonian aquatic system using molluscan isotopic signatures. Palaeogeography, Palaeoclimatology, Palaeoecology, 141, 85-93. https://doi.org/10.1016/S0031-0182(98)00010-8