The uplift of the Sierra de Perijá and Mérida Andes and the marine connection with the Falcón Basin ultimately controlled the distribution of shallow-marine, coastal, and nonmarine sedimentary rocks in the Maracaibo Basin during the early and middle Miocene. These rocks contain the most important shale top seal in the basin and nearly three-quarters of the produced plus proven reserves of the supergiant Bolívar coastal fields.
The Maracaibo Basin has been isolated from extrabasinal drainage systems since the late Oligocene, and sediments derived from the surrounding highlands were either deposited in the basin or delivered into the neighboring Falcón Basin through a narrow marine passage (the westward extension of the Falcón Channel). Four unconformity-bounded sequences mapped in the northeastern sector of the Maracaibo Basin help recreate its regional paleogeography as it was flooded from the northeast through this passage. In the early Miocene, part of the basin became a semi-enclosed shallow-marine gulf, and wave- and tide-modified deltas prograded across the temporarily inactive Lama-Icotea fault system. As sea level dropped, the shoreline advanced eastward of the Falcón Channel, and valleys were incised and subsequently filled by transgressive estuarine sediments. In the next sea level highstand, tidal-bar complexes of a tide-dominated delta system prograded and filled all available accommodation space. In the middle Miocene, relative sea level dropped into the Falcón Basin, and the Maracaibo area became a mixed-load fluvial drainage basin. By late middle Miocene, the two basins were separated, and the Maracaibo Basin became an intermontane fluvial-lacustrine depression.