A rare example of Miocene reef-rimmed carbonate platform in Central America, Abrojo, southern Costa Rica

The Early Miocene Abrojo platform, located in the southwestern Fila Costeña of Costa Rica, represents a rare reef-rimmed carbonate system within the Central American forearc. Detailed sedimentological and petrographic analyses of seven stratigraphic sections and 42 thin sections identify seven microfacies (MA1–MA7) recording environments from open-marine to shallow, reefal settings. These include siliciclastic and nummulitic facies (MA1–MA2), coral- and red-algae–dominated carbonates (MA3–MA5), macrolith–acervulinid rudstones-bindstone (MA6), and coral–algal boundstones (MA7). The vertical and lateral facies relationships document the evolution from mixed siliciclastic–carbonate deposition to a coral–algal reef buildup, followed by platform drowning due to relative sea-level rise and increased terrigenous influx. The macrolith–acervulinid facies represents the first record of such assemblages in Central America, indicating warm, back-reef conditions comparable to Tethyan Miocene reefs. Facies distribution and stratigraphic correlations define a laterally variable platform about 1.5 km wide and 4 km long, bordered by open-marine and lagoonal deposits. The coexistence and interdigitation of these carbonates with siliciclastic beds of the Térraba Formation demonstrate that Early Miocene sedimentation in the Fila Costeña occurred on a shallow mixed shelf rather than in a deep bathyal basin, refining the paleogeographic interpretation of the southern Costa Rican forearc.