Neogene Foraminifera of the Limon Basin of Costa Rica.

Abstract

The distribution of foraminifera in the Uscari, Rio Banano, Moin Formations of the Limon Basin, Costa Rica, is the subject of this study. Heterostegina antillea and Lepidocyclina (L.) waylandvaughani place the basal Uscari Formation of Quebrada Terciopelo in Zone N3 of the latest Oligocene. The overlap of Orbulina universa and Globorotalia fohsi peripheroronda places the type section Uscari Formation in the early Middle Miocene (zones N9 to N10). The overlapping ranges of planktonic foraminifera and the ostracods Radimella ovata in the sandstones places that unit in Zone N18 of Early Pliocene. Globorotalia truncatulinoides excelsa indicates a Pleistocene age for the Moin Formation. The algal-foraminiferal limestones of the basal Uscari Formation were deposited on a shallow shelf receiving volcaniclastic sediment from the emerging islands of the Central American arc. During the Late Oligocene and Early Miocene the Linon Basin quickly subsided. In the Quebrada Terciopelo reference section of the Uscari Formation, Early Miocene foraminiferal indicators of a middle to upper bathyal environment include Siphogenerina transversa and Melonis pompilioides; late early and early Middle Miocene faunas (Uvigerina peregrina, Bolivina pseudoplicata, and Buliminella bassendorfensis indicate progressive shoaling. For the Early Miocene Rio Reventazon section of the Uscari Formation, species such as Cibicides wuellerstorfi and Melonis pompilioides indicate depths near 2000 meters. The type section of the Uscari Formation in Quebrada Uscari is dominated by an outer shelf assemblage containing Hanzawaia concentrica and Fursenkoina pontoni. The Early Pliocene sandstones of the Rio Banano Formation were deposited on a shallow, current-swept continental shelf. These rocks contain a mixture of open marine, nearshore, and a few reefal species, including Cibicidoides floridanus, Amphistegina gibbosa, Articulina mayori, and Elphidium discoidale. The claystones of the Pleistocene Moin Formation contain outer shelf to upper slope species, such as Gyroidina soldani, Cibicidoides floridanus, and Cassidulina curvata. The foraminiferal record of the Rio Banano sandstones suggests land was emergent in the southern Limon Basin, separating the Caribbean and Pacific basins, by 5 mybp, before such an event occurred in Panama.