In northern Costa Rica, the upper part of a volcaniclastic lithostratigraphic unit (Venado Formation) contains low-diversity benthic foraminiferal assemblages. The geologic age of these sediments is interpreted to be late Tortonian–Messinian (N17), on the basis of the 6.1 ± 0.6 Ma K-Ar age of a younger trachyandesite and an ostracode index genus from an older unit. The benthic foraminifera, in particular two ecophenotypes of Ammonia parkinsoniana (d'Orbigny), indicate that in the late Miocene, a shallow, brackish bay existed there, the shore lying to the north and west. The bay was an extension of the Caribbean Sea, already separated from the Pacific in this part of Costa Rica.

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