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Middle to upper Miocene volcaniclastic strata of the Curre Formation of southwestern Costa Rica form the upper part of an uplifted forearc basin sequence deposited between the Middle America Magmatic Arc and the Middle America Trench. The rocks consist of first-cycle, quartz-poor lithic sandstone, conglomerate, breccia, and mudstone. Detrital modes of 50 sandstone samples from seven stratigraphic sections suggest that the sediment was derived from an undissected magmatic arc. Microprobe analyses of plagioclase and augite from 3^ samples indicate calc-alkaline magma parentage for the volcanic rocks. The most likely source for the Curre sandstones was the Middle America Magmatic Arc, today represented by the uplifted volcanically inactive Cordillera de Talamanca. The Curre Formation is here divided into k members, A through D from the base upward. The members are interpreted as follows: Member A: juvenile sediment gravity flow breccia and conglomerate deposited under conditions ranging from deep-sea channels to flanks of active volcanoes, Member B: shallow marine sandstone, conglomerate, and mudstone deposited along a narrow unstable island coast, Member C: shallow marine sandstone and siltstone and fluvial sandstone, mudstone, and conglomerate deposited along a gently subsiding coast, and Member D: fluvial and lacustrine volcaniclastic units and inter- bedded airfall volcanic ash. The sedimentation history of the forearc basin during middle to upper Miocene time is represented by the vertical succession of members, A-B-C- D, which reflects basin shoaling probably in response to increased sediment input related to active volcanism and plutonism in the volcanic arc.