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Abstract: Evidence of Late Pliocene-Early Pleistocene Environmental Change and Possible Coastal Upwelling Based on Marine Ostracodes from the Moin Formation, Costa Rica

Pamela F. Borne

The Moin Formation (1.9-< 0.8 Ma) of the Limon Basin on the Caribbean coast of Costa Rica preserves assemblages of marine ostracodes which reveal that a major environmental change occurred in the western Caribbean during the early Pleistocene. The lower portion of the Moin Formation contains the ostracode genera Krithe, Bradleya, Cytherella, and Munseyella. These ostracode genera typically occur in outer shelf-lower slope environments. In particular, Bradleya cf. B. acceptabilis has been documented in the Gulf of Mexico from middle-slope and greater depths. Previous estimates based on benthic foraminiferal data place the water depth of the Moin Formation in the outer shelf or upper slope. The presence of Bradleya cf. B. cceptabilis suggests the possibility that upwelling of cooler waters may have allowed these normally deeper-water taxa to migrate up the slope into relatively shallow outer shelf waters. These deeper-water ostracodes have their nearest modern analog faunas along the Venezuelan coast in regions of modern oceanic upwelling of cooler, deeper waters onto the Trinidad-Orinoco shelf. A major environmental shift occurred in the western Caribbean (estimated to be around 1.6-1.3 Ma) and is recorded in the upper portion of the Moin Formation. The deeper-water ostracode assemblage of the lower Moin is replaced by marine ostracode assemblages representative of inner shelf, carbonate platform faunas. The carbonate platform faunas are dominated by the ostracode genera Jugosocythereis, Orionin , Occultocythereis, and Caudites and have their nearest modern analog faunas on the Belize carbonate platform.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana