ABSTRACT

The Moin Formation (1.9-1.5 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 (Cangrejos Creek type section) contains ostracode genera typical of outer shelf-lower slope environments: Krithe, Bradleya, Echinocythereis, and Cytherella, as well as smaller numbers of carbonate platform taxa: Loxocorniculum, Radimella, and Caudites. These deeper water ostracodes have their nearest modern analog faunas along the Venezuelan coast where modern oceanic upwelling brings cooler, nutrient-rich waters onto the Trinidad-Orinoco shelf. The presence of Bradleya cf. B. acceptabilis suggests the possibility that upwelling of cooler waters may have allowed the normally outer shelf-lower slope taxa to migrate up the slope into relatively shallower shelf waters. A dramatic environmental shift occurred in the western Caribbean at approximately 1.7-1.6 Ma and is recorded in the upper portion of the Moin Formation (Lomas del Mar section). The deeper water ostracode assemblage of the Cangrejos Creek section is replaced by marine ostracode assemblages of the Lomas del Mar section representative of inner-shelf, carbonate-platform faunas. These carbonate-platform ostracode faunas are composed of the genera Loxocorniculum, Jugosocythereis, Orionina, Occultocythereis, and Caudites and have their nearest modern analog faunas on the Belize carbonate platform.