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Micropaleontological analyses based on calcareous nannofossils were conducted on samples taken from the Agencia Nacional de Hidrocarburos (ANH)-Piedras Blancas-1 Well, which was drilled in the Sinú–San Jacinto Basin, Caribbean Region of Colombia. A summary of published literature on fossil coccolithophores from the marine Cenozoic record of South America reveals that our study is the first to report Lower Eocene coccolith-bearing sedimentary rocks in the substratum of the Caribbean inland margin. The assemblages are characterized by common occurrences of Reticulofenestra dictyoda and Sphenolithus radians and sporadic occurrences of Girgisia gammation, Heliodiscoaster lodoensis, and Sphenolithus orphanknolli. The co-occurrence of H. lodoensis, R. dictyoda, and S. orphanknolli indicates that the core between 1190 ft (362 m) and 279 ft (85 m) encompasses Zone NP12 to Subzone NP14a and spans the interval between ~53 to ~48.3 Ma of the latest Early Eocene (Late Ypresian Age). Small placoliths (R. dictyoda and Reticulofenestra <3 μm) and sphenoliths (S. radians and Sphenolithus spp.) dominate the tropical assemblages. Other species include Campylosphaera dela, Coccolithus formosus, Coccolithus pelagicus, Helicosphaeroides lophota, Helicosphaeroides seminulum, and Pontosphaera spp. Investigations on calcareous nannofossil distribution in Cenozoic marine sediments from South America are few, and most published information concerns the Brazilian continental margin and inland Argentina. The Man Aike Formation from the Austral Basin of Argentina is the only Lower Eocene formation available for comparison.

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