Abstract
Well-site biostratigraphy using planktonic foraminifera, palynomorphs, and calcareous nannofossils was performed in Chuira-2ST well, Middle Magdalena Valley Basin, Colombia. Biostratigraphic analysis was focused in the latest Early Campanian marly and sandy-shale deposits of the Umir Formation and the limestone and shale deposits of the La Luna Formation. In this contribution, foraminifera results are discussed and correlated with palynomorphs and calcareous nannofossil data. A foraminifera zonation for the Middle Magdalena Valley Basin is proposed, which is based on the Chuira-2 and Chuira-2ST wells and other exploration wells and outcrop sections in the area. In the La Luna Formation, a total of six globotruncanids zones were established: Whiteinella archaeocretacea, Helvetoglobotruncana helvetica, Marginotruncana sigali, Dicarinella primitiva, Dicarinella concavata, Dicarinella concavata–Rugoglobigerina pilula, and Dicarinella asymetrica (earliest part). In the Umir Formation, Globotruncanita elevata (latest part), Globotruncana ventricosa, and Globotruncanita calcarata zones were recognized. The Dicarinella asymetrica and Globotruncanita elevata zones are not developed in its total range since there is a regional unconformity in the Santonian/Campanian boundary, which corresponds to the La Luna and the Umir Formations contact and involves a 4.4 Myr hiatus. In the La Luna Formation, paleoenvironments correspond to a marine inner to outer neritic, epicontinental sea with oxygen-depleted bottom waters and episodic upwelling events. Anoxia was more intense at the Middle Turonian and the Coniacian/Santonian boundary. In the Umir Formation, a shallowing trend is observed that ends in littoral to marginal marine paleoenvironments.
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