Ten planktonic foraminiferal zones are identified and six stratigraphic sequences are interpreted in the Turonian–Maastrichtian interval of the Michu-1 well in the Upper Magdalena Basin. Anoxic bottom-water conditions prevailed during the Turonian and Coniacian followed by dysoxic conditions in the Santonian and Campanian. Oxic conditions were established during the Maastrichtian age. Upwelling occurred for most of the Late Cretaceous. Coccolithophorids, planktonic foraminifera, radiolaria, ammonites, clupeoid fish, and mosasaurs dominated the food-web structure of the Turonian–Coniacian interval. Following a major turnover during the Coniacian–Santonian boundary interval, dinoflagellates were the primary producers that sustained radiolarians, a sparse population of planktonic foraminifera, clupeoid fish, and ammonites. Dinoflagellate blooms (peridinoids) were frequent during the Campanian interval and sustained a sparse population of planktonic foraminifera and abundant clupeoid fish whose feces (phosphatic pellets) were grazed extensively by a specialized population of buliminids dominated by the genus Siphogenerinoides.

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