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Seventeen samples bracketing the Cenomanian -Turonian stage boundary in the Bridge Creek Limestone (Greenhorn Formation), Pueblo, Colorado, were analyzed for planktonic and benthic foraminifera. A major turnover in the biostratigraphically important, deep-dwelling planktonic foraminifera occurred across this boundary. The Cenomanian rotaliporids were succeeded by the Turonian globotruncanids, perhaps due in part to dynamic global oceanographic developments including the expansion of an oceanic oxygen-minimum zone. Shallow-dwelling planktonic foraminifera on the other hand, were strongly influenced by regional factors (e.g. salinity) affecting the uppermost water column of the Greenhorn sea.

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