The 1- to 3-m-thick Tres Esquinas sequence of Santonian to Campanian age is a foraminiferal pelletal packstone consisting of shells, intraclasts, coated grains, pellets, and minor detrital quartz—all diagenetically replaced to varying degrees by apatite, glauconite, dolomite, pyrite, and silica. The phosphatization and glauconitization of the unit may be related to early diagenesis in a geologic setting in which upwelled waters were combined with low influx of detrital sediments during a marine depositional stillstand coincident with an expanded oxygen-minimum layer. Dolomite formed in the postlithification stage mainly at the expense of calcite cement and glauconite. Finally, pyrite and granular quartz formed diagenetically from precursor opal and black mono-sulfides, respectively. The condensed sequence, probably 8 to 10 m.y. long, is significant in the context of Cretaceous sedimentation in that it separates the Coniacian La Luna Formation representing the culmination of Cretaceous transgression from the overlying Colon Formation of normal marine shelf environment. The regressive trend, first documented in the Colon Formation, reached its peak during the Tertiary Period.