Planktonic foraminifera and calcareous nannoplankton data define the chronostratigraphy of intervals examined in the study area. These intervals, each comprising more than one third-order stratigraphic sequence, are bounded by sequence boundaries (unconformities) and maxi-mum flooding surfaces correlated and delineated by integrated biostratigraphic, seismic, and well-data analysis.

Ages were assigned on the basis of interpreted species extinction (downhole first appearance) and/or planktonic foraminifera and calcareous nannoplankton biozone tops. The microfossils used for Pliocene age determinations were Discoaster brouweri, Discoaster pentaradiatus, Reticulofenestra pseudoumbilica, Sphenolithus abies, Globigerinoides obliquus extremus, and Globorotalia margaritae margaritae. Similarly, Sphaeroidinellopsis disjuncta, Globorotaloides variabilis, Discoaster quinqueramus, Triquetrorhabdulus rugosus, and Discoaster berggrenii were used to define the late Miocene. The middle Miocene was defined by Globorotalia mayeri, Globorotalia fohsi s.l., and Discoaster hamatus, and the early Miocene by Globigerinoides bisphericus, Catapsydrax dissimilis, and Sphenolithus dissimilis.

Comparison and integration of the results of this study with those of other coastal basins in eastern and southeastern Mexico, and in the adjacent Texas coastal plain and offshore areas, should result in unified biostratigraphic criteria for future age determinations of Neogene intervals in the southwestern Gulf of Mexico.