Upper Cretaceous and lower Paleogene sedimentary rocks in southeastern Missouri record the northwest extension of the Mississippi Embayment, yet very little information exists about them due to rarity of outcrops and borehole material. This has hindered a clearer understanding of the depositional conditions before and after the terminal Cretaceous impact event. Access to freshly excavated sediments of the Owl Creek, Clayton and Porters Creek formations and to material from several US Geological Survey boreholes has provided a wealth of data to study this time interval in the area. In this study, detailed palynomorph and palynofacies data have been used to infer palynostratigraphy and paleovegetation. Characteristics of lithology, ichnofossils, invertebrate fossils, foraminifera and well logs have provided the framework for reconstructing the depositional history. Palynomorphs indicate Late Maastrichtian, Danian and Thanetian–Selandian ages for the Owl Creek, Clayton and Porters Creek formations, respectively. Although the Cretaceous–Paleogene (K–Pg) boundary itself is marked by an unconformity, the basal part of the overlying Clayton Formation was apparently deposited as a single megatsunamite following the Chicxulub impact event. Palynofloras suggest mixed coastal and inland subtropical forest vegetation dominated by angiosperm taxa, although abundant records of gymnosperm pollen production occurred at various times during the deposition of the Porters Creek. While terrestrially derived organic components dominate the four palynofacies assemblages (A–D) identified by cluster analysis, lithologic, ichnofacies, macrofossil and dinoflagellate cysts suggest that assemblages C and D, which are characterized by higher percentages of amorphous organic matter, represent more distal marine depositional environments.