The Cretaceous geological record of the Western Interior Seaway of North America has been extensively studied and suggests a complex and dynamic history of interrelated tectonic, eustatic, oceanographic and climatic changes. This seaway linked the Arctic ocean with the subtropical Tethyan ocean and eustatic fluctuations caused mixing of the two very different water masses affecting the sedimentological and palaeontological record. Extensive work on the lithostratigraphy, biostratigraphy. and geochemistry has resulted in a highresolution chronostratigraphic framework allowing precise bed-by-bed correlation between sections hundreds of kilometres apart. Cenomanian to Turonian strata from four sections and one core, from Arizona, Utah, Colorado and Kansas have been studied for their marine palynomorph and palynofacies assemblages and the biostratigraphy is described. Precise correlation of samples between the sites shows that many biostratigraphically useful taxa have synchronous ranges and that many taxa also had environmental preferences. These are subsequently used for palaeoenvironmental interpretation. Strata from the core record the second-order transgressive-regressive eustatic Greenhorn marine cycle with superimposed third- and fourth-order fluctuations. The range of palaeoenvironments studied here include probable freshwater through near-shore to distal pelagic environments. Foraminifera have long been used in the basin for palaeoenvironmental interpretation and a direct comparison shows that the dinoflagellate cyst assemblages are equally as good at tracing sea-level and water mass fluctuations. A global marine oxygen depletion event is suggested to have occurred at the Cenomanian - Turonian boundary. Integration of palynological data with previous work suggests that burial of amorphous organic matter at this time was related to primary productivity which increased with expansion of the Tethyan water mass at times of third- and fourth-order transgression. This integrated, high-resolution study with tight chronostratigraphic control credits the use of dinoflagellate cysts, not only as a biostratigraphic tool but also with a very real and precise potential for palaeoenvironmental and palaeoecological interpretation.